AC- and DC-Side Start-Up Strategies for Half-/Full-Bridge Hybrid Modular Multilevel Converter

The hybrid modular multilevel converter (MMC) composed of half-bridge submodules (HBSMs) and full-bridge submodules (FBSMs) is considered as a promising topology to deal with dc fault in high-voltage direct-current (HVDC) transmission systems. Compared with conventional HB-MMC, start-up process in hybrid MMC is different due to SMs topology difference. For ac-side start-up, the voltages of HBSM capacitors and FBSM capacitors are unbalanced after uncontrolled pre-charge process. For dc-side start-up, applying current-limiting resistor to restrict the inrush current can be costly. Aiming to charge the SMs capacitors to rated value smoothly, start-up strategies are proposed in this paper. The proposed ac start-up method can eliminate the unbalance voltage during closed loop pre-charge control stage. The dc start-up method without current-limiting resistor is proposed by regulating dc-link voltage. Simulation in PSCAD/EMTDC is implemented to verify the feasibility and validity of the proposed start-up strategies.