Parallel running of GTO PWM inverters

A new parallel running system of GTO PWM inverters is proposed to achieve a high reliability and large capacity inverter system. In the system, a separate control circuit with a local processing unit is used for each inverter to prevent total system breakdown. A dc power source is also separated for each inverter. A circulating current which is caused by frequency differences of the oscillators in the control circuits and dc voltage differences is reduced by a feedback control. From amplitude and phase of the circulating current, a supervisory processing unit decides which control variable should be changed between amplitude and phase of the output voltages, and sends a signal to the local processing unit. As a result, load balance of the inverters is achieved.