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Title: Electric Transmission of Water Power
Author: Adams, Alton D.
Language: English
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Transcriber’s Notes
Small capitals have been transcribed as ALL CAPITALS; text in
italics has been transcribed between underscores, as _text_. The
symbol ∩ is used to represent an upside-down U.
More Transcriber’s Notes may be found at the end of this text.
Published by the
McGraw-Hill Book Company
New York
Successors to the Book Departments of the
McGraw Publishing Company
Hill Publishing Company
Publishers of Books for
Electrical World The Engineering and Mining Journal
The Engineering Record Power and The Engineer
Electric Railway Journal American Machinist
ELECTRIC TRANSMISSION
OF
WATER POWER
_By_
ALTON D. ADAMS, A.M.
MEMBER AMERICAN INSTITUTE OF ELECTRICAL ENGINEERS
NEW YORK
MCGRAW-HILL BOOK CO.
1906
Copyrighted, 1906, by the
McGRAW PUBLISHING COMPANY
NEW YORK
TABLE OF CONTENTS
CHAPTER PAGE
I. WATER-POWER IN ELECTRICAL SUPPLY 1
II. UTILITY OF WATER-POWER IN ELECTRICAL SUPPLY 10
III. COST OF CONDUCTORS FOR ELECTRIC-POWER TRANSMISSION 19
IV. ADVANTAGES OF THE CONTINUOUS AND ALTERNATING CURRENT 31
V. THE PHYSICAL LIMITS OF ELECTRIC-POWER TRANSMISSION 44
VI. DEVELOPMENT OF WATER-POWER FOR ELECTRIC STATIONS 51
VII. THE LOCATION OF ELECTRIC WATER-POWER STATIONS 64
VIII. DESIGN OF ELECTRIC WATER-POWER STATIONS 83
IX. ALTERNATORS FOR ELECTRICAL TRANSMISSION 103
X. TRANSFORMERS IN TRANSMISSION SYSTEMS 122
XI. SWITCHES, FUSES, AND CIRCUIT-BREAKERS 135
XII. REGULATION OF TRANSMITTED POWER 155
XIII. GUARD WIRES AND LIGHTNING ARRESTERS 168
XIV. ELECTRICAL TRANSMISSION UNDER LAND AND WATER 187
XV. MATERIALS FOR LINE CONDUCTORS 200
XVI. VOLTAGE AND LOSSES ON TRANSMISSION LINES 215
XVII. SELECTION OF TRANSMISSION CIRCUITS 233
XVIII. POLE LINES FOR POWER TRANSMISSION 246
XIX. ENTRIES FOR ELECTRIC TRANSMISSION LINES 261
XX. INSULATOR PINS 270
XXI. INSULATORS FOR TRANSMISSION LINES 287
XXII. DESIGN OF INSULATOR PINS FOR TRANSMISSION LINES 298
XXIII. STEEL TOWERS 306
INDEX 327
ELECTRIC TRANSMISSION OF WATER-POWER.
CHAPTER I.
WATER-POWER IN ELECTRICAL SUPPLY.
Electrical supply from transmitted water-power is now distributed in
more than fifty cities of North America. These include Mexico City, with
a population of 402,000; Buffalo and San Francisco, with 352,387 and
342,782 respectively; Montreal, with 266,826, and Los Angeles, St. Paul,
and Minneapolis, with populations that range between 100,000 and 200,000
each. North and south these cities extend from Quebec to Anderson, and
from Seattle to Mexico City. East and west the chain of cities includes
Portland, Springfield, Albany, Buffalo, Hamilton, Toronto, St. Paul,
Butte, Salt Lake City, and San Francisco. To reach these cities the
water-power is electrically transmitted, in many cases dozens, in a
number of cases scores, and in one case more than two hundred miles. In
the East, Canada is the site of the longest transmission, that from
Shawinigan Falls to Montreal, a distance of eighty-five miles.
From Spier Falls to Albany the electric line is forty miles in length.
Hamilton is thirty-seven miles from that point on the Niagara
escarpment, where its electric power is developed. Between St. Paul and
its electric water-power station, on Apple River, the transmission line
is twenty-five miles long. The falls of the Missouri River at Cañon
Ferry are the source of the electrical energy distributed in Butte,
sixty-five miles away. Los Angeles draws electrical energy from a plant
eighty-three miles distant on the Santa Ana River. From Colgate
power-house, on the Yuba, to San Francisco, by way of Mission San José,
the transmission line has a length of 220 miles. Between Electra
generating station in the Sierra Nevada Mountains and San Francisco is
154 miles by the electric line.
[Illustration: FIG. 1.--Spier Falls Transmission Lines.]
These transmissions involve large powers as well as long distances. The
new plant on the Androscoggin is designed to deliver 10,000
horse-power for electrical supply in Lewiston, Me. At Spier Falls, on
the Hudson, whence energy goes to Albany and other cities, the electric
generators will have a capacity of 32,000 horse-power. From the two
water-power stations at Niagara Falls, with their twenty-one electric
generators of 5,000 horse-power each, a total of 105,000, more than
30,000 horse-power is regularly transmitted to Buffalo alone; the
greater part of the capacity being devoted to local industries.
Electrical supply in St. Paul is drawn from a water-power plant of 4,000
and in Minneapolis from a like plant of 7,400 horse-power capacity. The
Cañon Ferry station, on the Missouri, that supplies electrical energy in
both Helena and Butte, has a capacity of 10,000 horse-power. Both
Seattle and Tacoma draw electrical supply from the 8,000 horse-power
plant at Snoqualmie Falls. The Colgate power-house, which develops
energy for San Francisco and a number of smaller places, has electric
generators of 15,000 horse-power aggregate capacity. At the Electra
generating station, where energy is also transmitted to San Francisco
and other cities on the way, the capacity is 13,330 horse-power.
Electrical supply in Los Angeles is drawn from the generating station of
4,000 horse-power, on the Santa Ana River, and from two stations, on
Mill Creek, with an aggregate of 4,600, making a total capacity of not
less than 8,600 horse-power. Five water-power stations, scattered within
a radius of ten miles and with 4,200 horse-power total capacity, are the
source of electrical supply in Mexico City.
The foregoing are simply a part of the more striking illustrations of
that development by which falling water is generating hundreds of
thousands of horse-power for electrical supply to millions of
population. This application of great water powers to the industrial
wants of distant cities is hardly more than a decade old. Ten years ago
Shawinigan Falls was an almost unheard-of point in the wilds of Canada.
Spier Falls was merely a place of scenic interest; the Missouri at Cañon
Ferry was not lighting a lamp or displacing a pound of coal; that
falling water in the Sierra Nevada Mountains should light the streets
and operate electric cars in San Francisco seemed impossible, and that
diversion of Niagara, which seems destined to develop more than a
million horse-power and leave dry the precipices over which the waters
now plunge, had not yet begun. In some few instances where water-power
was located in towns or cities, it has been applied to electrical supply
since the early days of the industry. In the main, however, the supply
of electrical energy from water-power has been made possible only by
long-distance transmission. The extending radius of electrical
transmission for water-powers has formed the greatest incentive to
their development. This development in turn has reacted on the
conditions that limit electrical supply and has materially extended the
field of its application. Transmitted water-power has reduced the rates
for electric service. It may not be easy to prove this reduction by
quoting figures for net rates, because these are not generally
published, but there are other means of reaching the conclusion.
[Illustration: FIG. 2.--Snoqualmie Falls Transmission Lines.]
In the field of illumination electricity competes directly with gas, and
in the field of motive power with coal. During the past decade it is
well known that the price of gas has materially declined and the price
of coal, barring the recent strike period, has certainly not increased.
In spite of these reductions electrical supply from water-power has
displaced both gas and coal in many instances.
Moreover, the expansion of electric water-power systems has been
decidedly greater, as a rule, than that of electrical supply from
steam-driven stations. An example of the fact last stated may be seen in
Portland, Me. In the spring of 1899, a company was formed to transmit
and distribute electrical energy in that city from a water-power about
thirteen miles distant. For some years, prior to and since the date just
named, an extensive electric system with steam-power equipment has
existed in Portland. In spite of this, the system using water-power, on
January 1st, 1903, had a connected load of 352 enclosed arcs and 20,000
incandescent lamps, besides 835 horse-power in motors.
Comparing the expansion of electric water-power systems with those
operated by steam, when located in different cities, Hartford and
Springfield may be taken on the one hand and Fall River and New Bedford
on the other. The use of water-power in electrical supply at Hartford
began in November, 1891, and has since continued to an increasing
extent. Throughout the same period electrical supply in Fall River has
been derived exclusively from steam. In 1890 the population of Hartford
was 53,230, and in 1900 it stood at 79,850, an increase of 50 per cent.
At the beginning of the decade Fall River had a population of 74,398,
and at its close the figures were 104,863, a rise of 40.9 per cent. In
1892 the connected load of the electric supply system at Fall River
included 451 arc and 7,800 incandescent lamps, and motors aggregating
140 horse-power. By 1901 this load had increased to 1,111 arcs, 24,254
incandescent lamps, and 600 horse-power in motors. The electric supply
system at Hartford in 1892 was serving 800 arcs, 2,000 incandescent
lamps, and no motors. After the use of transmitted water-power during
nine years the connected load of the Hartford system had come to include
1,679 arcs, 68,725 incandescent lamps, and 3,476 horse-power of motor
capacity in 1901. At the beginning of the decade Hartford was far
behind Fall River in both incandescent lamps and motors, but at the end
Hartford had nearly three times as many incandescent lamps and nearly
six times as great a capacity in connected motors. As Fall River had a
population in 1900 that was greater by thirty-one per cent. than the
population of Hartford, and the percentage of increase during the decade
was only 9.1 lower in the former city, water-power seems to have been
the most potent factor in the rise of electric loads in the latter.
Electric gains at Hartford could not have been due to the absence of
competition by gas, for the price of gas there in 1901 was $1 per 1,000
cubic feet, while the price in Fall River was $1.10 for an equal amount.
Water-power began to be used in electrical supply at Springfield during
the latter half of 1897. In that year the connected load of the
Springfield electric system included 1,006 arcs, 24,778 incandescent
lamps, and motors with a capacity of 647 horse-power. Five years later,
in 1902, this connected load had risen to 1,399 arc lamps, 45,735
incandescent lamps, and a capacity of 1,025 horse-power in electric
motors. At New Bedford, in 1897, the electric system was supplying 406
arc and 22,122 incandescent lamps besides motors rated at 298
horse-power. This load, in 1902, had changed to 488 arcs, 18,055
incandescent lamps, and 432 horse-power in capacity of electric motors.
From the foregoing figures it appears that while 82 arc lamps were added
in New Bedford, 393 such lamps were added in Springfield. While the
electric load at New Bedford was increased by 134 horse-power of motors,
the like increase at Springfield was 378 horse-power, and while the
former city lost 4,067 from its load of incandescent lamps, the latter
gained 20,957 of these lamps. During all these changes electrical supply
in Springfield has come mostly from water-power, and that in New Bedford
has been the product of steam. Population at Springfield numbered 44,179
in 1890 and 62,059 in 1900, an increase of 40.5 per cent. In the earlier
of these years New Bedford had a population of 40,733, and in the later
62,442, an increase of 53.3 per cent. In 1902 the average price obtained
for gas at Springfield was $1.04 and at New Bedford $1.18 per 1,000
cubic feet.
Springfield contains a prosperous gas system, and the gross income there
from the sale of gas was thirty-one per cent greater in 1902 than in
1897. During this same period of five years the gross income from sales
of electrical energy, developed in large part by water-power, increased
forty-seven per cent. For the five years of general depression, ending
in 1897 gross annual income of gas sales in Springfield rose only five
per cent, and the like electric income nine per cent. In the five years
last named the electrical supply system was operated with coal.
The application of transmitted water-power in electrical supply has
displaced steam as a motive power in many large industrial plants that
never would have been operated from steam-driven electric stations. An
example of this sort exists at Portland, where one of the motors
operated by the electric water-power system, in an industrial plant, has
a capacity of 300 horse-power. Every pound of coal burned in Concord, N.
H., is hauled by the single steam railway system entering that city,
which railway operates large car and repair shops there. Some years ago
the railway installed a complete plant of engines, dynamos, and motors
for electric-driving throughout these shops. These engines and dynamos
now stand idle and the motor equipment, with an aggregate capacity of
590 horse-power, is operated with energy purchased from the local
electrical supply system and drawn from water-power.
Another striking example of the ability of electric water-power systems
to make power rates that are attractive to large manufacturers may be
seen at Manchester, N. H. One of the largest manufacturing plants in
that city purchases energy for the operation of the equivalent of more
than 7,000 incandescent lamps, and of motors rated at 976 horse-power,
from the electrical supply system there, whose generating stations are
driven mainly by water-power. The Manchester electrical supply system
also furnishes energy, through a sub-station of 800-horse-power
capacity, to operate an electric railway connecting Manchester and
Concord. This electric line is owned and operated in common with the
only steam railway system of New Hampshire, so that the only inducement
to purchase energy from the water-power system seems to be one of price.
In Buffalo the electric transmission system from Niagara Falls supplies
large motors of about 20,000 horse-power capacity in manufacturing and
industrial works, and 7,000 horse-power to the street railway system,
besides another 4,000 horse-power for general service in lighting and
small motors. Few large cities in the United States have cheaper coal
than Buffalo, and in Portland, Concord, and Manchester coal prices are
moderate. In the Rocky Mountain region, where coal is more expensive,
the greater part of the loads of some electric water-power systems is
made up of large industrial works. In Salt Lake City the electrical
supply system, which draws its energy almost exclusively from
water-powers, had a connected load of motors aggregating 2,600
horse-power as far back as 1901, and also furnished energy to operate
the local electric railway, and several smelters six miles south of the
city, besides all the local lighting service. As good lump coal sells
in Salt Lake for $4.50 per ton, slack at less than one-half this figure,
and the population there by the late census was only 53,531, the figures
for the load of motors are especially notable. At Helena energy from the
10,000 horse-power station at Cañon Ferry operates the local lighting
and power systems, two smelting and a mining plant.
CITIES WITH ELECTRICAL SUPPLY FROM WATER-POWER.
+------------------+-----------------+--------------+-----------+
| |Miles from Water-|Horse-Power of| |
| City. | Power to City. | Water-Driven |Population.|
| | | Stations. | |
+------------------+-----------------+--------------+-----------+
|Mexico City | 10 to 15 | 4,200 | 402,000 |
|Buffalo | 23 | [A]30,000 | 352,387 |
|Montreal | 85 | -- | 266,826 |
|San Francisco | 147 | 13,330 | 342,782 |
|Minneapolis | 10 | 7,400 | 202,718 |
|St. Paul | 25 | 4,000 | 163,065 |
|Los Angeles | 83 | 8,600 | 102,479 |
|Albany | 40 | 32,000 | 94,151 |
|Portland, Ore. | -- | -- | 90,426 |
|Hartford | 11 | 3,600 | 79,850 |
|Springfield, Mass.| 6 | 3,780 | 62,059 |
|Manchester, N. H. | 13.5 | 5,370 | 59,987 |
|Salt Lake City | 36.5 | 10,000 | 53,531 |
|Portland, Me. | 13 | 2,660 | 50,145 |
|Seattle | -- | 8,000 | 80,671 |
|Butte | 65 | 10,000 | 30,470 |
|Oakland | 142 | 15,000 | 66,900 |
|Lewiston, Me. | 3 | 3,000 | 23,761 |
|Concord, N. H. | 4 | 1,000 | 19,632 |
|Helena, Mont. | 20 | -- | 10,770 |
|Hamilton, Ont. | 35 | 8,000 | |
|Quebec | 7 | 3,000 | |
|Dales, Ore. | 27 | 1,330 | |
+------------------+-----------------+--------------+-----------+
[A] Power received.
In Butte, energy from the station just named operates the works of five
smelting and mining companies, driving motors that range from 1 to 800
horse-power in individual capacity. The capacity of the Butte
sub-station is 7,600 horse-power.
The great electric water power system marked by the Santa Ana station at
one end and the city of Los Angeles at the other, eighty-three miles
distant, includes more than 160 miles of transmission lines, several
hundred miles of distribution circuits, and supplies light and power in
twelve cities and towns. Among the customers of this system are an
electric railway, a number of irrigation plants, and a cement works.
These works contain motors that range from 10 to 200 horse-power each
in capacity. Motors of fifty horse-power or less are used at pumping
stations in the irrigation systems.
Applications of water-power in electrical supply during the past decade
have prepared the way for a much greater movement in this direction.
Work is now under way for the electric transmission of water-power,
either for the first time or in larger amounts, to Albany, Toronto,
Chicago, Duluth, Portland, Oregon, San Francisco, Los Angeles, and
dozens of other cities that might be named.
Another ten years will see the greater part of electrical supply on the
American continent drawn from water-power.
Only the largest city supplied from each water-power is named above.
Thus the same transmission system enters Albany, Troy, Schenectady,
Saratoga, and a number of smaller places.
CHAPTER II.
UTILITY OF WATER-POWER IN ELECTRICAL SUPPLY.
In comparatively few systems is the available water-power sufficient to
carry the entire load at all hours of the day, and during all months of
the year, so that the question of how much fuel can be saved is an
uncertain one for many plants. Again, the development of water-power
often involves a large investment, and may bring a burden of fixed
charges greater than the value of the fuel saved.
In spite of these conflicting opinions and factors, the application of
water-power in electrical systems is now going on faster than ever
before. If a saving of fuel, measured by the available flow of water
during those hours when it can be devoted directly to electrical supply,
were its only advantage, the number of cases in which this power could
be utilized at a profit would be relatively small. If, on the other
hand, all of the water that passes down a stream could be made to do
electrical work, and if the utilization of this water had other
advantages nearly or quite as great as the reduction of expense for
coal, then many water-powers would await only development to bring
profit to their owners.
No part of the problem is more uncertain than the first cost and
subsequent fixed charges connected with the development of water-power.
To bring out the real conditions, the detailed facts as to one or more
plants may be of greater value than mere general statements covering a
wide range of cases.
On a certain small river the entire water privilege at a point where a
fall of fourteen feet could be made available was obtained several years
ago. At this point a substantial stone and concrete dam was built, and
also a stone and brick power-house with concrete floor and steel truss
roof. In this power-house were installed electric generators of 800
kilowatts total capacity, direct-connected to horizontal turbine wheels.
The entire cost of the real estate necessary to secure the water-power
privilege plus the cost of all the improvements was about $130,000. More
than enough water-power to drive the 800-kilowatt generators at full
load was estimated to be available, except at times of exceptionally low
water. At this plant the investment for the water-power site,
development, and complete equipment was thus $162 per kilowatt capacity
of generators installed.
Allowing 65 days of low water, these generators of 800 kilowatts
capacity may be operated 300 days per year. If the running time averages
ten hours daily at full load, the energy delivered per year is 2,400,000
kilowatt hours. Ten per cent of the total investment should be ample to
cover interest and depreciation charges, and this amounts to $13,000
yearly. It follows that the items of interest and depreciation on the
original investment represent a charge of 0.54 cent per kilowatt hour on
the assumed energy output at this plant. This energy is transmitted a
few miles and used in the electrical supply system of a large city.
On another river the entire water privilege was secured about four years
ago at a point where a fall of more than 20 feet between ledges of rock
could be obtained and more than 2,000 horse-power could be developed. At
this point a masonry dam and brick power-house were built, and
horizontal turbine wheels were installed, direct-connected to electric
generators of 1,500 kilowatts total capacity. The entire cost of real
estate, water rights, dam, building, and equipment in this case was
about $250,000.
Assuming, as before, that generators may be operated at full capacity
for 10 hours per day during 300 days per year, the energy delivered by
this plant amounts to 4,500,000 kilowatt hours yearly. The allowance of
10 per cent on the entire investment for interest and depreciation is
represented by $25,000 yearly in this case, or 0.56 cent per kilowatt
hour of probable output. Energy from this plant is transmitted and used
in a large system of electrical supply.
If, through lack of water or inability to store water or energy at times
when it is not wanted, generators cannot be operated at full capacity
during the average number of hours assumed above, the item of interest
and depreciation per unit of delivered energy must be higher than that
computed. With the possible figure for this item at less than six-tenths
of a cent per kilowatt hour, there is opportunity for some increase
before it becomes prohibitive. At the plant last named the entire
investment amounted to $166 per kilowatt capacity of connected
generators, compared with $162 in the former case, and these figures may
be taken as fairly representative for the development of water-power in
a first-class manner on small rivers, under favorable conditions. In
both of these instances the power-houses are quite close to the dams. If
long canals or pipe lines must be built to convey the water, the expense
of development may be greatly increased.
One advantage of water- over steam-power is the smaller cost of the
building with the former for a given capacity of plant. The building for
direct-connected electric generators, driven by water-wheels, is
relatively small and simple. Space for fuel, boilers, economizers,
feed-water heaters, condensers, steam piping, and pumps is not required
where water-power is used. No chimney or apparatus for mechanical
draught is needed.
The model electric station operated by water-power usually consists of a
single room with no basement under it. One such station has floor
dimensions 27 by 52 feet, giving an area of 1,404 square feet, and
contains generators of 800 kilowatts capacity. This gives 1.75 square
feet of floor space per kilowatt of generators. In this station there is
ample room for all purposes, including erection or removal of machinery.
Next to the saving of fuel, the greatest advantage of water-power is due
to the relatively small requirements for labor at generating stations
where it is used. This is well illustrated by an example from actual
practice. In a modern water-power station that contributes to electrical
supply in a large city the generator capacity is 1,200 kilowatts. All of
the labor connected with the operation of this station during nearly
twenty-four hours per day is done by two attendants working alternate
shifts.
These attendants live close to the station in a house owned by the
electric company, and receive $60 each per month in addition to house
rent. Considering the location, $12 per month is probably ample
allowance for the rent. This brings the total expense of operation at
this station for labor up to $132 per month, or $1,584 per year, a sum
corresponding to $1.32 yearly per kilowatt of generator capacity.
At steam-power stations of about the above capacity, operating
twenty-four hours daily, $6 is an approximate yearly cost of labor per
kilowatt of generators in use. It thus appears that water-power plants
may be operated at less than one-fourth of the labor expense necessary
at steam stations per unit of capacity. On an average, the combined cost
of fuel and labor at electric stations driven by steam-power is a little
more than 76 per cent of their total cost of operation. Of this total,
labor represents about 28, and fuel about 48 per cent. Water-power, by
dispensing with fuel and with three-fourths of the labor charge, reduces
the expense of operation at electric stations by fully 69 per cent.
But this great saving in the operating expenses of electric stations can
be made only where water entirely displaces coal. If part water-power
and part coal are used, the result depends on the proportion of each,
and is obviously much affected by the variations of water-power
capacity. In such a mixed system the saving effected by water-power must
also depend on the extent to which its energy can be absorbed at all
hours the day. By far the greater number of electric stations using
water-power are obliged also to employ steam during either some months
in the year or some hours in the day, or both.
[Illustration: ENERGY CURVES FROM WATER POWER ELECTRIC STATIONS.
FIG. 3.]
It is highly important, therefore, to determine, as nearly as may be,
the answers to three questions:
First, what variations are to be expected in the capacity of a
water-power during the several months of a year?
Second, if the daily flow of water is equal in capacity to the daily
output of electrical energy, how far can the water-power be devoted to
the development of that energy?
Third, with a water-power sufficient to carry all electrical loads at
times of moderately high water, what percentage of the yearly output of
energy in a general supply system can be derived from the water?
To the first of these questions experience alone can furnish an answer.
Variations in the discharge of rivers during the different months of a
year are very great. In a plant laid out with good engineering skill
some provision will be made for the storage of water, and the capacity
of generating equipment will correspond to some point between the
highest and lowest rates of discharge.
Curve No. 1 in the diagram on the opposite page represents the energy
output at an electric station driven entirely by water-power from a
small stream during the twelve months of 1901, the entire flow of the
stream being utilized. During December, 1901, the output of this station
was 527,700 kilowatts, and was greater than that in any other month of
the year. Taking this output at 100 per cent, the curve is platted to
show the percentage attained by the delivered energy in each of the
other months. At the lowest point on the curve, corresponding to the
month of February, the output of energy was only slightly over 33 per
cent of that in December. During nine other months of the year the
proportion of energy output to that in December was over 60 and in three
months over 80 per cent. For the twelve months the average delivery of
energy per month was 73.7 per cent of that during December.
PERCENTAGES OF ENERGY DELIVERED IN DIFFERENT MONTHS, 1901.
January 68.0
February 33.1
March 80.5
April 81.7
May 77.9
June 58.6
July 67.7
August 75.8
September 79.3
October 65.9
November 95.8
December 100.0
At a somewhat small water-power station on another river with a
watershed less precipitous than that of the stream just considered, the
following results were obtained during the twelve months ending June
30th, 1900. For this plant the largest monthly output of energy was in
November, and this output is taken at 100 per cent. The smallest
delivery of energy was in October, when the percentage was 53.1 of the
amount for November. In each of seven other months of the year the
output of energy was above 80 per cent of that in November. During
March, April, May, and June the water-power yielded all of the energy
required in the electrical supply system with which it was connected,
and could, no doubt, have done more work if necessary. For the twelve
months the average delivery of energy per month was 80.6 per cent of
that in November, the month of greatest output.
PERCENTAGES OF ENERGY DELIVERED IN DIFFERENT MONTHS, 1899 AND 1900.
July 68.6
August 69.1
September 73.3
October 53.1
November 100.0
December 87.0
January 84.9
February 91.3
March 98.5
April 85.7
May 80.8
June 74.9
The gentler slopes and better storage facilities of this second river
show their effect in an average monthly delivery of energy 6.9 per cent
higher as to the output in a month when it was greatest than the like
percentage for the water-power first considered. These two water-power
illustrate what can be done with only very moderate storage capacities
on the rivers involved. At both stations much water escapes over the
dams during several months of each year. With enough storage space to
retain all waters of these rivers until wanted the energy outputs could
be largely increased.
As may be seen by inspection of curve No. 2, the second water-power has
smaller fluctuations of capacity, as well as a higher average percentage
of the maximum output than the water-power illustrated by curve No. 1.
If the discharge of a stream during each twenty-four hours is just
sufficient to develop the electrical energy required in a supply system
during that time, the water may be made to do all of the electrical work
in one of two ways. If the water-power has enough storage capacity
behind it to hold the excess of water during some hours of the day, then
it is only necessary to install enough water-wheels and electric
generators to carry the maximum load. Should the storage capacity for
water be lacking, or the equipment of generating apparatus be
insufficient to work at the maximum rate demanded by the electrical
system, then an electric storage battery must be employed if all of the
water is to be utilized and made to do the electrical work.
The greatest fluctuations between maximum and minimum daily loads at
electric lighting stations usually occur in December and January. The
extent of these fluctuations is illustrated by curve No. 3, which
represents the total load on a large electrical supply system during a
typical week-day of January, 1901. On this day the maximum load was
2,720 and the minimum load 612 kilowatts, or 22.5 per cent of the
highest rate of output. During the day in question the total delivery
of energy for the twenty-four hours was 30,249 kilowatt hours, so that
the average load per hour was 1,260 kilowatts. This average is 46 per
cent of the maximum load.
Computation of the area included by curve No. 3 above the average load
line of 1,260 kilowatts shows that about 17.8 per cent of the total
output of energy for the day was delivered above the average load, that
is, in addition to an output at average load. It further appears by
inspection of this load curve that this delivery of energy above the
average load line took place during 12.3 hours of the day, so that its
average rate of delivery per hour was 438 kilowatts.
If a water-power competent to carry a load of 1,260 kilowatts
twenty-four hours per day be applied to the system illustrated by curve
No. 3, then about 17.8 per cent of the energy of the water for the
entire day must be stored during 11.7 hours and liberated in the
remaining 12.3 hours. This percentage of the total daily energy of the
water amounts to 36 per cent of its energy during the hours that storage
takes place.
If all of the storage is done with water, the electric generators must
be able to work at the rate of 2,720 kilowatts, the maximum load. If all
of the storage is done in electric batteries, the use of water may be
uniform throughout the day, and the generator capacity must be enough
above 1,260 kilowatts to make up for losses in the batteries. Where
batteries are employed the amount of water will be somewhat greater than
that necessary to operate the load directly with generators, because of
the battery losses.
In spite of the large fluctuations of electrical loads throughout each
twenty-four hours, it is thus comparatively easy to operate them with
water-powers that are little, if any, above the requirements of the
average loads.
Perhaps the most important question relating to the use of water-power
in electrical supply is what percentage of the yearly output of energy
can be derived from water where this power is sufficient to carry the
entire load during a part of the year. With storage area for all surplus
water in any season, the amount of work that could be done by a stream
might be calculated directly from the records of its annual discharge of
water. As such storage areas for surplus water have seldom, or never,
been made available in connection with electrical systems, the best
assurance as to the percentage of yearly output that may be derived from
water-power is found in the experience of existing plants.
The question now to be considered differs materially from that
involving merely the variations of water-power in the several months,
or even the possible yearly output from water-power. The ratio of output
from water-power to the total yearly output of an electrical system
includes the result of load fluctuations in every twenty-four hours and
the variable demands for electrical energy in different months, as well
as changes in the amount of water-power available through the seasons.
In order to show the combined result of these three important factors
curve No. 4 has been constructed. This indicates the percentages of
total semi-yearly outputs of electrical energy derived from water-power
in two supply systems. Each half-year extends either from January to
June, inclusive, or from July to December, inclusive, and thus covers a
wet and dry season. Each half-year also includes a period of maximum and
one of minimum demand for electrical energy in lighting. The period of
largest water supply usually nearly coincides with that of heaviest
lighting load, but this is not always true.
Electrical systems have purposely been selected in which the water-power
in at least one month of each half-year was nearly or quite sufficient
to carry the entire electrical load. The percentage of energy from
water-power to the total energy delivered by the system is presented for
each of five half-years. Three of the half-years each run from July to
December, and two extend from January to June, respectively. The half
years that show percentages of 66.8, 80.2, and 95.6, respectively, for
the relation of energy from water-power to the total electrical output
relate to one system, and the half years that show percentages of 81.97
and 94.3 for the energy from water-power relate to another system.
For the half-year when 66.8 per cent. of the output of the electrical
system was derived from water-power, the total output of the system was
3,966,026 kilowatt hours. During the month of December in this half-year
more than 98 per cent of the electrical energy delivered by the system
was from water-power, though the average for the six months was only
66.8 per cent from water.
In the following six months, from January to June, the electrical supply
system delivered 4,161,754 kilowatt hours, and of this amount the
water-power furnished 80.2 per cent. For the six months just named, one
month, May, saw 99 per cent of all the delivered energy derived from
water-power.
The same system during the next half-year, from July to December,
without any addition to its water-power development or equipment, got
95.6 per cent of its entire energy output from water-power, and this
output amounted to 4,415,945 kilowatt hours. In one month of the
half-year just named only 0.2 per cent of the output was generated with
steam-power.
These three successive half years illustrate the fluctuations of the
ratio between water-power outputs and the demands for energy on a single
system of electrical supply. The percentage of 81.9 for energy derived
from water-power during the half-year from July to December represents
the ratio of output from water to the total for an electrical supply
system where water generated 94 per cent of all the energy delivered in
one month.
In the same system during the following six months, with exactly the
same water-power equipment, the percentage of output from water-power
was 94.3 of the total kilowatt-hours delivered by the system. This
result was reached in spite of the fact that the total outputs of the
system in the two half-years were equal to within less than one per
cent.
The lesson from the record of these five half-years is that
comparatively large variations are to be expected in the percentage of
energy developed by water-power to the total output of electrical supply
systems in different half-years. But, in spite of these variations, the
portion of electrical loads that may be carried by water-power is
sufficient to warrant its rapidly extending application to lighting and
power in cities and towns.
CHAPTER III.
COST OF CONDUCTORS FOR ELECTRIC-POWER TRANSMISSION.
Electrical transmission of energy involves problems quite distinct from
its development. A great water-power, or a location where fuel is cheap,
may offer opportunity to generate electrical energy at an exceptionally
low cost. This energy may be used so close to the point of its
development that the cost of transmission is too small for separate
consideration.
An example of conditions where the important problems of transmission
are absent exists in the numerous factories grouped about the great
water-power plants at Niagara and drawing electrical energy from it. In
such a case energy flows directly from the dynamos, driven by
water-power, to the lamps, motors, chemical vats, and electric heaters
of consumers through the medium, perhaps, of local transformers. Here
the costs and losses of transmitting or distributing equipments are
minor matters, compared with the development of the energy.
If, now, energy from the water-power is to be transmitted over a
distance of many miles, a new set of costs is to be met. In the first
place, it will be necessary to raise the voltage of the transmitted
energy much above the pressure at the dynamos in order to save in the
weight and cost of conductors for the transmission line. This increase
of voltage requires transformers with capacity equal to the maximum rate
at which energy is to be delivered to the line. These transformers will
add to the cost of the energy that they deliver in two ways: by the
absorption of some energy to form heat, and by the sum of annual
interest, maintenance, and depreciation charges on the price paid for
them. Other additions to the cost of energy delivered by the
transmission line must be made to cover the annual interest,
maintenance, and depreciation charges on the amount of the line
investment, and to pay for the energy changed to heat in the line.
Near the points where the energy is to be used, the transmission line
must end in transformers to reduce the voltage to a safe figure for
local distribution. This second set of transformers will further add to
the cost of the delivered energy in the same ways as the former set.
From these facts it is evident that, to warrant an electrical
transmission, the value of energy at the point of distribution should at
least equal the value at the generating plant plus the cost of the
transmission. Knowing the cost of energy at one end of the transmission
line and its value at the other, the difference between these two
represents the maximum cost at which the transmission will pay.
Three main factors are concerned in the cost of electric power
transmission, namely, the transformers, the pole line, and the wire or
conductors. These factors enter into the cost of transmitted energy in
very different degrees, according to the circumstances of each case. The
maximum and average rates of energy transmission, the total voltage, the
percentage of line loss, and the length of the line mainly determine the
relative importance of the transformers, pole line, and conductors in
the total cost of delivered energy.
First cost of transformers varies directly with the maximum rate of
transmission, and is nearly independent of the voltage, the length of
the transmission, and the percentage of line loss. A pole line changes
in first cost with the length of the transmission, but is nearly
independent of the other factors. Line conductors, for a fixed maximum
percentage of loss, vary in first cost directly with the square of the
length of the transmission and with the rate of the transmission; but
their first cost decreases as the percentage of line loss increases and
as the square of the voltage of transmission increases.
If a given amount of power is to be transmitted, at a certain percentage
of loss in the line and at a fixed voltage, over distances of 50, 100,
and 200 miles, respectively, the foregoing principles lead to the
following conclusions: The capacity of transformers, being fixed by the
rate of transmission, will be the same for either distance, and their
cost is therefore constant. Transformer losses, interest, depreciation,
and repairs are also constant. The cost of pole line, depending on its
length, will be twice as great at 100 and four times as great at 200 as
at 50 miles. Interest, depreciation, and repairs will also go up
directly with the length of the pole lines.
Line conductors will cost four times as much for the 100- as for the
50-mile transmission, because their weight will be four times as great,
and the annual interest and depreciation will go up at the same rate.
For the transmission of 200 miles the cost of line conductors and their
weight will be sixteen times as great as the cost at 50 miles. It
follows that interest, depreciation, and maintenance will be increased
sixteen times with the 200-mile transmission over what they were at 50
miles, if voltage and line loss are constant.
A concrete example of the cost of electric power transmission over a
given distance will illustrate the practical application of these
principles. Let the problem be to deliver electrical energy in a city
distant 100 miles from the generating plant. Transformers with
approximately twice the capacity corresponding to the maximum rate of
transmission must be provided, because one set is required at the
generating and another at the delivery station. The cost of these
transformers will be approximately $7.50 per horse-power for any large
capacity.
Reliability is of the utmost importance in a great power transmission,
and this requires a pole line of the most substantial construction. Such
a line in a locality where wooden poles can be had at a moderate price
will cost, with conductors in position, about $700 per mile, exclusive
of the cost of the conductors themselves or of the right of way but
including the cost of erecting the conductors. The 100 miles of pole
line in the present case should, therefore, be set down at a cost of
$70,000.
A large delivery of power must be made to warrant the construction of so
long and expensive a line, and 10,000 horse-power may be taken as the
maximum rate of delivery. On the basis of two horse-power of transformer
capacity for each horse-power of the maximum delivery rate, transformers
with a capacity of 20,000 horse-power are necessary for the present
transmission. At $7.50 per horse-power capacity, the first cost of these
transformers is $150,000.
Before the weight and cost of line conductors can be determined, the
voltage at which the transmission shall be carried out and the
percentage of the energy to be lost in the conductors at periods of
maximum load must be decided on. The voltage to be used is a matter of
engineering judgment, based in large part on experience, and cannot be
determined by calculation. In a transmission of 100 miles the cost of
conductors is certain to be a very heavy item, and, as this cost
decreases as the square of the voltage goes up, it is desirable to push
the voltage as high as the requirements for reliable service permit.
A transmission line 142 miles long, from the mountains to Oakland, Cal.,
has been in constant and successful use for several years with 40,000
volts pressure. This line passes through wet as well as dry climate. It
seems safer to conclude, therefore, that 40,000 volts may be used in
most places with good results.
Having decided on the amount of power and the voltage and length of the
transmission, the required weight of conductors will vary inversely as
the percentage of energy lost as heat in the line. The best percentage
of loss depends on the number of factors, some of which, such as the
cost of energy at the generating plant, are peculiar to each case.
As a provisional figure, based in part on the practice elsewhere, the
loss on the line here considered may be taken at 10 per cent. when
transmitting the full load of 10,000 horse-power. If the line is
constructed on this basis the percentage of loss will be proportionately
less for any smaller load. Thus, when the line is transmitting only
5,000 horse-power, the loss will amount to 5 per cent. During the
greater portion of each day the demand for power is certain to be less
than the maximum figure, so that a maximum loss of 10 per cent will
correspond to an average loss on all the power delivered to the line of
probably less than 7 per cent.
In order to deliver 10,000 horse-power by the transformers at a
receiving station from a generating plant 100 miles distant where the
pressure is 40,000 volts, the copper conductors must have a weight of
about 1,500,000 pounds, if the loss of energy in them is 10 per cent of
the energy delivered to the line. Taking these conductors at a medium
price of 15 cents per pound, their cost amounts to $225,000.
The combined cost of the transformers, pole line, and line conductors,
as now estimated, amounts to $445,000. No account is taken of the
right-of-way for the pole line, because in many cases this would cost
nothing, the public roads being used for the purpose; in other cases the
cost might vary greatly with local conditions.
The efficiency of the transmission is measured by the ratio of the
energy delivered by the transformers at the receiving station for local
distribution to the energy delivered by the generating plant to the
transformers that supply energy to the line for transmission. If worked
at full capacity the large transformers here considered would have an
efficiency of nearly 98 per cent; but as they must work, to some extent,
on partial loads, the actual efficiency will hardly exceed 96 per cent.
The efficiency of the line conductors rises on partial loads, and may be
safely taken at 93 per cent for all of the energy transmitted, though it
is only 90 per cent on the maximum load. The combined efficiencies of
the two sets of transformers and the line give the efficiency of the
transmission, which equals the product of 0.96 × 0.93 × 0.96, or almost
exactly 85.7 per cent. In other words, the transformers at the
water-power station absorb 1.17 times as much energy as the transformers
at the receiving station deliver to distribution lines in the place of
use.
Interest, maintenance, and depreciation of this complete transmission
system are sufficiently provided for by an allowance of 15 per cent
yearly on its entire first cost. As the total first cost of the
transmission system was found to be $445,000, the annual expense of
interest, depreciation, and repairs at 15 per cent of this sum amounts
to $66,750.
In order to find the bearings of this annual charge on the cost of power
transmission the total amount of energy transmitted annually must be
determined. The 10,000 horse-power delivered by the system at the
sub-station is simply the maximum rate at which energy may be supplied,
and the element of time must be introduced in order to compute the
amount of transmitted energy. If the system could be kept at work during
twenty-four hours a day at full capacity, the delivered energy would be
represented by the product of the numbers which stand for the capacity
and for the total number of hours yearly.
Unfortunately, however, the demands for electric light and power vary
through a wide range in the course of each twenty-four hours, and the
period of maximum demand extends over only a small part of each day. The
problem is, therefore, to find what relation the average load that may
be had during the twenty-four hours bears to the capacity required to
carry this maximum load. As the answer to this question depends on the
power requirements of various classes of consumers, it can be obtained
only by experience. It has been found that some electric stations,
working twenty-four hours daily on mixed loads of lamps and stationary
motors, can deliver energy to an amount represented by the necessary
maximum capacity during about 3,000 hours per year. Applying this rule
to the present case, the transformers at the sub-station, if loaded to
their maximum capacity of 10,000 horse-power by the heaviest demands of
consumers, may be expected to deliver energy to the amount of 3,000 ×
10,000 = 30,000,000 horse-power hours yearly.
The total cost of operation for this transmission system was found above
to be $66,750 per annum, exclusive of the cost of energy at the
generating plant. This sum, divided by 30,000,000, shows the cost of
energy transmission to be 0.222 cent per horse-power hour, exclusive of
the first cost of the energy. To obtain the total cost of transmission,
the figures just given must be increased by the value of the energy lost
in transformers and in the line conductors. In order to find this value,
the cost of energy at the generating plant must be known.
The cost of electrical energy at the switchboard in a water-power
station is subject to wide variations, owing to the different
investments necessary in the hydraulic work per unit of power developed.
With large powers, such as are here considered, a horse-power hour of
electrical energy may be developed for materially less than 0.5 cent in
some plants. As the average efficiency of the present transmission has
been found to be 85.7 per cent of the energy delivered by the
generators, it is evident that 1.17 horse-power hours must be drawn from
the generators for every horse-power hour supplied by the transformers
at the sub-station for distribution. In other words, 0.17 horse-power
hour is wasted for each horse-power hour delivered.
The cost of 0.17 of a horse-power hour, or say not more than 0.5 × 0.17
= 0.085 cent, must thus be added to the figures for transmission cost
already found, that is, 0.222 cent per horse-power hour, to obtain the
total cost of transmission. The sum of these two items of cost amounts
to 0.307 cent per horse-power hour, as the entire transmission expense.
It may now be asked how the cost of transmission just found will
increase if the distance be extended. As an illustration, assume the
length of the transmission to be 150 instead of 100 miles. Let the
amount of energy delivered by the sub-station, the loss in line
conductors, and the energy drawn from the generating plant remain the
same as before. Evidently the cost of the pole line will be increased 50
per cent, that is, from $70,000 to $105,000. Transformers, having the
same capacity, will not be changed from the previous estimate of
$150,000. If the voltage of the transmission remain constant, as well as
the line loss at maximum load, the weight and cost of copper conductors
must increase with the square of the distances of transmission. For 150
miles the weight of copper will thus be 2.25 times the weight required
for the 100-mile transmission.
Instead of an increase in the weight of conductors a higher voltage may
be adopted. The transformers for the two great transmission systems that
extend over a distance of about 150 miles, from the Sierra Nevada
Mountains to San Francisco Bay, in California, are designed to deliver
energy to the line at either 40,000 or 60,000 volts, as desired. Though
the regular operation at first was at the lower pressure, the voltage
has been raised to 60,000.
The lower valleys of the Sacramento and the San Joaquin rivers, which
are crossed by these California systems, as well as the shores of San
Francisco Bay, have as much annual precipitation and as moist an
atmosphere as most parts of the United States and Canada. Therefore
there seems to be no good reason to prevent the use of 60,000 volts
elsewhere.
The distance over which energy may be transmitted at a given rate, with
a fixed percentage of loss and a constant weight of copper, goes up
directly with the voltage employed. This rule follows because, while the
weight of conductors to transmit energy at a given rate, with a certain
percentage of loss and constant voltage, increases as the square of the
distance, the weight of conductors decreases as the square of the
voltage when all the other factors are constant.
Applying these principles to the 150-mile transmission, it is evident
that an increase of the voltage to 60,000 will allow the weight of
conductors to remain exactly where it was for the transmission of 100
miles, the rate of working and the line loss being equal for the two
cases.
The only additional item of expense in the 150-mile transmission, on the
basis of 60,000 volts, is the $35,000 for pole line. Allowing 15 per
cent on the $35,000 to cover interest, depreciation, and maintenance, as
before, makes a total yearly increase in the costs of transmission of
$5,250 over that found for the transmission of 100 miles. This last sum
amounts to 0.0175 cent per horse-power hour of the delivered energy.
The cost of transmission is thus raised to 0.307 + 0.0175 = 0.324 cent
per horse-power hour of delivered energy on the 150-mile system with
60,000 volts.
Existing transmission lines not only illustrate the relations of the
factors named above to the cost and weight of conductors, but also show
marked variations of practice, corresponding to the opinions of
different engineers. In order to bring out the facts on these points,
the data of a number of transmission lines are here presented. On these
lines the distance of transmission varies between 5 and 142 miles, the
voltage from 5,000 to 50,000, and the maximum rate of work from a few
hundred to some thousands of horse-power. For each transmission the
single length and total weight of conductors, the voltage, and the
capacity of the generating equipment that supplies the line is recorded.
From these data the volts per mile of line, weight and cost of
conductors per kilowatt capacity of generating equipment, and the weight
of conductors per mile for each kilowatt of capacity in the generating
equipment are calculated. In each case the length of line given is the
distance from the generating to the receiving station. The capacity
given for generating equipment in each case is that of the main dynamos,
where their entire output goes to the transmission line in question, but
where the dynamos supply energy for other purposes also, the rating of
the transformers that feed only the particular transmission line is
given as the capacity of generating equipment.
DISTANCE AND VOLTAGE OF ELECTRICAL TRANSMISSION.
+------------------------------------+-----------+--------+---------+
| |Distance in| Volts. |Volts per|
| | Miles. | | Mile. |
+------------------------------------+-----------+--------+---------+
|Colgate to Oakland, Cal. | 142 | 60,000 | 422 |
|Cañon Ferry to Butte, Mont. | 65 | 50,000 | 769 |
|Santa Ana River to Los Angeles | 83 | 33,000 | 397 |
|Ogden to Salt Lake City, Utah | 36.5 | 16,000 | 438 |
|Madrid to Bland, N. M. | 32 | 20,000 | 625 |
|Welland Canal to Hamilton, Can. | { 35 | 22,500 | 643 |
| | { 37 | | |
|San Gabriel Cañon to Los Angeles | 23 | 16,000 | 695 |
|Cañon City to Cripple Creek, Colo. | 23.5 | 20,000 | 851 |
|Apple River to St. Paul, Minn. | 25 | 25,000 | 1,000 |
|Yadkin River to Salem, N. C. | 14.5 | 12,000 | 827 |
|Into Victor, Colo. | 8 | 12,600 | 1,575 |
|Montmorency Falls to Quebec | 7 | 5,500 | 785 |
|Farmington River to Hartford | 11 | 10,000 | 909 |
|Sewall’s Falls to R.R. shops Concord| 5.5 | 10,000 | 1,818 |
|Wilbraham to Ludlow Mills | 4.5 | 11,500 | 2,555 |
|To Dales, Ore. | 27 | 22,000 | 814 |
+------------------------------------+-----------+--------+---------+
The transmission systems here considered have been selected because it
was possible to obtain the desired data as to each, and it may be
presumed that they fairly illustrate present practice. It may be noted
at once that in general the line voltage is increased with the length of
the transmission. Thus, the transmission for the Ludlow Mills over a
distance of 4.5 miles is carried out at 11,500 volts. On the other hand,
the transmission between Cañon Ferry and Butte, a distance of 65 miles,
employs 50,000 volts and represents recent practice. The system from
Colgate to Oakland, a distance of 142 miles, the longest here
considered, now has 60,000 volts on its lines. In spite of the general
resort to high pressures with greater distances of transmission, the
rise in voltage has not kept pace with the increasing length of line.
For the Wilbraham-Ludlow transmission the total pressure amounts to
2,555 volts per mile, while the line from Colgate to Oakland with 31.5
times the length of the former operates at an average of only 422 volts
per mile. Of the fifteen transmissions considered, six are over
distances of less than 15 miles, and for four of the six the voltage is
more than 900 per mile. Eight transmissions range from 23 to 83 miles in
length, with voltages that average between 1,000 volts per mile at 25
miles and only 397 per mile on the 83-mile line. The volts per mile are
6 times as great in the Ludlow as in the Oakland transmission.
CAPACITY OF GENERATING STATIONS AND WEIGHT OF CONDUCTORS.
+---------------------------------+-----------+------------+------------+
| | Kilowatt |Total Weight| Pounds of |
| |Capacity at| of | Conductors |
| Location of Transmission. |Generators.|Conductors. |per Kilowatt|
| | | | Capacity. |
+---------------------------------+-----------+------------+------------+
|Wilbraham to Ludlow | 4,600 | 17,820 | 3.7[A] |
|Sewall’s Falls to railroad shops | 50 | 6,914 | 15 |
|Into Victor, Colo. | 1,600 | 15,960 | 10 |
|To Dales, Ore. | 1,000 | 33,939 | 34 |
|Apple River to St. Paul | 3,000 | 159,600 | 53 |
|Farmington River to Hartford | 1,500 | 54,054 | 36 |
|Cañon City to Cripple Creek | 1,500 | 59,079 | 39 |
|Yadkin River to Salem | 1,500 | 58,073 | 39 |
|Montmorency Falls to Quebec | 2,400 | 189,056 | 79 |
|Cañon Ferry to Butte | 5,700 | 658,320 | 115 |
|San Gabriel Cañon to Los Angeles | 1,200 | 73,002 | 61 |
|Welland Canal to Hamilton | 6,000 | 376,494 | 63 |
|Madrid to Bland, N. M. | 600 | 127,680 | 212 |
|Ogden to Salt Lake City | 2,250 | 292,365 | 129 |
|Santa Ana River to Los Angeles | 2,250 | 664,830 | 295 |
|Colgate to Oakland | 11,250 | {906,954 | 81 |
| | | {446,627 | 40[A] |
+---------------------------------+-----------+------------+------------+
[A] Aluminum.
These wide variations in the volts per mile on transmission lines and in
length of lines lead to different weights of conductors per kilowatt of
generator capacity. All other factors remaining constant, the weight of
conductors per kilowatt of generator capacity would be the same whatever
the length of the transmission, provided that the volts per mile were
uniform for all cases. One important factor, the percentage of loss for
which the line conductors are designed at full load, is sure to vary in
different cases, and lead to corresponding variations in the weights of
conductors per kilowatt of generator capacity. In conductors of equal
length one pound of aluminum has nearly the same electrical resistance
as two pounds of copper, and this ratio must be allowed for when copper
and aluminum lines are compared.
From the table it may be seen that the weight of conductors per kilowatt
of generator capacity for the transmission from Santa Ana River is 29.5
times as great as the like weight for the line into Victor. But the
volts per mile are four times as great on the Victor as they are on the
Santa Ana River line. The extreme range of the cases presented is that
between the Ludlow plant, with the equivalent of 7.4 pounds, and the
Santa Ana River system with 295 pounds of copper conductors per kilowatt
of generator capacity. Three transmissions with 1,575 to 2,555 volts
per mile have the equivalent of 7.4 to 15 pounds of copper each, per
kilowatt of generator capacity.
WEIGHT AND COST OF CONDUCTORS.
+--------------------------------+----------+-----------+
| |Pounds per|Dollars per|
| | Kilowatt | Generator |
| | Mile. | Kilowatt. |
+--------------------------------+----------+-----------+
|Wilbraham to Ludlow | 0.86[A] | 1.11 |
|Sewall’s Falls to railroad shops| 2.7 | 2.25 |
|Into Victor, Colo. | 0.9 | 1.50 |
|To Dales, Ore. | 1.2 | 5.10 |
|Apple River to St. Paul | 2.1 | 7.95 |
|Farmington River to Hartford | 3.2 | 10.80 |
|Cañon City to Cripple Creek | 1.6 | 5.85 |
|Yadkin River to Salem | 2.6 | 5.85 |
|Montmorency Falls to Quebec | 11.2 | 11.85 |
|Cañon Ferry to Butte | 1.7 | 17.25 |
|San Gabriel Cañon to Los Angeles| 2.6 | 9.85 |
|Welland Canal to Hamilton | 1.7 | 9.45 |
|Madrid to Bland, N. M. | 6.6 | 31.80 |
|Ogden to Salt Lake City | 3.5 | 19.35 |
|Santa Ana River to Los Angeles | 3.5 | 44.25 |
|Colgate to Oakland | { .56 | 24.15 |
| | { .27[A] | |
+--------------------------------+----------+-----------+
[A] Aluminum.
Of the seven transmissions using between 36 and 79 pounds of copper for
each kilowatt of generator capacity, four have voltages ranging from 827
to 1,000 per mile, and on only one is the pressure as low as 643 volts
per mile. Five transmission lines vary between 115 and 295 pounds of
copper, or its equivalent, per kilowatt of generator capacity, and their
voltages per mile are as high as 769 in one case and down to 281 in
another. Allowing for some variations in the percentages of loss in
transmission lines at full load, the fifteen plants plainly illustrate
the advantage of a high voltage per mile, as to the weight of
conductors. This advantage is especially clear if the differences due to
the lengths of the transmissions are eliminated by dividing the weight
of conductors per kilowatt of generator capacity in each case by the
length of the transmission in miles. This division gives the weight of
conductors per kilowatt of generators for each mile of the line, which
may be called the weight per kilowatt mile. For the Ludlow transmission
this weight is only 0.86 pound of aluminum, the equivalent of 1.72
pounds of copper, while the like weight for the line into Quebec is 11.2
pounds of copper, or 6.5 times that for the former line. But the
voltage per mile on the Ludlow is 3.2 times as great as the like voltage
on the Quebec line.
The weight of conductor per kilowatt mile in the Victor line is only 0.9
pound, and the like weight for the line between Madrid and Bland is 6.6
pounds, or 7.3 times as great. On the Victor line the voltage per mile
is 2.5 times as great as the voltage for each mile of the Bland line.
Comparing systems with nearly equal voltages per mile, it appears in
most cases that only such difference exists in their pounds of
conductors per kilowatt mile as may readily be accounted for by designs
for various percentages of loss at full load. Though the transmission
line into Butte is nearly twice as long as the one entering Hamilton,
the weight of conductors for each is 1.7 pounds per kilowatt mile. The
line from Santa Ana River is more than twice as long as the one entering
Salt Lake City, but its voltage per mile is only nine per cent less, and
there are 3.5 pounds of copper in each line per kilowatt mile.
The final, practical questions as to conductors in electrical
transmission relate to their cost per kilowatt of maximum working
capacity, and per kilowatt hour of delivered energy. If the cost of
conductors per kilowatt of generator capacity is greater than that of
all the remaining equipment, it is doubtful whether the transmission
will pay. If fixed charges on the conductors more than offset the
difference in the cost of energy per kilowatt hour at the points of
development and delivery, it is certain that the generating plant should
be located where the power is wanted. The great cost of conductors is
often put forward as a most serious impediment to long-distance
transmission, and the examples here cited will indicate the weight of
this argument. In order to find the approximate cost of conductors per
kilowatt of generator capacity for each of the transmission lines here
considered, the price of bare copper wire is taken at 15 cents, and the
price of bare aluminum wire at 30 cents per pound. In each case the
weight of copper or aluminum conductor per kilowatt of generator
capacity is used to determine their costs per kilowatt of this capacity
at the prices just named. This process when carried out for the 15
transmission lines shows that their cost of conductors per kilowatt of
generator capacity varies between $1.11 for the 4.5 mile line into
Ludlow and $44.25 for the line of 83 miles from the Santa Ana River. It
should be noted that the former of these lines operates at 2,555 and the
latter at 397 volts per mile. The line into Madrid shows an investment
in conductors of $31.80 per kilowatt of generator capacity with 625
volts per mile. That a long transmission does not necessarily require a
large investment in conductors per kilowatt of generator capacity is
shown by the line 65 miles long into Butte, for which the cost is
$17.25 per kilowatt, with 769 volts per mile. For the transmission to
St. Paul, a distance of 25 miles, at 1,000 volts per mile, the cost of
conductors is $7.95 per kilowatt of generator capacity. The seven-mile
line into Quebec shows an investment of $11.85 per kilowatt of generator
capacity.
CHAPTER IV.
ADVANTAGES OF THE CONTINUOUS AND ALTERNATING CURRENT.
Electrical transmissions over long distances in America have been mainly
carried out with alternating current. In Europe, on the other hand,
continuous current is widely used on long transmissions at high
voltages. So radical a difference in practice seems to indicate that
neither system is lacking in points of superiority.
A fundamental feature of long transmissions is the high voltage
necessary for economy in conductors, and this voltage is attained by
entirely different methods with continuous and alternating currents. In
dynamos of several hundred or more kilowatts capacity the pressure of
continuous current has not thus far been pushed above 4,000 volts,
because of the danger of sparking and flashing at the commutator. Where
10,000 or more volts are required on a transmission line with continuous
current a number of dynamos are connected in series so that the voltage
of each is added to that of the others. In this way the voltage of each
dynamo may be as low as is thought desirable without limiting the total
line voltage. There is no apparent limit to the number of
continuous-current dynamos that may be operated in series or to the
voltage that may be thus obtained. In the recently completed
transmission from St. Maurice to Lausanne, Switzerland, with continuous
current, ten dynamos are connected in series to secure the line voltage
of 23,000. When occasion requires twenty or thirty or more dynamos to be
operated in series, giving 50,000 or 75,000 volts on the line, machines
exactly like those in the transmission just named, may be used. No
matter how many of these dynamos are operated in series the electric
strain on the insulation of the windings of each dynamo remains
practically constant, because the iron frame of each dynamo is insulated
in a most substantial manner from the ground. The electric strain on the
insulation of the windings of each dynamo in the series is thus limited
to the voltage generated by that dynamo. There is no practical limit to
the thickness or strength of the insulation that may be interposed
between the frame of each dynamo and the ground, and hence no limit to
line voltage as far as dynamo insulation is concerned.
It is impracticable to operate alternating dynamos in series so as to
add their voltages, and the pressure available in transmission with
alternating current must be that of a single dynamo or must be obtained
by the use of transformers. The voltage of an alternating may be carried
much higher than that of a continuous-current dynamo of very large
capacity, and in many cases pressures of 13,200 volts are now supplied
to transmission lines by alternating dynamos. Just how high the voltage
of single alternating dynamos will be carried no one can say, but it
seems probable that the practical limit will prove to be much less than
the voltages now employed in some transmissions. As the voltage of
alternating dynamos is carried higher the thickness of insulation on
their armature coils and consequently the size or number of slots in
their armature cores and the size of these cores increase rapidly. The
dimensions and weight of an alternating dynamo per unit of its capacity
thus go up with the voltage, and at some undetermined point the cost of
the high-voltage dynamo is greater than that of a low-voltage dynamo of
equal capacity with raising transformers. To the voltage that may be
supplied by transformers there is no practical limit now in sight. Lines
have been in regular operation from one to several years on which
transformers supply 40,000 to 50,000 volts; some large transformers have
been built for commercial use at 60,000 volts, and other transformers
for experimental and testing purposes have been employed in a number of
cases for pressures of 100,000 volts and more.
Available voltages for continuous- and alternating-current transmissions
are thus on a practically equal footing as to their upper limit. The
amount of power that may be generated and delivered with either the
alternating- or continuous-current system of transmission is practically
unlimited. Single alternating dynamos may be had of 5,000 or even 8,000
kilowatts capacity if desired, but it is seldom that these very large
units are employed, because the capacity of a generating station should
be divided up among a number of machines. It is perhaps impracticable to
build single continuous-current dynamos with capacities equal to those
of the largest alternators, but as any number of the continuous-current
machines may be operated either in series or multiple, the power that
may be applied to a transmission circuit is unlimited.
At the plant or plants where the power transmitted by continuous current
is received, a number of motors must be connected in series to operate
at the high-line voltage. These motors may all be located in a single
room, may be connected to machinery in different parts of a building, or
may be in use at points miles apart. The vital requirement is that the
motors must be in series with each other so that the line voltage
divides between them. If simply mechanical power is wanted at the places
where the motors are located, they complete the transmission system and
no further electrical apparatus is required. Where, however, as at
Lausanne, the transmitted power is to be used in a system of general
electrical supply, the motors that receive the current at the line
voltage must drive dynamos that will deliver energy of the required
sorts. In the station at Lausanne four of the motors to which the
transmission line is connected each drives a 3,000-volt three-phase
alternator for the distribution of light and power. The fifth motor at
this station drives a 600-volt dynamo which delivers continuous current
to a street railway. A sixth motor in the same series drives a cement
factory some distance from the station. Neglecting minor changes in
capacity due to losses in the line and motors, this continuous-current
system must thus include three kilowatts in motors and dynamos for each
kilowatt delivered for general electrical distribution at the receiving
station. In a case in which only mechanical power is wanted at the
receiving station, the dynamos and motors concerned in the transmission
must have a combined capacity of two horse-power for each horse-power
delivered at the motor shaft. In contrast with these figures, the
electrical equipment in a transmission with alternating current for
mechanical power alone includes two kilowatts capacity in generators and
motors, besides two kilowatts capacity in transformers for each
corresponding unit of power delivered at the motor shaft unless
generators and motors operate at the full line voltage. If a general
electrical supply is to be operated by the alternating system of
transmission, either motors and dynamos or rotary converters must be
added to transformers where continuous current is required. An
alternating transmission may thus include as little as one kilowatt in
dynamos and one in transformers, or as much as two kilowatts capacity in
dynamos, two in transformers, and one in motors for each kilowatt
delivered to distribution lines at the receiving station.
Line construction from the continuous-current transmission is of the
most simple character apart from the necessity of high insulation. Only
two wires are necessary and they may be of any desired cross-section,
strung on a single pole line and need not be transposed. On these wires
the maximum voltage for which insulation must be provided is the nominal
voltage of the system. It is possible under these conditions to build a
single transmission line with two conductors of such size and strength
and at such a distance apart that a high degree of reliability is
attained against breaks in the wires or arcing between them. In a
transmission of power by two- or three-phase alternating current at
least three wires are necessary and six or more are often employed. If
six or more wires carrying current at the high voltages required by long
transmissions are mounted on a single line of poles, it is not
practicable to obtain such distances between the wires as are desirable.
The repair of one set of wires while the other set is in operation is a
dangerous task, and an arc originating between one set of the wires is
apt to be communicated to another set. For these reasons two pole lines
are frequently provided for a transmission with alternating current, and
three or more wires are then erected on each line. Compared with a
continuous-current transmission, one with alternating current often
requires more poles and is quite certain to require more cross-arms,
pins, insulators, and labor of erection. For a given effective voltage
of transmission it is harder to insulate an alternating- than a
continuous-current line. In the first place the maximum voltage of the
alternating line with even a true sine curve of pressure is 1.4 times
the nominal effective voltage, but the insulation must withstand the
maximum pressure. Then comes the matter of resonance, which may carry
the maximum voltage of an alternating circuit up to several times its
normal amount, if the period of electrical vibration for that particular
circuit should correspond to the frequency of the dynamos that operate
it. Even where the vibration period of a transmission circuit and the
frequency of its dynamos do not correspond, and good construction should
always be planned for this lack of agreement, resonance may and often
does increase the normal voltage of an alternating transmission by a
large percentage. The alternating system of transmission must work at
practically constant voltage whatever the state of its load, so that the
normal stress on the insulation is always at its maximum. In a
transmission with continuous current on the other hand, if the
prevailing practice of a constant current and varying pressure on the
line is followed, the insulation is subject to the highest voltage only
at times of maximum load on the system. Lightning is a very real and
pressing danger to machinery connected to long transmission lines, and
this danger is much harder to guard against in an alternating system
than in a system with continuous constant current. The large degree of
exemption from damage by lightning enjoyed by series arc dynamos is well
known, the magnet windings of such machines acting as an inductance that
tends to keep lightning out of them. Moreover, with any
continuous-current machines lightning arresters having large
self-induction may be connected in circuit and form a most effective
safeguard against lightning, but this plan is not practicable on
alternating lines.
In the matter of switches, controlling apparatus, and switchboards, an
alternating transmission requires much more equipment than a system
using continuous, constant current. The ten dynamos in the generating
station at St. Maurice, with a capacity of 3,450 kilowatts at 23,000
volts, are each connected and disconnected with the transmission by a
switch in a small circular column of cast-iron that stands hardly breast
high. An amperemetre and voltmetre are mounted on each dynamo. The
alternating generators in a station of equal capacity and voltage would
require a large switchboard fitted with bus-bars, oil switches, and
automatic circuit-breakers. Relative efficiencies for the
continuous-current and the alternating-transmission systems vary with
the kind of service required at receiving stations and with the extent
to which transformers are used in the alternating system, other factors
being constant. For purposes of comparison the efficiency at full load
of both alternating- and continuous-current dynamos and motors, also of
rotary converters, may be fairly taken at 92 per cent, and the
efficiency of transformers at 96 per cent.
For the line an efficiency of 94 per cent may be assumed at full load,
this being the actual figure in one of the Swiss transmissions of 2,160
kilowatts at 14,400 volts to a distance of 32 miles. Where the
continuous current system must simply deliver mechanical power at the
receiving stations, its efficiency under full load amounts to 92 × .94 ×
.92 = 79.65 per cent from dynamo shaft to motor shaft. An alternating
system delivering mechanical power will have an efficiency of 92 × .94 ×
.96 × .92 = 76.46 per cent between dynamo shaft and motor shaft, if the
line voltage is generated in the armature coils of the dynamo and the
line loss is 6 per cent. If step-up transformers are employed to secure
the line voltage the efficiency of the alternating transmission
delivering mechanical power drops to the figure of 92 × .96 × .94 × .96
× .92 = 73.40 per cent. It thus appears that for the simple delivery of
mechanical power the continuous current transmission has an advantage
over the alternating of three to six per cent in efficiency, depending
on whether step-up transformers are employed.
When the receiving station must deliver a supply of either continuous or
alternating current for general distribution, the efficiency of the
continuous-current transmission amounts to 92 × .94 × .92 × .92 = 73.27
per cent. The alternating-transmission system in a case in which no
step-up transformers are employed will deliver alternating current of
the same frequency as that on the transmission line at any desired
pressure for general distribution at an efficiency of 92 × .94 × .96 =
83.02 per cent, if step-down transformers are used, but the efficiency
drops to 83.02 × .96 = 79.70 per cent. when step-up transformers are
introduced. If the alternating transmission uses no step-up transformers
and delivers either alternating or continuous current by means of motor
generators, its efficiency at full load is 83.02 × .92 × .92 = 70.26 per
cent, but with step-up transformers added the efficiency drops to 70.26
× .96 = 67.43 per cent. In a transmission where electrical energy must
be delivered for general distribution, the full-load efficiency of an
alternating system ranges either higher or lower than that of a
continuous-current system depending on whether the current from the
transmission line must be converted or not.
Line loss is the same whatever the load in a constant-current
transmission, so that line efficiency falls rather rapidly with the
load. On the other hand, at constant pressure the percentage of energy
loss on the line varies directly with the load, but the actual rate of
energy loss with the square of the load. On partial loads the line
efficiency is thus much higher with alternating than with continuous
constant current.
Efficiency of electrical machinery is generally low at partial loads, so
that in cases in which the number or capacity of alternating dynamos,
transformers, motors, or rotary converters for a transmission would be
greater per unit of delivered power than the corresponding number or
capacity of machines for a transmission by continuous current, the
latter would probably have the advantage in the combined efficiency of
machinery at partial loads. In this way the lower-line efficiency of one
system might offset the lower efficiency of machinery in the other.
Energy is usually very cheap at the generating station of a transmission
system. For this reason small differences in the efficiencies of
different systems should be given only moderate weight in comparison
with the items of first cost, reliability, and expense of operation.
In the matter of first cost at least the continuous-current system seems
to have a distinct advantage over the alternating. Without going into a
detailed estimate, it is instructive to consider the figures given by a
body of five engineers selected to report on the cost of continuous- and
alternating-current equipments for the St. Maurice and Lausanne
transmission. According to the report of these engineers, a three-phase
transmission system would have cost $140,000 more than the
continuous-current system actually installed, all other factors
remaining constant. It should be noted that the conditions of this
transmission are favorable to three-phase working and unfavorable to
continuous-current equipment, because all of the energy except that
going to the 400 horse-power motor at the cement mill must be delivered
at the receiving station for general distribution. Moreover, four out of
the five motors at Lausanne drive three-phase generators, and only one
drives a continuous-current dynamo for the electric railway, so that a
three-phase transmission would have required only one rotary converter.
Had the transmission been concerned merely with the delivery of
mechanical power, as at the cement mill, the advantage of the
continuous- over the alternating-current system in the matter of first
cost would have been much greater than it was.
Long-distance transmission with three-phase current began at Frankfort,
in 1891, when 58 kilowatts were received over a 25,000-volt line from
Lauffen, 109 miles away. Shortly after this historic experiment,
three-phase transmission in the United States began on a commercial
scale, and plants of this sort have multiplied rapidly here. Meantime
very little has been done in America with continuous currents in long
transmissions. In Europe, the birthplace of the three-phase system, it
has failed to displace continuous current for transmission work. About a
score of these continuous-current transmissions are already at work
there. If the opinion of European engineers as to the lower cost of the
continuous-current system, all other factors being equal, is confirmed
by experience, this current will yet find important applications to long
transmissions in the United States.
Systems of transmission with continuous-current may operate at constant
voltage and variable current, at constant current and variable voltage,
or with variations of both volts and amperes to correspond with changes
of load. Dynamos of several thousand kilowatts capacity each can readily
be had at voltages of 500 to 600, but the attempt to construct dynamos
to deliver more than two or three hundred kilowatts each at several
thousand volts has encountered serious sparking at the commutator. Thus
far, dynamos that yield between 300 and 400 kilowatts each have been
made to give satisfactory results at pressures as high as 2,500 volts.
