ALTERNATORS FOR ELECTRICAL TRANSMISSION.
Dynamos in the generating station of an electric transmission system
should be so numerous that if one of them is disabled the others can
carry the maximum load. If only two generators are installed, it is thus
desirable that each be large enough to supply the entire output, so that
the dynamo capacity exceeds the greatest demand on the station by 100
per cent. To avoid so great excess of dynamo capacity it is common
practice to install more than two generators.
Other considerations also tend to increase the number of dynamos in the
generating station of a transmission system. Thus one transmission line
may be devoted exclusively to lighting, another to stationary motors,
and a third to electric railway service; and it may be desirable that
each line be supplied by an independent dynamo to avoid any effect of
fluctuations of railway or motor load on the lighting system.
At the generating station of the transmission system that supplies
electric light and power in Portland, Me., the idea of independent units
has been carried out with four 500-kilowatt dynamos, each driven by a
pair of wheels fed with water through a separate penstock from the dam.
Each of these dynamos operates one of the four independent transmission
circuits. Where a number of water-power stations feed into a single
sub-station the requirement that each generating station have its
capacity divided up among quite a number of dynamos may not exist, since
one station may be entirely shut down for repairs and the load carried
meantime by the other stations. A good illustration of this point may be
seen at Manchester, where a single sub-station receives energy
transmitted from four water-power plants. At one of these plants the
entire capacity of 1,200 kilowatts is in a single generator.
The foregoing considerations as to the number of dynamos apply with
equal force to both steam- and water-driven stations, but other factors
tend to increase the number of dynamos in water-power plants where the
head of water is comparatively small. This tendency is due to the fact
that the peripheral speeds of pressure turbine water-wheels should be
about twenty-five per cent less than the velocity at which water would
issue from an opening under the head of water at which these wheels
operate in order to secure high efficiency. This velocity of water and
therefore the peripheral speed of pressure turbine wheels varies with
the square root of the head of water.
[Illustration: FIG. 40.--Generators at Sault Ste. Marie Power Plant.]
Since the peripheral speed of turbines is thus determined by the heads
of water under which they operate, and since the diameters of turbines
must increase with their capacities, the rate of revolution for pressure
turbines under any given head decreases as the power goes up. For this
reason it is often desirable to use a larger number of dynamos in a
water-power plant than would otherwise be required in order to avoid
very low speeds of revolution on the direct-connection to the turbines.
A notable illustration of this practice exists in the great water-power
plant of the Michigan-Lake Superior Power Company, at Sault Ste. Marie,
Mich., where a generating capacity of 32,000 kilowatts is divided up
between 80 dynamos of 400 kilowatts each. The head of water available at
the pressure turbines in this plant is about 16 feet, and their speed is
180 revolutions per minute. In order to obtain even this moderate speed
under the head of 16 feet it was necessary to select turbines of only
140 horse-power each. Four of these turbines are mounted on each shaft
that drives a 400-kilowatt dynamo, direct-connected, so that there are
320 wheels in all. Had a smaller number of wheels been employed to yield
the total power their speed and that of direct-connected dynamos must
have been less than 180 revolutions per minute. As the cost of dynamos
increases with very low speeds it is often cheaper to install a larger
number of dynamos at a higher speed than a smaller number at a lower
speed for a given total capacity.
The use of a larger number of units than would otherwise be necessary in
order to avoid a very low speed is further illustrated by the
7,500-kilowatt plant of the Missouri River Power Company, at Cañon
Ferry, Mont. This capacity is made up of ten generators, each rated at
750 kilowatts and direct-connected to a pair of pressure turbine wheels
operating at 157 revolutions per minute, under a head of about 32 feet.
Under comparatively high heads of water pressure turbines operate at
speeds that are ample for direct-connection to even the largest dynamos.
[Illustration: FIG. 41.--Interior of Power-house No. 2, Niagara Falls.]
Thus in the Niagara Falls plant, where the head of water is 136 feet,
each pair of turbines drives a direct-connected dynamo of 3,750
kilowatts at 250 revolutions per minute. In the rare case where the
power to be developed is so great that the number of generators
necessary to give security and reliability to the service leaves each
generator with a capacity larger than is desirable for structural
reasons, the number must be increased simply to reduce the size of each
generator. Such a state of facts existed at Niagara Falls, where the
first station contains ten dynamos of 3,750 kilowatts each, and the
second station contains eleven units of like capacity.
