New control strategy of decoupling the AC/DC voltage offset for modular multilevel converter

For its notable advantages, the modular multilevel converter (MMC) has become a hot topic in high-voltage direct current (HVDC) research and construction. Based on the existing MMC configuration and double-loop control strategy, there is a coupling between AC/DC voltage offset, which brings a series of problems in different engineering background. This study proposes solutions to decouple this coupling with two steps: (i) a principle of the arm sub-module configuration is proposed to change the topology of MMC arms; MMC with the new changes of configuration is defined as asymmetrical MMC. (ii) The asymmetrical MMC is modelled and analysed, and a new control strategy is designed for it. Two two-terminal active MMC-HVDC simulation models with different configuration are established in PSCAD/EMTDC and the efficiency of the proposed topology and control strategy is validated.