An Active Snubber Circuit for High-Capacity DC Chopper to Achieve Soft-Switching Operation for Offshore Wind VSC-HVdc System

The dc chopper (DCC) is the key equipment to realize the fault ride through (FRT) of the offshore wind voltage source converter based high voltage direct current (VSC-HVdc) system. In the existing projects, the solid-state switch is directly connected in parallel with the metal oxide varistor. However, the FRT process lasts for several seconds and the solid-state switch operates at high frequency. The hard-switching operation results in an increase in the number of switches and the limited usage capacity, respectively. In this letter, an active snubber circuit for high-capacity DCC is proposed to achieve soft-switching operation. By actively controlling the capacitor discharge by thyristor, the turn-<sc>off</sc> voltage and the loss under the multipulse condition are reduced. Furthermore, the working process of the active snubber circuit is analyzed and the parameter selection method is proposed. Finally, the 2 kV/600 A equivalent experimental platform is built, and the experimental results show that the turn-<sc>off</sc> overvoltage is eliminated, and the turn-<sc>on</sc> loss and turn-<sc>off</sc> loss of insulated gate bipolar transistor (IGBT) are reduced by 78%, which verifies the effectiveness of the proposed method.