PURPOSE: To enable to suppress an erroneous current variation detecting operation occurred upon protecting operation in a substation by controlling a chopper ON or OFF in response to the terminal voltage of a filter condenser upon reception of a detection signal from a discharge voltage detector.

CONSTITUTION: When a load 9 is interrupted, a regenerative current IR flows to a filter condenser 10 and the overvoltage of the terminal of a filter condenser 10 is detected by an overvoltage detector 11, a thyristor 13 is conducted to form a discharge circuit. A comparison arithmetic unit 24 outputs a deviation signal Idef when the terminal voltage Vc of the condenser 10 detected by a discharge voltage detector 23 exceeds the set value Iref. A chopper controller 25 inputs the signal Idef, prevents the ON pulse of the chopper 2 and turns the chopper 2 OFF.