Another one of the Swiss transmissions takes place over a distance of
thirty-two miles at 14,400 volts, the capacity being 2,160 kilowatts. To
give this voltage and capacity, eight dynamos are connected in series at
the generating station, each dynamo having an output of 150 amperes at
1,800 volts, or 216 kilowatts.
Continuous-current motors are, of course, subject to the same
limitations as dynamos in the matter of capacity at high voltage, so
that a series of motors must be employed to receive the high-pressure
energy from the line. The number of these motors may just equal, or may
be less or greater than the number of dynamos, but the total working
voltage of all the motors in operation at one time must equal the total
voltage of the dynamos in operation at that time minus the volts of drop
in the line.
Each constant-current motor may have any desired capacity up to the
practicable maximum, but it must be designed for the current of the
system. The voltage at the terminals of each motor varies with its load,
being greatest when the motor is doing the most work. Constant speed is
usually attained at each motor by means of a variable resistance
connected across the terminals of the magnet coils. The amount of this
resistance is regulated by a centrifugal governor, driven by the motor
shaft. This governor also shifts the position of the brushes on the
commutator to prevent sparking as the current flowing through the magnet
coils is changed.
For a constant-current transmission the magnet and armature windings of
both dynamos and motors are usually connected in series with each other
and the line so that the same current passes through every element of
the circuit, except that each motor may have some current shunted out of
its magnet coil for the purpose of speed regulation.
In some cases, however, the magnet coils of the dynamos are connected in
multiple with each other and receive their current from a separate
dynamo designed for the purpose. With this separate excitation of the
magnet coils, the dynamo armatures are still connected in series with
each other and the line.
The total voltage at the generating station and on the line of a
constant-current system varies with the rate at which energy is
delivered, and has its maximum value only at times of full load. To
obtain this variation of voltage, it is the general practice to change
the speed of the dynamos by means of an automatic regulator which is
actuated by the line current. Any increase of the line current actuates
the regulator and reduces the speed of the dynamos, while a decrease of
the line current raises the dynamo speed. With a good regulator the
variations of the line current are only slight. Under this method of
regulation the dynamos in operation have a substantially constant
current in both armature and magnet coils at all times, so that there is
no reason to shift the position of the brushes on the commutator.
Generating stations of constant current transmission systems are
generally driven by water-power and the speed regulator operates to
change the amount of water admitted to each wheel. Each turbine wheel
usually drives a pair of dynamos, but one or any number of dynamos might
be driven by a single wheel. The two dynamos driven by a single wheel
are generally connected in series at all times, and are cut in or out of
the main circuit together. When the load on a constant-current
generating station is such that the voltage can be developed by less
than all the dynamos, one or more dynamos may be stopped and taken out
of the circuit.
To do this the dynamo or pair of dynamos to be put out of service may be
stopped, their magnet coils having first been short-circuited, and then
a switch across the connections of their armatures to the lines closed,
after which the connections of the armatures to the line are opened. By
a reverse process, any dynamo or pair of dynamos may be cut into the
operating circuit.
At the terminals of each dynamo in the series, while in operation, the
voltage is simply that developed in its armature, so that the insulation
between the several windings is subject to only a corresponding stress.
The entire voltage of the line, however, tends to force a current from
the coils of the dynamo at one end of the series into its frame, thence
to any substance on which that frame rests, and so on to the frame and
coils of the dynamo at the other end of the series. To protect the
insulation of the dynamo coils from the line voltage, thick blocks of
porcelain are placed beneath the dynamo frames, and the armature shafts
are connected to those of the turbines by insulating couplings.
Besides the switches, already mentioned, a voltmeter and ammeter should
be provided for each dynamo and also for the entire series of machines.
This completes the switchboard equipment, which is, therefore, very
simple. As the line loss of a constant-current system is the same
whatever the load that is being operated, this loss may be a large
percentage of the total output when the load is light. If, for
illustration, five per cent of the maximum voltage of the station is
required to force the constant current through the line, the percentage
of line loss will rise to ten when the station voltage is one-half the
maximum, and to twenty when the station is delivering only one-quarter
of its full capacity.
In view of this property of constant-current working, the line loss
should be made quite small in its ratio to the maximum load, as most
stations must work on partial loads much of the time. Five per cent of
maximum station voltage is a fair general figure for the line loss in a
constant-current transmission, but the circumstances of a particular
case may dictate a higher or a lower percentage.
On the 32-mile transmission, above named, the loss in the line is six
per cent of the station output at full load.
If a transmission with continuous current is to be carried out at
constant pressure the limitation as to the capacity and voltage of each
dynamo is about the same as with constant current. Probably more energy
is now transmitted by continuous current at constant pressure than by
any other method, the greater part being devoted to electric railway
work at 500 to 600 volts. Dynamos for about these voltages can readily
be had in capacities up to several thousand kilowatts each, but the
length of transmission that can be economically carried out at this
pressure is comparatively small. For each kilowatt delivered to a line
at 500 volts and to be transmitted to a distance of five miles at a ten
per cent loss in the line, the weight of copper conductors must be 372
pounds, costing $56.80 at 15 cents per pound. This sum is twice to four
times the cost of good continuous-current dynamos per kilowatt of
capacity. If the distance of transmission is ten miles and the voltage
and line loss remain as before, the weight of copper conductor must be
increased to 1,488 pounds per kilowatt delivered to the line, costing
$227.20.
Experience has shown that in sizes of not more than 400 kilowatts,
continuous-current dynamos may safely have a voltage of 2,000 each, and
any number of such dynamos may be operated in multiple, giving whatever
capacity is desired. At 2,000 volts and a loss of 10 per cent in the
line the weight of copper conductors per kilowatt would be 93 pounds,
costing $13.95, for each kilowatt delivered to the line on a 10-mile
transmission. With 2,000 volts on a 20-mile transmission the weight of
conductors per kilowatt would be the same as their weight on a 5-mile
transmission at 500 volts, the percentage of loss being equal in the two
cases. Large continuous-current motors of, say, 50 kilowatts or more can
be had for a pressure of 2,000 volts, so that any number of such motors
might be operated from a 2,000-volt, constant-pressure line entirely
independent of each other. From these figures it is evident that a
transmission of 10 miles may be carried out with continuous-current at
constant pressure from a single dynamo with good efficiency and a
moderate investment in conductors.
When the distance is such that much more than 2,000 volts are required
for the constant-pressure transmission, with continuous current, resort
must be had to the connection of dynamos and motors in series. Any
number of dynamos may be so connected as in the case of constant-current
work. The combined voltages of the series of motors connected to the
constant-pressure transmission line must equal the voltage of that
line, so that the number of motors in any one series must be constant.
If the voltage of transmission is so high that more than two or three
motors must be connected in each series, there comes the objection that
motors must be operated at light loads during much of the time.
Moreover, each series of motors must be mechanically connected to the
same work, as that of driving a single dynamo or other machine, because
if the loads on the motors of a series vary differently, these motors
will not operate at constant speed. Continuous-current transmission at
constant pressure with motors in series thus lacks the flexibility of
transmission at constant current where any motor may be started and
stopped without regard to the others in the series, the line voltage
being automatically regulated at the generating station according to the
number of motors in use at any time and to the work they are doing.
In the efficiency of its dynamos, motors and line, a constant-pressure
system of transmission is substantially equal to one with constant
current at full load. At partial loads the constant-pressure line has
the advantage because the loss of energy in it varies with the square of
the load. Thus at constant pressure the line loss in energy per hour at
half-load is only one-fourth as great as the loss at full load. On the
other hand, the energy loss in the constant-current line is the same at
all stages of load. Because of these facts it is good practice to allow,
say, a ten-per-cent loss in a constant-pressure line and only five per
cent in a constant-current line at full load.
In a generating station at 2,000 volts or more constant pressure, it is
desirable to have the magnet coils of the main dynamos connected in
multiple and separately excited by a small dynamo at constant pressure.
This plan is especially desirable when the armatures of several dynamos
are connected in series to obtain the line voltage. Separately excited
magnet coils make it easier to control the operation of the several
dynamos, coils of low-voltage are cheaper to make than coils of high
voltage, and the low voltage windings are less liable to burn out. If a
series of constant-pressure motors is in use at one point, it may be
cheaper and safer to excite its magnet coils from a special dynamo than
from the line.
In a transmission carried out with series-wound dynamos and motors, the
speed of the motors may be constant at all loads without any special
regulating mechanism. To attain this result it is necessary that all the
motors be coupled so as to form a single unit mechanically and that the
dynamos be driven at constant speed. A transmission system of this sort
may include a single dynamo and a single motor, or two or more dynamos,
and two or more motors may be used in series.
When the dynamos of such a system are driven at constant speed and a
variable load is applied to the single motor, or to the mechanically
connected motors, both the voltage of the system and the amperes flowing
in all its parts change together so that practically constant speed is
maintained at the motors, provided that the design of both the dynamos
and motors is suitable for the purpose. With the maximum load on the
motors the volts and amperes of the system have their greatest values,
and these values both decline with smaller loads. The chief disadvantage
of this system lies in the fact that where more than one motor is
employed all the motors must be mechanically joined together so as to
work on the same load.
Compared with the constant-current system, this combination of series
dynamos with mechanically connected series motors has the distinct
advantage that neither the dynamos nor motors require any sort of
regulators in order to maintain constant motor speed. It is only
necessary that the dynamos be driven at constant speed and that both the
dynamos and motors be designed for the transmission. In comparison with
a constant-pressure system, the one under consideration has the
advantage that neither its dynamos nor motors require magnet coils with
a high voltage at their terminals and composed of fine wire or separate
excitation by a special dynamo. These features of the system with series
dynamos and motors, the latter being joined as a mechanical unit, make
it cheaper to install and easier to operate than either of the other
two. This system is especially adapted for the delivery of mechanical
power in rather large units. The voltage available may be anything
desired, but is subject to the practical limitations that all the motors
must deliver their power as a mechanical unit, so that unless the power
is quite large the number of motors in the series and, therefore, the
voltage is limited.
An interesting illustration of the system of transmission just described
exists between a point on the River Suze, near Bienne, Switzerland, and
the Biberest paper mills. At the river a 400 horse-power turbine
water-wheel drives a pair of series-wound dynamos, each rated at 130
kilowatts and 3,300 volts. These dynamos are connected in series, giving
a total capacity of 260 kilowatts and a pressure of 6,600 volts. At the
Biberest mills are located two series-wound motors, mechanically coupled
and connected in series with each other and with the two-wire
transmission line, which extends from the two dynamos at the River Suze.
Each of these motors has a capacity and voltage equal to that of either
of the dynamos previously mentioned. The coupled motors operate at the
constant speed of 200 revolutions per minute at all loads and deliver
over 300 horse-power when doing maximum work. Between the generating
plant at the river and the Biberest mills the distance is about 19
miles, and the two line wires are each of copper, 275 mils, or a little
more than one-fourth inch in diameter. The dynamos and motors of this
system are mounted on thick porcelain blocks in order to protect the
insulation of their windings from the strain of the full-line voltage.
Either of the three systems of transmission by continuous-current that
have been considered requires a smaller total capacity of electrical
apparatus for a given rate of mechanical power delivery than any system
using alternating current except that where both the dynamos and motors
operate at line voltage.
CHAPTER V.
THE PHYSICAL LIMITS OF ELECTRIC-POWER TRANSMISSION.
Electrical energy may be transmitted around the world if the line
voltage is unlimited. This follows from the law that a given power may
be transmitted to any distance with constant efficiency and a fixed
weight of conductors, provided the voltage is increased directly with
the distance.
The physical limits of electric-power transmission are thus fixed by the
practicable voltage that may be employed. The effects of the voltage of
transmission must be met in the apparatus at generating and receiving
stations on the one hand, and along the line on the other. In both
situations experience is the main guide, and theory has little that is
reliable to offer as to the limit beyond which the voltage will prove
unworkable.
Electric generators are the points in a transmission system where the
limit of practical voltage is first reached. In almost all high-voltage
transmissions of the present day in the United States alternating
generators are employed. Very few if any continuous-current dynamos with
capacities in the hundreds of kilowatts and voltages above 4,000 have
been built in Europe, and probably none in the United States. Where a
transmission at high voltage is to be accomplished with continuous
current, two or more dynamos are usually joined in series at the
generating station, and a similar arrangement with motors is made at the
receiving station, so that the desired voltage is available at the line
though not present at any one machine.
Alternating dynamos that deliver current at about 6,000 volts have been
in regular use for some years, in capacities of hundreds of kilowatts
each, and may readily be had of several thousand kilowatts capacity. But
even 6,000 volts is not an economical pressure for transmissions over
fifteen to fifty miles, such as are now quite common; consequently in
such transmissions it has been the rule to employ alternators that
operate at less than 3,000 volts, and to raise this voltage to the
desired line pressure by step-up transformers at the generating station.
More recently, however, the voltage of alternating generators has been
pushed as high as 13,000 in the revolving-magnet type where all the
armature windings are stationary. This voltage makes it practicable to
dispense with the use of step-up transformers for transmissions up to or
even beyond 30 miles in some cases. This voltage of 13,000 in the
armature coils is attained only by constructions involving some
difficulty because of the relatively large amount of room necessary for
the insulating materials on coils that develop this pressure. The
tendency of this construction is to give alternators unusually large
dimensions per given capacity. It seems probable, moreover, that the
pressures developed in the armature coils of alternating generators must
reach their higher limits at a point much below the 50,000 and 60,000
volts in actual use on present transmission lines. In the longest
transmissions with alternating current there is, therefore, little
prospect that step-up transformers at the generating stations and
step-down transformers at receiving stations can be dispensed with. The
highest voltage that may be received or delivered at these stations is
simply the highest that it is practicable to develop by transformers and
to transmit by the line.
A very high degree of insulation is much more easy to attain in
transformers than in generator armatures, because the space that can be
readily made available for insulating materials is far greater in the
transformers, and further because their construction permits the
complete immersion of their coils in petroleum. This oil offers a much
greater resistance than air to the passage of electric sparks, which
tend to set up arcs between coils at very high voltages and thus destroy
the insulation. Danger to insulation from the effect known as creeping
between coils at widely different pressures is largely avoided by
immersion of the coils in oil. For several years groups of transformers
have been worked regularly at 40,000 to 60,000 volts, and in no instance
is there any indication that the upper limit of practicable voltage has
been reached. On the contrary, transformers have repeatedly been worked
experimentally up to and above 100,000 volts.
From all these facts, and others of similar import, it is fair to
conclude that the physical limit to the voltages that it is practicable
to obtain with transformers is much above the 50,000 or 60,000 volts now
in practical use on transmission systems. So far as present practice is
concerned, the limit to the use of high voltages must be sought beyond
the transformers and outside of generating and receiving stations. As
now constructed, the line is that part of the transmission system where
a physical limit to the use of higher voltages will first be reached.
The factors that tend most directly to this limit are two: temporary
arcing between the several wires on a pole, and the less imposing but
constant passage of energy from one wire to another. On lines of very
high voltage arcing is occasionally set up by one of several causes. At
a point where one or more of the insulators on which the wires are
mounted become broken or defective, the current is apt to flow from one
wire to another along a wet cross-arm, until the wood grows carbonized
and an arc is formed that ends by burning up the cross-arm or even the
pole. Where lines are exposed to heavy sea fog, the salt is in some
cases deposited on the insulators and cross-arms to an extent that
starts an arc between the wires, and ends often in the destruction of
the cross-arm. In some instances the glass and porcelain insulators
supporting wires used with high voltages are punctured by sparks that
pass right through the material of the insulator to the pin on which it
is mounted, thus burning the pin and ultimately the cross-arm. This
trouble is easily met, however, by the adoption of a better grade of
porcelain or of an insulator with a greater thickness of glass or
porcelain between the wires and the supporting pin. Arcs between lines
at high voltages usually start by sparks that jump from the lower edges
of insulators, when they are wet or covered with salt deposit, to the
cross-arm. As the lower edges of insulators are only a few inches from
their cross-arms, the sparks find a path of comparatively low resistance
by passing from insulator to cross-arm and thence to the other insulator
and wire. The wood of a wet cross-arm is a far better conductor than the
air. Where wires are several feet or more apart, sparks probably never
jump directly through the air from one to the other. Large birds flying
close to such wires, however, have in some instances started momentary
arcs between them. The treatment of cross-arms with oil or paraffine
reduces the number of arcs that occur on a line of high voltage, but
does not do away with them.
As the voltages of long transmissions have gone up, the distance through
the air between wires and the distances between the lower wet edges of
insulators and the cross-arms have been much increased. Most of the
earlier transmission lines for high voltages were erected on insulators
spaced from one to two feet apart. In contrast with this practice, the
three wires of the transmission line in operation at 50,000 volts
between Cañon Ferry and Butte are arranged at the corners of a triangle
seventy-eight inches apart, one wire at the top of each pole and the
other two at opposite ends of the cross-arm. A voltage that would just
start an arc along a wet cross-arm between wires eighteen inches apart
would be quite powerless to do so over seventy-eight inches of
cross-arm, the lower wet edges of insulators being equidistant from
cross-arms in the two cases. To reach the cross-arm, the electric
current passes down over the wet or dirty outside surface of the
insulator to its lower edge. In the older types of insulators the lower
wet edge often came within two inches of the cross-arm. For the
50,000-volt line just mentioned the insulators (see illus.) are mounted
with their lower wet edges about eight inches above the cross-arms. At
its lower edge each insulator has a diameter of nine inches, and a small
glass sleeve extends several inches below this edge and close to the
wooden pin, to prevent sparking from the lower wet edge of the insulator
to the pin. These increased distances between wires in a direct line
through the air, and also the greater distances between the lower wet
edges of insulators and their pins and cross-arms, are proving fairly
effective to prevent serious arcing under good climatic conditions, for
the maximum pressures of 50,000 to 60,000 volts now in use. If these
voltages are to be greatly exceeded it is practically certain that the
distance between wires, and from the lower wet edges of insulators to
the wood of poles or cross-arms, must be still further increased to
avoid destructive arcing.
The nearest approach to an absolute physical limit of voltage with
present line construction is met in the constant current of energy
through the air from wire to wire of a circuit. A paper in vol. XV.,
Transactions American Institute Electrical Engineers, gives the tests
made at Telluride, Col., to determine the rates at which energy is lost
by passing through the air from one wire to another of the same circuit.
The tests at Telluride were made with two-wire circuits strung on a pole
line 11,720 feet in length, at first with iron wires of 0.165 inch
diameter and then with copper wires of 0.162 inch diameter. Measurements
were made of the energy escaping from wire to wire at different voltages
on the line, and also with the two wires at various distances apart. It
was found that the loss of energy over the surfaces of insulators was
very slight, and that the loss incident to the passage of energy
directly through the air is the main one to be considered. This leakage
through the air varies with the length of the line, as might be
expected. Tests were made with pairs of wires running the entire length
of the pole line and at distances of 15, 22, 35, and 52 inches apart
respectively. Losses with wires 22 or 35 inches apart were intermediate
to the losses when wires were 15 and 52 inches apart respectively.
Results given in the original paper for the pair of wires that were 15
inches apart and for the pair that were 52 inches apart are here reduced
to approximate watts per mile of two-wire line. At 40,000 volts the loss
between the two wires that were 15 inches apart was about 150 watts per
mile, and between the two wires that were 52 inches apart the loss was
84 watts per mile. The two wires 15 inches apart showed a leakage of
approximately 413 watts per mile when the voltage was up to 44,000, but
the wires 52 inches apart were subject to a leakage of only 94 watts per
mile at the same voltage. At 47,300 volts, the highest pressure recorded
for the two wires 15 inches apart, the leakage between them was about
1,215 watts per mile, while an equal voltage on the two wires 52 inches
apart caused a leakage of only 122 watts per mile, or one-tenth of that
between the wires that were 15 inches apart. When about 50,000 volts
were reached on the two wires 52 inches apart, the leakage between them
amounted to 140 watts per mile; but beyond this voltage the loss went up
rapidly, and was 225 watts per mile at about 54,600 volts. For higher
pressures the loss between these two wires still more rapidly increased,
and amounted to 1,368 watts per mile with about 59,300 volts, the
highest pressure recorded. With a loss of about 1,215 watts per mile
between the two wires 52 inches apart, the voltage on them was 58,800,
in contrast with the 47,300 volts producing the same leakage on the two
wires 15 inches apart.
Evidently, however, at even 52 inches between line wires the limit of
high voltage is not far away. When the voltage on the 52-inch line was
raised from 54,600 to 59,300, the leakage loss between the two wires
increased about 1,143 watts per mile. If the leakage increases at least
in like proportion, as seems probable, for still higher pressures, the
loss between the two wires would amount to 6,321 watts per mile with
80,000 volts on the line. On a line 200 miles long this loss by leakage
between the two wires would amount to 1,264,200 watts. Any such leakage
as this obviously sets an absolute, physical limit to the voltage, and
consequently the length of transmission.
Fortunately for the future delivery of energy at great distances from
its source, the means to avoid the limit just discussed are not
difficult. Other experiments have shown that with a given voltage and
distance between conductors the loss of energy from wire to wire
decreases rapidly as their diameters increase. The electrical resistance
of air, like that of any other substance, increases with the length of
the circuit through it. The leakage described is a flow of electrical
energy through the air from one wire to another of the same circuit. To
reduce this leakage it is simply necessary to give the path from wire to
wire through the air greater electrical resistance by increasing its
length, that is, by placing the wires at greater distances apart. The
fact demonstrated at Telluride, that with 47,300 volts on each line the
leakage per mile between the two wires 15 inches apart was ten times as
great as the leakage between the two wires 52 inches apart, is full of
meaning. Evidently, leakage through the air may be reduced to any
desired extent by suitable increase of distance between the wires of the
same circuit. But to carry this increase of distance between wires very
far involves radical changes in line construction. Thus far it has been
the uniform practice to carry the two or three wires of a transmission
circuit on a single line of poles, and in many cases several such
circuits are mounted on the same pole line. For the 65 mile transmission
into Butte, Mont., only the three wires of a single circuit are mounted
on one pole line, and this represents the best present practice. The
cross-arms on this line are each 8 feet long, and one is attached to
each pole. The poles are not less than 35 feet long and have 8-inch
tops. One wire is mounted at the top of each pole, and the other two
wires near the ends of the cross-arm, so that the three wires are
equidistant and 78 inches apart. By the use of still heavier poles the
length of cross-arms may be increased to 12 or 14 feet, for which their
section should be not less than 4 by 6 inches. Placing one wire at the
pole top, the 12-foot cross-arm would permit the three wires of a
circuit to be spaced about 10.5 feet apart. The cost of extra large
poles goes up rapidly and there are alternative constructions that seem
better suited to the case. Moreover, a few tens of thousands of volts
above present practice would bring us again to a point where even 10.5
feet between wires would not prevent a prohibitive leakage. Two poles
might be set 20 feet apart, with a cross-piece between them, extending
out 5 feet beyond each pole and having a total length of 30 feet. This
would permit three wires to be mounted along the cross-piece at points
about 14 feet apart.
If the present transmission pressures of 50,000 to 60,000 volts are to
be greatly exceeded, the line structure may involve the use of a
separate pole for each wire of a circuit, each wire to be mounted at the
top of its pole. This construction calls for three lines of poles to
carry the three wires of a three-phase transmission. Each of these poles
may be of only moderate dimensions, say 30 feet long with 6- or 7-inch
top. The cost of three of these poles will exceed by only a moderate
percentage that of a 35- or 40-foot pole with an 8- to 10-inch top, such
as would be necessary with 12-foot cross-arms. The distance between
these poles at right angles to the line may be anything desired, so that
leakage from wire to wire through the air will be reduced to a trifling
matter at any voltage. Extra long pins and insulators at the pole tops
will easily give a distance of two feet or more between the lower wet
edge of each insulator and the wood of pin or pole. Such line
construction would probably safely carry two or three times the maximum
voltage of present practice, and might force the physical limit of
electrical transmission back to the highest pressure at which
transformers could be operated. With not more than 60,000 volts on the
line the size of conductors is great enough in many cases to keep the
loss of energy between them within moderate limits when they are six
feet apart, but with a large increase of voltage the size of conductors
must go up or the distances between them must increase.
CHAPTER VI.
DEVELOPMENT OF WATER-POWER FOR ELECTRIC STATIONS.
Electrical transmission has reduced the cost of water-power development.
Without transmission the power must be developed at a number of
different points in order that there may be room enough for the
buildings in which it is to be utilized. This condition necessitates
relatively long canals to conduct the water to the several points where
power is to be developed, and also a relatively large area of land with
canal and river frontage.
With electrical transmission the power, however great, may well be
developed at a single spot and on a very limited area of land. The canal
in this case may be merely a short passageway from one end of a dam to a
near-by power-house, or may disappear entirely when the power-house
itself forms the dam, as is sometimes the case.
These differences between the distribution of water for power purposes
and the development by water of electrical energy for transmission may
be illustrated by many examples.
A typical case of the distribution of water to the points where power is
wanted may be seen in the hydraulic development of the Amoskeag
Manufacturing Company at Manchester, N. H. This development includes a
dam across the Merrimac River, and two parallel canals that follow one
of its banks for about 3,400 feet down stream. By means of a stone dam
and a natural fall a little beyond its toe a water head of about
forty-eight feet is obtained at the upper end of the high-level canal.
Below this point there is little drop in the bed of the river through
that part of its course that is paralleled by the two canals. All of the
power might be thus developed within a few rods of one end of the dam,
if means were provided for its distribution to the points where it must
be used.
Years ago, when this water-power was developed, the electrical
transmission or distribution of energy was unheard of, and distribution
of the water itself had therefore to be adopted. For this purpose the
two canals already mentioned were constructed along the high bank of the
river at two different levels.
[Illustration: FIG. 4.--Hydraulic Development on the Merrimac River.]
The high-level canal, so called, was designed to take water directly
from the basin or forebay a little below one end of the dam, so that
between this canal and the river there is a full water head of about
forty-eight feet. Over nearly its entire course the nearer side of this
high-level canal runs between 450 and 750 feet from the edge of the
river wall, and thus includes between it and the river a large area on
which factories to be driven by water-wheels may be located. It was
thought, however, that this strip of land between the high-level canal
and the river was too wide for a single row of mill sites, and the lower
level canal was therefore constructed parallel with that on the higher
level, but with about twenty-one feet less elevation.
Between these two canals a strip of land about 250 feet wide was left
for the location of mills. By this arrangement of canals it is possible
to supply wheels located between the high and the low levels with water
under a head of about twenty-one feet, and to supply wheels between the
lower canal and the river with water under a head of about twenty-nine
feet. The entire area of land between the high canal and the river is
thus made readily available for factory buildings.
Water for the lower canal is drawn mainly from the high canal through
the wheels in buildings that are located between the two canals. It is
desirable in a case of this sort to have as much water flow through the
wheels between the high and low canal as flows through the wheels
between the low canal and the river, but this is not always possible. A
gate is therefore provided at the forebay where the two canals start, by
which water may pass from the forebay directly into the low canal when
necessary, but the head of twenty-one feet between the forebay and the
low canal is lost as to this water. Between the high and low canal, and
between the low canal and the river twenty-three turbine wheels or pairs
of wheels have been connected, and these wheels have a combined rating
of 9,500 horse-power.
To carry out this hydraulic development it thus appears that about 1.3
miles of canal have been constructed; one-half this length of
river-front has been required, and about one-sixth square mile of
territory has been occupied. Contrast with this result what might have
been done if electrical transmission of power had been available at the
time when this water-power was developed. All but a few rods in length
of the existing 1.3 miles of canal might have been omitted, and an
electric generating station with wheels to take the entire flow of the
river might have been located not far from one end of the dam. Factories
utilizing the electric power thus developed might have been located at
any convenient points along the river-front or elsewhere, and no space
would have been made unavailable because of the necessity of head- and
tail-water connections to scattered sets of wheels.
Compare with the foregoing hydraulic development that at Cañon Ferry on
the Missouri River, in Montana, where 10,000 horse-power is developed
under a water-head of 32 feet. At Cañon Ferry the power-house is 225
feet by 50 feet at the floor level inside, contains turbine wheels
direct-connected to ten main generators of 7,500 kilowatts, or 10,000
horse-power combined capacity, and is built on the river bank close to
one end of the 500-foot dam. The canal which runs along the land side of
the power-house, and takes water at the up-stream side of the abutment,
is about twice the length of the power-house itself. The saving in the
cost of canal construction alone, to say nothing of the saving as to the
required area of land, is evidently a large item in favor of the
electrical development and transmission. In its small area and short
canal the Cañon Ferry plant is not an exception, but is rather typical
of a large class of electric water-power plants that operate under
moderate heads.
A like case may be seen in the plant at Red Bridge, on the Chicopee
River, in Massachusetts, where a canal 340 feet long, together with
penstocks 100 feet long, convey water from one end of the dam and
deliver it to wheels in the electric station with a head of 49 feet.
This station contains electric generators with a combined capacity of
4,800 kilowatts or 6,400 horse-power, and its floor area is 141 by 57
feet.
[Illustration: FIG. 5.--Canal at Red Bridge on Chicopee River.]
So, again, at Great Falls, on the Presumpscot River, in North Gorham,
Me. (see cut), the electric station sets about 40 feet in front of the
forebay wall, which adjoins one abutment of the dam, and there is no
canal whatever, as short penstocks bring water to the wheels with a head
of 35 feet. In ground area this station is 67.5 by 55 feet, and its
capacity in main generators is 2,000 kilowatts or 2,700 horse-power.
A striking illustration of the extent to which electrical transmission
reduces the cost of water-power development may be seen at Gregg’s Falls
on the Piscataquog River, in New Hampshire, where an electric station of
1,200 kilowatts capacity has been built close to one end of the dam, and
receives water for its wheels under a head of 51 feet through a short
penstock, 10 feet in diameter, that pierces one of the abutments.
[Illustration: FIG. 6.]
Perhaps the greatest electric water-power station anywhere that rests
close to the dam that provides the head for its wheels is that at
Spier Falls (see cut), on the upper Hudson. One end of this station is
formed by the high wall section of the dam, and from this wall the
length of the station down-stream is 392 feet, while its width is 70
feet 10 inches, both dimensions being taken inside. The canal or forebay
in this case, like that at Cañon Ferry, lies on the bank side of the
power-station, and is about equal to it in length. From this canal ten
short penstocks, each 12 feet in diameter, will convey water under a
head of 80 feet to as many sets of turbine wheels in the station. These
wheels will drive ten generators with an aggregate capacity of 24,000
kilowatts or 32,000 horse-power.
Sometimes the slope in the bed of a river is so gradual or so divided up
between the number of small falls, or the volume of water is so small,
that no very large power can be developed at any one point without the
construction of a long canal. In a case of this sort electrical
transmission is again available to reduce the expense of construction
that will make it possible to concentrate all the power from a long
stretch of the river at a single point. This is done by locating
electric generating stations at as many points as may be thought
desirable along the river whose energy is to be utilized, and then
transmitting power from all of these stations to the single point where
it is wanted.
A case in point is that of Garvins Falls and Hooksett Falls on the
Merrimac River and four miles apart. At the former of these two falls
the head of water is twenty-eight feet, and at the latter it is sixteen
feet. To unite the power of both these falls in a single water-driven
station would obviously require a canal four miles long whose expense
might well be prohibitive. Energy from both these falls is made
available at a single sub-station in Manchester, N. H., by a generating
plant at both points and transmission lines thence to that city.
At Hooksett the present capacity of the electric station is 1,000
horse-power, and at Garvins Falls the capacity is 1,700 horse-power. The
river is capable of developing larger powers at both of these falls,
however, and construction is now under way at Garvins that will raise
its station capacity to 5,000 horse-power.
A similar result in the combination of water-powers without the aid of a
long canal is reached in the case of Gregg’s Falls and Kelley’s Falls,
which are three miles apart on the Piscataquog River. At the former of
these two falls the electric generating capacity is 1,600 horse-power,
as previously noted, and at the latter fall the capacity is 1,000
horse-power. In each case the station is close to its dam, and no canal
is required. Electrical transmission unites these two powers in the same
sub-station at Manchester that receives the energy from the two
stations above named on the Merrimac River.
Instead of transmitting power from two or more waterfalls to some point
distant from each of them, the power developed at one or more falls may
be transmitted to the site of another and there used. This is, in fact,
done at the extensive Ludlow twine mills on the Chicopee River, in
Massachusetts. These mills are located at a point on the river where its
fall makes about 2,500 horse-power available, and this fall has been
developed to its full capacity. After a capacity of 2,400 horse-power in
steam-engines had been added, more water-power was sought, and a new dam
was located on the same river at a point about 4.5 miles up-stream from
the mills. The entire flow of the river was available at this new dam,
and a canal 4.5 miles long might have been employed to carry the water
down to wheels at the mills in Ludlow.
Such a canal would have meant a large investment, not only for land and
construction, but also, possibly, for damages to estates bordering on
the river, if all of its water was diverted. Instead of such a canal, an
electric generating station was located close to the new dam with a
capacity of 6,400 horse-power, and this power is transmitted to motors
in the mills at Ludlow.
Even where the power is to be utilized at some point distant from each
of several waterfalls, it may be convenient to combine the power of all
at one of them before transmitting it to the place of use. This is
actually done in the case of two electric stations located respectively
at Indian Orchard and Birchem Bend on the Chicopee River, whose energy
is delivered to the sub-station of the electrical supply system in
Springfield, Mass. At the Indian Orchard station the head of water is 36
feet, and at Birchem Bend it is 14 feet, while the two stations are
about 2 miles apart. A canal of this length might have been built to
give a head of 50 feet at the site of the Birchem Bend dam, but instead
of this an electric station was located near each fall, and a
transmission line was built between the two stations. Each generating
station was also connected with the sub-station in Springfield by an
independent line, and power is now transmitted from one generating plant
to the other, as desired, and the power of both may go to the
sub-station over either line. In the Indian Orchard station the dynamo
capacity is about 2,000 kilowatts, and at Birchem Bend it is 800
kilowatts.
Another case showing the union of two water-powers by electrical
transmission, where the construction of an expensive canal was avoided,
is that of the electrical supply system of Hartford, Conn. This system
draws a large part of its energy from two electric plants on the
Farmington River, at points that are about 3 miles apart in the towns of
Windsor and East Granby, respectively. At one of these plants the head
of water is 32 feet, and at the other it is 23 feet, so that head of 55
feet might have been obtained by building a canal 3 miles long. Each of
these stations is located near its dam, and the generator capacity at
one station is 1,200 and at the other 1,500 kilowatts. Transmission
lines deliver power from both of these plants to the same sub-station in
Hartford.
Sometimes two or more water-powers on the same river that are to be
united are so far apart that any attempt to construct a canal between
them would be impracticable. This is illustrated by the Spier and
Mechanicsville Falls on the Hudson River, which are 25 miles apart in a
direct line and at a greater distance by the course of the stream. At
Spier Falls the head is 80 feet, and at Mechanicsville it is 18 feet.
Union of the power of these two falls is thus out of the question for
physical reasons alone. Electrical transmission, however, brings energy
from both of these water-powers to the same sub-stations in Schenectady,
Albany, and Troy.
In another class of cases electrical transmission does what could not be
done by any system of canals, however expensive; that is, unites the
water-powers of distinct and distant rivers at any desired point. Thus
power from both the Merrimac and the Piscataquog rivers is distributed
over the same wires in Manchester; the Yuba and the Mokelumne contribute
to electrical supply along the streets of San Francisco; and the Monte
Alto and Tlalnepantla yield energy in the City of Mexico.
It does not follow from the foregoing that it is always more economical
to develop two or more smaller water-powers at different points along a
river for transmission to some common centre than it is to concentrate
the water at a single larger station by more elaborate hydraulic
construction, and then deliver all of the energy over a single
transmission line. The single larger hydraulic and electric plant will
usually have a first cost larger than that of the several smaller ones,
because of the required canals or pipe lines. A partial offset to this
larger hydraulic investment is the difference in cost between one and
several transmission lines, or at least the cost of the lines that would
be necessary between the several smaller stations in order to combine
their energy output before its transmission over a single line to the
point of use.
Against the total excess of cost for the single larger hydraulic and
electrical plant there should be set the greater expense of operation
at several smaller and separate plants. Even a small water-driven
electric station that can be operated by a single attendant at any one
time must have two attendants if it is to deliver energy during the
greater part or all of every twenty-four hours. But a single attendant
can care for a water-power plant of 2,000 horse-power or more capacity,
so that two plants of 750 horse-power each will require double the
operating force of one plant of 1,500 horse-power. If two such plants
are constructed instead of one that has their combined capacity, the
monthly wages of the two additional operators will amount to at least
one hundred dollars. If money is worth six per cent yearly, it follows
that an additional investment of $1,200 ÷ 0.06 = $20,000 might be made
in hydraulic work to concentrate the power at one point before the
annual interest charge would equal the increase of wages made necessary
by two plants.
Reliability of operation is one of the most important requirements in an
electric water-power plant, and its construction should be carried out
with this in view. Anchor ice is a serious menace to the regular
operation of water-wheels in cold climates, because it clogs up the
openings in the racks and in the wheel passages. Anchor ice is formed in
small particles in the water of shallow and fast-flowing streams, and
tends to form into masses on solid substances with which the water comes
in contact.
At the entrance to penstocks or wheel chambers, steel racks with long,
narrow openings, say one and one-quarter inches wide, are regularly
placed to keep all floating objects away from the wheels. When water
bearing fine anchor or frazil ice comes in contact with these racks, it
rapidly clogs up the narrow openings between the bars, unless men are
kept at work raking off the ice as it forms. At the Niagara Falls
electric station, in some instances, when the racks become clogged, they
have been lifted, and the anchor ice allowed to pass down through the
wheels. This is said to have proved an effective remedy, but it would
obviously be of no avail in a case where the ice clogged the passages of
the wheels themselves.
The best safeguard against anchor ice is a large, deep forebay next to
the racks, where the water, being comparatively quiet, will soon freeze
over after cold weather commences. The anchor ice coming down to this
forebay and losing most of its forward motion, soon rises to the surface
or to the under side of the top coating of solid ice, and the warmer
water sinks to the bottom. Good construction puts the entrance ends of
penstocks well below the surface of water in the forebay, so that they
may receive the warmer water that contains little or no anchor ice.
[Illustration: FIG. 7.--Cross Section of Dike on Chicopee River at Red
Bridge.]
Illustrations of practice along these lines, as to size, depth of
forebay, and location of penstocks may be seen in many well-designed
plants. One instance is that at Garvins Falls, on the Merrimac River,
where the new hydraulic development for 5,000 horse-power is now under
way. Water from the river in this case comes down to the power-station
through a canal 500 feet long, and of 68 feet average width midway
between the bottom and the normal flow line. In depth up to his flow
line the canal is 12 feet at its upper and 13 feet at its lower end,
just before it widens into the forebay. In this forebay the depth
increases to 17 feet, and the width at the rack is double that of the
canal. The steel penstocks, each 12 feet in diameter, terminate in the
forebay wall at an average distance of 7 feet behind the rack, and each
penstock has its centre 10.6 feet below the water level in the forebay.
As there is a large pond created by the dam in this case, and as the
flow of water in the canal is deep rather than swift, enough opportunity
is probably afforded for any anchor ice to rise to the surface before it
reaches the forebay in this case.
Penstocks for the electric station at Great Falls, on the Presumpscot
River, whence energy is drawn for lighting and power in Portland, Me.,
are each 8 feet in diameter, and pierce the forebay wall behind the rack
with their centres 15 feet below the normal water level in the forebay.
In front of the forebay wall the water stands 27 feet deep, and the pond
formed by the dam, of which the forebay wall forms one section, is 1,000
feet wide and very quiet. Though the Maine climate is very cold in
winter and the Presumpscot is a turbulent stream above the dam and pond,
there has never been any trouble with anchor ice at the Great Falls
plant. An excellent illustration is thus presented of the fact that
deep, still water in the forebay is a remedy for troubles with ice of
this sort.
Maximum loads on electrical supply systems are usually from twice to
four times as great as the average loads during each twenty-four hours.
A pure lighting service tends toward the larger ratio between the
average and maximum load, while a larger motor capacity along with the
lamps, tends to reduce the ratio. Furthermore, by far the greater part
of the energy output of an electrical supply system during each
twenty-four hours must be delivered between noon and midnight. For these
reasons there must be enough water stored, that can flow to the station
as wanted, to carry a large share of the load during each day, unless
storage batteries are employed to absorb energy at times of light load,
if the entire normal flow of the river is to be utilized.
It is usually much cheaper to store water than electrical energy for the
daily fluctuations of load, and the only practicable place for this
storage is most commonly behind the dam that maintains the head for the
power-station. This storage space should be so large that the drain upon
it during the hours of heavy load will lower the head of water on the
wheels but little, else it may be hard to maintain the standard speed of
revolution for the wheels and generators, and consequently the
transmission voltage.
[Illustration: FIG. 8.--Valley Flooded above Spier Falls on the Hudson
River.]
At the Great Falls plant, water storage to provide for the fluctuations
of load in different parts of the day takes place back of the dam, and
for about one mile up-stream. This dam is 450 feet long in its main
part, and a retaining wall increases the total length to about 1,000
feet. For half a mile up-stream from this dike and dam the average width
of the pond is 1,000 feet, and its greatest depth is not less than 27
feet. As the station capacity is 2,700 horse-power in main generators,
with a head of 35 feet at the wheels the storage capacity is more than
ample for all changes of load at different times of day.
The dam at Spier Falls, on the Hudson River, is 1,820 feet long between
banks, 155 feet high above bedrock in its deepest section, and raises
the river 50 feet above its former level. Behind the dam a lake is
formed one-third of a mile wide and 5 miles long. Water from this
storage reservoir passes down through the turbines with a head of 80
feet, and is to develop 32,000 horse-power. As a little calculation will
show, this lake is ample to maintain the head under any fluctuation in
the daily load. At Cañon Ferry, where electrical energy for Butte and
Helena, Mont., is developed, the dam, which is 480 feet long, crosses
the river in a narrow canyon that extends up-stream for about half a
mile. Above this canyon the river valley widens out, and the dam, which
maintains a head of 30 feet at the power-station, sets back the water in
this valley, and thus forms a lake between two and three miles wide and
about seven miles long. At the station the generator equipment has a
total rating of 10,000 horse-power. From these figures it may be seen
that the storage lake would be able to maintain nearly the normal head
of water for some hours, when the station was operating under full load,
however small the flow of the river above.
[Illustration: FIG. 9.--Canal at Bulls Bridge on Housatonic River.]
CHAPTER VII.
THE LOCATION OF ELECTRIC WATER-POWER STATIONS.
Cost of water-power development depends, in large measure, on the
location of the electric station that is to be operated. The form of
such a station, its cost, and the type of generating apparatus to be
employed are also much influenced by the site selected for it. This site
may be exactly at, or far removed from, the point where water that is to
pass through the wheels is diverted from its natural course.
A unique example of a location of the former kind is to be found near
Burlington, Vt., where the electric station is itself a dam, being built
entirely across the natural bed of one arm of the Winooski River at a
point where an island near its centre divides the stream into two parts.
The river at this point has cut its way down through solid rock, leaving
perpendicular walls on either side. Up from the ledge that forms the bed
of the stream, and into the rocky walls, the power-station, about 110
feet long, is built. The up-stream wall of this station is built after
the fashion of a dam, and is reënforced by the down-stream wall, and the
water flows directly through the power-station by way of the wheels. A
construction of this sort is all that could well be attained in the way
of economy, there being neither canal nor long penstocks, and only one
wall of a power-house apart from the dam. On the other hand, the
location of a station directly across the bed of a river in this way
makes it impossible to protect the machinery if the up-stream wall,
which acts as the dam, should ever give way. The peculiar natural
conditions favorable to the construction just considered are seldom
found.
[Illustration: FIG. 10.--Power-house on the Winooski River, near
Burlington, Vt.]
One of the most common locations for an electric water-power station is
at one side of a river, directly in front of one end of the dam and
close to the foot of the falls. A location of this kind was adopted for
the station at Gregg’s Falls, one of the water-powers included in the
electric system of Manchester, N. H., where the spray of the fall rises
over the roof of the station. Two short steel penstocks, each ten feet
in diameter, convey the water from the forebay section of the dam to
wheels in the station with a head of fifty-one feet.
[Illustration: FIG. 11.--Canal and Power-house on St. Joseph River,
Buchanan, Mich.]
A similar location was selected for the station at Great Falls, on
the Presumpscot River (see cuts), whence electrical energy is
delivered in Portland, Me. Four steel penstocks, a few feet long and
each eight feet in diameter, bring the water in this case from the
forebay section of the dam to the wheel cases in the power-house.
[Illustration: FIG. 12.--Power-house on Hudson River at Mechanicsville.]
Where the power-station is located at the foot of the dam, as just
described, that part which serves as a forebay wall usually carries a
head gate for each penstock. The overfall section of a dam may give way
in cases like the two just noted without necessarily destroying the
power-station, but in times of freshet or very high water the station
may be flooded and its operation stopped. The risk of any such flooding
will vary greatly on different rivers, and in particular cases may be
very slight. Location of the generating station close to the foot of the
dam at one end obviously avoids all expense for a canal and cuts the
cost of penstocks down to a very low figure.
Such locations for stations are not limited to falls of any particular
height, and the short penstocks usually enter the dam nearer its base
than its top and pass to the station at only a slight inclination from
the horizontal. At Great Falls, above mentioned, the head of water is
thirty-seven feet.
A short canal is constructed in some cases from one end of a dam to a
little distance down-stream, terminating at a favorable site for the
electric station. Construction of this sort was adopted at the Birchem
Bend Falls of the Chicopee River, whence energy is supplied to
Springfield, Mass. These falls furnish a head of fourteen feet, and the
water-wheels are located on the floor of the open canal at its end. The
power-station is on the shore side of this canal, and the shafts of the
water-wheels extend through bushings in the canal wall, which forms the
lower part of one side of the station, to connect with the electric
generators inside.
This rather unusual location of water-wheels has at least the obvious
advantage that they require no room inside of the station. Furthermore,
as the canal is between the station and the river, any break in the
canal is not apt to flood the station.
[Illustration: FIG. 13.--York Haven Power-house, on Susquehanna River,
Pennsylvania.]
An illustration of the use of a very short canal to convey water from
one end of a dam to a power-station exists in the 10,000 horse-power
plant at Cañon Ferry, Mont., where the head of water is thirty feet. In
this case the masonry canal is but little longer than the power-house,
and this latter sits squarely between the canal and the river, virtually
at the foot of the falls. Other examples of the location of generating
stations between short canals and the river may be seen at Concord, N.
H., where the head of water is sixteen feet; at Lewiston, Me., where the
head is thirty-two feet; and at Spier Falls, on the Hudson River, New
York, where there is a head of eighty feet.
There is some gain in security in many cases by locating the
power-station several hundred feet from the dam and a little to one side
of the main river channel. For such cases a canal may be cheaper than
steel penstocks when the items of depreciation and repairs are taken
into account. Aside from the question of greater security for the
station in the event of a break in the dam, it is necessary in many
cases to convey the water a large fraction of a mile, or even a number
of miles, from the point where it leaves its natural course to that
where the power-station should be located. An example in point exists at
Springfield, Mass., where one of the electric water-power stations is
located about 1,400 feet down-stream from a fall of thirty-six feet in
the Chicopee River, because land close to the falls was all occupied at
the time the electric station was built.
[Illustration: FIG. 14.--Power-house at Cañon Ferry on the Missouri
River.]
[Illustration: FIG. 15.--Shawinigan Falls Power-plant.]
The Shawinigan Falls of the St. Maurice River in Canada occur at two
points a short distance apart, the fall at one point being about 50 and
at the other 100 feet high. A canal 1,000 feet long takes water from the
river above the upper of these falls and delivers it near to the
electric power-house on the river bank below the lower falls. In this
way a head of 125 feet is obtained at the power-house. The canal in this
case ends on high ground 130 feet from the power-house, and the water
passes down to the wheels through steel penstocks 9 feet in diameter.
[Illustration: FIG. 16.--Power-house on White River, Oregon.]
Another interesting example of conditions that require a power-house to
be located some distance from the point where water is diverted from its
natural course may be seen at the falls on the Apple River, whence
energy is transmitted to St. Paul, Minn. By means of a natural fall of
30 feet, a dam 47 feet high some distance up-stream, and some rapids in
the river, it was there possible to obtain a total fall of 82 feet. To
utilize this entire fall a timber flume, 1,550 feet in length, was built
from the dam to a point near the power-house on the river bank and below
the falls and rapids. The flume was connected with the wheels, 82 feet
below, by a steel penstock 313 feet long and 12 feet in diameter.
As the St. Mary’s River leaves Lake Superior it passes over a series of
rapids about half a mile in length, falling twenty feet in that
distance. To make the power of this great volume of water available, a
canal 13,000 feet long was excavated from the lake to a point on the
river bank below the rapids. Between the end of the canal and the river
sits the power-station, acting as a dam, and the water passes down
through it and the wheels under a head of twenty feet.
[Illustration: FIG. 17.--Power-house Across Canal at Sault Ste. Marie,
Mich.]
By means of a canal 16,200 feet long from the St. Lawrence River a head
of water amounting to fifty feet has been made available at a point on
the bank of Grass River near Massena, N. Y. There again the
power-station acts as a dam, and the canal water passes down through it
to reach the river.
[Illustration: FIG. 18.--Canal and Station on Payette River, Idaho.]
From these illustrations it may be seen that in many cases, in
comparatively level country, a water-power can be fully developed only
by means of canals or pipe lines, and the generating stations cannot be
located at the points where the water is diverted.
Thus far the cases considered have been only those with moderate heads
and rather large volumes of water. In mountainous country, where rivers
are comparatively small and their courses are marked by numerous falls
and rapids, it is generally necessary to utilize the fall of a stream
through some miles of its length in order to effect a satisfactory
development of power. To reach this result, rather long canals, flumes,
or pipe lines must be utilized to convey the water to power-stations and
deliver it at high pressures.
In cases of this kind the cost of the canal or pipe line may be the
largest item in the power development, and it may be an important
question whether this cost should be reduced or avoided by the erection
of several small generating plants instead of one large one. California
offers numerous examples of electric-power development with water that
has been carried several miles through artificial channels. An
illustration of this class of work exists at the Electra power-house on
the bank of the Mokelumne River, in the Sierra Nevada Mountains. Water
is supplied to the wheels in this station under a head of 1,450 feet
through pipes 3,600 feet long leading to the top of a near-by hill. To
reach this hill the water, after its diversion from the Mokelumne River
at the dam, flows twenty miles through a canal or ditch and then through
3,000 feet of wooden stave pipe.
Another example of the same sort may be seen in the power-house at
Colgate, on the North Yuba River, in the chain of mountains above named.
Water taken from this river passes through a wooden flume nearly eight
miles long to the side of a hill 700 feet above the power-house, and
thence down to the wheels through steel and cast-iron pipes, five in
number and thirty inches each in diameter.
Even with long flumes, canals, and pipe lines, it may be necessary to
locate a number of generating stations along a single river of the class
now under consideration in order to utilize its entire power. Thus on
the Kern River, which rises in the Sierra Nevada Mountains and empties
into Tulare Lake, two electric power-stations are under construction,
and surveys are being made for three more. Of these stations, the one at
the lowest level will operate under an 872-foot head of water, and this
water, after its diversion from the river, will pass through twenty-one
tunnels, with an aggregate length of about ten miles, and through six
flumes mounted on trestles and having a total length of 1,703 feet.
Next up-stream is a station near the point where water is diverted for
the plant just named. This second station will work under a head of 317
feet, and water for it will come from a point farther up-stream by
canals, tunnels, and flumes, with an aggregate length of eleven and
one-half miles. At three points still higher up on this river it is the
intention to locate three other power-stations by conducting the water
in artificial channels, about twelve and one-half, fifteen, and twenty
miles in length respectively.
Farther south in California, on the Santa Ana River and Mill Creek,
extensive power developments on the lines just indicated have been
carried out. On Mill Creek, about six miles from the city of Redlands,
is an electric station operating under a head of 530 feet, with water in
part diverted from the stream a little less than two miles above and
brought down through a steel pipe 10,250 feet long and thirty inches
in diameter. This pipe line also takes water from the tail race of
another generating plant at its upper end. With some additions and
modifications, the station just described is the famous Redlands plant,
built in 1893, and believed to be the first for three-phase transmission
in the United States.
[Illustration: FIG. 19.--Canal and Power-station on Neversink River, New
York.]
At the upper end of the pipe line just named the second station
operates, in part, with water drawn from Mill Creek through a
combination of tunnels, flumes, and cement and steel pipes, with a
combined length of about three miles, and delivered to some of the
wheels with a head of 627 feet. The other wheels at this plant receive
water drawn from the same creek by a pipe line about six miles long. A
large part of this line is composed of 31-inch cement pipe, laid in
trenches and tunnels. The water in the 8,000 feet of pipe next to the
power-house has a fall of 1,960 feet, and this pipe is of steel and 24
and 26 inches in diameter. The head of 1,960 feet, minus friction losses
in the steel pipes, is delivered at the wheels.
From the foregoing it appears that in a space of eight miles along Mill
Creek there is a fall of more than 2,490 feet. To utilize this fall,
water is diverted from the creek at three points within a distance of
six miles and delivered in two power-stations under three different
heads. As the stream gathers in volume between the upper and the lower
intakes, an equal amount of power could have been developed in a single
station only by taking the three separate conduits or pipe lines to it
and delivering their water there at three heads.
Whether the expense of extending conduits and pipe lines to a single
generating station will more than offset the advantages to be gained
thereby is a question that should be decided on a number of factors
varying with each case. In general, it may be said that the smaller the
volume of water to be handled and the greater its head, the more
advantageous is it to concentrate the generating machinery in the
smallest practicable number of stations.
On the Santa Ana River, into which Mill Creek flows, the Santa Ana
plant, whence energy is transmitted to Los Angeles, is located. Water
reaches this plant through a conduit of tunnels, flumes, and pipes, with
a total length of about three miles from the point where the flow of the
river is diverted. The 2,210 feet of this conduit nearest the
power-plant are composed of 30-inch steel pipe, with a fall of 728 feet.
Within fifteen miles of Mexico City are five water-power stations that
supply energy for its electrical system. Two of these stations are on
the Monte Alto and three are on the Tlalnepantla River, the two former
stations being about three miles, and the more distant of the three
latter stations five miles, apart. At a distance of several miles above
the highest station on each river the water is diverted by a canal, and
the water of each of these canals, after passing through the wheels of
the highest station, goes on to the remaining station, or stations, on
the same river by a continuation of the canal.
[Illustration: FIG. 20.--Wood Pipe Line to Pike’s Peak Power-house.]
By placing the stations so short a distance apart the head of water at
each station is reduced. On one stream these heads are 492 and 594 feet
respectively, and at two of the stations on the other stream they are
547 and 295 feet respectively. This division of the total head of water
afforded by each river results in a rather small capacity for each
station, the total at the five plants being only 4,225 kilowatts.
In contrast with this figure the already mentioned Electra plant has
generators of 10,000, the Santa Ana plant generators of 3,000, and the
larger of the two Mill Creek plants generators of 3,500 kilowatts
capacity. It should be noted that the cost of operation, as well as that
of original construction, will vary materially between one large and
several smaller stations of equal total capacity, the advantage as to
operative cost being obviously with the one large plant.
[Illustration: FIG. 21.--Power-house at Great Falls, Presumpscot
River.]
All of the power-stations here considered have been equipped with
water-wheels and generators operating on horizontal shafts, and this is
the general practice. This arrangement brings the generators and the
floor of the power-station within a few feet of the level of the
tail-water. By the general use of draught tubes with turbine wheels the
floors of stations are often kept twenty feet or more above the
tail-water level.
Where the total available head of water is quite small, as is often the
case with rivers where the volume of water is great, it is generally
necessary to bring the level of the station floor down to within a few
feet of the tail-water. The Birchem Bend station of the Springfield,
Mass., electric system affords a good example of this sort, the floor of
this station being only 2.6 feet above the ordinary level of the
tail-water. At this station the difference of level between the head-
and tail-water is only fourteen feet, and even with the low floor level
named the top sides of the horizontal turbine wheels are covered only by
4.5 feet of water.
At the Garvin’s Falls station of the Manchester, N. H., electric system
the level of the floor of the generator room is thirteen feet above the
ordinary level of the Merrimac River, on the bank of which this station
is located; but in this case the total head of water is about
twenty-eight feet. The high water of the Merrimac in 1896, before the
Garvin’s Falls station was built, reached a point 5.24 feet above its
present floor level, and 18.24 feet above the ordinary level of the
river at the point where the station is located.
Under the Red Bridge electric station of the Ludlow Manufacturing
Company, on the Chicopee River, in Massachusetts, the tail-water is
twenty feet below the level of the floor and twenty-four feet below the
centres of the water-wheel and generator shafts. The difference between
wheel-shaft and tail-water levels at this station is near the maximum
that can be attained with horizontal pressure turbines, because a
draught tube much longer than twenty-five feet does not give good
results.
In a pressure turbine the guides and wheel must be completely filled
with water, as must also the draught tube, for efficient operation. If
draught tubes are much more than twenty-five feet long, it is hard to
keep a solid column of water from turbine to tail-water in each, and if
this is not done a part of the head of water becomes ineffective. As
pressure turbines are employed almost exclusively at electric stations
with low heads of water, it is frequently impossible to locate such
stations above the possible level of tail-water in times of flood if
horizontal wheels direct-connected to generators are employed.
[Illustration: FIG. 22.--Power-house at Garvin’s Falls on the Merrimac
River.]
If turbines with vertical shafts are to be used, a power-station may
be so located or constructed that all the electrical equipment will be
above the highest known water-mark. With vertical shafts, connecting
wheels, and generators, the main floor of an electric station may be
located above the crest of the falls where the power is developed
instead of at or near their base.
[Illustration: FIG. 23.--Power-house No. 2 at Niagara Falls.]
By far the most important examples of electric stations laid out on this
plan are those at Niagara Falls, where there are four such plants. Two
of these generating plants, with an aggregate capacity of 105,000
horse-power, stand a mile above the falls, and are supplied with water
through a short canal from Niagara River. Beneath each of these two
stations a long, narrow wheel pit has been excavated through rock to a
depth of 172 feet below the level of water in the canal. Both wheel pits
terminate in a tunnel 7,000 feet long that opens into the river below
the falls.
In this wheel pit the tail-water level is 161 feet below that of the
water in the canal, and 166 feet below the floor of the power-station.
Water passes from the canal down the wheel pits to the wheels near the
bottom through steel penstocks, each seven feet in diameter, and a
vertical shaft extends from each wheel case to a generator in the
station above.
Locations like that at Niagara give great security against high water
and washouts, but are seldom adopted because of the large first cost of
plant construction. With heads of water from several hundred to 2,000
feet the loss of a few feet of head reduces the available power to only
a very slight extent, and impulse wheels are usually employed. Draught
tubes are not available to increase the heads at such wheels, and any
fall of the water after it leaves the wheels does no useful work.
[Illustration: FIG. 24.--Colgate Power-house.]
Electric stations driven by impulse wheels under great heads, like those
at Colgate, Electra, Kern River, Santa Ana River, and Mill Creek, may be
located far enough above the beds of their water-courses to avoid
dangers from freshets, without serious loss of available power.
CHAPTER VIII.
DESIGN OF ELECTRIC WATER-POWER STATIONS.
[Illustration: FIG. 25.--Cross Section of Columbus, Ga., Power-station.]
Water-wheels must be located at some elevation between that of head- and
tail-water. With horizontal shafts and direct-connected wheels and
generators the main floor of the station is brought below the level of
the wheel centres. This is much the most general type of construction,
and was followed in the Massena, Sault Ste. Marie, Cañon Ferry, Colgate,
Electra, Santa Ana, and many other well-known water-power stations. If
horizontal shafts are employed for wheels and generators with belt or
rope connections between them the floor of the generator room may be
elevated a number of feet above the wheels. This difference of elevation
is usually provided for either by upper and lower parts of the same
room, or by separate rooms one above the other and a floor between them.
A two-story construction of this latter sort was frequently adopted in
the older water-power stations, and good examples of it may be seen in
connection with the electrical supply system at Burlington, Vt., and the
Indian Orchard station in the Springfield, Mass., system. Vertical wheel
shafts make the elevation of the main or generator floor of a station
independent of that of the wheels, and thus give the highest degree of
security against high water. After the vertical wheel shaft reaches the
generator room, it may be geared to a horizontal shaft that has one or
more dynamos directly mounted on it, or drives dynamos through belts or
ropes. Belt-driving in this way, from horizontal shafts connected by
bevel gears with vertical wheel shafts, is not uncommon in the older
class of water-power stations. Generators mounted singly or in pairs on
horizontal shafts that are driven by gearing on vertical wheel shafts
have been adopted at the Lachine Rapids and South Bend plants, and it
seems to offer a desirable method of connection in cases where vertical
wheels are necessary and the cost of generators must be kept at a low
figure. With this method of driving the generators can be designed for
any economical speed and step bearings avoided.
[Illustration: FIG. 26.--Cross Section of Combined Steam- and
Water-power Station at Richmond, Va.]
[Illustration: FIG. 27.--Cross Section of Wheel House at Buchanan,
Mich.]
The most desirable method of driving generators with vertical wheels,
where the expense is not too great, is the direct mounting of each
generator on the upper end of a wheel shaft (see cut). This method of
connection not only requires a special type of generator, but may put
serious limits on its speed. In general, the peripheral speed of a
pressure turbine should be about 75 per cent of the theoretical velocity
of water issuing under a head equal to that at which the wheel operates,
in order to give the best efficiency. The rotative speeds of turbines,
operating under any given head, should thus increase as their capacities
and diameters decrease. Because of these principles it is the common
practice, with horizontal wheels, to mount two or more on each shaft to
which a generator is direct-connected in order to obtain a greater speed
of rotation than could be obtained with a single wheel of their combined
power. Thus, at Sault Ste. Marie the horizontal shaft on which each
400-kilowatt generator is mounted is driven at 180 revolutions per
minute by four turbines under a head of about 20 feet. At Massena the
head of water is 50 feet, and each 5,000 horse-power generator is driven
at 150 revolutions per minute by six turbines on a horizontal shaft.
Vertical turbines are sometimes mounted singly on their shafts, as was
done in the hydroelectric plant at Oregon City on the Willamette River,
and this practice gives speeds that are too low for direct-connected
dynamos of moderate cost, unless the head of water is unusually great.
At the Oregon City plant the head of water is only 40 feet, and yet a
single 42-inch turbine was mounted on the vertical shaft that drives
each generator.
[Illustration: FIG. 28.--Longitudinal Section of Buchanan, Mich.,
Power-house.]
The most notable examples of direct-connected generators and vertical
turbines is that at Niagara Falls, where twenty-one generators of 5,000
horse-power each are mounted at the tops of as many vertical wheel
shafts in two of the four stations. Each vertical shaft in the Niagara
stations is driven at 250 revolutions per minute by a pair of turbines,
one above the other. The maximum head between the water in the Niagara
canal and that in the tunnel which forms the tail-race is 161 feet. On
ten shafts the centres of the wheel cases are 136 feet below the level
of water in the canal, and no draft tubes are used.
[Illustration: FIG. 29.--Section of Power-house No. 2 at Niagara Falls.]
The eleven pairs of wheels at the second Niagara power-house have their
centre line 128.25 feet below the canal level and a draft tube for each
pair of wheels extends to a point below the tail-water level. It is
entirely practicable to use more than a single pair of turbines on the
same vertical shaft, as is shown at the Hagneck station on the Jura,
in Switzerland, where the head of water is about twenty-one feet and
four turbines are mounted on each vertical shaft. The combined capacity
of these four wheels on each shaft is 1,500 horse-power and its speed is
100 revolutions per minute. At the top of each shaft an 8,000-volt
generator, with external, revolving magnet frame, is mounted. The use of
four wheels per vertical shaft presents no great difficulty and should
be resorted to more frequently in the future.
[Illustration: FIG. 30.--Interior of Power-house, Buchanan, Mich.]
For horizontal, direct-connected turbine wheels and generators the
nearly uniform practice is to locate the generators in a single row from
one end of a station to the other, and this brings the turbines into a
parallel row. On this plan the shaft of each connected generator and its
group of turbines sets at right angles to the longer sides of a station
and approximately parallel with the direction in which water flows to
the wheels. The typical water-power station with direct-connected units
is thus a rather long, narrow building into which water enters on one
side through penstocks and leaves on the other through tail-races. Such
stations usually set with one of the longer sides parallel to the river
into which the tail-water passes and between this river and the canal or
pipe line. At Massena the electric station occupies the position of a
dam between the end of the power canal and the Grass River, being about
150 feet wide and 550 feet long. Canal water entering this station
passes through its wheels to the river under a head of about 50 feet. A
similar construction was followed at Sault Ste. Marie, where the
power-station separates the end of the canal from the St. Mary’s River.
This station is 100 feet wide, 1,368 feet long, and is to contain 80
sets of horizontal wheels, each set being connected to its own
generator, and through these wheels the canal water passes under a head
of approximately 20 feet. Ten generators are placed in line at the Cañon
Ferry station which is 225 by 50 feet inside, and each generator is
driven by a pair of horizontal wheels under a head of 30 feet. This
station sets between a short canal and the Missouri River, near one end
of the dam. Passing from water-heads of less than 50 to those of several
hundred or even more than 1,000 feet, the general type of station
building remains about the same, but there is an important change in the
arrangement of direct-connected wheels and generators. With these high
heads of water, wheels of the impulse type, to which the water is
supplied in the form of jets from nozzles, are employed. These jets pass
to the wheels in planes at right angles to their shafts, instead of
flowing in lines parallel to these shafts like water to pressure
turbines. The shafts of impulse wheels and their direct-connected
generators are consequently arranged parallel with the longer instead
of the shorter sides of their stations. This plan results in long,
narrow stations with water entering at one and leaving at the other of
the longer sides, just as in the case of direct-connected turbines under
moderate heads. Stations with direct-connected impulse wheels are even
longer for a given number and capacity of units than are stations with
pressure turbines. Colgate power-house, on the North Yuba River,
contains seven generators, each direct-connected to an impulse wheel and
shafts all parallel to its longer sides. This station is 275 feet long
by 40 feet wide, and the water which enters one side by five iron pipes,
30 inches each in diameter, under a head of about 700 feet, is
discharged from the other side into the river.
[Illustration: FIG. 31.--Plan of Generating Station near Cedar Lake for
City of Seattle, Wash.]
[Illustration: FIG. 32.--Foundation of Power-station at Spier Falls.]
[Illustration: FIG. 33.--Plan of Power-station at Great Falls.]
At Electra station on the Mokelumne River five pairs of impulse wheels
are direct-connected to five generators, each unit having its shaft
diagonal with the walls of the building, and pipes deliver water to the
wheels under a head of 1,450 feet. The ground plan of the generator room
at this plant is 40 by 208 feet. The power-station on Santa Ana River,
whence energy is transmitted 83 miles to Los Angeles, measures 127 feet
long and 36 feet wide inside, and contains four generating units in
line, each of which consists of a direct-connected dynamo and impulse
wheel, with shafts parallel to the longer sides of the station. Jets
driving the wheels in this station are delivered under a head of 728
feet minus the loss by friction in a penstock 2,210 feet long.
[Illustration: FIG. 34.--Power-house at Red Bridge on Chicopee River.]
Both of the first Niagara plants, with vertical wheels far below the
stations in the pits, are long and narrow and have their generators in a
single row. The later of these two stations has a ground area of
approximately 72 by 496 feet outside, and contains eleven generators all
in line. From these examples it may be seen that the prevailing type of
electric water-power station, whether designed for horizontal or
vertical wheels of either the pressure or impulse type, is wide enough
for only a single row of generators and wheels, and has sufficient
length to accommodate the required number of units.
A few modern stations that depart from this general plan will be found,
as that at Great Falls, on the Presumpscot River, whence electrical
supply for Portland, Me., is drawn. This station sets about forty feet
in front of the forebay end of the dam, and two penstocks enter the rear
wall, while the other two enter one each through two of the remaining
opposite sides. Of the four generators, with their direct-connected
wheels, two are arranged with parallel shafts, while the other two have
their shafts in line and at right angles to the lines of the former two.
The station containing these generating sets has a floor area of 55 by
67.5 feet.
[Illustration: FIG. 35.--Plan and Elevation of Red Bridge Station on the
Chicopee River.]
Modern electric stations driven by water-power are usually but one story
in height and are clear inside from floor to roof, save for cranes and
roof trusses. This construction may be seen in the Niagara, Spier Falls,
Cañon Ferry, Colgate, Electra, Santa Ana River and many other notable
plants. In spite of this one-story style of construction, the electric
stations reach fair elevations because of the necessity for head room to
operate cranes in placing and removing generators. At Garvin’s Falls, on
the Merrimac River, the electric station contains generators of 650
kilowatts each and the distance from floor to the lower cords of roof
trusses is 27 feet. In the station at Red Bridge, on the Chicopee River,
where generators are of 1,000 kilowatts capacity each, the distance
between floor and the under side of roof beams is 30.66 feet. Between
the floor and roof trusses at the Birchem Bend station, on the river
last named, the distance is 26.25 feet, but each generator is rated at
only 400 kilowatts. In the Cañon Ferry plant, with its generators of 750
kilowatts each, the distance from floor to roof trusses is 28 feet. At
the plant on Santa Ana River, the 750-kilowatt generators, being
connected to impulse-wheels, operate at 300 revolutions per minute, have
relatively small diameters and are mounted over pits in the floor so
that their shaft centres are only about two feet above it. By these
means the distance from floor to roof trusses was reduced to 18.25 feet.
All these examples of elevations between floors and roof supports are
for stations with direct-connected generators and horizontal wheels. In
the new Niagara station, where generators of 3,750 kilowatts each are
mounted on vertical wheel shafts that rise from the floor, the distance
between the floor and roof trusses is 39.5 feet.
