A power control apparatus which includes a cycloconverter having thyristors in which a circulating current flows, for supplying a compensating current to AC power lines; capacitors for supplying a reactive current component of the compensating current to the thyristors; a trigger control circuit for controlling the triggering of the thyristors; a detector coupled to the cycloconverter, for detecting from switching currents flowing through the thyristors a circulating signal corresponding to the circulating current. The compensating current serves to improve the power factor of the AC lines as well as to reduce higher harmonic currents on the AC lines. The circulating signal is fed back to the trigger control circuit so that the amount of the circulating current is kept substantially constant according to a negative feedback operation.