Power Losses Control for Modular Multilevel Converters Under Capacitor Deterioration

The modular multilevel converter (MMC) is attractive for medium-/high-voltage and high-power applications because of the advantages of its high modularity, availability, and high power quality. Due to the chemical process, aging effect, etc., the capacitor in the submodule (SM) of the MMC would gradually deteriorate and the capacitance would drop, which would cause unbalanced SM power losses distribution in the same arm of the MMC and affect the reliability of the MMC. This paper proposed an equivalent-reference (ER) control method, which can effectively realize balanced SM power losses distribution in the same arm of the MMC through the voltage-balancing control for the virtual capacitor voltages (VCVs) in the MMC under the capacitor deterioration. The proposed ER control can effectively improve the reliability of the MMC with the balanced SM power losses distribution in the MMC under capacitor deterioration. The simulation studies with the time-domain professional tool PSCAD/EMTDC are conducted, and a down-scale MMC prototype is also tested with the proposed control strategy. The study results confirm the effectiveness of the proposed control strategy.