PURPOSE: To suppress over current from a power inverter by operating the power inverter with zero AC output current when the voltage at the side of the AC system of the power inverter drops.

CONSTITUTION: When AC voltage drops to operate an AC under voltage relay 4, switches 44 and 7 function to bring the set values 15, 43 for APR and AQR to zero. Consequently, the control angle of the power inverter is determined such that AC secondary voltage Vi at the side of the power inverter has same magnitude as that of AC primary voltage Vs at the side of the power inverter. In other words, the AC secondary voltage Vi at the side of the power inverter has same phase and amplitude as those of the AC primary voltage Vs at the side of the power inverter, and the effective and reactive output powers P, Q at the side of the power inverter are brought to zero, thus bringing out an operating state similar to that where AC output current is zero. By such arrangement, over current from the power inverter can be suppressed effectively for AC voltage drop.