PURPOSE:To optimize the controllability of a converter by providing a means varying the gains of a converter control system in response to the state of operation of an inverter. CONSTITUTION:Electromagnetic switches 241-24n are each mounted to DC paths to n inverters 221-22n from a converter 21, and the switches are ON-OFF cotrolled in response to the commands of the inverters of operation or stoppage by an operation sequence circuit 25. The sequence circuit 25 is provided with a control means changing over and controlling the gains of a voltage control amplifier incorporated into a control circuit 21A for the converter 21 at the same time as the ON-OFF control of the switches 241-24n. Accordingly, even when capacitor capacity viewed from the converter 21 alters in the separation or juxtaposition of DC side capacitors for the inverters 22, the gains of a converter control system are controlled automatically, thus eliminating trouble on operation due to the transient characteristics of the converter.