Novel low loss active voltage clamp circuit for series connection of GCT thyristors

This paper describes novel low loss active voltage clamp circuits for the series connection of GCT thyristors. For high-voltage converters the series connection of power semiconductor devices is an essential technique for direct switching of high voltages. Several protection circuits have been applied to the series connection of GCT thyristors such as CRD snubber circuits which suppress overvoltages across GCT thyristors, and voltage balancing resistors to equalize voltage sharing in steady states. However, significant losses in these protection circuits lower the converter´s efficiency. The authors propose a novel low-loss protection circuits, which have active voltage clamp snubber circuits and static voltage balancing circuits. The clamp capacitor voltage of the active voltage clamp snubber circuits are designed to be higher than the equally divided DC-link voltage. This method can reduce the loss of the clamp circuit to no more than 1/10 of that of the conventional CRD snubber. Also the static voltage balancing circuits compensate for the voltage imbalance generated by the difference in the leakage current between the series connection GCT thyristors.