Lightning Faults on Distribution Lines

A distribution line subjected to a direct lightning strike can be expected to result in an insulation failure (flashover). Typical distribution line basic insulation levels (BIL) are less than 300 kV; often, the BILs for individual structures are below 100 kV. Nearby strikes can cause flashover from induced voltages on these low-BIL structures. One of the objectives in EPRI Research Project 2542-1 Characteristics and system damage caused by lightning. An important parameter is strikes to the lines. Although EPRI had a parallel project in progress to measure ground flash density (Ng), no low-cost instrumentation was a vailable to measure strikes to the line. The coincident lightning events detector (CLED) was developed for this purpose. The CLED allows circuit breaker operations to be categorized into lightning-caused operations and nonlightning-caused operations. The lightning sensing is provided by a modified RAS 10 flash counter. The standard RAS 10 Uses a 4-m vertical aerial with a range of 22 km. The modified RSA 10 has a 2-m aerial with a calculated range of 9.6 km. This range was selected to be larger than the normal distribution substation service area but smaller than the standard RAS 10 range so that the lightning flash data would be recorded in an area approximately the same size as the substation service area. The RSA 10 is triggered by the radiated energy from the lightning return stroke current; this triggering is essentially instantaneous. A distribution circuit breaker takes 4 to 6 cycles to trip after fault initiation.