Performance Analysis and Optimization of Circulating Current Control for Modular Multilevel Converter

The circulating current in phase legs is an inherent phenomenon of modular multilevel converters (MMCs). The circulating current affects various aspects of MMC performance, such as power losses, capacitor voltage ripples, and the required current rating of semiconductors. Existing circulating current control methods can improve MMC performance in one aspect, but they may not implement the optimal performance in various aspects. Analytically analyzing how circulating current control affects MMC performance is essential. This paper proposes an improved steady-state analysis method that considers circulating current control. The influence of circulating current control is included by considering the effects of the second-harmonic component on reference voltages. Explicit analytical expressions for various electrical quantities can be derived, and the analytical calculation curves of some MMC performance indexes concerned under varying second-harmonic reference voltages can be obtained. Accordingly, the effects of circulating current control on MMC performance can be concluded, where a theoretical foundation for the accurate design of MMC parameters is provided. In addition, the obtained analytical calculation curves can be used to evaluate different circulating current control targets and find appropriate control targets to comprehensively improve MMC performance. Both simulation and experimental results verify the validity of the proposed steady-state analysis method and the corresponding conclusions.