[Task] In a power conversion system, a variation in PCS alternating current voltage is suppressed during a change operation to a self-contained run according to an instantaneous voltage drop-service interruption.[Means for solving the task] During an ordinary system voltage stabilization, with change switch 25 changed to the output side of APR control circuit 22 and change switch 400 changed to the output side of charge/discharge run PWM command value preparing section 200, the power conversion system is run in a charge/discharge run mode. If the instantaneous voltage drop or the service interruption occurs, change switch 25 is changed to the output side of the self-contained run change prior current command value preparing section 60 and the constant current discharge run is carried out in which the electric power is discharged for a predetermined time duration in response to the current command value of the self-contained run change prior current command value preparing section 60. Thus, after a predetermined time has elapsed, change switch 400 is changed to the output side of the self-contained run PWM command value preparing section 30 to perform the self-contained run. At this time, the variation in PCS alternating current voltage in the power conversion system is suppressed.