Analytical Studies of Lightning Performance of 1- and 2-Ground-Wire 138-Kv Double-Circuit Lines of the Commonwealth Edison Company

An unusual number of flashovers due to lightning have occurred on the 138-kv lines of Commonwealth Edison Company on a relatively new type of 138-kv double-circuit tower designed for only one overhead ground wire. Calculations based on the most recent AIEE Committee Report1 for estimating lightning performance indicate that these towers should have tripout rates very much similar to the type of double-circuit towers that are designed for two overhead ground wires. This paper goes a step beyond the AIEE Committee Report in that it considers the voltage surge induced on the conductors at the instant of field collapse through cloud discharge. This is done on a comparative basis for the new and old towers, as well as for modifications of the new towers. In addition, all towers are compared on a direct-stroke basis, in line with the AIEE Committee Report. A curve is presented which indicates a region where a second ground wire can be added to the new towers so that the resulting induced surge voltage and direct-stroke voltage compare favorably with previous designs. Actual modifications made in tower designs are discussed. These consist of: 1. changing the tower head to provide for two ground wires in a horizontal plane, and 2. adding a second ground wire at the height of the top phase conductor through the center of the tower.