Modelling of Safe Operation Area for Capacitor under Mission Profile in Modular Multilevel Converter

Capacitors are the key components in the Modular Multilevel Converter (MMC). The proper design of the capacitor is becoming a major concern for the MMC system due to the high reliability requirements. However, there is little research on the accurate safe operation area of capacitors in the MMC, resulting in the lack of theoretical guidance and sudden failures in practical use. Considering the thermal stress is the main reason for capacitor aging, a novel method for the capacitor loss evaluation is first proposed to estimate the hot-spot temperature. Then the thermal stress of capacitors in an arm is analyzed under different modulations and working conditions of the MMC. A time-varying safe operation area is thereby established in consideration of the electrical and thermal stress during the degradation process. Conclusions are demonstrated by the simulation of MMC. Experimental validation is also conducted on the submodule mission profile emulator to verify the analysis.