TBANSMISSION OF POWER. 418 TRANSMISSION OF POWER. 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 tiie 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 1.500 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 ma.vimum voltage; this limits their use to places where skilled supervision can he exercised. There are a few instances of the use of direct-current transmission in Europe, but in the United States there is none. The maximum permissible voltage then limits the distance of transmission. What, in turn, limits the maximum 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 "5,000 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 he made with as good economy in copper by the single-phase as by the three-phase system (see Dynamo- Electric Machinery ) , for the same stress on the insulator, and requires only tw^o wires against three for the three-phase system; but 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 sin-rle-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 an'd 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 are supported by porcelain or glass insulators fixed to cross-arms on poles, which, in the United States, are generally of wood. 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 kilow att. The cost of a water-power plant, excluding the transmission, may vary from $75 to $200 per kilowatt. The cost of tlie 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 $0 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 80 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 tiie plants of the St-andard 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
