ELECTRIC POWER TRANSMIS

SION OF. Strictly speaking, any transmi ion of electric current, regardle of what it is to be used for should be included under this head; but the conventional meaning of the terms limited to the transmi ion of current from a relatively distant point, where the power is generated, to the point where the power is distributed to the various users. There are two cla es of electric power transmi ion, namely short distance, where direct or alternating current is applicable; and long distance transmis+ sion where the use of high voltage and alternating current predominates. Owing to the fact that in many systems the use of voltage transforming devices is not economical, the voltage in short distance work in general does not exceed 500 V. D. O. or 2300 V. A.C. This relatively low limit is nece ary since full transmi ion voltage must be applied to motors, and the use of high (over 2300 V.) voltage machines is not good prac tive around industrial plants. Both A.C. and D.C. systems have their advantages, and a careful study of conditions must be made before a system is chosen. Long distance transmi ion dictates high voltages to reduce both the power lo in the line, and the amount of copper required to transmit a given quantity of power. V oltages as high as 220,000 are now being used. The transmi ion voltage is limited only by the insulators obtainable, the lo due to ‘corona’ discharges and the insulation of the transformers available; Where voltages higher than 13,000 volts are used, it is usual to operate the generators at this low voltage transformers | being employed to obtain transmi ion voltage. The question of frequency of power systems has been much discu ed. 25,40,133 and 60 cycles per second have been used; present practice, however has standardized on 60 cycles (although the other frequencies may be found to a limited extent). A.C. transmi iom usually employs the three wire, three phase system owing to its great economy of copper.