TRANSMISSION OF POWER ay ELEC TRICITY. The electrical transmission of energy in considerable quantities from the point of generation to one or more distant points of delivery. This excludes telegraphy and telephony (qq.v.), where energy is transmitted over a wire, but in small quantities.
Energy is transmitted in order to utilize a cheap source of energy supply, and the total cost of generation and transmission to the point of use must be less than would be the cost of generation at the point of use, otherwise such transmission would not be profitable. Therefore, transmission plants are usually supplied by water power or by cheap coal, and the electricity is derived from generators. (See DYNAMO ELECTRIC MACHINERY.) The cost of the trans mission depends upon the distance and upon the permissible electrical pressure, or voltage, of the transmission line. The weight of copper re quired to transmit any amount of energy for any distance at any particular efficiency is directly proportional to the square of the distance, and inversely proportional to the square of the volt age. if the distance be doubled, with the same voltage, the weight of copper will be four times as great; if the voltage be doubled, with the same distance, the weight of copper will be one quarter as great; if the loss be doubled, the weight of copper, for the same distance and same voltage, will be halved.
This is the fundamental fact in the trans mission of energy by electricity; great distances can be economically overcome by the use of high voltages only. This explains the practical elimination of the direct current as a means of transmission; direct-current generators cannot be built in large sizes for more than 1500 volts, on account of trouble at the commutator; to connect a large number of such generators in series involves great difficulties in the use of the energy at the receiving end; the motors must also be connected in series and be insulated to stand the maximum voltage; this limits their use to places where skilled supervision can be exercised. There are a few instances of the use of direct-current transmission in Europe, hut in the United States there is none.
The maximum permissible voltage then limits the distance of transmission. What, in turn, limits the maxinunn voltage? Transformers (g.v.) can be built to operate satisfactorily at 75,000 volts; insulators are now built for use at 60,000 volts and for a test pressure of 120,000 volts; the insulator is at present the weak point of the transmission system, yet it is probable that improvements in insulators can be made at reasonable cost to permit their use up to 75,000 volts. There is, however, another limitation due
to the material loss of energy between two wires suspended in air, when the voltage between them exceeds a certain value; this loss is compara tively 'small up to about 50,000 volts, but in creases very rapidly above that point; it can be diminished by increasing the distance apart of the wires, but great increase on a single pole is impracticable, hence it would become neces sary to build separate pole lines for the several wires; this added cost may easily be greater than the value of the energy saved by such con struction, and a lower voltage may be more economical. The practical limit to the voltage is fixed by the excessive cost of the insulation of transformers and line, rather than by the dielec tric loss, and at present 75,000 volts is probably the maximum.
The transmission of energy can be made with as good economy in copper by the single-phase as by the three-phase system (see DYNAMO ELECTRIC MACIwcERY), for the same stress on the insulator, and requires only two wires against three for the three-phase system; hut this advantage is more than offset by the fact that there is no good and simple motor that can be used, under ordinary conditions, with the single-phase system. The polyphase system, on the other hand, has come into general use on account of the induction motor, which is self starting and of extreme simplicity in construc tion and operation; its commercial introduction gave the first great impetus to the transmission of energy by electricity. Compared with the two-phase system, the three-phase has the ad vantage of requiring only three wires as against four, and 25 per cent. less copper, hence it is practically the only system used for transmis sion. Copper wire is generally used, although aluminum has some advantages; the wires are always bare, and arc supported by porcelain or glass insulators fixed to cross-arms on poles, which, in the United States, are generally of wood.