Electric stations driven by water-power are now constructed almost
entirely of materials that will not burn--that is, stone, brick, tile,
concrete, cement, iron, and steel. Stone masonry laid with cement mortar
or concrete masonry is very generally employed for all those parts of
the foundations that come in contact with the tail-water. For
sub-foundations bedrock is very desirable, but where this cannot be
reached piles are driven closely and their tops covered with several
feet of cement concrete as a bedding for the stone foundation. Where
stone is plenty or bricks hard to obtain, the entire walls of a
water-power station are frequently laid entirely with stone in concrete
mortar. If bricks can readily be had they are more commonly used than
stone for station walls above the foundations. Concrete formed into a
monolithic mass is a favorite type of construction for the foundations,
walls and floors of water-power plants in Southern California. Cement
and concrete are much used for station floors in all parts of the
country, and these floors are supported by masonry arches in cases where
the tail-water flows underneath the station after leaving the wheels.
Station roofs are usually supported by steel trusses or I-beams, and
slate and iron are favorite roof materials. With iron roof-plates an
interior lining of wood, asbestos, or some other poor conductor of heat
is much used to prevent the condensation of water on the under side of
the roof in cold weather. Walls of water-power stations are usually
given sufficient thickness of masonry to support all loads that come
upon them without the aid of steel columns. In some cases where cranes
do not extend entirely across their stations, one end of each crane is
supported by one of the station walls and the other end by a row of
iron or steel columns rising from the floor. Where the generator-room of
a station has its floor level below high-water mark especial care should
be taken to make the walls water-proof to an elevation above this mark.
As the travelling-crane and the loads which it carries in erecting
wheels and generators form a large part of the weight on the station
walls, these walls are often reduced as much as one-half in thickness at
the level of the crane, thus forming benches on which the ends of the
cranes rest.
[Illustration: FIG. 36.--Steel Penstocks at Chamblay Power-house.]
The Garvin’s Falls station, on the Merrimac River, rests on arches of
stone masonry through which the tail-water passes, and the brick walls
are water-proofed to an elevation eight feet above the floor. At twenty
feet above the floor the twenty-four-inch brick walls on the two longer
sides are reduced to eight inches in thickness, thus forming benches
each sixteen inches wide on which the crane travels. Arches of stone
masonry support the twenty-four-inch brick walls of the station at Red
Bridge, on the Chicopee River, and these walls on the two longer sides
decrease in thickness to twelve inches at an elevation of twenty-one
feet above the floor, thus forming benches twelve inches wide for the
ends of the crane.
One concrete wall of the Santa Ana station is 2.5 feet thick to a
distance of 13.5 feet above the floor, and then shrinks to a thickness
of 1.5 feet, corresponding to that of the opposite wall, thus forming a
bench twelve inches wide for one end of the crane. The other end of the
crane in this case is supported by an I-beam on a row of iron columns.
It is not uncommon to locate horizontal turbines in a room separate from
that occupied by the generators to which they are direct-connected, in
order to protect the latter from water in the event of a break in
penstocks or wheel cases. In cases of this sort the shafts connecting
wheels and generators pass through the wall between them. The horizontal
turbines may be located at the bottom of a canal whose water presses
against the wall through which the wheel shafts pass, or they may be
contained in iron cases at the ends of penstocks. In this latter case an
extension of the station is often provided for a wheel room to contain
these cases. Such wheel rooms are long, narrow, low-roofed and parallel
to the generator rooms of their stations. The floors of these wheel
rooms are at nearly the same levels as the floors of generator rooms,
but elevations of their roofs above the floors are much less than like
elevations in the main parts of the stations. The Garvin’s Falls, Red
Bridge, and Apple River stations have wheel rooms of the type just
described. With impulse-wheels to which water passes in planes at right
angles to their shafts it is desirable, in order to avoid changes in the
direction of water pipes, that direct-connected wheels and generators
occupy the same room, and this is the arrangement at the Colgate,
Electra, Santa Ana, Mill Creek, and many other power-houses using such
equipments. The area of a wheel room may frequently be reduced at
stations operating direct-connected horizontal-pressure turbines under
low heads by placing the wheels at the bottom of the canal which has one
side of the station or generator room for a retaining wall. This plan
was adopted at the Birchem Bend plant with a head of fourteen feet, and
at the Sault Ste. Marie station where the head of water is about twenty
feet. Vertical wheels direct-connected to generators must be directly
underneath the main room of their station, and may be in a canal over
which the station is built, in a wheel room that forms its lower part,
or in a wheel pit and supplied with water through penstocks, as at the
Niagara Falls plants.
Step-up transformers developing very high voltages are not an element of
safety in a generator room, and the better practice is to locate them in
a separate apartment by themselves, if not in a separate building. For
the Niagara Falls plant, the transformers that deliver three-phase
current at 22,000 volts are located in a building across the canal from
the generating plant. At Cañon Ferry the transformers operating at
50,000 volts, three-phase, are located in a steel and iron addition to
the power-house. Transformers at Electra station, which are intended to
work ultimately at 60,000 volts, are located in an extension of the main
building and are separated from the generator-room by a wall. At the
Santa Ana plant the 33,000-volt transformers are grouped in one corner
of the generator room, but no partition separates their space from the
remainder of the room. In the Colgate plant the transformers, working at
40,000 volts, are spaced along one of the longer sides of the station
opposite to and only a few feet from the row of generators. One end of
the main room in the Apple River plant is devoted exclusively to the
25,000-volt transformers, and there is a distance of about twenty-seven
feet between them and the nearest generator. The highest degree of
safety for transformers at these great voltages seems to require that
they be located in a separate room where the floor, walls, and roof are
made entirely of incombustible material.
[Illustration: FIG. 37.--One of the Turbine Wheels at Spier Falls on the
Hudson River.]
Water supplied to horizontal turbine wheels under moderate heads usually
enters the station by penstocks on one side and leaves it by the
tail-race on the other, but this is not true in every case. At the
Birchem Bend plant, the canal in which the wheels are located being
between the station and the river, water never enters or passes under
the station, which has a continuous foundation. So again at the Apple
River plant the single supply pipe, twelve feet in diameter and
delivering water under a head of eighty-two feet, lies parallel with the
greater length of the station and between it and the river. Short
penstocks pass from this supply pipe into the wheel section of the
power-house, and the water after passing through the wheels flows out to
the river between the masonry piers that support the twelve-foot pipe.
The generator section of this station has thus no water flowing under
it. An interesting distinction may be noted between the conditions as to
the tail-water about the foundations of stations working under low and
those under great water heads. In cases of the former sort the volumes
of water are relatively great and the foundations of stations are
usually submerged, and much reduced in area to make room for the
tail-races. Thus, the foundations of the station at Red Bridge, where
there is 49 feet head, have nearly all of their footings under water,
and of a total length of 145 feet at the top of these foundations the
six tail-races underneath cut out 92 feet. These tail-races extend
underneath both the wheel and generator rooms.
Where power is derived from water delivered under great head from pipe
nozzles to impulse-wheels, stations are usually well above the water
levels of streams into which they discharge, and passages for tail-water
underneath the station shrink to small tunnels through their
foundations. Seven of these tunnels have a total width of less than 25
feet at the Santa Ana River station, which is 127 feet long, and where
the head of water is 728 feet. At the Colgate plant, with its head of
700 feet, the water, at times of light load, instead of flowing out of
its passages underneath the station, shoots from the pipe nozzles clear
across the North Yuba River on the bank of which the station stands.
[Illustration: FIG. 38.--Power Plant of Ludlow Manufacturing Company.]
In a comparison of floor areas per kilowatt of main generator capacities
in electric stations using water- and those using steam-power, the
matter of space for transformers may be entirely omitted, because the
extent of this space is independent of the type or location of
water-wheels, or the difference of water and steam as motive powers.
Where water-wheels and their connected generators occupy separate rooms,
as is often the case with turbines under low pressures, the wheel room
has a little less length, and is generally narrower than the generator
room. Thus, at the Red Bridge station the generator room is 141 feet
long and the wheel room about 127 feet, while the former is 33.33 feet
and the latter 24 feet wide. So again at Apple River Falls the generator
room is 140 by 30 feet and the wheel room 106 by 22 feet, the generator
room in this case containing also transformers. It follows that if
wheels can be located outside of the station, as in a canal, quite a
reduction in its total floor area can be made, which may easily range
from 20 to 40 per cent. The kilowatt capacity per square foot of floor
area in both wheel and generator rooms combined tends to increase with
the individual capacity of the generating units. Generators on vertical
shafts seem to require about as much floor space per unit of capacity as
do generators on horizontal shafts. In the Red Bridge station the total
capacity is 4,800 kilowatts of main generators in six horizontal units,
and the area of the generator room alone is 0.96 square foot per
kilowatt of this capacity. The second station with vertical units at
Niagara Falls has a capacity of 41,250 kilowatts in eleven generators on
vertical shafts, and its floor area amounts to 0.86 square foot per
kilowatt; narrow impulse-wheels of large diameter tend to economy of
floor space, as in Electra station, where the room containing wheels and
generators has an area of only 0.83 square foot per unit of its 10,000
kilowatts capacity. At the Colgate plant, where the total rating of
generators is 11,250 kilowatts, the floor area under wheels and
generators is almost exactly one square foot per kilowatt. The Santa Ana
station, with a total capacity of 3,000 kilowatts, has 1.52 square feet
of floor area for each unit of capacity. This last figure may be
compared with the 1.72 square feet per kilowatt of generator rating for
the 4,800-kilowatt station at Red Bridge and the 1.75 square feet per
unit of capacity in the 800-kilowatt plant at Birchem Bend.
[Illustration: FIG. 39.--Power-house on Payette River, Idaho.]
All types of water-power stations with direct-connected wheels and
generators have much smaller floor areas per unit capacity than do
steam-power stations with direct-connected horizontal units. Thus, the
modern steam-driven station at Portsmouth, N. H., has a plan area in
engine- and boiler-rooms of 16,871 square feet, and its total capacity
in four direct-connected units is 4,400 kilowatts, so that the area
amounts to 3.82 square feet per kilowatt rating of its generators. Of
this area about 46 per cent is in the boiler-room.
FLOOR DIMENSIONS FOR DIRECT-CONNECTED, HORIZONTAL WATER-WHEELS AND
GENERATORS AT ELECTRIC STATIONS.
+-----------------+----------+------------+-----------+---------+
| | | | Number | Total |
| Station. |Feet Long.| Feet Wide. | of |Kilowatt |
| | | |Generators.|Capacity.|
+-----------------+----------+------------+-----------+---------+
|[A]Niagara, No. 2| 496 | 72 | 11 | 41,250 |
|Sault Ste. Marie | 1,368 | 100 | 80 | 32,000 |
|Colgate | 275 | 40 | 7 | 11,250 |
|Electra | 208 | 40 | 5 | 10,000 |
|Cañon Ferry | 225 | 50 | 10 | 7,500 |
|Red Bridge | 141 | 57 | 6 | 4,800 |
|Apple River | { 140 | 30 } | 4 | 3,000 |
| | { 106 | 22 } | | |
|Santa Ana River | 127 | 36 | 4 | 3,000 |
|Great Falls | 67.5 | 55 | 4 | 2,000 |
|Garvin’s Falls | { 62 | 30 } | 2 | 1,300 |
| | { 50 | 23 } | | |
|Birchem Bend | 56.6 | 26.7 | 2 | 800 |
|Portsmouth |{ 14.4 | 119.66 }| | |
| (steam-driven)|{inside, but minus 360}| 5 | 4,400 |
| |{ square feet. }| | |
+-----------------+----------+------------+-----------+---------+
[A] Vertical wheel shafts.
Some of these dimensions apply to the inside and some to the outside
of stations. Some small projections are not included.
CHAPTER IX.
ALTERNATORS FOR ELECTRICAL TRANSMISSION.
Dynamos in the generating station of an electric transmission system
should be so numerous that if one of them is disabled the others can
carry the maximum load. If only two generators are installed, it is thus
desirable that each be large enough to supply the entire output, so that
the dynamo capacity exceeds the greatest demand on the station by 100
per cent. To avoid so great excess of dynamo capacity it is common
practice to install more than two generators.
Other considerations also tend to increase the number of dynamos in the
generating station of a transmission system. Thus one transmission line
may be devoted exclusively to lighting, another to stationary motors,
and a third to electric railway service; and it may be desirable that
each line be supplied by an independent dynamo to avoid any effect of
fluctuations of railway or motor load on the lighting system.
At the generating station of the transmission system that supplies
electric light and power in Portland, Me., the idea of independent units
has been carried out with four 500-kilowatt dynamos, each driven by a
pair of wheels fed with water through a separate penstock from the dam.
Each of these dynamos operates one of the four independent transmission
circuits. Where a number of water-power stations feed into a single
sub-station the requirement that each generating station have its
capacity divided up among quite a number of dynamos may not exist, since
one station may be entirely shut down for repairs and the load carried
meantime by the other stations. A good illustration of this point may be
seen at Manchester, where a single sub-station receives energy
transmitted from four water-power plants. At one of these plants the
entire capacity of 1,200 kilowatts is in a single generator.
The foregoing considerations as to the number of dynamos apply with
equal force to both steam- and water-driven stations, but other factors
tend to increase the number of dynamos in water-power plants where the
head of water is comparatively small. This tendency is due to the fact
that the peripheral speeds of pressure turbine water-wheels should be
about twenty-five per cent less than the velocity at which water would
issue from an opening under the head of water at which these wheels
operate in order to secure high efficiency. This velocity of water and
therefore the peripheral speed of pressure turbine wheels varies with
the square root of the head of water.
[Illustration: FIG. 40.--Generators at Sault Ste. Marie Power Plant.]
Since the peripheral speed of turbines is thus determined by the heads
of water under which they operate, and since the diameters of turbines
must increase with their capacities, the rate of revolution for pressure
turbines under any given head decreases as the power goes up. For this
reason it is often desirable to use a larger number of dynamos in a
water-power plant than would otherwise be required in order to avoid
very low speeds of revolution on the direct-connection to the turbines.
A notable illustration of this practice exists in the great water-power
plant of the Michigan-Lake Superior Power Company, at Sault Ste. Marie,
Mich., where a generating capacity of 32,000 kilowatts is divided up
between 80 dynamos of 400 kilowatts each. The head of water available at
the pressure turbines in this plant is about 16 feet, and their speed is
180 revolutions per minute. In order to obtain even this moderate speed
under the head of 16 feet it was necessary to select turbines of only
140 horse-power each. Four of these turbines are mounted on each shaft
that drives a 400-kilowatt dynamo, direct-connected, so that there are
320 wheels in all. Had a smaller number of wheels been employed to yield
the total power their speed and that of direct-connected dynamos must
have been less than 180 revolutions per minute. As the cost of dynamos
increases with very low speeds it is often cheaper to install a larger
number of dynamos at a higher speed than a smaller number at a lower
speed for a given total capacity.
The use of a larger number of units than would otherwise be necessary in
order to avoid a very low speed is further illustrated by the
7,500-kilowatt plant of the Missouri River Power Company, at Cañon
Ferry, Mont. This capacity is made up of ten generators, each rated at
750 kilowatts and direct-connected to a pair of pressure turbine wheels
operating at 157 revolutions per minute, under a head of about 32 feet.
Under comparatively high heads of water pressure turbines operate at
speeds that are ample for direct-connection to even the largest dynamos.
[Illustration: FIG. 41.--Interior of Power-house No. 2, Niagara Falls.]
Thus in the Niagara Falls plant, where the head of water is 136 feet,
each pair of turbines drives a direct-connected dynamo of 3,750
kilowatts at 250 revolutions per minute. In the rare case where the
power to be developed is so great that the number of generators
necessary to give security and reliability to the service leaves each
generator with a capacity larger than is desirable for structural
reasons, the number must be increased simply to reduce the size of each
generator. Such a state of facts existed at Niagara Falls, where the
first station contains ten dynamos of 3,750 kilowatts each, and the
second station contains eleven units of like capacity.
In the greater number of transmission systems the generators are
direct-connected to either steam-engines or water-wheels, and their
speeds of rotation are largely determined by the requirements of these
prime movers. Steam-engines can be designed with some regard to the
desirable speeds for direct-connection to dynamos, but water-wheels are
less flexible in this particular. Each type of wheel has its peripheral
speed mainly determined by the head of water under which it may be
required to operate, and variation from this speed means serious loss of
efficiency.
[Illustration: FIG. 42.--10,000 H. P. 12,000 Volt Generator in Canadian
Power-house at Niagara Falls.]
Under heads of much more than 100 feet pressure turbines operate at
rather high speeds in all except very large sizes. It is much the more
common to see water-wheels at a lower speed belted to dynamos at a
higher speed; but in some instances, as at the lighting plant of
Spokane, Wash., wheels of a higher speed are belted to dynamos of a
lower speed. Another plan by which moderate dynamo speeds are obtained
with water-wheels under rather high heads mounts a dynamo at each end of
the shaft of a large turbine or pair of turbines. This plan is followed
at the plant of the Royal Aluminum Company, Shawinigan Falls, Quebec,
where there are two pairs of horizontal turbine wheels, each pair
developing 3,200 horse-power under a head of 125 feet, and driving a
dynamo direct-coupled on each end of its shaft. Where vertical wheels
are employed it is sometimes more desirable to drive some standard type
of dynamo with horizontal shaft by means of bevel gears than to design a
special dynamo to mount directly on the vertical shaft. This latter plan
is warranted in very large work like that at two of the Niagara Falls
generating stations, where the twenty-one 3,750-kilowatt dynamos are
direct-connected, each on the vertical shaft of a turbine. This type of
connection is not one that will be frequently followed, but at one other
point--Portland, Ore.--each dynamo is mounted directly on the shaft of
its vertical turbine wheel.
Where water-wheels must operate under heads of several hundred feet, it
is usually necessary to abandon pressure turbines and to adopt one of
the types of impulse-wheels. In this class of wheels the peripheral
speed of highest efficiency is only one-half the spouting velocity of
the water under any particular head. This gives the impulse-wheels about
two-thirds the peripheral speed of pressure turbines of equal diameter
and consequently about two-thirds as many revolutions per minute. But as
the water may be applied at one or more points on the circumference of
an impulse-wheel, as desired, such wheels may have much greater
diameters than pressure turbines for equal power under a given head.
[Illustration: FIG. 43.--Generators in Power-station at Mechanicsville
on the Hudson River.]
[Illustration: FIG. 44.--Generators at Chamblay, Quebec, Power-house.]
These properties of low peripheral speed, as to head and great diameter,
as to power developed, fit impulse-wheels for direct-connection to
dynamos where great heads of water must be employed, and they are
generally used in such cases. This is particularly true for the Pacific
coast, where water-powers depend more on great heads than on large
volumes. In the generating plant of the Bay Counties’ Power Company, at
Colgate, Cal., the dynamos are direct-connected to impulse-wheels that
operate under a head of 700 feet. The three 2,250-kilowatt dynamos in
this plant are each mounted on a wheel shaft operating at 285
revolutions per minute, and each of the four 1,125-kilowatt dynamos is
direct-driven by an impulse-wheel at 400 revolutions per minute. At the
Electra, Cal., plant of the Standard Electric Company the impulse-wheels
operate at 240 revolutions per minute under a head of 1,450 feet. Each
of the five pairs of these wheels drives a 2,000-kilowatt generator,
direct-connected. As the head of water at these wheels is 1,450 feet,
its spouting velocity is about 300 feet per second, or 18,000 feet per
minute. Each wheel is eleven feet in diameter, so that a speed of 240
revolutions per minute gives the periphery a little less than 9,000 feet
per minute, or about one-half of the spouting velocity of the water.
These two great plants are excellent illustrations of the way in which
impulse-wheels, under great heads, may be given speeds that are suitable
for direct-connected dynamos.
[Illustration: FIG. 51_a_. Plan and Elevation of Water Wheels and
Generators at Power Station on Burrard Inlet, near Vancouver, B. C.]
Three types of alternators, the revolving armature, the revolving
magnet, and the inductor, are used in the generating plants of electric
transmission systems.
Revolving armatures are used in the dynamos of comparatively few
transmission systems and hardly at all in those of recent date. The
prevailing type of alternator for transmission work is that with
internal revolving magnets and external stationary armature. This type
is employed in the great water-power plants at Cañon Ferry, Mont.; Sault
Ste. Marie, Mich., and for all of the generators installed in the later
Niagara Falls plants. For the sixteen earlier vertical generators at
Niagara Falls the revolving magnets are external to the stationary
armatures, but this construction has the disadvantage of high first cost
and inaccessibility of the internal armature, and is not likely to be
often adopted elsewhere.
[Illustration: FIG. 46.--Elevations of Water-wheels and Generators at
Power-station on Burrard Inlet, near Vancouver, B. C.]
Inductor alternators are those in which both the armature and magnet
coils are stationary and only a suitable structure of iron revolves;
they are employed in a comparatively small number of transmission
systems, but this number includes some of the largest plants. The seven
alternators in the Colgate, Cal., plant aggregating 11,250 kilowatts
capacity, and the five alternators in the plant at Electra in the same
State, with a capacity of 10,000 kilowatts, are all of the inductor
type. As more commonly constructed the magnet winding of the inductor
alternator consists of only one or two very large coils, which are in
some cases as much as ten feet in diameter. The repair of these large
magnet coils seems to present a more serious problem, in case of
accident, than the repair of the small coils used on interval, revolving
magnets. As far as satisfactory operating qualities are concerned,
inductor alternators and those with revolving magnets seem to be on an
equality, but for structural reasons inductor alternators will probably
be built less freely in the future than in the past.
Nearly all long transmissions are now carried out with either two- or
three-phase current. The most notable two-phase installation is that at
Niagara Falls, where the original ten generators, as well as the eleven
dynamos later added in two of the large plants, are all of the two-phase
type. At Cañon Ferry, Mont., the first four of the 750-kilowatt
generators were two-phase, but the six machines of like capacity
installed later are three-phase. In the latest plants of large capacity
or involving very long transmissions three-phase machines have been
generally employed. This is true of the Colgate and Electra plants in
California, and of that at Sault Ste. Marie, Mich.
[Illustration: FIG. 47.--Interior of Power-house at Garvin’s Falls on
the Merrimac River.]
[Illustration: FIG. 48.--500-Kilowatt Generator in Station at Great
Falls on the Presumpscot River.]
As to frequency, existing practice extends all the way from 133 cycles
per second on the lines at Marysville, Cal., down to only 15 cycles on
the transmission for the Washington & Baltimore Electric Railway.
More common practice ranges between 25 and 60 cycles. Niagara Falls saw
the first great plant installed for 25 cycles, but others of that
frequency are now engaged in the supply of light and power for general
distribution. For transmission to electric railway lines a frequency of
25 cycles has been and is being widely used, prominent examples of which
may be seen in the New Hampshire traction, the Berkshire, and the Albany
& Hudson systems.
[Illustration: FIG. 49.--Columbus, Ga., Water-power Station.]
The strong feature of a system at 25 cycles is that it is well suited to
the supply of continuous currents through rotary converters with
reasonable numbers of poles, armature slots, and commutator bars.
[Illustration: FIG. 50.--1065-Kilowatt, 2300 volt Generator Connected to
Motor in Shawinigan Sub-station at Montreal.]
On the other hand, the cost of transformers is greater with current at
25 cycles per second than with a higher frequency, and this current is
only just bearable for incandescent lighting and quite unsuited for arc
lamps, because of the fluctuating character of the light produced. At 15
cycles per second a current can be employed for incandescent lighting
with satisfactory results only by means of some special devices, as
lamps with very thick filaments, to avoid the flicker. Very low
fluctuations cut down undesirable effects in the way of inductance and
resonance, but these effects can be avoided to a large degree in other
ways.
Where power is the most important element in the service of an electric
water-power and transmission system there is a decided tendency to adopt
a rather small number of periods for the system, even at some
disadvantage as to lighting facilities. This is illustrated by the
transmission from St. Anthony’s Falls, Minn., at 35 cycles, from Cañon
City to Cripple Creek, Col., at 30 cycles, by the Sault Ste. Marie plant
of 32,000 kilowatts at 30 cycles, as well as by the two Niagara Falls
plants of 78,750 kilowatts at 25 cycles.
Where the main purpose of a transmission system is the supply of light
and power for general distribution, sixty periods per second are adopted
as the standard in many cases. This number of periods in comparison with
a smaller one tends to increase the cost of rotary converters but
decreases the cost of transformers, and is suitable for both
incandescent and arc lighting.
[Illustration: FIG. 51.--Efficiency Curves for Motor Generators at
Montreal Sub-station of the Shawinigan Transmission Line.]
Few, if any, transmission systems have recently been installed for
frequencies above sixty cycles, and the older plants that worked at
higher figures have in most cases been remodelled.
During the past decade the voltages of alternators have been greatly
increased, but have not caught up with the demand for high pressures on
long-transmission lines. Ten years ago when the first long transmissions
were going into operation 2,000 volts was considered high for an
alternator. As this voltage is too low for economy of conductors longer
than three or four miles, the important early transmissions were all
carried out with the aid of step-up transformers at generating stations.
The practice then was, and to a large extent still is, to design the
alternators for a transmission with a voltage well suited to their
economical construction, and then give the step-up transformers any
ratio necessary to attain the required line voltage.
ALTERNATORS IN TRANSMISSION SYSTEMS.
+----------------------+----------------------------------------------+
| |Number at Plant. |
| | |Kilowatts Each. |
| | | |Alternator Voltage. |
| | | | |Phase. |
| | | | | |Cycles. |
| | | | | | |R. P. M. |
| | | | | | | |Type of |
|Location of System. | | | | | | |Magnet. |Method |
| | | | | | | | |of |
| | | | | | | | |Connec-|
| | | | | | | | |tions. |
+----------------------+--+-----+------+-+----+-----+---------+-------+
|Niagara Falls[A] |16|3,750| 2,300|2|25 |250 |External |Direct |
| | | | | | | |revolving| |
|Niagara Falls[A] | 5|3,750| 2,300|2|25 |250 |Internal | „ |
|Colgate to Oakland | 3|2,250| 2,400|3|60 |285 |Inductor | „ |
|Colgate to Oakland | 4|1,125| 2,400|3|60 |400 | „ | „ |
|Electra to S.Francisco| 5|2,000| ....|3|60 |240 | „ | „ |
|Portsmouth to Pelh’m | 1|2,000|13,200|3|25 | 83.3|Internal | „ |
|Portsmouth to Pelh’m | 2|1,000|13,200|3|25 | 94 | „ | „ |
|Virginia City | 2| 750| 500|3|60 |400 |External | „ |
|Ogden & Salt Lake | 5| 750| 2,300|3|60 |300 |Internal | „ |
|Chaudière Falls | 2| 750|10,500|3|66.6|400 | „ | „ |
|Yadkin River Falls | 2| 750|12,000|3|66 |166 | „ | „ |
|Lewiston, Me. | 3| 750|10,000|3|60 |180 | „ | „ |
|Farmington River }| | | | | | | | |
| to }| 2| 750| 500|3|60 |... | „ | „ |
|Hartford, Conn. }| 2| 600| 500|2|60 |... | „ | „ |
|Cañon Ferry to Butte |10| 750| 500|3|60 |157 | „ | „ |
|Apple Riv. to St. Paul| 4| 750| 800|3|60 |300 | „ | „ |
|Edison Co., L. Angeles| 4| 700| 750|3|50 |... | „ | „ |
|Madrid to Bland | 2| 600| 605|3|60 | 90 | „ | „ |
|Cañon City to Cripple | | | | | | | | |
|Creek | 3| 450| 500|3|30 |... | ..... | „ |
|Sault Ste. Marie |80| 400| 2,400|3|30 |180 | „ | „ |
|St. Hyacinthe, Que. | 3| 180| 2,500|3|60 |600 | „ | „ |
|Great Falls to | | | | | | | | |
|Portland, Me. | 4| 500|10,000|3|60 |225 | „ | „ |
+----------------------+--+-----+------+-+----+-----+---------+-------+
[A] Niagara Falls Power Company.
Thus in the two water-power plants connected with the electrical supply
system of Hartford, Conn., the alternators operate at 500 volts with
transformers that put the line voltage up to 10,000. In the station on
Apple River that supplies the lighting system of St. Paul, Minn., the
alternators operate at 800 volts, and this is raised to 25,000 volts for
the line. At Cañon Ferry the alternator voltage of 500 is multiplied by
100 in the transformers giving 50,000 on the line.
[Illustration: FIG. 52.--Transmission Line of New Hampshire Traction
Company.]
Where the generating station of a transmission system is located close
to a part of its load the alternators are given a voltage suitable for
distribution, say about 2,400, and any desired pressure on the line is
then obtained by means of step-up transformers. Two of the Niagara
Falls plants are an illustration of this practice, the voltage of all
the alternators there being 2,200, which is raised to 22,000 for the
transmission of a part of the energy to Buffalo. A similar practice is
followed in the water-power plant at Ogden, where the generators furnish
current at 2,300 volts for local distribution, and transformers raise
the pressure to 26,000 volts for the transmission to Salt Lake City. In
the 32,000-kilowatt plant at Sault Ste. Marie, Mich., the alternators
operate at 2,400 volts and a large part of their load is local, but this
voltage will no doubt be raised by transformers when transmission lines
are operated.
For generating stations that carry little or no local loads the cost of
transformers can be saved if the generators develop the voltage required
on the transmission lines. This possible saving has led to the
development of alternators that generate voltages as high as 15,000 in
their armature coils. Such alternators have stationary armatures in all
cases and are of either the revolving magnet or inductor type.
At the present time many transmission systems in the United States
operating at 10,000 or more volts develop these pressures in the
armature coils of their alternators, and the number of such systems is
rapidly increasing. It is now the rule rather than the exception to
dispense with step-up transformers on new work where the line voltage is
anything under 15,000. Perhaps the longest transmission line now in
regular operation with current from the armature coils of an alternator
is that at 13,200 volts between the generating station at Portsmouth and
one of the sub-stations of the New Hampshire Traction system at Pelham,
a distance of forty-two miles.
In at least one transmission system now under construction, that of the
Washington, Baltimore & Annapolis Electric Railway, the voltage of
generators to supply the line without the intervention of step-up
transformers will be 15,000.
The company making these alternators is said to be ready to supply
others that generate 20,000 volts in the armature coils whenever the
demand for them is made. In quite a number of cases alternators of about
13,000 volts have been installed for transmissions along electric
railway lines.
Alternator
Systems Using High-voltage Alternators. Voltages.
Electrical Development Co. of Ontario, Niagara Falls 12,000
Lighting and Street Railway, Manchester, N. H. 10,000
Lighting and Street Railway, Manchester, N. H. 12,500
Lighting and Power, Portland, Me. 10,000
Lighting and Power, North Gorham, Me. 10,000
Mallison Power Co., Westbrook, Me. 10,000
Lighting and Power, Lewiston, Me. 10,000
Electric Railway, Portsmouth, N. H. 13,200
Electric Railway, Pittsfield, Mass. 12,500
Ludlow Mills, Ludlow, Mass. 13,200
Electric Railway, Boston to Worcester, Mass. 13,200
Electric Railway, Albany & Hudson, N. Y. 12,000
Empire State Power Co., Amsterdam, N. Y. 12,000
Lehigh Power Co., Easton, Pa. 12,000
Hudson River Power Co., Mechanicsville, N. Y. 12,000
Light and Power, Anderson, S. C. 11,000
Fries Mfg. Co., Salem, N. C. 12,000
Light and Power, Ouray, Col. 12,000
Washington & Baltimore Electric Railway 15,000
Canadian Niagara Power Co., Niagara Falls 12,000
Ontario Power Co., Niagara Falls 12,000
This list of high-voltage alternators is not intended to be exhaustive,
but serves to indicate their wide application. If such alternators can
be purchased at a lower price per unit of capacity than alternators of
low voltage plus step-up transformers, there is an apparent advantage
for transmission systems in the high-voltage machines. This advantage
may rest in part on a higher efficiency in the alternators that yield
the line voltage than in the combination of low-voltage alternators plus
step-up transformers. It is not certain, however, that depreciation and
repairs on the generators of high voltage will not be materially greater
than the like charges on generators of low voltage, and some advantage
in price should be required to cover this contingency.
Just how far up the voltage of alternators can be pushed for practical
purposes is uncertain, but it seems that the limit must be much below
that for transformers where there is ample room for solid insulation and
the coils can be immersed in oil. The use of generators at 10,000 volts
and above tends to lower the volts per mile on transmission lines,
because it seems better in some cases to increase the weight of line
conductors rather than to add step-up transformers, as in the 42-mile
transmission from Portsmouth to Pelham.
CHAPTER X.
TRANSFORMERS IN TRANSMISSION SYSTEMS.
Transformers are almost always necessary in long electric systems of
transmission, because the line voltage is greater than that of
generators, or at least that of distribution. As transformers at either
generating or receiving stations represent an increase of investment
without corresponding increase of working capacity, and also an
additional loss in operation, it is desirable to avoid their use as far
as is practicable. In short transmissions over distances of less than
fifteen miles it is generally better to avoid the use of transformers at
generating stations, and in some of these cases, where the transmission
is only two or three miles, it is even more economical to omit
transformers at the sub-stations.
Thus, where energy is to be transmitted two miles and then applied to
large motors in a factory, or distributed at 2,500 volts, the cost of
bare copper conductors for the three-phase transmission line will be
only about $6 per kilowatt of line capacity at 2,500 volts, with copper
at 15 cents per pound, and a loss of 5 per cent at full load. The
average loss in such a line will probably be as small as that in one set
of transformers and a line of higher voltage. Furthermore, the first
cost of the 2,500-volt generators and line without transformers will be
less than that of generators and line of higher voltage with step-down
transformers at the sub-station.
As generators up to 13,500 volts are now regularly manufactured, it is
quite common to omit step-up transformers at the main stations of rather
short transmission systems. This practice was followed in the
13,500-volt transmission to Manchester, N. H., the 10,000-volt
transmission to Lewiston, Me., and the 12,000-volt transmission to
Salem, N. C.
In most transmission over distances of twenty-five miles or more,
step-up transformers at generating stations as well as step-down
transformers at sub-stations are employed. As yet the highest voltages
that have been put into practical use on transmission lines (that is,
50,000 to 60,000) are much below the pressures that have been yielded by
transformers in experimental work. These latter voltages have in a
number of instances gone above 100,000. The numbers and capacities of
transformers used at main stations vary much in their relation to the
numbers and individual capacities of generators there. In some cases
there are three times as many transformers as three-phase generators,
and the capacity of each transformer is either equal to or somewhat
greater than one-third of the capacity of each generator.
[Illustration: FIG. 53.--Transformers at Central Sub-station, Montreal.]
Thus in the station at Spier Falls on the Hudson, whence power is
transmitted to Albany and other cities, the number of step-up
transformers will be thirty and their aggregate capacity will be 24,014
kilowatts, while the total number of three-phase generators will be ten,
with a combined capacity of 24,000 kilowatts. Another practice is to
give each transformer a capacity greater than one-third of that of the
three-phase generator with which it is to be connected, and make the
total number of transformers less than three times as great as the
number of generators. An example of this sort exists in the station on
Apple River, whence power is transmitted to St. Paul. This station
contains four three-phase generators of 750 kilowatts each, and six
transformers of 500 kilowatts each, these latter being connected in two
sets of three each. The use of three transformers for each three-phase
generator instead of three transformers for each two or three
generators, tends to keep transformers fully loaded when in use, and
therefore to increase their efficiency. On the other hand, efficiency
increases a little with the size of transformers, and the first cost per
unit capacity is apt to be less the greater the size of each.
Another solution of the problem is to provide one transformer for each
three-phase generator, each transformer being wound with three sets of
coils, so that the entire output of a generator can be sent into it.
This practice is followed at the Hochfelden water-power station, whence
power is transmitted to Oerlikon, Switzerland, also in the water-power
station at Grenoble, France, whence energy at 26,000 volts is
transmitted to a number of factories. With three-phase transformers each
generator and its transformer may form an independent unit that can be
connected with the line at pleasure, thus tending to keep transformers
at full load.
Though three-phase transformers are much used in Europe, they have thus
far had little application in the United States. Single-phase
transformers may, of course, be limited in number to that of the
three-phase generators with which they are used, but such transformers
must regularly be connected to the generators and line in groups of two
or three. Such an equipment was provided in part at the 7,500-kilowatt
station on the Missouri River at Cañon Ferry, which contains ten
three-phase generators of 750 kilowatts each. The transformers at this
station include twelve of 325 kilowatts each, connected in four groups
of three each, also six transformers of 950 kilowatts each which are
also connected in groups of three. Three of these larger transformers
have a capacity of 2,850 kilowatts, or nearly equal to that of four
generators.
With two-phase generators single-phase transformers must be connected in
pairs, and it is common to provide two transformers for each generator.
Thus, in the Rainbow station on the Farmington River, whence energy is
transmitted to Hartford, there are two generators of the two-phase type
and rated at 600 kilowatts each, also four transformers rated at 300
kilowatts each.
As the regulation of transformers on overloads is not as good as that of
generators, it seems good practice to give each group of transformers a
somewhat greater capacity than that of the generator or generators whose
energy is to pass through it. This plan was apparently followed at the
Cañon Ferry station, where the total generator capacity is 7,500
kilowatts and the total capacity of step-up transformers is 9,600
kilowatts. Each group of the 325-kilowatt transformers there has a
capacity of 975 kilowatts, while each generator is only of 750
kilowatts. Usually the number of groups of transformers at a two-phase
or three-phase generating station is made greater than the number of
transmission circuits supplied by the station, for some of the reasons
just considered. When this is not the case it is commonly desirable in
any event to have as many groups of step-up transformers as there are
transmission circuits, so that each circuit may be operated with
transformers that are independent of the other circuits.
At sub-stations it is desirable to have a group of transformers for each
transmission circuit, and it may be necessary to subdivide the
transformer capacity still further in order to keep transformers in
operation at nearly full load, or to provide a group of transformers for
each sort of service or for each distribution circuit. All of the
transformers at a sub-station should have a total capacity at least
equal to that of the generators whose energy they are to receive, minus
the losses in step-up transformers and the line. Transformers at
sub-stations do not necessarily correspond in number or individual
capacity with those at generating stations, and the number of
sub-station transformers bears no necessary relation to the number of
generators by which they are fed.
Two transmission circuits extend from Cañon Ferry to a sub-station at
Butte, and in that sub-station there are six transformers divided into
two groups for three-phase operation, each transformer being rated at
950 kilowatts. This sub-station equipment thus corresponds to only the
six 950-kilowatt transformers in the generating station, because the
four groups of smaller transformers there are used to supply the
transmission line to Helena.
In the sub-station at St. Paul that receives the entire output of the
plant on Apple River, where the six transformers of 500 kilowatts each
are located, ten transformers receive energy from two three-phase
transmission circuits. Six of these transformers are rated at 300
kilowatts each. The 300-kilowatt transformers are connected in two
groups of three each, and the 200-kilowatt in two groups of two each,
transforming current from three-phase to two-phase. The aggregate
capacity of the sub-station transformers is thus 2,600 kilowatts, while
that of transformers at the generating station is 3,000 kilowatts. With
four generators at the water-power plant there are ten transformers at
the sub-station, where all the energy, minus losses, is delivered.
At Watervliet, where one of the several sub-stations of the system with
its larger generating plant at Spier Falls is located, the capacity of
each transformer is 1,000 kilowatts, though each transformer at Spier
Falls has a rating below this figure.
In the sub-station at Manchester, N. H., that receives nearly all of the
energy from four water-power plants, containing eight generators with an
aggregate capacity of 4,030 kilowatts, there are located twenty-one
step-down transformers that have a total rating of 4,200 kilowatts.
These twenty-one transformers are fed by six circuits, of which five are
three-phase and one is two-phase. A part of the transformers supply
current to motor-generators, developing 500-volt current for a street
railway, and the remaining transformers feed circuits that distribute
alternating current.
From these examples it may be seen that in practice either one or more
groups of transformers are employed in sub-stations for each
transmission circuit, that the total number of these transformers may be
just equal to or several times that of the generators from which they
receive energy, and that the individual capacities of the transformers
range from less than one-third to more than that of a single generator.
Groups of transformers at a main station must correspond in voltage with
that of the generators in the primary and that of the transmission line
in the secondary windings. Sub-station transformers receive current at
the line voltage and deliver it at any of the pressures desired for
local distribution. Where step-up transformers are employed the
generator pressure in nearly all cases is at some point between 500 and
2,500 volts.
At the Cañon Ferry station the voltage of transformers is 550 in in the
primary and 50,000 in the secondary windings. In the Colgate
power-house, whence energy is transmitted to Oakland, the generator
pressure of 2,400 volts is raised to 40,000 volts by transformers.
Generator voltage in the power-house on Apple River is 800 and
transformers put the pressure up to 25,000 for the line to St. Paul.
Transformers at the Niagara Falls station raise the voltage from 2,200
to 22,000 for the transmission to Buffalo.
As transformers can be wound for any desired ratio of voltages in their
primary and secondary coils, a generator pressure that will allow the
most economical construction can be selected where step-up transformers
are employed. In general it may be said that the greater the capacity of
each generator, the higher should be its voltage and that of the primary
coils of step-up transformers, for economical construction. At
sub-stations the requirements of distribution must obviously fix the
secondary voltages of transformers.
Weight and cost of transformers depend in part on the frequency of the
alternating current employed, transformers being lighter and cheaper the
higher the number of cycles completed per second by their current, other
factors remaining constant. In spite of this fact the tendency during
some years has been toward lower frequencies, because the lower
frequencies present marked advantages as to inductive effects in
transmission systems, the distribution of power through induction
motors, the construction and operation of rotary converters, and the
construction of generators. Instead of the 133 cycles per second that
were common in alternating systems when long transmissions first became
important, sixty cycles per second is now the most general rate of
current changes in such transmission systems. But practice is constantly
extending to still lower frequencies. The first Niagara Falls plant with
its twenty-five cycles per second reached the lower limit for general
distribution, because incandescent lighting is barely satisfactory and
arc lighting decidedly undesirable at this figure.
In contrast with the great transmissions from Cañon Ferry to Butte,
Colgate to Oakland, and Electra to San Francisco, which operate at sixty
cycles, the system between Cañon City and Cripple Creek, in Colorado, as
well as the great plant at Sault Ste. Marie, employs thirty-cycle
current, and the lines from Spier Falls to Schenectady, Albany, and Troy
are intended for current at forty cycles per second. From these examples
it may be seen that the bulk and cost of transformers is not the
controlling factor in the selection of current frequency in a
transmission system.
[Illustration: FIG. 54.--First Floor of Saratoga Sub-station.]
Transformers used at either generating or sub-stations are cooled by
special means in many cases.
The advantages of so-called artificial cooling are smaller weight and
first cost in transformers, and perhaps longer life for the insulation
of windings. For these advantages a small increase in the cost of
operation must be paid. Station transformers are usually cooled either
by forcing air through their cases under pressure, or else by passing
water through pipes in the oil with which the transformer cases are
filled. If cooling with air-blast is adopted, a blower, with electric
motor or some other source of power to operate it, must be provided.
Where transformers are oil-insulated and cooled with water there must be
some pressure to maintain the circulation. If free water under a
suitable head can be had for the cooling of transformers, as in most
water-power plants, the cost is very slight. Where water must be
purchased and pumped through the transformers its cost will usually be
greater than that of cooling with air-blast. One manufacturer gives the
following as approximate figures for the rate at which water at the
temperature of 15° centigrade must be forced through his transformers to
prevent a rise of more than 35° centigrade in their temperature,
probably when operating under full loads.
Transformers--Kilowatts. Gallons per minute.
150 0.5
400 .75
400 1.00
1,000 1.5
75 .37
An air-blast to cool transformers at main or sub-stations may be
provided in either of two ways. One plan is to construct an air-tight
compartment, locate the transformers over openings in its top, and
maintain a pressure in the compartment by means of blower-fans that draw
cool air from outside. Such an arrangement has been carried out at the
sub-station in Manchester, N. H. The basement underneath this
sub-station is air-tight, and in the concrete floor over it there are
twenty-seven rectangular openings, each twenty-five by thirty inches,
and intended for the location of a 200-kilowatt transformer. Aggregate
transformer capacity over these openings will thus be 5,400 kilowatts.
Pressure in this basement is maintained by drawing outside air through a
metal duct that terminates in a hood on the outside of the sub-station
about nine feet above the ground. In the roof of this sub-station there
are ample skylight openings to permit the exit of hot air that has been
forced through the transformers. In the air-tight basement are two
electric motors of ten horse-power each, connected to the blower that
maintains the pressure. It may be noted that in this case there is less
than one-horse power of motor capacity for each 200 kilowatts capacity
in transformers.
Where there are not more than six or nine transformers to be cooled, it
is common practice to provide a separate motor and blower for each group
of three transformers, and lead the air directly from each blower to its
group of transformers by a metal duct, thus avoiding the necessity for
an air-chamber. In such cases a blower giving a three-eighth-ounce air
pressure per square inch and a motor of one horse-power capacity are
generally provided for each group of three transformers rated at 100 to
150 kilowatts each. Where cooling with air-blast is adopted,
oil-insulation cannot be carried out because the air must come into
intimate contact with the transformer coils and core. Both
oil-insulation with water cooling and dry insulation with cooling by
air-blast have been widely used in transmission systems of large
capacity and high voltage.
In the Colgate plant, where the line pressure is 40,000 volts, the
700-kilowatt transformers are oil-insulated and water-cooled, and this
is also true of the 950-kilowatt transformers in the 50,000-volt
transmission between Cañon Ferry and Butte. On the other hand, the
transmission system between Spier Falls, Schenectady, and Albany,
carried out at 26,500 volts, includes transformers that range from
several hundred to 1,000 kilowatts each in capacity and are all
air-cooled. Either a water-cooled transformer or one cooled by air-blast
may be safely overloaded to some extent, if the circulation of air or
water is so increased that the overload does not cause heating beyond
the allowable temperature.
The circulation of air or water through a transformer should never be
forced to an extent that cools the transformer below the temperature of
the air in the room where it is located, as this will cause the
condensation of water on its parts.
In some cases it is desirable that means for the regulation of
transformer voltages through a range of ten per cent or more each way
from the normal be provided. This result is reached by the connection of
a number of sections at one end of the transformer winding to a terminal
board, where they may be cut in or out of action at will. Regulation is
usually desired, if at all, in a secondary winding of comparatively low
voltage, and the regulating sections generally form a part of such
winding, but these sections may be located in the primary winding.
In order to keep the number of transformers smaller and the capacity of
each larger than it would otherwise be, it is practicable to divide the
low-voltage secondary winding of each transformer into two or more
parts that have no electrical connection with each other. These
different parts of the winding may then be connected to distinct
distribution lines or other services. An example of this sort exists in
the Hooksett sub-station of the Manchester, N. H., transmission system.
Three-phase current at about 11,000 volts enters the primary windings of
three transformers at this sub-station. Each of these transformers has a
single primary, but two distinct secondary windings. Three of these
secondaries, one on each transformer, are connected together and feed a
rotary converter at about 380 volts, three-phase. The other three
secondary windings are connected in like manner to a second rotary
converter. Each of these transformers is rated at 250 kilowatts, and
each rotary is rated at 300 kilowatts, so that the transformer capacity
amounts to 750 kilowatts and that of the converters to 600 kilowatts,
giving a desirable margin of transformer capacity for railway service.
With the ordinary method of connection and windings, six transformers of
125 kilowatts each would have been required in this sub-station.
High voltage for transmission lines may be obtained by the combination
of two or more transformers with their secondary coils in series. This
method was followed in some of the early transmissions, as in that at
10,000 volts to San Bernardino and Pomona, begun in 1891, where twenty
transformers, giving 500 volts each, were used with their high-voltage
coils in series. Some disadvantages of such an arrangement are its high
cost per unit of transformer capacity and its low efficiency.
In a single-phase system the maximum line pressure must be developed or
received in the coils of each transformer, unless two or more are
connected in series. This is also true as to either phase of a two-phase
system with independent circuits. In three-phase circuits the coils of a
transformer connected between either two wires obviously operate at the
full line pressure. The same result is reached when the three
transformers of a group are joined to the line in mesh or Δ-fashion. If
the three transformers of a group are joined in star or Y-fashion, the
coils of each transformer are subject to fifty-eight per cent of the
voltage between any two wires of the three-phase line on which the group
is connected. It is no longer the practice to connect two or more
transformers in series either between two wires of a two-phase or
between two wires of a three-phase circuit, because it is cheaper and
more efficient to use a single transformer in each of these positions.
Where very high voltage must be developed or received with a three-phase
system, the star or Y-connection of each group of three transformers has
the advantage of a lower strain on the insulation of each transformer
than that with the mesh or Δ-grouping. Thus if the Δ-grouping is used,
the line pressure equals that of each transformer coil, but if the
Y-grouping is used the line voltage is 1.73 times that of each
transformer coil.
At the Colgate power-house, the 700-kilowatt transformers are designed
for a maximum pressure of 60,000 volts on the three-phase line when
Y-connected, so that the corresponding voltage is 34,675 in their
secondary coils. The primary coils of these same transformers are
connected in mesh or Δ-form and each coil operates at 2,300 volts, the
generator pressure.
Transformers are in some cases provided with several sets of connections
to their coils so that they may be operated at widely different
pressures. Thus, in the Colgate plant, each transformer has taps brought
out from its secondary coils so that it can be operated at either
23,175, 28,925, or 34,675, with 2,300 volts at its primary coil.
Corresponding to the three voltages named in each secondary coil are
voltages of 40,000, 50,000, and 60,000 on a three-phase line connected
with three of these transformers in Y-fashion.
The mesh or Δ-connection is used between the coils of transformers on
some transmission lines of very high voltage. The 950 kilowatt
transformers in the system between Cañon Ferry and Butte illustrate this
practice, being connected Δ-fashion to the 50,000-volt line.
When transformers that will operate at the desired line voltage on
Δ-connection can be obtained at slight advance over the cost of
transformers requiring Y-connections, it is often better practice to
select the former, because this will enable an increase of seventy-three
per cent in the voltage of transmission to be made at any future time by
simply changing to Y-connections. Such an increase of voltage may become
desirable because of growing loads or extension of transmission lines.
An example of this sort came up some time ago in connection with the
transmission between Ogden and Salt Lake City, which was operating at
16,000 volts, three-phase, with the high-pressure coils of transformers
connected in Δ-form. By changing to Y-connections the line voltage was
raised seventy-three per cent without increasing the strain on
transformer insulation.
In some cases it is desirable to change alternating current from
two-phase to three-phase, or _vice versa_, for purposes of transmission
or distribution, and this can readily be done by means of static
transformers. One method often employed to effect this result includes
the use of two transformers connected to opposite phases of the
two-phase circuit. The three-phase coil of one of these transformers
should be designed for the desired three-phase voltage, and should have
a tap brought out from its central point. The three-phase coil of the
other transformer should be designed for 87 per cent of the desired
three-phase voltage. One end of the coil designed for 87 per cent of the
three-phase voltage should be connected to the centre tap of the
three-phase coil in the other transformer. The other end of the 87 per
cent coil goes to one wire of the three-phase circuit. The other two
wires of this circuit should be connected, respectively, to the outside
end of the coil that has the central tap. As a matter of illustration it
may be required to transform 500-volt, two-phase current from
generators, to 20,000-volt, three-phase current for transmission. Two
transformers designed for 500 volts in their primary coils are necessary
for this work. One of these transformers should have a secondary coil
designed for 20,000 volts, so that the ratio of transformation is 20,000
÷ 500 or 40 to 1, and a tap should be brought out from the centre of
this coil. The other transformer should have a secondary voltage of 0.87
× 20,000 = 17,400, so that its ratio of transformation is 34.8 to 1.
These two transformers, with the connections above indicated, will
change the 500-volt, two-phase current to 20,000 volts, three-phase.
At one of the water-power stations supplying energy for use in Hartford,
four transformers of 300 kilowatts each change 500-volt, two-phase
current from the generators to 10,000-volt, three-phase, for the
transmission line.
In the Niagara water-power station the generators deliver two-phase
current at 2,200 volts, and 975-kilowatt transformers are connected in
pairs to change the pressure to 22,000 volts, three-phase, for
transmission to Buffalo.
A transformer is used in some cases to raise the voltage and compensate
for the loss in a transmission line. For this purpose the secondary of a
transformer giving the number of volts by which the line pressure is to
be increased is connected in series with the line. The primary winding
of this transformer may be supplied from the line boosted or from
another source.
Transformers ranging in capacity from 100 to 1,000 kilowatts each, such
as are commonly used for transmission work, have efficiencies of 96 to
98 per cent at full loads, when of first-class construction. Efficiency
increases slowly with transformer capacity within the limits named, and
98 per cent can be fairly expected in only the larger sizes. In any
given transformer the efficiency may be expected to fall a little, say
one or two per cent, between full load and half load, and another one
per cent between half load and quarter load. These figures for
efficiencies at partial loads vary somewhat with the design and make of
transformers. In general, it may be said that step-up or step-down
transformers will cost approximately $7.50 per kilowatt capacity, or
about one-half of the like cost of low-voltage dynamos. If dynamos of
voltage sufficiently high for the transmission line can be had at a
figure below the combined cost of low-volt dynamos and raising
transformers, it will usually pay to avoid the latter and develop the
line voltage in the armature coils. This plan avoids the loss in one set
of transformers.
TRANSFORMERS IN TRANSMISSION SYSTEMS.
+-------------------------------+----------+---------+---------+
| | Trans- | Trans- | Genera- |
| | formers | formers | tors at |
| |at Power- |at Sub- | Power- |
|Transmission System. |stations. |stations.|stations.|
| +----+-----+---+-----+---+-----+
| |No. | Kw. |No.| Kw. |No.| Kw. |
| | |Each.| |Each.| |Each.|
+-------------------------------+----+-----+---+-----+---+-----+
|Cañon Ferry to Butte | 12| 325 |[A]| [A] |...| ... |
| | 6| 950 | 6| 950 | 10| 750 |
|Apple River to St. Paul | ...| ... | 6| 300 |...| ... |
| | 6| 500 | 4| 200 | 4| 750 |
|White River to Dales | 3| 400 | 3| 375 | 2| 500 |
|Farmington River to Hartford | 4| 300 |...| ... | 2| 600 |
|Ogden to Salt Lake |[B]9| 250 |...| ... | 5| 750 |
|Colgate to Oakland | ...| 700 |...| ... | {3|1125 |
| | | | | | {4|2250 |
|Presumpscot River to Portland | ...| ... | {6| 200 |...| ... |
| | | | {3| 150 | 4| 500 |
| | | | | | {1| 180 |
| | | | | | {3| 300 |
|Four water-powers to Manchester| ...| ... | 21| 200 | {1| 450 |
| | | | | | {4| 650 |
| | | | | | {1|1200 |
+-------------------------------+----+-----+---+-----+---+-----+
[A] Other transformers at Helena sub-stations.
[B] Part of energy distributed directly from generators.
CHAPTER XI.
SWITCHES, FUSES, AND CIRCUIT-BREAKERS.
Electrical transmission has worked a revolution in the art of switching.
As long as the distances to be covered by distribution lines required
pressures of only a few hundred volts, the switch contacts for
generators and feeders could well be exposed in a row on the surface of
vertical marble slabs and separated from each other by distances of only
a few inches. These switches were capable of manual operation even at
times of heavy overload without danger of personal injury to the
operator or of destructive arcing between the parts of a single switch
or from one switch to another near-by. On the back of these marble slabs
one or more sets of bare bus-bars could be located without much
probability that an accidental contact between them would start an arc
capable of destroying the entire switchboard structure and shutting down
the station.
The rise of electric pressures to thousands and tens of thousands of
volts in distribution and transmission systems has vastly increased the
difficulty of safe and effective control with open-air switches. The
higher the voltage of the circuit to be operated under load the greater
must be the distance between the contact parts of each switch and also
between adjacent switches. Such switches must also be farther removed
from the operators as the voltages of their circuits go up, as a person
cannot safely stand very close to an electric arc of several feet or
even yards in length. In the West, where long transmissions are most
common, long break-stick switches have been much used with high
voltages. These switches depend on the length of the break to open the
circuit and on the length of the stick that moves the switch-jaw or plug
to insure the safety of the operator. Where switches of this sort are
used it is highly important to have ample distances between the contact
points of each switch and also between the several switches. On circuits
of not more than 10,000 volts an arc as much as a yard long will in some
cases follow the opening switch blade and hold on for several seconds.
On the 33,000-volt transmission line at Los Angeles a peculiar form of
switch is used which makes a break between a pair of curved wire horns
that are ten inches apart at their nearest points. When the contact
between these horns is broken the arc travels up between portions of the
horns that curve apart and is thus finally ruptured. Besides the very
large space required for open switches on circuits of 5,000 to 10,000
volts or more, there is a further objection that the arcs developed by
opening such switches under heavy loads rapidly destroy the contact
parts and produce large quantities of metallic vapor that is
objectionable in a central station. In some experiments performed at
Kalamazoo (A. I. E. E., vol. xviii., p. 407) with open-air switches the
voltages ranged from 25,000 to 40,000. The loads on circuits broken by
the switches were highly inductive and mounted from 1,200 to 1,300
kilovolt-amperes. At 25,000 volts the arc produced by the open-air
switch held on for several seconds. At 40,000 volts the arc following
the opening of this switch was over thirty feet long, and being out of
doors near the pole line the arc struck the line wires and
short-circuited the system. It has been shown that the oscillations of
voltage occurring when a circuit under heavy load is opened by an
open-air switch may be very dangerous to insulation (A. I. E. E., vol.
xviii., p. 383). In the Kalamazoo test the oscillations of this sort
were reported to have reached two or three times the normal voltage of
the system when the open-air switch was used.
[Illustration: FIG. 55.--Connections between Power-houses 1 and 2 at
Niagara Falls.]
Facts of the nature just outlined have led to the development of oil
switches. The general characteristic of oil switches is that the contact
parts are immersed in, and the break between these contacts takes place
under, oil. Two types of the oil switch are made, one having all of its
contact parts in the same bath of oil and the other having a separate
oil-bath for each contact. Compared with those of the open-air type, oil
switches effect a great saving of space, develop no exposed arcs or
metallic vapors, cause little if any oscillation or rise of voltage in
an alternating circuit, and can be depended on to open circuits of any
voltage and capacity now in use. In the tests above mentioned at
Kalamazoo, a three-phase oil switch making two breaks in each phase and
with all the six contacts in a single oil-bath was used to open circuits
of 25,000 volts and 1,200 to 1,300 kilovolt-arcs with satisfactory
results. At 40,000 volts, however, this type of switch spat fire and
emitted smoke, indicating that it was working near its ultimate
capacity. A three-phase switch with each of its six contacts in a
separate cylindrical oil-chamber was used to open the 40,000-volt 1,300
kilovolt-arc circuit at Kalamazoo with perfect success even under
conditions of short-circuit and without the appearance of fire or smoke
at the switch. The three-phase switch used in the tests at Kalamazoo
and having each of its contacts in a separate oil-chamber was similar in
construction to the switches used in the Metropolitan and Manhattan
railway stations in New York City. In each of these switches the two
leads of each phase terminate in two upright brass cylinders. These
cylinders have fibre linings to prevent side-jumping of the arcs when
the switch is opened, and each cylinder is filled with oil. Into the two
brass cylinders of each phase dips a ∩-shaped contact piece through
insulating bushings, and the ends of this contact piece fit into
terminals at the bottom of the oil pots. A wooden rod joins the centre
or upper part of the ∩-contact piece, and the three rods of a
three-phase switch pass up through the switch compartment to the
operating mechanism outside. The six brass cylinders and their three
∩-contact pieces are usually mounted on a switch cell built entirely of
brickwork and stone slabs. For a three-phase switch the brick and stone
cell has three entirely separate compartments, and each compartment
contains the two brass cylinders that form the terminals of a single
phase. On top of and outside the cell the mechanism for moving the
wooden switch rods is mounted. In the Metropolitan station, where the
voltage is 6,000, the vertical movement of the ∩-shaped contact piece
with its rod is twelve inches. At the Manhattan station, where the
operating voltage is 12,000, the vertical movement of the ∩-contacts in
opening a switch is seventeen inches. The total break in each phase in a
switch at the Metropolitan station is thus twenty-four inches, or four
inches per 1,000 volts, and the total break per phase in switches at
the Manhattan station is thirty-four inches, or 2.66 inches per 1,000
volts total pressure.
Oil switches are now very generally employed on alternating circuits
that operate at 2,000 volts or more for purposes of general
distribution. On circuits of moderate voltage like that just named, and
even higher, it is common practice to use oil switches that have only a
single reservoir of oil each, the entire six contacts in the case of a
three-phase switch being immersed in this single reservoir. Such
switches are usually operated directly by hand and are located on the
backs of or close to the slate or marble boards on which the handles
that actuate the switch mechanism are located. A good example of this
sort of work may be seen at the sub-station in Manchester, N. H., where
energy from four water-power stations is delivered over seven
transmission lines and then distributed by an even larger number of
local circuits at 2,000 volts three-phase. At the Garvin’s Falls
station, one of the water-power plants that delivers energy to the
sub-station in Manchester, the generators operate at 12,000 volts
three-phase, and these generators connect directly with the bus-bars
through hand-operated oil switches on the back of the marble
switchboard. These last-named switches, like those at the Manchester
sub-station, have all the contacts of each in a single reservoir of oil.
With very high voltages, where only a few hundred kilowatts are
concerned, and also with powers running into thousands of kilowatts at
as low a pressure as 2,000 volts, it is very desirable to remove even
oil switches from the switchboard and the vicinity of the bus-bars.
Great powers as well as very high voltages not only increase the element
of personal danger to an attendant who must stand close to a switch
while operating it, but also render the damage to other apparatus that
may result from any failure of or short-circuit in a switch much more
serious.
[Illustration: FIG. 56.--Wire-room Back of Switchboard in Power-station
on French Broad River, North Carolina.]
As soon as the switches are removed to a distance from the operating
board the necessity for some method of power control becomes evident,
since the operator at the switchboard should be able to make or break
connections of any part of the apparatus quickly. The necessity for the
removal of switches for very large powers to a distance from the
operating boards and for the application of mechanical power to make and
break connections was met before the development of oil switches. Thus
at the first Niagara (A. I. E. E., vol. xviii., p. 489) power-house, in
1893, the switches for the 3,750-kilowatt, 2,200-volt generators, though
of the open-air type, were located in a special switch compartment
erected in the generator room and over a cable subway at some distance
from the operating board. These switches were actuated through
compressed-air cylinders into which air was admitted by the movement of
levers near the switchboard. Evidently a switch of this capacity--1,000
amperes per pole and 2,200 volts, two-phase--could not well be operated
by hand-power wherever located, because of the large effort required. In
the second generating station at Niagara Falls oil switches similar to
those used at the Manhattan Elevated Railway plant in New York, but
two-phase, were employed. Each of these oil switches at Niagara Falls
has a capacity of 5,000 horse-power, like the previous open-air
switches, and is electrically actuated.
[Illustration: FIG. 57.--Section through Cable Subway under Oil Switches
in Niagara Power-house No. 2.]
In these electrically operated oil switches a small motor is located on
top of the brick cell that contains the contact parts, and this motor
releases and compresses springs that open and close the switch. While it
is not desirable to employ open-air switches to open circuits of several
thousand or even hundreds of kilowatts at voltages of 2,000 or more, it
is nevertheless possible to do so. This is shown by the experience of
the first Niagara Falls station, where the 2,200-volt two-phase switches
are reported to have opened repeatedly currents of more than 600 amperes
per phase without injurious sparking. The great rise of voltage that was
shown by the experiments at Kalamazoo to follow the opening of a simple
open-air switch was avoided at the first Niagara switches by a simple
expedient. In these 5,000 horse-power open-air switches a shunt of high
resistance was so connected between each pair of contacts that the
blades and jaws that carried the main body of the current never
completely opened the circuit. When the main jaws of one of these
switches were opened the shunt resistance continued in circuit until
subsequently broken at auxiliary terminals. That no excessive rise of
voltage took place when one of these switches was open was shown by
connecting two sharp terminals in parallel with the switch and by
adjusting these terminals to a certain distance apart. Had the voltage
risen on opening the switch above the predetermined amount there would
have been an arc formed by a spark jumping the distance between the
pointed terminals.
[Illustration: FIG. 58.--Schenectady Switch-house on Spier Falls Line.]
[Illustration: FIG. 59.--Second-floor Plan of Saratoga Switch-house on
Spier Falls Line.]
Safety and reliability of operation at high voltages, say of 5,000 or
more, require that each element of the equipment be so isolated as well
as insulated from every other element that the failure or even
destruction of one element will not seriously endanger the others. With
this end in view the cables from each generator to its switch should be
laid in a conduit of brick or concrete that contains no other cables.
The brick or stone compartment for each phase of each switch should be
so substantial that the contacts of that phase may arc to destruction
without injury to the contacts of another phase. Bus-bars, like
switches, should be removed from the operating switchboard, because an
arc between them might destroy other apparatus thereon, and even the
board itself. It is not enough to remove bus-bars from the switchboard
where very high voltages are to be controlled, but each bar should be
located in a separate brick compartment so that an arc cannot be started
by accidental contact between two or more of the bars. It is convenient
to have the brick and stone compartments for bus-bars built horizontally
one above the other. The top and bottom of each compartment may
conveniently be formed of stone slabs with brick piers on one side and a
continuous brick wall on the other to hold the stone slabs in position.
Connections to the bus-bars should pass through the continuous brick
wall that forms what may be termed the back of the compartments. To
close the openings between the brick piers at the front of the
compartments movable slabs of stone may be used. Feeders passing away
from the bus-bars, like dynamo cables running to these bars, should not
be grouped close together in a single compartment, but each cable or
circuit should be laid in a separate fireproof conduit to the point
where it passes out of the station.
[Illustration: FIG. 60.--Ground Floor of Saratoga Switch-house.]
The folly of grouping a large number of feeders that transmit great
powers together in a single combustible compartment was well illustrated
by the accident that destroyed the cables that connected the first
Niagara power-station with the transformer-house on January 29th, 1903.
On the evening of that day lightning short-circuited one of the cables
in the short bridge that connects No. 1 station with the
transformer-house, and all the cables in this bridge, supplying local
consumers as well as railways and lighting in Buffalo, were destroyed.
This bridge contained probably more than thirty-six cables, as that
number of new cables was put in position within twenty-four hours after
the accident, and these cables, covered with inflammable insulation,
were close together. The result was not only the loss of the cables, but
also the damage to power users. If these cables had been located in
separate fire-proof conduits, it is highly probable that only the one
directly affected by lightning would have been destroyed.
The brick and stone compartments for bus-bars may be located in the
basement underneath the switchboard, as at the Portsmouth station of the
New Hampshire Traction Company, or at any other place in a station where
they are sufficiently removed from the other apparatus. In power-house
No. 2 at Niagara Falls a cable subway beneath the floor level runs the
entire length, parallel with the row of generators (A. I. E. E., vol.
xix., p. 537). In this subway, which is thirteen feet nine and
three-quarter inches wide and ten feet six inches high, the two
structures for bus-bar compartments are located. Each of these
structures measures about 6.6 feet high and 1.8 feet wide, and contains
four bus-bar compartments. In each compartment is a single bar, and the
four bars form two sets for two-phase working. Above the bus-bar
compartments and rising from the floor level are the oil switches. A
space over the cable subway midway of its length and between the two
groups of oil switches is occupied by the switchboard gallery which is
raised to some elevation above the floor and carries eleven generator,
twenty-two feeder, two interconnecting, and one exciter panels. In
power-house No. 1 the bus-bars are located in a common space above the
5,000 horse-power open-air switches already mentioned, and each bar has
an insulation of vulcanized rubber covered with braid and outside of
this a wrapping of twine. Of course; an insulation of this sort would
amount to nothing if by any accident an arc were started between the
bars. Where each bus-bar is located in a separate fireproof compartment,
as at Niagara power-house No. 2, the application of insulation directly
to each bar is neither necessary nor desirable. Consequently the general
practice where each bar has its own fireproof compartment is to
construct the bars of bare copper rods.
With main switches for generators and feeders removed from the operating
board and actuated by electric motors or magnets, the small switches at
the board with which the operator is directly concerned must of course
control these magnets or motors. The small switches at the operating
board are called relay switches, and the current in the circuits opened
and closed by these switches and used to operate the magnets or motors
of the oil switches may be conveniently obtained from a storage battery
or from one of the exciting dynamos.
Probably the best arrangement of the relay switches is in connection
with dummy bus-bars on the face of the switchboard, so that the
connections on the face of the board constitute at all times a diagram
of the actual connections of the generator and feeder circuits. It is
also desirable for quick and correct changes in the connections of the
main apparatus that all the relay switches and instruments necessary for
the control of any one generator or any one feeder be brought together
on a single panel of the switchboard. If this plan is followed, the
operator at any time will have before him on a single panel all of the
switches and instruments involved in the connections then to be made,
and the chance for mistakes is thus reduced to a minimum. The plan just
outlined was that adopted at the Niagara power plant No. 2, where a
separate panel is provided for each of eleven generators and each of
twenty-two feeders. On each of the eleven generator panels there are two
selector relay switches, one generator relay switch, and one relay
generator field switch. On each of the twenty-two feeder panels there
are two relay selector switches. The relay switches on the two
interconnecting panels serve to make connections between the two groups
of five and six generators respectively in power-house No. 2 and the ten
generators of power-house No. 1. On each panel there are relay
indicators to show whether the oil switches that carry the main current
respond to the movements of their relay switches.
Where the electric generators operate at the maximum voltage of the
system, as at Garvin’s Falls and in the power-house of the Manhattan
Elevated Railway, there may be said to be only one general plan of
connections possible. That is, the generators must connect directly with
the main bus-bars at the voltage of the system, and the feeders or
transmission lines must also connect to these same bars. Of course there
may be several sets of bus-bars for different circuits or classes of
work, but this does not change the general plan of through connections
from generators to lines. So, too, the arrangement of switches is
subject to variations, as by placing two switches in series with each
other in each dynamo or feeder cable, or by connecting a group of
feeders through their several switches to a particular set of bus-bars
and then supplying this set of bars from the generator bus-bars through
a single switch.