In the greater number of transmission systems the generators are
direct-connected to either steam-engines or water-wheels, and their
speeds of rotation are largely determined by the requirements of these
prime movers. Steam-engines can be designed with some regard to the
desirable speeds for direct-connection to dynamos, but water-wheels are
less flexible in this particular. Each type of wheel has its peripheral
speed mainly determined by the head of water under which it may be
required to operate, and variation from this speed means serious loss of
efficiency.
[Illustration: FIG. 42.--10,000 H. P. 12,000 Volt Generator in Canadian
Power-house at Niagara Falls.]
Under heads of much more than 100 feet pressure turbines operate at
rather high speeds in all except very large sizes. It is much the more
common to see water-wheels at a lower speed belted to dynamos at a
higher speed; but in some instances, as at the lighting plant of
Spokane, Wash., wheels of a higher speed are belted to dynamos of a
lower speed. Another plan by which moderate dynamo speeds are obtained
with water-wheels under rather high heads mounts a dynamo at each end of
the shaft of a large turbine or pair of turbines. This plan is followed
at the plant of the Royal Aluminum Company, Shawinigan Falls, Quebec,
where there are two pairs of horizontal turbine wheels, each pair
developing 3,200 horse-power under a head of 125 feet, and driving a
dynamo direct-coupled on each end of its shaft. Where vertical wheels
are employed it is sometimes more desirable to drive some standard type
of dynamo with horizontal shaft by means of bevel gears than to design a
special dynamo to mount directly on the vertical shaft. This latter plan
is warranted in very large work like that at two of the Niagara Falls
generating stations, where the twenty-one 3,750-kilowatt dynamos are
direct-connected, each on the vertical shaft of a turbine. This type of
connection is not one that will be frequently followed, but at one other
point--Portland, Ore.--each dynamo is mounted directly on the shaft of
its vertical turbine wheel.
Where water-wheels must operate under heads of several hundred feet, it
is usually necessary to abandon pressure turbines and to adopt one of
the types of impulse-wheels. In this class of wheels the peripheral
speed of highest efficiency is only one-half the spouting velocity of
the water under any particular head. This gives the impulse-wheels about
two-thirds the peripheral speed of pressure turbines of equal diameter
and consequently about two-thirds as many revolutions per minute. But as
the water may be applied at one or more points on the circumference of
an impulse-wheel, as desired, such wheels may have much greater
diameters than pressure turbines for equal power under a given head.
[Illustration: FIG. 43.--Generators in Power-station at Mechanicsville
on the Hudson River.]
[Illustration: FIG. 44.--Generators at Chamblay, Quebec, Power-house.]
These properties of low peripheral speed, as to head and great diameter,
as to power developed, fit impulse-wheels for direct-connection to
dynamos where great heads of water must be employed, and they are
generally used in such cases. This is particularly true for the Pacific
coast, where water-powers depend more on great heads than on large
volumes. In the generating plant of the Bay Counties’ Power Company, at
Colgate, Cal., the dynamos are direct-connected to impulse-wheels that
operate under a head of 700 feet. The three 2,250-kilowatt dynamos in
this plant are each mounted on a wheel shaft operating at 285
revolutions per minute, and each of the four 1,125-kilowatt dynamos is
direct-driven by an impulse-wheel at 400 revolutions per minute. At the
Electra, Cal., plant of the Standard Electric Company the impulse-wheels
operate at 240 revolutions per minute under a head of 1,450 feet. Each
of the five pairs of these wheels drives a 2,000-kilowatt generator,
direct-connected. As the head of water at these wheels is 1,450 feet,
its spouting velocity is about 300 feet per second, or 18,000 feet per
minute. Each wheel is eleven feet in diameter, so that a speed of 240
revolutions per minute gives the periphery a little less than 9,000 feet
per minute, or about one-half of the spouting velocity of the water.
These two great plants are excellent illustrations of the way in which
impulse-wheels, under great heads, may be given speeds that are suitable
for direct-connected dynamos.
[Illustration: FIG. 51_a_. Plan and Elevation of Water Wheels and
Generators at Power Station on Burrard Inlet, near Vancouver, B. C.]
Three types of alternators, the revolving armature, the revolving
magnet, and the inductor, are used in the generating plants of electric
transmission systems.
