研究模块化多电平变流器(modular multilevel converter，MMC)中桥臂的功率特性，并求得桥臂的瞬时功率解析表达式和 MMC 稳态运行时流过桥臂电流的直流分量。通过研究桥臂的等效充放电电路，求出充放电电流的解析表达式，并据此导出电容电压波动分量的解析表达式。根据电容电压波动分量的表达式，画出电容电压波动分量的幅值与调制比和功率因数角的关系曲线，并分析波动分量的来源及特性，求取波动分量的幅值与调制比和功率因数角的解析表达式，进而提出电容大小的设计方法。通过算例验证了电容电压波动分量的表达式和电容设计方法的正确性。“,”In this paper, the power characteristics of the arm in the Modular Multilevel converter (MMC) was studied, and the mathematical equation of the instantaneous power of the arm and the DC component of the arm current were acquired. By studying the equivalent circuit of the capacitor charge circuit, the mathematical equation of the equivalent charge current and capacitor AC voltage component were acquired. Based on the mathematical equation of the capacitor AC voltage component, the relationship of magnitude of B with modulate ratio and power factor angle were acquired. Furthermore, the mathematical equation of the magnitude of capacitor AC voltage component were worked out and the design approach of the sub-module capacitor were put forward. The simulation results show the accuracy of the mathematical equation of capacitor AC voltage component and the design approach of the sub-module capacitor.