模块化多电平变换器自2002年提出以来，引起了众多学者的关注。MMC具有易于扩展、模块化、适用于大容量场合、谐波含量低等优点，在电力电子变压器、能源路由器和能源互联网等领域具有良好的应用前景。特别是由于其特殊的级联拓扑，输出波形相对平滑，开关损耗小，因此在高压直流输电中得到了广泛的应用。

Different from traditional two-level and three-level converters, modular multilevel converters adopt modular cascade mode and have strong scalability. By connecting different number of sub-modules in MMC intermediate, it can meet the requirements of various voltage level applications. When applied to high-voltage and high-power applications, MMC avoids the series connection between a large number of power switching devices, which greatly reduces the difficulty of converter manufacturing.

However, compared with the traditional two-level and three-level converters, the MMC topology also has some shortcomings, because the uneven energy distribution between phases leads to the formation of circulations in the structure, and the bridge arm current will be distorted, resulting in greater loss in the converter. On the one hand, the circulation will increase the current distortion of the bridge arm, improve the rated current capacity of the switch components, and increase the system cost. On the other hand, the circulation will increase the loss of the system, the power switch tube heating seriously, affect the service life of the system. At present, in the application and research of MMC devices at home and abroad, many literatures have studied the internal circulation of MMC, but it has not been fully solved.

In this paper, an optimization method of MMC circulation suppression strategy based on PIR virtual impedance is proposed for typical MMC systems. In this paper, the topological structure and working principle of MMC are analyzed from the perspective of suppressing the double frequency circulation, and the loss composition of MMC is deduced and calculated. Based on the specific mathematical analytical formula of the double frequency circulation of MMC, a circulation suppression control strategy combining virtual impedance and proportional integral resonance control is proposed. Finally, a typical MMC circuit simulation model is built with MATLAB platform to verify the effectiveness of the proposed loss reduction strategy.