[Illustration: FIG. 61.--Switchboard Wiring, Glens Falls Sub-station on
Spier Falls Line.]
Where the voltage of transmission is obtained by the use of step-up
transformers, the connections of these transformers may be such as to
require nearly all switching to be done on either the high- or
low-tension circuits. The more general practice was formerly to do all
switching in the generator circuits and on the low-tension side of
transformers, except in the connection and disconnection of transformers
and transmission lines with the high-tension bus-bars, when not in
operation. Where generators operate at the maximum voltage of the system
only two main groups of switches are necessary, one group connecting
generators to bus-bars, and the other group connecting bus-bars to the
transmission lines. As soon as step-up transformers are introduced the
number of switch groups must be increased to four if the usual method of
connection is followed, and there must be both a high voltage and a low
voltage set of bus-bars. That is, one set of switches must connect
generators with low-tension bus-bars, another group must connect
low-tension bars with the primary coils of transformers, a third group
joins the secondary coils of transformers with the high-tension bars,
and the fourth group of switches joins the transmission lines to the
high-tension bus-bars. Switches connecting the secondary coils of
step-up transformers to the high-tension bus-bars, and also the
transmission lines to these same bars, have often been of the simple
open-air type with short knife-blade construction. These switches have
been used to disconnect the secondary coils of transformers and also the
transmission lines from the high-tension bus-bars when no current was
flowing, and switches of the simple knife-blade construction with short
breaks could of course be used for no other purpose. With switches of
this sort on the high-tension side of apparatus the practice is to do
all switching of line circuits on the low-tension side.
It is possible to avoid some of this multiplication of switches if each
generator with its transformers is treated for switching purposes as a
unit and the switching for this unit is done on the secondary or
high-voltage side of the step-up transformers. The adoption of this
plan, of course, implies the use of switches that are competent to break
the secondary circuit of any group of transformers under overload
conditions and at the maximum voltage of the system, but oil switches as
now made are competent to meet this requirement. When all switching of
live circuits is confined to those of high voltage there is also the
incidental advantage that heavy contact parts carrying very large
currents are avoided in the operating switches. Where each generator is
connected directly to its own group of transformers the secondary coils
of these transformers will pass through oil switches to high-tension
bus-bars, and the use of low-tension bus-bars may be avoided. From these
high-tension bus-bars the transmission lines will pass through oil
switches, so that on this plan there are only two sets of oil switches,
namely, those connecting the secondary coils of transformers to the
high-tension bus-bars, and those connecting the transmission lines to
the same bars. Each group of two or three transformers, according as
two or three are used with each generator, should be connected to its
generator through short-break, open-air knife switches for convenience
in disconnecting and changing transformers that are not in operation,
but these switches are not intended or required to open the circuit of
the generators and primary coils when in operation.
[Illustration: FIG. 62.--Distributing Switchboard, Central Sub-station,
Montreal.]
A plan similar to that just outlined was followed at the station of the
Independent Electric Light and Power Company, San Francisco, where each
of the 550-volt generators is ordinarily connected directly to the
primary coils of two transformers that change the current from two-phase
to three-phase and then deliver it through oil switches to the
high-tension bus-bars at 11,000 volts. To these bus-bars the 11,000-volt
feeders for five sub-stations are connected through switches. At this
station there is a set of 550-volt bus-bars to which any of the
generators may be connected, but to which no generator is connected in
ordinary operation. The generators alone have switches connecting with
these bars. When it is desirable to operate any particular generator on
some pair of transformers other than its own, that generator is
disconnected from its own transformers and connected to the 550-volt
bus-bars. The generator whose transformers are to be operated by the
generator before mentioned next has its switch connected to the 550-volt
bus-bars, while the brushes of the contact rings of the former generator
are raised. As the leads from each generator to its two switches are
permanently joined, the switching operations just named connect the
transformers of one generator with the other generator that has its
switch closed on the 550-volt bars.
[Illustration: FIG. 63.--Switchboard at Chambly Power-station.]
Where it is desired that a single reserve transformer may be readily
substituted for any one of a number of transformers in regular use, the
connections to each of these latter transformers may be provided with
double-pole double-throw knife switches on both the primary and
secondary sides, so that when these switches are thrown one way at any
transformer in regular use the reserve transformer will be connected in
its place.
Fuses and automatic circuit-breakers alike are intended to break
connections without the intervention of human agency under certain
predetermined conditions. In the fuse the heat generated by a certain
current is sufficient to melt or vaporize a short length of special
conductor. In the circuit-breaker a certain current gives a magnet or
motor sufficient strength to overcome the pressure of a spring, and
contact pieces through which the current is passing are pulled apart.
The primary object of both the fuse and the circuit-breaker is thus to
open connections and stop the flow of energy when more than a certain
current passes. When any current passes through a circuit in the reverse
of its regular direction the circuit-breaker can be arranged to break
the connections, though the fuse cannot. A fuse must carry the current
at which it is designed to melt during some seconds before enough heat
is developed to destroy it, and the exact number of seconds for any
particular case is made a little uncertain by the possibility of loose
connections at the fuse tips which develop additional heat and also by
the heat-conducting power of its connecting terminals. A circuit-breaker
may be set so as to open its connections in one or more seconds after a
certain current begins to flow. When connections are broken by a fuse
the molten or vaporized metal forms a path that an arc may easily
follow. A circuit-breaker with its contacts under oil offers a much
smaller opportunity than a fuse for the maintenance of an arc. These
qualities of fuses and circuit-breakers form the basis of their general
availability and comparative advantages in transmission circuits.
Much variation exists in practice as to the use of fuses and
circuit-breakers on transmission circuits. One view often followed is
that fuses and circuit-breakers should be entirely omitted from the
generator and transmission lines. The argument in favor of this practice
is that temporary short circuits due to birds that fly against the lines
or to sticks and loose wires that are thrown onto them will interrupt
all or a large part of the transmission service if fuses or
circuit-breakers that operate instantly are employed. On the other hand,
it may be said that if fuses and circuit-breakers are omitted from the
generator and transmission circuits a lasting short circuit will make it
necessary to shut down an entire plant in some cases until it can be
removed. Electric transmission at high voltages became important before
magnetic circuit-breakers competent to open overloaded circuits at such
voltages were developed. Consequently the early question was whether a
transmission line and the generators that fed it should be provided with
fuses or be solidly connected from generators to the distribution
circuits of sub-stations. The original tendency was strong to use fuses
in accord with the practice at low voltages. The great importance of
continuous service from transmission systems and the many interruptions
caused by temporary short circuits where fuses were used led to their
abandonment in some cases. An example of this sort may be seen at the
first Niagara station. In 1893, when this station was equipped, no
magnetic circuit-breaker was available for circuits of either 11,000 or
2,200 volts, carrying currents of several thousand horse-power, and
fuses were employed in lines at both these pressures (A. I. E. E., vol.
xviii., pp. 495, 497). The fuses adopted in this case were the same for
both the 2,200 and the 11,000-volt lines and were of the explosive type.
Each complete fuse consisted of two lignum-vitæ blocks that were hinged
together at one end and were secured when closed at the other. In these
blocks three parallel grooves for fuses were cut and in each groove a
strip of aluminum was laid and connected to suitable terminals at each
end. Vents were provided for the grooves in which the aluminum strips
were placed so that the expanding gas when a fuse was blown would
escape. When these fuse blocks were new and the blocks of lignum vitæ
made tight joints the metallic vapor produced when a fuse was blown was
forced out at the vents and the connections of the line were thus
broken. After a time, however, when the joints between the blocks were
no longer tight because of shrinkage, the expanding gas of the fuse
would reach the terminals and an arc would continue after the fuse had
blown. These aluminum fuses, which were adopted about 1893, were
abandoned at the Niagara plant in 1898. Since this later date the
2,200-volt feeders from the No. 1 power-house to the local consumers
have had no fuses at the power-house, nor have circuit-breakers been
installed there in the place of the fuses that were removed. At the
large manufacturing plants supplied through these local Niagara feeders,
the feeders formerly terminated in fuses, but these have since been
displaced by circuit-breakers. In the second Niagara power-station,
completed in 1902, the local 2,200-volt feeders are provided with
circuit-breakers, but no fuses. Between the generators and bus-bars of
the first Niagara plant the circuits were provided with neither fuses
nor automatic circuit-breakers, and this practice continues there to the
present time.
Besides the aluminum fuses in the 11,000-volt transmission line at the
first Niagara station, there were lead fuses in the 2,200-volt primary
circuits of the step-up transformers that supplied these lines. At the
other end of these lines, in the Buffalo sub-station, another set of
aluminum fuses was inserted before connection was made with the
step-down transformers. Between the secondary coils of these
transformers and the 550-volt converters there were no fuses, but these
converters were connected to the railway bus-bars through direct current
circuit-breakers. These lead fuses, which contained much more metal than
those of aluminum, when blown set up arcs that lasted until power was
cut off by opening a switch, and usually destroyed their terminals. An
effort was made to so adjust the sizes of the fuses in this transmission
system that in case of a short circuit in distribution lines at Buffalo
only the fuses in the sub-station would be blown, leaving those at
Niagara entire. This plan did not prove effective, however, and a severe
overload on the distribution lines in Buffalo would blow out fuses clear
back to the generator bus-bars at the Niagara station.
In order to accomplish the opening of overloaded circuits with greater
certainty, to delay such opening where the overload might be of only a
momentary nature, and to confine the open circuit to the lines where the
overload existed, automatic circuit-breakers were substituted for the
fuses named in the Niagara and Buffalo transmission system. This system
was also changed from 11,000 to 22,000 volts on the transmission lines,
thus rendering the requirements as to circuit-opening devices more
severe. These circuit-breakers were fitted with time-limit attachments
so that any breaker could be set to open at the end of any number of
seconds after the current flowing through it reached a certain amount. A
circuit-breaker with such a time-limit attachment will not open until
the time for which it is set, after the amperes flowing through it reach
a certain figure, has elapsed, no matter how great the current may be.
Moreover, if the overload is removed from a line before the number of
seconds for which its time-limit circuit-breaker is set have elapsed,
the circuit-breaker resets itself automatically and does not open the
connections. If a circuit-breaker is set to open a line after an
interval of say three seconds from the time when its current reaches the
limit, the line will not be opened by a mere momentary overload such as
would blow out a fuse. By setting the time-limit relays of
circuit-breakers in transmission lines to actuate the opening mechanism
after three seconds from the time that an overload comes on, and then
leaving the breakers on distribution lines to operate without a
time-limit, it seems that the opening of breakers on the distribution
lines should free the system from an overload there before the breakers
on the transmission lines have time to act. Such a result is very
desirable in order that the entire service of a transmission system may
not be interrupted every time there is a fault or short circuit on one
of its distribution lines. This plan was followed in the Niagara and
Buffalo system. In the 22,000-volt lines at the Niagara station the time
relays were set to actuate the breakers after three seconds, at the
terminal house in Buffalo, where the transformers step down from 22,000
to 11,000 volts, the circuit-breakers in the 11,000 volt lines to
sub-stations had their relays set to open in one second. Finally the
circuit-breakers in the distribution lines from the several sub-stations
were left to operate without any time limit. By these means it was
expected that a short circuit in one of the distribution circuits from a
sub-station would not cause the connections of the underground cable
between that sub-station and the terminal house to be broken, because of
the instant action of the circuit-breaker at the sub-station.
Furthermore, it was expected that a short circuit in one of the
underground cables between the terminal house and a sub-station would be
disconnected from the transmission line at that house and would not
cause the circuit-breakers at the Niagara station to operate. It is
reported that the foregoing arrangement of circuit-breakers with time
relays failed of its object because the breakers did not clear their
circuits quick enough and that the time-limit attachments on the 22,000
and 11,000 volt lines are no longer in use (A. I. E. E., vol. xviii., p.
500). As the circuits under consideration convey thousands of
horse-power at 11,000 and 22,000 volts it may be that time-limit devices
with circuit-breakers would give good results under less exacting
conditions. Time-limit relays are perhaps an important aid toward
reliable operation of transmission systems, but they are subject to the
objection that no matter how great the overload they will not open the
circuit until the time for which they are set has run. In the case of a
short circuit the time-limit relay may lead to a prolonged drop in
voltage throughout the system, which is very undesirable for the
lighting service and also allows all synchronous apparatus to fall out
of step. With a mere momentary drop in voltage the inertia of the
rotating parts of synchronous apparatus will keep them in step. For
these reasons it is desirable to have circuit-breakers that will act
immediately to open a line on which there is a short circuit or very
great overload, but will open the line only after an interval of one or
more seconds when the overload is not of a very extreme nature. This
action on the part of circuit-breakers at the second Niagara
power-station was obtained by the attachment of a dash-pot to the
tripping plunger of each circuit-breaker (A. I. E. E., vol. xviii., p.
543). With moderate overloads of a very temporary nature this dash-pot
so retards the action of a tripping plunger that the circuit-breaker
does not open. When a short circuit or great overload comes onto a line
the pull on the tripping plunger or the circuit-breaker on that line is
so great that the resistance of the dash-pot to the movement is overcome
at once and the line is disconnected from the remainder of the system.
The fact that a circuit-breaker may be designed to open the line which
it connects, whenever the direction from which the flow of energy takes
place is reversed, is taken advantage of at some sub-stations to guard
against a flow of energy from a sub-station back toward the generating
station. By this means a flow of energy from a sub-station to a short
circuit in one of the lines or cables connecting it with the generating
plant is prevented.
CHAPTER XII.
REGULATION OF TRANSMITTED POWER.
Regulation of voltage at incandescent lamps is a serious problem in the
distribution of electrically transmitted energy. Good regulation should
not allow the pressure at incandescent lamps rated at 110 to 120 volts
to vary more than one volt above or below the normal.
Electric motor service is much less exacting as to constancy of voltage,
and the pressure at motor terminals may sometimes be varied as much as
ten per cent without material objection on the part of users. A mixed
service to these three classes of apparatus must often be provided where
transmitted energy is used, and the limitations as to variations at
incandescent lamps are thus the ones that must control the regulation of
pressure.
Transmission systems may be broadly divided into those that have no
sub-stations and must therefore do all regulation at the generating
plant, and those that do have one or more sub-stations so that
regulation of voltage may be carried out at both ends of the
transmission line.
[Illustration: FIG. 64.--Arc-lighting Switchboard at Central
Sub-station, Montreal.]
As a rule, a sub-station with an operator in attendance is highly
desirable between transmission and distribution lines, and this is the
plan generally followed at important centres of electrical supply, even
though the transmission is a short one. One example of this sort may be
noted at Springfield, Mass., where energy for electrical supply is
transmitted from two water-power plants on the Chicopee River only about
four and a half and six miles, respectively, from the sub-station in the
business centre of the city. The voltage of transmission for two-phase
current in this case is 6,000, and is reduced to about 2,400 volts at
the sub-station for the general distribution of light and power. A
similar instance may be seen at Concord, N. H., where electrical energy
at both 2,500 and 10,000 volts is delivered to a sub-station in the
business section from a water-power plant at Sewall’s Falls, on the
Merrimac River, four and one-half miles distant. From this sub-station
the current is distributed at about 2,500 volts for the supply of lamps
and motors. A sub-station was found desirable at Concord for purposes
of regulation before the voltage of transmission was raised above that
of distribution. Subsequently, when the load increased, the voltage of
10,000 was adopted on a part of the transmission circuit in order to
avoid an increase in the size of their conductors.
[Illustration: FIG. 65.--Area of Electrical Distribution at Montreal.]
[Illustration: FIG. 66.]
[Illustration: FIG. 67.]
[Illustration: FIG. 68.]
In some instances, however, transmission and distribution lines are
joined without the intervention of a sub-station, where regulation of
voltage can be accomplished, though this practice has little to
recommend it aside from the savings in first cost of installation and
subsequent cost of operation. These savings are more apparent than real
if fairly constant pressure is to be maintained at the lamps, because
what is gained by the omission of sub-stations will be offset, in part
at least, by additional outlays on the lines if good regulation is to be
maintained. This fact may be illustrated by reference to Figs. 66, 67,
and 68, in each of which _D_ represents a generating station and _A_,
_B_, and _C_ towns or cities where energy from the station is to be
distributed. In the case of each figure it is assumed that the distance
between the generating station and each of the cities or towns is such
that distributing lines with a loss of, say, not more than two per cent
in voltage at full load cannot be provided between the generating
station and each city or town because of the cost of conductors. This
being so, one or more centres of distribution must be located in each
town, and the transmission lines must join the distribution lines at
these centres either on poles or in sub-stations. If several of these
towns are in the same general direction from the generating plant so as
to be reached by the same transmission line, as _A_, _B_, and _C_ in
Fig. 66, this one line will be all that is necessary with a sub-station
in each town. Where sub-stations are not employed a separate
transmission circuit must be provided between the generating plant and
each town for reasons that will appear presently. The percentage of
voltage variation in a transmission line under changing loads will be
frequently from five to ten, and is thus far beyond the allowable
variations at incandescent lamps. To give good lighting service the
centre of distribution, where the transmission line joins the
distribution circuits, must be maintained at very nearly constant
voltage if no sub-station is located there. Regulation at a generating
station will compensate for the changing loss of pressure in a line
under varying loads so as to maintain a nearly constant voltage at any
one point thereon. No plan of station regulation, however, can maintain
constant voltages at several points on the same transmission line when
there is a varying load at each. The result is that even though the
several towns served are in the same general direction from the
generating station, as in Fig. 67, yet each town should have its
separate transmission line where no sub-stations in the towns are
provided. In the case illustrated by Fig. 68, where the towns served are
in very different directions from the generating station, there should
be a separate transmission line to each, regardless of whether there is
a sub-station or only a centre of distribution there.
Even in the case illustrated by Fig. 68, as in each of the others, there
is a large saving effected in the cost of distribution lines by the
employment of a sub-station at the point where these lines join the
transmission circuit, provided that the variation of pressure at lamp
terminals is to be kept within one volt either way from the standard.
With the variations of loads the loss of pressure in the distribution
lines will range from zero to its maximum amount and the connected lamps
will be subjected to the change of voltage represented by this total
loss, unless the distribution start from a sub-station where the loss in
distribution lines can be compensated for by regulation. To give good
service the distribution lines should be limited to a loss of one per
cent at full load if there is no sub-station where they join
transmission lines. With opportunity for regulation at a sub-station the
maximum loss in distribution lines may easily be doubled, thus reducing
their weight by one-half in comparison with that required where there is
no sub-station.
Another advantage of connecting transmission and distribution lines in a
sub-station, where regulation of voltage can be had, lies in the fact
that it is practically impossible to maintain an absolutely constant
pressure miles from a generating plant at the end of a transmission line
that is carrying a mixed and varying load. A result is that without the
intervention of regulation at a sub-station it is almost impossible to
give good lighting service over a long transmission line. Furthermore,
the labor of regulation at a generating station is much increased where
there are no sub-stations, because it must be much more frequent and
accurate. The absence of sub-stations from a transmission system thus
implies more transmission circuits, heavier distribution circuits, more
labor at the generating plant, and a poor quality of lighting service.
Where stationary motors form the great bulk of the load on a
transmission system, and good lighting service is of small importance,
it may be well to omit sub-stations at some centres of distribution.
This is a condition that sometimes exists in the Rocky Mountain region
where the main consumers of power along a transmission line may be mines
or works for the reduction of ores. An example of this sort exists in
the system of the Telluride Power Transmission Company, in Utah, which
extends from Provo Cañon, on the river of the same name, entirely around
Utah Lake by way of Mercur, Eureka, and Provo, and back to the
power-house in Provo Cañon, a continuous circuit of 105 miles.
The transmission voltage on this line is 40,000, and at intervals where
there are distributing points the voltage is reduced to about 5,000 by
transformers on poles, and without the aid of regulation at sub-stations
in some cases. The power thus transmitted is largely used in mines and
smelters for the operation of motors, but also for some commercial
lighting.
Regulation at generating stations of the voltage on transmission lines
may be accomplished by the same methods whether there are sub-stations
at centres of distribution or not. In any such regulation the aim is to
maintain a certain voltage at some particular point on the transmission
line, usually its end, where the distribution circuits are connected. If
more than one point of distribution exists on the same transmission
line, the regulation at the generating plant must be designed to
maintain the desired pressure at only one of these points, leaving
regulation at the others to be accomplished by local means. One method
of regulation consists in the overcompounding of each generator so that
the voltage at its terminals will rise at a certain rate as its load
increases. If a generator and transmission line are so designed that the
rise of voltage at the generator terminals just corresponds with the
loss of voltage on the line when the output of that generator alone
passes over it to some particular point, then the pressure at that point
may be held nearly constant for all loads if no energy is drawn from the
line elsewhere. These several conditions necessary to make regulation by
the compounding of generators effective can seldom be met in practice.
If a varying number of generators must work on the same transmission
line, or if varying loads must be supplied at different points along the
line, no compound winding of generators will suffice to maintain a
constant voltage at any point on the line that is distant from the
power-station. For these reasons the compound winding of generators is
of minor importance so far as the regulation of voltage on transmission
lines is concerned, and on large alternators is not generally attempted.
An example may be noted on the 3,750-kilowatt generators at Niagara
Falls, where the single magnet winding receives current from the
exciters only.
A much more effective and generally adopted method of regulation of
voltage at the generating plants of transmission systems is based on the
action of an attendant who varies the current in the magnet coils of
each generator so as to raise or lower its voltage as desired. The
regulation must be for some one point on the transmission line, and the
attendant at the generating plant may know the voltage at that point
either by means of a pair of pressure wires run back from that point to
a voltmeter at the generating plant, by a meter that indicates the
voltage at the point in question according to the current on the line,
or by telephone connection with a sub-station at the point where the
constant voltage is to be maintained. Pressure wires are a reliable
means of indicating in the generating station the voltage at a point of
distribution on the line, but the erection of these wires is quite an
expense in a long transmission, and in such cases they are only
occasionally used. Owing to inductive effects and to variable
power-factors the amperes indicated on a line carrying alternating
current are far from a certain guide as to the drop in voltage between
the generating station and the distant point. In long transmissions,
telephone communication between the generating plant and the
sub-stations is the most general way in which necessary changes to
maintain constant voltage at sub-stations are brought to the attention
of the attendant in the generating plant. Few, if any, extensive
transmission systems now operate without telephone connection between a
generating plant and all of its sub-stations, or between a single
sub-station and the several generating plants that may feed into it.
Thus, the generating plant at Spier Falls, on the Hudson River, will be
connected by telephone with sub-stations at Schenectady, Albany, Troy,
and some half-dozen smaller places. On the other hand, the single
sub-station in Manchester, N. H., that receives the energy from four
water-power plants has a direct telephone line to each.
Where two or more transmission lines from the same power-station are
operated from the same set of bus-bars the voltage at a distant point on
each line cannot be held constant by changes of pressure on these
bus-bars. One generator only may be connected to each transmission line
and be regulated for the loss on that line, but this loses the
advantages of multiple operation. Another plan is to connect a
regulator in each transmission line before it goes from the generating
plant. One type of regulator for this purpose consists of a transformer
with its secondary coil divided into a number of sections and the ends
of these sections brought out to a series of contact segments. The
primary coil of this transformer may be supplied with current from the
bus-bars and the secondary coil is then connected in series with the
line to be regulated, so that the secondary voltage is added to or
subtracted from that of the main circuit. A movable contact arm on the
segments to which the sections of the secondary coil are connected makes
it possible to vary the secondary voltage by changing the number of
these sections in circuit. In another transformer used for regulating
purposes the primary coil is connected to the bus-bars as before and the
movable secondary coil is put in series with the line to be regulated.
The regulation is accomplished in this case by changing the position of
the secondary relative to that of the primary coil and thus raising or
lowering the secondary voltage. Both of these regulators require hand
adjustment, and the attendant may employ the telephone, pressure wires,
or the compensating voltmeter above mentioned, to determine the voltage
at the centre of distribution. The voltage indicated by this so-called
“compensator” is that at the generating station minus a certain amount
which varies with the current flowing in the line to be regulated. The
voltmeter coil of the compensator is connected in series with the
secondary coils of two transformers, which coils work against each
other. One transformer has its secondary coil arranged to indicate the
full station voltage, and the other secondary coil is actuated by a
primary coil that carries the full current of the regulated line. By a
series of contacts the effect of this last-named coil can be varied to
correspond with the number of volts that are to be lost at full load
between the generating station and the point on the transmission line at
which the voltage is to be held constant. If there is no inductive drop
on the transmission line, or if this drop is of known and constant
amount, the compensator may give the actual voltage at the point for
which the regulation is designed.
Automatic regulators are used in some generating stations to maintain a
constant voltage either at the generating terminals or at some distant
distributing point on a line operated by a single generator. These
regulators may operate rheostats that are in series with the magnet
windings of the generators to be regulated, and raise or lower the
generator voltage by varying the exciting current in these windings.
These regulators are much more effective to maintain constant voltage at
generating stations than at the distributing end of long transmission
lines with variable power-factors. In spite of the compound winding of
generators, of automatic regulators for the exciting currents in their
magnet coils, and of regulating transformers in the transmission
circuits, hand-adjustment of rheostats in series with the magnet coils
of generators remains the most generally used at the generating stations
of long transmission systems. Automatic regulators at the ends of
transmission lines in sub-stations are now being introduced, and may
prove very desirable.
[Illustration: FIG. 69.--Motor-generators in Shawinigan Sub-station at
Montreal.]
The more exacting and final work of regulation in transmission systems
is usually done at the sub-stations. After a nearly constant voltage is
delivered at the high-pressure coils of step-down transformers in a
sub-station, there remains the varying losses in these transformers, in
motor-generators or converters, in distribution lines and in service
transformers, to be compensated for. In general, three or four sorts of
loads must be provided for, namely, arc or incandescent lamps for street
lighting on series circuits, usually of 4,000 to 10,000 volts. Arc and
incandescent lamps on constant-pressure circuits of 2,000 to 2,500 volts
for commercial lighting, direct-current stationary motors on
constant-pressure circuits of about 500 volts, and alternating motors
which may be served at either 2,500 or 500 volts according to their
sizes and locations. To these loads may be added that of street-car
motors of 500 volts, direct current. Both the stationary and the
street-car motors, but more especially the latter, by their changes of
load give rise to large and rapid fluctuations of voltage on the
distribution lines to which they are connected. The problem of
regulation with combined lamp and motor loads is not, therefore, so much
to maintain a nearly constant voltage at the motors as to protect the
lamps from the fluctuations of voltage which the motors set up.
[Illustration: FIG. 70.--One of the 1,065-kilowatt Motor-generators in
the Shawinigan Sub-station at Montreal.]
For street-car motors using direct current at about 500 volts, the
sub-station equipment includes either step-down transformers and
converters or motor-generators with or without transformers. It is the
practice in some cases where both lighting and street-railway service
are drawn from the same transmission system to keep these two kinds of
service entirely separate, devoting independent generators and
transmission lines, as well as independent transformers and converters
or motor-generators, to the street-car work. This is done in the
transmission system centring at Manchester, N. H., in which each one of
the four water-power plants, as well as the sub-station, has a double
set of bus-bars on the switchboard; and from each water-power plant to
the sub-station there are two transmission circuits. In operation, one
set of generators, bus-bars, transmission circuits, and transformers
supply converters or motor-generators for the street-car motors; and
another set of generators, bus-bars, transmission circuits, and
transformers are devoted to lighting and stationary motors in this
system. Where street-car motors draw their energy from the same
generators and transmission lines that supply commercial incandescent
lamps, some means must be adopted to protect the lighting circuits from
the fluctuations of voltage set up by the varying street-car loads. One
way to accomplish this purpose is to operate the lighting circuits with
generators driven by synchronous motors in the sub-stations. These
generators may, of course, be of either direct or alternating type and
of any desired voltage. The synchronous motors driving these generators
take their current from the transmission line either with or without the
intervention of step-down transformers. By this use of synchronous
motors the lighting circuits escape fluctuations of voltage
corresponding to those on the transmission line, because synchronous
motors maintain constant speeds independently of the voltage of the
circuits to which they are connected. This plan was followed at Buffalo,
where the street-car system and the lighting service are operated with
energy from the Niagara Falls stations over the same transmission line.
In one of the sub-stations at Buffalo, both 2,200-volt, two-phase
alternators, and 150-volt continuous-current generators for lighting
service, are driven by synchronous motors connected to the Niagara
transmission line through transformers. At other sub-stations in
Buffalo, the 500-volt continuous current for street-car motors is
obtained from the same transmission system through transformers and
converters. Another solution of the problem of voltage regulation where
street-railway and commercial lighting service are to be drawn from the
same transmission line is found in the operation of 500-volt
continuous-current generators in the sub-stations by synchronous motors
fed from the line either directly or through transformers. This plan has
been adopted in the transmission system of the Boston Edison Company,
which extends to a number of cities and towns within a radius of
twenty-five miles. The sub-stations at Natick and Woburn in this system,
where there are street-railway as well as lighting loads, contain
500-volt continuous-current generators driven by synchronous motors
connected directly to the three-phase transmission lines. In a case like
this the synchronous motors maintain their speed irrespective of the
voltage on the line and thus tend to hold that voltage steady in spite
of the variable losses due to fluctuating loads.
Stationary motors should not as a rule be operated from the same
distribution lines that supply incandescent lamps, especially in sizes
above one horse-power, and this is the better practice. Motor circuits
of about 2,400 volts and two- or three-phase, alternating, or 500 volts,
alternating or direct current, may be supplied at a sub-station either
by transformers alone in the first case or by transformers and
converters in the second. In either case no especial provision is
usually necessary for the regulation of constant pressure on the motor
circuits.
[Illustration: FIG. 71.--1,100-kilowatt, 2,300-volt, Three-phase,
30-cycle, Synchronous Motor at Sub-station of Shawinigan Line in
Montreal.]
In some transmission systems the distribution circuits for stationary
motors are not fed by the same transmission lines that carry the
lighting load, but draw their energy from lines that do no other work.
This practice is certainly desirable, as it frees the lighting circuits
from all fluctuations of voltage due to line losses with changing motor
loads. Examples of this sort may be seen at Springfield, Mass., and
Portland and Lewiston, Me., in each of which the load of stationary
motors is operated over independent transmission as well as distribution
lines.
In transmission systems series arc and incandescent lamps for street
lighting are commonly operated either by direct-current arc dynamos or
by constant-current transformers or constant-pressure transformers with
automatic regulators at the sub-stations. The arc dynamos are driven by
either induction or synchronous motors supplied directly from the
transmission line or through transformers. As the arc dynamos regulate
automatically for constant current no further regulation is required. If
the series arc and incandescent lamps are to be supplied with
alternating current, the constant-current transformer or the
constant-current regulator come into use. This type of transformer and
regulator alike depend for their regulating effect on the movement of a
secondary coil on a transformer core in such a way that the current in
this coil, which is in series with the lamps, is held nearly constant.
Such constant-current transformers and regulators are usually supplied
from the transmission line through regular constant-pressure
transformers, and they hold their currents sufficiently constant for the
purposes of their use.
The main problem of regulating thus comes back to the 250- or
2,200-volt, constant-pressure circuits for incandescent lighting,
supplied from transmission lines through transformers or motor
generators or both at the sub-station. For this regulation one of the
most reliable instruments is the hand of a skilful attendant, guided by
voltmeters connected with pressure wires from minor centres of
distribution, and adjusting the regulating transformers above mentioned,
or other regulating devices.
CHAPTER XIII.
GUARD WIRES AND LIGHTNING ARRESTERS.
Lightning in its various forms is the greatest danger to which
transmission systems are exposed, and it attacks their most vulnerable
point, that is, insulation. The lesser danger as to lightning is that it
will puncture the line insulators and shatter or set fire to the poles.
The greater danger is that the lightning discharge will pass along the
transmission wires to stations and sub-stations and will there break
down the insulation of generators, motors, or transformers. Damage by
lightning may be prevented in either of two ways, that is, by shielding
the transmission line so completely that no form of lightning charge or
discharge can reach it, or by providing so easy a path from line
conductors to earth that lightning reaching these conductors will follow
the intended path instead of any other. In practice the shielding effect
is sought by grounded guard wires, and the easy path for discharge takes
the form of lightning arresters, but neither of these devices is
entirely effective.
Aerial transmission lines are exposed to direct discharges of lightning,
to electromagnetic charges due to lightning discharges near by, and to
electrostatic charges that are brought about by contact with or
induction from electrically charged bodies of air. It is evidently
impracticable to provide a shield that will free overhead lines from all
these influences. To cut off both electrostatic and electromagnetic
induction from a wire and also to free it from a possible direct
discharge of lightning, it seems that it would at least be necessary
completely to incase the wire with a thick body of conducting material.
This condition is approximated when an electric circuit is entirely
beneath the surface of the ground, but would be hard to maintain with
bare overhead wires. It seems, however, that grounded guard wires near
to and parallel with long aerial circuits should tend to discharge any
high electrostatic pressures existing in the surrounding air, and
materially to reduce the probability that a direct discharge of
lightning will choose the highly insulated circuits for its path to
earth. Lightning arresters may conduct induced and direct lightning
discharges to earth, without damage to transmission lines, so that both
arresters and guard wires may logically be used in the same system.
Wide differences of opinion exist as to the general desirability of
grounded guard wires on transmission lines, both because of their
undoubted disadvantages and because the degree of protection that they
afford is uncertain. It seems, however, that the defects of guard wires
depend in large degree on the kind of wire used for the purpose, and the
method of its erection. Galvanized iron wire with barbs every few inches
has been more generally used for guard wires along transmission lines
than any other sort. Sometimes a single guard wire of this sort has been
run on a pole line carrying transmission circuits, and the more common
location of this single wire is on the tops of the poles. In other cases
two guard wires have been used on the same pole line, one of these wires
being located at each end of the highest cross-arm and outside of the
power wires. Besides these guard wires at the ends of the top cross-arms
of a pole line, a third wire has in some systems been added to the tops
of the poles. These guard wires have sometimes been secured to the poles
and cross-arms by iron staples driven over the wire and into the wood,
and in other cases the guard wires are mounted on small glass
insulators. Much variation in practice also exists as to the ground
connections of guard wires, such connections being made at every pole in
some systems, and much less frequently in some others.
With all these differences in the practical application of guard wires
it is not strange that opinions as to their utility do not agree.
Further reason for differences of opinion as to the practical value of
guard wires exists in the fact that in some parts of the country the
dangers from lightning are largely those of the static and inductive
sort, that are most effectively provided for by lightning arresters,
while in other parts of the country direct lightning strokes are the
greatest menace to transmission systems. At the present time, knowledge
of the laws governing the various manifestations of energy that are
known under the general head of lightning is imperfect, and the most
reliable rules for the use of guard wires along transmission lines are
those derived from practical experience.
A case where a guard wire did not prove effective as a protection
against lightning is that of the San Miguel Consolidated Gold Mining
Company, of Telluride, Col., whose three transmission lines ran from the
water-power plant to points from three to ten miles distant, as
described in A. I. E. E., vol. xi., p. 337, and following pages. This
transmission operated at 3,000 volts, single-phase, alternating, and the
pole lines ran over the mountains at elevations of 8,800 to 12,000 feet
above sea-level, passing across bare ridges and tracts of magnetic
material. It was stated that the country over which the circuits ran is
so dry and rocky that it was practically impossible to secure good
ground connections along the line, and no mention was made of the way in
which the ground wire was grounded, or of the number of its ground
connections. Furthermore, it does not appear that there was more than
one guard wire on each pole line. Under these circumstances, and with a
certain make of lightning arresters in use at the station, lightning was
a frequent cause of damage to the connected apparatus. The insulation of
some of the machinery is described as honeycombed with perforations
which led to continual leakage, grounds, and short-circuits, which seems
to indicate that the damage was being done by static and inductive
discharges rather than by direct lightning strokes, one of which would
have disabled a machine at once. The type of lightning arrester in use
on this system was changed, and thorough ground connections were
provided for the new arresters, after which the damage by lightning came
to an end. It is not stated, however, that the guard wires were removed.
This case has been referred to as one in which guard wires failed to
give protection, but, as may be seen from the above facts, such a
statement is hardly fair. In the first place, it does not appear that
the single guard wire on each pole line was effectively grounded
anywhere. Again, a large part of the damage to apparatus appears to have
been the result of static or inductive discharges that could not in the
nature of things have been prevented by a guard wire. Finally, as the
guard wire was not removed after the new lightning arresters were
erected, it is possible that this wire prevented some direct discharges
over the transmission wires that would have been destructive.
On page 381 of the volume of A. I. E. E. above cited, it is stated that
the frequency and violence of lightning discharges that entered a
certain electric station on Staten Island were much less after guard
wires had been erected along the connected circuits than they were
before the guard wires were put up.
It is also stated on page 385 of the same volume that examination of
statistics of a number of stations in this country and Europe had shown
that in every case where an overhead guard wire had been erected over
power circuits, or where these circuits ran for their entire distance
beneath telegraph wires, lightning had given no trouble on the circuits
so protected. Unfortunately, the speaker who made this statement did not
tell us where the interesting statistics mentioned could be consulted.
On the first pole line erected for power transmission from Niagara Falls
to Buffalo, two guard wires were strung at opposite ends of the top
cross-arm on guard irons there located. This cross-arm also carried
parts of two power circuits, and the nearest wires of these circuits
were distant about thirteen inches from the guard irons. These guard
wires were barbed, and grounded at every fifth pole, according to an
account given in A. I. E. E., vol. xviii., at 514 and following pages.
The character of the ground connections is not stated. Much trouble in
the way of grounds and short circuits on the transmission lines was
caused by these guard wires at times when they were broken by the weight
of ice coatings and wind pressure. As a result of these troubles the
guard wires were removed in 1898. Since that date it appears that the
transmission lines between Niagara Falls and Buffalo have been without
guard wires. Up to 1901, according to page 537 in the volume just cited,
twenty per cent of the interruptions in operation at the Niagara plant
were caused by lightning, and it seems probable that this record applies
to the period after 1898, when the guard wires were removed. It is also
stated that during a single storm the line was struck five times, and
that five poles with their cross-arms were destroyed. If these direct
lightning strokes occurred while there were no guard wires along the
line, as seems to have been the fact, it is a fair question whether such
wires well grounded would not have carried off the discharges without
damage. In California, the country of long transmissions, the use of
guard wires along the pole lines is quite general. Many of these lines
run east and west across the State, and a single line may thus have
elevations in its different parts all the way from that of tide-water up
to several thousand feet above sea-level. Unless guard wires are strung
with these lines there is much manifestation of induced or static
electricity, according to an account at page 538, in vol. xviii., A. I.
E. E., where it is said that in the absence of guard wires a person will
be knocked off his feet every time he touches a transmission wire that
is entirely disconnected from the source of power. It is also said that
this static charge on idle power lines is sufficient, in time, to
puncture the insulation of the connected apparatus. On the other hand,
where the grounded and barbed guard wires are strung over the entire
lengths of these long power lines, these lines may be handled with
impunity when they are idle. Ground connections to the guard wire are
said to be made at about every fourth pole, and to consist of a wire
stapled down the face of the pole and joined to an iron plate beneath
its butt. The barbed guard wire itself, of which each pole line appears
to have but one, is regularly stapled to the tops of the poles.
At the reference just named it is related that on a certain
transmission line running east and west across the State for a distance
of forty-six miles, and protected by a guard wire, no trouble was
experienced during a severe storm that swept north and south over the
line. Meantime the damage on other lines in the same neighborhood, and
presumably not protected by guard wires, was large.
[Illustration: FIG. 72.--Transposition of Wires on Chambly Montreal
Line.]
Between the electric plant at Chambly, on the Richelieu River, and
Montreal, Quebec, a distance of 16.6 miles, a transmission line of three
circuits on two pole lines, with guard wires, was operated from some
time in 1898 to December 1st, 1902, or somewhat more than four years. On
the date last named the dam that maintained the head of water at the
Chambly station gave way, and the plant was shut down during nearly a
year for repairs. For as much as three years this line was operated at
12,000 volts, sixty-six cycles per second, two-phase. During the
remainder of the period up to the failure of the dam the line was
operated at 25,000 volts, sixty-three cycles, three-phase. In each
transmission two pole lines were employed with two cross-arms per pole.
One two-phase, four-wire circuit was carried on each of three of these
cross-arms. At each end of the upper cross-arm on each pole, and at a
distance of fifteen inches from the nearest power wire, a guard wire was
mounted on a glass insulator. A third guard wire was mounted on a glass
insulator at the top of each pole, and this third guard wire was about
twenty inches from the nearest power wire. Each of these guard wires was
made up of two No. 12 B. W. G. galvanized iron wires twisted together,
with a four-point barb every five inches of length. Poles carrying these
lines were ninety feet apart, and at each pole all three of the guard
wires were connected by soldered joints to a ground wire that was
stapled down the side of the pole, passed through an iron pipe eight
feet long, and was then twisted several times about the butt of the pole
before it was set in the ground. At three points along the line the
conductors consisted of single-conductor underground or submarine cables
that had an aggregate length of about twenty-five miles. No lightning
arresters were employed at the points where the overhead transmission
wires joined the underground cables.
These two-phase, 12,000-volt circuits were operated from some time in
1898 to some time in 1902, and during that time there was no damage done
by lightning either at the Chambly plant, on the overhead line or the
underground cable, or at the Montreal sub-station. This record is not
due to lack of thunder-storms, for in the territory where the line is
located these storms are frequent and severe. One very severe storm
during the period in question resulted in serious damage on distribution
lines at Chambly and Montreal, where the guard wires were not in use,
but the transmission line and its connected apparatus escaped unharmed.
The path of this storm was in the direction of the transmission line
from Montreal to Chambly, and several trees were struck on the way. At
the time of this storm and during an entire summer there were no
lightning arresters in the power-house at Chambly.
In 1902, when the transmission line just considered was changed from
two-phase to three-phase, and its voltage raised from 12,000 to 25,000,
the method of protection by grounded, barbed guard wires, as above
described, was retained. Two three-phase circuits were arranged on each
of the two pole lines, with one wire of each circuit on an upper
cross-arm and two wires of each circuit on a lower cross-arm, so that
the nearest power wire on the upper cross-arm is thirty-two inches from
the guard wire, and the nearest power wire on the lower cross-arm is
about thirty inches from the guard wire at each end of the upper
cross-arm. The guard wire at the tops of the poles is about thirty-three
inches from each of the power wires on the upper cross-arm. In this
three-phase line there is about 1,440 feet of three-conductor
underground cable, and this cable lies between the end of the overhead
line and the sub-station in Montreal. At the juncture of the overhead
line and the cables there is a terminal house containing lightning
arresters, and there are also arresters at the Chambly plant and the
Montreal sub-station. No lightning arresters are connected to this line
save those at the generating plant, the terminal house and the
sub-station.
During that part of the year 1902 in which the new 25,000-volt line was
in operation--that is, after the change and up to the time of the
failure of the dam--this line and its connected apparatus were not
damaged in any way by lightning, and the same is true for the period in
which the line was idle pending repairs on the dam. The experience on
this Montreal and Chambly transmission is probably among the best
evidence to be found anywhere as to the degree of protection from
lightning that may be had by the use of guard wires. In spite of cases
like that just considered, where guard wires appear to have given a
large degree of protection to transmission systems, many important
transmissions are operated without them.
An example of this sort may be seen in the transmission line between the
10,000-horse-power plant at Electra, in the Sierra Nevada Mountains of
California, and San Francisco, a distance of 154 miles, where it seems
that no guard wires are in use. Another important transmission line that
appears to get along without guard wires is that between the
10,000-horse-power plant at Cañon Ferry, on the Missouri River, and
Butte, Mont., sixty-five miles away. On the transmission line between
the power-station on Apple River, in Wisconsin, and the sub-station at
St. Paul, Minn., about twenty-seven miles long, there are no guard wires
for lightning protection. Further east, on the large, new transmission
system that stretches from Spier Falls and Glens Falls on the north to
Albany on the south, a distance in a direct line of forty miles, no
guard wires are employed. On its way the transmission system just named
touches Saratoga, Schenectady, Mechanicsville, Troy, and a number of
smaller places, thus forming a network with several hundred miles of
overhead wire. Examples of this sort might be multiplied, but those
already named are sufficient to show that it is entirely practicable to
operate long transmission systems without guard wires as a protection
against lightning.
With these examples of transmission systems both with and without guard
wires, the expediency of their use on any particular line should be
determined by weighing their supposed advantages against their known
disadvantages, under the existing conditions. It seems fairly certain
from all the evidence at hand that if guard wires are to offer any large
degree of protection to transmission systems such wires must be
frequently and effectively grounded. There is certainly some reason to
think that the failures of guard wires to protect transmission systems
in some instances may have been due to the lack of numerous and
effective ground connections. Such, for example, may have been the case
above mentioned, at Telluride, Col. On the other hand, it seems
reasonable to believe that the apparently high degree of protection
afforded by the guard wires on the Chambly and Montreal line is due to
the fact that these wires are connected through soldered joints at every
pole with a ground wire that is wound about its base. The nearer the
guard wires are located to the power wires on a line the greater is the
danger that a guard wire will come into contact with a power wire by
breaking or otherwise. It is probable that the protection given by a
guard wire does not increase nearly as fast as the distance between it
and a power wire is diminished. Even if one guard wire on a line is
thought to be desirable, it does not follow that two or more such wires
should be used, for the additional protection given by two or three
guard wires beyond that given by one wire may be trifling, while the
cost of erection and the danger of crosses with the power circuits
increase directly with the number of guard wires. At one time it was
thought very desirable to have barbs on guard wires, but now the better
opinion seems to be that, as barbs tend to weaken the wire, they lead to
breaks and cause more trouble than they are worth. The point where the
barbs are located seems to rust more quickly than do other parts of the
wire. In some cases barbed guard wires that have given trouble by
breaking have been taken down and smooth wires put up instead. If a
guard wire is well grounded at least as often as every other pole, its
size may be determined largely on considerations of mechanical strength
and lasting qualities. For ordinary spans a No. 4 B. & S. G. galvanized
soft iron wire seems to be about right for guarding purposes. Iron seems
to be the most desirable material for guard wires because it gives the
required mechanical strength and sufficient conductivity at a less cost
than copper, aluminum, or bronze, and is easier to handle and less
liable to break than steel. It was formerly the practice to staple guard
wires to the tops of poles or to the ends of cross-arms, but it was
found that the wire was more apt to rust and break at the staple than
elsewhere, and in the better class of work such wires are now mounted on
small insulators. This practice, as stated above, was followed on the
Montreal and Chambly line. In all cases the connection between the guard
wire and each of its ground wires should be soldered, and the ground
wire should have a large surface in contact with damp earth, either
through a soldered joint with a ground plate, by winding a number of
turns about the butt of the pole, or by other means.
It is thought by some telegraph engineers that the use of a separate
ground wire running to the top of each pole is quite as effective as a
protection against lightning as is a guard wire that runs to all of the
poles and is frequently connected to the ground.
This practice is mentioned at page 26 of “Culley’s Handbook of Practical
Telegraphy.” Such ground wires are free from most of the objections to
the ordinary guard wires. It seems quite certain that a guard wire along
an alternating-current line, and grounded at frequent intervals, must
act as a secondary circuit of a transformer by reason of its ground
connections, and thus absorb some energy from the power circuits. No
experimental data are yet available, however, to show how large this
loss may be in an ordinary case. It is fairly evident that there must be
some electrostatic effects between the working conductors and a guard
wire, but here again data are lacking as to the amount of any such
effect. On most, if not all, transmission lines the guard wire or wires,
if used at all, are placed either above or on a level with the highest
power conductors. With one conductor of a three-phase circuit mounted on
a pin set in the top of a pole, and the two remaining conductors on a
two-pin cross-arm beneath, in the method most frequently adopted for
transmission lines of very high voltage, it is obviously impracticable
to put guard wires either above or on a level with the power circuits.
In the latest transmissions there is a strong tendency to omit guard
wires entirely and rely on lightning arresters for protection.
Lightning arresters are wrongfully named, for their true purpose is not
to arrest or stop lightning, but to offer it so easy a path to the
ground that it will not force its way through the insulation of the line
or of machinery connected to the system. The requirements of a lightning
arrester are in a degree conflicting, because the resistance of the path
it offers must be so low as to allow discharges of atmospheric
electricity to earth and so high as to prevent any flow of current
between the transmission lines. In other words, the insulation of the
line conductors must be maintained at a high standard in spite of the
connection of lightning arresters between each conductor and the earth;
but the resistance to the arrester must not be so high that lightning
will pierce the insulation of the line or machinery at some other point.
When a lightning discharge takes place through an arrester the
resistance which the arrester offers to a flow of current is for the
moment greatly reduced by the arcs which the lightning sets up in
jumping the air-gaps of the arrester. Each wire of a transmission
circuit must be connected alike to arresters, and the paths of low
resistance through arcs in these arresters to the earth would obviously
short-circuit the connected generators unless some construction were
adopted to prevent this result. In some early types of lightning
arresters magnetic or mechanical devices were resorted to in order to
break arcs formed by the discharge of lightning.
The type of lightning arrester now in common use on transmission lines
with alternating current includes a row of short, parallel, brass
cylinders mounted on a porcelain block and with air-gaps of
one-thirty-second to one-sixteenth of an inch between their parallel
sides. The cylinder at one end of the row is connected to a line wire
and the cylinder at the other end to the earth, when a 2,000 or
2,500-volt circuit is to be protected. For higher voltages a number of
these single arresters are connected in series with each other and with
the free ends of the series to a line wire and to the earth,
respectively. Thus, for a 10,000-volt line, four or, better, five single
arresters are connected in series to form a composite arrester for each
line conductor. For any given line voltage the number of single
arresters going to make up the composite arrester should be so chosen
that the regular working voltage will not jump the series of air-gaps
between the little brass cylinders, but yet so that any large rise of
voltage will be sufficient to force sparks across these gaps. A
variation of this practice by one large manufacturing company is to
mount the group of single arresters on a marble board in series with
each other and with an adjustable air-gap. This gap is intended to be so
adjusted that any large increase of voltage on the lines will be
relieved by a spark discharge. An arrester made up entirely of the brass
cylinders and air-gaps has the disadvantage that an arc once started
between all the cylinders by a lightning discharge so lowers the
resistance between each line wire and the earth that the generating
equipment is short-circuited and the arcs may not cease with the escape
of atmospheric electricity. To avoid this difficulty it is the practice
to connect a conductor of rather large ohmic resistance such as a rod of
carborundum in series with the brass cylinders and air-gaps of lightning
arresters. This resistance should be non-inductive so as not to offer a
serious obstacle to lightning discharge, and its resistance should be
great enough to prevent a flow of current from the generators that will
be large enough to maintain the arcs started in the arrester by the
lightning discharge. Accurate data are lacking as to the amount of this
resistance that should be employed with arresters for any given voltage.
As a rough, approximate rule it may be said that in some cases good
results will be obtained with a resistance in ohms in series with a
group of lightning arresters that represents one per cent of the
numerical value of the line voltage. That is, for a 10,000-volt line the
group of arresters for each wire may be connected to earth through a
resistance of, say, 100 ohms, so that if the generator current follows
the arc of a lightning discharge through the arresters it must pass
through a fixed resistance of 200 ohms in going from one line wire to
another. This rule is given merely as an illustration of the resistance
that will work well in some cases, and should not be taken to have a
general application. If the resistance connected in series with
lightning arresters is high, the tendency is a little greater for
lightning to go to earth at some point in the apparatus where the
insulation is low. If only a small resistance is employed to connect
lightning arresters with the earth, the danger is that arcs formed by
lightning discharges will be followed and maintained by the dynamo
currents. In one make of lightning arrester the row of little brass
cylinders is connected at the ends to carbon rods which form a
resistance for the purpose just mentioned. Two of these carbon rods are
contained in each arrester for 2,000 or 2,500 volts, and the resistance
of each rod may be anywhere from several score to several hundred ohms
as desired. This form of arrester may be connected directly from line to
earth without the intervention of any outside resistance, since the
carbon rods may easily be given all the resistance that is desirable.
One of the most important features in the erection of a lightning
arrester is its connection to earth. If this connection is poor it may
render the arrester useless so far as protection from lightning is
concerned. It need hardly be said that ground connections formed by
driving long iron spikes into the walls of buildings or into dry earth
are of slight value as far as protection from lightning is concerned. A
good ground connection for lightning arresters may be formed with a
copper or galvanized iron plate, which need not be over one-sixteenth of
an inch thick, but should have an area of, say, ten to twenty square
feet. This plate may be conveniently made up into the form of a cylinder
and should have a number of half-inch holes punched in a row down one
side into which one or more copper wires with an aggregate area equal to
that of a No. 4 or No. 2 wire, B. & S. gauge, should be threaded and
then soldered. This plate or cylinder should be placed deep enough in
the ground to insure that the earth about it will be constantly moist,
and the connected copper wire should extend to the lightning arresters.
It is a good plan to surround this cylinder with a layer of coke or
charcoal.
A good earth connection for lightning arresters may be made through
large water-pipes, but to do this it is not enough simply to wrap the
wire from the lightning arresters about the pipe. A suitable contact
with such a pipe may be made by tapping one or two large bolts into it
and then soldering the wires from lightning arresters into holes drilled
in the heads of these bolts. A metal plate laid in the bed of a stream
makes a good ground.
With some of the older types of lightning arresters it was the custom
to insert a fuse between the line wire and the ground, but this practice
defeats the purpose for which the arrester is erected because the fuse
melts and leaves the arrester disconnected and the circuit unprotected
with the first lightning discharge. The modern arresters for
alternating-current circuits are made up of a series of metal cylinders
and short air-gaps and are connected solidly without fuse between line
and earth.
[Illustration: FIG. 73.--Entry of Lines at the Power-house on Neversink
River.]
It was once the practice to locate lightning arresters almost entirely
in the stations, but this has been modified by experience and
consideration of the fact that as the line acts as a collector of
atmospheric electricity, paths for its escape should be provided along
the line. Consideration fails to reveal any good reason why lightning
that reaches a transmission line some miles from a station should be
forced to travel to the station, where it may do great damage before it
finds an easy path to earth. It is, therefore, present practice to
connect lightning arresters to each wire at intervals along some lines
as well as at stations and sub-stations. The main purpose of arresters
is to offer so easy a path to earth that lightning discharges along the
lines will not flow to points of low insulation in generators,
transformers, or even the line itself. Practice is far from uniform as
to the distance between lightning arresters on transmission lines, the
distances varying from less than one to a large number of miles apart.
In general the lines should be provided with lightning arresters at
least where they run over hilltops and at any points where lightning
strokes are unusually frequent. Where a long overhead line joins an
underground cable arresters should always be connected, and the same is
true as to transformers located on the transmission line. The
multiplication of arresters along pole lines should be avoided as far as
is consistent with suitable protection, because every bank of arresters
may develop a permanent ground or short-circuit, unless frequently
inspected and kept clean and in good condition.
Arresters, besides those connected along the lines, should be located
either in or just outside of stations and sub-stations. If the buildings
are of wood, the arresters had better be outside in weather-proof cases,
but in brick or stone buildings the arresters may be properly located
near an interior wall and well removed from all other station equipment.
Transmission lines, on entering a station or sub-station, should pass to
the arresters at once and before connecting with any of the operating
machinery.
To increase the degree of protection afforded by lightning arresters
choke-coils are frequently used with them. A choke-coil for this purpose
usually consists of a flat coil of copper wire or strip containing
twenty to thirty or more turns and mounted with terminals in a wooden
frame. This coil is connected in series with the line wire between the
point where the tap for the lightning arrester is made and the station
apparatus. Lightning discharges are known to be of a highly oscillatory
character, their frequency being much greater than that of the
alternating currents developed in transmission systems. The
self-induction of a lightning discharge in passing through one of these
choke-coils is great, and the consequent tendency is to keep the
discharge from passing through the choke-coil and into the station
apparatus and thus to force the discharge to pass to earth through the
lightning arrester. The alternating current employed in transmission has
such a comparatively low frequency that its self-induction in a
choke-coil is small. Increased protection against lightning is given by
the connection of several groups of lightning arresters one after
another on the same line wire at an electric station. This gives any
lightning discharge that may come along the wire several paths to earth
through the different groups of arresters, and a discharge that passes
the first group will probably go to earth over the second or third
group. In some cases a choke-coil is connected into a line wire between
each two groups of lightning arresters as well as between the station
apparatus and the group of arresters nearest thereto.
An electric transmission plant at Telluride, Col., where thunder-storms
are very frequent and severe, was equipped with arresters and
choke-coils of the type described, and the results were carefully noted
(vol. xi., A. I. E. E., p. 346). A small house for arresters and
choke-coils was built close to the generating station of this system and
they were mounted therein on wooden frames. Four choke-coils were
connected in series with each line wire, and between these choke-coils
three lightning arresters were connected, while a fourth arrester was
connected to the line before it reached any of the choke-coils. These
arresters were watched during an entire lightning season to see which
bank of arresters on each wire discharged the most lightning to earth.
It was found that, beginning on the side that the line came to the
series of arresters, the first bank of arresters was traversed by only a
few discharges of lightning, the second bank by more discharges than any
other, the third bank by quite a large number of discharges, and the
fourth bank seldom showed any sign of lightning discharge. Over the
second bank of arresters the lightning discharges would often follow
each other with great rapidity and loud noise. The obvious conclusion
from these observations seems to be that three or four banks of
lightning arresters connected in succession on a line at a station
together with choke-coils form a much better protection from lightning
than a single bank. At the plant in question, that of the San Miguel
Consolidated Gold Mining Company, the entire lightning season after the
erection of the arresters in question was passed without damage by
lightning to any of the equipment. During the two lightning seasons
previous to that just named the damage by lightning to the generating
machinery at the plant had been frequent and extensive.
A good illustration of the high degree of security against lightning
discharges that may be attained with lightning arresters and choke-coils
exists at the Niagara Falls plants and the terminal house in Buffalo,
where the step-up and step-down transformers have never been damaged by
lightning though the transmission line has been struck repeatedly and
poles and cross-arms shattered (vol. xviii., A. I. E. E., p. 527). This
example bears out the general experience that lightning arresters,
though not an absolute protection, afford a high degree of security to
the apparatus at electric stations.
Lightning arresters are in some cases connected across high-voltage
circuits from wire to wire so that the full line pressure tends to force
a current across the air-gaps. The object of this practice is to guard
against excessive voltages on the circuit such as might be due to
resonance. In such a case, as in that where arresters are connected from
line wire to earth as a protection against lightning, the number of
air-gaps should be such that the normal line voltage will not force
sparks across the air-gaps and thus start arcs between the cylinders.
The number and total length of air-gaps in a bank of arresters necessary
to prevent the formation of arcs by the regular line voltage depends on
a number of factors besides the amount of that voltage.
According to the report of the Committee on Standardization of the
American Institute of Electrical Engineers, the sparking distances in
air between opposed sharp needle points for various effective sinusoidal
voltages are as follows (vol. xix., A. I. E. E., p. 1091):
+--------------------+---------------+
|Kilovolt Square |Inches Sparking|
|Root of Mean Square.| Distance. |
+--------------------+---------------+
| 5 | 0.225 |
| 10 | .47 |
| 15 | .725 |
| 20 | 1.0 |
| 25 | 1.3 |
| 30 | 1.625 |
| 35 | 2.0 |
| 40 | 2.45 |
| 45 | 2.95 |
| 50 | 3.55 |
| 60 | 4.65 |
| 70 | 5.85 |
| 80 | 7.1 |
| 90 | 8.35 |
| 100 | 9.6 |
| 110 | 10.75 |
| 120 | 11.85 |
| 130 | 12.95 |
| 140 | 13.95 |
| 150 | 15.0 |
+--------------------+---------------+
It may be noted at once from this table that the sparking distance
between the needle points increases much faster than the voltage between
them. Thus, 20,000 volts will jump an air-gap of only an inch between
the points, but seven times this pressure, or 140,000 volts, will force
a spark across an air-gap of 13.95 inches. Two cylinders or other blunt
bodies show smaller sparking distances between them at a given voltage
than do two needle points, but when a number of cylinders are placed in
a row with short air-gaps between them the aggregate length of these
gaps that will just prevent the passage of sparks at a given voltage may
be materially greater or less than the sparking distance of that voltage
between needle points. It has been found by experiment that the numbers
one-thirty-second-inch spark-gaps between cylinders of non-arcing alloy
necessary to prevent the passage of sparks with the voltages named and a
sine wave of electromotive force are as follows (vol. xix., A. I. E. E.,
p. 1026):
+----------+---------+
|Number of | Normal |
|1/32-Inch | Voltage |
| Air-Gaps |Withheld.|
|in Series.| |
+----------+---------+
| 5 | 6,800 |
| 10 | 10,000 |
| 15 | 12,500 |
| 20 | 14,500 |
| 25 | 16,400 |
| 30 | 18,200 |
| 35 | 19,300 |
| 40 | 20,500 |
| 45 | 21,700 |
| 50 | 22,600 |
| 55 | 23,900 |
| 60 | 25,000 |
| 65 | 26,000 |
| 70 | 27,000 |
| 75 | 28,000 |
| 80 | 29,000 |
+----------+---------+
According to these data, only ten air-gaps of one-thirty-second of an
inch each and 0.3125 inch combined length are required between cylinders
to prevent a discharge at 10,000 volts, though opposed needle points may
be 0.47 inch apart when a spark is obtained with this voltage. On the
other hand, eighty air-gaps of one-thirty-second of an inch each between
cylinders of non-arcing metal, or a total gap of 2.5 inches, are
necessary to prevent a discharge at 29,000 volts, though 30,000 volts
can force a spark across a single gap of only 1.625 inches between
opposed needle points.
Under the conditions that existed in the test just recorded the pressure
at which the aggregate length of one-thirty-second of an inch air-gaps
that just prevents a discharge equals the single sparking distance
between needle points seems to be about 18,000 volts.
The object of dividing the total air-gap in a lightning arrester for
lines that carry alternating current up into a number of short gaps is
to prevent the continuance of an arc by the regular generator or line
current after the arc has been started by a lightning discharge. As soon
as an electric spark leaps through air from metal to metal, a path of
low electrical resistance is formed by the intensely heated air and
metallic vapor. If the arc thus formed is, say, two inches long it will
cool a certain amount as the passing current grows small and drops to
zero. If, however, this total arc of two inches is divided into
sixty-four parts by pieces of metal, the process of cooling as the
current decreases will go on much more rapidly than with the single arc
of two inches because of the great conducting power of the pieces of
metal. As an alternating current comes to zero twice in each period, the
many short arcs formed in an arrester by a lightning discharge are so
far cooled during the small values of the following line current that
the resistance quickly rises to a point where the regular line voltage
cannot continue to maintain them, if the arrester is properly designed
for the system to which it is connected. In this way the many-gap
arrester destroys the many small arcs started by lightning discharges
that would continue and short-circuit the line if they were combined
into a single long arc.
When an electric arc passes between certain metals like iron and copper
a small bead is raised on their surfaces. If these metals were used for
the cylinders of arresters the beads on their surface would quickly
bridge the short air-gaps. Certain other metals, like zinc, bismuth, and
antimony, are pitted by the passage of arcs between their surfaces. By
suitable mixture of metals from these two classes, an alloy is obtained
for the cylinders of lightning arresters that pits only slightly and is
thus but little injured by lightning discharges. After long use and many
discharges an arrester of the class here considered gradually loses its
power to destroy electric arcs. This may be due to the burning out of
the zinc and leaving a surface of copper on the cylinders.
Aside from the structure of an arrester and the normal voltage of the
circuit to which it is connected, its power to destroy arcs set up by
lightning discharges depends on the capacity of the connected generators
to deliver current on a short-circuit through the gaps, and upon the
inductance of the circuit. The greater the capacity of the generators
connected to a system the more trying are the conditions under which
arresters must break an arc because the current to be broken is greater.
So, too, an increase of inductance in a circuit adds to the work of an
arrester in breaking an arc.
An arc started by lightning discharge at that period of a voltage phase
when it is at or near zero is easily destroyed by the arrester, but an
arc started at the instant when the regular line voltage has its maximum
value is much harder to break because of the greater amount of heat
generated by the greater current sent through the arrester. For this
reason the arcs at arresters will hold on longer in some cases than in
others, according to the portion of the voltage phase in which they are
started by the lightning discharge. Lightning discharges, of course, may
occur at any phase of the line voltage, and for this reason a number of
discharges must take place before it can be certain from observation
that a particular arrester will always break the resulting arc. Between
twenty-five and sixty cycles per second there is a small difference in
favor of the latter in the power of a given arrester to break an arc,
due probably to the fact that more heat in the arcs is developed per
phase with the lower than with the higher frequency.
It will now be seen that while increase of the regular line voltage
requires a lengthening of the aggregate air-gap in lightning arresters
to prevent the formation of arcs by this voltage alone, the increase of
generating capacity requires more subdivisions of the total air-gap in
order that the arcs maintained by the larger currents may be cooled with
sufficient rapidity. These two requirements are to some extent
conflicting, because the subdivision of the total air-gaps renders it
less effective to prevent discharges due to the normal line voltage, as
has already been shown. The result is that the more an air-gap is
subdivided in order to cool and destroy arcs that have been started by
lightning, the longer must be the aggregate air-gap in order to prevent
the development of arcs directly by the normal line voltage.
Furthermore, the practical limit of subdivision of the air-gap is soon
reached because of the difficulty of keeping very short gaps clean and
of nearly constant length. As a resistance in series with an arrester
cuts down the generator current that can follow a lightning discharge,
such a resistance also decreases the number of air-gaps necessary to
give an arrester power to destroy arcs on a particular circuit.
The increase of resistance in series with a lightning arrester as well
as the increase in the aggregate length of its air-gaps subjects the
insulation of connected apparatus to greater strains at times of
lightning discharge. On systems of large capacity the number and
aggregate length of air-gaps in arresters necessary to destroy arcs must
be greater than the number or length of these air-gaps necessary to
prevent the development of arcs by the normal line voltage, unless a
relatively large resistance is connected in series with each arrester.
To reduce the strains produced on the insulation of line and connected
apparatus under these conditions by lightning discharges, a resistance
is connected in shunt with a part of the air-gaps in one make of
lightning arrester. The net advantage claimed for this type of arrester
is that a lower resistance may be used in series with all the air-gaps
than would otherwise be necessary. One-half of the total number of
air-gaps in this arrester are shunted by the shunt resistance and the
series and shunt resistance are in series with each other. Only the
series air-gaps or those that are not shunted must be jumped in the
first instance by the lightning discharge, which thus passes to earth
through these air-gaps and the shunt and series resistance in series. An
arc is next started in the shunted air-gaps, and this arc is in turn
destroyed because the shunt weakens the current in these gaps. This
throws the entire current of the arc through the series air-gaps and
the shunt and series resistance all in series with each other. As the
shunt resistance is comparatively large, the current maintaining the arc
in the series air-gaps is next so reduced that this arc is broken.
Taking the claims of its makers just as they stand, the advantages of
the shunted air-gaps are not very clear. The series air-gaps alone must
evidently be such that the normal line voltage will not start an arc
over them, and these same series gaps must be able to break the arcs of
line current flowing through them and the shunt and series resistance
all in series. Evidently the greatest strain on the insulation of the
line and apparatus occurs at the instant when the lightning discharge
takes place through the series gaps and the shunt and series resistances
all in series with each other.
Why develop subsequent arcs in the shunted air-gaps? Why not throw the
shunted air-gaps away and combine the shunt and series resistances?
CHAPTER XIV.
ELECTRICAL TRANSMISSION UNDER LAND AND WATER.
Energy transmitted over long distances must sometimes pass through
conductors that are underground or beneath water. In some other cases it
is a question of relative advantages merely, whether portions of a
transmission line go under water or overhead. Where the transmitted
energy must enter a sub-station in the heart of a large city, it not
infrequently must go by way of underground conductors without regard to
the voltage employed. In some cities the transmission lines may be
carried overhead, provided that their voltage is within some moderate
figure, but not otherwise. Here it becomes a question whether
transmission lines at high voltage shall be carried underground, or
whether transforming stations shall be established outside of the
restricted area and then low-pressure lines brought into the business
section overhead or underground, as desired. Where a transmission line
must cross a steam railway track it may be required to be underground,
whether the voltage is reduced or not. The distance across a body of
water in the path of a transmission line may be so great that a span is
impossible and a cable under the water therefore necessary. Such a cable
may work at the regular line voltage, or a transformer station may be
established on one side or on each side of the body of water. Even where
it is possible to span a body of water with a transmission line, the
cost of the span and of its supports may be so great that a submarine
cable is more desirable. A moderate increase in the length of a
transmission line in order to avoid the use of a submarine cable is
almost always advisable, but where rivers are in the path of the line it
is generally impossible to avoid crossing them either overhead or
underneath. Thus, St. Paul could only be reached with the 25,000-volt
line from the falls on Apple River by crossing the St. Croix River,
one-half mile wide, on the way. In order to carry out the 40,000-volt
transmission between Colgate and Oakland, the Carquinez Straits, which
intervened with nearly a mile of clear water, were crossed. Sometimes,
as in the former of the two cases just named, an existing bridge may be
utilized to support a transmission line, but more frequently the choice
lies between an overhead span from bank to bank of a river and a
submarine cable between the same points.
The prime advantage of an overhead line at high voltage is its
comparatively small first cost, which is only a fraction of that of an
underground or submarine cable in the great majority of instances. At
very high voltages, like 40,000 to 50,000 or more, the overhead line
must also be given first place on the score of reliability, since the
lasting qualities of underground and submarine cables at such pressures
is as yet an unknown quantity. On the other hand, at voltages in which
cable insulation has been shown by experience to be thoroughly
effective, underground or submarine cables may be more reliable than
overhead lines because of the greater freedom from mechanical
disturbances which these cables enjoy.
