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 sub-modules 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.