PURPOSE: To prevent the flow of unnecessary current to a system and quickly recover an exciting system by detecting the recovery of a system accident on the basis of a voltage on the high voltage side of a main transformer and removing the accident point of the system and at the same time resetting the operation of converting into an alternating current.

CONSTITUTION: A level comparator CP compares the output detected by a voltage detector S provided on the high voltage side of a main transformer 7M with a predetermined set value to output a signal when the detected output is larger than the set value. When a signal is outputted from the comparator CP, the function of a power converter 3 is stopped by a controller 55, and a d. c. current is supplied to a secondary winding through a switch 9 from a d.c. power supply 10 to perform the operation of converting into an alternating current, so that the a.c. component of the primary side of a generator motor is secured. When the accident point is removed from a transmission line by the release of a breaker 6, the system voltage is recovered in the case of two- network transmission line. Thereby, an unnecessary current can be prevented from flowing into the system.