DC fault protection for modular multilevel converter HVDC using asymmetrical unipolar full-bridge submodule

This paper proposes an asymmetrical unipolar full-bridge submodule to protect modular multilevel converter from DC fault. The asymmetrical unipolar full-bridge submodule consists of 3 IGBTs, 4 diodes, and 2 capacitors. Central capacitor generates submodule voltage and works during normal operation. The other capacitor named a supplemental capacitor is located in the short circuit current path and charges during DC fault, effectively generating higher opposite voltage than the conventional full-bridge submodule. It thus reduces the required number of full-bridge submodules in each arm and consequently converter loss. This paper investigates DC fault blocking principles and procedures with the asymmetrical unipolar full-bridge submodule applied hybrid MMC. This paper explores the requirements of the full-bridge submodule of MMC to handle DC fault current and compares the semiconductor losses in the cases of using conventional full-bridge submodule and the asymmetrical unipolar full-bridge submodule. Simulation studies validate the efficacy of the proposed scheme.