where there is a heavy load only a small portion of the time. These
boosters are direct-current dynamos that are connected in series with
the feeder upon which the voltage is to be raised above the regular
power-house voltage. The booster may be driven either by a small
steam engine or by an electric motor. The simplest form of booster
is a series-wound dynamo. A booster armature must, of course, be of
sufficient current capacity to pass all the current that will be
required on its feeder. The voltage yielded by this dynamo, plus the
power-station voltage, is the voltage of the boosted feeder as it
leaves the power house. Supposing that a series-wound booster will give
125 volts at full load; it is obvious that being series-wound it will
give no voltage at no load. The voltage will increase approximately
as the load on the feeder increases; and since the drop in voltage on
the feeder for which the booster is to compensate also varies with the
load, the action of the booster is simply to add sufficient voltage to
its feeder at any instant to compensate for the line loss upon that
feeder and to maintain approximately constant potential at the far end
of the feeder. Boosters raising the power-station voltage of a feeder
more than 250 volts above the normal power-station voltage, are not
common, though cases are on record where a feeder has been boosted as
high as 1,100 volts above the power-station voltage. Since all the
power used in driving a booster is wasted in line loss, this method of
feeding is not economical; but where used only a few days out of the
year it is sometimes to be preferred to a heavy investment in feeders.
The investment in feeders might involve more interest charges than the
cost of power wasted in booster feeding would amount to.
=Alternating-Current Transmission.= High-tension alternating-current
transmission _to_ substations, with direct-current distribution _from_
substations, is extensively used on long interurban roads, and on
large city street-railway systems where power is to be distributed
over a wide area. In such cases the power house is equipped with
alternating-current dynamos supplying high-tension three-phase
alternating current to high-tension transmission lines or feeders.
These high-tension feeders are taken to substations located at
various points on the road, where the voltage is reduced by step-down
transformers; and these transformers supply current to operate rotary
converters, which convert from alternating to direct current for use on
the trolley.
The advantage of this system of high-tension distribution is that,
owing to the high transmission voltage, there is but a small loss
in the high-tension lines, which lines can be made very small, and
will thus involve but little copper investment. The substations can
be located at frequent intervals, so that the distance the 500-volt
direct-current must be conducted to supply the cars is not great.
Current from one power house can thus be distributed over a very
large system in cases where, if the 500-volt direct-current system of
distribution were used, the cost of feeders for distributing such a
low-voltage current would be prohibitive. Were the alternating-current
high-tension scheme of distribution not used, it would be necessary
to have a number of small power houses at various points on the
system instead of one large power house. The cost of operation of
several small power plants per kilowatt output, is likely to be
much greater than that of one large power plant. The first cost of
the alternating-current distributing system, including power house
and substations, is likely to be considerably higher than would
be the cost of a number of small power houses; but in cases where
alternating-current distribution has been installed, it has been
figured that the cost of operation of the central power house with
alternating-current distribution would be sufficiently low as compared
with several small ones to pay more than the interest on this extra
investment.
[Illustration: Fig. 87. Diagram of Distributing System.]
=A System of Distribution for an Interurban Railway.= The typical
features of a high tension system of distribution for an extensive
interurban railway system are shown in Fig. 87, which represents the
electrical transmission and distribution system of the Indiana Union
Traction Company. The central power station at Anderson feeds into
thirteen rotary converter substations from 7 to 65 miles distant from
the power house. The substations east of Indianapolis are fed at 16,000
volts and are placed about 11 miles apart. The substations due north of
Indianapolis are located at intervals of about 17 miles and are fed at
30,000 volts.
The power station at Anderson has a total capacity of 5,000 K. W. The
substations vary in capacity from 250 to 1,500 K. W.
=Efficiency of Transmission Systems.= The average efficiency of a high
tension transmission system for a certain interurban electric railway
system are given below. Current was generated at 380 volts. The step-up
transformers raised it to a potential of 16,000 volts at which pressure
it was transmitted to eight substations at distances from 10 to 40
miles from the power station. It was then stepped down to 380 volts and
converted to direct current by a rotary converter. The tests extended
over a period of three days. The efficiency of the step-up transformers
was 95 per cent; of the high tension line 92.9 per cent; of the
step-down transformers 95 per cent; and of the rotary converters 88 per
cent; giving a total efficiency of the transmission system of 73.5 per
cent.
=Power House Location.= A power house is usually located where coal
and water supply can be cheaply obtained. For this reason it is placed
either on some line of railroad or where coal can be taken to it over
the electric railway.
