115-Kv Bus Switching Surges

In 1955, at the Black Dog Steam Station of Northern States Power Company (NSP), a 97-kv lightning arrester on the 115-2.4-kv reserve auxiliary transformer failed while de-energizing the 115-kv transfer bus with an air-break switch. The replacement of this arrester failed in 1957 when the 115-kv transfer bus was being energized through an air-break switch. These two arresters were not manufactured by the companies with which the authors are associated. Considering these failures as more than coincidental, a thorough investigation was initiated by NSP to attempt to explain how this transfer bus switching influenced the arrester failures. Analytical and Anacom (General Purpose Transient Analyzer) studies supplemented by field tests indicated sufficiently high overvoltages could be created by the transfer bus switch operation, due to a prestrike or restrike, to cause the lightning arrester to operate.