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.

中文翻译：

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变频情况下MMC子模块电容电压抑制控制策略

模块化多电平变换器（MMC）拓扑因其高度模块化和可扩展性的特点，在柔性直流输电、电能质量控制等领域受到广泛关注，成为最有前途的拓扑之一。但由于MMC本身的特性，MMC子模块的电容电压波动与MMC工作频率成反比。 MMC的主要技术挑战之一是有效抑制低频工作时MMC子模块电容电压的波动。本研究首先分析了MMC子模块电容电压低频波动的原因。然后，基于分析结果，提出了变频工作情况下MMC子模块电容电压波动的抑制控制策略。最后利用MATLAB/Simulink环境搭建MMC模块进行仿真验证，并搭建五级MMC物理实验平台进行实验验证。仿真和实验结果表明，所提出的MMC子模块电容电压抑制控制策略有效抑制了变频工作情况下电容电压的波动。