Inverter control system

An inverter in the D.C. power transmission is controlled by gate pulses which are synchronized with voltages of an A.C. system and whose interval is constant for respective arms of the inverter. Simultaneously therewith, in order to prevent the inverter from causing a commutation failure due to a fault in the A.C. system, a control voltage for making the gate pulses is corrected by the use of the lowest one of the phase voltages or line voltages of the A.C. system. The control voltage becomes greater as the phase voltage or the line voltage lowers more from a rated voltage. When the control voltage increases, the gate pulses are provided at a timing of a larger advanced control angle so as to maintain the extinction angle constant.