Design and DC fault clearance of modified hybrid MMC with low proportion of full-bridge submodules

Abstract

The hybrid modular multilevel converter (MMC) consisting of half bridge submodules (HBSMs) and full bridge submodules (FBSMs) combines the respective advantages of both submodules. In order to further reduce the manufacture cost and power loss, this paper proposed a modified hybrid MMC with lower proportion of FBSMs than the conventional hybrid MMC. First, three main conditions for successful DC fault clearing of the hybrid MMC are analysed, which are: (1) Blocking AC fault current; (2) suppression of DC fault current; (3) capacitor voltage under threshold value. Then the topology and the DC fault clearing strategy of the proposed modified hybrid MMC are introduced. Since the installed AC current blocking switches (ACBSs) block the AC current feed during the fault, the number of FBSMs is only limited by third condition. Furthermore, the maximum capacitor voltage after the DC fault is analysed to obtain the minimum required number of FBSMs. Finally, the DC fault responses of the modified hybrid MMC are compared with the conventional hybrid MMC in a three-terminal MMC-HVDC test system in PSCAD/EMTDC. Simulation results demonstrate the effectiveness and feasibility of the proposed modified hybrid MMC.