MMC sub-module capacitor voltage fluctuation suppression

To sovle the problem that the modular multi-level converter(MMC) is easily affected by the voltage fluctuation of the sub-module(SM) capacitance during operation, with the energy analysis of the arm, the main component of the voltage fluctuation of SM capacitance is found to be the fundamental frequency fluctuation. To reduce SM capacitor voltage fluctuation, a method is proposed based on second-harmonic circulating current injection combined with high frequency circulating current and high frequency output voltage injection to reduce the fundamental frequency fluctuation of SM capacitor voltage. The fundamental frequency power of the arm energy was first reduced by injecting second-harmonic circulating current, and then the injected high-frequency circulating current and high-frequency output voltage were used to work together to further reduce the fundamental frequency power of the arm, so as to reduce the fundamental frequency fluctuations of SM capacitor voltage. Finally, by building a simulation circuit, with the capacitor voltage fluctuations and harmonic analysis in the three cases of no fluctuation suppression, it compared only injection of second-harmonic circulating current, injection of second-harmonic circulating current combined with injection of high frequency circulating current and high frequency output voltage. The result proves that the method is effective. © 2021, Harbin University of Science and Technology Publication. All right reserved.