As it is always desirable to operate the engines in connection with
condensers, on account of the saving in fuel, which is approximately
20 per cent with condensers, power stations are located, when
possible, near rivers and ponds from which a large supply of cold
water for condensation of exhaust steam can be obtained. Where no
such natural water supply is available, it has become customary to
provide means for artificially cooling a sufficiently large supply of
water for condensation. One method is to erect a number of towers, so
constructed that the water when pumped to the top will fall through a
structure that breaks the water up into fine spray as it falls, thus
allowing it to cool by evaporation so that it can be used again for the
condensers when it arrives at the bottom of the tower. Where more room
is available, ponds are sometimes excavated near the power house, and
the water is made to flow back and forth through a series of troughs
located above the pond, and it is thus cooled.
Where a power station is of the direct-current type, operating at
500 to 600 volts, it is desirable to have it as near the center of
electrical distribution as possible, in order to keep down the amount
of investment in the feed wire; but it is more important to have it
located near a cheap coal and water supply than exactly at the center
of distribution.
It is also desirable to have the station located where there is room
for coal storage, on account of the chances for interruption of the
coal supply by strikes, railroad blockades, and other causes beyond the
company’s control. The continuity of the coal supply is also another
argument against placing the station where dependence must be placed
upon wagons or inadequate railroad facilities.
Coal handling, after the coal has reached the station, is done by hand
in the smaller power stations; but in larger power stations it has come
to be the general practice to do as much of the handling as possible by
means of automatic coal conveyors. The most elaborate power stations
have means for dumping coal from cars into hoppers, from which it is
conveyed by an endless chain provided with buckets, called a _coal
conveyor_, to storage bins. Coal conveyors also take the coal from the
storage bins, and deposit it in the hoppers of mechanical stokers in
front of the boilers. Ashes are conveyed from under the boilers by the
same kind of conveyors, and are dumped into hoppers, whence they are
drawn into cars or wagons to be hauled away. The coal, having been
deposited in hoppers at the boiler front, is automatically fed into the
furnaces by automatic stokers. One type of automatic stoker in common
use is of the chain-grate or link-belt type, which is constructed like
an endless sprocket chain, with links composed of heavy cast-iron
blocks that serve as grate bars. This link belt or chain is kept in
constant, slow motion by a small stoker engine or motor which operates
all the stokers of a line of boilers. The coal is fed from the hopper
on to the chain grate, and the chain is slowly moved under the boilers.
As the coal on that part of the grate under the boilers is on fire, the
fresh coal as it enters the furnaces is soon ignited. The grate is run
at such a rate, and the thickness of the coal is so adjusted, that the
coal is burned to an ash by the time it has traveled to the back of
the furnace. There the grate turns down over a sprocket wheel, and the
ashes are dumped into the ash pit as the grate revolves.
The boilers in most common use in large American electric-railway power
houses are of the water-tube type, in which water is contained inside
of a bank of tubes, the ends of these tubes being connected to drums or
headers. The horizontal return-tubular type of boiler is used in many
of the smaller power stations, and vertical boilers are also in use.
The engines in the larger and more economical stations are generally
of the Corliss compound-condensing type, running at speeds of from
60 to 120 revolutions per minute, according to the size of the unit.
The smaller the unit, the higher the speed. In the smaller and older
stations, simple Corliss engines belted to generators are frequently
found, and high-speed engines also are used. It is the almost universal
custom now, to place the generator directly on the engine shaft, making
a direct-connected unit.
Steam turbines, in which the steam acts in jets against the blades of
a turbine wheel, are beginning to come into use at the present time.
These turbines rotate at very high speed, the largest and slowest
speed-units running 600 r.p.m., and others at higher rates. As the
output of any generator varies directly according to its speed, a
very much smaller generator can be used when coupled to a high-speed
steam turbine, to obtain a given output, than if the generator must be
coupled to a Corliss steam engine which revolves at very low speed.
The economy of the steam turbine at full load is about that of a
compound-condensing Corliss engine, but is better on light loads than
the engine. The turbine requires less building space and a much less
expensive foundation.
[Illustration: Fig. 88. Plan of Power House.]
Railway generators or dynamos for direct current are usually built
with compound-wound fields, so that, as the load increases, they will
automatically raise the voltage at their terminals to compensate for
the drop in the feeders and to maintain a constant potential at the
cars. Thus, if the line loss on a system is 10 per cent, or 50 volts
at full load, the generators will be provided with shunt fields of
sufficient strength to give 500 volts at no load, and with series
field coils which will add to the field strength enough to give 550
volts at full load. The amount of “compounding”—which is the term
applied to this method of increasing voltage—may be any amount within
reasonable limits. The pressure maintained at different companies’
electric-railway power houses varies, but is usually between 500 and
600 volts.
