TRANSMISSION OF ELECTRIC POWER. Under this heading are con sidered methods of conveying electrical energy from one locality to another. The need for such transmission arises chiefly from economic causes. In the center of a city, electricity can seldom be gener ated except at high cost. It is a common practice to produce the electricity at some point where there is a cheap source of power (such, for instance, as water power, or a readily available and abun• dant supply of coal) and then to convey it to the city in the manner outlined below. It is obvious that the site of the central supply station will depend not only on the cost of generating the elec tricity, but also on the cost of transmit ting it to the point where it is to be used. Before considering the methods of transmitting, therefore, it will be advis able to examine, briefly, the different fac tors determining its cost.
The first factor will clearly be the dis tance which has to be covered. The greater the distance, the more metal will have to be used for conveying the cur rent, and the greater will be the expendi ture on constructing and maintaining the service lines. The second factor is the voltage at which the current is supplied. Electric power is the product of the volt age and the current, and the greater the voltage, the smaller may be the cur rent, and, consequently, the smaller the cross-section of the cable carrying the current. In other words, by using a high voltage and a low the amount of copper required will be much smaller than when the current is greater and the voltage power lower, while the amount of power supplies will be no less. As suming that the efficiency remains con stant, the amount of copper required will be proportional to the square of the dis tance and inversely proportional to the square of the voltage. It is, therefore, economical to have the sources of supply located at a great distance from the point of consumption only when the permis sible voltage is very high. It is for this reason that alternating current is always used when supplied over long distances. Large direct current generators cannot supply current at a higher voltage than 1,500 volts, and so alternating current generators are used. (See DYNAMO ELECTRIC.) A typical high voltage trans mission system consists of the power station in which are located the genera tors and the "step-up transformers," and the cables to transmit the current to the terminal station, at which are located the "step-down transformers," from which the current is supplied to various sub-stations. The transformer is a piece
of apparatus which receives electricity at one voltage and delivers it at another, the "step-up transformers" raising the voltage, while the "step-down transform ers" lower it. For many purposes, alter nating current is unsuitable or undesira ble, and it is frequently necessary to transform the current from alternating to direct. This is done by means of a motor generator set. The motor is driven by the alternating current, and in turn drives a direct current dynamo.
The cable for conveying the current is nearly always of copper, but aluminum is occasionally used. The wires are bare and in overhead systems are supported on wooden poles fitted with cross-arms or on steel towers. The former are used for lower voltage systems, the wires be ing supported on glass or porcelain in sulators fixed on to the cross-arms. For higher voltages, however, the steel tower is now commonly used, a suspension type of insulator replacing the so-called pin insulators. Occasionally the cable is laid underground, especially in parts of Eu rope, where the overhead system is com paratively uncommon. The cable in this case is insulated with paper impregnated with rosin oil, or some similar substance, and is then frequently sheathed in lead.
From what has been said regarding the economy of high voltages, it would seem that the logical procedure would be to generate electricity at the highest pos sible voltage. There are certain prac tical considerations, however, which limit the permissible voltage. In the first place, the insulators are not entirely satisfactory at any higher voltage than 60,000. Moreover, when two parallel wires carrying a current are suspended in air, it is found that there is a con siderable loss of energy between them, and this loss increases rapidly with volt ages above 50,000. The only way to overcome this loss is to keep the wires widely separated, but it is obvious that there is a limit to the possible separation where only one line of poles is used. To obtain satisfactory separation, two or more lines of poles or towers would be necessary, and it will be at once seen that a great increase in cost would re sult. For this reason, it is found cheaper to limit the voltage, and at the present time 75,000 is the highest voltage that can be considered economical.