DC-Link Voltage Fluctuation Suppression Method for Modular Multilevel Converter Based on Common-Mode Voltage and Circulating Current Coupling Injection under Unbalanced Grid Voltage

Grid voltage imbalance conditions often occur. Modular multilevel rectifiers (MMCs) have high DC-link voltage fluctuation under an unbalanced grid, which affects the normal operation of DC-side equipment. To suppress voltage fluctuation under an unbalanced grid, a coupling injection strategy composed of third zero-sequence common-mode voltage (TZCV) and secondary circulating current (SCC) was designed in this paper. In this paper, we calculated the coupling time-domain expression of the TZCV and SCC under an unbalanced grid voltage. Then, the influence of an SCC and TZCV coupling injection on DC-link voltage fluctuation was analyzed. The converter power flow of different system control objectives under an unbalanced grid was calculated, and the overall control method of the converter based on the arm current was proposed. The advantage of the method proposed in this paper is that it can realize online control under different grid voltages and input power conditions in real time and effectively suppress DC-link voltage fluctuation. The simulation was carried out on the MATLAB/Simulink platform, and a hardware-in-the-loop experimental platform was built; the results verify the effectiveness of the proposed strategy.