Abstract:  ABSTRACT： To transmit the bulk renewable power using overhead lines, this paper proposes a VSC-HVDC grid transmission system based on hybrid modular multilevel converter （hybrid MMC） and DC switch. According to the hybrid MMC composed of half bridge sub-modules and full bridge sub-modules, this paper analyzes its topology, basic operating principle and DC voltage operating region, proposes a control framework embedding three-channel control freedom for hybrid MMC, analyzes the DC fault ride through control strategy in detail, and then designs the fault clearing method and multiple restart sequence of the VSC-HVDC DC grid based on hybrid MMC. During the DC fault, hybrid MMC can control the fault current to be zero without tripping the IGBTs. Therefore, the DC grid remains continuous operation to provide reactive power to the AC system. During post-fault, if the system fails to restart after three restart attempts, the DC switches installed at the overhead lines will be tripped to isolate the fault path at zero DC current. After fault clearance, as the change of network topology, the power flow will be re-distributed. At last, the feasibility of the fault clearing strategy and restart sequence is validated in PSCAD/EMTDC.

Key words: overhead lines, VSC-HVDC grid, hybrid MMC, fault protection