A Capacitor Voltage Sorting Algorithm for Modular Multilevel Converters(MMC) under Low-Frequency Carrier Modulation

MMC has shown great advantages in medium voltage and high-power applications, but not yet wide speared because of its voltage ripples. In the low-frequency carrier modulation, the submodule’s (SM) capacitor voltage are not stable under the common-mode voltage injection method and the output voltage contains a large number of harmonic components. This paper presents a new voltage sorting algorithm for suppressing submodule’s (SM) capacitor voltage fluctuations and minimizing voltage difference in Modular Multilevel Converters (MMC) under Low-Frequency Carrier Modulation. The switching state of each submodule depends on the direction of the circulating current and the voltage of the submodule capacitor. The comparisons are made between the existing control method and the improved one of adding this algorithm. The simulation results show the validity of the introduced control scheme.