The items of the cost of a 10,000 kilowatt transmission plant for 150 miles, at 75,000 volts, with a 10 per cent. loss are: Copper, $20; pole line, $15; transforming stations, $10—a total of $45 per kilowatt. The cost of a water-power plant, excluding the transmission, may vary from $75 to $200 per kilowatt. The cost of the trans mission system is then from 18 per cent. to 38 per cent. of the total cost. The annual charges for interest, depreciation, and maintenance of this line will be $6 per kilowatt. Hence there must be at least this difference in the cost of generation at the two points to make the trans mission pay. At 37,500 volts, the copper would cost $80—making the total $105 per kilowatt for the transmission; the annual charges would then be $10 per kilowatt. Accurate statistics are not available, and the line of demarcation between a transmission plant and an alternating-current central station is difficult to draw. There are, however, about 200 water-driven electric plants in America, aggregating approximately 600,000 kilowatts, with single stations up to 75,000 kilowatts capacity, and with single generators up to 7500 kilowatts capacity. The maximum distance in daily service is 220 miles; the maxi mum voltage 60,000; the frequency varies from 25 to 60 cycles, with an increasing tendency to the lower value.
The efficiency of a transmission system is largely determined by the amount of copper, which is in turn determined by economical con siderations; it usually falls between SO and 00 per cent., and is about as high at half load as at
full load. The increasing prominence of the steam turbine and the gas engine will probably retard the development of transmission plants in the immediate future, but, looking further, it is inevitable that their importance and value will increase as the coal supply diminishes in quan tity and increases in cost.
The best practice in the transmission of energy by electricity is found on the Pacific Coast, and the plants of the Standard and Bay Counties Company, in California, are typical. This com pany supplies Oakland, San Francisco, Stockton, and a large number of smaller places with elec tric energy for all purposes. It operates two main power houses at Colgate and at Electra; it also has the Yuba and the Nevada power houses. It has more than 650 miles of pole line and some 800 miles of three-phase circuit, nearly all of aluminum cable; it uses voltages up to 60,000, and operates to a distance of 220 miles, from Col gate to Stockton. The poles are of Oregon cedar or sawn California redwood, of an average height of 40 feet, and carrying either one of two three phase circuits, the wires being supported on porcelain or glass insulators. The capacity of these power plants exceeds 25,000 kilowatts. For detailed descriptions of modern transmission plants, reference should be made to the files of the electrical engineering journals and the Pro ceedings of the American Institute of Electrical Engineers. See DYNAMO-ELECTRIC MACHINERY; TRANSFORMER.