Suppression control strategy of MMC sub-module capacitor voltage in variable-frequency situation

Modular multilevel converter (MMC) topology has received widespread attention in flexible DC transmission, power quality control, and other fields because of its high modularity and malleability characteristics, and has become one of the most promising topologies. However, owing to the characteristics of the MMC itself, the fluctuation of the capacitor voltage of the MMC sub-module is inversely proportional to the MMC working frequency. One of the main technical challenges for MMC is effectively suppressing the fluctuation of the capacitor voltage of the MMC sub-module when working at low frequencies. This study first analyzes the reasons for the fluctuation of the MMC sub-module capacitor voltage at low frequency. Then, based on the results of this analysis, we propose a suppression control strategy for the capacitor voltage fluctuation of the MMC sub-module in variable-frequency working situations. Finally, an MMC module was built using the MATLAB/Simulink environment for simulation verification, and a five-level MMC physical experimental platform was built for experimental verification. The simulation and experimental results demonstrate that the proposed suppression control strategy for the MMC sub-module capacitor voltage effectively suppresses the fluctuation of the capacitor voltage under variable-frequency working situations.