PURPOSE: To prevent the generation of an overvoltage by a method wherein a capacitor interceptor is provided with a tripping command by a signal when a trouble in an AC system is detected by an Ac undervoltage relay.

CONSTITUTION: When a trouble such as grounding and the like is generated in an Ac system connected to a converter 3, the voltage of an Ac bus 1 is reduced while the reduction of the voltage is detected by an undervoltage relay 11. An interceptor 13, putting a phase regulating capacitor 14 ON/OFF by a trouble signal is provided with a tripping command. when the command is provided, it takes tens to handreds of milliseconds until the interception is finished actually due to the delay of transmission, the operation of the interceptor and the like. During this period, the operation of the converter is continued and ineffective power is consumed. On the other hand, when the trouble is removed by separating the transmission line in the troubling division or the like, an Ac voltage, deteriorated during the trouble, is restored suddenly and exciting rush current is conducted through a transformer or a system, in which the trouble is removed, becomes unbalance in the case of an unbalance trouble whereby an overvoltage is generated.