In the business portion of many cities a transmission line must go
underground, whether its voltage is high or low. Under these conditions,
it may be desired either to transmit energy to a sub-station for
distribution within the area where conductors must be underground, or to
transmit energy from a generating station there located to outside
points. If the transmitted energy is reduced in pressure before reaching
such a sub-station, a transforming station must be provided, and this
will allow the underground cables to operate at a moderate voltage. For
such a case the advantages as to insulation at the lower voltage should
be compared with the additional weight of conductors in the cable and
the cost of the transforming apparatus and station. If the voltage at
which current is delivered from the transforming station does not
correspond with the required voltage of distribution at the sub-station,
the necessary equipment of step-down transformers is doubled in capacity
by lowering the voltage of the transmitted energy where it passes from
the overhead line to the underground cables. Conditions of just this
sort exist at Buffalo in connection with the delivery of energy from the
power-stations at Niagara Falls. This transmission was first carried out
at 11,000 volts, and a terminal station was located at the Buffalo city
limits where the overhead lines joined underground cables that continued
the transmission at the same voltage to several sub-stations in
different parts of the city. Later the voltage of the overhead
transmission line was raised to 22,000, and it not being thought
advisable to subject the insulation of the underground cables to this
higher pressure, transformers were installed at the terminal station to
lower the line voltage to 11,000 for the underground cables. As the
sub-stations in this case also have transformers, there are two
kilowatts of capacity in step-down transformers for each kilowatt of
delivery capacity at the sub-stations.
[Illustration: FIG. 74.--Cable Terminal House for the 25,000-volt
Chambly Line at Montreal.]
The saving effected in capacity of transformers and in the weight of
cables by continuing the full transmission voltage right up to the
sub-stations whence distribution takes place furnishes a strong motive
to work underground cables at the pressure of the overhead transmission
line of which they form a continuation. Thus, at Hartford, the
10,000-volt overhead lines that bring energy from water-power stations
to the outskirts of the city connect directly in terminal houses there
with underground cables that complete the transmission to the
sub-station at the full line voltage. In Springfield, Mass., the
overhead transmission lines from water-power stations connect directly
with underground cables at a distance of nearly two miles from the
sub-station, and these cables are thus subject to the full line pressure
of 6,000 volts. The overhead line that brings energy at 25,000 volts
from Apple River falls to St. Paul terminates about three miles from the
sub-station there, and the transmission is completed by underground
cables that carry current at the 25,000-volt pressure.
In these and similar cases the relative advantages of underground cables
at the full voltage of transmission and of overhead lines at a much
lower pressure, in the central portions of cities, must be compared. The
overhead lines at moderate voltage will no doubt cost less in almost
every case than underground cables of equal length and at the full
transmission voltage.
As an offset to the lower cost of overhead city lines at moderate
voltage, where they are permitted by local regulations, comes the
increase in weight of conductors due to the low pressure on the overhead
lines, and also the cost of additional transformer capacity, unless the
lines that complete the transmission operate at the voltage of
distribution. The 10,000-volt lines that transmit energy from Great
Falls to Portland, Me., terminate in two transformer houses that are
distant about 0.5 mile and 2.5 miles, respectively, from the sub-station
there. In these transformer houses the voltage is reduced to 2,500, and
the transmission is then continued at this pressure to the sub-station
whence distribution takes place without further transformation.
Where a river or other body of water must be crossed by a transmission
line, either of three plans may be followed. The overhead line may be
continued as such across the water, either by a single span or by two or
more spans supported by one or more piers built for that purpose in the
water. The overhead line may connect directly with a submarine cable,
this cable being thus exposed to the full voltage of the transmission.
As a third expedient, a submarine cable may be laid and connected with
step-down transformers on one bank and with step-up transformers on the
other bank of the river or other body of water to be crossed. The
overhead lines connecting with these transformers can obviously be
operated at any desired voltage, and this is also true of the cable.
Even though the distance across a body of water is not so great that a
transmission line can not be carried over it in a single span, the cost
of such a span may be large. A case in point is that of the Colgate and
Oakland line, where it crosses the Carquinez Straits by a span of 4,427
feet. These straits are about 3,200 feet wide where the transmission
line crosses, and overhead lines were required to be not less than 200
feet above high water so as not to impede navigation. In order to gain
in ground elevation and thus reduce the necessary height of towers, two
points 4,427 feet apart on opposite sides of the straits were selected
for their location. Under these circumstances two steel towers, one
sixty-five feet and the other 225 feet high, were required to support
the four steel cables that act as conductors across the straits. To take
the strain of these four cables, each with a clear span nearly three
times as great as that of the Brooklyn Bridge, eight anchors with housed
strain insulators were constructed, four on the land side of each tower.
On each of these anchors the strain is said to be 24,000 pounds. At each
end of the cables making this span is a switch-house where either of the
two three-phase transmission lines may be connected to any three of the
four steel cables, thus leaving one cable free for repairs. It is not
possible to state here the relative cost of these steel towers and
cables in comparison with that of submarine cables for the same work,
but at first glance the question appears to be an open one. The voltage
of 40,000, at which this transmission is carried out, is probably higher
than that on any submarine cable in use, but it is possible that a
suitable cable can be operated at this voltage. Whatever the limitations
of voltage as applied to submarine cables, it would, of course, have
been practicable to use step-up and step-down transformers at the
switch-houses and thus operate a submarine cable at any voltage desired.
In another case, on a transmission between Portsmouth and Dover, N. H.,
it was necessary to cross an arm of the sea on a line 4,811 feet long
with a three-phase circuit operating at 13,500 volts. It was decided to
avoid the use of either a great span or of raising and lowering
transformers at this crossing, and to complete the line through a
submarine cable operating at the full voltage of transmission. To this
end a brick terminal house six by eight feet inside, and with an
elevation of thirteen feet from the concrete floor to the tile roof, was
erected on each bank of the bay at the point where the submarine cable
came out of the water. The lead-covered cable pierced the foundation of
each of these terminal houses at a point four feet below the floor level
and rose thence on one wall to an elevation eleven feet above the floor
to a point where connection was made with the ends of the overhead
lines. From this connection on each of the three conductors a tap was
carried to a switch and series of lightning arresters. A single
lead-covered cable containing three conductors makes connection between
these two terminal houses. At each end of this cable the lead sheath
joins a terminal bell one foot long and 2.5 inches in outside diameter,
increasing to four inches at the end where the three conductors pass
out. This terminal bell is filled nearly to the flaring upper end with
an insulating compound.
In the instance just named it is possible that the cost of the submarine
cable was less than would have been the outlay for shore supports and a
span nearly a mile long across this body of water.
Underground and submarine cables have been operated at voltages suitable
for transmission during periods sufficiently long to demonstrate their
general reliability. The Ferranti underground cables between Deptford
and London have regularly carried current at 11,000 volts since a date
prior to 1890. During about five years cables with an aggregate length
of sixteen miles have transmitted power from St. Anthony’s Falls to
Minneapolis. At Buffalo, some thirty miles of rubber-insulated cables
have been in use for underground work at 11,000 volts since 1897, and
eighteen miles of paper-insulated cable since the first part of 1901.
These examples are enough to show that transmission through underground
cables at 11,000 to 12,000 volts is entirely practicable. At Reading,
Pa., an underground cable one mile long has carried three-phase current
at 16,000 volts for the Oley Valley Railway since some time in 1902. The
cables in the transmission from Apple River to St. Paul, which carry
three-phase current at 25,000 volts, have a total length of three miles,
and have been in use since 1900. This voltage of 25,000 is probably the
highest in regular use on any underground or submarine cable conveying
energy for light or power. From the experience thus far gained there is
much reason to think that the voltages applied to underground cables may
be very materially increased before a prohibitive cost of insulation is
reached.
On submarine cables the voltage of 13,000 in the Portsmouth and Dover
transmission, above mentioned, is perhaps as great as any in use. It
does not appear, however, that any material difference exists, as to the
strain on its insulation at a given voltage, between a cable when laid
in an underground conduit and when laid under water. In either case the
entire stress of the voltage employed operates on the insulation between
the several conductors in the cable and between each conductor and the
metallic sheath. Underground conduits have little or no value as
insulators of high voltages, because it is practically impossible to
keep them water-tight and prevent absorption or condensation of moisture
therein. For these reasons a cable that gives good results at 25,000
volts in an underground conduit should be available for use at an equal
voltage under water. The standard structure of high-voltage cables for
either underground or submarine work includes a continuous metallic
sheath outside of each conductor or of each group of conductors that
goes to make up a circuit. As most transmissions are now carried out
with three-phase current, the three conductors corresponding to a
three-phase circuit are usually contained in a single cable and covered
by a single sheath. The cables used in transmission systems at
Portsmouth, Buffalo, and St. Paul are of this type. If single-phase or
two-phase current is transmitted, each cable should contain the two
conductors that go to make up a circuit. In work with alternating
currents the use of only one conductor per cable should be avoided
because of the loss of energy that results from the currents induced in
the metallic sheath of such a cable.
[Illustration: FIG. 75.--Cable Terminal House at Piscataqua River
Crossing.]
Where the two, three, or more conductors that form a complete circuit
for alternating current are included in a single metallic sheath, the
inductive effects of currents in the several conductors tend to
neutralize each other and the waste of energy in the sheath is in large
part avoided. To neutralize more completely the tendency to local
currents in their metal sheath, the several insulated conductors of an
alternating circuit are sometimes twisted together, after being
separately insulated, before the sheath is put on. Distribution of power
at Niagara Falls was at first carried out through single-conductor,
lead-covered cables with two-phase current at 2,200 volts. One objection
to this plan was the loss of energy by induced currents in the lead
coverings of the cables. It was later decided to adopt three-phase
distribution at 10,000 volts for points distant more than two miles from
the power-station. Each three-phase circuit for this purpose was made up
of three conductors separately insulated and then covered with a single
lead sheath, so as to avoid losses through induced currents in the
latter. Underground and submarine cables for operation at high voltages
are generally covered with a continuous lead sheath and sometimes with a
spiral layer of galvanized iron wire. For high-voltage work underground
the lead covering is generally preferred without iron wire, but in
submarine work coverings of both sorts are employed. The lead sheath of
a cable being continuous completely protects the insulation from contact
with gases or liquids. As ducts of either tile, wood, or iron form a
good mechanical protection for a cable, the rather small strength of a
lead sheath is not a serious objection in conduit work. Submarine
cables, on the other hand, depend on their own outer coverings for
mechanical protection, and may be exposed to forces that would rapidly
cut through a lead sheath. Cables for operation under water should
usually be covered, therefore, with a layer of galvanized iron wires
outside of the lead sheath. These wires are laid closely about the cable
in spiral form and are usually between 0.12 and 0.25 inch in diameter
each, depending on the size of the cable and its location.
Underground conduits cannot be relied on to exclude moisture and acids
of the soil from the cables which they contain, and either of these
agents may lead to destructive results. If cables insulated with rubber,
but without a protecting covering outside of it, are laid in underground
conduits, the rubber is apt to be rapidly destroyed by fluids and gases
that find their way into the conduit. If a plain lead-covered cable is
employed the acids of the soil attack it, and if stray electric currents
from an electric railway find the lead a convenient conductor it is
rapidly eaten away where they flow out of it. To avoid both of these
results the underground cable should have a lead sheath, and this sheath
may be protected by an outside layer of hemp or jute treated with
asphaltum.
Rubber, paper, and cotton are extensively used as insulation for
underground and submarine cables, but the three are not usually employed
together. As a rule, the insulation is applied separately to each
conductor, and then an additional layer of insulation may be located
about the group of conductors that go to make up the cable. Where rubber
insulation is employed, a lead sheath may or may not be added, but where
insulation depends on cotton or paper the outer covering of lead is
absolutely necessary to keep out moisture. The radial thickness of
insulation on each conductor and of that about the group of conductors
in a cable should vary according to the voltage of operation.
Cables employed between the generating and sub-stations of the Manhattan
Elevated Railway, to distribute three-phase current at 11,000 volts, are
of the three-conductor type, rubber insulated, lead covered, and laid in
tile conduits. Each cable contains three No. 000 stranded conductors,
and each conductor has its own insulation of rubber. Jute is laid on to
give the group of conductors an outer circular form, and outside of the
group a layer of insulation and then a lead sheath is placed. Outside
diameter of this cable is nearly three inches, and the weight nine
pounds per linear foot.
The 11,000-volt, three-phase current from Niagara Falls is distributed
from the terminal house to seven sub-stations in Buffalo through about
30 miles of rubber-insulated and 18 miles of paper-insulated,
three-conductor, lead-covered cables, all in tile conduits. In each
cable the three No. 000 stranded conductors are separately insulated and
then twisted into a rope with jute yarn laid in to give an even round
surface for the lead sheath to rest on. A part of the rubber-insulated
cables have each conductor covered with 9/32-inch of 30 per cent pure
rubber compound, and the remaining rubber cables have 8/32-inch
covering on each conductor of 40 per cent pure rubber compound. The
paper-insulated cable has 13/64-inch of paper around each conductor, and
also another 13/64-inch of paper covering about the group of three
conductors and next to the lead sheath. In outside diameter the
rubber-insulated cable is 2-3/8 inches, and of the paper-insulated cable
2-5/8 inches, the radial thickness of the lead sheath being 1/8-inch in
each case. It is reported that the cables insulated with 9/32-inch of
the mixture, said to be 30 per cent pure rubber, have proved to be more
reliable than the cables insulated with 8/32-inch of a mixture said to
be 40 per cent pure rubber. Vol. xviii., A. I. E. E., 136, 836.
The six miles of underground cables that carry three-phase, 25,000-volt
current in St. Paul are of the three-conductor type, lead covered, and
laid in a tile conduit. One of the two three-mile cables is insulated
with rubber and the other with paper. In the former cable each conductor
is separately insulated with a compound containing about 35 per cent of
pure rubber and having a radial thickness of 7/32-inch. The three
conductors after being insulated are laid up with jute to give a round
surface, tape being used to hold them together, and then a rubber cover
5/32-inch thick is placed about the group, after which comes the lead
sheath over all. In the three miles of paper-insulated cable each
conductor is separately covered with paper to a thickness of 9/32-inch,
then the three conductors are laid together with jute and taped, and
next a layer of paper 4/32-inch thick is put on over the group. Outside
of all comes the lead sheath, which has an outer coating of tin. The
paper insulation in these cables was saturated with a secret insulating
compound. The lead sheath on both the rubber and paper insulated cables
is 1/8-inch thick and the sheath of the former contains 3 per cent of
tin. Each of the three conductors in each cable consists of 7 copper
strands and has an area of 66,000 circular mils. Outside of the lead
sheath each of these cables has a diameter of about 2-1/4 inches. By the
manufacturer’s contract these cables were tested up to 40,000 volts
before shipment, and might be tested up to 30,000 volts in their
conduits during any time within five years from their purchase. In first
cost the cable with rubber insulation was said to be about 50 per cent
more expensive than the cable in which paper was used. Vol. xvii., A. I.
E. E., 650.
Underground cables in which the separate conductors are covered with
cotton braid treated with an insulating compound, and then the group of
conductors going to make up the cable enclosed in a lead sheath, are
extensively used in Austria and Germany. For cables that operate at
10,000 to 12,000 volts the radial thickness of cotton insulation on each
conductor is said to be within 3/16-inch, and these cables are tested up
to 25,000 volts by placing all of the cable except its ends in water,
and then connecting one end of the 25,000-volt circuit to the water and
the other end to the conductors of the cable.
A test on the paper-insulated cable at St. Paul showed its charging
current to be 1.1 amperes at 25,000 volts for each mile of its length.
For the cable with rubber insulation the charging current per mile of
length was found to be about twice as great as the like current for the
paper-insulated cable. Each of the two overhead transmission lines
connected with these cables consisted of three solid copper wires with
an area of 66,000 circular mils each, and all three so mounted on the
poles as to form the corners of an equilateral triangle twenty-four
inches apart. The charging current of one of these three-wire, overhead
circuits was found to be about 0.103 ampere per mile, at 25,000 volts,
or a little less than one-tenth of the like current for the paper cable.
These tests were made with three-phase current of sixty cycles per
second.
Where overhead transmission lines join underground or submarine cables,
either with or without the intervention of transformers, lightning
arresters should be provided to intercept discharges of this sort that
come over the overhead wires. Lightning arresters were provided in the
terminal house at Buffalo, where the 22,000-volt overhead lines feed the
11,000-volt cables through transformers, also at the terminal house in
St. Paul, where the 25,000-volt overhead lines are electrically
connected to the underground cables. If an underground or submarine
cable connects two portions of an overhead line, as in the Portsmouth
and Dover transmission above mentioned, lightning arresters should be
provided at each end of the cable, as was done in that case. One
advantage of a high rather than a low voltage on underground cables,
where power is to be transmitted at any given rate, lies in the fact
that the amperes flowing at a fault in the cable determine the
destructive effect there, rather than the voltage of the transmission.
It is reported that a fault or short-circuit in one of the 11,000-volt
cables at Buffalo usually melts off but little lead at the sheath and
does not have enough explosive force to injure the cable or its duct.
Ozone seems to destroy the insulating properties of rubber very rapidly,
and as it is well known that the silent electric discharge from
conductors at high voltages develops ozone, care should be taken to
protect rubber insulation from its action. This is especially true at
the ends of cables where connections are made with switches or other
apparatus, and the rubber insulation is exposed. To protect the rubber
at such points it is the practice to solder a brass cable head or
terminal bell to the lead sheath near its end, this head having a
diameter perhaps twice as great as that of the sheath, and then to fill
the space about the cable conductors in this head with an insulating
compound. Heads of this sort were used on the 11,000-volt cables at
Buffalo as well as on the 13,500-volt cable in the Portsmouth and Dover
transmission.
As insulating materials, whether rubber, cotton, or paper, may be
impaired or destroyed by heat, it is necessary that the temperature of
underground cables under full load be kept within safe limits. Rubber
insulation can probably be raised to 125° or 150° Fahrenheit without
injury, and paper and cotton may go a little higher. For a given size
and make of leaded cable the rise of temperature in its conductors above
that of the surrounding air, for a given loss in watts per foot of the
cable, may be determined by computation or experiment. The next step is
to find out how much the temperature of the air in the conduits where
the cable is to be used will rise above the temperature of the earth in
which the conduits are laid, with the given watt loss per foot of cable.
On this point there are but little experimental data. Obviously, the
material of which ducts are made, the number of ducts grouped together
with cables operating at the same time, and the extent to which ducts
are ventilated must have an important bearing on this question. At
Niagara Falls some tests were made to show the rise of air temperature
in a section of thirty-six-duct conduit lying between two manholes about
140 feet apart. For the purpose of this test twenty-four of the
thirty-six ducts in the conduit had one No. 6 drawing-in wire passed
through each of them. These twenty-four wires were connected into three
groups of eight wires each, so that one group was all in ducts next to
the surrounding earth, another group was one-half in ducts next to the
earth and the other half in ducts separated from the earth by at least
one duct, while the third group of wires was entirely in ducts separated
from the earth by at least one duct. It was found that when enough
current was sent through these wires to represent a loss of 5.5 watts
per foot of ducts in which they were located, the rise of temperature in
the air of the ducts next to the earth was about 108° Fahrenheit above
that of the earth. For the ducts separated from the earth by at least
one other duct the rise in temperature of contained air was 144°
Fahrenheit above the earth. If the earth about the ducts reached 70° in
hot weather, the temperature of air in the inner ducts, with a loss of
5.5 watts per duct foot, would thus be 214°. This temperature is too
high for either rubber, cotton, or paper insulation, to say nothing of
the amount by which the temperature of the conductors and insulation of
a cable in operation must exceed that of the surrounding air in its
duct. The cables actually installed in the ducts just considered were
designed for a loss of 2.34 watts per foot. As the No. 6 wire used in
the test did not nearly fill each duct as a cable would do, it would be
very interesting to know how much ventilation took place while the test
was going on. Unfortunately, this point was not reported. Vol. xviii.,
A. I. E. E., 508.
CHAPTER XV.
MATERIALS FOR LINE CONDUCTORS.
Copper, aluminum, iron, and bronze are all used for conductors in
long-distance electric transmissions, but copper is the standard metal
for the purpose. An ideal conductor for transmission lines should
combine the best electrical conductivity, great tensile strength, a high
melting point, low coefficient of expansion, hardness, and great
resistance to oxidation. No one of the metals named possesses all of
these properties in the highest degree, and the problem is to select the
material best suited to each case. Aluminum suffers very slightly by
exposure to the weather, copper and bronze suffer a little more, while
iron and steel wire are attacked seriously by rust.
Iron, copper, and bronze are all so hard that little or no trouble has
occurred from wires of these metals cutting or wearing away at the
points of attachment to insulators. Aluminum, on the other hand, is so
soft that swaying of the wire may, in time, cause material wear at the
supports, or it may be cut by tie wires. But lines of aluminum wire have
not been in use long enough to determine how much trouble is to be
expected from its lack of hardness.
A small coefficient of expansion is desirable in transmission wires,
because the strain on the wire itself and on its supports varies rapidly
with the amount of vertical deflection of each span, becoming greater as
the deflection decreases. Taking the expansion of copper as unity, that
of aluminum is 1.4; of bronze, 1.1; and of iron and steel, 0.7. From
these figures it follows that iron and steel wires show the least
variation in the amount of sag between supports, and aluminum wire shows
the most.
Wrought iron melts at about 2,800°, steel at 2,700°, copper at 1,929°,
bronze at about the same point as copper, and aluminum at 1,157°
Fahrenheit. This low melting point of aluminum may prove a source of
trouble by opening a line of that material where some foreign wire falls
on it. This, according to a report, was illustrated at a sub-station on
a 30,000-volt transmission line where a destructive arc was started at
the switchboard. Not being able to extinguish the arc in any other way,
a lineman threw an iron wire across the aluminum lines just outside of
the sub-station, and these lines were immediately melted through by the
iron wire, thus opening the circuit. The trouble may have warranted so
desperate a remedy in this case; but, as a rule, it does not pay to cut
a transmission line in order to get rid of a short circuit.
In the ordinary construction of transmission lines on land the tensile
strength of wire is secondary in importance to its electrical
conductivity, because supports can be spaced according to the strength
of the conductor used. When large bodies of water must be crossed,
tensile strength is a prime requirement. Thus a 142-mile line from
Colgate to Oakland, in California, crosses the Straits of Carquinez in
the form of steel cables, each seven-eighths of an inch in diameter and
4,427 feet long. Steel wire was selected for this long span, probably
because it can be given a greater tensile strength than that of any
other metal. Annealed iron wire has a tensile strength between 50,000
and 60,000 pounds per square inch. Steel wires vary all the way from
50,000 to more than 350,000 pounds per square inch in strength, but mild
steel wire with a strength ranging from 80,000 to 100,000 pounds per
square inch is readily obtained.
Soft copper shows a tensile strength between 32,000 and 36,000 pounds
per square inch, and hard-drawn copper between 45,000 and 70,000 pounds,
depending on the degree of hardness. Silicon-bronze wires vary in
strength from less than 60,000 to more than 100,000 pounds per square
inch, and phosphor-bronze has a tensile strength of about 100,000
pounds. Bronze wires, like those of most alloys, show a much wider range
of strength than those of iron or copper.
In silicon-bronze wire the electrical conductivity decreases as the
tensile strength increases. The tensile strength of aluminum wire is
lower than that of any other used in transmission lines, being only
about 30,000 pounds per square inch. Solid aluminum wires of large size
have given trouble by breaking under strains well within their nominal
strength, due probably to imperfections or twists. This trouble is now
generally avoided by the use of aluminum cables.
In that most necessary property of a transmission
line--conductivity--copper excels all other metals except silver. Taking
the conductivity of soft copper wire at 100, the conductivity of
hard-drawn copper is 98; that of silicon-bronze ranges from 46 to 98;
that of aluminum is 60; of phosphor-bronze, 26; of annealed iron wire,
14; and of steel wire of 100,000 pounds tensile strength per square
inch, 11. Copper wire, both soft and hard, as regularly made, does not
vary more than one per cent from the standard, and aluminum and annealed
iron wires also show high uniformity as to resistance. Silicon-bronze
and steel wires, on the other hand, fluctuate much in electrical
conductivity. For any particular transmission line the resistance is
usually determined by considerations apart from the metal to be used as
a conductor, so that a line of given resistance or conductivity must be
constructed of that material which best conforms to the requirements as
to size of wire, weight, strength, and cost.
Allowing the weight of any definite mass of copper to represent unity,
the weight of an equal mass of wrought iron is 0.87; of steel, 0.89; of
aluminum, 0.30; while that of bronze is very nearly equal to that of the
copper. The smallest line wire that can be used for a given length and
resistance is one of pure, soft copper. Next in cross-sectional area
come hard-drawn copper and some silicon-bronze, either of which need be
only two per cent larger than the soft copper for an equal resistance.
Some other silicon-bronze wire of greater tensile strength per square
inch would require a sectional area of 2.17 times that of the soft
copper.
Aluminum wire with 60 per cent of the conductivity of copper requires
1.66 of its section for wires of equal resistance. As phosphor-bronze
has only 26 per cent of the conductivity of copper, the section of the
bronze must be 3.84 times that of the copper wire if their lengths and
resistance are to be equal. An annealed iron wire is equal in resistance
to a copper wire of the same length when the iron has 7.14 times the
section of the copper. Steel, with 11 per cent of the conductivity of
copper, must have 9.09 times the copper section in order that wires of
the same length may have equal resistances.
It is not desirable to use a copper wire smaller than No. 4 B. & S.
gauge for transmission lines, because of the lack of tensile strength in
smaller sizes. When the conductivity of a copper wire smaller than No. 4
is ample, an iron wire will give the required conductivity, with a
strength far greater than that of the copper. For a line of given length
and conductivity of any other metal the weight compared with that of a
copper line is represented by the product of the figures for relative
section of the two lines and of the weight of unit mass of the metal in
question compared with that of copper.
Thus, for the same conductivity the weight of a certain length of iron
wire is 0.87 × 7.14 = 6.21 times the weight of a copper wire. For the
steel wire above named the weight is 0.89 × 9.09 = 8.09 times that of a
copper line of equal conductivity. Phosphor-bronze in a line of given
length and resistance has 3.84 times the weight of soft copper.
Silicon-bronze for a transmission line must weigh from 1.02 to 2.17
times as much as soft copper for a given length and conductivity.
Aluminum for a line of fixed length and conductivity will weigh 1.66 ×
0.3 = 0.5 times as much as copper. For a line of fixed length and
resistance, hard-drawn copper will weigh about two per cent more than
soft copper.
Taking the tensile strength of soft copper at 34,000 pounds per square
inch, hard-drawn copper at 45,000 to 70,000, silicon-bronze at 60,000 to
100,000, phosphor-bronze at 100,000, iron at 55,000, steel at 100,000,
and aluminum at 30,000 pounds, the relative strengths of wires with
equal sectional areas compared with the soft copper are, for hard-drawn
copper, 1.32 to 2.06; silicon-bronze, 1.76 to 2.94; phosphor-bronze,
2.94; iron, 1.62; steel, 2.94; and for aluminum, 0.88.
Comparing wires on the basis of equal resistances for equal lengths,
with soft copper again the standard, the tensile strength of each as to
it is as follows: A hard-drawn copper line has 1.02 × 1.32 = 1.34 to
1.02 × 2.06 = 2.10 times the strength of a line of soft copper. With
silicon-bronze the strength of line wire would range between 1.02 × 1.76
= 1.79 and 2.17 × 2.94 = 6.38 times that of copper. Iron would give the
line a strength as to soft copper represented by 7.14 × 1.62 = 11.56.
Steel of 100,000 pounds tensile strength per square inch will give a
line 9.09 × 2.94 = 26.70 times as strong as it would be if composed of
soft copper. With aluminum the strength of the line would be 1.66 × 0.88
= 1.46 times that of copper. For phosphor-bronze the figures are 3.84 ×
2.94 = 11.29.
From the foregoing it may be shown how many times the price of soft
copper per pound may be paid for each of the other metals to form a line
of given length and resistance at a cost equal to that of a soft copper
line. These prices per pound for the several metals relative to that of
soft copper are as follows: Taking the price of soft copper as one, the
price for hard-drawn copper must be 1 ÷ 1.02 = 0.98. For silicon-bronze
the price may be as high as 1 ÷ 1.02 = 0.98, or as low as 1 ÷ 2.17 =
0.46 of the price of soft copper wire. Phosphor-bronze may have a price
represented by only 1 ÷ 3.84 = 0.26 that of copper. The price of iron
wire should be 1 ÷ 6.21 = 0.16 of that of copper, and for steel wire of
the quality stated the price can only be 1 ÷ 8.01 = 0.12. Aluminum wire
alone may have a higher price per pound than soft copper for the same
resistance and cost of line, the figure for the relative cost of this
metal being 1 ÷ 0.5 = 2.
From the foregoing it appears that for a line of given cost, length, and
resistance, soft copper has the least cross-section and tensile
strength; steel, the greatest cross-section, weight, tensile strength,
and lowest permissible price per pound; and aluminum, the least weight
and highest price per pound.
RELATIVE PROPERTIES OF WIRES HAVING EQUAL LENGTHS AND RESISTANCES.
+--------------------+---------+--------+----------+----------+
| |Relative |Relative| Relative | Relative |
| Metal in Wire. | Cross |Weights.| Tensile |Prices per|
| |Sections.| |Strengths.|Pound for |
| | | | |Same Total|
| | | | | Cost. |
+--------------------+---------+--------+----------+----------+
|Soft Copper | 1.00 | 1.00 | 1. | 1.00 |
|Hard Copper | 1.02 | 1.04 | 1.34 | .98 |
|Very Hard Copper | 1.02 | 1.02 | 2.10 | .98 |
|No. 1 Silicon-Bronze| 1.02 | 1.02 | 1.79 | .98 |
|No. 2 Silicon-Bronze| 2.17 | 2.17 | 6.38 | .46 |
|Aluminum | 1.66 | .50 | 1.46 | 2.00 |
|Phosphor-Bronze | 3.84 | 3.84 | 11.29 | .26 |
|Annealed Iron | 7.14 | 6.21 | 11.56 | .16 |
|Mild Steel | 9.09 | 8.09 | 26.70 | .12 |
+--------------------+---------+--------+----------+----------+
The relative cross sections and weights of both iron and steel wires are
so great as to prevent their general use because of the labor and cost
of their erection.
So far as the first cost of the wire alone is concerned, iron may be
approximately equal to copper in some metal markets. The only practical
place for an iron wire, however, is one where copper would be too small
or not strong enough. Steel wire finds a place, in spite of its high
resistance, in those exceptional cases where a single span of several
thousand feet must be made, requiring high tensile strength. In such
cases it is usually better to give the steel span a greater resistance
than an equal length of the main portion of the line, so as to avoid
excessive size and weight of the span. Even when this is done the
resistance of the steel span would be very small compared with that of a
long transmission line.
Phosphor-bronze finds little use as conductors in transmission systems
because of its relatively high electrical resistance. If great tensile
strength is wanted, iron or steel will supply it at a fraction of the
cost of phosphor-bronze. As a conductor simply, phosphor-bronze is worth
only 0.26 as much per pound as soft copper, while its actual market
price is greater than that of copper.
Silicon-bronze of relatively high resistance, requiring 2.17 times the
section and weight of copper for equal conductivity, is entitled to
little or no consideration as a transmission line material. This alloy,
in order to give equal conductivity at equal cost with copper, must sell
at only 0.46 of the price of copper per pound. But the price of
silicon-bronze is equal to, or greater than, the price of copper, so
that the cost of the high-resistance silicon-bronze for a line of given
resistance will be more than twice that of copper. For this more than
double cost the bronze gives 6.38 times the tensile strength of a soft
copper line of equal conductivity.
Taking the market price of steel at one-fifth that of copper, which is
amply high for the steel, as a rule, a steel wire of equal conductivity
with the copper will cost only 1.6 times as much and will have 26.7
times the tensile strength of the copper, or four times the tensile
strength of a wire of equal conductivity made from the high-resistance
silicon-bronze. From this it is clear that steel offers a cheaper
combination of conductivity and strength than does silicon-bronze of
high resistance. That grade of silicon-bronze having the lowest
resistance may cost 0.98 as much per pound as soft copper, and will have
1.79 times the strength of the copper for equal conductivity. This
bronze actually costs more per pound than copper, so that it cannot give
equal conductivity at equal cost.
Very hard-drawn copper has a conductivity equal to that of the best
silicon-bronze, and the tensile strength of this copper is seventeen per
cent greater than that of the bronze. Silicon-bronze costs more per
pound than hard copper, but even with equal prices the hard copper gives
equal conductivity and higher strength at the same cost. Furthermore,
the conductivity of silicon-bronze is much more liable to serious
variations than that of hard copper. Between hard-drawn copper and steel
there is very little apparent place for any grade of bronze in electric
transmission lines.
The hardest copper wire is very stiff, and is more liable to crack when
twisted or bent than is wire of only medium hardness. Such medium-hard
copper has a tensile strength of thirty-four per cent greater than soft
copper of equal conductivity, and is much used on long transmission
lines. Aluminum is the only metal which, for given conductivity in a
transmission line, combines a smaller weight, a greater tensile
strength, and a higher permissible price than soft copper for the same
total cost. For equal conductivity an aluminum wire has a greater
tensile strength than one of medium-hard copper, and costs less than
copper of any grade when the price per pound of the aluminum is less
than twice that of copper, which is usually the case.
These properties make aluminum by far the most important competitor of
copper in electric transmission and have led to its use in a number of
cases, notably for the two longest lines in the world, namely, between
Colgate and Oakland and between Electra and San Francisco, in
California.
It has not been found practicable to solder joints in aluminum wires
because of the resulting electrolytic action when aluminum is in contact
with other metals. Joints of aluminum wires are usually made by slipping
the ends past each other in an oval aluminum sleeve and then giving the
sleeve and wires two or three complete twists, or by a process of cold
welding with a sleeve joint.
Long transmission lines are in nearly all cases run with bare wire
supported by poles. Where very high voltages are employed no insulation
that can be put on the wire will make it safe to handle, and the cost of
such insulation would add materially to that of the entire line. It is,
therefore, the practice to run transmission lines above all other wires
and to rely entirely on the supports for insulation.
The considerations thus far noted apply alike to wires carrying
continuous and alternating currents, but there are some other factors
that apply solely to alternating lines. Owing to the inductive effects
of alternating currents in long, parallel wires, such wires should be
transposed between their supports at frequent intervals. The induction
between wires increases with the frequency of the current carried, and
decreases with the distance between the wires. According to these
conditions, wires should be transposed as often as every eighth of a
mile in some cases, and at intervals of one mile or more in others.
An alternating current when passing along a line tends to concentrate
itself in the outer layers of the wire, leaving the centre idle. This
unequal current distribution increases with the frequency of the current
and with the area of the cross section of the wire. The practical effect
of this unequal distribution is to make the resistance of a wire a
little higher for alternating than for continuous currents. In existing
transmission lines the increase of resistance due to this cause seldom
amounts to one per cent.
When an alternating current passes through a circuit, the action termed
self-induction sets up an electromotive force in the circuit that
opposes the flow of current, as does the resistance of the wire, and
this is called the inductance of the circuit. The ratio of this
inductance to the resistance of a circuit increases with the number of
periods per second of the alternating current used and with the
sectional area of the wires composing the circuit. For a circuit of No.
6 B. & S. gauge wire the inductance amounts to only five per cent of the
line resistance, but for a circuit of No. 000 wire the inductance
consumes as much of the applied voltage as does the resistance, with
60-cycle current.
Both the unequal distribution of alternating current over the
cross-section of a conductor and the inductance of circuits make it
desirable to keep the diameters of transmission wires as small as other
considerations permit. As soft copper has greater conductivity per unit
of area than any of the other available metals, it clearly has an
advantage over all of them as to inductance and increase of resistance
with alternating current.
At very high voltages there is an important leakage of energy between
the conductors of a circuit, and this loss varies inversely with the
distance between these conductors. Thus it happens that inductance makes
it desirable to bring the parallel wires of a circuit close together,
while the leakage of energy from wire to wire makes it desirable to
carry them far apart.
To provide greater security from interruption, the conductors for
important transmissions are in some cases carried on two independent
pole lines. Even where all the conductors are on a single line of poles
it is frequent practice to divide them up into a number of comparatively
small wires, and this decreases inductance.
Data of a number of transmission lines presented in the appended table
illustrate the practice in some of the more recent and important cases
as to the materials, size, number, and arrangement of the wires. The
plants of which particulars are given include the greatest power
capacities, the longest distances, and the highest voltages now involved
in electrical transmissions. Each of the lines named is worked with
alternating current of two- or three-phase. Each three-phase line must
have at least three wires, and each two-phase line usually has four
wires.
On ten of the lines the number of wires is greater than three or four,
thus reducing the necessary size of each wire for a given conductivity
of the line. The Butte, Oakland, and Hamilton lines are run on two sets
of poles for greater security, and a second pole line has been added to
the Niagara and Buffalo system to carry additional wires.
The largest wire used in any of these lines is the aluminum cable of
500,000 circular mils between Niagara Falls and Buffalo. This cable has
1.66 times the area in cross section of a copper cable of equal
conductivity.
SIZES AND MATERIALS OF WIRES ON SOME AMERICAN TRANSMISSION LINES.
+----------------------------+----+------+----------+--------+-------+
| | | | | |Length |
| | |Number| Size of | | of |
| Location of Transmission. |Line|Wires.| Each Wire|Metal in|Trans- |
| | +-+ +--+ B. & S. | Wire. | mis- |
| |Volt- | | Gauge. | | sion. |
| |age. | | | |Miles. |
+----------------------------+------+-+-------------+--------+-------+
|Cañon Ferry to Butte |50,000|6| 0 | Copper | 65 |
|Colgate to Oakland |40,000|3| 00 | Copper |142 |
| | |3| 000 |Aluminum|142 |
|Electra to San Francisco |40,000|3|471,034 C. M.| „ |147 |
|Santa Ana R. to Los Angeles |33,000|6| 1 | Copper | 83 |
|Apple River to St. Paul |25,000|6| 2 | „ | 25 |
|Welland Canal to Hamilton |22,500|3| 1 | „ | 35 |
| | |3| 00 | „ | 37 |
|Cañon City to Cripple Creek |20,000|3| 3 | „ | 23-1/2|
|Madrid to Bland |20,000|6| 4 | „ | 32 |
|White River to Dales |22,000|3| 6 | „ | 27 |
|Ogden to Salt Lake City |16,000|6| 1 | „ | 36-1/2|
|San Gabriel Cañon to Los | | | | | |
| Angeles |16,000|6| 5 | „ | 23 |
|To Victor, Col |12,600|3| 4 | „ | 8 |
|Niagara Falls to Buffalo |22,000|6|350,000 C. M.| „ | 23 |
| „ „ „ |22,000|3|500,000 C. M.|Aluminum| 20 |
|Yadkin River to Salem |12,000|3| 1 | Copper | 14.5 |
|Farmington Riv’r to Hartford|10,000|3|336,420 C. M.|Aluminum| 11 |
|Wilbraham to Ludlow Mills |11,500|6|135,247 C. M.| „ | 4.5 |
|Niagara Falls to Toronto |60,000|6|190,000 C. M.| Copper | 75 |
+----------------------------+------+-+-------------+--------+-------+
Aluminum lines are now employed for the three longest electrical
transmissions in North America. In the longest single line, that from
Electra power-house to San Francisco, a distance of 147 miles, aluminum
is the conductor used. The 142-mile transmission between Colgate and
Oakland is carried out with three aluminum and three copper line wires.
For the third transmission in point of length, that from Shawinigan
Falls to Montreal, a distance of 85 miles, three aluminum conductors are
employed.
The three transmissions just named have unusually large capacities as
well as superlative lengths, the generators in the Electra plant being
rated at 10,000, in the Colgate plant at 11,250, and in the Shawinigan
plant at 7,500 kilowatts. Weight and cost of such lines are very large.
For the three No. 0000 aluminum conductors, 142 miles each in length,
between Colgate and Oakland, the total weight must be about 440,067
pounds, costing $132,020 at 30 cents per pound. Between Electra and
Mission San José, where the line branches, is 100 miles of the 147-mile
transmission from Electra to San Francisco. On the Electra and Mission
San José section the aluminum conductors comprise three stranded cables
of 471,034 circular mils each in sectional area and with a total weight
of about 721,200 pounds. This section alone of the line in question
would have cost $216,360 at 30 cents per pound. The 85-mile aluminum
line from Shawinigan Falls to Montreal is made up of three-stranded
conductors each with a sectional area of 183,708 circular mils. All
three conductors have a combined weight of about 225,300 pounds, and at
30 cents per pound would have cost $67,590.
Aluminum lines are not confined to new transmissions, but are also found
in additions to those where copper conductors were at first used. Thus,
the third transmission circuit between the power-house at Niagara Falls
and the terminal house in Buffalo, a distance of 20 miles by the new
pole line, was formed of three aluminum cables each with an area of
500,000 circular mils, though the six conductors of the two previous
circuits were each 350,000 circular mils copper.
From these examples it may be seen that copper has lost its former place
as the only conductor to be seriously considered for transmission
circuits. Aluminum has not only disputed this claim for copper, but has
actually gained the most conspicuous place in long transmission lines.
This victory of aluminum has been won in hard competition. The decisive
factor has been that of cost for a circuit of given length and
resistance.
From the standpoint of cross-sectional area aluminum is inferior to
copper as an electrical conductor. Comparing wires of equal sizes and
lengths, the aluminum have only sixty per cent of the conductivity of
the copper, so that an aluminum wire must have 1.66 times the sectional
area of a copper wire of the same length in order to offer an equal
electrical resistance. As round wires vary in sectional areas with the
squares of their diameters, an aluminum wire must have a diameter 1.28
times that of a copper wire of equal length in order to offer the same
conductivity.
The inferiority of aluminum as an electrical conductor in terms of
sectional area is more than offset by its superiority over copper in
terms of weight. One pound of aluminum drawn into a wire of any length
will have a sectional area 3.33 times as great as one pound of copper in
a wire of equal length. This follows from the fact that the weight of
copper is 555 pounds while that of aluminum is only 167 pounds per cubic
foot, so that for equal weights the bulk of the latter is 3.33 times
that of the former metal. As the aluminum wire has equal length with and
3.33 times the sectional area of the copper wire of the same weight,
the electrical conductivity of the former is 3.33 ÷ 1.66 = 2 times that
of the latter. Hence, for equal resistances, the weight of an aluminum
is only one-half as great as that of a copper wire of the same length.
From this fact it is evident that when the price per pound of aluminum
is anything less than twice the price of copper, the former is the
cheaper metal for a transmission line of any required length and
electrical resistance.
The tensile strength of both soft copper and of aluminum wire is about
33,000 pounds per square inch of section. For wires of equal length and
resistance the aluminum is therefore sixty-six per cent stronger because
its area is sixty-six per cent greater than that of a soft copper wire.
Medium hard-drawn copper wire such as is most commonly used for
transmission lines has a tensile strength of about 45,000 pounds per
square inch, but even compared with this grade of copper the aluminum
wire of equal length and resistance has the advantage in tensile
strength. While the aluminum line is thus stronger than an equivalent
one of copper, the weight of the former is only one-half that of the
latter, so that the distance between poles may be increased, or the
sizes of poles, cross-arms, and pins decreased with aluminum wires. In
one respect the strain on poles that carry aluminum may be greater than
that on poles with equivalent copper lines, namely, in that of wind
pressure. A wind that blows in a direction other than parallel with a
transmission line tends to break the poles at the ground and prostrate
the line in a direction at right angles to its course. The total wind
pressure in any case is obviously proportional to the extent of the
surface on which it acts, and this surface is measured by one-half of
the external area of all the poles and wires in a given length of line.
As the aluminum wire must have a diameter twenty-eight per cent greater
than that of copper wire of equal length, one-half of the total wire
surface will also be twenty-eight per cent greater for the former metal.
This carries with it an increase of twenty-eight per cent in that
portion of the wind pressure due to wire surface. In good practice the
number of transmission wires per pole line is often only three, and
seldom more than six, so that the surface areas of these wires may be no
greater than that of the poles. It follows that the increase of
twenty-eight per cent in the surface of wires may correspond to a much
smaller percentage of increase for the entire area exposed to wind
pressure. Such small difference as exists between the total wind
pressures on aluminum and copper lines of equal conductivity is of
slight importance in view of the general practice by which some straight
as well as the curved portions of transmission lines are now secured by
guys or struts at right angles to the direction of the wires.
Vibration of transmission lines and the consequent tendency of
cross-arms, pins, insulators, and of the wires to work loose is less
with aluminum than with copper conductors as ordinarily strung, because
of the greater sag between poles given the former and also probably
because of their smaller weight. An illustration of this sort may be
seen on the old and new transmission lines between Niagara Falls and
Buffalo. The two old copper circuits consist of six cables of 350,000
circular mils section each on one line of poles, and are strung with
only a moderate sag. In a strong wind these copper conductors swing and
vibrate in such a way that their poles, pins, and cross-arms are thrown
into a vibration that tend to work all attachments loose. The new
circuit consists of three 500,000 circular mil aluminum conductors on a
separate pole line strung with a large sag between poles, and these
conductors take positions in planes at large angles with the vertical in
a strong wind, but cause little or no vibration of their supports. One
reason for the greater sag of the aluminum over that of the copper
conductors in this case is the fact that the poles carrying the former
are 140 feet apart while the distance between the poles for the latter
is only seventy feet, on straight sections of the line.
The necessity for greater sag in aluminum than in copper conductors,
even where the span lengths are equal, arises from the greater
coefficient of expansion possessed by the former metal. Between 32° and
212° Fahrenheit aluminum expands about 0.0022, and copper 0.0016 of its
length, so that the change in length is 40 per cent greater in the
former than in the latter metal. The conductors in any case must have
enough sag between poles to provide for contraction in the coldest
weather, and it follows that the necessary sag of aluminum wires will be
greater than that of copper at ordinary temperature.
In pure air aluminum is even more free from oxidation than copper, but
where exposed to the fumes of chemical works, to chlorine compounds, or
to fatty acids the metal is rapidly attacked. For this reason aluminum
conductors should have a water-proof covering where exposed to any of
these chemicals. The aluminum line between Niagara Falls and Buffalo is
bare for most of its length, but in the vicinity of the large chemical
works at the former place the wires are covered with a braid treated
with asphaltum. Aluminum alloyed with sodium, its most common impurity,
is quickly corroded in moist air, and should be carefully avoided. All
of the properties of aluminum here mentioned relate to the pure metal
unless otherwise stated, and its alloys should not, as a rule, be
considered for transmission lines. As aluminum is electropositive to
most other metals the soldering of its joints is quite sure to result in
electrolytic corrosion, unless the joints are thoroughly protected from
moisture, a result that is hard to attain with bare wires. The regular
practice is to avoid the use of solder and rely on mechanical joints. A
good joint may be made by slipping the roughened ends of wires to be
connected through an aluminum tube of oval section, so that one wire
sticks out at each end, then twisting the tube and wires and giving each
of the latter a turn about the other. Aluminum may be welded
electrically and also by hammering at a certain temperature, but these
processes are not convenient for line construction. Especial care is
necessary to avoid scarring or cutting into aluminum wires, as may be
done when they are tied to their insulators. Aluminum tie wires should
be used exclusively. To avoid the greater danger of damage to solid
wires and also to obtain greater strength and flexibility, aluminum
conductors are most frequently used in the form of cables. The sizes of
wires that go to make up these cables commonly range from No. 6 to 9 B.
& S. gauge for widely different cable sections. Thus the 183,708
circular mil aluminum cable between Shawinigan Falls and Montreal is
made up of seven No. 6 wires, and the 471,034 circular mil cable between
Electra and Mission San José contains thirty-seven No. 9 wires. From the
Farmington River to Hartford each 336,420 circular mils cable has
exceptionally large strands of approximately No. 3 wire. It appears from
the description of a 43-mile line in California (vol. xvii., A. I. E.
E., p. 345) that a solid aluminum wire of 294 mils diameter, or No. 1 B.
& S. gauge, can be used without trouble from breaks. This wire was
tested and its properties reported as follows:
Diameter, 293.9 mils. Pounds per mile, 419.4. Resistance per mil
foot, 17.6 ohms at 25° C. Resistance per mile at 25° C., 1.00773
ohms. Conductivity as to copper of same size, 59.9 per cent. Number
of twists in six inches for fracture, 17.9. Tensile strength per
square inch, 32,898 pounds.
This wire also stood the test of wrapping six times about its own
diameter and then unwrapping and wrapping again. It was found in tests
for tensile strength that the wire in question took a permanent set at
very small loads, but that at points between 14,000 and 17,000 pounds
per square inch the permanent set began to increase very rapidly. From
this it appears that aluminum wires and cables should be given enough
sag between poles so that in the coldest weather the strains on them
shall not exceed about 15,000 pounds per square inch. This rather low
safe working load is a disadvantage that aluminum shares with copper.
From the figures just given it is evident that the strains on aluminum
conductors during their erection should not exceed one-half of the
ultimate strength in any case, lest their sectional areas be reduced.
ALUMINUM CABLES IN TRANSMISSION SYSTEMS.
+----------------------------+-------+-----+--------+-------+----------+
| | | | | | Size of |
| | Number|Miles|Circular|Strands| Strands. |
| Locations. | of | of |Mils of | per |B. & S. G.|
| |Cables.|Each.| Each. |Cable. | Approx- |
| | | | | | imate. |
+----------------------------+-------+-----+--------+-------+----------+
|Niagara Falls to Buffalo | 3 | 20 | 500,000| .. | .. |
|Shawinigan Falls to Montreal| 3 | 85 | 183,708| 7 | 6 |
|Electra to Mission San José | 3 |100 | 471,034| 37 | 9 |
|Colgate to Oakland | 3 |144 | 211,000| 7 | 5-6 |
|Farmington River to Hartford| 3 | 11 | 336,420| 7 | 3 |
|Lewiston, Me. | 3 | 3.5| 144,688| 7 | 8 |
|Ludlow, Mass. | 6 | 4.5| 135,247| 7 | 7 |
+----------------------------+-------+-----+--------+-------+----------+
This table of transmission systems using aluminum conductors is far from
exhaustive. Aluminum is also being used to distribute energy to the
sub-stations of long electric railways, as on the Aurora and Chicago
which connects cities about forty miles apart. The lower cost of
aluminum conductors is also leading to their adoption instead of copper
in city distribution of light and power. Thus at Manchester, N. H., the
local electric lines include about four miles each of 500,000 and
750,000 circular mil aluminum cable with weather-proof insulation. The
larger of these cables contains thirty-seven strands of about No. 7
wire.
As may be seen from the foregoing facts, the choice of copper or
aluminum for a transmission line should turn mainly on the cost of
conductors of the required length and resistance in each metal. So
nearly balanced are the mechanical and electrical properties of the two
metals that not more than a very small premium should be paid for the
privilege of using copper. As already pointed out, the costs of aluminum
and copper conductors of given length and resistance are equal when the
price per pound of aluminum wire is twice that of copper. During most of
the time for several years the price of aluminum has been well below
double the copper figures, and the advantage has been with aluminum
conductors. With the two metals at the same price per pound aluminum
would cost only one-half as much as equivalent copper conductors. When
the price of aluminum is fifty per cent greater per pound than that of
copper, the use of the former metal effects a saving of twenty-five per
cent. For the new Niagara and Buffalo line, completed early in 1901,
aluminum was selected because it effected a saving of about twelve per
cent over the cost of copper. All of the aluminum lines here mentioned,
except the short one near Hartford, were completed during or since 1900.
Most of the facts here stated as to the line between Niagara Falls and
Buffalo are drawn from vol. xviii., A. I. E. E., at pages 520 and 521.
The greater diameter of aluminum over equivalent copper conductors has
advantages in transmission with alternating current at very high
voltages. At high voltages, say of 40,000 or more, the constant silent
loss of energy from one conductor to another of the same circuit through
the air tends to become large and even prohibitive in amount. This loss
is greater, other factors being constant, the smaller the diameter of
the conductors in the line. It follows that this loss is more serious
the smaller the power to be transmitted, because the smaller the
diameter of the line wires. The silent passage of energy from wire to
wire increases directly with the length of line and thus operates as a
limit to long transmissions.
CHAPTER XVI.
VOLTAGE AND LOSSES ON TRANSMISSION LINES.
The voltage on a transmission line may be anything up to at least
60,000, and the weight of conductors varies inversely with the square of
the figures selected, the power, length and loss being constant.
Whatever the total line pressure, the weight of conductors varies
inversely with the percentage of loss therein.
The case of maximum loss and minimum weight of conductors is that in
which all of the transmitted energy is expended in heating the line
wires. Such a case would never occur in practice, because the object of
power transmission is to perform some useful work.
Minimum loss is theoretically zero, and the corresponding weight of
conductors is infinite, but these conditions obviously cannot be
attained in practice. Between these extremes of minimum and of infinite
weights of conductors comes every practical transmission with a line
loss greater than zero and less than 100 per cent.
To determine the weight and allowable cost of conductors, the cost of
the energy that will be annually lost in them enters as one of the
factors. At this point the distinction between the percentage of power
lost at maximum load and the percentage of total energy lost should come
into view.
Line loss ordinarily refers to the percentage of total power consumed in
the conductors at maximum load. This percentage would correspond with
that of total energy lost if the line current and voltage were constant
during all periods of operation, but this is far from the case.
A system of transmission may operate with either constant volts or
constant amperes on the line conductors, but in a practical case
constancy of both these factors is seldom or never to be had. This is
because the product of the line volts and amperes represents accurately
in a continuous-current system, and approximately in an
alternating-current system, the amount of power transmitted. In an
actual transmission system, the load--that is, the demand for power--is
subject to more or less variation at different times of the day, and the
line volts or amperes, or both, must vary with it.
If the transmission system is devoted to the operation of one or more
factories the required power may not vary more than twenty-five per cent
during the hours of daily use; but if a system of general electrical
supply is to be operated, the maximum load will usually be somewhere
between twice and four times as great as the average load for each
twenty-four hours. Such fluctuating loads imply corresponding changes in
the volts or amperes of the transmission line.
A number of rather long transmissions is carried out in Europe with
continuous, constant current, and in such systems the line voltage
varies directly with the load. As the loss of power in an electrical
conductor depends entirely on its ohms of resistance, which are constant
at any given temperature, and on the amperes of current passing through
it, the line loss in a constant-current system does not change during
the period of operation, no matter how great may be its changes of load.
For this reason the percentage of power loss in the line at maximum load
is usually smaller than the percentage of energy loss for an entire day.
If, for example, the constant-current transmission line is designed to
convert into heat 5 per cent of the maximum amount of energy that will
be delivered to it per second--that is, to lose 5 per cent of its power
at maximum load--then, when the power which the line receives drops to
one-half of its maximum, the percentage of loss will rise to 10, because
0.05 ÷ 0.5 = 0.1. So again, when the power sent through the line falls
to one-quarter of the full amount, the line loss will rise to 0.05 ÷
0.25 = 0.2, or 20 per cent.
From these facts it is clear that a fair all-day efficiency for a
constant-current transmission line can be obtained only in conjunction
with a high efficiency at maximum load, if widely varying loads are to
be operated. It does not necessarily follow from these facts as to
losses in constant-current lines that such losses should always be small
at maximum loads, for if a large loss may be permitted at full load a
still greater percentage of loss at partial loads may not imply bad
engineering.
In a large percentage of electric water-power plants some water goes
over the dam during those hours of the day when loads are light, the
storage capacity above the dam not being sufficient to hold all of the
surplus water during most seasons of the year. If, therefore, the line
loss in a constant-current transmission, where all of the daily flow of
water cannot be used, is not great enough to reduce the maximum load
that would otherwise be carried, then the fact that the percentage of
line loss at small loads is still larger is not very important.
Obviously, it makes little difference whether water goes over a dam or
through wheels to make up for a loss in the line. In a case where all
the water can be stored during small loads and used during heavy loads,
it is clearly desirable to keep the loss in a constant-current line down
to a rather low figure, say not more than five per cent, at maximum
load.
Much the greater number of electrical transmissions are carried out with
nearly constant line voltage, mostly alternating, and the line current
in such cases varies directly with the power transmitted, except as to
certain results of inductance on alternating lines. As line resistance
is constant, save for slight variations due to temperature, the rate of
energy loss on a constant-pressure line varies with the square of the
number of amperes flowing, and the percentage of loss with any load
varies directly as the number of amperes.
These relations between line losses and the amperes carried follow from
the law that the power, or rate of work, is represented by the product
of the number of volts by the number of amperes, and the law that the
power actually lost in the line is represented by the product of the
number of ohms of line resistance and the square of the number of
amperes flowing in it. In each of these cases the power delivered to the
line is, of course, measured in watts, each of which is 1/746 of a
horse-power.
Applying these laws, it appears that if the loss of a certain
constant-pressure transmission line is 10 per cent of the power
delivered to it at full load, then, when the power, and consequently the
amperes, on the line is reduced one-half, the watts lost in the line as
heat will be (1/2)² = 1/4 of the watts lost at full load, because the
number of amperes flowing has been divided by 2.
But the amount of power delivered to the line at full load having been
reduced by 50 per cent, while the power lost on the line dropped to
one-fourth of 10 per cent, or to 2.5 per cent of the full line load, it
follows that the power lost on the line at half-load is represented by
0.025 ÷ 0.5 = 0.05, or 5 per cent of the power then delivered to it.
This rise in the efficiency of a constant-pressure transmission line as
the power delivered to it decreases, together with the fact that maximum
loads on such lines continue during hardly more than one to two hours
daily, tends to raise the allowable percentage of line loss at maximum
loads.
This is so because a loss of fifteen per cent at maximum load may easily
drop to an average loss of somewhere between five and ten per cent for
the entire amount of energy delivered to a line during each day under
ordinary conditions in electrical supply. In the practical design of
transmission lines, therefore, the sizes of conductors are influenced by
the relation of the largest load to be operated to the greatest amount
of power available for its operation, and by questions of regulation, as
well as by considerations of all-day efficiency.
If the maximum load that must be carried by a transmission system during
a single hour per day requires nearly as much power as can be delivered
to the line conductors, either because of lack of water storage or of
water itself, even if it is stored, it may be desirable to design these
conductors for a small loss at maximum load, rather than to install a
steam plant.
So again, as the fluctuation in voltage at the delivery end of a
transmission line between no load and full load will amount to the
entire drop of volts in the line at full load, if the pressure at the
generating end is constant, the requirements of pressure regulation on
distribution circuits limit the drop of pressure in the transmission
conductors. For good lighting service with incandescent lamps at about
110 volts, the usual pressure, it is necessary that variations be held
within one volt either way of the pressure of the lamps--that is,
between 109 and 111 volts.
Every long-transmission system for general electrical supply necessarily
includes one or more sub-stations where the distribution lines join the
transmission circuits, and where the voltage for lighting service is
regulated. As the limits of voltage variations on lighting circuits are
so narrow, it is necessary to keep the changes of pressure on the
transmission lines themselves within moderate limits, or such as can be
compensated for at sub-stations.
This is particularly true in cases where energy transmitted over a
single circuit is distributed for both incandescent lamps and large
electric motors, because the starting and operation of such motors
causes large fluctuations of amperes and terminal voltage on the
transmission circuits. To hold such fluctuations within limits which a
sub-station can readily compensate for, it is necessary that the loss in
the transmission line be moderate, say often within ten per cent of the
total voltage delivered to it at maximum load.
Capacity and cost of equipment at generating stations go up with the
percentage of line loss, and thus serve to limit its economical amount.
For every horse-power delivered to a transmission line at a water-power
station there must be somewhat more than one horse-power of capacity in
water-wheels, at least one horse-power in generators, and frequently a
further capacity of one horse-power in step-up transformers. Every
additional horse-power lost in the line at maximum load, if the
generating plant is to be worked up to its full capacity, implies an
addition of somewhat more than one horse-power capacity in water-wheels,
one horse-power in generators, and one horse-power in transformers.
Since the cost of a generating station is thus increased as the maximum
line loss is raised, a point may be reached where any further saving in
the cost of the line is more than offset by the corresponding addition
to the cost of the station and of its operation. Just where this point,
as indicated by a percentage of line loss, is to be found depends on the
factors of each case, important among which is the length of the
transmission line.
Much effort has been made to fix some exact relation for maximum economy
between the first cost of conductors for a transmission line and the
amount of energy annually lost as heat therein. The best-known statement
applying to this case is that of Lord Kelvin, made in a paper read
before the British Association in 1881. According to the rule there laid
down, the most economical size for the conductors of a transmission line
is that for which the annual interest on first cost equals the cost of
the energy annually wasted in them.
If transmission systems were designed for the sole purpose of wasting
energy in their line conductors this rule would exactly apply, for it
simply shows how the cost of energy wasted, plus the interest on the
cost of the conductor in which it is wasted, may be brought to a
minimum. As a matter of fact, transmission systems are primarily
intended to deliver energy rather than to waste it; but of the
proportions of the entire energy to be delivered and wasted (which is
exactly what we want to know), the rule of Kelvin takes no account.
According to his rule, the cheaper the cost of power where it is
developed, the less should be paid for conductors to bring it to market.
The obvious truth is that the less the cost of power development at a
particular point, the more may be invested in a line to bring it to
market. If power cost nothing whatever at its source it would not be
worth while to build any transmission line at all if this rule is
correct.
A modification of Lord Kelvin’s rule has been proposed by which it is
said that the interest on the cost of the conductors and the annual
value of the energy lost in them should be equal, value here meaning
what the energy can be sold for. This rule would make an investment in
line conductors too large.
The entire cost of production and transmission for the delivered energy
should not be greater than the cost of a like amount of energy
developed at the point where the delivery is made. In this entire cost
of production and transmission, interest on the investment in line
conductors is only one item.
It is perhaps impossible to state any exact rule for the most economical
relation between the cost of conductors and the loss of energy therein
that will apply to every transmission. A maximum limit to the weight of
conductors may, however, be set for most cases. This limit should not
allow the annual interest and depreciation charges on the investment in
line conductors, plus all other costs of development and transmission,
to raise the total cost of the transmitted energy above the cost of
development for an equal amount of energy at the point where the
transmitted energy is delivered.
While the maximum investment in transmission conductors may be properly
limited in the way just stated, it by no means follows that this maximum
limit should be reached in every case. In the varying requirements of
actual cases, the problem may be to deliver a fixed amount of power at
the least possible cost, or to deliver the largest possible amount of
power at a cost per unit under that of development at the point of use.
Frequently a transmission system has a possible capacity in excess of
present requirements, and a line that would not be too heavy for future
business might put an unreasonable burden of interest charges on present
earnings.
The foregoing considerations apply to the design of conductors for a
transmission line after the voltage at which it is to operate has been
decided on. Quite a different set of facts should influence the
selection of this voltage. A transmission that would be entirely
impracticable with any percentage of line loss that might be selected,
if carried out at some one voltage, might represent a paying business at
some higher voltage and any one of several sizes of line conductors. The
power that could be delivered by a line of practicable cost, operated at
one voltage, might be too small for the purpose in hand, while the
available power at a higher voltage might be ample.
If any given power is to be transmitted with a given percentage of
maximum loss in line conductors, the weight of these conductors will
increase as the square of their length, and decrease as the square of
the full voltage of operation in every case.
Thus, if the length of this transmission is doubled, the weight of the
conductors must be multiplied by four, the voltage remaining the same;
but if the voltage is doubled and the line length remains unchanged, the
weight of conductors must be divided by four. With the length of line
and the voltage of transmission either lowered or raised together, the
weight of the conductors remains fixed, for constant power and loss.
An illustration of this last rule may be drawn from the case of lines
designed to transmit any given power a distance of ten miles at 10,000
volts, and a distance of fifty miles at 50,000 volts, in which the total
weight of conductors would be the same for each line if the percentage
of loss was constant.
This statement of the rule as to proportionate increase of voltage and
distance presents the advantages of high voltages in their most
favorable light. Though a uniform ratio between the voltage of operation
and the length of line allows a constant weight of conductors to be
employed for the transmission of a given power with unchanging
efficiency of conductors, yet other considerations soon limit the
advantage thus obtained.
Important among these considerations may be mentioned the mechanical
strength of line conductors, difficulties of line insulation, losses
between conductors through the air, limits of generator voltages, and
the cost of transformers.
If the ten-mile transmission at 10,000 volts, above mentioned, requires
a circuit of two No. 1/0 copper wires, the total weight of these wires
will be represented by (5,500 × 10 × 2 × 320) ÷ 1,000 = 35,200 pounds,
allowing 5,500 feet of wire per mile of single conductor to provide
something for sag between poles, and 320 pounds being the weight of bare
No. 1/0 copper wire per 1,000 feet.
When the length of line is raised to 50 miles, the two-wire circuit will
contain 5,500 × 50 × 2 = 550,000 feet of single conductor, and since the
voltage is raised to 50,000 at the same time, the total weight of
conductors will be 35,200 pounds as before. The weight of single
conductor per 1,000 feet is therefore only 64 pounds in the 50-mile
line.
A No. 7 copper wire, B. & S. gauge, has a weight of 63 pounds per 1,000
feet, and is the nearest regular size to that required for the 50-mile
line as just found. It would be poor policy to string a wire of this
size for a transmission line, because it is so weak mechanically that
breaks would probably be frequent in stormy weather. The element of
unreliability introduced by the use of this small wire on a 50-mile line
would cost far more in the end than a larger conductor.
As a rule, No. 4 B. & S. gauge wire is the smallest that should be used
on a long transmission line in order to give fair mechanical strength,
and this size has just twice the weight of a No. 7 wire of equal
length. Here, then, is one of the practical limits to the advantages
that may be gained by increasing the voltage with the length of line.
As line voltage goes up, the strain on line insulation increases
rapidly, and the insulators for a circuit operated at 50,000 volts must
be larger and of a much more expensive character than those for a
10,000-volt circuit. In this way a part of the saving in conductors
effected by the use of very high voltages on long lines is offset by the
increased cost of insulation.
Another disadvantage that attends the operation of transmission lines at
very high voltages is the continuous loss of energy by the silent
passage of current through the air between wires of a circuit. This loss
increases at a rapid rate after a pressure between 40,000 and 50,000
volts is reached with ordinary distances between the wires of each
circuit. To keep losses of this sort within moderate limits, and also to
lessen the probability of arcs on a circuit at very high voltage, the
distance of eighteen inches or two feet between conductors that carry
current at 10,000 volts should be increased to six feet or more on
circuits that operate at 50,000 volts.
Such an increase in the distance between conductors makes the cost of
poles and cross-arms greater, either by requiring them to be larger than
would otherwise be necessary or by limiting the number of wires to two
or three per pole and thus increasing the number of pole lines. These
added expenses form another part of the penalty that must be paid for
the use of very high voltages and the attendant saving in the cost of
conductors.
Apparatus grows more expensive as the voltage at which it is to operate
increases, because of the cost of insulating materials and the room
which they take up, thereby adding to the size and weight of the iron
parts.
Generators for alternating current can be had that develop as much as
13,500 volts, but such generators cost more than others of equal power
that operate at between 2,000 and 2,500 volts. These latter voltages are
as high as it is usually thought desirable to operate distribution
circuits and service transformers in cities and towns, so that if more
than 2,500 volts are employed on the transmission line, step-down
transformers are required at a sub-station. For a transmission of more
than ten miles the saving in line conductors by operation at 10,000 to
12,000 volts will usually more than offset the additional cost of
generators designed for this pressure and of step-down transformers. If
the voltage of transmission is to exceed that of distribution, it will
generally be found desirable to carry the former voltage up to 10,000
or 12,000, at least.
As the cost of generators designed for the voltage last named is less
than that of lower voltage generators plus transformers, step-up
transformers should usually be omitted in systems where these pressures
are not exceeded. For alternating pressures above 13,000 to 15,000
volts, step-up transformers must generally be employed. In order that
the saving in the weight of line conductors may more than offset the
additional cost of transformers when the voltage of transmission is
carried above 15,000, this voltage should be pushed on up to as much as
25,000 in most cases.
Power transmission with continuous current has the advantage that the
cost of generators remains nearly the same whatever the line voltage,
and that no transformers are required. Such transmissions are common in
Europe, but have hardly a footing as yet in the United States. The
reason for the uniform cost of continuous-current generators is found in
the fact that they are connected in series to give the desired line
voltage, and the voltage of each machine is kept under 3,000 or 4,000.
As a partial offset to the low cost of the continuous-current generators
and to the absence of transformers, there is the necessity for
motor-generators in a sub-station when current for lighting as well as
power is to be distributed.
In spite of the various additions to the cost of transmission systems
made necessary by the adoption of very high voltages, these additions
are much more than offset by the saving in the cost of conductors on
lines 30, 50, or 100 miles in length. In fact, it is only by means of
voltages ranging from 25,000 to 50,000 that the greatest of these
distances, and others up to more than 140 miles, have been successfully
covered by transmission lines. Above 60,000 volts there has been but
slight practical experience in the operation of transmission lines.
Calculations to determine the sizes of conductors for electric
transmission lines are all based on the fundamental law discovered by
Ohm, which is that the electric current flowing in a circuit at any
instant equals the electric pressure that causes the current divided by
the electric resistance of the circuit itself, or current = pressure ÷
resistance.
Substituting in this formula the units that have been selected because
of their convenient sizes for practical use, it becomes, amperes = volts
÷ ohms, in which the ohm is simply the electrical resistance, taken as
unity, of a certain standard copper bar with fixed dimensions.
