Electricity

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reciprocating steam engines were always employed, and though these are still in very extensive use, they are being superseded in many cases by steam turbines.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.

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.

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.