Lifetime enhancement with stress control for submodule capacitors of a modular multilevel converter

This paper proposes a stress control for submodule (SM) capacitors of a modular multilevel converter with SMs used for dc-voltage sources intended for battery and renewable energy applications. Capacitors are among the most fragile components. The SM capacitors in the MMC experiences degradation imbalance among the SMs due to the deviation of the source (load) characteristics, which shorten the lifetime of the MMC. The proposed control improves the lifetime of the MMC through a capacitor current control according to a state of health of the capacitors estimated by a condition monitoring. Experimental results of the proposed control demonstrate a suppression of the SM capacitor current by 18.2%. Reliability analyses show that the proposed control improves the lifetime of the MMC 6.03 times as long as conventional control.