To protect thyristors from damages caused by application of surge voltages having excessive increase rates of forward voltages.

A voltage detector is connected in parallel with the thyristor to measure the voltage between the anode and cathode of the thyristor. A voltage increase rate calculator obtains a forward voltage increase rate from an applied voltage. When a protection operation determination device determines that a voltage reaches or exceeds a forward voltage increase rate prescribed for each applied voltage from the applied voltage and the forward voltage increase rate, a pulse generator outputs a protection gate trigger to the thyristor and all thyristors connected to the thyristor in series, thus protecting the thyristors from the excessive forward voltage increase rate. The forward voltage increase rate is determined for each applied voltage, thus reducing the number of unnecessary protection operation times as compared with a case where the forward voltage increase rate is determined by one applied voltage.

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