PURPOSE: To absorb fluctuating current for zero point detection and eliminate output ripple, by finding out the peak value of the output current of a thyristor converter, and by finding out the correction control angle of lag according to the unevenness of the current, to be bias added to the control angle of lag.

CONSTITUTION: In a power converter for detecting the output current of a thyristor converter 1 with a current detector 4 and comparing the detected value with the reference value and applying gate pulse from a gate pulse generator 10 to the converter 1 according to the deviation, the peak value of each phase current is found out of the detected value of the current detector 4. Then, if any lag is present in the zero point detection of a synchronous power source, unevenness is generated thereby on the ripple value of the current of the converter 1, and so the unevenness of the peak value of current is found out, and the correction and control angle Δα of lag is determined according to the unevenness. The correction angle Δα is bias added to the control angle α of lag.