ELECTRIC TRANSMISSION OF EN ERGY, Long Distance. An electric trans mission of energy obviously occurs when the relay of a Morse telegraph circuit, or the sensi tive mirror used in submarine cable telegraphy, responds to the feeble current impulse origi nated at the transmitting end of the wire. It is also obvious that we have the electric trans mission of power in the ordinary use of electric light or electric motors even when the generator is stationed in the building in which that light or power is used. But generally spealcing the term electric transmission of energy denotes the transmission of energy on a large scale by means of overhead or underground conductors or cables and its transformation into light, heat, chemical energy. or mechanical power at the remote end of the conductors. INhen the dis tance to which this electric power is trans mitted exceeds say 15 or 20 miles it is usually spoken of as the long distance transmission of electric energy or power.
In cities like New York where electric energy for lighting, power and railway traction is trans mitted distances ranging from less than a mile to 15 or 18 miles from the power house the conductors are usually placed in cables in under ground conduits and the maximum electro motive force transmitted is about 11,000 volts. This pressure is directly generated by a steam driven alternating current generator and is transmitted over the conductors to sub-stations, where by.means of step-down transformers the pressure is dropped to say 600 volts alternating current which when direct current is desired by rotary converters is converted into direct current for the street mains, the feeders of the railway system and for charging storage bat teries which in turn give out direct current at times of heavy demand or when otherwise re quired; or when alternating current is required for service mains the 11,000 volts is transformed to say 240 volts. When water power is avail able, as at Niagara Falls and innumerable other places in this and other countries, the electric transmission of power on a large scale to dis tances of 25, 50, 100 and 200 miles from its source is not uncommon.
In order that electric power may be trans mitted economically to long distances the use of high electric pressure or tension is essential, since otherwise the cost of copper in the con ductors would be excessive. For example: It
has been calculated, on the basis of 5,000 volts, that to transmit 2,356 kilowatts a distance of 100 miles would require about 22,862,737 pounds of copper in the conductors, assuming a line drop of 750 volts, or 15 per cent of the total electro-motive force, whereas with 40,000 volts the total amount of copper required would be about 357,230 pounds. Even doubling the pres sure would quarter the amount of metal re quired for a given distance and given line drop. In the electric transmission of power to long distances the use of alternating current is gen eral, and transformers are utilized to increase the electro-motive force on the transmission line and for reducing it at the points of distribution. Where the electro-motive force on the line does not exceed 10,000 volts generators developing that electro-motive force may be employed. When that line voltage is exceeded the station voltage adopted is about 2,300 volts; this being stepped up by transformers to the electro-motive force desired on the line. This moderate voltage admits of the use of lighter copper wires or conductors in the station apparatus than higher voltages would require. In present day prac tice, however, the use of 150,000 volts is not uncommon and line insulators and transformers to withstand this pressure are now employed. Indeed, manufacturers are prepared to supply transformers capable of operating at 200,000 volts. In North Carolina and South Carolina over 1,000 miles of 100,000-volt transmission circuits are in operation on the system of the Southern Power Company, which derives its power mainly from hydro-electric developments. In California there is a 150,000-volt circuit from Big Creek to Los Angeles, a distance of 240 miles, on the system of the Pacific Light and Power Corporation. The conductors of this circuit are supported on steel towers. In addi tion this company has in operation 105 miles of 60,000-volt circuits, 240 miles of 50,000-volt cir cuits and 831 miles of 15,000-volt circuits. These last circuits are supplied with current from the 150,000-volt circuits at various distribution points by means of step-down transformers in sub-stations.