Revolving armatures are used in the dynamos of comparatively few
transmission systems and hardly at all in those of recent date. The
prevailing type of alternator for transmission work is that with
internal revolving magnets and external stationary armature. This type
is employed in the great water-power plants at Cañon Ferry, Mont.; Sault
Ste. Marie, Mich., and for all of the generators installed in the later
Niagara Falls plants. For the sixteen earlier vertical generators at
Niagara Falls the revolving magnets are external to the stationary
armatures, but this construction has the disadvantage of high first cost
and inaccessibility of the internal armature, and is not likely to be
often adopted elsewhere.
[Illustration: FIG. 46.--Elevations of Water-wheels and Generators at
Power-station on Burrard Inlet, near Vancouver, B. C.]
Inductor alternators are those in which both the armature and magnet
coils are stationary and only a suitable structure of iron revolves;
they are employed in a comparatively small number of transmission
systems, but this number includes some of the largest plants. The seven
alternators in the Colgate, Cal., plant aggregating 11,250 kilowatts
capacity, and the five alternators in the plant at Electra in the same
State, with a capacity of 10,000 kilowatts, are all of the inductor
type. As more commonly constructed the magnet winding of the inductor
alternator consists of only one or two very large coils, which are in
some cases as much as ten feet in diameter. The repair of these large
magnet coils seems to present a more serious problem, in case of
accident, than the repair of the small coils used on interval, revolving
magnets. As far as satisfactory operating qualities are concerned,
inductor alternators and those with revolving magnets seem to be on an
equality, but for structural reasons inductor alternators will probably
be built less freely in the future than in the past.
Nearly all long transmissions are now carried out with either two- or
three-phase current. The most notable two-phase installation is that at
Niagara Falls, where the original ten generators, as well as the eleven
dynamos later added in two of the large plants, are all of the two-phase
type. At Cañon Ferry, Mont., the first four of the 750-kilowatt
generators were two-phase, but the six machines of like capacity
installed later are three-phase. In the latest plants of large capacity
or involving very long transmissions three-phase machines have been
generally employed. This is true of the Colgate and Electra plants in
California, and of that at Sault Ste. Marie, Mich.
[Illustration: FIG. 47.--Interior of Power-house at Garvin’s Falls on
the Merrimac River.]
[Illustration: FIG. 48.--500-Kilowatt Generator in Station at Great
Falls on the Presumpscot River.]
As to frequency, existing practice extends all the way from 133 cycles
per second on the lines at Marysville, Cal., down to only 15 cycles on
the transmission for the Washington & Baltimore Electric Railway.
More common practice ranges between 25 and 60 cycles. Niagara Falls saw
the first great plant installed for 25 cycles, but others of that
frequency are now engaged in the supply of light and power for general
distribution. For transmission to electric railway lines a frequency of
25 cycles has been and is being widely used, prominent examples of which
may be seen in the New Hampshire traction, the Berkshire, and the Albany
& Hudson systems.
[Illustration: FIG. 49.--Columbus, Ga., Water-power Station.]
The strong feature of a system at 25 cycles is that it is well suited to
the supply of continuous currents through rotary converters with
reasonable numbers of poles, armature slots, and commutator bars.
[Illustration: FIG. 50.--1065-Kilowatt, 2300 volt Generator Connected to
Motor in Shawinigan Sub-station at Montreal.]
On the other hand, the cost of transformers is greater with current at
25 cycles per second than with a higher frequency, and this current is
only just bearable for incandescent lighting and quite unsuited for arc
lamps, because of the fluctuating character of the light produced. At 15
cycles per second a current can be employed for incandescent lighting
with satisfactory results only by means of some special devices, as
lamps with very thick filaments, to avoid the flicker. Very low
fluctuations cut down undesirable effects in the way of inductance and
resonance, but these effects can be avoided to a large degree in other
ways.
Where power is the most important element in the service of an electric
water-power and transmission system there is a decided tendency to adopt
a rather small number of periods for the system, even at some
disadvantage as to lighting facilities. This is illustrated by the
transmission from St. Anthony’s Falls, Minn., at 35 cycles, from Cañon
City to Cripple Creek, Col., at 30 cycles, by the Sault Ste. Marie plant
of 32,000 kilowatts at 30 cycles, as well as by the two Niagara Falls
plants of 78,750 kilowatts at 25 cycles.
