Lightning performance of 138-kv double circuit lines

THE COMMONWEALTH Edison Company operates a 138-kv and a 230-kv overhead transmission system in northern Illinois. Prior to 1954, most of the 138-kv circuits were carried on double circuit steel towers designed for two overhead ground wires. A typical 138-kv tower is shown as type A in Fig. 1. This tower has given very good lightning performance based on cumulative 6,351 year-miles of experience with circuits on towers where both circuits and both ground wires were installed, and 3,782 cumulative year-miles where only one circuit and one ground wire were installed. However, the type A tower had not given satisfactory service with respect to flashovers caused by galloping conductors under ice and wind conditions; for this reason, the type C tower in Fig. 1 was developed.