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-
off
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-
off
overvoltage is eliminated, and the turn-
on
loss and turn-
off
loss of insulated gate bipolar transistor (IGBT) are reduced by 78%, which verifies the effectiveness of the proposed method.