Electric Transmission of Water Power
by Alton D. Adams · English · Gutenberg #48134 · 725,394 chars · public domain (US)
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Title: Electric Transmission of Water Power
Author: Alton D. Adams
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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
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