Where the main purpose of a transmission system is the supply of light
and power for general distribution, sixty periods per second are adopted
as the standard in many cases. This number of periods in comparison with
a smaller one tends to increase the cost of rotary converters but
decreases the cost of transformers, and is suitable for both
incandescent and arc lighting.
[Illustration: FIG. 51.--Efficiency Curves for Motor Generators at
Montreal Sub-station of the Shawinigan Transmission Line.]
Few, if any, transmission systems have recently been installed for
frequencies above sixty cycles, and the older plants that worked at
higher figures have in most cases been remodelled.
During the past decade the voltages of alternators have been greatly
increased, but have not caught up with the demand for high pressures on
long-transmission lines. Ten years ago when the first long transmissions
were going into operation 2,000 volts was considered high for an
alternator. As this voltage is too low for economy of conductors longer
than three or four miles, the important early transmissions were all
carried out with the aid of step-up transformers at generating stations.
The practice then was, and to a large extent still is, to design the
alternators for a transmission with a voltage well suited to their
economical construction, and then give the step-up transformers any
ratio necessary to attain the required line voltage.
ALTERNATORS IN TRANSMISSION SYSTEMS.
+----------------------+----------------------------------------------+
| |Number at Plant. |
| | |Kilowatts Each. |
| | | |Alternator Voltage. |
| | | | |Phase. |
| | | | | |Cycles. |
| | | | | | |R. P. M. |
| | | | | | | |Type of |
|Location of System. | | | | | | |Magnet. |Method |
| | | | | | | | |of |
| | | | | | | | |Connec-|
| | | | | | | | |tions. |
+----------------------+--+-----+------+-+----+-----+---------+-------+
|Niagara Falls[A] |16|3,750| 2,300|2|25 |250 |External |Direct |
| | | | | | | |revolving| |
|Niagara Falls[A] | 5|3,750| 2,300|2|25 |250 |Internal | „ |
|Colgate to Oakland | 3|2,250| 2,400|3|60 |285 |Inductor | „ |
|Colgate to Oakland | 4|1,125| 2,400|3|60 |400 | „ | „ |
|Electra to S.Francisco| 5|2,000| ....|3|60 |240 | „ | „ |
|Portsmouth to Pelh’m | 1|2,000|13,200|3|25 | 83.3|Internal | „ |
|Portsmouth to Pelh’m | 2|1,000|13,200|3|25 | 94 | „ | „ |
|Virginia City | 2| 750| 500|3|60 |400 |External | „ |
|Ogden & Salt Lake | 5| 750| 2,300|3|60 |300 |Internal | „ |
|Chaudière Falls | 2| 750|10,500|3|66.6|400 | „ | „ |
|Yadkin River Falls | 2| 750|12,000|3|66 |166 | „ | „ |
|Lewiston, Me. | 3| 750|10,000|3|60 |180 | „ | „ |
|Farmington River }| | | | | | | | |
| to }| 2| 750| 500|3|60 |... | „ | „ |
|Hartford, Conn. }| 2| 600| 500|2|60 |... | „ | „ |
|Cañon Ferry to Butte |10| 750| 500|3|60 |157 | „ | „ |
|Apple Riv. to St. Paul| 4| 750| 800|3|60 |300 | „ | „ |
|Edison Co., L. Angeles| 4| 700| 750|3|50 |... | „ | „ |
|Madrid to Bland | 2| 600| 605|3|60 | 90 | „ | „ |
|Cañon City to Cripple | | | | | | | | |
|Creek | 3| 450| 500|3|30 |... | ..... | „ |
|Sault Ste. Marie |80| 400| 2,400|3|30 |180 | „ | „ |
|St. Hyacinthe, Que. | 3| 180| 2,500|3|60 |600 | „ | „ |
|Great Falls to | | | | | | | | |
|Portland, Me. | 4| 500|10,000|3|60 |225 | „ | „ |
+----------------------+--+-----+------+-+----+-----+---------+-------+
[A] Niagara Falls Power Company.
Thus in the two water-power plants connected with the electrical supply
system of Hartford, Conn., the alternators operate at 500 volts with
transformers that put the line voltage up to 10,000. In the station on
Apple River that supplies the lighting system of St. Paul, Minn., the
alternators operate at 800 volts, and this is raised to 25,000 volts for
the line. At Cañon Ferry the alternator voltage of 500 is multiplied by
100 in the transformers giving 50,000 on the line.
