ELECTRIC POWER STATIONS
It is apparently a very simple matter to fit up a power station with
a number of very large dynamos driven by powerful engines, and to
distribute the current produced by these dynamos to all parts of a town
or district by means of cables, but as a matter of fact it is a fairly
complicated engineering problem. First of all the source of power for
driving the dynamos has to be considered. In private and other small
power plants, gas, petrol or oil engines are generally used, but for
large stations the choice lies between steam and water power. In this
country steam power is used almost exclusively. Formerly the ordinary
reciprocating steam engines were always employed, and though these are
still in very extensive use, they are being superseded in many cases
by steam turbines. The turbine is capable of running at higher speeds
than the reciprocating engine, and at the greatest speeds it runs with
a great deal less noise, and with practically no vibration at all.
More than this, turbines take up much less room, and require less oil
and attendance. The turbines are coupled directly to the dynamos, so
that the two machines appear almost as one. In the power station shown
on Plate V. a number of alternating current dynamos coupled to steam
turbines are seen.
A large power station consumes enormous quantities of coal, and
for convenience of supply it is situated on the bank of a river or
canal, or, if neither of these is available, as close to the railway
as possible. The unloading of the coal barges or trucks is done
mechanically, the coal passing into a large receiving hopper. From here
it is taken to another hopper close to the furnaces by means of coal
elevators and conveyors, which consist of a number of buckets fixed
at short intervals on an endless travelling chain. From the furnace
hopper the coal is fed into the furnaces by mechanical stokers, and the
resulting ash and clinker falls into a pit below the furnaces, from
which it is carted away.
The heat produced in the furnaces is used to generate steam, and
from the boilers the steam passes to the engines along a steam pipe.
After doing its work in the engines, the steam generally passes to a
condenser, in which it is cooled to water, freed from oil and grease,
and returned to the boilers to be transformed once more into steam.
As this water from the condenser is quite warm, less heat is required
to raise steam from it than would be the case if the boiler supply
were kept up with cold water. The power generated by the engines is
used to drive the dynamos, and stout copper cables convey the current
from these to what are called “bus” bars. There are two of these,
one receiving the positive cable from the dynamos, and the other the
negative cable, and the bars run from end to end of a large main
switchboard. From this switchboard the current is distributed by other
cables known as feeders.
The nature of the current generated at a power station is determined to
a great extent by the size of the district to be supplied. Generally
speaking, where the current is not to be transmitted beyond a radius
of about two miles from the station, continuous current is generated;
while alternating current is employed for the supply of larger areas.
In some cases both kinds of current are generated at one station.
[Illustration: PLATE V.
_By permission of_ _C. A. Parsons & Co._
LOTS ROAD ELECTRIC POWER STATION, CHELSEA.]
If continuous current is to be used, it is generated usually at a
pressure of from 400 to 500 volts, the average being about 440 volts;
and the supply is generally on what is known as the three-wire system.
Three separate wires are employed. The two outer wires are connected
respectively to the positive and the negative bus bars running along
the main switchboard, these bars receiving positive or negative current
directly from the dynamos. The outer wires therefore carry current at
the full voltage of the system. Between them is a third and smaller
wire, connected to a third bar, much smaller than the outer bars, and
known as the mid-wire bar. This bar is not connected to the dynamos,
but to earth, by means of a large plate of copper sunk into the ground.
Connexion between the mid-wire bar and the outer bars is made by two
machines called “balancers,” one connecting the mid-wire bar and the
positive bus bar, and the other the mid-wire bar and the negative bus
bar. If the pressure between the outer bars is 440 volts, then the
pressure between the mid-wire bar and either of the outer bars will be
220 volts, that is just half.
The balancers serve the purpose of balancing the voltage on each side,
and they are machines capable of acting either as motors or dynamos. In
order to comply with Board of Trade regulations, electric appliances of
all kinds intended for ordinary domestic purposes, including lamps, and
heating and cooking apparatus, are supplied with current at a pressure
not exceeding 250 volts. In a system such as we are describing, all
these appliances are connected between the mid-wire and one or other of
the outer wires, thus receiving current at 220 volts. In practice it is
impossible to arrange matters so that the lamps and other appliances
connected with the positive side of the system shall always take the
same amount of current as those connected with the negative side, and
there is always liable to be a much greater load on one side or the
other. If, for instance, a heavy load is thrown on the negative side,
the voltage on that side will drop. The balancer on the positive side
then acts as an electric motor, drives the balancer on the negative
side as a dynamo, and thus provides the current required to raise the
voltage on the negative side until the balance is restored. The working
of the balancers, which need not be described in further detail, is
practically automatic. Electric motors, for driving electric trams or
machinery of any kind, are connected between the outer wires, so that
they receive the full 440 volts of the system.
