For a hybrid high voltage direct current(HVDC)transmission system connected to the active power grid

the pre-charging and start-up process of the half-bridge sub-module and full-bridge sub-module based hybrid modular multilevel converter(MMC)is analyzed in detail. The general mathematical expressions of DC voltage in each stage are derived

and the internal relation between the hybrid MMC and the MMC composed of half-bridge sub-module or the full-bridge sub-module is justified mathematically. The over-voltage and under-voltage problems in the pre-charging process for the hybrid MMC under the different mix proportions are analyzed

and five feasible measures are proposed. Meanwhile

for the hybrid HVDC applied to the passive system and the application for black start

the pre-charging and start-up process are analyzed

and the concrete implementation method is proposed. Finally

the proposed strategy is verified by PSCAD/EMTDC simulation.