Thermal Balancing Strategy Based on Voltage Compensation Method for Capacitors in Modular Multilevel Converter

Abstract

The capacitor is one of the key components in the Modular Multilevel Converter (MMC) system. Its thermal stress has an important effect on the reliability of the MMC system. Under normal operations, significant temperature distributions exist in capacitors among different submodules (SMs) of one MMC arm due to the capacitance variations. However, existing research mainly focuses on the thermal balance of power semiconductors, but less on capacitors. To solve this problem, this paper first reveals the relationship between the capacitor parameter distribution and the thermal stress distribution. Then a method for realizing the temperature balance of capacitors by controlling the equivalent modulation indexes of SMs with compensation voltage is proposed. The hot-spot temperatures of capacitors in an arm can be balanced to the average value, which relieves the thermal stress. The simulation analyses with the proposed method are conducted with the professional tool PLECS. Experimental results on a mission profile emulator for MMC capacitors also validate the effectiveness of the proposed method.