=Alternating-Current Generators.= Alternating-current generators used
for generating alternating current to be distributed at high tension,
are generally constructed to give a three-phase current at 25 cycles
per second. The voltage of these alternating-current generators
is sometimes the voltage at which the power is to be transmitted,
if the distances are not too great. A number of stations have
alternating-current generators giving 6,600 volts at their terminals,
which is a voltage well adapted to high-tension distribution within
the limits of a large city. However, generators giving 11,000 volts
at their terminals are now becoming common. For higher voltages than
this, it is considered necessary to use step-up transformers, in order
to raise the voltage to the proper pressure for transmission over long
distances. In such cases there is no object in having a high generator
voltage. At such stations the voltage of the generators adopted may
be anything desired, and it varies according to the ideas of the
constructing engineer. Voltages of 400, 1,000, and 2,300 are among
those in most common use.
=Double-Current Generators.= Double-current generators are sometimes
used, which generators will give direct current at a commutator at one
end of the armature for use on a 500-volt direct-current distribution
system supplying the trolley direct. The other end of the armature
has collector rings from which the three-phase alternating current is
obtained, which can be taken to step-up transformers and raised to a
sufficient pressure, for high-tension transmission to substations at
distant parts of the road. The same generator can therefore be used
on both the direct-current and the high-tension alternating-current
distribution.
=General Plan of Power Stations.= The general plan of an
electric-railway power station is usually such that the building can
be extended and more boilers, engines and generators added without
disturbing the symmetrical design of the station. Thus, the boilers
and engines are placed as in Fig. 88, in parallel rows, although
almost invariably in different rooms separated by a fire wall. By
adding to the row of engines and to the row of boilers, the station
capacity can be increased. Other arrangements are sometimes required
by circumstances; but this is the most common arrangement and gives
the greatest capacity with the minimum amount of steam piping. Large
stations are sometimes constructed with a boiler room of several floors
and with boilers on each floor, in order to save ground space and bring
the boilers near to the large engine units so that there will not be an
excessive amount of steam piping.
[Illustration: Fig. 89_a_. G. E. Circuit Breaker.]
=Switchboards.= Direct-current stations have switchboards, which may
be considered under two general classes—_generator boards_ and _feeder
boards_. Each board consists of panels.
=Generator D. C. Panels.= The generator panel usually contains an
automatic circuit breaker which will open the main circuit to the
generator in case of an overload due to a short circuit. These circuit
breakers consist of a coil in the main circuit, which acts upon a
solenoid. When the current in the coil exceeds a certain amount, the
solenoid is drawn in, and a trigger is tripped which allows the circuit
breaker to fly open under the pressure of a spring. In the General
Electric circuit breaker, the main contact is made by heavy copper
jaws, but the last breaking of the contact is made between points
which are under the influence of a magnetic field. This magnetic field
blows out the heavy arc that would otherwise be established. On the
I-T-E, the Westinghouse and most other types of circuit breaker, the
breaking of the contact takes place between carbon points, which are
not so readily destroyed by an arc as are copper contacts, and which
are more cheaply renewed. The main contact through the circuit breaker,
in either type, is made between copper jaws of sufficient cross-section
for carrying the current without heating. These jaws open before the
current is finally broken by the smaller contacts which take the final
arc.
In Fig. 89_a_ is seen a General Electric circuit breaker with the
magnetic blow-out coils at the top, the solenoid at the left, and the
handle for resetting the circuit breaker at the bottom. The small
handle for tripping the circuit breaker, when it is desired to open the
circuit by hand, is shown just under the solenoid.
An I-T-E circuit breaker is shown in Fig. 89_b_. This is of the type
previously mentioned, in which the break occurs between carbon contacts
and there is no magnetic blow-out.
[Illustration: Fig. 89_b_. I-T-E Circuit Breaker.]
In addition to the circuit breaker there is usually an ammeter, to
indicate the current passing from the generator; and a rheostat handle,
geared to a rheostat back of the board, for cutting in and out more
or less resistance in the shunt field coils of the generator so as to
reduce or raise the voltage. There is a small switch for opening and
closing the circuit through the shunt field coils.
The main leads from the generator pass through two single-pole
quick-break knife switches. The most recent practice is to have the
switches on the switchboard in only the positive and negative leads
from the generator, leaving connection to the equalizer to be made by a
switch located on or near the generator. However, all three leads may
be taken to the switchboard, and a three-pole knife switch may be used
instead of the positive and negative switches spoken of.
In Fig. 90 is given a simple diagram of the general relative connection
of generators and feeders in a direct-current railway power station.
It is seen that the generators are connected in parallel across the
positive and negative bus bar. There is a third bus bar—called an
“equalizing bus”—which connects in parallel the series coils of all
the generator fields. The object of this equalizer is to prevent the
weakening of the series field of any one generator, so as to allow it
to take current and to act as a motor instead of as a generator.
