The modular multilevel converter (MMC) has become one of the most promising converter technologies in the high voltage direct current transmission system. However

like the low level voltage source converter (VSC)

the half bridge MMC

a topical subject at present

is unable to block the direct current (DC) fault. In light of the short-circuit situations of different MMC structures under the DC fault

an improved MMC topology is proposed. A hybrid MMC topology based on mismatched-cascade mechanism and the DC fault blocking principle of converter is designed

which is very suitable for the VSC based multi-terminal DC (VSC-MTDC) transmission system. The process of switching off the faulty line and recovering the normal line is explained with the “handshaking method” when a DC fault happens in the VSC-MTDC transmission system. Finally

a typical four-terminal DC system is introduced and simulations are performed in PSCAD/EMTDC to verify the system characteristics under a bipolar short-circuit fault which is the most serious DC fault. The results show that the multi-terminal DC system based on the topology proposed is able to quickly block the converter under a DC fault and recover the normal line within a short time.This work is supported by State Grid Corporation of China.