Research on Commutating Over-Voltage Protection for High Power Converter

In the course of converter normal operation, large transient surges called commutating over-voltage may be generated internally during the commutating process of thyristor. The commutating voltage can effectively be reduced by the RC snubber circuit parallel to thyristor. In the RC snubber circuit design of high power thyristor, the reverse recovery process of thyristor must be considered in detail. Based on the analysis about reverse recovery transient of thyristor, the mathematical model of the converter thyristor reverse recovery current is created. Through the equivalent transformation of converter circuit during thyristor reverse recovery period, the analytical expressions about the relationship between commutating over-voltage and RC value are derived based on the Kirchhoff's equation. Utilizing the analysis results, the optimal resistance and capacitance value can be selected by considering the reduction of peak over-voltage value and snubber power loss. It turns out that the proposed approach is available through the commutating over-voltage simulation and experiment results of high power converter