A Novel Suppression Strategy Design Method for Capacitor Voltage Fluctuation of Half-Bridge MMC

The half-bridge modular multilevel converter (MMC) is widely employed in high-voltage direct current (HVDC) systems. However, the large volume and high cost of submodule capacitors remain critical challenges. Additional control strategies to suppress capacitor voltage fluctuation can effectively achieve lightweight MMC. This paper proposes a novel parameter design method for harmonic composite injection strategy of half-bridge MMC based on third-harmonic voltage and second-harmonic circulating current. First, the mathematical models of submodule capacitor voltage and its various harmonic components are constructed based on bridge arm current and switch function. On this basis, the impact of harmonic composite injection on voltage fluctuation of submodule capacitors is analyzed. Second, the optimal parameters of the third-harmonic voltage injection are chosen to expand the range of modulation index, and the mathematical relationship between modulation index and the amplitude of fundamental frequency and second-harmonic components of capacitor voltage are determined. Then, the parameters of the second-harmonic circulating current injection are optimized via a traversal search algorithm to minimize the voltage fluctuation of submodule capacitors. Finally, an MMC-HVDC simulation model is established in PSCAD/EMTDC to verify the effectiveness of the proposed method, and the results show the excellent performance of the proposed method in suppressing the voltage fluctuation.