Charging Strategy for Hybrid MMC with DC Link Short-Connected

For the problem of voltage imbalance among different types of sub-module in VSC-UHVDC hybrid MMC group with DC link short-connected, a controllable pre-charging strategy was proposed. Firstly, the locked-charging characteristic of hybrid MMC group with HBSM and FBSM was analyzed, and the problem that different types of submodules can't be charged equally as well as HBSM can't be powered up were pointed out. Therefore, to pre-charge both sub-modules balanced and controllable, a strategy that switch-off some sub-modules in the special arms was proposed, and the selection criteria for arm and calculating method for sub-module's cut-off number were given. Finally, simulation in Matlab/Simulink was conducted. The results show that the controllable pre-charging strategy can effectively respond to the DC link short-connected start-up condition of hybrid MMC and has engineering feasibility.