=Starting Up a Generator.= Suppose that a new generator is to be
started up and connected to the bus bars in addition to others
already in operation. The engine of that generator is first brought
up to speed. The switch controlling the shunt field circuit is then
closed, causing current to flow through the shunt fields; and the
generator begins to “build up,” its voltage gradually rising until it
approximates that upon the bus bars. Before the generator is thrown
in parallel with the others by connecting it with the bus bars, it is
important that its voltage be nearly the same as that of the bus bars.
Otherwise, when connected to the bus bars, it might take more than its
share of the load; while, on the other hand, if its voltage were too
low, it might act as a motor, taking current from the bus bars. The
voltage of the bus bars in a railway station is constantly fluctuating,
owing to the varying load and to the fact that generators are often
compounded, as before mentioned, in order to compensate for the line
loss.
[Illustration: Fig. 90. Connection of Generators and Feeders.]
In order that the voltage of the generator to be thrown in shall vary
in accordance with the bus bar voltage, the next step in the operation
is to close the positive switch, assuming that the equalizer switch on
the generator has already been closed. This throws the series field
of the new generator in parallel with the series fields of the other
generators. The voltage of the new generator will therefore vary just
as the voltage on the bus bars; and, by adjusting the resistance of the
shunt field, this voltage can be adjusted so as to be the same as that
on the bus bars. The voltages on the bus bars and on the new generator
are measured usually by a large voltmeter on a bracket at the end of
the generator switchboard. By means of a voltmeter plug or of a push
button on the generator panel, the voltmeter can be connected either
to the bus bars or to the new generator. When the two voltages are the
same, the negative switch of the new generator can be closed, and it
will operate in parallel with the other generators, taking its share of
the load. If the attendant sees that any generator is not taking its
share, he can raise its voltage by cutting out some of the resistance
in series with its shunt field, and this makes that generator take more
load.
[Illustration: Fig. 91. Railway Switchboard.]
=Feeder Panel.= The feeder panel is simpler than the generator panel,
since it usually handles only the positive side of the circuit.
Frequently two feeders are run on a single panel side by side.
The feeder panel has an automatic circuit breaker, an ammeter for
indicating the current on that feeder, and a single-pole switch for
connecting the feeder to the bus bar. All generators feed into a common
set of bus bars; and the positive bus bar continues back of the feeder
panels so that all feeders can draw current from the bus bars. Fig.
91 shows a railway switchboard with 7 feeder panels at the right; 4
generator panels at the left; and, in the middle, a panel with an
ammeter and recording wattmeter for measuring total output.
In some stations two and even three sets of bus bars are used, as it
may be desired to operate different parts of the system at different
voltages or to feed a higher voltage to the longer lines than to those
near the station. In such a case double-throw switches are provided for
connecting feeders and generators to either set of bus bars.
=Alternating-Current Switchboards.= In an alternating-current station,
generator switchboards are radically different from those in a
direct-current station. Practice in alternating-current generator
switchboards has not yet been so fully standardized and is not so
uniform as in direct-current railway switchboards. There is always,
however, a three-pole main switch for opening and closing the main
three wires from the three-phase generator. Automatic circuit breakers
are usually provided, as well as indicating ammeters and wattmeters to
show the output.
Indicating wattmeters, recording the number of watt hours passing
through them, are frequently used both on alternating and
direct-current generator panels.
A station usually has what is called a “total load” panel, which has
a recording wattmeter measuring the total output of the station in
kilowatt hours. This panel also has an ammeter indicating the total
station load.
=High-Tension Oil Switches.= Alternating-current generators for high
voltages usually have oil switches to interrupt the main circuit, that
is, switches in which the contact is made and broken under oil. These
switches have been found very efficient in preventing the formation
of a destructive arc upon the opening of a high-voltage circuit, on
circuits up to 60,000 volts. Some of the larger oil switches are
operated by electric motors or solenoids. The machine-type oil switch
of the General Electric Company has the motive power for operating the
switches, stored up in a spring. The spring is wound up by a small
electric motor. This motor operates every time the switch is opened or
closed, and winds up the spring enough to compensate for the amount it
was unwound in operating the switch. Each circuit is broken under oil
in a long tube, and these tubes are mounted in individual cells, each
cell being separated from the next by a masonry wall so that there can
be no flashing across from one leg of the circuit to another in case
of any defect in the switch. All the high-tension wiring to and from
such switches, is taken either in lead-covered cables, or on bus bars
separated from each other by masonry walls to prevent the spread of
short circuits. These precautions are necessary because of the great
length of arc that may be established between adjacent high-tension
conductors.
Where alternating-current generators of low voltage are used in
connection with step-up transformers, one practice is to have the
switches for each generator directly in the generator leads, between
the generators and the step-up transformers, in the low-voltage circuit.