A Bypass LCC-Based DC Fault Isolation Scheme for Bipolar MMC–HVDC

This paper proposes a topology that can quickly eliminate DC short-circuit faults for a bipolar modular-multilevel-converter based high-voltage direct-current (MMC–HVDC) transmission. The additional thyristors are augmented to each half-bridge submodule, which will be reconfigured to form a bypass line commutated converter (LCC). The bypass LCC can be paralleled with one pole of bipolar MMC–HVDC through a hybrid switch, and MMC can be switched to the bypass LCC during the DC fault. The bypass LCC is used to eliminate the DC fault given its ability to output an inverse voltage. The tripped sequence of the hybrid switch is designed and the corresponding equivalent circuits during different stages are analyzed. Afterward, the expressions of the DC fault current in different stages are deduced by the equivalent circuits. The firing angle of the bypass LCC and the conduction resistance of the hybrid switch MOA are then designed. The additional cost and power loss of the proposed scheme are compared with those of other DC fault isolation schemes. The simulation results of the DC fault case obtained by PSCAD/EMTDC are used to verify the effectiveness of the proposed scheme.