Effect of snubber circuit parameters on the turn-off voltage spike of large GTO thyristors

The authors present results obtained from the testing of candidate 4500-V, 4000-A GTO (gate turn-off) thyristors for use in high-power inverters for electric utility application. Factors contributing to the voltage spike on turn-off are discussed with emphasis on the interpretation of GTO specifications. In order to successfully use large GTO thyristors in high-power switching circuits, it is essential that the device manufacturers´ specifications with regard to snubber circuit inductance should be correctly interpreted. Testing of devices from different manufacturers has shown that such specifications can be misleading because there are several mechanisms that contribute to spike voltage on turn-off. It is pointed out that, by properly understanding these mechanisms, it is possible to arrive at a cost-effective, reliable, and manufacturable snubber circuit assembly for 4500-V, 4000-A GTOs. The turn-off voltage spike is one of the main possible causes of GTO failure. The constitution of the spike voltage has been considered in detail with reference to laboratory measurements that allow a unique comparison of devices, obtained from different sources, under comparable full power switching conditions