Online Monitoring and Balancing for MMC Capacitor Aging

Metallized film capacitor (MFC) is widely used to maintain the stability of sub-module voltage in modular multilevel converter (MMC). Capacitor aging is likely to lead to a decrease in the capacitance value and an increase in equivalent series resistance (ESR). And it will increase the voltage fluctuation and rise the temperature, which may accelerate the aging of capacitors and cause the open-circuit of power electronic devices in serious circumstances. However, the traditional on-line monitoring methods for capacitor aging are not suitable for MMC because of the higher occupation for computing resources and hardware resources. To solve this problem, this paper proposes an on-line monitoring method for capacitor aging degree without installing any additional device. Then a balancing strategy is designed to balance the relative aging degree of capacitors, which can reduce the switching frequency of sub-module with sever aging state and extending the lives of capacitors. The simulation results confirm the effectiveness of proposed method.