Three-phase series hybrid modular multilevel converter

Traditional modular multilevel converter (MMC) systems are often characterized by low submodule utilization, leading to high component costs and a large physical footprint, which limits their applicability in converter stations. To address these limitations and reduce the overall weight and size of the converter, this paper proposes a three-phase arm multiplexing series MMC (AM-SMMC) topology suitable for voltage-source converter high-voltage direct current (VSC-HVDC) stations. The proposed topology builds on the principles of three-phase leg cascading and arm time-division multiplexing (TDM). By reorganizing the arm structure of the conventional series MMC (S-MMC) into upper, multiplexed, and lower arms and incorporating arm switching devices to enable multiplexing of the intermediate arm, the number of required submodules is significantly reduced. Furthermore, a zero-voltage switching modulation strategy is developed specifically for the AM-SMMC to fulfill the modulation requirements for connverter stations in VSC-HVDC transmission applications. Simulation results obtained from MATLAB/Simulink validate the feasibility of the AM-SMMC architecture and the effectiveness of the proposed modulation strategy. Compared to the conventional S-MMC, the AM-SMMC achieves a 25% reduction in submodule capacitance while maintaining the same number of IGBT switches, thereby reducing both the manufacturing cost and physical volume of the converter station.