A control apparatus for controlling a three- or single-phase semiconductor power converter employing fixed-pulse switching patterns superposes a DC component on an AC output voltage, to suppress asymmetrical magnetization of a transformer connected to the power converter.The three-phase semiconductor power converter 1 provides an AC output. The voltage of the AC output is changed by the transformer 12. The control apparatus 14 for controlling the power converter 1 includes an asymmetrical magnetization suppressive controller 15 to generate a correction value for each of basic fixed-pulse switching patterns to be applied to three-phase switching elements of the power converter 1, a phase corrector 16 to switch the correction values from one to another at timing when no switching operation is carried out at any one of the switching elements, and a pattern generator 17 to correct the basic fixed-pulse switching patterns according to the correction value provided by the phase corrector 16 and provide the corrected switching patterns.