Temperature-Balancing Control for Modular Multilevel Converters Under Unbalanced Grid Voltages

The modular multilevel converter (MMC) is attractive for medium-voltage and high-power applications because of the advantages of its high modularity, availability, and high power quality. Due to unbalanced grid voltages, the arm current among three phases of the MMC would unbalance, which would cause unbalanced temperature distribution among three phases of MMCs and affect the reliability of MMCs. This article proposes a variable frequency (VF) control strategy, which can effectively realize balanced temperature distribution among three phases of MMCs through adjusting carrier frequency in each phase to regulating device's switching loss in each phase of the MMC under unbalanced grid voltages. The proposed VF control can effectively improve the reliability of the MMC with the balanced temperature distribution among three phases of the MMC under unbalanced grid voltages. Simulation studies with the time-domain professional tool PSCAD/EMTDC and experimental studies with a down-scale MMC prototype are conducted to confirm the effectiveness of the proposed control.