To dispense with a special generator, when the short circuit capacity of a power supply bus is insufficient as compared with that in a generator and to prevent a voltage stop or loss or deterioration in static stability when a current-limiting reactor is provided.

By providing a current-limiting reactor 7 between a generator 6 and a bus 2 and providing a normally-closed AC switching circuit 8 parallel to the reactor 7, a voltage drop and the like by the reactor 7 is eliminated at normal times. When the bus voltage drops and a short circuit current is about to flow, a voltage drop detecting relay 9 detects the voltage drop to open the switching circuit 8, thereby controlling the short-circuit current from the generator.