The ampere is the unit flow of current that is maintained with the unit
pressure of one volt between the terminals of a one-ohm conductor. When
this formula is applied to the computation of transmission lines the
volts represent the electrical pressure that is required to force the
desired amperes of current through the ohms of resistance in any
particular line, and these volts have no necessary or fixed relation to
the total voltage at which the line may operate. Thus, if the total
voltage of a transmission system is 10,000, it may be desirable to use
500, 1,000, or even 2,000 volts to force current through the line, so
that one of these numbers will represent the actual drop or loss of
volts in the line conductors when the number of amperes that represent
full load is flowing. As it is a law of every electric circuit that the
rate of transformation of electric energy to heat or work in each of its
several parts is directly proportional to the drop of voltage therein,
it follows that a drop of 500 or 1,000 or 2,000 volts in the conductors
of a 10,000-volt transmission line at full load would correspond to a
power loss of five to ten or twenty per cent respectively. Any other
part of 10,000 volts might be selected in this case as the pressure to
be lost in the line. Evidently no formula can give the number of volts
that should be lost in line conductors at full load for a given power
transmission, but this number must be decided on by consideration of the
items of line efficiency, regulation, and the ratio of the available
power to the required load.
Having decided on the maximum loss of volts in the line conductors, and
knowing the full voltage of operation, the power and consequently the
number of amperes delivered to the line at maximum load, the resistance
of the conductors may then be calculated by the formula, amperes = volts
÷ ohms. Thus, if the proposition is to deliver 2,000,000 watts or 2,000
kilowatts to a two-wire transmission line with a voltage of 20,000, the
amperes in each wire must be represented by 2,000,000 ÷ 20,000 = 100.
With a drop of ten per cent or 2,000 volts in the two line conductors,
their combined resistance must be found from 100 = 2,000 ÷ ohms, and the
ohms are therefore twenty. If the combined length of the two conductors
is 200,000 feet, corresponding to a transmission line of a little under
twenty miles, the resistance of these conductors must be 20 ÷ 200 = 0.1
ohm per 1,000 feet. From a wire table it may be seen that a No. 1/0 wire
of copper, B. & S. gauge, with a diameter of 0.3249 inch, has a
resistance of 0.1001 ohm per 1,000 feet at the temperature of 90°
Fahrenheit, a little less at lower temperatures, and is thus the
required size. Obviously, the resistance of twenty ohms is entirely
independent of the length of the line, all the other factors being
constant, and wires of various sizes will be required for other
distances of transmission.
It is often convenient to find the area of cross section for the desired
transmission conductor instead of finding its resistance. This can be
done by substituting in the formula, amperes = volts ÷ ohms, the
expression for the number of ohms in any conductor, and then solving as
before.
Electrical resistance in every conductor varies directly with its
length, inversely with its area of cross section, and also has a
constant factor that depends on the material of which the conductor is
composed. This constant factor is always the same for any given
material, as pure iron, copper, or aluminum, and is usually taken as the
resistance in ohms of a round wire one foot long and 0.001 inch in
diameter, of the material to be used for conductors. Such a wire is said
to have an area in cross section of one circular mil, because the square
of its diameter taken as unity is still unity, that is, 1 × 1 = 1. In
like manner, for the convenient designation of wires by their areas of
cross-section, each round wire of any size is said to have an area in
circular mils equal to the square of its diameter measured in units of
0.001 inch each. Thus, a round wire of 0.1 inch diameter has an area of
100 × 100 = 10,000 circular mils, and a round wire one inch in diameter
has an area of 1,000 × 1000 = 1,000,000 circular mils. The circular mils
of a wire do not express its area of cross section in terms of square
inches, but this is not necessary since the resistance of a wire of one
circular mil is taken as unity. Obviously, the areas of all round wires
are to each other as are their circular mils.
From the foregoing it may be seen that the resistance of any round
conductor is represented by the formula, ohms = _l_ × _s_ ÷ circular
mils, in which _l_ represents the length of the conductor in feet, _s_
is the resistance in ohms of a wire of the same material as the
conductor but with an area of one circular mil and a length of one foot,
and the circular mils are those of the required conductor. Substituting
the quantity, _l_ × _s_ ÷ circular mils, for ohms in the formula,
amperes = volts ÷ ohms, the equation, amperes = volts ÷ (_l_ × _s_ ÷
circular mils), is obtained, and this reduces to circular mils = amperes
× _l_ × _s_ ÷ volts. For any proposed transmission all of the quantities
in this formula are known, except the desired circular mils of the line
conductors. The constant quantity s is about 10.8 for copper, but is
conveniently used as eleven in calculation, and this allows a trifle for
the effects of impurities that may exist in the line wire.
The case above mentioned, where 2,000 kilowatts were to be delivered to
a transmission line at 20,000 volts, and a loss of 2,000 volts at full
load was allowed in the line conductors, may now be solved by the
formula for circular mils. Taking the resistance of a round copper wire
0.001 inch in diameter and one foot long as eleven ohms, and
substituting the 100 amperes, 2,000 volts, and 200,000 feet of the
present case in the formula, gives circular mils = (100 × 200,000 × 11)
÷ 2,000 = 110,000. The square root of this 110,000 will give the
diameter of a copper wire that will exactly meet the conditions of the
case, or the more practical course of consulting a table of standard
sizes of wire will show that a No. 1-0 B. & S. gauge, with a diameter of
0.3249 inch, has a cross section of 105,500 circular mils, or about five
per cent less than the calculated number, and is the size nearest to
that wanted. As this No. 1-0 wire will give a line loss at full load of
about 10.5 per cent, or only one-half of one per cent more than the loss
at first selected, it should be adopted for the line in this case.
The formula just made use of is perfectly general in its application,
and may be applied to the calculation of lines of aluminum or iron or
any other metal just as well as to lines of copper. In order to use the
formula for any desired metal, it is necessary that the resistance in
ohms of a round wire of that metal one foot long and 0.001 inch in
diameter be known and substituted for _s_ in the formula. This
resistance of a wire one foot long and 0.001 inch in diameter is called
the specific resistance of the substance of which the wire is composed.
For pure aluminum this specific resistance is about 17.7, for soft iron
about sixty, and for hard steel about eighty ohms. The use of these
values for _s_ in the formula will therefore give the areas in circular
mils for wires of these three substances, respectively, for any proposed
transmission line. In the same way the specific resistance of any other
metal or alloy, when known, may be applied in the formula.
The foregoing calculations apply accurately to all two-wire circuits
that carry continuous currents, whether these circuits operate with
constant current, constant pressure, or with pressure and current both
variable. Where circuits are to carry alternating currents, certain
other factors may require consideration. Almost all transmissions with
alternating currents are carried out with three-phase three-wire, or
two-phase four-wire, or single-phase two-wire circuits. Of the entire
number of such transmissions, those with the three-phase three-wire
circuits are in the majority, next in point of number come the two-phase
transmissions, and lastly a few transmissions are carried out with
single-phase currents. The voltage of a continuous-current circuit, by
which the power of the transmission is computed and on which the
percentage of line loss is based, is the maximum voltage operating
there; but this is not true for circuits carrying alternating currents.
Both the volts and amperes in an alternating circuit are constantly
varying between maximum values in opposite directions along the wires.
It follows from this fact that both the volts and amperes drop to zero
as often as they rise to a maximum. It is fully demonstrated in books on
the theory of alternating currents, that with certain ideal
constructions in alternating generators, and certain conditions in the
circuits to which they are connected, the equivalent or, as they are
called, the virtual values of the constantly changing volts and amperes
in these circuits are 0.707 of their respective maximum values. Or, to
state the reverse of this proposition, the maximum volts and amperes
respectively in these circuits rise to 1.414 times their equivalent or
virtual values. These relations between maximum and virtual volts and
amperes are subject to some variations with actual circuits and
generators, but the virtual values of these factors, as measured by
suitable volt- and amperemeters, are important in the design of
transmission circuits, rather than their maximum values. When the volts
or amperes of an alternating circuit are mentioned, the virtual values
of these factors are usually meant unless some other value is specified.
Thus, as commonly stated, the voltage of a single-phase circuit is the
number of virtual volts between its two conductors, the voltage of a
two-phase circuit is the number of virtual volts between each pair of
its four conductors, and the voltage of a three-phase circuit is the
number of virtual volts between either two of its three conductors.
Several factors not present with continuous currents tend to affect the
losses in conductors where alternating currents are flowing, and the
importance of such effects will be noted later. In spite of such
effects, the formula above discussed should be applied to the
calculation of transmission lines for alternating currents, and then the
proper corrections of the results, if any are necessary, should be made.
With this proviso as to corrections, the virtual volts and amperes of
circuits carrying alternating currents may be used in the formula in the
same way as the actual volts and amperes of continuous current circuits.
Thus, reverting to the above example, where 2,000 kilowatts was to be
delivered at 20,000 volts to a transmission line in which the loss was
to be 2,000 volts, the kilowatts should be taken as the actual rate of
work represented by the alternating current, and the volts named as the
virtual volts on the line. The virtual amperes will now be 100, as were
the actual amperes of continuous current, and the size of line conductor
for a single-phase alternating transmission will therefore be 1-0, the
same as for the continuous-current line. If the transmission is to be
carried out on the two-phase four-wire system, the virtual amperes in
each of these wires will be fifty instead of 100, as the power will be
divided equally between the two pairs of conductors, and each of these
four wires should have a cross-section in circular mils just one-half as
great as that of the No. 1-0 wire. The required wire will thus be a No.
3 B. & S. gauge, of 52,630 circular mils, this being the nearest
standard size. In weight the two No. 1-0 wires and the four No. 3 wires
are almost equal, and they should be exactly equal to give the same loss
in the single-phase and the two-phase lines. For a three-phase circuit
to make the transmission above considered, each of the three conductors
should have an area just one-half as great as that of each of the two
conductors for a single phase circuit, the loss remaining as before, and
the nearest standard size of wire is again No. 3, as it was for the
two-phase line. This is not a self-evident proposition, but the proof
can be found in books devoted to the theory of the subject. From the
foregoing it is evident that while the single-phase and two-phase lines
require equal weights of conductors, all other factors being the same,
the weight of conductors in the three-phase line is only seventy-five
per cent of that in either of the other two. Neglecting the special
factors that tend to raise the size and weight of alternating-current
circuits, the single-phase and two-phase lines require the same weight
of conductors as does a continuous-current transmission of equal power,
voltage, and line loss. It should be noted that in each of these cases
the factor _l_ in the formula for circular mils denotes the entire
length of the pair of conductors for a continuous-current line, or
double the distance of the transmission with either of the
alternating-current lines.
Having found the circular mils of any desired conductor, its weight per
1,000 feet can be found readily in a wire table. In some cases it is
desirable to calculate the weight of the conductors for a transmission
line without finding the circular mils of each, and this can be done by
a modification of the above formula. A copper wire of 1,000,000 circular
mils weighs nearly 3.03 pounds per foot of its length, and the weight of
any copper wire may therefore be found from the formula, pounds =
(circular mils × 3.03 × _l_) ÷ 1,000,000, in which pounds indicates the
total weight of the conductor, _l_, its total length, and the circular
mils are those of its cross-section. This formula reduces to the form,
circular mils = (1,000,000 × pounds) ÷ (3.03 × _l_) and if this value
for circular mils is substituted in the formula above given for the
cross-section of any wire, the result is (1,000,000 × pounds) ÷ (3.03 ×
_l_) = (_l_ × amperes × 11) ÷ volts. Transposition of the factors in
this last equation brings it to the form, pounds = (3.03 × _l_² ×
amperes × 11) ÷ (1,000,000 × volts), which is the general formula for
the total weight of copper conductors when _l_, the length of one pair,
the total amperes flowing, and the volts lost in the conductors are
known for either a continuous-current, a single-phase, or a two-phase
four-wire line.
If the value of _l_, 200,000, of amperes, 100, and of volts, 2,000, for
the transmission above considered are substituted in the formula for
total weight, just found, the result is pounds = (3.03 (200,000)² × 100
× 11) ÷ (1,000,000 × 2,000), which reduced to pounds = 66,660, the
weight of copper wire necessary for the transmission with either
continuous, single-phase or two-phase current. With three-phase current
the weight of copper in the line for this transmission will be 75 per
cent of the 66,660 pounds just found. One or more two-wire circuits may
be employed for the continuous current or for the single-phase
transmission, and if one such circuit is used the weight for each of the
two wires is obviously 33,660 pounds. For a two-phase transmission two
or more circuits of two wires each will be used, and in the case of two
circuits, if all four of the wires are of equal cross section, as would
usually be the case, the total weight of each is 16,830 pounds. If the
transmission is made with one three-phase circuit, the weight of each of
the three wires is 16,830 pounds, and their combined weight, 50,490
pounds of copper. In each of these transmission lines the length of a
single conductor in one direction is 100,000 feet, or one-half of the
length of the wires in a single two-wire circuit. For the two-wire line
the calculated weight of each conductor amounts to 66,660 ÷ 200 = 333.3
pounds per 1,000 feet. For a two-phase four-wire line and also for a
three-phase three-wire line, the weight of each conductor is 16,830 ÷
100 = 168.3 pounds per 1,000 feet. On inspection of a table of weights
for bare copper wires it may be seen that a No. 1-0 B. & S. gauge wire
has a weight of 320 pounds per 1,000 feet, and being much the nearest
size to the calculated weight of 333 pounds should be selected for the
two-wire circuit. It may also be seen that a No. 3 wire, with a weight
of 159 pounds per 1,000 feet, is the size that comes nearest to the
calculated weight of 168 pounds, and should therefore be employed in the
three-wire and the four-wire circuits, for two- and three-phase
transmissions. Either a continuous-current, single-phase, two-phase, or
three-phase transmission line may of course be split up into as many
circuits as desired, and these circuits may or may not be designed to
carry equal portions of the entire power. In either case the combined
weights of the several circuits should equal those above found, the
conditions as to power, loss, and length of line remaining constant. It
will be noted that the formulæ for the calculation of the circular mils
and for the weight of the conductors in the transmission line lead to
the selection of the same sizes of wires, as they obviously should do.
Several laws governing the relations of volts lost, length and weight of
line conductors, may be readily deduced from the above formulæ.
Evidently the circular mils and weight of line conductors vary inversely
with the number of volts lost in them when carrying a given current, so
that doubling this number of volts reduces the circular mils and weight
of conductors by one-half. If the length of the line changes, the
circular mils of the required conductors change directly with it, but
the weight of these conductors varies as the square of their length.
Thus, if the length of the line conductors is doubled, the cross-section
in circular mils of each conductor is also doubled, and each conductor
is therefore four times as heavy as before for the same current and loss
in volts. Should the length of the conductors and also the number of
volts lost in them be varied at the same rate, the circular mils of each
conductor remain constant, and its weight increases directly with the
distance of transmission. Thus, with the same size of line wire, both
the number of volts lost and the total weight are twice as great for a
100- as for a fifty-mile transmission. If the total weight of conductors
is to be held constant, then the number of volts lost therein must vary
as the square of their length, and their circular mils must vary
inversely as the length. So that if the length of a transmission line is
doubled, the circular mils for conductors of constant weight are divided
by two, and the volts lost are four times as great as before. Each of
these rules assumes that the watts and percentage of loss in the line
are constant.
The above principles and formulæ apply to the design of transmission
lines for either continuous or alternating currents, but where the
alternating current is employed certain additional factors should be
considered. One of these factors is inductance, by which is meant the
counter-electromotive force that is always present and opposed to the
regular voltage in an alternating current circuit. One effect of
inductance is to cut down the voltage at that end of the line where the
power is delivered to a sub-station, just as is also done by the ohmic
resistance of the line conductors. Between the loss of voltage due to
line resistance and the loss due to inductance there is the very
important difference that the former represents an actual conversion of
electrical energy into heat, while the latter is simply the loss of
pressure without any material decrease in the amount of energy. While
the loss of energy in a transmission line depends directly on its
resistance, the loss of pressure due to inductance depends on the
diameter of conductors without regard to their resistance, on the length
of the circuit, the distance between the conductors, and on the
frequency or number of cycles per second through which the current
passes. As a result of these facts, it is not desirable or even
practicable to use inductance as a factor in the calculation of the
resistance or weight of a transmission line. On transmission lines, as
ordinarily constructed, the loss of voltage due to inductance generally
ranges between 25 and 100 per cent of the number of volts lost at full
load because of the resistance of the conductors. This loss through
inductance may be lowered by reducing the diameter of individual wires,
though the resistance of all the circuits concerned in the transmission
remains the same, by bringing the wires nearer together and by adopting
smaller frequencies. In practice the volts lost through inductance are
compensated for by operating generators or transformers in the
power-plant at a voltage that insures the delivery of energy in the
receiving-station at the required pressure. Thus, in a certain case, it
may be desirable to transmit energy with a maximum loss of ten per cent
in the line at full load, due to the resistance of the conductors, when
the effective voltage at the generator end of the line is 10,000, so
that the pressure at the receiving-station will be 9,000 volts. If it
appears that the loss of pressure due to inductance on this line will be
1,000 volts, then the generators should be operated at 11,000 volts,
which will provide for the loss of 1,000 volts by inductance, leave an
effective voltage of 10,000 on the line, and allow the delivery of
energy at the sub-station with a pressure of 9,000 volts, when there is
a ten-percent loss of power due to the line resistance.
Inductance not only sets up a counter-electromotive force in the line,
which reduces the voltage delivered to it by generators or transformers,
but also causes a larger current to flow in the line than is indicated
by the division of the number of watts delivered to it by the virtual
voltage of delivery. The amount of current increase depends on both the
inductance of the line itself and also on the character of its connected
apparatus. In a system with a mixed load of lamps and motors there is
quite certain to be some inductance, but it is very hard to predetermine
its exact amount. Experience with such systems shows, however, that the
increase of line current due to inductance is often not above five and
usually less than ten per cent of the current that would flow if there
were no inductance. To provide for the flow of this additional current,
due to inductance, without an increase of the loss in volts because of
ohmic resistance, the cross section of the line conductors must be
enlarged by a percentage equal to that of the additional current. This
means that in an ordinary case of a transmission with either single,
two, or three-phase alternating current, the circular mils of each line
wire, as computed with the formulæ above given, should be increased by
five to ten per cent. Such increase in the cross section of wires of
course carries with it a like rise in the total weight of the conductors
for the transmission. If wire of the cross section computed with the
formulæ is employed for the alternating current transmission, inductance
in an ordinary case will raise the assumed line loss of power by five to
ten per cent of what it would be if no inductance existed. Thus, with
conductors calculated by the formulæ for a power loss of ten per cent at
full load, inductance in an ordinary case would raise this loss to
somewhere between 10.5 and eleven per cent. As a rule it may therefore
be said that inductance will seldom increase the weight of line
conductors, or the loss of power therein, by more than ten per cent.
When an alternating current flows along a conductor its density is not
uniform in all parts of each cross section, but the current density is
least at the centre of the conductor and increases toward the outside
surface. This unequal distribution of the alternating current over each
cross section of a conductor through which it is passing increases with
the diameter or thickness of the conductor and with the frequency of the
alternating current. By reason of this action the ohmic resistance of
any conductor is somewhat greater for an alternating than for a
continuous current, because the full cross section of the conductor
cannot be utilized with the former current. Fortunately, the practical
importance of this unequal distribution of alternating current over each
cross section of its conductor is usually slight, so far as the sizes of
wires for transmission lines are concerned, because the usual
frequencies of current and diameters of conductors concerned are not
great enough to give the effect mentioned a large numerical value. Thus,
sixty cycles per second is the highest frequency commonly employed for
the current on transmission lines. With a 4-0 wire, and the current
frequency named, the increase in the ohmic resistance for alternating
over that for continuous current does not reach one-half of one per
cent.
Having calculated the circular mils of weight of a transmission line by
the foregoing formulæ, it appears that the only material increase of
this weight required by the use of alternating current is that due to
inductance. This increase cannot be calculated exactly beforehand
because of the uncertain elements in future loads, but experience shows
that it is seldom more than ten per cent of the calculated size or
weight of conductors.
CHAPTER XVII.
SELECTION OF TRANSMISSION CIRCUITS.
Maximum power, voltage, loss, and weight of conductors having been fixed
for a transmission line, the number of circuits that shall make up the
line, and the relations of these circuits to each other, remain to be
determined.
In practice wide differences exist as to the number and relations of
circuits on a single transmission line between two points. Cases
illustrating this fact are the 147-mile transmission from Electra
power-house to San Francisco and the 65-mile transmission between Cañon
Ferry, on the Missouri River and Butte, Mont. At the Electra plant the
generator capacity is 10,000 kilowatts, and the transmission to San
Francisco is carried out over a single pole line that carries one
circuit composed of three aluminum conductors, each with an area in
cross section of 471,000 circular mils. From the generators at Cañon
Ferry, which have an aggregate capacity of 7,500 kilowatts, a part of
the energy goes to Helena over a separate line, and the transmission to
Butte goes over two pole lines that are 40 feet apart. Each of these two
pole lines carries a single circuit composed of three copper conductors,
and each conductor has a cross section of 105,600 circular mils. The
difference in practice illustrated by these two plants is further
brought out by the fact that their voltages are not far apart, as the
Cañon Ferry and Butte line operates at 50,000, and the Electra and San
Francisco line at 60,000 volts.
Economy in the construction of a transmission line points strongly to
the use of a single circuit, because this means only one line of poles,
usually but one cross-arm for the power wires per pole, the least
possible number of pins and insulators, and the smallest amount of labor
for the erection of the conductors. In favor of a single circuit there
is also the argument of greatest mechanical strength in each conductor,
since the single circuit is to have the same weight as that of all the
circuits that may be adopted in its place. Where each conductor of the
single circuit would have a cross section of less than 83,690 circular
mils, if of copper, corresponding to a No. 1 B. & S. gauge wire, the
argument as to mechanical strength is of especial force, since two equal
circuits instead of one, in the case where one circuit of No. 1 wires
would have the required weight, reduce the size of each conductor to No.
4 wire, of 41,740 circular mils cross section, and this is the smallest
wire that it is practicable to use on long lines for mechanical reasons.
Opposed to these arguments for a single circuit are those based on the
supposed greater reliability of two or more circuits, their greater ease
of repair, their more effective means of regulation, and the influence
on inductance of a reduction in the size of conductors.
In spite of the consequent reduction in the size of each conductor, the
use of two or more separate circuits for the same transmission is
sometimes thought to increase its reliability, because in case of a
break or short-circuit on one of the circuits the other will still be
available. Breaks in transmission conductors are due either to
mechanical strains alone, as wind pressure, the falling of trees, or the
accumulation of ice, or else to an arc between the conductors that tends
to melt them at some point. As a smaller conductor breaks or melts more
readily than a large one, the use of two or more circuits instead of a
single circuit tends to increase troubles of this sort. It thus seems
that while two or more circuits give a greater chance of continued
operation after a break in a conductor actually occurs, the use of a
single circuit with larger conductors makes any break less probable.
When repairs must be made on a transmission line, as in replacing a
broken insulator or setting a pole in the place of one that has burned,
it is certainly convenient to have two or more circuits so that one may
be out of use while the repairs on it are made. It is practicable,
however, to make such repairs on any high-voltage circuit, even when it
is in use, provided the conductors are spaced so far apart that there is
no chance of making a contact or starting an arc between them. To get
such distance between conductors there should be only one circuit per
pole, and even then more room should be provided for that circuit than
is common in this type of construction. On each of the two pole lines
between Cañon Ferry and Butte there is a single circuit of three
conductors arranged in triangular form, two at the opposite ends of a
cross-arm and one at the top of the pole, and the distance from each
conductor of a circuit to either of the other two is 6.5 feet. This
distance between conductors is perhaps as great as that on any
transmission circuit now in use, but it seems too small to make repairs
on the circuit reasonably safe when it is in operation at a pressure of
50,000 volts. There seems to be no good reason why the distance between
the conductors of a single circuit to which a pole line is devoted might
not be increased to as much as ten feet, at the slightly greater
expense of longer cross-arms. With as much as ten feet between
conductors, and special tools with long wooden handles to grasp these
conductors, there should be no serious danger about the repair of even
60,000-volt lines when in operation. As the 60,000-volt line between
Electra and San Francisco consists of only one circuit, it seems that
repairs on it must be contemplated during operation.
Another example of a high-voltage transmission carried out with a single
circuit is that between Shawinigan Falls and Montreal, a distance of
eighty-five miles. In this case the circuit is made up of three aluminum
conductors, each of which has an area in cross section of 183,750
circular mils, and these conductors are located five feet apart, one at
the top of each pole, and two at the ends of a cross-arm below. This
single circuit is in regular operation at 50,000 volts for the supply of
light and power in Montreal, and it is hard to see how repairs while
there is current on the line are to be avoided.
Inductance varies with the ratio between the diameter of the wires in
any circuit and the distance between these wires, but as inductance
simply raises the voltage that must be delivered by generators or
transformers, and does not represent a loss of energy, it may generally
be given but little weight in selecting the number of circuits, the
distance between conductors, and the size of each conductor. If two or
more circuits with smaller conductors have a combined resistance in
multiple equal to that of a single circuit with larger conductors, the
loss of voltage due to inductance may be greater on the single circuit
than the corresponding loss on the multiple circuits, but the advantages
due to the single circuit may more than compensate for the higher
pressure at generators or transformers. That such advantages have been
thought to exist in actual construction may be seen from the fact that
the 147-mile line from Electra power-house to San Francisco, and the
83-mile line from Shawinigan Falls to Montreal, are composed of one
circuit each. As inductance increases directly with the length of
circuits, these very long lines are especially subject to its influence,
yet it was thought that the advantages of a single circuit more than
offset its disadvantages in each case.
Where several sub-stations, widely separated, are to be supplied with
energy by the same transmission line, another argument exists for the
division of the line conductors into more than one circuit, so that
there may be an independent circuit to each sub-station. As the pressure
for local distribution lines must be regulated at each sub-station, it
is quite an advantage to have a separate transmission circuit between
each sub-station and the power plant, so that the voltage on each
circuit at the power-house may be adjusted as nearly as possible to the
requirements of its sub-station. An interesting illustration of this
practice may be noted in the design of transmission circuits for the
line between Spier Falls on the Hudson River and the cities of
Schenectady, Troy, and Albany, located between thirty and forty miles to
the south, which passes through Saratoga and Ballston on the way. When
this transmission line is completed, four three-phase circuits, one of
No. 0 and three of No. 000 copper wire, will run to the Saratoga
switch-house from the generating plant at the Falls, a distance of some
eight miles.
From this switch-house two circuits of No. 0 conductors go to the
Saratoga sub-station, a little more than one mile away, two circuits of
No. 000 wires run to the Watervliet sub-station, across the river from
Troy and thirty-five miles from the generating station, and one circuit
of No. 0 and one circuit of No. 000 wires are carried to Schenectady,
thirty miles from Spier Falls, passing through and supplying the
Ballston sub-station on the way. Other circuits connect the sub-station
at Watervliet with that at Schenectady and with the water-power station
at Mechanicsville. From the Watervliet sub-station secondary lines run
to sub-stations that control the local distribution of light and power
in Albany and Troy. This network of transmission circuits was made
desirable by the conditions of this case, which include the general
supply of light and power in three large and several smaller cities, the
operation of three large electric railway systems, and the delivery of
thousands of horse-power for the motors in a great manufacturing plant.
In not every transmission system with different and widely scattered
loads it is thought desirable to provide more than one main circuit.
Thus, the single circuit eighty-three miles long that transmits energy
from Shawinigan Falls to Montreal is designed to supply power also in
some smaller places on the way.
So again, the 147-mile circuit from Electra power-house to San Francisco
passes through a dozen or more smaller places, including Stockton, and
is tapped with side lines that run to Oakland and San José. In cases
like this, where very long lines run through large numbers of cities and
towns that sooner or later require service, it is obviously
impracticable to provide a separate circuit for each centre of local
distribution. It may well be in such a case that a single main
transmission circuit connected to a long line of sub-stations will
represent the best possible solution of the problem. At the power-house
end of such a circuit the voltage will naturally be regulated to suit
that sub-station where the load is the most important or exacting, and
each of the other sub-stations will be left to do all of the regulating
for its own load.
[Illustration: FIG. 76.--Connections at Watervliet Sub-station on Spier
Falls Line.]
The greater the total loss of voltage on a transmission line supplying
sub-stations that are scattered along much of its length, the larger
will be the fluctuations of voltage that must be compensated for at all
of the sub-stations save one, under changing loads, if only one circuit
is employed between the power-plant and these sub-stations. Suppose, for
example, that a transmission line 100 miles long is composed of a single
circuit, and supplies two sub-stations, one located 50 miles and the
other 100 miles from the power-plant. Assume at first that there is no
load whatever at the intermediate sub-station. If the single
transmission circuit operates with 50,000 volts at the power-plant, and
45,000 volts at the sub-station 100 miles away when there is a full load
there, corresponding to a loss of ten per cent, then the pressure at the
intermediate sub-station will be 47,500 volts. If, now, the load at the
sub-station 100 miles from the power-house drops to a point where the
entire line loss is only 1,000 volts, and the pressure at the generating
plant is lowered to 46,000 volts so as to maintain 45,000 volts at the
more distant sub-station, then the pressure at the intermediate
sub-station will be 45,500 volts, or 2,000 volts less than it was
before. If the loss on the entire line at full load were only five per
cent, making the voltage at the sub-station 100 miles away 47,500 when
that at the generating station is 50,000, then the pressure at the
intermediate sub-station will be 48,750 volts. Upon a reduction of the
loss on the entire length of line to one-fifth of its maximum amount, or
to 500 volts, the pressure at the generating station must be reduced to
48,000 volts, if that at the more distant sub-station is to be held
constant at 47,500. At the intermediate sub-station the pressure will
then be 47,750 volts, or 1,000 volts less than it was at full load. From
these two examples it may be seen that the extent of pressure variation
at the intermediate sub-station will depend directly on the maximum line
loss, if the regulation at the generating station is such as to maintain
a constant voltage at the sub-station 100 miles away.
[Illustration: FIG. 77.--Sections of Switch-house on New Hampshire
Traction System.]
All the foregoing has assumed no load to be connected at the
intermediate sub-station, and with a load there the fluctuations of
pressure will of course depend on its amount as well as on the load at
the more distant sub-station.
One of the strongest reasons for the use of two or more circuits in the
same transmission line arises from the rapid fluctuations of load where
large stationary motors or an electric railway system is operated. When
a transmission line must carry a load of stationary or railway motors,
it is a common practice to divide the line into at least two circuits,
and to devote one circuit exclusively to railway or motor work and
another to lighting, at any one time. In some cases this division of the
transmission system into two parts, one devoted to the lighting and the
other to the motor load, is carried out not only as to the sub-station
apparatus and the line, but also as to the transformers, generators,
water-wheels, and even the penstocks at the power-plant. It is possible
even to carry this division of the transmission system still further,
and to separate either the motor or the lighting load, or both, into
sections, and then to devote a distinct transmission circuit, group of
transformers, generator, and water-wheel to the operation of each
section. An example of the complete division of generating and
transmitting apparatus into independent units may be noted in the case
of the system that supplies light and power in Portland, Me., from a
generating plant on the Presumpscot River, thirteen miles away. At this
station four steel penstocks, each provided with a separate gate at the
forebay wall, bring water to as many pairs of wheels, and each pair of
wheels drives a direct-connected generator. Four three-phase circuits
connect the generating plant with the sub-station at Portland, and each
circuit between the generating plant and a transformer-house outside the
business section of the city is made up of No. 2 solid soft-drawn copper
wires.
Each of these four sets of apparatus, from head-gate to sub-station, is
usually operated independently of the others, and supplies either the
motor load or a part of the electric lighting. In this way changes in
the amount of one section of the load cause no fluctuation of the
voltage on the other sections. At Manchester, N. H., the sub-station
receives energy from four water-power plants, and is provided with two
sets of low-tension, 2,300-volt, three-phase bus-bars, one set of these
bus-bars being devoted to the operation of the local electric railway
system, and the other set to the supply of lamps and stationary motors.
Each set of these bus-bars is divided into a number of sections, and by
means of these sections different transmission circuits are devoted to
different portions of the lighting and motor loads. As three of the four
water-power plants are connected to the sub-station by two circuits
each, the division of loads in this case is often carried clear back to
the generators, one generator in a power-house being operated, for
instance, on railway work and another on a lighting load at the same
time. This plan has the obvious advantage that much of the regulation
for the several parts of the entire load may be done at the generators,
thus reducing the amount of regulation necessary at the sub-station, and
that fluctuating motor loads do not affect the lamps. In this case the
conductors of the several transmission circuits are all of moderate
size, and the division of the lines was evidently adopted for purposes
of regulation, rather than to reduce the amount of inductance. Thus the
line between Gregg’s Falls and the sub-station, a distance of six miles,
is made up of one three-phase circuit of No. 4 and one circuit of No. 6
bare copper wires. The fourteen-mile line between the plant at Garvin’s
Falls and the sub-station, the longest of the four transmissions, is
made up of two three-phase circuits, each composed of No. 0 bare copper
wires. In the case of the Gregg’s Falls plant the subdivision of the
line has gone further than that of the generating equipment, for the
station there contains only a single generator, the rating being 1,200
kilowatts, while two circuits run thence to the sub-station. Another
instance showing extensive subdivision of a line into separate circuits
may be noted in the seven-mile transmission from Montmorency Falls to
Quebec, Canada, where sixteen conductors, each No. 0 copper wire, make
up four two-phase circuits that connect a plant of 2,400 kilowatts
capacity with its sub-station.
Such multiplication of transmission circuits has some advantages from
the standpoint of regulation, but there are good reasons for limiting it
to rather short lines, where it is, in fact, almost exclusively found.
On very long lines the use of numerous circuits composed of rather small
conductors would obviously increase the constant expense of inspection
and repairs and add materially to uncertainty of the service. Very few,
if any, transmission lines of as much as twenty-five miles in length are
divided into more than two circuits, and in several instances lines of
superlative length have only a single circuit each. The greatest single
power transmission in the world, that between Niagara Falls and Buffalo,
is carried out with two pole lines, one of which is about twenty and the
other about twenty-three miles long. The longer pole line, which is also
the older, carries two three-phase circuits, each of which is made up of
three 350,000 circular mil copper conductors. The shorter pole line
carries a single three-phase circuit composed of aluminum conductors,
each of which has an area in cross section of 500,000 circular mils. In
electrical conductivity the aluminum circuit is intended to be equal to
each of the two that are composed of copper. According to the
description of the Niagara Falls and Buffalo transmission system in vol.
xviii., A. I. E. E., pages 518 to 527, each of these three circuits is
designed to transmit about 7,500 kilowatts, and the maximum power
transmitted up to August, 1901, was 15,600 kilowatts, or about the
calculated capacity of two of the circuits. According to the description
just mentioned, the transmission circuits used to supply energy for use
at Buffalo are regularly operated in parallel, and this is also true of
the generators and the step-down transformers, though the uses to which
this energy is applied include lighting, large stationary motors, and
the electric railway system. Apparatus in the generating station at
Niagara Falls and in the terminal-house near the city limits of Buffalo
is so arranged, however, that two of the 3,750 kilowatt generators and
eight step-up transformers at the power-house, together with one
transmission circuit and three step-down transformers in the
terminal-house at Buffalo, may be operated independently of all the
other apparatus.
As already pointed out, the use of separate circuits for each
sub-station, and for lighting and power loads at each sub-station in
very long transmission systems, is often impracticable. Even in
comparatively short transmissions the multiplication of circuits and the
use of rather small and mechanically weak conductors increased the first
cost of installation and the subsequent expense of inspection and
repairs. An objection to operation with a single circuit in a
transmission line that supplies widely separated sub-stations with
lighting, power, and railway loads is the consequent difficulty of
pressure regulation on the distribution lines at each sub-station. Such
a transmission line necessarily delivers energy at different and
fluctuating voltages at the several sub-stations, and these fluctuations
are of course reproduced on the secondary side of the step-down
transformers. Fortunately, however, the use of synchronous motor
generators, either in place of or in connection with static
transformers, goes far to solve the problem of pressure regulation for
distribution circuits supplied with energy from transmission lines. This
is due to the well-known fact that with constant frequency the speed of
rotation for a synchronous motor is constant without regard to
fluctuations in the applied voltage or changes in its load. With a
constant speed at the motor and its connected generator it is of course
easy to deliver current at constant voltage to the distribution lines.
This constancy of speed makes the synchronous motor generator a favorite
in large transmission systems with both power and lighting loads. The
satisfactory lighting service in Buffalo, operated with energy
transmitted from Niagara Falls, seems to be due in some measure to the
use of synchronous motor generators at the sub-station in Buffalo,
whence lighting circuits are supplied. As above stated, the three
circuits that make up the transmission line between Niagara Falls and
Buffalo are operated in multiple, and in the latter place there is a
large load of both railway and stationary motors. As the three circuits
are operated in multiple, they of course amount to only a single
circuit so far as fluctuations of voltage due to changes in these
several sorts of loads are concerned. According to vol. xviii., A. I. E.
E., pages 125 and following, the load on the transmission system at
Buffalo in 1901 was made up of about 7,000 horse-power in railway
motors, 4,000 horse-power in induction motors, and 4,000 horse-power
divided up between series arc lamps, constant pressure incandescent
lamps, and continuous current motors. The railway load is operated
through step-down transformers and rotary converters. The induction
motors are connected either to the 2,000-volt secondary circuits of the
step-down transformers or to service transformers supplied by these
circuits. On these railway and stationary motor loads there is of course
no necessity for close pressure regulation. Series arc lamps are
operated through step-down transformers and synchronous motors
direct-connected to constant continuous current dynamos. Continuous
current stationary motors draw power from the transmission lines through
step-down transformers and rotary converters, like the railway load. For
the 2,200 volt circuits that supply service transformers for commercial
arc and incandescent lighting the transmitted energy passes through
step-down transformers and synchronous motor-generators. These
motor-generators raise the frequency from twenty-five to sixty cycles
per second. Finally the continuous current three-wire system for
incandescent lighting at about 250 volts between outside wires is
operated through step-down transformers and synchronous motors
direct-connected to continuous current generators. For this last-named
service rotary converters were at first tried, but were found to be
impracticable because voltage fluctuations on the transmission line (due
largely to the railway and motor loads) were reproduced on the
continuous-current circuits by the rotary converters. Since the adoption
of motor-generators this fluctuation of the service voltage is no longer
present.
Another case in which synchronous motor-generators deliver power from a
transmission line that carries both a lighting and a motor load is that
of the Shawinigan sub-station in Montreal. At this sub-station the
85-mile transmission line from the generating plant at Shawinigan Falls
terminates. As already pointed out, this line is composed of a single
three-phase circuit of aluminum conductors, each of which has a cross
section of 183,750 circular mils. In the Montreal sub-station the
thirty-cycle, three-phase current from Shawinigan Falls is delivered to
transformers that lower the voltage to 2,300. The current then goes to
five synchronous motor-generators of 1,200 horse-power capacity each,
and is there converted to sixty-three cycles per second, two-phase, at
the same voltage. This converted current passes onto the distribution
lines of the local electrical supply system in Montreal, which also
draws energy from two other water-power plants, and is devoted to
lighting, stationary motors, or to the street railway work, as may be
required. Though separate local distribution circuits are devoted to
these several loads, the fluctuations in the stationary and railway
motor work necessarily react on the voltage of the transmission line and
transformers at the sub-station. By the use of the synchronous
motor-generators the lighting circuits are protected from these pressure
variations.
As the numbers of sub-stations at different points on long transmission
lines increase, and stationary motor and railway loads at each become
more common, it is to be expected that the use of synchronous
motor-generators for lighting service will be much more frequent than at
present. With such use there will disappear one of the reasons for the
multiplication of transmission circuits.
[Illustration: FIG. 78.--Transfer Switches at Saratoga Switch-house on
Spier Falls Line.]
Where several transmission circuits connect a generating plant with a
single sub-station, or with several sub-stations in the same general
direction, it is desirable to have switches so arranged that two or more
circuits may be combined as one, or so that any circuit that ordinarily
operates a certain load or sub-station may be devoted to another when
occasion requires. For this purpose transfer switches on each circuit
are necessary at generating plants, sub-stations, and often at
switch-houses. These transfer switches will ordinarily be of the knife
type, and intended for manual operation when the circuits to which they
are connected are not in use. As such switches are exposed to the full
voltage of transmission, the insulation of their conducting parts should
be very high. In the extensive transmission system between the
power-plants at Spier Falls and Mechanicsville and the sub-stations at
Troy, Albany, and Schenectady, N. Y., a transfer switch of highly
insulated construction has been much used. The two blades of this switch
move independently of each other, but both are mounted between the same
metal clips. Each blade is of two by one-quarter inch drawn copper rod,
and the clips supporting the two blades are mounted on top of a circular
metal cap four and three-quarter inches in outside diameter and two
inches high, that is cemented over the top of a large, double petticoat,
porcelain line insulator.
[Illustration: FIG. 79.--Cross Section of Schenectady Switch-house on
Spier Falls Line.]
Clips into which these copper blades are swung in closing the switch are
also mounted in caps carried by insulators in the way just described.
Each of these insulators is mounted on a large wooden pin, and these
pins are secured in timbers at the points where the switches are wanted.
This construction of switches gives ample insulation for the line
voltage of 30,000 in this system. By means of the transfer switches
just described, either of the transmission circuits leaving the Spier
Falls power-plant may be connected to any one of the ten generators and
ten groups of transformers there. At the Saratoga switch-house, any one
of the twelve conductors, making up the four three-phase circuits from
Spier Falls may be connected to any one of the eighteen conductors
making up the six three-phase circuits that go south to Saratoga,
Watervliet, and Schenectady sub-stations, in the way indicated by the
drawing. So again at the Watervliet sub-station, where energy at 26,500
volts is received from Spier Falls and energy at 10,800 volts from
Mechanicsville, any single conductor from either of these water-power
plants may be connected, either directly or through a transformer, with
either conductor running to the railway and lighting sub-stations about
Albany and Troy. Where several transmission circuits are employed, this
complete flexibility of connection evidently adds materially to the
convenience and reliability of operation.
CIRCUITS IN TRANSMISSION LINES.
+----------------------------+------+------+------+----------+--------+
| | | | | | Cycles |
| |Length|Number|Number| Circular | per |
| Location of Lines. | in | of | of | Mils | Second |
| |Miles.|Cir- | Pole | per | of |
| | |cuits.|Lines.| Wire. |Current.|
+----------------------------+------+------+------+----------+--------+
|Electra to San Francisco | 147 | 1 | 1 |[A]471,034| 60 |
|Colgate to Oakland, Cal. | 142 | 2 | 2 | 133,100| 60 |
| | | | |[A]211,000| |
|Santa Ana River to Los | | | | | |
|Angeles | 83 | 2 | 1 | 83,690| 60 |
|Shawinigan Falls to Montreal| 85 | 1 | 1 |[A]183,750| 30 |
|Cañon Ferry to Butte | 65 | 2 | 2 | 106,500| 60 |
|Welland Canal to Hamilton | 35 | 1 | 1 | 83,690| 60 |
|Welland Canal to Hamilton | 37 | 1 | 1 | 133,100| 60 |
|Spier Falls to Schenectady | 30 | 2 | 1 | 105,600| 40 |
| | | | | 167,800| |
|Spier Falls to Watervliet, | | | | | |
|N. Y. | 35 | 2 | 1 | 167,800| 40 |
|Ogden to Salt Lake City | 36 | 2 | 1 | 83,690| 60 |
|Apple River Falls to St. | | | | | |
|Paul | 27 | 2 | 1 | 66,370| 60 |
|Niagara Falls to Buffalo | 23 | 2 | 1 | 350,000| 25 |
|Niagara Falls to Buffalo | 20 | 1 | 1 |[A]500,000| 25 |
|Farmington River to Hartford| 11 | 1 | 1 |[A]364,420| 60 |
|Niagara Falls to Toronto | 75 | 2 | 1[B]| 190,000| 25 |
+----------------------------+------+------+------+----------+--------+
[A] Aluminum conductor.
[B] Steel towers.
CHAPTER XVIII.
POLE LINES FOR POWER TRANSMISSION.
Long transmission lines should follow the most direct routes between
generating and sub-stations as far as practicable. The number of poles,
cross-arms, and insulators increases directly with the length of line,
and the weight of conductors with the square of that length, other
factors remaining equal. Every material deviation from a straight line
must therefore be paid for at a rather high rate.
Distribution lines necessarily follow the public streets in order to
reach consumers, but the saving of the cost of a private right of way
and ease of access are the main considerations which tend to keep
transmission lines on streets and highways. Except in very rough or
swampy country, the difficulty of access to a pole line on a private
right of way is not a serious matter and should be given but little
weight. The cost of a private right of way may be more important, and
should be compared with the additional cost of the pole line and
conductors if erected on the public highway. In this additional cost
should be included any items for paving about the poles, extra pins,
insulators, and guys made necessary by frequent turns in the highway,
and the sums that may be required to secure the necessary franchises.
There is also the possible contingency of future legislation as to the
voltage that may be maintained on wires located over public streets.
These considerations taken together give a strong tendency to the
location of long transmission lines on private rights of way, especially
where the amount of power involved is great and the voltage very high.
A transmission line 80.3 miles in length recently erected between
Rochester and Pelham, N. H., by way of Portsmouth, where the generating
station is located, to feed an electric railway system, operates at
13,200 volts and is mainly located on private rights of way. Deeds
conveying the easements for this right of way provide that all trees or
branches within one rod on either side of the line may be cut away. The
transmission line between Niagara Falls and Buffalo, about twenty-three
miles long and operating at 22,000 volts, is largely on a private way
thirty feet wide.
[Illustration: FIG. 80.--Transmission Line of New Hampshire Traction
Company over Hampton River Bridge, 4,623 Feet Long.]
For the transmission between Cañon Ferry and Butte the line is mainly
located on a private way. Between Colgate and Oakland the transmission
line is mostly on private way, and this is also true of the greater part
of some other high-pressure lines in California. These private rights of
way range from fifty to several hundred feet wide, it being necessary in
forests to cut down all trees that are tall enough to fall onto the
wires.
In some cases of transmission at very high voltage two independent pole
lines are erected and one or more circuits are then run on each set of
poles. This construction has been followed on the transmission line
between Niagara Falls and Buffalo, Cañon Ferry and Butte, Welland Canal
and Hamilton, and between Colgate and Oakland. Such double pole lines
are more usually located on the same right of way, this being true of
the Cañon Ferry and Colgate systems, but this is not always the case. In
the Hamilton system the two lines of poles, one thirty-five miles and
the other thirty-seven miles in length, are located several miles apart.
The two sets of poles on a part of the Buffalo line are less than thirty
feet, on the Colgate line are twenty-five feet, and on the Cañon Ferry
line are forty feet apart.
The main reasons for the use of two pole lines instead of one are the
probability that an arc started on one circuit will be communicated to
another on the same poles, and the greater ease and safety of repairs
when each circuit is on a separate line of poles. On each pole line of
the Cañon Ferry transmission, and also on each pole line of the Colgate
transmission, there is only one three-wire circuit. On the Cañon Ferry
line each wire of the two circuits has a cross-section of only 106,500
circular mils, and on the Colgate line one circuit is of 133,225
circular mils wire and the other circuit is of 211,600 circular mils
cable. In contrast with these figures the line of the Standard Electric
Company between Electra and Mission San José, a distance of ninety-nine
miles, is made up of only three conductors, each being an aluminum cable
of 471,034 circular mils section. Inductance increases with the
frequency of the current in a conductor, and in each of the three
systems just considered the frequency is sixty cycles per second.
The use of one circuit of larger wire instead of two circuits of smaller
wire has the obvious advantage of greater mechanical strength in each
conductor, saves the cost of one pole line and of the erection of the
second circuit. With voltages above 40,000 to 50,000 on long
transmission lines there is a large loss of energy by leakage directly
through the air from wire to wire. To keep this loss within desirable
limits it may be necessary to give each wire of a circuit a greater
distance from the others of the same circuit than can readily be had if
all the wires of each circuit are mounted on one line of poles. If there
is only one three-wire circuit to be provided for, three lines of poles
or two lines with a long crosspiece between them may be set with any
desired distance between the lines so that the leakage through the air
with one wire on each pole will be reduced to a small quantity. On a
line built in this way it would be practically impossible for an arc to
start between the wires by any of the usual means.
Distances from pole to pole in the same line vary somewhat with the
number, size, and material of the conductors to be carried. On ordinary
construction in a straight line poles are often spaced from 100 to 110
feet apart--that is, about fifty poles per mile. On curves and near
corners the spacing of poles should be shorter. Poles for the 80.3
miles, mentioned in New Hampshire, are regularly located 100 feet apart.
Of the two pole lines between Niagara Falls and Buffalo, the older was
designed to carry twelve copper cables of 350,000 circular mils each,
and its poles were spaced only 70 feet apart. The newer line is designed
to carry six aluminum cables of 500,000 circular mils each and its poles
are 140 feet apart. Poles in each of the lines between Cañon Ferry and
Butte are regularly spaced 110 feet apart and each pole carries three
copper cables of 106,500 circular mils.
[Illustration: FIG. 81.--Chambly-Montreal Line Crossing the Chambly
Canal.]
[Illustration: FIG. 82.--Special Wooden Structures on Line Between Spier
Falls and Schenectady.]
The two 142-mile lines between Colgate and Oakland are each made up of
poles 132 feet apart, and one line of poles carries the three copper
conductors and the other line of poles the aluminum conductors already
named. As aluminum wire has only one-half the weight of copper wire of
equal conductivity, the length of span between poles carrying aluminum
wire may be greater than that where copper is used. Only a part of the
strain on poles is due to the weight of wires carried, however. Where a
body of water must be crossed, a very long span, with special supports
for the wires at each side, may be necessary. A case of this sort was
met where the Colgate and Oakland line crosses the Carquinez Straits at
a point where the waterway is 3,200 feet wide. It was necessary to have
the lowest part of the cables across these straits at least 200 feet
above the surface of the water so that vessels with the tallest masts
could pass underneath. To secure the necessary elevation for the cables
a steel tower was built on each bank of the straits at such a point that
the distance between the points for cable support on the two towers is
4,427 feet apart. As the banks rise rapidly from the water level, one
steel tower was given a height of only 65 feet, while the height of the
other was made 225 feet. Between these two towers four steel cables were
suspended, each cable being made up of nineteen strands of galvanized
steel wire, having an outside diameter of seven-eighths inch and
weighing 7,080 pounds for the span. The breaking strain of each cable is
98,000 pounds, and it has the electrical conductivity of a No. 2 copper
wire. The cables are simply supported on the towers by steel rollers,
and the pull of each cable, amounting to twelve tons, is taken by an
anchorage some distance behind each tower, where the cable terminates.
Each anchorage consists of a large block of cement deeply embedded in
the ground, and with anchor bolts running through it. Each cable is
secured to its anchorage through a series of strain insulators, and the
regular line cables of copper and aluminum are connected with the steel
cables just outside of the shelter built over the strain insulators of
each anchor. Steel cables were used for the long span across the straits
because of the great tensile strength that could be had in that metal.
This span is, no doubt, the longest and highest that has ever been
erected for electrical transmission at high voltage.
[Illustration: FIG. 83.--Special Structure on Line Between Spier Falls
and Schenectady.]
It has been suggested in one instance that steel towers ninety feet high
and 1,000 feet apart be substituted for pole lines and the wires strung
from tower to tower. Such construction would increase the difficulty of
insulation and would cost more at the start than a line of wooden poles.
The question is whether a lower maintenance and depreciation rate for
the steel towers would offset their disadvantages compared with poles.
Pole lines should be staked out with a transit, and the same instrument
can be used to give a perpendicular position to each pole and bring it
into line. Wooden poles are used in most cases of high-voltage
transmission lines. Iron poles would make it unsafe to work on any
circuit carried by them when it was transmitting current at high
voltage. With iron poles a defective insulator might lead to the
destruction of the conductors at that point through continuous arcing on
to the iron.
[Illustration: FIG. 84.--Crossing of Delaware and Hudson Railway Tracks
by 30,000-volt Line at Saratoga, N. Y.]
The kinds of wood used for poles vary in different sections of the
country. In New England, chestnut poles are a favorite and were used on
the 80.3 miles of transmission line mentioned in New Hampshire. Cedar
poles are used to some extent in nearly all parts of the country,
including Canada. Spruce and pine poles are employed to some extent,
especially in lengths of more than fifty feet. In the Rocky Mountain
region and in California round cedar poles from the forests of Oregon,
Washington, and Idaho are much used. Sawed redwood poles from the trunks
of large trees were erected on the 147-mile line between Electra
power-house and San Francisco. For the Colgate and Oakland line Oregon
cedar poles were selected, and the transmission between Cañon Ferry and
Butte was carried out with cedar poles from Idaho. For transmission
circuits it is far more important at most points to have poles very
strong rather than very long. Where wires or obstructions must be
crossed by the high-voltage circuits the poles should be long enough to
carry these circuits well above everything else. In the open country,
where no obstructions are to be avoided, it does not pay to use poles
with a length greater than thirty-five feet.
[Illustration: FIG. 85.--Pole Line from Spier Falls over Mount
McGregor.]
Short poles offer less surface to the wind, the length of the lever
through which wind pressure acts to break the pole at the ground
decreases with the length of pole, and the shorter the poles the smaller
is the strain on struts and guy wires. If poles are only thirty or
thirty-five feet long, they may be large in diameter without excessive
cost. As a rule, no pole should be used with a top less than seven
inches in diameter, and a pole with this top should not be required to
carry more than three wires. A pole with seven- or eight-inch top and
thirty feet long should measure not less than twelve inches in diameter
at the butt. For longer poles the diameters at the butt should increase
up to at least eighteen inches for a round pole sixty feet long.
In the New Hampshire transmission above named the standard length of
poles is thirty-five feet. On the line between Cañon Ferry and Butte the
poles range from thirty-five to ninety feet in length. The round cedar
poles used in the Colgate and Oakland line range from twenty-five to
sixty feet in length, from eight to twelve inches diameter at the top,
and from twelve to eighteen inches diameter at the butt. On the line
between Electra and San Francisco the square-sawed redwood poles are
reported to have the following dimensions, in a paper read at the annual
convention of Edison Illuminating Companies in 1902.
+-------+-------+---------------+----------+
|Height,| Top, | Butt, | Depth |
| Feet. |Inches.| Inches. |in Ground.|
+-------+-------+---------------+----------+
| | | | |
| 35 | 7 × 7 |12 × 12 | 5.5 |
| 40 | 8 × 8 |13-1/2 × 13-1/2| 6 |
| 45 | 9 × 9 |15 × 15 | 6.5 |
| 50 |10 × 10|16 × 16 | 7 |
| 60 |11 × 11|17 × 17 | 8 |
+-------+-------+---------------+----------+
The relative dimensions of these poles are of interest because, being
sawed from the trunks of large trees, they could have any desired
measurements at the tops and butts. These poles, over the greater part
of the line, carried the three aluminum cables of 471,034 circular mils
each, previously mentioned. Depth to which poles are set in the ground
ranges from about five feet for twenty-five- or thirty-foot poles to
eight feet for poles sixty feet long. In locations where the soil is
very soft or where poles must resist heavy strains the stability of each
pole may be much increased by digging the hole two feet or more larger
in diameter than the butt of the pole, and then filling in cement
concrete--one part, by measure, of Portland cement, three of sand and
five of broken stone--all around the butt of the pole after it is in the
hole. The butts of poles up to a point one foot or more above the ground
line are frequently treated with hot tar, pitch, asphalt, or carbolineum
before the poles are erected, and in Salt Lake City salt is said to be
used around pole butts after they are in the hole.
In some cases the poles of transmission lines are painted over their
entire length. Pole tops should always be pointed or wedge-shaped, to
shed water, and paint or tar should be applied to these tops. In some
cases poles are filled with crude petroleum or other preservative
compound in iron retorts, where moisture is withdrawn from the pole by
exhausting the air, and then, after treatment with dry steam, the poles
have the compound forced into them by hydraulic pressure.
In favorable soils cedar poles may remain fairly sound for twenty years,
chestnut poles more than one-half of that time, and spruce and pine
about five years. Poles up to forty feet in length may be readily set
with pike poles, but when they are much longer than this a derrick will
save time and labor. A derrick should have a little more than one-half
the length of the poles to be set.
[Illustration: FIG. 86.--Chambly-Montreal Line Crossing the Richelieu
River.]
Poles should be guyed or braced at all points where there are material
changes in the direction of the line, and on long straight stretches
about every fifth pole should be guyed or braced in both directions to
prevent the poles setting back when the line wire is cut or broken at
any point. Where there is room for wooden struts, as on a private right
of way, they should be used instead of guys because of their more
substantial character and the higher security of insulation thus
obtained. Ordinary strain insulators cannot be relied on with lines that
operate at very high voltages, and where guys must be used a timber four
by six inches and ten to twenty feet long may have the guy twisted
about each end of it and be used as a strain insulator. A guy or strut
should come well up under the lower cross-arm on a pole to avoid
breaking of the pole at the point of attachment.
Where poles have heavy circuits and several cross-arms each it is
sometimes desirable to attach a guy or strut beneath the lowest arm and
also a guy close to the pole top. Galvanized iron or steel wire is the
material best suited for guys, and the cable form has greater strength
and is more flexible than solid wire.
[Illustration: FIG. 87.--Cross-arms and Insulators on the Line Between
the Chambly Plant and Montreal.]
On the transmission line between Electra and San Francisco, which is
intended to operate at 60,000 volts, the use of guys has been mostly
avoided and struts employed instead. Where a guy had to be used, a
strain insulator of wood six by six inches and twenty feet long was
inserted in it.
The number and spacing of cross-arms on the poles of transmission lines
are regulated by the number of circuits that each pole must carry and by
the desired distance apart of the wires. Formerly it was common to carry
two or more circuits on a single line of poles, but now a frequent
practice is to give each pole line only one circuit and each pole only
one cross-arm, except that a small cross-arm for a telephone circuit is
placed some feet below the power wires. With only one transmission
circuit per pole line, one wire is usually placed at the top of the pole
and the other two wires at opposite ends of the single cross-arm. The
older pole line for the transmission between Niagara Falls and Buffalo
carried two cross-arms per pole for the power wires, these cross-arms
being two feet apart. Each cross-arm was of yellow pine, twelve feet
long, four by six inches in section, and intended to carry four
three-wire circuits, but only two circuits have been erected on these
two cross-arms. On the later pole line for this same transmission each
pole carries two cross-arms, the upper intended for four and the lower
cross-arm for two wires, so that one three-wire circuit may be strung on
each side of the poles, two wires on the upper and one on the lower arm
in the form of an equilateral triangle. The pole lines between Cañon
Ferry and Butte, Colgate and Oakland, and Electra and San Francisco all
have only one cross-arm for power wires per pole, and the third wire of
the circuit in each case is mounted at the top of the pole so that the
three conductors are at the corners of an equilateral triangle.
This relative position of the conductors makes it easy to transpose them
as often as desired. On the line from Cañon Ferry to Butte the
cross-arms are each eight feet long with two holes for pins
seventy-eight inches apart, and are attached to the pole five feet ten
and one-half inches from the top. Gains for cross-arms should be cut
from one to two inches deep in poles before they are raised, and one
hole for three-quarters or seven-eighths-inch bolt should be bored
through the centre of the cross-arm and of the pole at the gain. Each
cross-arm should be attached to the pole by a single bolt passing
entirely through the pole and cross-arm with a washer about three inches
in diameter next to the cross-arm. One large through bolt weakens the
pole and arm less than two smaller bolts or lag-screws, and the arm can
be more easily replaced if there is only one bolt to remove. Alternate
poles in a line should have their cross-arms bolted on opposite sides,
and at corners double arms should be used.
Yellow pine is a favorite wood for cross-arms, though other varieties
are also used. The large, long pins necessary on high voltage lines tend
to increase the sectional area of cross-arms, and a section less than
five and one-half by four and one-half inches is seldom desirable. On
the line between Electra and San Francisco, which carries the three
aluminum cables of 471,034 circular mils each, the cross-arms of Oregon
pine have a section of six by six inches each. Standard dimensions of
some smaller cross-arms are four and three-quarters by three and
three-quarters inches, but it may be doubted whether these arms are
strong enough for long transmission work. Cross-arms should be surfaced
all over and crowned one-quarter to one-half inch on top so as to shed
water. After being kiln dried, cross-arms should be boiled in asphaltum
or linseed oil to preserve the wood and give it higher insulating
properties. Cross-arms longer than five feet should be secured by braces
starting at the pole some distance below each arm and extending to
points on the arm about half-way between the pole and each end of the
arm.
[Illustration: FIG. 88.--Tail Race and Pole Line at Chambly, Quebec
Power-station.]
Each brace may be of flat bar iron about one and one-half by one-quarter
inch in section, or the brace for both ends of an arm may be made of a
single piece of angle-iron bent into the proper shape. For high-voltage
lines it is undesirable to employ iron braces of any sort, since these
braces form a path of low resistance that comes much too close to the
pins on which the insulators and wires are mounted. Braces formed of
hard wood are much better as to insulation, and such braces of maple are
in use on the line between Butte and Cañon Ferry where the voltage is
50,000. Each brace on that line is thirty-six inches long and three
inches wide, with one end bolted to the centre of its pole and the other
end to the cross-arm twenty-three inches from the pole centre.
The line from Electra has hard-wood braces secured with wood pins.
Wood is the most common material for pins on which to mount the
insulators of high-voltage transmission circuits. Iron has been used for
pins to some extent, and its use is on the increase. Oak and locust pins
are generally used, the latter being stronger and more lasting. In
California, pins of eucalyptus wood are much used and are said to be
stronger than locust. All wooden pins should be boiled several hours in
linseed oil after being well dried. This increases the insulating and
lasting properties of the pins.
High-voltage lines require long pins to hold the lower edges of
insulators well above the cross-arms, and these pins must be much
stronger than those used on ordinary lines, because of the increased
leverage of each wire.
A pin twelve inches long over all and having a diameter of one and
one-half inches in the part that enters the cross-arm has been much used
for transmission circuits, but is much too short and weak for high
voltages. On the 50,000-volt line between Cañon Ferry and Butte the pins
are seasoned oak boiled in paraffin. Each of these pins is seventeen and
one-half inches long, two and one-half inches in diameter for a length
of four and one-half inches in the middle part, two inches in diameter
for a length of five and one-half inches that fits into the cross-arm or
pole top, and one and one-half inches in diameter at the top of the
thread inside of the insulator. These pins hold the outside edges of the
insulators nine inches above the tops of cross-arms. Each of these pins
is held in its socket by a three-eighths-inch bolt that passes entirely
through the pin and the cross-arm or pole top.
On the line between Electra and San Francisco the pins are each sixteen
and seven-eighths inches long, two and three-quarters inches in diameter
at the largest central part, and two and one-quarter inches in diameter
in the lower part, five inches long, that fits into the cross-arm or
pole top. One of these pins broke at the shoulder with a pull of 2,200
pounds at the threaded part. Carriage bolts one-half inch in diameter
pass through the cross-arm and pin two inches from the top of the arm,
and one bolt three inches from the pin on each side. Without these bolts
the arms split on test with a pull of 1,200 pounds on the pin, but with
the bolts the pin broke as above.
CHAPTER XIX.
ENTRIES FOR ELECTRIC TRANSMISSION LINES.
The entrance of transmission lines into generating plants and
sub-stations presents special problems in construction and insulation.
One of these problems has to do with the mechanical security of each
conductor at the point where it passes through the side or roof of the
station. Conductors are sometimes attached to the station so that the
strain of the line is borne by the side wall where they enter and tends
to pull it out of line.
This practice has but little to commend it, aside from convenience, for
unless the conductors are rather small, or the wall of the station is
unusually heavy, the pull of the former is apt to bulge the latter in
the course of time. For any heavy line the end strain is ultimately most
suitably taken by an anchor securely fixed. As special insulators must
be used where a conductor is secured directly to such an anchor, it is
usually more convenient to set one or more heavy poles with double
cross-arms at the end of a line, and then to make these poles secure by
large struts, or by guys attached to anchors. Extra heavy cross-arms on
these end poles should be provided with iron pins for the line
insulators; two or more of the insulators mounted in this way within a
few feet of each other, for each wire, will stand up against the end
strain on almost any line.
Insulators that are to take the end strain of a line in this way should
allow attachment of the wire at the side, so that the force exerted by
each conductor tends to press the insulator against the side of its pin,
rather than to pull off the top of the insulator. The end strain of the
line having been taken on poles close to the station, the conductors may
be attached to insulators on the wall, the latter thus being subjected
to very little mechanical strain.
Overhead lines usually enter a station through one of its side walls,
but an entry may be made in the roof. It is desirable to have a side
entry on the gable end of a building rather than on a side below the
eaves where there will be much dripping of water. If an entry must be
made below the eaves, a shelter should be provided above the entry, and
the roof of this shelter should have a gutter that will carry water
away from the wires.
Entrance of each conductor into a station must be effected in such a way
that ample insulation of the circuit will be maintained, and in some
cases so that rain, snow, and wind will be excluded. The line voltage
and the climate where the station is located thus have an important
bearing on the form of entry that is suitable in any particular case.
The simplest form of entry for a high-voltage line is a clear opening,
usually circular in form, through the wall of the station for each wire.
Insulators for each wire should be provided both inside and outside of
the wall to hold the wire at the centre of this opening. Such insulators
are usually most conveniently supported by fixtures attached to both
sides of the wall, and insulators on the outside should of course be
kept in an upright position, unless completely protected from rain and
snow.
The diameter of the openings through the wall should be great enough to
prevent any visible discharge of current between the wire and wall under
the worst conditions of snow, rain, fog, or dust. Such an opening must,
therefore, increase in diameter with the voltage of the line. The larger
these openings for the line wires, the greater is the opportunity for
rain, snow, dust, and cold air to enter the station through them.
Openings may be so protected as to keep out snow and rain by means of
shelters on the outside of the wall on which they are placed, but such
shelters cannot keep out the cold air. If the openings for the entrance
of wires are located in the wall of a room that contains air-blast
transformers, the area of openings for circuits of very high voltage may
be no greater than is necessary to allow the escape of heated air from
the transformers.
The milder the climate, other factors being the same, the higher the
voltage of circuits which may enter a station through openings that are
free for the movement of air. With circuits of only moderate voltage,
say less than 15,000, it is quite practicable to admit wires to a
station through perfectly free openings, in the coldest parts of the
United States. With voltages of 20,000 to 60,000 it is often necessary,
in the colder parts of the country, to close the opening in the wall
through which each wire enters with a disc of insulating material.
In order to keep the current leakage over these discs within proper
limits, the diameters of the discs must increase with the voltage of the
circuit. This increase of disc diameter obviously lengthens the path of
leakage current over the disc surface. Where the openings in a wall for
the entrance of high-voltage circuits are closed by insulating discs
about the wires, these discs may make actual contact with bare wires,
or the wire at each entry may have some special insulation.
In the side wall of the sub-station at Manchester, N. H., the entrance
of transmission lines from four water-power plants is provided for by
circular openings in slate slabs that are built into the brickwork. The
transmission circuits from three of the water-power plants operate at
10,000 to 12,000 volts, and the circuit from the fourth plant at about
6,000 volts. Circular openings in the slate slabs are each five inches
in diameter, and they are spaced twelve to fifteen inches between
centres. A single wire enters through each of these openings and is held
at the centre by insulators both inside and outside of the wall. Each
wire is bare where it passes through the slate slab, and the circular
openings are not closed in any way. The largest wires passing through
these five-inch circular openings in the slate slabs are of solid
copper, No. 0, of 0.325-inch diameter each.
Before passing through the opening in the slate slabs the wires of these
transmission circuits are tied to regular line insulators supported by
cross-arms secured to the outside of the brick wall by iron brackets.
The point of attachment of each wire to its insulator is about nine
inches below the centre of the circular hole by which it enters the
sub-station.
This Manchester sub-station is equipped with air-blast transformers from
which the hot air is discharged into the same room that the transmission
lines enter. Along one side of the sub-station there are twenty-seven of
these five-inch circular openings in the slate slabs for entrance of the
high-voltage lines, and on another side of the sub-station there are a
greater number of smaller openings for the distribution circuits. Were
it not for the air-blast transformers, all of these openings would
probably admit more air than would be desirable in a climate as cold as
that at Manchester.
Another example of openings in the walls of a station for the entrance
of transmission circuits, where there is free movement of the air
between the inside and outside of the building, is that of the
33,000-volt line between Santa Ana River and Los Angeles, Cal. In this
case a sewer pipe twelve inches in diameter is built into the wall of
the station for each wire of the line, so that there is a free opening
of this size from inside to outside.
Each wire of the 33,000-volt circuit enters the station through the
centre of one of these twelve-inch pipes, and is thus surrounded by six
inches of air on every side. As the temperature near Los Angeles seldom
or never goes down to zero, these large openings do not admit enough
air to be objectionable. Besides this mild climate, air-blast
transformers add to the favorable features in the stations having the
twelve-inch openings.
In another case, however, where the openings for the entrance of wires
of very high voltage allow free movement of air between the inside and
outside of the station, the climate is cold and the winter temperatures
go down to 30° or more below zero. This condition exists on the
25,000-volt line between Apple River Falls and St. Paul, where six No. 2
wires enter the generating station through plain circular openings in
the brick side wall of a small extension where the lightning arresters
are located. Air-blast transformers are located in the end of the
station next to this lightning-arrester house, but it is not certain
that the hot air from them escapes through the openings for the wires.
In another case where the climate is about as cold as that just named, a
gallery is built along one side of the exterior of the station at some
distance above the ground, and two openings are provided for each wire
of the high-tension line. One of these two openings is in the horizontal
floor of the gallery and allows the entrance of the wire from the
outside, and the other opening is in the side wall of the station
against which the gallery is built. The two openings for each wire being
thus at right angles to each other, and the opening to the outside air
being protected from the wind by its horizontal position, no more than a
permissible amount of cold air, it is said, finds its way into the
station.
In some cases with lines of moderate voltage, say 10,000 to 15,000, and
in probably the majority of cases with lines of 25,000 volts or more,
the entry for the high-tension wires is entirely closed. An example of
this practice may be seen at the various sub-stations of the New
Hampshire Traction Company, which are located along their 12,000-volt
line between Portsmouth and Pelham, in that State.
