220-Kv. Transmission: Southern California Edison company system, and some 220-Kv. researches

The rapid growth in the demand for electric power and the increasing distances to which transmission is desirable, have constantly forced the use of higher transmission voltages. The Southern California Edison Company, having two single-circuit tower lines 241 miles long from its Big Creek hydroelectric plants to near Los Angeles, had the alternative of either duplicating these lines or of raising the voltage upon them. The latter procedure was found to be vastly the more economical. Other things being equal, the amount of power that can be transmitted varying with the square of the voltage, and the existing voltage being 150 kv., a doubling of capacity will result by raising the voltage to 220 kv. To avoid the difficulties inherent in changing over generating and substations built for the lower voltage, and in which adequate clearances would be very difficult to obtain, it was decided to use auto-transformers at each such station, transforming between 150 and 220 kv. Additional sectionalizing switching stations will be built in the line, making six in all, so that the rebuilding of the line may be done without crippling service, and insulator testing can be done at any convenient time. An extension of the line 30 miles in length will be built so that the completed 220-kv. system will be 270 miles long. Preparatory to the final design, a considerable amount of investigation and research was carried on. The best form of insulation that would fit existing towers had to be determined, standard suspension insulators being preferred over new untried designs. Laboratory high-voltage tests of insulation at oscillator frequencies of 30,000 and 50,000 cycles and at continuous 60 cycles were undertaken in order to get as much information as possible, all such tests being made in dummy towers so as to duplicate actual conditions as nearly as possible. The next step was to equip 27 miles of one Big Creek line with additional insulators and shield rings and carry out field tests. This section of line was energized to 280 kv. for one month, and to 241 kv. for about five months, extending through the greater part of the rainy season. Considerable care was taken to obtain reliable measurements of voltage, current and corona losses. The results of the laboratory and field tests lead to the firm belief that nothing extraordinary will happen with 220-kv. transmission. The difference between operation at this voltage and existing voltages will be only of degree. There seems to be no pressing need of new designs of insulator so that as new designs are developed they may be given the acid test of time on unimportant lines where their failure will be of small moment.