Parallel Operation Techniques of GTO Inverter Sets for Large AC Motor Drives

Techniques are presented for using a high-power GTO in a PWM inverter and operating inverter sets in parallel to increase the inverter equipment capacity. The high-power GTO has both a very large turn-off gate current and turn-on gate current for overdrive. It is best to employ an isolation pulse transformer for current amplification as the gate circuit. Also, the high-power GTO, which has a relatively small nonrepetitive controllable current compared with that of a medium- or low-power GTO, in principle can be protected effectively against short circuit faults using fuses. In parallel set operation, the inverter equipment has four typical behavior patterns which arise from differences in GTO switching characteristics. Of these, the behavior pattern in which the current balance is optimized is explained. In addition, the method for designing an interphase reactor as a current balancer and the combination region of GTO characteristics have been given. By means of a parallel set operation test, it has been confirmed that a current unbalance can be suppressed to below ten percent of the peak load current.