For the entry of each wire on these lines a sixteen-inch square opening
is made in the brick wall of the sub-station. On the outside of this
wall a box is built about a group of three or more of these openings
located side by side. The top or roof of this box is formed by a slab of
bluestone three inches thick, which is set into the wall and extends
twenty-six inches from the face of the wall, with a slight slope from
the horizontal.
The ends, the bottom, and the outer side of this box are formed by slabs
of slate one inch thick, so that the enclosed space has an area in
vertical cross section at right angles to this building 15.5 inches high
and twenty-two inches wide.
[Illustration: FIG. 89.--Cable Entering Building.]
In the bottom of this box there is a circular opening for each wire, and
into this opening fits a heavy glass or porcelain bushing through which
the wire passes. After reaching the inside of the box the wire turns at
right angles and passes through the sixteen-inch square opening into the
sub-station. Beneath the box a special insulator is secured by an iron
bracket to the outside of the brick wall for each line wire, and this
insulator takes the strain of the wire before it is carried up through
the bushing in the bottom of the box. This form of entry is permissible
where the desire is to exclude cold air from the station, and where the
voltage is not high enough to cause serious leakage over the surface of
the bushing and the slate forming the bottom of the box. In all of the
cases above mentioned the wires used to enter the stations were the
regular line conductors and were bare.
Another type of entry in sub-stations is that employed on the extensive
transmission system between Spier Falls, Schenectady, and Albany, N. Y.
The maximum voltage on this system is 30,000, and the lines usually
enter each sub-station through the brick wall at one of its gable ends.
Outside of and about the entry of each circuit or group of circuits a
wooden shelter is built on the brick wall of the sub-station. Each
shelter has a slanting roof that starts from the brick wall at some
distance above the openings for the entrance of the line, and terminates
in a gutter. The front of each shelter is carried down three feet below
the centre of the openings in the brick wall, and the ends go still
lower. The front of each shelter is four feet in height, is four feet
from the face of the brick wall, and has a circular opening of 10-inch
diameter for each wire of the transmission line.
In line with each circular opening in the wooden shield there is an
opening of 15-inch diameter in the brick wall of the sub-station, and
into this opening in the brickwork fits a ring of wood with 15-inch
outside and 11-inch inside diameter. To this wooden ring a 15-inch disc
of hard fibre 1/8-inch thick is secured, and a porcelain tube 24 inches
long and of 2-inch inside diameter passes through a hole in the centre
of this disc. Within the wooden shield and in line with each circular
opening in it and with the corresponding porcelain tube through the
fibre disc a line insulator is secured. Within the sub-station and in
line with each tube there is also an insulator, and the two insulators
near opposite ends of each tube hold the line wire that passes through
it in position.
Each wire of the transmission lines, of which the largest is No. 000
solid of 0.410-inch diameter, terminates at one of the insulators within
the wooden shield, and is there connected to a special insulated wire
that passes through one of the porcelain tubes into the sub-station. A
copper trolley sleeve 12 inches long is used to make the soldered
connection between the bare line wire and the insulated conductor that
passes through the porcelain tube. Each of these entry cables, whatever
its size, is insulated first with a layer of rubber 9/32-inch thick,
then with varnished cambric wound on to a thickness of 9/32-inch, and
lastly with two layers of weather-proof braid outside of the cambric.
This form of closed entry for the transmission lines obviously excludes
snow, rain, cold air, and dust from the station. Whether the fibre discs
and wooden rings, together with the insulation on the entry cables, are
as desirable as a glass disc at the entry is another question.
Another instance where the entry for a high-tension line is closed with
the aid of combustible material is that of the 25,000-volt transmission
between the water-power plant at Chambly, on the Richelieu River, and
the sub-station in Montreal. The four three-phase circuits of this line
are made up of No. 00 wires of 0.365-inch diameter each, which enter the
power-station at Chambly and the terminal-house in Montreal bare, as
they are outside.
At each end of the line the wires are secured to insulators on a
horizontal arm with their centres twenty-two inches outside of an end
wall of the station or terminal building. The insulators are mounted
with their centres thirty inches apart, and a few inches above the tops
of these insulators a corresponding row of wooden bushings pass through
the wall with an outward slant.
At the Chambly end of the line each of these bushings is of oak, boiled
in stearin, four inches in diameter and twelve inches long. At the
Montreal end the wall bushings are of boxwood, and each is four inches
square and twelve inches long. Each of the wooden bushings carries a
glass tube, and is itself held in position by the concrete of the wall
in which it is located. Entrance to the station by each of the bare No.
00 wires is gained through one of these glass tubes, and cold air is
excluded.
Quite a different type of closed entry for the wires of a transmission
line is in use on that between Shawinigan Falls and Montreal, which
operates at 50,000 volts. For the entry of each of the three aluminum
cables that make up this line, each cable being composed of seven No. 6
B. & S. gauge wires, a tile pipe of twenty-four-inches diameter was set
into the station wall. The end of each tile pipe is closed by a glass
plate, with a small hole at its centre, through which the cable passes.
As the cable is thus held twelve inches from the terra cotta pipe all
the way around, any leakage of current must pass over this length of
glass surface at each cable or through the air.
A heavy coating of frost sometimes collects on these plates, and this
increases the amount of current leakage over them. Surface leakage in a
case of this sort, of course, varies with the size of the glass plate,
and if a tile pipe is used the limit of size is soon reached.
There seems to be no good reason, however, why a glass plate of any
desired dimensions should not be set directly into the brick wall of a
station for each line wire and the tile pipes entirely omitted. This
plan is followed on the system of the Utah Light & Power Company, which
extends to Salt Lake City, Ogden, Provo, and a number of other points in
that State.
On the 40,000-volt line of that system an entry for each wire is
provided by setting two plates of glass into the brick wall, one plate
being flush with the inner surface and the other with the outer surface
of the wall.
In the centre of each plate there is a hole of about 2.5-inch diameter,
into which a glass or porcelain tube fits. The line wire enters the
station through this tube, and it does not appear that any shelter for
the glass plates is located outside of the building. An entry of this
type for the 40,000-volt line with glass plates in a brick wall at a
gable end of the Murphy mill is said to have given satisfactory results
during four years, though that wall faces the southwest, from which
direction most of the storms come. At this entry each glass plate is not
more than eighteen inches in diameter, and the wires are about four feet
apart. On a 16,000-volt line of the same company, a glass plate twelve
inches square with a three-quarter-inch hole at its centre, and the bare
wire passing through without a tube, has given results that were
entirely satisfactory.
Two quite different types of entry to stations are used on the
50,000-volt line between Cañon Ferry and Butte, Mont. One type, employed
at the side wall of a corrugated iron building, consists of a thick
bushing of paraffined wood carrying a glass tube two inches in diameter,
four feet long, with a side wall of five-eighths to three-quarter-inch,
through which the line conductor passes.
On the roof of the power-station at Cañon Ferry a vertical entry is made
with the 50,000-volt circuit. For this purpose each line wire is brought
to a dead end on three insulators carried by a timber fixture on the
roof. A vertical tap drops from each line wire and passes through the
roof and into the station. This roof is of wood, covered with tin
outside and lined with asbestos inside. Each tap is an insulated wire,
and elaborate methods are adopted in the way of further insulation, and
to prevent water from following the wire down through the roof.
Over the point of entrance sits a large block of paraffined wood with a
central hole, and down through this hole passes a long cylinder of paper
that extends some distance above the block. Into the top end of this
cylinder fits a wood bushing, and a length of the tap wire that has been
served with a thick layer of rubber is tightly enclosed by this bushing.
The rubber-covered portion of the tap wire also extends above the
bushing, and has taped to it a paper cone that comes down over the top
of the paper cylinder to keep out the water. On the outside of this
paper cylinder, at a lower point, a still larger paper cone is attached
to prevent water from following the cylinder down through the wooden
block. At the lower end of the paper cylinder, within the station, there
is another bushing of wood, and between this and the wooden bushing at
the top of the cylinder and inside of the paper cylinder there is a long
glass tube. Down through this tube and into the station the insulated
tap wire passes.
From the experience thus far gained with high-voltage lines, it seems
that their entrance into stations should always be at a side wall,
unless there is some imperative reason for coming down through the roof.
If climatic conditions permit, no form of entry can be more reliable
than a plain, ample opening through the wall with a large air-space
about each wire. If the opening must be closed, it had better be done
with one or more large plates of thick glass set directly into the
brickwork of the wall. Some additional insulation is obtained by placing
a long glass or porcelain tube over each wire where it passes through
the central hole in the glass plates. Each conductor should be bare at
the entry, as it is on the line. Some of the above examples of existing
practice in entries for transmission lines are taken from Vol. xxii., A.
I. E. E.
CHAPTER XX.
INSULATOR PINS.
Wooden insulator pins are among the weakest elements in electric
transmission systems. As line voltages have gone up it has been
necessary to increase the distances between the outside petticoats of
insulators and their cross-arms and to lengthen the insulators
themselves in order to keep the leakage of current between the
conductors within permissible limits. To reduce the leakage, the wires
on most lines are located at the tops instead of in the old position at
the sides of their insulators.
All this has tended to a large increase of the mechanical strains that
operate to break insulator pins at the point where they enter the
cross-arm, because the strain on each line wire acts with a longer
leverage. Again, it is sometimes necessary that transmission lines make
long spans across rivers or elsewhere, and a very unusual strain may be
put on the insulator pins at these places.
As long as each electric system was confined to a single city or town a
broken insulator pin could be quickly replaced, and any material
interruption of service from such a cause was improbable. Where the
light and power supply of a city, however, depends on a long
transmission line, as is now the case in many instances, and where the
line voltage is so great that contact between a wire and a cross-arm
will result in the speedy destruction of the latter by burning, a broken
pin may easily lead to a serious interruption of the service.
Besides the increase of mechanical strains on insulator pins, there is
the danger of destruction of wooden pins by charring, burning, and other
forms of disintegration due to leakage of current over the insulators.
This danger was entirely absent in the great majority of cases so long
as lines were local and operated at only moderate voltages. These
several factors combined are bringing about marked changes in design.
On straight portions of a transmission line the insulator pins are
subject to strains of two principal kinds. One of these is due directly
to the weight of the insulators and line wire, and acts vertically to
crush the pins by forcing them down onto the cross-arm. The other is due
to the horizontal pull of the line wire, which is often much increased
by coatings of ice and by wind pressure, tending to break the pins by
bending--most frequently at the point where they enter the cross-arm. A
strain of minor importance on pins is that encountered where a short
pole has been set between two higher ones, and the line at the short
pole tends to lift each insulator from its pin, and each pin from the
cross-arm.
Where the line changes its direction, as on curves and at corners, the
side strain on pins is greatly increased, and such places give by far
the largest amount of trouble through the breaking of pins. The latter
seldom fail by crushing through the weight of the lines they support,
because the size of pin necessary to withstand the bending strain has a
large factor of safety as to crushing strength. Insulators are sometimes
lifted from wooden pins, and the threads of these pins stripped where a
short pole is used, as already noted; but failure of this kind is not
common.
Iron pins are either screwed or cemented into their insulators, but the
cemented joint is much more desirable, because where a screw joint is
made the unequal expansion of the iron and the glass or porcelain is apt
to result in breakage of the insulator. Where cement is used, both the
pins and insulators should be threaded or provided with shoulders of
some sort, so that, although the shoulders of threads do not come into
contact with each other, they will, nevertheless, help to secure a
better hold. Pure Portland cement, mixed with water to a thick liquid,
has been used with success, the insulator being placed upside down and
the pin held in a central position in the hole of the insulator while
the cement is poured in. Another cement that has been used for the same
purpose is a mixture of litharge and glycerin. Melted sulphur is also
available.
The same forces that tend to lift an insulator from its pin tend also to
pull the pin from its socket in the cross-arm or pole top. With wooden
pins the time-honored custom has been to drive a nail into the side of
the cross-arm so that it enters the shank of the pin in its socket. This
plan is good enough so far as immediate mechanical strength is
concerned, but is not desirable, because it is hard to remove a nail
when a pin is to be removed, and also because the rust of the nail rots
the wood. A better plan is to have a small hole entirely through each
cross-arm and insulator pin at right angles to the shank of that pin in
its socket, and then to drive a small wooden pin entirely through from
side to side.
Some of the important factors affecting the strains on insulator pins
vary much on different transmission lines, as may be seen from the
following table of lines on which wooden pins are used. On the older
line between Niagara Falls and Buffalo, the regular length of span is 70
feet, and each copper conductor of 350,000 circular mils is attached to
its insulator 7.5 inches above the cross-arm. On the new line the length
of span is 140 feet, and each aluminum conductor of 500,000 circular
mils is attached to its insulator 10 inches above the cross-arm.
TABLE I.--DATA OF LINES ON WOODEN PINS.
+----------------------------+-------------+-----------+-------------+
| |Circular Mils|Feet Length| Inches from |
|Location of the Lines. | of Each | of Span | Wire to |
| | Conductor. | Between |Shank of Pin.|
| | | Poles. | |
+----------------------------+-------------+-----------+-------------+
|Colgate to Oakland | [B]133,100 | ... | 13 |
|Electra to San Francisco | [A]471,034 | 130 | 15 |
|Cañon Ferry to Butte | [B]105,600 | 110 | 13-1/2 |
|Shawinigan Falls to Montreal| [A]183,750 | 100 | 16-1/4 |
|Niagara Falls to Buffalo | [B]350,000 | 70 | 7-1/2 |
|Niagara Falls to Buffalo | [A]500,000 | 140 | 10 |
|Chambly to Montreal | [B]133,100 | 90 | 8-1/2 |
|Colgate to Oakland | [A]211,600 | ... | 13 |
+----------------------------+-------------+-----------+-------------+
[A] Aluminum conductors.
[B] Copper conductors.
TABLE II.--DIMENSIONS OF WOODEN PINS IN INCHES.
+-----------------------+------+------+------+------+--------+--------+
| |Length|Length|Diam- | Diam-|Diameter| Length |
| Location of Lines. | of | of | eter | eter | of | of |
| |Stem. |Shank.| of | of |Threaded|Threaded|
| | | |Shank.|Shoul-| End. | Part. |
| | | | | der. | | |
+-----------------------+------+------+------+------+--------+--------+
|Colgate to Oakland |10-3/8| 5-3/8| 2-1/8| 2-1/2| 1-3/8 | 2 |
|Electra to San | | | | | | |
|Francisco |12 | 4-7/8| 2-1/4| 2-3/4| 1-3/8 | 2 |
|Cañon Ferry to Butte |12-1/2| 5-1/8| 2 | 2-1/2| 1-1/8 | 3 |
|Shawinigan Falls to | | | | | | |
|Montreal |13-1/2| 5 | 2-3/4| 3 | 1 | .. |
|Niagara Falls to | | | | | | |
|Buffalo[A] | 5-1/4| 6 | 2 | 2-3/4| 7/8 | 1-1/2 |
|Niagara Falls to | | | | | | |
|Buffalo[B] | 7-3/4| 6 | 2-1/4| 2-3/4| 1-1/2 | 2-1/2 |
|Chambly to Montreal[C] | 7 | 5 | 1-1/2| 1-7/8| .. | .. |
|Cañon Ferry to Butte[D]|12-3/8| 7-7/8| 2-1/8| 2-1/2| 1-1/8 | 3 |
+-----------------------+------+------+------+------+-------+---------+
[A] Pins on old line.
[B] Pins on new line.
[C] Approximate dimensions.
[D] Pole top pins.
To compensate for the greater strains introduced by doubling the length
of span and using pins of longer stem, the diameter of the shank of the
new pins was increased to two inches. One line between Colgate and
Oakland is of copper, and the other is of aluminum conductors, but the
same pins appear to be used for each. On the line between Cañon Ferry
and Butte, Mont., the pin used in pole tops has a shank 2-3/4 inches
longer and 1/8-inch greater in diameter than the pin used in cross-arms.
The weakest pin included in the table seems to be that in use on the
line between Chambly and Montreal, which is of hickory wood, about 1-1/2
inches in diameter at the shank, and carries its No. 00 copper wire
8-1/2 inches above the cross-arm.
The following dimensions for standard wooden insulator pins to be used
on all transmission lines are proposed in vol. xxi., page 235, of the
Transactions of the American Institute of Electrical Engineers. These
pins are designed to resist a uniform pull at the smaller end and at
right angles to the axis in each case. The length of each pin, in inches
between the shoulder and the threaded end, is represented by L, and the
diameter of each pin at its shank by D.
L. D.
1 0.87
2 1.10
3 1.26
4 1.39
5 1.50
6 1.59
7 1.67
8 1.75
9 1.82
10 1.88
11 1.95
13 2.06
15 2.17
17 2.25
19 2.34
21 2.42
The two strongest pins in Table II. appear to be those in use on the
line between Shawinigan Falls and Montreal and on the line from Niagara
Falls to Buffalo. The former have a diameter of 2-3/4 inches at the
shank, and the wire is carried 16-1/4 inches above the shoulder of the
pin. On the new Niagara line the shank diameter of each pin is only
2-1/4 inches, but the line wire is only 10 inches above the shoulder. It
was found by tests that a strain of 2,100 pounds at the top of the
insulator and at right angles to the axis of this Niagara pin was
necessary to break it at the shank. This strain is about six times as
great as the calculated maximum strain that will occur in service on the
line.
Some of the pins here noted are much stronger than those proposed in the
above specifications for standard pins. The pins on the old Niagara line
have a shank diameter of 2 inches, with a stem only 5-1/4 inches long,
while the proposed pin of 2 inches diameter at the shank has a stem 11
inches long. On the Colgate and Oakland line a shank diameter of 2-1/8
inches goes with a length of 10-3/8 inches in the stem, but the proposed
pin with this size of shank has a stem 13 inches long. For a shank of
2-1/4 inches diameter the proposed pin has a stem 15 inches long, but
the pins with this diameter of shank on the Electra line are only 12
inches long in the stem.
The 2-1/4-inch diameter of shank in the pins on the new Niagara line
goes with a length of only 7-3/4 inches in the stem. The new Niagara pin
is thus almost exactly twice as strong as the proposed pin, since the
strength of a pin where the shank joins the stem varies inversely as the
length of the stem, all other factors being the same.
Pins on the Shawinigan Falls line have a shank 2-3/4 inches in diameter,
with a length of 13-1/2 inches in the stem; but the largest of the
proposed pins, that with a stem 19 inches long, has a diameter of only
2-1/2 inches in the shank.
It is hardly too much to say in the interest of good engineering that
the wooden pin of about 5 inches length of stem and 1-1/2 inches
diameter of shank, as well as all longer pins of no greater strength,
should be discarded for long transmission lines of high voltage. These
pins have done good service on telegraph and telephone lines, and on
local lighting circuits of No. 6 B. & S. gauge wire or smaller, and they
may well be left for such work.
To meet the conditions of transmission work a change in both the shape
and size of pins is necessary. In the first place, the shoulder on pins
where the shank and stem meet, that relic of telegraph practice, should
be entirely discarded. This change will save considerable lumber on pins
of a given diameter at the shank, and will increase the strength of the
pin by avoiding the sharp corner at the junction of the shank and stem.
Another change of design should leave an excess of strength in the stem
of the pin, to provide for deterioration of the wood, and particularly
for charring by current breakage. This increase of diameter and strength
near the top of the pin will cost nothing in lumber, for the wood is
necessarily there unless it is turned off. The shank of each pin should
be proportionately shorter than in the older type, and the pin hole
should be bored only part way through the cross-arm. A saving in lumber
for pins and for cross-arms will thus be made, since the size of the
cross-arm may be less for a given resistance to splitting.
With these changes in general design the pin is a simple cylinder in the
shank, with a gentle taper from the shank to form the stem. An example
of this design, which might well serve as a basis for a line of standard
pins, would be a pin 2 inches in diameter and 3-1/2 inches long in the
shank, and tapering for a length of 5 inches from the shank to form the
stem, with a diameter of 1-1/2 inches at the top. The hole in a
cross-arm for this pin should be 3-1/2 inches deep, and this, in an arm
4-3/4 inches deep, would leave 1-1/4 inches of wood below the pin. From
the lower end of the pin hole, a hole 1/4-inch in diameter should run to
the bottom of the cross-arm to drain off water. A line of longer pins
designed to resist the same line pull as this short one would be strong
enough for small conductors, say up to No. 1 B. & S. gauge wire.
For larger wires, long spans and sharp angles in a line, a pin 2-1/4
inches in diameter and 4-1/2 inches long in the shank, tapering for 5
inches to a diameter of 1-3/4 inches at the top, or longer pins of equal
strength, should be used.
Where the pin holes do not extend through the cross-arm there is no need
of a shoulder on the pin to sustain the weight of the line wire. In the
cross-arm on the new Niagara Falls line each pin hole is bored to a
depth of 5 inches, leaving 1 inch of wood below the hole. On the line
from Electra to San Francisco the depth of each pin hole is again 5
inches, and the depth of the cross-arm 6 inches.
The pins for use on the Electra line were kept for several hours in a
vat of linseed oil at a temperature of 210° F. The pins for the
Shawinigan line were boiled in stearic acid. All wooden pins should be
treated chemically, but the object of this treatment should be to
prevent decay rather than to give them any particular insulating value.
The lack of strength in wooden pins and their destruction in some cases
by current leakage are leading to the use of iron and steel pins. Such a
pin, in use on the lines of the Washington Power Company, of Spokane,
Wash., is made up of a mild steel bar 17-1/2 inches long and 1-1/8
inches in diameter, cast into a shank at one end, so that the total
length is 18 inches. The cast-iron shank has a diameter of 2-1/16
inches, with a shoulder of 2-1/2 inches diameter at its upper end. To
prevent the pin from lifting out of its hole a small screw enters the
top of the cross-arm and bears on the top end of the shank. Above the
cast-iron shank the length of the steel rod is 12 inches, and starting
3/4 inch down from its top a portion of the rod 3/4 inch long is turned
to a diameter of one inch.
It is said that this pin begins to bend with a pull of 1,000 pounds at
its top, but that it will support the insulator safely even when badly
bent.
Insulators may resist puncture and prevent surface arcing from wire to
pin, but still allow a large though silent flow of energy over the pins
and cross-arms between the conductors of a transmission circuit. The
rate at which current flows from one wire of a transmission circuit to
another in this way depends on the total resistance of each path over
insulator surfaces and through air to the pins and cross-arm, and then
over these parts.
If the pins and cross-arm are entirely of iron, the total resistance of
the path through them from wire to wire is practically that of the
insulator surfaces. If the pins and cross-arm are of wood which is dry,
they may offer an appreciable part of the total resistance of the path
through them between the wires of a circuit; but if the wood be wet, its
resistance is very much reduced.
The resistance of wooden pins and cross-arm may be so small compared
with that of the air and insulator surfaces that complete the path
through them from wire to wire of a circuit, that the effect of these
wooden parts in checking the flow of current between conductors is
relatively unimportant, and yet the resistances of these pins and the
cross-arm may affect their lasting qualities.
The current that flows over the pins and cross-arms from one wire to
another of a high-tension circuit may be so small as not to injure these
wooden parts when evenly distributed over them, and yet this same
current may char or burn the wood if confined to a narrow path. Such a
leakage current will naturally cease to be evenly distributed over pins
and their cross-arms when certain portions of their surfaces are of much
lower resistance than others, because an electric current divides and
follows several possible paths in the inverse ratio of their
resistances.
These narrow paths of relatively low resistance along wooden pins and
cross-arms are heated and charred by the very current that they attract,
so that the conductivity of the path and the heat developed therein
react mutually to increase each other, and tend toward the destruction
of the wood.
Among causes that tend to make some parts of pins and cross-arms better
conductors than others, there may be mentioned cracks in the wood, where
dirt and moisture collect, dust, with a mixture of salt deposited on the
wood by the winds at certain places, and sea fogs that are often blown
only against one side of the pins and arms and deposit salt.
To make matters worse, the same cause that creates a path of relatively
good conductivity along wooden pins and cross-arms often materially
lowers the resistance offered to the leakage of current by the insulator
surfaces. Thus an increase of the rate at which energy passes from wire
to wire of a circuit, and the concentration of this energy in certain
parts of the wooden path, are sometimes brought about at the same time.
Where the line insulators employed are so designed that the resistance
of the dry wooden pins and cross-arms forms a material part of the total
resistance between the wires of a circuit, a rain or heavy fog may cause
a very large increase in the rate at which energy passes over these
wooden parts between the conductors.
As long as only moderate voltages were carried on line conductors, the
charring and burning of their pins and cross-arms was a very unusual
matter; but with the application of very high pressures on long circuits
the destruction of these wooden parts by the heat of leakage currents
has become a serious menace to transmission systems. Even with low
voltages there may be charring and burning of pins and cross-arms if the
line insulators are very poor or if the conditions as to weather and
flying dust are sufficiently severe.
In vol. xx. of the Transactions of the American Institute of Electrical
Engineers, pages 435 to 442 and 471 to 479, an account of the charring
and burning of pins on several transmission lines is given, from which
some of the following examples are taken.
In one case a line that ran near a certain chemical factory was said to
be much troubled by the burning of its pins, though the voltage employed
was only 440, and the insulators were designed for circuits of 10,000
volts. In rainy weather, when insulators, pins, and cross-arms were
washed clear of the chemical deposits, there was no pin burning. Similar
trouble has been met with on sections of the 40,000-volt Provo line, in
Utah, where dust, mixed with salt, is deposited on the insulators, pins,
and cross-arms. On page 708 a 2,000-volt line is mentioned on which fog,
dust, and rain caused much burning of pins.
When circuits are operated at voltages of 40,000 to 60,000, no very
severe climatic conditions are necessary to develop serious trouble in
the wooden pins by leakage currents, even where the transmission lines
are supported in insulators of the largest and best types yet developed.
Striking examples along this line may be seen in the transmission
systems between Colgate and Oakland, Cal., and between Electra and San
Francisco. Both of these systems were designed to transmit energy at
60,000 volts, but the actual pressure of operation seems to have been
limited to about 40,000 volts during much of their period of service.
Insulators of a single type and size are used on both of these
transmission lines, and are among the largest ever put into service on
long circuits. Each of these insulators is 11 inches in diameter, and
11-1/4 inches high from the lower edge to the top, the line wire being
carried in a central top groove. The wooden pins used on the two lines
vary a little in size, so that on the Electra line each pin stands
11-1/2 inches above its cross-arm, while on the Colgate line the
corresponding distance is 12 inches. As the insulators are of the same
size in each case, the length of the pin between the lower edge of each
insulator and the top of the cross-arm is 4 inches on the Colgate line
and 3-1/2 inches on the Electra line.
On the latter line a porcelain sleeve, entirely separate from and making
no contact with the insulator, covers each pin from the top of its
cross-arm to a point above the lower edge of the insulator. On the
Colgate line each insulator makes contact with its pin for a length of
2-1/2 inches down from the top of its thread, and on the Electra line
the contact of each insulator with its pin runs down 3-1/2 inches below
the top of the thread. This leaves 9 inches in the length of the pin
between the insulator contact and the top of each cross-arm on the
Colgate line, and a corresponding length of pin amounting to 8-1/2
inches on the Electra line. Of this 8-1/2 inches of pin surface, about 6
inches is covered by the porcelain insulating sleeve used on each pin of
the Electra line, so that only about 2-1/2 inches of the length of each
pin on that line is exposed to the leakage of current from the insulator
directly through the air. Both the sizes of pins just mentioned were
made of eucalyptus wood, boiled in linseed oil.
Each one of three pins taken from a pole, between North Tomer and
Cordelia, on the Colgate line, was badly charred and burned on its side
that faced the damp ocean winds. This charring extended all the way down
each pin from the point where the insulator made contact with it, a
little under the threads, to the top of the cross-arm nine inches below.
Two of these pins were located at the opposite ends of a cross-arm, and
the third was fixed in the top of the pole. This cross-arm was charred
or burnt, as well as the pin, but no defects could be detected in the
insulators that the pins supported.
As to these three pins, the most reasonable explanation seems to be that
enough current leaked over both the outside and inside surfaces of each
insulator and through the air to char the pin and cross-arm. In flowing
down each pin, the current was naturally concentrated on the side
exposed to the damp winds of the ocean, because the deposit of moisture
by these winds lowered the resistance on that side. When these winds
were not blowing, and before a pin became charred on one side, its
resistance was probably about the same all the way around, and the
leakage current, being distributed over the pin, was not sufficient to
char it. The damp wind would, of course, lower the surface resistance of
each insulator, and this, with the deposit of moisture on the pins and
cross-arm, many have made a very material reduction in the total
resistance from wire to wire.
The insulators used on these pins each had two petticoats, an upper one,
11 inches in diameter, and a lower one, 6-1/2 inches in diameter, the
lower edge of the smaller petticoat being 7-1/2 inches beneath the lower
outside edge of the larger petticoat. As the inner surface of the larger
petticoat was much nearer to a horizontal plane than the inner surface
of the smaller petticoat, moisture would have been more readily retained
on it, and the greater part of the surface resistance of the insulator
during wet weather must therefore have been on the inside of the smaller
petticoat. At its lower edge the smaller petticoat was distant radially
about 1-3/4 inches from the pin, and the distance between the pin and
the inside surface of the smaller petticoat gradually decreased to
actual contact at a point 5-1/2 inches above this lower edge.
The path of the current from the line wire to the pin in this case seems
to have been first over the entire insulator surface to the lower edge
of the smaller petticoat and then partly up over the inner surface of
this petticoat and partly from that surface through the air. On each of
these three pins the charring was quite as bad just below the thread as
it was further down, so that a large part of the leakage current seems
to have gone up over the interior surface of the smaller petticoat. The
charred portion of these pins extended but little, if at all, into the
threads near the tops or into the part of the pin fitting into the
cross-arm. The preservation of the part of each pin that entered the
cross-arm seems to have been due to the increase of surface and decrease
of resistance of the cross-arm in comparison with the pin. Preservation
of the threaded part of each pin seems to have been due to its
protection from moisture and its high resistance, so that little or no
current passed over it.
Another pin taken from the same line as the three just considered was
badly burned at a point about 1.75 inches below the threads, but on
sawing it completely across at two points below the charred spot the
entire section was found to be perfectly sound and free from any sign of
burning. The explanation of the condition of this pin is, perhaps, that
the resistance of the burned part, owing to its additional protection
and dryness, was high compared with that of the lower part of the pin,
and thus developed most of the heat on the passage of current. It is not
clear, however, why this pin should burn only just below the thread,
while other pins of the same kind on the same line were charred all the
way down from the thread to the cross-arm.
Another curious result noticed in some pins on this same line is the
softening of the threads so that they can be rubbed off with the
fingers.
RELATION OF PINS AND INSULATORS.
+------------------------------+-------+--------+--------+-------------+
| |Voltage|Diameter| Height |Length of Pin|
| Location of Line. | of | of | of | Covered by |
| | Line. |Insula- | Insula-| Insulator. |
| | | tor. | tor. | |
+------------------------------+-------+--------+--------+-------------+
| | | Inches.| Inches.| Inches. |
|Electra to San Francisco | 60,000| 11 | 11-1/4 | 12 |
|Colgate to Oakland | 60,000| 11 | 11-1/4 | 8 |
|Cañon Ferry to Butte | 50,000| 9 | 12 | 10-1/2 |
|Shawinigan Falls to Montreal | 50,000| 10 | 13 | 10-1/4 |
|Santa Ana River to Los Angeles| 33,000| 6-3/4 | 4-7/8 | 2-1/2 |
|Provo around Utah Lake | 40,000| 7 | 5-3/4 | 4-3/4 |
|Spier Falls to Schenectady | 30,000| 8-1/2 | 6-3/4 | 5-1/4 |
|Niagara Falls to Buffalo | 22,000| 7-1/2 | 7 | 5 |
+------------------------------+-------+--------+--------+-------------+
The softened wood of the threads is not charred, but is said to have a
sour taste and to resemble digested wood pulp. While the threads of a
wooden pin are destroyed in this way the remainder of the pin may still
remain perfect and show no charring.
RELATIONS OF PINS AND INSULATORS.
+------------------------------+----------+----------+-----------+
| |Length of | Distance | Distance |
| | Pin |from Outer|from Lowest|
| Location of Line. | Between | Petticoat| Petticoat |
| | Insula- | to Pin | to Pin |
| | tor and | Through | Through |
| |Cross-arm.| Air. | Air. |
+------------------------------+----------+----------+-----------+
| | Inches. | Inches. | Inches. |
|Electra to San Francisco | 0 | 10-1/2 | 3-1/2 |
|Colgate to Oakland | 3-1/2 | 10 | 2-1/2 |
|Cañon Ferry to Butte | 1-1/2 | 0 | 1-1/2 |
|Shawinigan Falls to Montreal | 3-1/4 | 9-1/2 | 1 |
|Santa Ana River to Los Angeles| 3-1/2 | 2-3/4 | .. |
|Provo around Utah Lake | 3-1/2 | 2-1/2 | .. |
|Spier Falls to Schenectady | 4 | 4 | 5/8 |
|Niagara Falls to Buffalo | 3 | 4-1/2 | 2 |
+------------------------------+----------+----------+-----------+
In explanation of this disintegration of the threads of wooden pins it
was stated that a number of these pins, the tops of which were reduced
to a white powder, had been taken from the line between Niagara Falls
and Buffalo, on which the voltage is 22,000, and that this powder proved
on analysis to be a nitrate salt. This salt was thought to be the result
of the action of nitric acid on the wood, it being supposed that the
acid was formed by a static discharge acting on the oxygen and nitrogen
of the air between the threads of the insulator and pin. In support of
this view it was stated that an experimental line of galvanized-iron
wire at Niagara Falls, which was operated at 75,000 volts continuously
during nearly four months, turned black over its entire length of about
two miles. This surface disintegration was not due to the normal action
of the air, for similar wire at the same place remained bright when not
used as an electrical conductor.
These facts seemed to indicate that the brush discharge from the wires
carrying the 75,000-volt current developed nitric acid from the oxygen
and nitrogen of the air, and that this acid attacked the wire.
One of the above-mentioned pins used on the Electra line was much
charred and burned away at a point a little below the threads. The
charred path of the current could also be traced down the side of the
pin to the cross-arm, but this path was not as badly burned as the spot
near the top of the pin.
A composite pin from a 33,000-volt line, probably a part of the
transmission system between the Santa Ana River and Los Angeles, was
burned through its wooden threads to the central iron bolt, along a
narrow strip at one side. Every pin burned on this line was said to show
the effects of the current in the way just described, but no cross-arms
were burned and very few insulators punctured.
The composite pin was made up of a central iron bolt 10-5/8 inches long,
1/2-inch in diameter, and with a thin head above the wooden threads, a
sleeve of wood 2-5/8 inches long and 1 inch in diameter in its threaded
portion, and a sleeve of porcelain 3-1/8 inches long and 1-1/4 inches in
diameter at its upper and 2-11/16 inches at its lower end. The sleeves
of wood and porcelain were slipped over the central iron bolt so that
the portions of the pin above the cross-arm measured 5-7/8 inches. In
this case the path of the leakage current seems to have been over both
the exterior and interior surface of the insulator and then through the
wooden sleeve to the central bolt and the cross-arm.
The facts just outlined certainly indicate a serious menace to the
permanence and reliability of long, high-voltage transmission lines
supported by insulators on wooden pins. If such results have been
encountered on the lines above named, where some of the largest and best
designs of insulators are employed, it is only fair to assume that
similar destructive effects of leakage currents are taking place on many
other lines that operate at high voltages.
It seems at least doubtful whether any enlargement or improvement of the
insulators themselves will entirely avoid the destruction of their
wooden pins in one of the ways mentioned. It is probable, but not
certain, that further extension of distances through air and over
insulator surfaces, both exterior and interior, between line wires and
wooden pins, will prevent charring and burning of the latter by leakage
currents. Much has already been done in the way of covering most of the
pin above its cross-arm with the insulator parts, but even those
portions of the pin that are best protected in this way are not free
from burning.
Thus, on the Colgate line, eight inches of each pin is protected by the
interior surface of its insulator, but these pins were charred quite as
badly where best protected, up close to the thread, as they were down
near the cross-arm. The same is true of the Electra line, where a
porcelain sleeve runs up about the pin from the cross-arm to a point
above the inner petticoat of each insulator, so that the entire length
of the pin above the cross-arm is protected. On the Cañon Ferry line, a
glass sleeve that virtually forms a part of each insulator, though
mechanically separate from it, protects the pin from its threaded
portion to within 1.5 inches of the cross-arm.
Insulators on the line from Shawinigan Falls to Montreal are each 13
inches long and extend down over the pin to within 1.5 inches of the
cross-arm. The burned portion of each pin from the Santa Ana line was
that carrying the threads, and thus in actual contact with that part of
the insulator which was separated by the greatest surface distance from
the line wire.
Aside from the burning of pins is the destruction of their threaded
parts by some chemical agency that is developed inside of the tops of
the insulators, as shown in the cases of the Colgate and Niagara lines.
It does not appear that any improvement of insulators will necessarily
prevent chemical action.
Though it may not be practicable to so increase the surface resistance
of each insulator that the burning of wooden pins by leakage current
will be prevented, the substitution of a conducting for an insulating
pin may remedy the trouble. As the insulators, pins, and cross-arm form
a path for the leakage current from wire to wire, the wooden pins by
their resistance, especially when dry, must develop heat. In pins of
steel or iron this heat would be trifling and would do no damage. With
pins of good conducting material, like iron, the amount of leakage from
wire to wire, with a given design of insulator, would, no doubt, be
somewhat greater than the leakage with wooden pins.
It will be cheaper, however, to increase the resistance of new
insulators up to the combined resistance of present insulators and
their wooden pins than it will be to replace these pins when they are
burned.
From all the evidence at hand, it seems that insulators which reduce the
leakage of current over their surfaces to permissible limits as far as
mere loss of energy is concerned, even with iron pins, will not prevent
the charring and destruction of wooden pins.
[Illustration: FIG. 90.--Glass Insulator and Sleeve on 50,000-volt Line
Between Cañon Ferry and Butte, Mont.]
When any suitable insulator is dry and clean it offers all necessary
resistance to the leakage of current over its surface, and any
resistance in the pin that carries the insulator is of small importance.
If the resistance of an insulator needs to be reinforced by that of its
pin in any case, it is when the surface of the insulator is wet or
dirty. Unfortunately, however, the same weather conditions that deposit
dirt or moisture on an insulator make similar deposits on its pin, and
the resistance of the pin is lowered much more than that of the
insulator by such deposits. The increase of current leakage over the
surface of an insulator during rains and fogs usually does no damage to
the insulator itself, but such leakage over the wet pin soon develops a
surface layer of carbon that continues to act as a good conductor after
the moisture that temporarily lowered the resistance has gone. Reasons
like these have led some engineers to prefer iron pins with insulators
that offer all of the resistance necessary for the voltage employed on
the line.
It may be suggested that the use of iron pins will transfer the charring
and burning to the wooden cross-arms, but this does not seem to be a
necessary result. The comparative freedom of cross-arms from charring
and burning where wooden pins are used seems to be due to the larger
surface and lower resistance of the cross-arms. With iron pins having a
shank of small diameter, so that the area of contact surface between the
pin and the wooden cross-arm is relatively small, there may be some
charring of the wood at this contact surface. Should it be thought
desirable to guard against any trouble of this sort, the surface of the
iron pin in contact with the cross-arm may be made ample by the use of
large washers, or by giving each pin a greater diameter at the shank
than elsewhere.
It may be noted that the pins with a central iron bolt only half an inch
in diameter, that were used on the 33,000-volt Santa Ana line, were said
to have caused no burning of their cross-arms in those cases in which
the wooden threads about the top of the central bolt were burned
through.
Another possible trouble with iron pins is that they, by their greater
rate of expansion than glass or porcelain, will break their insulators.
Such results can readily be avoided by cementing each iron pin into its
insulator, instead of screwing the insulator onto the pin. Iron pins
will, no doubt, cost somewhat more than those of wood, but this cost
will in any event be only a small percentage of the total investment in
a transmission line. Considering the cost of the renewals of wooden
pins, there seems little doubt that on a line where the voltage and
other conditions are such as to result in frequent burning, iron pins
would be cheaper in the end.
Iron pins have already been adopted on a number of high-voltage lines.
Not only iron pins, but even iron cross-arms and iron poles are in use
on a number of transmission lines. On a long line now under construction
in Mexico, iron towers, placed as much as 400 feet apart, are used
instead of wooden poles, and both the pins and cross-arms are also of
iron. The 75-mile line from Niagara Falls to Toronto is carried entirely
on steel towers.
The Vancouver Power Company, Vancouver, British Columbia, use a pin that
consists of a steel bolt about 12 inches long fitted with a sleeve of
cast iron 4-1/2 inches long to enter the cross-arm, and a lead thread to
screw into the insulator. On the 111-mile line of the Washington Power
Company, of Spokane, which was designed to operate at 60,000 volts and
runs to the Standard and Hecla mines, a pin consisting of a steel bar
1-1/8 inches in diameter, with a cast-iron shank 2-1/16 inches in
diameter to enter the cross-arm, and with the lead threads for the
insulator, is used.
[Illustration: FIG. 92.--Iron Pins on Spier Falls Line.]
On the network of transmission lines between Spier Falls, Schenectady,
Albany, and Troy, in the State of New York, the insulators are supported
on iron pins of two types. One of these pins, used at corners and where
the strain on the wire line is exceptionally heavy, is made up of a
wrought-iron bolt 3/4-inch in diameter and 16-1/2 inches long over the
head, and of a malleable iron casting 8-3/4 inches long. This casting
has a flange of 5 by 3-3/4 inches at its lower end that rests on the top
of the cross-arm, and the bolt passes from the top of the casting down
through it and the cross-arm. Threads are cut on the lower end of the
bolt, and a nut and washer secure it in the cross-arm. The total height
of this pin above the cross-arm is 9-1/4 inches.
For straight work on this line a pin with stem entirely of malleable
iron, and a bolt that comes up through the cross-arm and enters the base
of the casting, is used. The cast top of this pin has four vertical
webs, and its rectangular base, which rests on the top of the
cross-arm, is 3-1/2 by 4 inches. The bolt that comes up through the
cross-arm and taps into the base of the casting is 3/4-inch in diameter.
The cast part of this pin has such a length that the top of its
insulator is carried 10-3/4 inches above the cross-arm. For the casting
the length is 9-1/4 inches.
Both of the types of iron pins in use on the Spier Falls lines are
secured to their insulators with Portland cement poured into the pin
hole while liquid when the insulator is upside down and the pin is held
centrally in its hole. The top of each casting is smaller in diameter
than the hole in the insulator, and is grooved so as to hold the cement.
[Illustration: FIG. 93.--Standard Pin, Toronto and Niagara Line.]
On a long line designed for 60,000 volts, and recently completed in
California, wooden pins are used with porcelain insulators, each 14
inches in diameter and 12-1/2 inches high. Each of these pins is
entirely covered with sheet zinc from the cross-arm to the threaded end,
and it is expected that this metal covering will protect the wood of the
pin from injury by the leakage current.
CHAPTER XXI.
INSULATORS FOR TRANSMISSION LINES.
Line insulators, pins, and cross-arms all go to make up paths of more or
less conductivity between the wires of a transmission circuit. The
amount of current flowing along these paths from one conductor to
another in any case will depend on the combined resistance of the
insulators, pins, and cross-arm at each pole.
As a general rule, the wires of high-voltage transmission circuits are
used bare because continuous coverings would add materially to the cost
with only a trifling increase in effective insulation against high
voltages. In some instances the wires of high-pressure transmission
lines have individual coverings for short distances where they enter
cities, but often this is not the case. At Manchester, N. H., bare
conductors from water-power plants enter the sub-station, well within
the city limits, at 12,000 volts. From the water-power at Chambly the
bare 25,000-volt circuits, after crossing the St. Lawrence River over
the great Victoria bridge, pass overhead to a terminal-house near the
water-front in Montreal. In order to reach the General Electric Works,
the 30,000-volt circuits from Spier Falls enter the city limits of
Schenectady, N. Y., with bare overhead conductors.
Where transmission lines pass over a territory exposed to corrosive
gases, it is sometimes desirable to give each wire a weather-proof
covering. An instance of this sort occurs near Niagara Falls where the
aluminum conductors forming one of the circuits to Buffalo are covered
with a braid that is saturated with asphaltum for some distance.
Each path, formed by the surface of the insulators of a line and the
pins and cross-arm by which they are supported, not only wastes the
energy represented by the leakage current passing over it, but may lead
to the charring and burning of the pins and cross-arm by this current.
To prevent such burning, the main reliance is to be placed in the
surface resistance of the insulators rather than that of pins and
cross-arms. These insulators should be made of glass or porcelain, and
should be used dry--that is, without oil. In some of the early
transmission lines, insulators were used on which the lower edges were
turned inward and upward so that a circular trough was formed beneath
the body of the insulator, and this trough was filled with heavy
petroleum. It was found, however, that this trough of oil served to
collect dirt and thus tended to lower the insulation between wire and
cross-arm, so that the practice was soon abandoned. Glass and porcelain
insulators are rivals for use on high-tension lines, and each has
advantages of its own. Porcelain insulators are much stronger
mechanically than are those of glass, and are not liable to crack
because of unequal internal expansion, a result sometimes met with where
glass insulators are exposed to a hot morning sun. In favor of glass
insulators it may be said that their insulating properties are quite
uniform, and that, unlike porcelain, their internal defects are often
apparent on inspection. In order to avoid internal defects in large
porcelain insulators, it has been found necessary to manufacture some
designs in several parts and then cement the parts of each insulator
together.
Defective insulators may be divided into two classes--those that the
line voltage will puncture and break and those that permit an excessive
amount of current to pass over their surfaces to the pins and
cross-arms. Where an insulator is punctured and broken, the pin,
cross-arm, and pole to which it is attached are liable to be burned up.
If the leakage of current over the surface of an insulator is large, not
only may the loss of energy on the line where the insulator is used be
serious, but this energy follows the pins and cross-arm in its path from
wire to wire, and gradually chars the former, or both, so that they are
ultimately set on fire or break through lack of mechanical strength. The
discharge over the surface of an insulator may be so large in amount as
to have a disruptive character, and thus to be readily visible. More
frequently this surface leakage of current over insulators is of the
invisible and silent sort that nevertheless may be sufficient in amount
to char, weaken, and even ultimately set fire to pins and cross-arms.
All insulators, whether made of glass or porcelain, should be tested
electrically to determine their ability to resist puncture, and to hold
back the surface leakage of current, before they are put into practical
use on high-tension lines. Experience has shown that inspection alone
cannot be depended on to detect defective glass insulators. Electrical
testing of insulators serves well to determine the voltage to which they
may be subjected in practical service with little danger of puncture by
the disruptive passage of current through their substance. It is also
possible to determine the voltage that will cause a disruptive discharge
of current over the surface of an insulator, when the outer part of this
surface is either wet or dry. This is as far as electrical tests are
usually carried, but it seems desirable that such tests should also
determine the amount of silent, invisible leakage over the surface of
insulators both when they are wet and when they are dry, at the voltage
which their circuits are intended to carry. Such a test of silent
leakage is important because this sort of leakage chars and weakens
insulator pins, and sets fire to them and cross-arms, besides
representing a waste of energy.
The voltage employed to test insulators should vary in amount according
to the purpose for which any particular test is made. Glass and
porcelain, like many other solid insulators, will withstand a voltage
during a few minutes that will cause a puncture if continued
indefinitely. In this respect these insulators are unlike air, which
allows a disruptive discharge at once when the voltage to which it is
exposed reaches an amount that the air cannot permanently withstand.
Because of this property of glass and porcelain insulators, it is
necessary in making a puncture test to employ a voltage much higher than
that to which they are to be permanently exposed. In good practice it is
thought desirable to test insulators for puncture with at least twice
the voltage of the circuits which they will be required to permanently
support on transmission lines.
For the first transmission line from Niagara Falls to Buffalo, which was
designed to operate at 11,000 volts, the porcelain insulators were
tested for puncture with a voltage of 40,000, or nearly four times that
of the circuits they were to support.
Porcelain insulators for the second line between Niagara Falls and
Buffalo, after the voltage of transmission had been raised to 22,000,
were given a puncture test at 60,000 volts. Of these insulators tested
at 60,000 volts only about three per cent proved to be defective. These
puncture tests were carried out by placing each insulator upside down in
an open pan containing salt water to a depth of two inches, partly
filling the pin hole of the insulator with salt water, and then
connecting one terminal of the testing circuit with a rod of metal in
the pin hole, and the other terminal with the pan. Alternating current
was employed in these tests, as is usually the case (Volume xviii.,
Transactions A. I. E. E., pp. 514 to 520). For the transmission lines
between Spier Falls, Schenectady, Albany, and Troy, where the voltage is
30,000, the insulators were required to withstand a puncturing test with
75,000 volts for a period of five minutes after they had been soaked in
water for twenty-four hours.
There is some difference of opinion as to the proper duration of a
puncturing test, the practice in some cases being to continue the test
for only one minute on each insulator, while in other cases the time
runs up to five minutes or more. As a rule, the higher the testing
voltage compared with that under which the insulators will be regularly
used, the shorter should be the period of test. Instead of being tested
in salt water as above described, an insulator may be screwed onto an
iron pin of a size that fits its threads, and then one side of the
testing circuit put in contact with the pin and the other side connected
with the wire groove of the insulator. Care should be taken where an
iron pin is used either in testing or for regular line work, that the
pin is not screwed hard up against the top of the insulator, as this
tends to crack off the top, especially when the pin and insulator are
raised in temperature. Iron expands at a much higher rate than glass or
porcelain, and it is desirable to cement iron pins into insulators
rather than to screw them in. There seems to be some reason to think
that an insulator will puncture more readily when it is exposed to
severe mechanical stress by the expansion of the iron pin on which it is
mounted.
Tests of insulators are usually made with alternating current, and the
form of the voltage curve is important, especially where the test is
made to determine what voltage will arc over the surface of the
insulator from the line wire to the pin. The square root of the mean
square for two curves of alternating voltage or mean effective voltage,
as read by a voltmeter, may be the same though the maximum voltages of
the two curves differ widely. In tests for the puncture of insulators,
the average alternating voltage applied is more important than the
maximum voltage shown by the highest points of the pressure curve,
because of the influence of the time element with glass and porcelain.
On the other hand, when the test is to determine the voltage at which
current will arc over the insulator surface from the line wire to the
pin, the maximum value of the pressure curve should be taken into
consideration because air has no time element, but permits a disruptive
discharge under a merely instantaneous voltage.
Alternators used in transmission systems usually conform approximately
to a sine curve in the instantaneous values of the pressures they
develop, and it is therefore desirable that tests on line insulators be
made with voltages whose values follow the sine curve. Either a single
transformer or several transformers in series may be employed to step up
to the required voltage, but a single transformer will usually give
better regulation and greater accuracy. An air-gap between needle points
is not a very satisfactory means by which to determine the average
voltage on a testing circuit, because, as already pointed out, the
sparking distance between the needle points depends mainly on the
maximum instantaneous values of the voltage, which may vary with the
load on the generator, and the saturation of its magnets. For accurate
results a step-down voltmeter transformer should be used on the testing
circuit.
An insulator that resists a puncture test may fail badly when subjected
to a test as to the voltage that will arc over its surface from line
wire to pin. This arc-over test should be made with the outer surface of
the insulator both wet and dry. For the purpose of this test the
insulator should be screwed onto an iron pin, or onto a wooden pin that
has been covered with tinfoil. One wire of the testing circuit should
then be secured in the groove of the insulator, and the other wire
should be connected to the iron or tin foil of the pin. The voltage that
will arc over the surface of an insulator from the line wire to the pin
depends on the conditions of that surface and of the air. In light air,
such as is found at great elevations, an arc will jump a greater
distance than in dry air near the sea-level. A fog increases the
distance that a given voltage will jump between a line wire and its
insulator pin, and a heavy rain lengthens the distance still further.
The heavier the downpour of rain the greater is the distance over the
outside surface of an insulator that a given voltage will arc over. The
angle at which the falling water strikes the insulator surface also has
an influence on the voltage required to arc over that surface, a
deviation from a downpour perpendicular to the plane of the lower edge
of the petticoat of the insulator seeming to increase the arcing
distance for a given voltage.
An insulator should be given an arc-over test under conditions that are
approximately the most severe to be met in practice. These conditions
can perhaps be fairly represented by a downpour of water that amounts to
a depth of one inch in five minutes for each square inch of the plane
included by the edge of the largest petticoat of the insulator, when the
direction of the falling water makes an angle of forty-five degrees with
that plane. A precipitation of one inch in depth on a horizontal plane
during five minutes seems to be a little greater than any recorded by
the United States Weather Bureau. Under the severe conditions just
named, the voltage required to arc over the insulator surface from line
wire to pin should be somewhat greater at least than the normal voltage
of the circuit where the insulator is to be used. For the transmission
line between Spier Falls and Schenectady, on which the maximum voltage
is 30,000, the insulators were required to stand a test of 42,000 volts
when wet, without arcing over from line wire to pin. In these wet tests
the water should be sprayed evenly onto the insulator surface like rain,
and the quantity of water that strikes the insulator in a given time
should be measured.
When the outside of an insulator is wet with rain, it is evident that
most of the resistance between the line wire and the insulator pin must
be offered by the inside surface of the petticoat of the insulator. For
this reason an insulator that is to withstand a very high voltage so
that no arc will be formed over its wet outside surface must have a
wide, dry surface under its petticoat. In some tests of line insulators
reported in Volume xxi., Transactions A. I. E. E., p. 314, the results
show that the voltage required to arc over from line wire to pin depends
on the shortest distance between them, rather than on the distance over
the insulator surface. Three insulators, numbered 4, 5, and 7 in the
trial, were in each case tested by a gradual increase of voltage until a
discharge took place between the wire and pin. The pins were coated with
tinfoil, and the testing voltage was applied to the tie wire on each
insulator and to the tinfoil of its pin. Insulators 4, 5, and 7
permitted arcs from wire to pin when exposed to 73,800, 74,700, and
74,700 volts respectively, the surfaces of all being dry and clean. The
shortest distances between wires and pins over insulator surface and
through air were 6-5/8, 6-1/4, and 7-7/8 inches respectively for the
three insulators, so that the arcing voltages amounted to 11,140,
11,952, and 9,479 per inch of these distances. Measured along their
surfaces, the distances between wires and pins on these three insulators
were 8, 11-1/4, and 15-1/2 inches respectively, so that the three arcing
voltages, which were nearly equal, amounted to 9,225, 6,640, and 4,819
per inch of these distances. These figures make it plain that the arcing
voltage for each insulator depends on the shortest distance over its
surface and through the air, from wire to pin. It might be expected that
the voltage in any case would arc equal distances over clean, dry
insulator surface or through the air, and the experiments just named
indicate that this view is approximately correct. The sparking distance
through air between needle points, which is greater than that between
smooth surfaces, is 5.85 inches with 70,000 volts, and 7.1 inches with
80,000 volts according to the report in Volume xix., A. I. E. E., p.
721. Comparing these distances with the shortest distances between wires
and pins in the tests of insulators numbered 4, 5, and 7, which broke
down at 73,800 to 74,700 volts when dry, it seems that a given voltage
will arc somewhat further over clean, dry insulator surface than it will
through air. This view finds support from the fact that only a part of
each of the shortest distances between wire and pin was over insulator
surface, the remainder being through air alone.
The fact that the dry part of the surface of an insulator and the air
between its lower wet edge and the pin or cross-arm offer most of the
resistance between the line wire and the pin and cross-arm is plainly
brought out by the results of the tests above mentioned, in the cases of
insulators numbered 4 and 7. While 73,800 volts were required to arc
from line-wire to pin when the entire insulator was dry and clean, the
arc was formed at only 53,400 volts during a moderate rain-storm, in the
case of No. 4 insulator. With insulator No. 7 the arcing voltage was
74,700 when the entire surface was clean and dry, but the arc from wire
to pin was started at 52,800 volts during a moderate rain. No. 5
insulator seems to present an erratic result, for when dry and clean the
arc jumped from wire to pin at 74,700 volts, and yet during a moderate
rain no arc was formed until a voltage of 70,400 was reached. For each
of the seven insulators on which tests are reported as above, the
voltage required to arc from line wire to pin was nearly or quite as
great during a dry snow-storm as when the insulator surface was clean
and dry. When the insulators were covered with wet snow their surface
insulation broke down at voltages that were within ten per cent above or
below the arcing voltages during a moderate rain in five cases. With two
insulators the arcing voltages, when they were covered with wet snow,
were only about sixty per cent of the voltages necessary to break down
the surface insulation between wire and pin during a moderate rain.
When the outside surface of an insulator is wet, as during a moderate
rain, it seems that the under surface of the insulator, and the distance
through air from the lower wet edge of the insulator to the pin or
cross-arm, make up most of the insulation that prevents arcing over from
the wire to the pin or cross-arm. It further appears that it is useless
to extend the distance across the dry under surface of the insulator
indefinitely without a corresponding increase of the direct distance
through air from the lower wet edge of the insulator to the wood of
cross-arm or pin. Insulator No. 7 in the tests under consideration had a
diameter at the lower edge of its outer petticoat of seven inches, and
was mounted on a standard wooden pin. The diameter of this pin in the
plane of the lower edge of the insulator was probably about 1-1/4
inches, so that the radial distance through air from this edge to the
pin must have been 2-7/8 inches approximately. During a moderate rain
the surface insulation of this insulator broke down and an arc was
formed from wire to pin with 52,800 volts. The sparking distance between
needle points at 50,000 volts is 3.55 inches, according to Volume xix.,
A. I. E. E., p. 721, and must be shorter between smooth surfaces, such
as the wire and pin in question, so that nearly all of the 52,800 volts
in this case must have been required to jump the 2-7/8 inches of air,
leaving very little to overcome the slight resistance of the wet outside
surface of the insulator. On this insulator the surface distance from
wire to pin was 15-1/2 inches, while the shortest breaking distance was
only 7-7/8 inches, so that the distance across the dry under surface of
the insulator must have been 15-1/2 - (7-7/8 - 2-7/8) = 10-1/2 inches
approximately. It is evidently futile to put a path 10-1/2 inches long
across dry insulator surface in parallel with a path only 2-7/8 inches
long in air, as an arc will certainly jump this shorter path long before
one will be formed over the longer. The same line of reasoning applies
to No. 3 insulator in this test, which had a diameter of 6-3/4 inches, a
surface distance from wire to pin of 13 inches, and a minimum distance
of 7-1/4 inches, and whose surface insulation broke down at 48,600 volts
during a moderate rain. The necessity of increasing the distance between
the lower wet edges of insulators and the pins and cross-arm, as well as
the distance across the dry under surfaces of insulators, led to the
adoption of the so-called umbrella type for some high-voltage lines. In
this type of insulator the main or outer petticoat is given a relatively
great diameter, and instead of being bell-shaped is only moderately
concave on its under side. With an insulator of this type mounted on a
large, long pin, the lower edge of the umbrella-like petticoat may be
far removed from the pin and cross-arm. Beneath the large petticoat of
such insulators for high voltages there are usually one or more smaller
petticoats or sleeves that run down the pin, and increase the distance
between it and the lower edge of the largest petticoat.
INSULATORS ON TRANSMISSION LINES.
+----------------------------------+-------+---------+--------+-------+
| | |Material | Inches |Inches |
| |Voltage| of |Diameter|Height |
| Location of Line. | of | Insula- | of | of |
| | Line. | tor. |Insula- |Insula-|
| | | | tor. | tor. |
+----------------------------------+-------+---------+--------+-------+
|Electra to San Francisco | 60,000|Porcelain| 11 | 11-1/4|
|Colgate to Oakland | 60,000|Porcelain| 11 | 11-1/4|
|Cañon Ferry to Butte | 50,000|Glass | 9 | 12 |
|Shawinigan Falls to Montreal | 50,000|Porcelain| 10 | 13-1/2|
|Provo around Utah Lake | 40,000|Glass | 7 | 5-3/4|
|Santa Ana River to Los Angeles | 33,000|Porcelain| 6-3/4 | 4-7/8|
|Spier Falls to Schenectady | 30,000|Porcelain| 8-1/2 | 6-3/4|
|Apple River Falls to St Paul | 25,000|Glass | 7 | 5-3/4|
|Chambly to Montreal | 25,000|Porcelain| 5-1/2 | 6-1/2|
|Niagara Falls to Buffalo | 22,000|Porcelain| 7-1/2 | 7 |
|Portsmouth to Pelham, N. H. | 13,000|Porcelain| 5-1/4 | 3-3/4|
|Garvins Falls to Manchester, N. H.| 12,000|Glass | 5 | 4-3/4|
+----------------------------------+-------+---------+--------+-------+
The inner petticoat or sleeve that runs down over the pin and sometimes
reaches nearly to the cross-arm, of course becomes wet on its outside
surface and at its lower edge during a rain; but between this lower wet
part of the inner petticoat, or sleeve, and the lower wet edge of the
larger outside petticoat, there is a wide, dry strip of insulator
surface. A result is that an arc over the surface of the outside
petticoat can reach the wet edge of the sleeve only by crossing the
strip of dry under surface or jumping through the air.
The same type of insulator is used on the 60,000-volt lines between
Electra and San Francisco and between Colgate and Oakland, each
insulator having an outer petticoat 11 inches in diameter and one inner
petticoat or sleeve 6-1/2 inches in diameter. This inner petticoat runs
down the pin for a distance of 7-1/2 inches below the outer petticoat.
Slightly different pins are used for mounting the insulators on the two
transmission lines just named, so that on the former the distance
through air from the lower edge of the outer petticoat to the cross-arm
is 11 inches, and on the latter the corresponding distance is 11-1/2
inches. On the Electra line the lower edge of the inner petticoat of
each insulator is about 3-1/2 inches, and on the Colgate line about 4
inches above the cross-arm.
INSULATORS ON TRANSMISSION LINES.
+------------------------------+---------+---------+---------+----------+
| | Inches | Inches | Inches | Inches |
| | from | from | from | from |
| | Top of | Outside | Lowest | Edge of |
| Location of Line. |Insulator|Petticoat|Petticoat| Outside |
| | to | to | to | to Edge |
| | Cross- | Cross- | Cross- | of Lowest|
| | arm. | arm. | arm. |Petticoat.|
+------------------------------+---------+---------+---------+----------+
|Electra to San Francisco | 14-1/2 | 11 | 3-1/2 | 7-1/2 |
|Colgate to Oakland | 15 | 11-1/2 | 4 | 7-1/2 |
|Cañon Ferry to Butte | 13-1/2 | 7-3/4 | 1-1/2 | 6-1/4 |
|Shawinigan Falls to Montreal | 16-1/4 | 11-3/4 | 3-1/4 | 8-1/2 |
|Santa Ana River to Los Angeles| 8-5/8 | 3-3/4 | 3-3/4 | 0 |
|Spier Falls to Schenectady | 10-3/4 | 7-3/8 | 4-1/4 | 3-3/8 |
|Niagara Falls to Buffalo | 10 | 5-1/2 | 3 | 2-1/2 |
|Chambly to Montreal | 8-1/2 | 4-1/2 | 2 | 2-1/2 |
+------------------------------+---------+---------+---------+----------+
On each of the lines named in this table the wires are strung on the
tops of their insulators.
The Cañon Ferry line is carried on insulators each of which has three
short petticoats and a long separate sleeve that runs down over the pin
to within 1-1/2 inches of the cross-arm. This sleeve makes contact with
its insulator near the pin hole. The outside petticoat of each
insulator on this line is 7-3/4 inches above the cross-arm and 6-1/4
inches above the lower end of the sleeve. Both the main insulator and
the sleeve, in this case, are of glass.
White porcelain insulators are used to support the 50,000-volt
Shawinigan line, and are of a recent design. Each of these insulators
has three petticoats ranged about a central stem so that their lower
edges are 4-1/2 inches, 9 inches, and 13 inches respectively, below the
top. The highest petticoat is 10 inches, the intermediate 9-3/4 inches,
and the lowest 4-1/4 inches in diameter. The height of this insulator is
13 inches, compared with 11-1/4 inches for those used on the Electra and
Colgate lines and 12 inches for the combined insulator and sleeve used
on the Cañon Ferry line. When mounted on its pin, this insulator on the
Shawinigan line holds its wire 16-1/4 inches above the cross-arm,
compared with a corresponding distance of 14-1/2 inches on the Electra,
15 inches on the Colgate, and 13-1/2 inches on the Cañon Ferry line. The
two upper petticoats on each of these insulators are much less concave
than the lowest one, and the edges of all three stand respectively
11-3/4, 7-1/4, and 3-1/4 inches above the cross-arm. From the edge of
the top to the edge of the bottom petticoat the direct distance is 8-1/2
inches.
Of the three transmission lines above named that operate at 50,000 to
60,000 volts, that between Shawinigan Falls and Montreal leads as to
distances between the line wire and insulator petticoats and the
cross-arm. On the Santa Ana line, where the voltage is 33,000, the
insulator is of a more ordinary type, being of porcelain, 6-3/4 inches
in diameter, 4-7/8 inches high, and having the lower edges of its three
petticoats in the same plane. Each of these insulators holds its wire
8-5/8 inches above the cross-arm, and has all of its petticoats 3-1/2
inches above the cross-arm. Unlike the three insulators just described,
which are mounted on wooden pins, this Santa Ana insulator has a pin
with an iron core, wooden threads, and porcelain base. This base extends
up from the cross-arm a distance of 3-1/8 inches, and the wooden sleeve,
in which the threads for the insulator are cut, runs down over the
central bolt of the pin to the top of the porcelain base, which is
5/8-inch below the petticoats.
The 30,000-volt lines from Spier Falls are carried 10-3/4 inches above
their cross-arms by triple petticoat porcelain insulators. Each of these
insulators is 8-1/2 inches in diameter, 6-3/4 inches high, and is built
up of three parts cemented together. A malleable-iron pin cemented into
each insulator with pure Portland cement carries the outside petticoat
7-1/2 inches and its lowest petticoat 4-1/4 inches above the cross-arm.
When the voltage on the Spier Falls lines was raised from about 13,000
to 30,000, the circuits being carried in part by one-piece porcelain
insulators, a number of these insulators were punctured at the higher
pressures, and some cross-arms and poles were burned as a result. No
failures resulted on those parts of these lines where the three-part
insulators were in use. The second pole line between Niagara Falls and
Buffalo was designed to carry circuits at 22,000 volts, or twice that
for which the first line was built. Porcelain insulators were employed
on both of these lines, but while the 11,000-volt line was carried on
three-petticoat insulators, each with a diameter of 7 inches and a
height of 5-1/2 inches, the 22,000-volt line was mounted on insulators
each 7-1/2 inches in diameter and 7 inches high, with only two
petticoats. The older insulator has its petticoats 2 inches above the
cross-arm, and the lower petticoat of the new insulator is 3 inches
above the arm. These two insulators illustrate the tendency to lengthen
out along the insulator axis as the voltage of the circuits to be
carried increases.
[Illustration: FIG. 93A.--The Old and New Insulators on the Niagara
Falls-Buffalo Line.]
For future work at still higher voltages, the advantage as to both first
cost and insulating qualities seems to lie with insulators that are very
long in an axial direction, and which have their petticoats arranged one
below the other and all of about the same diameter, rather than with
insulators of the umbrella type, like those on the Electra and Colgate
lines.
CHAPTER XXII.
DESIGN OF INSULATOR PINS FOR TRANSMISSION LINES.
Bending strains due to the weights, degree of tension, and the
directions of line wires, plus those resulting from wind-pressure, are
the chief causes that lead to the mechanical failure of insulator pins.
Considering the unbalanced component of these forces at right angles to
the axis of the pin, which alone produce bending, each pin may be
considered as a beam of circular cross section secured at one end and
loaded at the other.
For this purpose the secured end of the beam is to be taken as the point
where the pin enters its cross-arm, and the loaded end of the beam is
the point where the line wire is attached to the insulator. The distance
between these two points is the length of the beam. The maximum strain
in the outside fibres of a pin measured in pounds per square inch of its
cross section, represented by S, may be found from the formula,
P X
S = -------
.0982 D³
where P is the pull of the wire in pounds, D is the diameter of the pin
at any point, and X is the distance in inches of that point from the
wire. Inspection of this formula shows that S, the maximum strain at any
point in the fibres of a pin, when the pull of the line-wire, P, is
constant, increases directly with the distance, X, from the wire to the
point where the strain, S, takes place. This strain, S, with a constant
pull of the line wire, decreases as the cube of the diameter, D, at the
point on the pin where S occurs increases. That cross section of a pin
just at the top of its hole in the cross-arm is thus subject to the
greatest strain, if the pin is of uniform diameter, because this cross
section is more distant from the line wire than any other that is
exposed to the bending strain. For this reason it is not necessary to
give a pin a uniform diameter above its cross-arm, and in practice it is
always tapered toward its top. Notwithstanding this taper, the weakest
point in pins as usually made is just at the top of the cross-arm, and
it is at this cross section where pins usually break. This break comes
just below the shoulder that is turned on each pin to prevent its
slipping down through the hole in its cross-arm. If the shoulder on a
pin made a tight fit all around down onto the cross-arm, the strength of
the pin to resist bending would be thereby increased, but it is hard to
be sure of making such fits, and they should not be relied on to
increase the strength of pins. By giving a pin a suitable taper from its
shoulder at the cross-arm to its top, the strain per square inch, S, in
the outside fibres of the pin may be made constant for every cross
section throughout its length above the cross-arm, whatever that length
may be. The formula above given may be used to determine the diameters
of a pin at various cross sections that will make the maximum stress, S,
at each of these cross sections constant. By transposition the formula
becomes
P
D³ = ------- X.
