Improved Capacitor Voltage Balancing Control for MMC Considering Voltage Convergence

Capacitor voltage balancing control is one of the important issues for a modular multilevel converter (MMC) to obtain high efficiency and reliability. In high-voltage MMCs using nearest level modulation (NLM) combined with the all-sorting algorithm, frequent and uncertain submodule (SM) switching actions lead to additional switching losses. To reduce the switching frequency of MMC, an improved capacitor voltage balancing method considering voltage convergence is proposed in this paper. The effectiveness of the SM switching is analyzed first. Then, the convergence of the capacitor voltage is predicted by analyzing the change of increment and sequence for capacitor voltage. According to the predicted results, the improved capacitor voltage balancing method is proposed, which can noticeably reduce the switching loss of the converter while not affecting the normal operation of MMCs. Finally, the effectiveness of the proposed method is verified by the simulation based on MATLAB/Simulink.