Closed Loop Energy Balancing Control of Modular Multilevel Converters Under Capacitor Degradation

Abstract

The modular multilevel converter (MMC) has emerged as a captivating multilevel converter topology for medium to high-power applications. Due to aging and chemical process, the impedance characteristics of the capacitor in the submodule (SM) changes. This can result in an increase of SM capacitor voltage ripple and unequal power distribution among the SMs in the arm. Furthermore, prolonged use of degraded capacitors could interrupt the normal operation of the MMC. Existing SM capacitor voltage balancing approaches for the MMC are based on an assumption that all SMs have equal capacitance and ignore the component degradation. Hence, there is a need for closed-loop balancing control techniques that use instantaneous monitoring of capacitor parameters to enhance the reliability of the MMC. This paper addresses this gap by introducing an energy-based balancing control of SM capacitors, which effectively balances and controls the energy distribution among the SMs in the arm under the capacitor degradation. For validation of the proposed control strategy, detailed simulation studies are carried out for half-bridge SM based MMC in PLECS software platform.