.0982 S
Where the pin is tapered so that S is constant for all cross sections,
then for any pull, P, of the line wire on the pin the quantity
(P/.0982 S) must be constant at every diameter, D, distant any number of
inches, X, from the point where the wire is attached. If the constant,
(P/.0982 S) is found for any one cross section of a pin, therefore, the
diameter at each other cross section with the same maximum stress, S,
may be readily found by substituting the value of this constant in the
formula. The so-called “standard” wooden pin that has been very
generally used for ordinary distribution lines, and to some extent even
on high-voltage transmission lines, has a diameter of nearly 1.5 inches
just below the shoulder. The distance of the line wire above this
shoulder varies between about 4.5 and 6 inches, according to the type of
insulator used, and to whether the wire is tied at the side or top of
the insulator. If the line wire is tied to the insulator 5 inches above
the shoulder of one of the standard pins, then X becomes 5, and D
becomes 1.5 in the formula last given. From that formula by
transposition and substitution
P D³ (1.5)³
------- = --- = ------ = 0.675.
.0982 S X 5
Substituting 0.675 for the quantity (P/0.0982 S) in the formula D³ =
(P/0.0982 S) X gives the formula D³ = 0.675 X, from which the diameters
at all cross sections of a tapered pin above its shoulder, that will
give it a strength just equal to that of a section of 1.5 inches
diameter and 5 inches from the line wire, may be found. To use the
formula for this purpose it is only necessary to substitute any desired
values of X therein and then solve in each case for the corresponding
values of D. Let it be required, for instance, to determine what
diameter a pin should have at a cross section one inch below the line
wire in order that the maximum strain at that cross section may equal
the corresponding strain at a cross section five inches below the line
wire and of 1.5 inch diameter. Substituting one as the value of X, the
last-named formula becomes D³ = 0.675, and from this, D = 0.877, which
shows that the diameter of the pin one inch below the line wire should
be 0.877-inch. A similar calculation will show that if a pin is long
enough so that a cross section above the cross-arm is 12 inches below
the line wire, the diameter of this cross section should be equal to the
cube root of 0.675 × 12 = 8.1, which is 2.008, or practically two
inches. It should be observed that the calculations just made have
nothing to do with the ability of a pin to resist any particular pull of
its line wire. These calculations simply show what diameters a pin
should have at different distances below its line wire in order that the
maximum stress at each of its cross sections may equal that at a cross
section 5 inches below the wire where the diameter is 1.5 inches. In
Vol. xx., A. I. E. E., pp. 415 to 419, specifications are proposed for
standard insulator pins based on calculations like those just made. As a
result of such calculations, the following table for the corresponding
values of X and D, as used in the above formula, are there presented,
each expressed in inches.
+----------+
| X D |
+----------+
| 1 0.877|
| 2 1.106|
| 3 1.263|
| 4 1.395|
| 5 1.500|
| 6 1.592|
| 7 1.678|
| 8 1.754|
| 9 1.825|
|10 1.888|
|11 1.95 |
|13 2.06 |
|15 2.17 |
|17 2.25 |
|19 2.34 |
|21 2.42 |
+----------+
A pin twenty-one inches long between the line wire and the cross-arm
will have a uniform strength to resist the pull of the wire if it has
the diameter given in this table at the corresponding distances below
the line wire. From this it follows that a pin of any length between
wire and cross-arm corresponding to X in the table will be equally
strong to resist a pull of the line wire as a standard 1.5-inch diameter
pin with its wire five inches above the cross-arm. In other words, if a
pin that is twenty-one inches long between the line wire and the
cross-arm has the diameters given in the table at the corresponding
distances below the wire, then a pin of equal strength to resist
bending, and of any shorter length, would correspond in the part above
the cross-arm to an equal length cut from the top end of the longer pin.
Designating that part of a pin that is above the cross-arm as the
“stem,” and that part in the cross-arm as the “shank,” each pin in the
specifications under consideration is named by the length of its stem,
as a 5-, 7- or 11-inch pin. It is proposed that each pin of whatever
length be threaded for a distance of 2.5 inches at the top of its stem
with four threads per inch, the sides of each thread being at an angle
of ninety degrees with each other. Each thread is to cut into the pin
about 3/32 inch, come to a sharp angle at the bottom, and be about 1/16
inch wide on top. At the end of the pin the proposed diameter over the
thread is one inch in all cases, and at the lower end of the threaded
portion the outside diameter is 1.25 inches. Near the end of the pin the
diameter at the bottom of the thread is thus only 13/16 inch, and the
corresponding diameter at the lower end of the threaded portion is about
1-1/16 inches on all pins. Each pin is to have a square shoulder to rest
on the cross-arm, and the diameter of this shoulder is to be 3/8 inch
greater than the nominal diameter of the shank of the pin. The proposed
length of this shoulder on all pins is 1/4 inch before the taper begins.
The actual diameter of the shank of each pin just below its shoulder is
to be 1/32 inch less than the nominal diameter, and the actual diameter
of the lower end of each shank is to be 1/16 inch less than the nominal
diameter. With these explanations the proposed sizes of pins have
dimensions as follows in inches:
+---------+---------+-----------+
| | | Nominal |
|Length of|Length of|Diameter of|
| Stem. | Shank. | Shank. |
+---------+---------+-----------+
| 5 | 4-1/4 | 1-1/2 |
| 7 | 4-1/4 | 1-3/4 |
| 9 | 4-1/4 | 1-7/8 |
| 11 | 4-3/4 | 2 |
| 13 | 4-3/4 | 2-1/8 |
| 15 | 4-3/4 | 2-1/4 |
| 17 | 5-3/4 | 2-3/8 |
| 19 | 5-3/4 | 2-1/2 |
+---------+---------+-----------+
In order rightly to appreciate the utility of this table of proposed
standard pins, it is necessary to have in mind the fact that all the
dimensions are based on the assumption that a wooden pin with a shank of
one and one-half inches diameter, and with its line wire attached five
inches above the cross-arm, is strong enough for general use on
transmission lines. Such an assumption covers a wide range of practice,
but its truth may well be doubted for many cases. That this assumption
does form the basis of the entire table is clearly shown by the fact
that the calculated diameter at the shank of each pin is made to depend
on a uniform pull, _P_, of the line wire, giving a uniform maximum
stress, _S_, in the outer fibres of the wood just where the shank joins
the stem. In other words, every pin in the table is designed to break
with a uniform pull of the line wire, provided that the point on the
insulator where the wire is attached is just on a level with the top of
its pin in each case. It will at once occur to practical men that while
a five-inch pin with one and one-half inch shank, or a larger pin of
equal ability to resist the pull of a line wire, may be strong enough
for the conductors of some transmission lines, this same pin may be
entirely too weak for the longer spans, sharper angles, and heavier
conductors of other lines.
Thus, on the sixty-five-mile line between Cañon Ferry and Butte, Mont.,
each conductor is of copper and has a cross section of 106,500 cm.,
while on the older line between Niagara Falls and Buffalo each copper
conductor has a cross section of 350,000 cm. Evidently with equal
conditions as to length of span, amount of sag, and sharpness of angles
on these two lines, pins ample in strength for the smaller wire might be
much too weak for the larger wire.
A little consideration will show that it is neither rational nor
desirable to adopt pins of uniform strength for all transmission lines,
but that several degrees of strength are necessary to correspond with
the range in sizes of conductors in regular use. The size of pins for
use on any transmission line, when the maximum bending strain exerted by
the conductors has been determined, should be found by calculation and
experiment, or by experiment alone. According to Trautwine, the average
compressive strength of yellow locust is 9,800 pounds, of hickory 8,000
pounds, and of white oak 7,000 pounds per square inch in the direction
of the grain. These compressive strengths are less than the tensile
strengths of the same woods, and should therefore be employed in
calculation, since the fibres on one side of a bending pin are
compressed while the fibres on the other side are elongated.
Substituting 1,000 for the value of S in the formula, S =
(P X/(.0982 D³)), and also 5 for the value of X, and 1-1/2 for the value
of D, the resulting value of P is found to be 736.5 pounds. This result
shows that with a locust pin of 1-1/2 inches diameter at the shank, and
with its line wire attached five inches above the shoulder, the
unbalanced side pull of the wire that will break the pin by bending is
736 pounds, provided that the wood of the pin has a strength of 1,000
pounds per square inch in compression. As all of the proposed standard
pins in the above table are designed for uniform strength to resist the
same pull of a line wire attached on a level with the top of the pin in
each case, it follows that the pull of 736 pounds by the wire will break
any one of these pins under the conditions stated.
The calculation just made takes no account of the fact that the actual
diameter of the shank of each pin just below the shoulder is 1/32 inch
less than the nominal diameter, but this of course reduces the strength
somewhat. Trautwine states that the figures above given for the
compressive strengths of wood are only averages and are subject to much
variation. Of course no pin should be knowingly loaded in regular
practice to the breaking point, and to provide against variations in the
strength of wood, and for unexpected strains, a liberal factor of
safety, say four, should be adopted in fixing the maximum strains on
insulator pins. Applying this factor to the calculations just made, it
appears that the maximum pull of the line wire at the top of any one of
the above proposed standard pins should not exceed 736 ÷ 4 = 184 pounds
in regular work. A little calculation will readily show that the side
pull of some of the larger conductors now in use on transmission lines
will greatly exceed 184 pounds under conditions, as to sag, angles and
wind pressure, that are frequently met in practice.
On page 448, Vol. xx., A. I. E. E., some tests are reported on six
locust wood pins with shank diameters of 1-7/16 to 1-1/2 inches. Each of
these pins was tested by inserting its shank in a hole of 1-1/2 inches
diameter in a block of hard wood, and then applying a strain at about
right angles to the pin and about 4-1/2 inches from the block by means
of a Seller’s machine. The pull on each pin was applied gradually, and
in most of the pins the fibres of the wood began to part when the side
pull reached 700 to 750 pounds, though the maximum loads sustained were
about ten per cent above these figures. The average calculated value of
S, the compressive strength of the wood in these pins, was 11,130 pounds
per square inch on the basis of the loads at which the fibres of the
wood began to break, and 13,623 pounds per square inch for the loads at
which the pins gave way. On pages 650 to 653 of the volume last cited,
results are reported of tests on twenty-two pins of eucalyptus wood,
which is generally used for this purpose in California. Twelve of these
pins were of a size much used in California on lines where the voltage
is not above 30,000. Each of the twelve pins was 6-7/8 inches long in
the stem, 4-5/8 inches long in the shank, 1-1/2 inches in diameter at
the shank, 2 inches in diameter at the square shoulder where the shank
joins the stem, and 1-3/8 inches in diameter at the top of the thread.
The pins were tested by mounting each of them in a cross-arm, securing
the cross-arm in a testing machine so that the pin was horizontal,
placing an insulator on the pin, and exerting the strain on a cable
wrapped around the side groove of the insulator. This cable varied a
little from right angles to the axis of each pin, but the component of
the strain at right angles to this axis was calculated and the breaking
load here mentioned is that component. Nearly all of these twelve pins
broke square off at the cross-arm.
For a single pin, the lowest breaking strain was 705 pounds, the largest
1,360 pounds, and the average for the twelve pins was 1,085 pounds.
Unfortunately, the exact distance of the cable from the cross-arm is not
stated, but as the cable was wound about the side groove of the
insulator it was probably either in line with or a little below the top
of the pin. It seems probable also that the diameter of these pins at
the shoulder--that is, two inches--may have increased the breaking
strain somewhat by giving the shoulder a good bearing on the cross-arm.
The ten other pins were of the size in use on the 60,000-volt line
between Colgate power-house and Oakland, Cal. Each of these pins had a
length of 5-3/8 inches and a maximum diameter of 2-1/8 inches in the
shank, and a length of 10-3/8 inches in the stem, with a diameter of
2-1/2 inches at the shoulder. This shoulder was not square, but its
surface formed an angle of forty-five degrees with the axis of the pin,
and this bevel shoulder took up 1/4 inch of the length just given for
the stem of the pin. At 2-1/2 inches from its threaded end the stem of
the pin had a diameter of 1-15/16 inches, and the diameter slopes to
1-3/8 inches at two inches from the end. The two inches of length at the
top of the stem has the uniform diameter of 1-3/8 inches, and is
threaded with four threads per inch for the insulator. Each of these ten
pins was tested, as already described, until it broke, but the break in
this case started as a split at the lower end of the threaded portion
and ran down the stem to the shoulder in a line nearly parallel with the
axis of the pin. The pull on the cable at right angles to the axis of
each pin had a maximum value of 1,475 pounds in one case, and a
corresponding value of 3,190 pounds in another, while the average
breaking strain for the ten pins was 2,310 pounds. Unfortunately, the
report of this test above named does not distinctly state just how far
the testing cable was attached above the shank of each of these large
pins; but it seems probable that the same insulator was used with the
larger as with the smaller pins, and if this was so the testing cable
was attached near the end of each pin, as this cable was wound about the
side groove of the insulator used on the smaller pins. With the types of
insulator in actual use on the Colgate and Oakland line the wire is
carried at the top groove and its centre is about two and a half inches
above the top of the pin. It is therefore probable that these pins would
not withstand as great strains on the lines as they did in these tests.
The bevel shoulder on each of these larger pins no doubt increases its
ability to resist a bending strain, because the bevel surface fits
tightly down into a counterbore in the cross-arm. Where the pin has a
shoulder at right angles with the axis, as is more usually the case, and
the top of the cross-arm is a little rounding, the square shoulder does
not have a firm seat and is of slight importance as far as the strength
of the pin to resist a bending strain is concerned. Evidently the
weakest point in the ten larger pins of this test was at the lower end
of the threaded portion, since in each case the break was in the form of
a long split starting where the thread ended. There seems to be no
sufficient reason for the reduction of the diameter of a pin intended
for a heavy line wire to a diameter as small as one inch at the threaded
end, or for limiting the length of the threaded portion to 2.5 inches,
as proposed in the specifications for standard pins. It is certain that
the cost of the pin would be no more if its diameter at the threaded end
were 1-1/4 or 1-3/8 inches with a uniform taper from the end of the pin
down to the shoulder and with the thread cut down the stem for three or
four inches. Furthermore, any increase in the cost of insulators for
these larger threaded ends of pins would no doubt be a small matter.
Some excess of strength in the stem of a pin over that of its shank is
to be desired, for the stem is more exposed to the weather and to
charring by leakage currents over the surface of the insulator. On
high-voltage lines, this charring is usually worse at that part of each
pin just below its thread, and the commonest breaks of pins on these
lines leave the insulators with the threaded portions of their pins
hanging on the wire, while the remainder of each pin remains on the
cross-arm. From the tests just noted it is evidently poor design to give
the threaded portion of a pin a short length of uniform diameter, and
then to increase the diameter at once by a shoulder, as was done with
the pins on the Colgate and Oakland line. This design evidently leads to
failure of pins by splitting from the lower end of the threads. The
better design is the more common one which gives the stem of the pin a
uniform taper from the shoulder to the top. Where the line wire is
secured to the top of its insulator, anywhere from one to three inches
above the top of the pin, there is a strong tendency for the insulator
to tip on its pin, and this tendency is more effectively met the longer
the joint between the pin and insulator.
CHAPTER XXIII.
STEEL TOWERS.
Steel towers are rapidly coming into use for the support of electric
transmission lines that deliver large units of energy at high voltages
to long distances from water-powers.
One case of this sort is the seventy-five-mile transmission of 24,000
horse-power at 60,000 volts from Niagara Falls to Toronto. Another
example may be seen in the seventy-five-mile line of steel towers which
carries transmission circuits of 60,000 volts to Winnipeg. Guanajuato,
Mexico, which is said to have produced more silver than any other city
in the world, receives some 3,300 electric horse-power over a
60,000-volt transmission line one hundred miles long on steel towers.
Between Niagara Falls and Lockport the electric circuits now being
erected are supported on steel towers. On a transmission line eighty
miles long in northern New York, for which plans are now being made,
steel towers are to support electric conductors that carry current at
60,000 volts.
For the elevations above ground at which it is common to support the
conductors of transmission lines--that is, from twenty-five to fifty
feet--a steel tower will cost from five to twenty times as much as a
wooden pole in various parts of the United States and Canada. It follows
at once from this fact that there must be cogent reasons, apart from the
matter of first cost, if the general substitution of steel towers for
wooden poles on transmission lines is to be justified on economic
grounds. During fifteen years the electric transmission of energy from
distant water-powers to important centres of population has grown from
the most humble beginnings to the delivery of hundreds of thousands of
horse-power in the service of millions of people, and the lines for this
work are supported, with very few exceptions, on wooden poles. Among the
transmissions of large powers over long distances at very high voltages
that have been in successful operation during at least several years
with wooden pole lines are the following: the 60,000-volt circuit that
transmits some 13,000 horse-power from Electra station across the State
of California to San Francisco, a distance of 147 miles, is supported
by wooden poles. In the same State, the transmission line 142 miles
long between Colgate power-house and Oakland, at 60,000 volts, and with
a capacity of about 15,000 horse-power, hangs on wooden poles, save at
the span nearly a mile long over the Straits of Carquinez. Wood is used
to carry the two 55,000-volt circuits that run sixty-five miles from the
10,000-horse-power station at Cañon Ferry on the Missouri River to
Butte. Between Shawinigan Falls and Montreal, a distance of eighty-three
miles, the conductors that operate at about 50,000 volts are carried on
wooden poles. Electrical supply in Buffalo to the amount of 30,000
horse-power depends entirely on circuits from Niagara Falls that operate
at 22,000 volts and are supported on lines of wooden poles.
In the operation of these and many other high-voltage transmissions
during various parts of the past decade some difficulties have been met
with, but they have not been so serious as to prevent satisfactory
service. Nevertheless, it is now being urged that certain impediments
that are met in the operation of transmission systems would be much
reduced by the substitution of steel towers for wooden poles, and it is
even suggested that perhaps the first cost, and probably the last cost,
of a transmission line would be less with steel than with wood for
supports. The argument for steel in the matter of costs is that while a
tower requires a larger investment than a pole, yet the smaller number
of towers as compared with that of poles may reduce the entire outlay
for the former to about that for the latter. More than this, it is said
that the lower depreciation and maintenance charges on steel supports
will make their final cost no greater than that of wooden poles.
In the present state of the market, steel towers can be had at from
three to three and one-half cents per pound, and the cost of a steel
tower or pole will vary nearly as its weight. During the first half of
1904 the quotations on tubular steel poles to the Southside Suburban
Railway Company, of Chicago, were between the limits just stated. That
company ordered some poles built up of steel sections about that time at
a trifle less than three cents per pound. Each of these poles was thirty
feet long and weighed 616 pounds, so that its cost was about eighteen
dollars (xxi, A. I. E. E., 754). For a forty-five-foot steel pole to
carry a pair of 11,000-volt, three-phase circuits along the New York
Central electric road the estimated cost was eighty dollars in the year
last named (xxi, A. I. E. E., 753). On the 100-mile line to Guanajuato,
Mexico, above mentioned, the steel towers were built up of 3″ × 3″ ×
3/16″ angles for legs, and were stayed with smaller angle sections and
rods. Each of these towers has four legs that come together near the
top, is forty feet high, weighs about 1,500 pounds, and carries a
single circuit composed of three No. 1 B. & S. gauge hard-drawn copper
cables. The weight of each of these cables is 1,340 pounds per mile, and
the forty-foot towers are spaced 440 feet apart, or twelve per mile,
over nearly the entire length of line. At three cents per pound, the
lowest figure at which these towers could probably be secured for use in
the United States, the approximate cost of each would be forty-five
dollars. Between Niagara Falls and Lockport each of the steel towers
that is to carry a single three-phase transmission circuit has three
legs built up of tubing that tapers from two and one-half inches to
smaller sizes and is braced at frequent intervals. The height of these
towers is forty-nine feet, and the weight of each is 2,800 pounds. At
three cents per pound the cost of each tower amounts to eighty-four
dollars. For a long transmission line in northern New York bids were
recently had on towers forty-five feet high to carry six wires, and the
resulting prices were $100 to $125 each for a tower weighing about 3,000
pounds. On the line between Niagara Falls and Toronto the standard tower
holds the lowest cables 40 feet above ground at the insulators, has a
weight of 2,360 pounds, and would cost $70.80 at 3 cents per pound.
In January, 1902, four steel towers were purchased to support
transmission circuits for two spans of 132 feet each over the Chambly
Canal, near Chambly Canton, Quebec. Each pair of these towers was
required to support eleven No. 2-0 B. & S. gauge bare copper wires with
the span of 132 feet between them. The vertical height of each of these
four towers is 144 feet above the foundation, and they were designed for
a maximum stress in any member of not more than one-fourth of its
ultimate strength, with wires coated to a diameter of one inch with ice
and under wind pressure. For these four steel towers erected on
foundations supplied by the purchasers the price was $4,670, and the
contract called for a weight in the four towers of not less than 121,000
pounds. On the basis of this weight the cost of the towers erected on
foundations was 3.86 cents per pound.
With these examples of the cost of steel towers a fair idea may be
gotten of the relative cost of wooden poles. For poles of cedar or other
desirable wood thirty-five feet long and with eight-inch tops fitted
with either one or two cross-arms an estimated cost of five dollars each
is ample to cover delivery at railway points over a great part of the
United States and Canada. This size of pole has been much used on the
long, high-voltage transmission systems that involve large power units
and use heavy conductors. Examples of lines where such poles are used
may be seen between Niagara Falls and Buffalo, between Colgate
power-house and Oakland, and between Cañon Ferry and Butte. Of course
some longer poles were used in special locations, like the crossing of
steam railways, but it is also true that on the lines supported by steel
towers such locations make exceptionally high towers necessary. The
thirty-five-foot poles will hold the electric lines about as high above
the ground level as the forty-nine-foot towers on the Niagara Falls and
Toronto transmission, because the former will be set so much closer
together. On the line just named the regular minimum distance of the
electric cables above the ground level at the centres of spans is
twenty-five feet. The standard towers on this line carry the lower
electric cables forty feet above the ground at the insulators, and it
was thought desirable to allow a sag of fifteen feet at the centres of
the regular spans of four hundred feet each. On these towers the
conductors that form each three-phase circuit are six feet apart, and
lines drawn between the three cables form the sides of an equilateral
triangle. With a pin fourteen and three-fourths inches long like that
used on these steel towers, and one conductor at the top of a
thirty-five-foot pole, where the other two are supported by a cross-arm
five feet three inches below, giving six feet between cables, the lower
cables are held by their insulators twenty-six feet above the ground,
when the poles are set five feet deep. Between thirty-five-foot poles
one hundred feet is a very moderate span, and one that is exceeded in a
number of instances. Thus on the 142-mile line from Colgate power-house
to Oakland the thirty-five-foot poles are 132 feet apart, and one line
of these poles carries three conductors of 133,000-circular-mil copper,
while the other pole line has three aluminum cables of 168,000 circular
mils. On the later transmission line from Niagara Falls to Buffalo,
which was designed for three-phase circuits of 500,000-circular-mil
cable, the regular distance between the thirty-five-foot poles is 140
feet.
A maximum sag of twenty-four inches between poles 100 feet apart under
the conditions named above brings the lowest points of the wire
twenty-four feet above the ground. The steel towers on the line to
Guanajuato being only forty feet in length, and spaced 440 feet apart,
it seems that the distance of conductors from the ground at the centres
of spans is probably no greater than that just named. Particular
attention is called to this point because it has been suggested that the
use of steel towers would carry cables so high that wires and sticks
could not be thrown onto them. It thus appears that thirty-five-foot
wooden poles set one hundred feet apart will allow as much distance
between conductors, and still keep their lowest points as far above the
ground, as will forty- to forty-nine-foot towers placed four hundred
feet or more apart. The two lines that have their conductors further
apart perhaps than any others in the world are the one from Cañon Ferry
to Butte, on thirty-five-foot wooden poles, and the one to Guanajuato,
on steel towers. In each of these cases the cables are seventy-eight
inches apart at the corners of an equilateral triangle. With steel
towers four hundred feet or wooden poles one hundred feet apart, four of
the latter must be used to one of the former. At $5 per pole this
requires an investment of $20 in poles as compared with at least $45 for
a tower like those on the Guanajuato line, $84 for a tower like those on
the line from Niagara Falls to Lockport, or $70 for one of the towers on
the Niagara and Toronto line. Each of the towers on the line to Toronto
carries two three-phase circuits, and the least distance between cables
is six feet. To reach the same result as to the distance between
conductors with the two circuits on poles, it would be desirable to have
two pole lines, so that $40 would represent the investment in the poles
to displace one tower for two circuits. The older pole line between
Niagara Falls and Buffalo carries two three-phase circuits on two
cross-arms, and the 350,000-circular-mil copper cables of each circuit
are at the angles of an equilateral triangle whose sides are each three
feet long. In this case, however, the electric pressure is only 22,000
volts.
The costs above named for poles and towers include nothing for erection.
Each tower has at least three legs and more commonly four, and owing to
the heights of towers and to the long spans they support it is the usual
practice to give each leg a footing of cement concrete. It thus seems
that the number of holes to be dug for a line of towers is nearly or
quite as great as that for a line of poles, and considering the concrete
footings the cost of erecting the towers is probably greater than that
for the poles. With wooden poles about four times as many pins and
insulators are required as with steel towers, or say twelve pins and
insulators on poles instead of three on a tower. For circuits of 50,000
to 60,000 volts the approximate cost of each insulator with a steel pin
may be taken at $1.50, so that the saving per tower reaches not more
than $13.50 in this respect. In the labor of erecting circuits there may
be a small advantage in favor of the towers, but the weight of the long
spans probably offsets to a large extent any grain of time due to fewer
points of support.
An approximate conclusion from the above facts seems to be that a line
of steel towers will probably cost from 1.5 to twice as much as a line
or lines of wooden poles to support the same number of conductors the
same distance apart, even when the saving of pins and insulators is
credited to the towers. This conclusion applies to construction over a
large part of the United States and Canada. It is known that wooden
poles of good quality retain enough strength to make them reliable as
supports during ten or fifteen years, and it is doubtful whether steel
towers will show enough longer life to more than offset their greater
first cost. It may be noted here that any saving in the cost of
insulators or other advantage that there may be in spans four hundred
feet or more long can be as readily secured with wooden as with steel
supports. With these long spans the requirements are greater height and
strength in the line supports, and these can readily be obtained in
structures each of which is formed of three or four poles with
cross-braces. Such wooden structures have long been in use at certain
points on transmission lines where special long spans were necessary or
where there were large angular changes of direction. In those special
cases where structures 75 to 150 or more feet in height are necessary to
carry a span across a waterway, as at the Chambly Canal above mentioned,
steel is generally more desirable than wood because poles of such
lengths are not readily obtainable. Neither present proposals nor
practice, however, contemplates the use of steel towers having a length
of more than forty to fifty feet on regular spans.
Much the strongest argument in favor of steel towers for transmission
lines is that these towers give a greater reliability of operation than
do wooden poles. It is said that towers will act as lightning-rods and
thus protect line conductors and station apparatus. As to static and
inductive influences from lightning, it is evident that steel towers can
give no protection. If each tower has an especial ground connection it
will probably protect the line to some extent against direct lightning
strokes, but there is no reason to think that this protection will be
any greater than that given by well-grounded guard wires, or even by a
wire run from a ground plate to the top of each pole or wooden tower. If
a direct lightning stroke passes from the line conductors to a wooden
support it frequently breaks the insulator on that support, and the pole
is often shattered or burned. Such a result does not necessarily
interrupt the transmission service, however, as the near-by poles can
usually carry the additional strain of the line until a new pole can be
set. Quite a different result might be reached if lightning or some
other cause broke an insulator on a steel tower, and thus allowed one of
the electric cables to come into contact with the metal structure, as
the conductor would then probably be burned in two. To repair a heavy
cable thus severed where the spans were as much as 400 feet long would
certainly require some little time. Where a conductor in circuits
operating at 20,000 to 35,000 volts has in many cases dropped onto a
wooden cross-arm, it has often remained there without damage until
discovered by the line inspector, but no such result could be expected
with steel towers and cross-arms (xxi, A. I. E. E., 760). Where steel
towers are employed it would seem to be safer to use wooden cross-arms,
for the reasons just stated. This is, in fact, the practice on the steel
towers before named that support 25,000-volt circuits over the Chambly
Canal, and also on the steel towers that carry the 60,000-volt circuits
from Colgate power-house over the mile-wide Straits of Carquinez.
On the 40,000-volt transmission line between Gromo and Nembro, Italy,
where timber is scarce and steel is cheap, both the poles and cross-arms
are of wood. It is thought that the comparatively small number of
insulators used where a line is supported at points about four hundred
feet apart should contribute to reliability in operation, but insulators
now give no more trouble than other parts of the line, and the leakage
of energy over their surfaces is very small in amount, as was shown in
the Telluride tests. Whatever benefits are to be had from long spans are
as available with wooden as with steel supports, and at less cost.
One advantage of steel towers over wooden poles or structures is that
the former will not burn and are probably not subject to destruction by
lightning. Where a long line passes over a territory where there is much
brush, timber or long grass, the fact that steel towers will not burn
may make their choice desirable. In tropical countries where insects
rapidly destroy wooden poles the use of steel towers may be highly
desirable even at much greater cost, and such a case was perhaps
presented on the line to Guanajuato, Mexico.
Mechanical failures of wooden insulator pins have been far more common
than those of poles, both as a direct result of the line strains and
because such pins are often charred and weakened by the leakage of
energy from the conductors. For these reasons the general use of iron or
steel pins for the insulators of long lines operating at high voltages
seems desirable. Such pins are now used to support the insulators on a
number of lines with wooden poles and cross-arms, among which may be
mentioned the forty-mile, 30,000-volt transmission between Spier Falls
and Albany and the forty-five-mile 28,000-volt line from Bear River to
Ogden, Utah. Iron or steel pins add very little to the cost of a line,
and materially increase its reliability. One of the cheapest and best
forms of steel pins is that swaged from a steel pipe and having a
straight shank and tapering stem with no shoulder. A pin of this sort
for the 400-foot spans of 190,000-circular-mil copper cable on the line
from Niagara Falls to Toronto measures three and one-quarter inches long
in the shank, eleven and one-half inches in the taper, and has diameters
of two and three-eighths inches at the larger and one and one-eighth
inches at the smaller end. On spans under 150 feet between wooden poles
pins of this type but with a much smaller diameter could be used to
advantage.
On long transmission lines where the amount of power involved is very
large the additional reliability to be had with steel towers is probably
great enough to justify their use. For the great majority of power
transmissions, however, it seems probable that wooden poles or
structures will long continue to be much the cheaper and more
practicable form of support.
The line of steel towers on a private right of way seventy-five miles
long, carrying two circuits for the transmission of 24,000 horse-power
at 60,000 volts from Niagara Falls to Toronto, is one of the most
prominent examples of this type of construction.
Eventually there will be two rows of steel towers along the entire
length of the line.
On the straight portions of the line the steel towers are regularly
erected 400 feet apart, but on curves the distances are less between
towers, so that their total number is about 1,400 for each line.
Standard curving along the line requires towers placed 50 feet apart,
and a change in the direction of not more than ten degrees at each
tower, except at the beginning and end of the curve, where the change in
direction is three degrees. When the change in the direction of the line
is not more than six degrees, the corresponding spans allowed with each
change are as follows:
Degrees Feet of
change. span.
1/2 300
1 286
1-1/2 273
2 259
2-1/2 246
3 232
3-1/2 219
4 205
4-1/2 192
5 178
5-1/2 165
6 151
At some points along the line conditions require a span between towers
of more than 400 feet, the regular distance for straight work. One
example of this sort occurs at Twelve-Mile Creek, where the stream has
cut a wide, deep gorge in the Erie plateau. At this point the lines make
a span of 625 feet between towers.
[Illustration: FIG. 94.--Transposition Tower (Second Tower).]
[Illustration: FIG. 95.--Elevations and Plan of Tower.]
The regular steel tower used in this transmission measures 46 feet in
vertical height from its foot to the tops of the lower insulators, and
51 feet 3 inches to the tops of the higher insulators. The lower six
feet of this tower are embedded in the ground, so that the tops of the
insulators measure about 40 feet and 45 feet 3 inches respectively above
the earth. At the ground the tower measures 14 feet at right angles to
the transmission line and 12 feet parallel with it. The width of each
tower at the top is 12 feet at right angles to the line, and the two
sides having this width come together at points about 40 feet above the
ground. Between the two L bars thus brought nearly together, at each
side of a tower a piece of extra heavy 3-inch steel pipe is bolted so as
to stand in a vertical position. Each piece of this pipe is about 3-1/2
feet long and carries a steel insulator pin at its upper end. The two
pieces of pipe thus fixed on opposite sides of the top of a tower carry
the two highest insulators. For the other four insulators of each tower,
pins are fixed on a piece of standard 4-inch pipe that serves as a
cross-arm, and is bolted in a horizontal position between the two
nearly rectangular sides of each tower, at a point two feet below the
bolts that hold the vertical 3-inch pipes, already named, in position.
Save for the two short vertical and one horizontal pipe, and the pins
they support, each tower is made up of L-shaped angle-bars bolted
together. Each of the two nearly rectangular sides of a tower consists
of two L bars at its two edges, three L bars for cross-braces at right
angles to the edges, and four diagonal braces also formed of L bars. The
lower halves of the L bars at the edges of each side of a tower have
sections of 3″ × 3″ × 1/4″, and the upper halves have sections of 3″ ×
3″ × 3/16″. This last-named cross-brace and the other two cross-braces
have a common section of 2″ × 1-1/2″ × 1/8″. For the lower set of
diagonal braces the common section is 2-1/2″ × 2″ × 1/8″, and the upper
set has a section of 2″ × 1-1/2″ × 1/8″ in each member. At the level of
the lowest cross-braces the two rectangular sides of a tower are tied
together by one member of 2″ × 1-1/2″ × 1/8″ of L section and at right
angles to the sides, and by two diagonal braces of 5/8″ round rod
between the corners of the tower. On each of its two triangular sides a
tower has four horizontal braces and three sets of diagonal braces. The
two upper horizontal braces are of 2″ × 1-1/2″ × 1/8″ L section, and the
lowest is the same, but the remaining horizontal brace has a section of
2-1/2″ × 2″ × 1/8″. Bars of 2″ × 1-1/2″ × 1/8″ L section are used for
the two upper sets of diagonal braces, and bars of 2-1/2″ × 2″ × 1/8″
for the lower set. In addition to the cross-braces named, each
triangular side of a tower near the top of the corner bars has two short
cross-pieces with the common L section of 3-1/2″ × 3-1/2″ × 5/8″, one
just above and the other just below the cross-arm of 4-inch pipe to hold
it in place. At the bottom of each of the four corner bars of a tower a
foot is formed by riveting a piece of 3″ × 1/4″ L section and 15 inches
long at right angles to the corner bar. On one corner bar of each tower
there are two rows of steel studs for steps, one row being located in
each flange of the L section. On the same flange these steps are two
feet apart, but taking both flanges they are only one foot apart. Every
part of each steel tower is heavily galvanized.
[Illustration: FIGS. 96, 97, 98.--Raising Towers on Niagara Transmission
Line.]
[Illustration: FIG. 99.--One of the Towers in Position.]
The labor of erecting these steel towers was reduced to a low figure by
the method employed, as shown in the accompanying illustration. Each
tower was brought to the place where it was to stand with its parts
unassembled. For erecting the towers a four-wheel wagon with a timber
body about thirty feet long was used. When it was desired to raise a
tower, two of the wheels, with their axle, were detached from the timber
body of the wagon, and this body was then stood on end to serve as a
sort of derrick. This derrick was guyed at its top on the side away from
the tower, and a set of blocks and tackle was then connected to the top
of the derrick and to the tower at a point about one-fourth of the
distance from its top. A rope from this set of blocks ran through a
single block fixed to the base of the derrick and then to a team of
horses. On driving these horses away from the derrick the steel tower
was gradually raised on the two legs of one of its rectangular sides
until it came to a vertical position. The next operation was to bring
the legs of the tower into contact with the extension pieces that were
fixed in the earth and then bolt them together.
[Illustration: FIG. 100.--Steel Tower for Transmission Line.]
The tops of the three pins that carry the insulators for each
three-phase circuit are at the corners of an equilateral triangle (Fig.
100), each of whose sides measures six feet. The six steel insulator
pins used on each tower are exactly alike, and each is swaged from extra
heavy pipe. Each finished pin is 2-3/8 inches in diameter for a length
of 3-1/4 inches, and then tapers uniformly to a diameter of 1-1/8 inch
at the top through a length of 11-1/2 inches. This gives the pin a total
length of 14-3/4 inches. In the larger part there are two 9/16-inch
holes from side to side, and within two inches of the top there are
three circular grooves each 3/16 inch wide and 1/16 inch deep. Forged
steel sockets of two types are employed to attach the steel pins with
the pipes. Each socket is made in halves, and these halves are secured
to both the pipe and the pin by through bolts. Like all other parts of
the towers, these steel pins and sockets are heavily galvanized. On
each of the four corner bars of a tower the lower six feet of its length
is secured to the upper part by bolts or rivets. This lower six feet of
each corner bar is embedded in the earth, and the construction just
named makes it easy to replace the bars in the earth when corrosion
makes it necessary.
Footings for each tower are provided by digging four nearly square holes
with their sides at approximately 45 degrees with the direction of the
transmission line, and the shortest side of each hole at least two feet
long. Centres of these holes are 14 feet 3 inches apart in a direction
at right angles to the line, and 13 feet 9 inches apart parallel with
the line. In hard-pan each one of the holes was filled to within 2 feet
6 inches of the top with stones, after the leg of the tower was in
position, and then the remainder of the hole was filled with cement
grouting mixed four to one.
At the bottom of each hole in marsh land a wooden footing 3 feet × 6
inches × 24 inches was laid flat beneath the leg of the tower, and then
the hole was filled to within 2-1/2 feet of the surface with the
excavated material. Next above this filling comes a galvanized iron
gutter-pipe, four inches in diameter, and filled with cement about the
leg of the tower for a length of two feet. Outside of this pipe the hole
is made rounding full of cement grouting.
[Illustration: FIG. 101.--Transmission Line at Welland Canal.]
At some points along the transmission line exceptionally high towers are
necessary, a notable instance being found at the crossing over the
Welland Canal, where the lowest part of each span must not be less than
150 feet above the water. For this crossing two towers 135 feet high
above ground are used, as seen in Fig. 101. Each of these towers is
designed to carry all four of the three-phase power circuits that are
eventually to be erected between Niagara Falls and Toronto. For this
purpose there was used a special design of tower with a width of about
48 feet at right angles to the direction of the line below the top
truss, and a width of about 68.5 feet at that truss where the two lower
conductors of each circuit are attached.
With all spans longer than 400 feet, a tower of heavier construction
than the standard type is used, and this tower provides three insulators
for the support of each conductor. A tower of this type that supports
the lowest conductors about 40 feet above the ground level has its
corner bars made up of 4″ × 4″ × 3/8″ and 4″ × 4″ × 5/16″ L sections,
has three cross-arms of extra heavy 4-inch pipe, and a 6-inch vertical
standard pipe to support each group of three insulators for the highest
conductor of each circuit. Each of the lower conductors of a circuit on
this tower is supported by an insulator on each of the three parallel
cross-arms. On some of these towers, for long spans, the two outside
insulators for the support of each conductor are set a little lower than
the insulator between them.
[Illustration: FIG. 102.--Heavy Tower at Credit River.]
[Illustration: FIG. 103.--Angle Tower Near Bronte.]
Angle towers, used where the line makes a large change in direction at a
single point, have three legs on each rectangular side, a width of 20
feet on each of these sides for some distance above the ground, and a
width of 27 feet 2 inches at the top. In these towers the two legs on
the triangular side that is in compression are each made up of four 3″ ×
3″ × 1/4″ L sections joined by 1-1/2″ × 1/4″ lattices and rivets. Towers
of this sort are used near the Toronto terminal-station, where the line
changes 35 degrees at a single point, and near the crossing of
Twelve-Mile Creek, where the angular change of the line on a tower is 45
degrees. Close to each terminal-station and division-house the
transmission line is supported by terminal towers. These towers differ
from the others in that each carries insulators for only three
conductors, and these insulators are all at the same level. Each
terminal tower has nine insulators, arranged in three parallel rows of
three each for the conductors of a single circuit, and each conductor
thus has its strain distributed between three pins. All three wires of a
circuit are held 40 feet above the ground by a terminal tower, and pass
to their entries in the wall of a station at the same level. As these
terminal towers must resist the end strain of the line, they are made
extra heavy, the four legs each being made up of 4″ × 4″ × 5/16″ and 4″
× 4″ × 3/8″ L sections. For the three cross-arms on one of these towers
three pieces of 4-inch pipe, each 15 feet 9 inches long, are secured at
its top with their parallel centre lines 30 inches apart in the same
plane. Each of these pipes carries three insulator pins with their
centres 7 feet 4-1/2 inches apart. On the bottom of each leg of a
terminal tower there is a foot, formed by riveting on bent plates, that
measure 15 and 18 inches, respectively, on the two longer sides. Each
foot of this tower is set in a block of concrete 5 feet square that
extends from 3.5 feet to 7.5 feet below the ground level.
Insulators for the transmission line, which are illustrated in Fig. 104,
are of brown, glazed porcelain, made in three parts, and cemented
together. The three parts consist of three petticoats or thimbles, each
of which slips over or into one of the others, so that there are three
outside surfaces and three interior or protected surfaces between the
top of an insulator and its pin.
From top to bottom the height of each insulator is 14 inches, and this
is also the diameter of the highest and largest petticoat. The next or
middle petticoat has a maximum diameter of 10 inches and the lowest
petticoat one of 8 inches. Cement holds the lowest petticoat of the
insulator on one of the steel pins previously described, and in this
position the edge of the lowest petticoat is about 2-1/2 inches from the
steel support. At the top of each insulator the transmission conductor
is secured, and the shortest distance from this conductor to any of the
steel parts through the air is about 17 inches.
From the step-up transformer house at Niagara Falls to the
terminal-station at Toronto, a distance of seventy-five miles, each
three-phase, 60,000-volt, 25-cycle circuit on the steel towers is made
up of three hard-drawn copper cables with a cross section of 190,000
circular mils each, and is designed to deliver 12,000 electric
horse-power with a loss of ten per cent, on a basis of 100 per cent
power factor. Six equal strands of copper make up each cable, and this
wire has been specially drawn with an elastic limit of more than 35,000
pounds and a tensile strength of over 55,000 pounds per square inch.
This cable is made in uniform lengths of 3,000 feet, and these lengths
are joined by twisting their ends together in copper sleeves, and no
solder is used. No insulation is used on these cables.
[Illustration: FIG. 104.--Insulators.]
Instead of a tie-wire, a novel clamp is employed to secure the copper
cable on each insulator. This complete clamp is made up of two separate
clamps that grasp the cable at opposite sides of each insulator and of
two half-circles of hard-drawn copper wire of 0.187 inch diameter. Each
half-circle of this wire joins one-half of each of the opposite clamps,
and fits about the neck of the insulator just below its head. Two bronze
castings, one of which has a bolt extension that passes through the
other, and a nut, make up each separate clamp. When the combined clamp
is to be applied, the sides are separated by removing the nut that holds
them together, the half-circles are brought around the neck of the
insulator, and each of the side clamps is then tightened on to the cable
by turning the nut that draws its halves together. This complete clamp
can be applied as quickly as a tie-wire, is very strong, and does not
cut into the cable.
Each of the regular steel towers is designed to withstand safely a side
strain of 10,000 pounds at the insulators, or an average of 1,666 pounds
per cable. With the 190,000-mil cable coated to a depth of 1/2 inch with
ice and exposed to a wind blowing 100 miles per hour, the estimated
strains on each steel pin for different spans and angular changes in the
direction of the line are given in the accompanying table:
POUNDS STRAIN ON PINS, 1/2-INCH SLEET, 100 MILES WIND.
=====+=========================================
| Degrees and Minutes.
Span,+-----+-----+-----+-----+-----+-----+-----
feet.| 0 | 0.30| 1 | 1.30| 2 | 2.30| 3
-----+-----+-----+-----+-----+-----+-----+-----
0| 0| 35| 69 | 104| 138| 173| 207
100| 256| 291| 325| 360| 394| 429| 463
200| 512| 547| 581| 616| 650| 685| 719
300| 768| 803| 837| 872| 906| 941| 975
400|1,024|1,059|1,093|1,128|1,162|1,197|1,231
500|1,280|1,315|1,349|1,384|1,418|1,453|1,487
600|1,536|1,571|1,605|1,640|1,674|1,709|1,743
700|1,792|1,827|1,861|1,896|1,930|1,965|1,999
800|2,048|2,083|2,117|2,152|2,186|2,221|2,255
900|2,304|2,339|2,373|2,408|2,442|2,477|2,511
1,000|2,560|2,595|2,629|2,664|2,698|2,733|2,767
-----+-----+-----+-----+-----+-----+-----+-----
=====+===================================
| Degrees and Minutes.
Span,+-----+-----+-----+-----+-----+-----
feet.| 3.30| 4 | 4.30| 5 | 5.30| 6
-----+-----+-----+-----+-----+-----+-----
0| 242| 276| 311| 345| 380| 414
100| 498| 532| 567| 601| 636| 670
200| 754| 788| 823| 857| 892| 926
300|1,010|1,044|1,079|1,113|1,148|1,182
400|1,266|1,300|1,335|1,369|1,404|1,438
500|1,522|1,556|1,591|1,625|1,660|1,694
600|1,778|1,812|1,847|1,881|1,916|1,950
700|2,034|2,068|2,103|2,137|2,172|2,206
800|2,290|2,324|2,359|2,393|2,428|2,462
900|2,546|2,580|2,615|2,649|2,684|2,718
1,000|2,802|2,836|2,871|2,905|2,940|2,974
-----+-----+-----+-----+-----+-----+-----
The copper cables were so strung as to have a minimum distance from the
ground of 25 feet at the lowest points of the spans. In order to do this
the standard steel towers that hold the lower cables 40 feet above the
ground level at the insulators are spaced at varying distances apart,
according to the nature of the ground between them. At each tower the
upper cable of each circuit is 5 feet 3 inches higher than the two lower
cables, and this distance between the elevations of the upper and the
lower cables is maintained whatever the amount of sag at the centre of
each span. If there is a depression between two standard towers on a
straight portion of the line, the sag in the centre of a span 400 feet
long may be as much as 18 feet. Where a rise and fall in the ground
between towers make it necessary to limit the sag to 14 feet in order to
keep the lowest cables 25 feet above the highest point of earth, the
length of span is limited to 350 feet. If the rise and fall of ground
level between towers allow a sag of only 11 feet with the lowest cable
25 feet above the earth, the length of span with 40-foot towers is
reduced to 300 feet; and if for a like reason the sag is limited to 8
feet, the span may only be 250 feet.
[Illustration: FIG. 105.--Take-up Arrangement on Terminal Tower.]
At each terminal tower, where the cables are secured before they pass
into a terminal-station, the three insulators for each cable are in a
straight line with their centres, 30 inches apart. When a line cable
reaches the first insulator of the three to which it is to be attached
on one of these towers, it is passed around the neck of this insulator
and then secured on itself by means of two clamps that are tightened
with bolts and nuts. See Fig. 105. The cable thus secured turns up and
back over the tops of the three insulators and goes to the
terminal-station. Around the neck of the insulator to which the line
cable has been secured in the way just outlined a short detached length
of the regular copper cable with the parts of a turnbuckle at each end
is passed, and this same piece of cable also passes around the neck of
the next insulator in the series of three. By joining the ends of the
turnbuckle and tightening it, a part of the strain of the line cable in
question is transferred from the first to the second insulator of the
series. In the same way a part of the strain of this same line cable is
transferred from the second insulator of the series to the third, or one
nearest to the terminal-station.
INDEX.
Air-blast cooled transformers, 129
Air-gap data, 183
Air gaps, number in series to stand given voltage, 183
Albany-Hudson Ry. Plant, 121
Alternating currents, 227
Alternator voltage, 118
Alternators, 103
data, 118
for high voltage, 120
inductor, 112
types of, 111
Aluminum as a conductor, 200, 209
cables in use, 213
conductor joints, 206
conductors, 27, 28
corrosion of, 211
properties of, 212
soldered joints, 206
_vs._ copper, 209
wire, cost of, 29
Amoskeag Mfg. Co. plant, 51, 52
Amsterdam (N. Y.) plant, 121
Anchor ice, 59
Anderson (S. C.) plant, 121
Apple River (Minn.) plant, 1, 26, 27, 28, 71, 97, 98, 99, 102, 118,
119, 124, 126, 127, 134, 174, 187, 190, 192, 208, 245, 264, 294
Arc lighting, 167
Arcing, 46
Automatic regulators, 162
Barbed wire, 169, 175
Belt drive, 83, 107
Bienne plant (Switzerland), 42
Birchem Bend, 57, 67, 79, 95, 97, 98, 102
Blower capacity necessary to cool transformers, 130
Boosters, 133
Boston-Worcester Ry. plants, 121
Braces for cross-arms, 259
Bronze conductors, 200
Brush discharge, 281
Buchanan (Mich.) plant, 88
Building materials, 95
Bulls Bridge plant, 63
Burrard Inlet (B. C.) plant, 111, 112
Bus-bars, 142, 147
dummy, 145
Cable insulation, 195
sheaths, 194
ways, 140
Cables, aluminum, 212
aluminum, in use, 213
charging current, 197
cost of, 188, 196
for alternating current, 194
high-voltage, 191
paper insulated, 196
protection against electrolysis, 195
rubber-covered, 195
submarine, 192
temperature of, 198
voltage in, 190, 196
Canadian-Niagara Falls Power Co., 121
Canals, 51, 53
long, 68
Cañon City plant, 26, 27, 28, 117, 118, 127, 208
Cañon Ferry plant, 1, 3, 26, 27, 28, 46, 49, 53, 62, 68, 69, 83, 89,
94, 95, 97, 102, 105, 112, 113, 118, 119, 124, 125, 126, 127, 130,
132, 134, 174, 208, 233, 234, 245, 246, 249, 254, 257, 259, 268, 272,
280, 282, 294, 295, 302
Cedar Lake plant, 90
Chambly plant, 96, 110, 149, 156, 172, 189, 249, 255, 256, 257, 267,
272, 287, 294, 295, 311, 312
Charging current for cable, 197
Charring of pins, 276, 278
Chaudière Falls plant, 118
Choke-coil used with lightning arresters, 180
Circuit breakers, 135, 150
breakers, time limit, 152
Circuits, selection of, 233
Coal, price of, in Salt Lake City, 8
Colgate plant, 1, 3, 26, 27, 28, 74, 82, 83, 90, 94, 97, 98, 99, 101,
102, 108, 112, 113, 118, 127, 130, 132, 134, 187, 190, 201, 206, 208,
213, 245, 246, 250, 254, 257, 272, 277, 280, 282, 294, 295, 304, 309
Columbus (Ga.) plant, 83, 115
Compounding, 160
Compressive strength of woods, 302
Conductivity of the conductor metals, 201
Conductors, 200
aluminum, 27, 28, 206
aluminum, properties of, 212
coefficients of expansion, 200
corrosion of, 211
cost of, 22, 29, 203, 204, 205
cost of aluminum, 29
cost of per k. w., 28
cost of copper, 29
data, 204
data from representative transmission plants, 208
expansion of aluminum and copper, 211
melting points, 200
minimum size for transmission line, 202
properties of ideal, 200
relative conductivity, 201
relative cost of, 20
relative properties for equal lengths and resistances, 204
relative strengths for given area, 203
relative weight for given conductivity, 202
relative weight of, 202
relative weights of three-phase, two-phase, and single-phase lines,
220
resistance of, 225
skin effect, 206, 233
weight per k. w., 27
Conduits, 195
radiation loss in, 198
temperature rise in, 198
Constant current regulator, 167
transformer, 167
Control equipment for d. c. and a. c. plants, 35
Copper conductors, 200
cost of, 22
_vs._ aluminum, 209
wire, cost of, 29
Corrosion of conductors, 211
Cross-arm braces, 258
iron, 284
location of, 257
material, 258
Cross-arms, 49, 256
Crossings, 187
Dales plant (White River), 26, 27, 28, 71, 134, 208
Dams, 62
Delta connection, 131
Depreciation, 11
Design of power-plant, 83
Dike, 60
Direct connection, 84
Discharge, static, 170
Distribution system, 158
Draught tubes, 79
Dummy bus-bars, 145
Easton (Pa.) plant, 121
Edison Co. (Los Angeles) plant, 118
Efficiency constant-current transmission, 216
curves, motor-generator set, 117
of constant-voltage transmission, 217
of transformers, 133
relative, of a. c. and d. c. transmission, 35
Electra plant, 1, 3, 74, 82, 83, 92, 94, 97, 98, 101, 102, 108, 112,
113, 118, 127, 174, 206, 208, 212, 213, 233, 235, 236, 245, 248, 253,
254, 256, 259, 272, 275, 277, 280, 281, 282, 294, 295
Electric power, market for, 7
Electrical Development Co., Niagara plant, 120
Electricity _vs._ gas, 6
Electrolysis, 195
Energy curves of hydro-electric stations, 13
electrical, cost of at switchboard, 23
Entrance end strain, 261, 325
insulating discs, 262
into buildings, 179
of lines, 179, 261, 265
through roof, 269
wall openings, 262
Entries for transmission lines, 261
Expansion, coefficient of, for copper and aluminum, 211
coefficients of, for various conductor metals, 200
Farmington River (Conn.) plant, 26, 27, 28, 58, 118, 125, 134, 208,
212, 213,
245
Feeders, 143
Ferranti cables, 192
Fire-proofing, 95
Floor, distance from roof to, 95
location of, 79
space, 12, 101, 102
space per k. w. of generators, 12
Floors, 95
Fog, 46, 277
Fore-bay, 59, 60
Foundations, 95
Frequency, 113, 127
effect on transformer cost, 116
Fuel, price of, in Salt Lake City, 8
Fuses, 135, 150
Garvins Falls plant, 56, 60, 79, 80, 94, 96, 97, 102, 113, 145, 240,
294
Gas _vs._ electricity, 6
Gears, 84, 108
Generators (a. c.), 103
d. c. _vs._ a. c., 31
Generators, belt-driven, 107
capacity of, 32
compounding of, 160
cost of, 40
(a. c.) cost, 32
(a. c.) data, 118
direct-connected to horizontal turbines, 89
to impulse wheels, 90
connection to vertical shafts, 84
(d. c.) field excitation of, 41
floor space, 101
per k. w., 12
gear-driven, 108
(a. c.) high-voltage, 120
(d. c.) in series, 31
(d. c.) installation of, 41
insulation of, 39, 45
lightning protection, 34
limiting voltage of, 44
(a. c.) limiting voltage of, 32
(d. c.) limiting voltage of, 31
overload capacity, 103
relation between voltage and capacity, 127
revolving armatures, 112
fields, 112
series-wound, 41
speed regulation, 38
Glass _vs._ porcelain insulators, 288
Great Falls plant, 54, 60, 61, 64, 67, 78, 92, 93, 102, 114, 118
Greggs Falls plant, 54, 56, 64, 240
Ground connections, 178
for guard wires, 171, 172
Grounded guard wires, 168
Guard wires, 168
installation of, 175
Guying of poles, 255
Hagneck (Switzerland) plant, 86
Hooksett Falls plant, 56, 131
Hydro-electric plants, 1
built at the dam, 64-67
canals, long, 68-73
long and short, 58
short, 53-56
capacity and weight of conductors per k. w. for various plants, 27
(800 k. w.) cost of, 10
(1500 k. w.) cost of, 11
cost of labor, 12
cost of operation, 12, 77
design of, 83
floor, 79
space per k. w., 101
interest and depreciation, 11
linked together, 56-58
load factors, 14, 15
location of, 64
model design, 12
operation, 59
_vs._ steam plant, 5, 12
with pipe-lines, 73-77
with steam auxiliary, 84
Ice, 59
Impulse wheel speed, 108
wheels, 82, 90
location of, 99
Indian Orchard plant, 57, 84
Inductance, 206, 230
Induction, electro-magnetic, electrostatic, 168
on lines, 206
regulator, 162
Inductor alternators, 112
Insulation, as affected by ozone, 197
cost of paper _vs._ rubber, 196
of a. c. and d. c. lines, 34
of apparatus, 142
of cables, 195
of electrical machines, 45
of generators, 39
protection against ozone, 198
Insulator arc-over test, 291
-pins, 270 (see Pins)
Insulators, 277, 282, 287, 322
and pins, data from various plants, 280
defective, 288
glass _vs._ porcelain, 288
in snow, 293
method of fastening to iron pins, 271
novel clamp, 323
on various transmission lines, 294
petticoats, 294
testing of, 288
tests, 290
test voltage, 289
with oil, 287
Iron conductors, 200
Kelley’s Falls plant, 56
Kelvin’s law, 219
Labor, cost of, 12
in hydro-electric stations, 12
Leakage, 275, 287
line, 207, 214
Lewiston (Me.) plant, 118, 120, 122, 167, 213
Lighting, incandescent, minimum frequency, 116
series distribution, 167
Lightning arrester, effect of series resistance, 185
arresters, 168, 176
ground connection, 178
multiple air-gap, 176, 183
non-arcing metals in, 184
series connection of, 180
shunted air-gaps, 185
with choke coil, 180
protection, 34
Line calculations, 221-232
charging current, 197
conductors, 200
conductors, cost of, 22
weight of, 21
construction, 222
cost, 310
cross-arms, 49
spacing of wires, 46
(a. c.) transmission, 34
(d. c.) transmission, 33
end strain, 325
leakage, 47
loss, 39
losses due to grounded guard wires, 176
Lines, sag, 309
transposition of, 314
Line voltages, 45
Load factors, 14, 15
lighting, 61
maximum, 60
motor, 160
railway, 164
Loss in conduits, 198
relation to weight of conductors, 215
Losses due to grounded guard wire, 176
on transmission lines, 215
Ludlow Mills plant, 26, 27, 28, 57, 79, 100, 121, 208, 213
Madrid (N. M.) plant, 26, 27, 28, 118, 208
Manchester (N. H.) plants, 120
Market for electric power, 7
Materials, building, 95
for line-conductors, 200
Mechanicsville plant, 58, 67, 109, 121, 174
Melting points of conductor metals, 200
Montmorency Falls plant, 26, 27, 28, 240
Motor load, 160
Motor-generator set efficiency curve, 117
Motors, series-wound, 41
(d. c.) speed regulation, 38
synchronous, 241
Multiple air-gap arrester, 176
Needle-point spark-gap for measuring pressure, 290
Neversink River plant, 75, 179
Niagara Falls Power Co., 3, 59, 81, 86, 87, 93, 94, 95, 97, 101, 102,
105, 106, 107, 108, 112, 113, 117, 118, 119, 127, 133, 137, 140, 143,
145, 151, 153, 161, 165, 170, 181, 188, 194, 195, 208, 211, 240, 245,
246, 257, 272, 273, 275, 280, 287, 289, 294, 295, 297
Nitric acid from air, 281
Non-arcing metals, 184
North Gorham (Me.) plant, 120
Ogden (Utah) plant, 26, 27, 28, 118, 120, 132, 134, 208, 245
Ohm’s law, 223
Oil switches, 136
Ontario Power Co., 121
Operating expenses, 59
Operation, cost of, 12, 77
Operations, reliability of, 311
Ouray (Col.) plant, 121
Overhead line connection to underground, 197
Overload capacity of generators, 103
Ozone, 197
Painting of poles, 255
Paper insulated cables, 196
_vs._ rubber insulation, 196
Payette River (Idaho) plant, 73, 101
Penstocks, 59, 98
Phase, 113
Pike’s Peak plant, 77
Pilot wires, 161
Pins, 259, 270
and insulators, data from various plants, 280
burning of, 270, 276, 278
charring of, 276, 278
composite, 281
compressive strength of woods, 302
design of, 298
dimensions of, 301
formula for diameter of, 299
iron, 275, 285, 286
expansion of, 290
method of fastening insulators, 271
method of fastening to cross-arms, 271
metal, 271, 275, 282, 285, 286
of uniform strength, 300, 302
proportions, 301
relative cost of metal and wooden, 284
shank, 274
shoulder, 275, 299, 305
softening of threads, 280
steel, 275, 312
strain with 1/2-inch sleet and 100-mile wind for different spans,
324
strains on, 270, 298
strength of, 303
table of standard, 301
treatment of, 259, 275
weakest point, 298
wooden, data from various plants, 272
dimensions of, 272
dimensions of standard, 273
Pipe-lines, 73
Pittsfield (Mass.) plant, 121
Pole line, cost of, 21
lightning arresters, 179
relative cost of, 20
lines, 246
Poles, cost, 310
depth in ground, 254
diameter of, 254
dimensions of, 254
guying of, 255
iron, 284
length of, 253, 309
life of, 255
setting of, 252
spacing of, 249
steel, cost of, 307
treatment of, 255
woods for, 252
Porcelain _vs._ glass insulators, 288
Portland (Me.) plant, 120, 166, 239
Portsmouth, N. H. plant (steam), 102, 118, 119, 120, 121, 144, 194,
264, 294
Power plant, relative cost of a. c. and d. c., 36
transmitted, total cost of, 24
Radiation loss in conduits, 198
Railway crossing, 187, 252
service, 164
Red Bridge plant, 53, 60, 79, 93, 94, 96, 97, 99, 101, 102
Regulation, 155, 239
as effected by synchronous motors, 165
at receiving end, 162
hand, 161
Regulator, automatic, 162
constant-current, 167
induction, 162
Relay-switches, 145
Resistance, 225
in series with lightning arrester, 185
Revolving armature alternators, 112
field alternator, 112
River crossings, 187, 190, 249
Roof, distance from floor, 95
Roofs, 95
Rope-drive, 83
Rotaries, cost of, 117
suitable frequency for, 115
Rubber-covered cables, 195
maximum temperature, 198
protection against ozone, 198
Sag in lines, 309
St. Hyacinthe (Que.) plant, 118
St. Joseph plant, 66
St. Maurice plant (Switzerland), 31
Salem (N. C.) plant, 121, 122
San Gabriel Cañon plant, 26, 27, 28, 208
Santa Ana plant, 1, 26, 27, 28, 74, 76, 82, 83, 92, 94, 95, 96, 97,
98, 99, 101, 102, 208, 245, 263, 280, 281, 294, 295, 296
Sault Ste. Marie plant, 72, 83, 85, 89, 97, 102, 104, 105, 112, 113,
117, 118, 120, 127
Scott system, 132
Series distribution, 167
machines, 41
Sewall’s Falls plant, 26, 27, 28, 155
Shawinigan Falls plant, 1, 70, 71, 107, 116, 117, 163, 164, 166, 209,
212, 213, 235, 236, 242, 245, 267, 272, 273, 280, 282, 294, 295, 296
Sheaths for cables, 194
Shunted air-gaps, 185
Skin effect, 206, 232
Snoqualmie Falls plant, 3, 4
map of transmission lines, 4
Snow, 293
Soldered joints, 206
Spacing of poles, 249
of wire, 234
Spans, long, 190, 250
strains for different lengths, 324
Sparking distances, 182
voltages, 182
Speed, peripheral, of impulse wheels, 108
peripheral of turbines, 85, 103
regulation, 38, 42
d. c. motors, 38
Spier Falls plant, 1, 2, 3, 54, 58, 61, 62, 68, 91, 94, 98, 124, 126,
127, 130, 141, 142, 146, 161, 174, 236, 237, 243, 244, 245, 250, 253,
266, 280, 285, 287, 289, 291, 294, 295, 296, 312
Star connection, 131
Static discharges, 170
Steam and water-power station combined, 84
electric plant, cost of labor, 12
cost of operation, 12
floor area per k. w., 102
_vs._ water-power, 5
Steel towers, 306
Storage capacity, 15
Strains on insulation as affected by resistance in series with
arrester, 185
Stray currents, protection against, 195
Submarine cables, 187, 192, 194
Sub-station, arrangement of apparatus, 128
Sub-stations, 157, 237
Surges, 136
Switchboard, 156
wiring, 146, 148, 149
Switches, 135, 244
arcing of, 135
electrically operated, 140
long break, 135
oil, 136
open-air, 136
pneumatically operated, 140
power operated, 138
relay, 145
Switch-houses, 141, 142, 238, 244
Switching, 146
high-tension, 147
Synchronous converters, 115
cost, 117
motors, 165, 241
Tail-race, 96
Telephone, 161
Telluride plant, 47, 160, 169, 181
Temperature of cables, 198
rise in conduits, 198
Tensile strength of conductor metals, 201
Time-limit circuit-breaker, 152
Time relays, 152, 153
Towers, 250, 306
angle, 320
cost, 310
dimensions, 314
erection of, 316-319
heavy, 320
reliability of operation, 311
spans, 313
steel, cost, 307, 308
steel pins, 312
strain on, 324
Transformers, 122
air-blast _vs._ water-cooled, 129
artificially cooled, 129
at sub-stations, 125
blower capacity necessary to cool, 130
constant-current, 167
cooling, quantity of water necessary, 129
cost, 21, 116, 124, 134
cost of operation, 129
cost of, relative, 20
delta and star connections, 131
efficiency, 133
frequency, effect of, 116
insulation, 45
in transmission systems, 134
limiting voltage for, 32
location of, 97
polyphase, 124
regulation, 125
reserve, 149
secondary, in series, 131
single-phase, 124
two- to three-phase, 132
used to compensate drop, 133
used to regulate voltage, 162
voltages, 45
when to use, 122
Transmission, constant-current, 38, 216
constant-voltage, 40, 217
continuous-current, 31, 32
control equipment, 35
cost of, 19, 40, 222
(d. c.) cost of, 40
efficiency, 35, 41
first long line, 37
frequency, 113
generator end, 103
lightning protection, 34
limiting voltage, 44
lines, arcing, 46
calculation of, 221-232
charging current, 197
construction, 222
cost, 310
cross-arms, 49, 256
crossings, 187, 190
data from various plants, 245
effect of length on cost, 20
effect of length on cost of power, 24
efficiency, 22, 24
end strain at entries, 325
entrance to buildings, 179, 261
inductance, 206
induction, 168
insulation, 34
insulators (see Insulators), 287
insulator-pins (see Pins), 270
interest, maintenance and depreciation, 22
leakage, 47, 207, 214
length of, capacity of, population supplied, 8
lightning arresters (see Lightning Arresters), 179
lightning protection, 118
long spans, 190
loss, 22, 39
losses, 215
maximum investment in, 220
method of fastening conductors to insulators, 323
operation, 311
pole spacing, 249
regulation with synchronous motors, 241
relative weights of three-phase, two-phase, and single-phase, 228
right-of-way, 246
sag in, 309
spacing of wire, 234
steel towers (see Towers), 306
switch-houses, 238
switches, fuses, and circuit-breakers, 135
take up, arrangement for, 325
total cost of, 22
total cost of operation, 23
transposition of wires, 206, 314
voltage, 21, 215
in cables, 190
regulation, 130
wind pressure, 210
long line, 221
minimum-sized wire, 202
physical limits of, 44
a. c. pole line construction, 34
d. c. pole line construction, 33
pole lines, 246
problems, 19
regulation, 155, 239
selection of circuits, 233
single _vs._ parallel circuits, 241
spacing of conductors, 46
submarine, 187
three-phase, 113
three-phase and two-phase, 228
two-phase, 113
underground, 187
without step-up transformers, 120
Transposition of wires, 206
Turbines, high-speed, 107
horizontal, 79, 83, 89, 97
impulse, 82, 90, 99
speed of, 108
low-head good speed, 105
peripheral, speed of, 85, 103
pressure, 79
several on same shaft, 85, 105
vertical, 79, 84, 85, 86, 97
Underground cable connected to overhead line, 197
cables, 187
Victor (Colo.) plant, 26, 27, 28, 208
Virginia City plant, 118
Voltage drop compensation, 133
fluctuations, 218
high, alternators, 120
measurements, 290
in cables, 190, 196
limiting, 44
for a. c. machines, 32
for d. c. machines, 31
of transmission lines, 21, 215
regulation, 130, 155
sparking, 182
test for insulators, 289
Volts per mile, 26
Wages paid attendants, 12
Walls, 95
Washington & Baltimore Ry., 121
Washouts, 81
Water-cooled transformers, 129
Water-power, development of, 51
high head, 74-77
low head, 51-74
per cent. of energy available, 16
pure hydraulic development, 51
stations (see Hydro-electric Stations)
storage capacity, 15, 61
utilization of, 10
_vs._ steam, 5
Water, storage of, 15, 61
Weight of the conductor metals, 202
Welland Canal plant, 1, 26, 27, 28, 208, 245, 248
Westbrook (Me.) plant, 120
White River to Dales plant, 26, 27, 28, 71, 134
Wind, 324
pressure on lines, 210
Winooski River plant, 64
Wire room, 139
Wood, compressive strength of, 302
Woods for poles, 252
Yadkin River (N. C.) plant, 26, 27, 28, 118, 208
Transcriber’s Notes
This transcription uses the text of the original work.
Inconsistencies (e.g., per cent. and per cent; Chambly and Chamblay;
Garvin’s and Garvins Falls; 1-0 and 1/0 B. & S. gauge; hyphenation;
capitalisation; use of italics, etc.) have been retained, except as
mentioned below. Some of the calculations in the book give different
results to the ones provided; these have not been corrected.
In this book, “cm.” stands for circular mils, not for centimeters.
Page 76, Fig. 16: the text in the centre of the illustration
probably reads 1200 feet Pipe Line.
Page 111: between figures 44 and 46 the original book has figure
51_a_; the numbering has not been changed.
Changes made:
Obvious minor typographical and punctuation errors have been
corrected silently.
Fractions have been standardised to x/y; all occurrences of _vs._
have been italicised.
In-line multi-line formulas have been changed to single-line
formulas.
Table of Contents: the Index has been added.
page 15: table header changed to small caps as others
page 31: Electrical transmission changed to Electrical transmissions
page 56: Canon changed to Cañon
page 77: Tlaluepantla changed to Tlalnepantla
page 312: Teluride changed to Telluride
page 332: Canon changed to Cañon.
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