[Illustration: FIG. 52.--Transmission Line of New Hampshire Traction
Company.]
Where the generating station of a transmission system is located close
to a part of its load the alternators are given a voltage suitable for
distribution, say about 2,400, and any desired pressure on the line is
then obtained by means of step-up transformers. Two of the Niagara
Falls plants are an illustration of this practice, the voltage of all
the alternators there being 2,200, which is raised to 22,000 for the
transmission of a part of the energy to Buffalo. A similar practice is
followed in the water-power plant at Ogden, where the generators furnish
current at 2,300 volts for local distribution, and transformers raise
the pressure to 26,000 volts for the transmission to Salt Lake City. In
the 32,000-kilowatt plant at Sault Ste. Marie, Mich., the alternators
operate at 2,400 volts and a large part of their load is local, but this
voltage will no doubt be raised by transformers when transmission lines
are operated.
For generating stations that carry little or no local loads the cost of
transformers can be saved if the generators develop the voltage required
on the transmission lines. This possible saving has led to the
development of alternators that generate voltages as high as 15,000 in
their armature coils. Such alternators have stationary armatures in all
cases and are of either the revolving magnet or inductor type.
At the present time many transmission systems in the United States
operating at 10,000 or more volts develop these pressures in the
armature coils of their alternators, and the number of such systems is
rapidly increasing. It is now the rule rather than the exception to
dispense with step-up transformers on new work where the line voltage is
anything under 15,000. Perhaps the longest transmission line now in
regular operation with current from the armature coils of an alternator
is that at 13,200 volts between the generating station at Portsmouth and
one of the sub-stations of the New Hampshire Traction system at Pelham,
a distance of forty-two miles.
In at least one transmission system now under construction, that of the
Washington, Baltimore & Annapolis Electric Railway, the voltage of
generators to supply the line without the intervention of step-up
transformers will be 15,000.
The company making these alternators is said to be ready to supply
others that generate 20,000 volts in the armature coils whenever the
demand for them is made. In quite a number of cases alternators of about
13,000 volts have been installed for transmissions along electric
railway lines.
Alternator
Systems Using High-voltage Alternators. Voltages.
Electrical Development Co. of Ontario, Niagara Falls 12,000
Lighting and Street Railway, Manchester, N. H. 10,000
Lighting and Street Railway, Manchester, N. H. 12,500
Lighting and Power, Portland, Me. 10,000
Lighting and Power, North Gorham, Me. 10,000
Mallison Power Co., Westbrook, Me. 10,000
Lighting and Power, Lewiston, Me. 10,000
Electric Railway, Portsmouth, N. H. 13,200
Electric Railway, Pittsfield, Mass. 12,500
Ludlow Mills, Ludlow, Mass. 13,200
Electric Railway, Boston to Worcester, Mass. 13,200
Electric Railway, Albany & Hudson, N. Y. 12,000
Empire State Power Co., Amsterdam, N. Y. 12,000
Lehigh Power Co., Easton, Pa. 12,000
Hudson River Power Co., Mechanicsville, N. Y. 12,000
Light and Power, Anderson, S. C. 11,000
Fries Mfg. Co., Salem, N. C. 12,000
Light and Power, Ouray, Col. 12,000
Washington & Baltimore Electric Railway 15,000
Canadian Niagara Power Co., Niagara Falls 12,000
Ontario Power Co., Niagara Falls 12,000
This list of high-voltage alternators is not intended to be exhaustive,
but serves to indicate their wide application. If such alternators can
be purchased at a lower price per unit of capacity than alternators of
low voltage plus step-up transformers, there is an apparent advantage
for transmission systems in the high-voltage machines. This advantage
may rest in part on a higher efficiency in the alternators that yield
the line voltage than in the combination of low-voltage alternators plus
step-up transformers. It is not certain, however, that depreciation and
repairs on the generators of high voltage will not be materially greater
than the like charges on generators of low voltage, and some advantage
in price should be required to cover this contingency.
Just how far up the voltage of alternators can be pushed for practical
purposes is uncertain, but it seems that the limit must be much below
that for transformers where there is ample room for solid insulation and
the coils can be immersed in oil. The use of generators at 10,000 volts
and above tends to lower the volts per mile on transmission lines,
because it seems better in some cases to increase the weight of line
conductors rather than to add step-up transformers, as in the 42-mile
transmission from Portsmouth to Pelham.