Abstract: Abstract:A tripout accidents of three 500kV output transmission lines of Togtoh Power Generation Company Limited occurred successively, four units were compelled to shut down urgently. The influence of voltage surge on the variable frequency equipments during trial transmitting power into the transmission line was expounded. The reasons of frequency converter tripping and the harm of the frequency converter tripping were analyzed. Concrete preventive measures were put forward so as to ensure the power system to operate safely and stably.

Key words: Key words:frequency converter, transmit power into transmission line, voltage surge, undervoltage trip