In any electric supply system the demand for current does not remain
constant, but fluctuates more or less. For instance, in a system
including an electric tramway, if a car breaks down and remains a
fixture for a short time, all cars behind it are held up, and a long
line of cars is quickly formed. When the breakdown is repaired, all
the cars start practically at the same instant, and consequently a
sudden and tremendous demand for current is made. In a very large
tramway system in a fairly level city, the fluctuations in the demand
for current, apart from accidents, are not very serious, for they tend
to average themselves; but in a small system, and particularly if the
district is hilly, the fluctuations are very great, and the current
demand may vary as much as from 400 to 2000 amperes. Again, in a
system supplying power and light, the current demand rises rapidly as
the daylight fails on winter afternoons, because, while workshop and
other motors are still in full swing, thousands of electric lamps are
switched on more or less at the same time. The power station must be
able to deal with any exceptional demands which are likely to occur,
and consequently more current must be available than is actually
required under average conditions. Instead of having generating
machinery large enough to meet all unusual demands, the generators at
a station using continuous current may be only of sufficient size to
supply a little more than the average demand, any current beyond this
being supplied by a battery of storage cells. The battery is charged
during periods when the demand for current is small, and when a heavy
load comes on, the current from the battery relieves the generators of
the sudden strain. To be of any service for such a purpose the storage
battery of course must be very large. Plate VI. shows a large battery
of no cells, and some idea of the size of the individual cells may be
obtained from the fact that each weighs about 3900 lb.
Alternating current is produced at almost all power stations supplying
large districts. It is generated at high pressure, from 2000 volts
upwards, the highest pressure employed in this country being about
11,000 volts. Such pressures are of course very much too high for
electric lamps or motors, and the object of generating current of
this kind is to secure the greatest economy in transmission through
the long cables. Electric energy is measured in watts, the watts
being obtained by multiplying together the pressure or voltage of the
current, and its rate of flow or amperage. From this it will be seen
that, providing the product of voltage and amperage remains the same,
it makes no difference, so far as electric energy is concerned, whether
the current be of high voltage and low amperage, or of low voltage and
high amperage. Now in transmitting a current through a long cable,
there is a certain amount of loss due to the heating of the conductor.
This heating is caused by the current flow, not by the pressure; and
the heavier the current, the greater the heating, and the greater the
loss. This being so, it is clear that by decreasing the current flow,
and correspondingly increasing the pressure, the loss in transmission
will be reduced; and this is why alternating current is generated at
high pressure when it is to be transmitted to a distance.
The kind of alternating current generated is usually that known as
three-phase current. Formerly single-phase current was in general
use, but it has been superseded by three-phase current because the
latter is more economical to generate and to distribute, and also more
satisfactory for electric motors. The actual voltage of the current
sent out from the station varies according to the distance to which the
current is to be conveyed. In the United States and in other countries
where current has to be conveyed to places a hundred or even more miles
from the station, pressures as high as 120,000 volts are in use. It
is possible to produce alternating current at such pressures directly
from the dynamos, but in practice this is never done, on account of the
great liability to breakdown of the insulation. Instead, the current
is generated at from 2000 to 10,000 or 11,000 volts, and raised to the
required pressure, before leaving the station, by means of a step-up
transformer. We have seen that an induction coil raises, or steps up,
the voltage of the current supplied to it. A step-up transformer works
on the same principle as the induction coil, and in passing through
it the current is raised in voltage, but correspondingly lowered in
amperage. Of course, if the pressure of the current generated by the
dynamos is already sufficiently high to meet the local requirements,
the transformer is not used.
[Illustration: PLATE VI.
_By permission of_ _Chloride Electrical Storage Co. Ltd._
POWER STATION BATTERY OF ACCUMULATORS.]
For town supply the current from the power station is led along
underground cables to a number of sub-stations, situated in different
parts of the town, and generally underground. At each sub-station the
current passes through a step-down transformer, which also acts on the
principle of the induction coil, but in the reverse way, so that the
voltage is lowered instead of being raised. From the transformer
the current emerges at the pressure required for use, but it is still
alternating current; and if it is desired to have a continuous-current
supply this alternating current must be converted. One of the simplest
arrangements for this purpose consists of an electric motor and a
dynamo, the two being coupled together. The motor is constructed to
run on the alternating current from the transformer, and it drives the
dynamo, which is arranged to generate continuous current. There is also
a machine called a “rotary converter,” which is largely used instead
of the motor generator. This machine does the work of both motor and
dynamo, but its action is too complicated to be described here. From
the sub-stations the current, whether converted or not, is distributed
as required by a network of underground cables.
In many parts of the world, especially in America, water power is
utilized to a considerable extent instead of steam for the generation
of electric current. The immense volume of water passing over the Falls
of Niagara develops energy equal to about seven million horse-power,
and a small amount of this energy, roughly about three-quarters
of a million horse-power, has been harnessed and made to produce
electric current for light and power. The water passes down a number
of penstocks, which are tubes or tunnels about 7 feet in diameter,
lined with brick and concrete; and at the bottom of these tubes are
placed powerful water turbines. The falling water presses upon the
vanes of the turbines, setting them revolving at great speed, and the
power produced in this way is used to drive a series of very large
alternating current dynamos. The current is conveyed at a pressure
of about 60,000 volts to various towns within a radius of 200 or 300
miles, and it is anticipated that before very long the supply will
be extended to towns still more distant. Many other American rivers
have been harnessed in a similar way, though not to the same extent;
and Switzerland and Norway are utilizing their water power on a
rapidly increasing scale. In England, owing to the abundance of coal,
little has been done in this direction. Scotland is well favoured in
the matter of water power, and it is estimated that the total power
available is considerably more than enough to run the whole of the
railways of that country. Very little of this power has been utilized
however, and the only large hydro-electric installation is the one at
Kinlochleven, in Argyllshire. It is a mistake to suppose that water
power means power for nothing, but taking things all round the cost of
water power is considerably lower than that of steam.