Condition Monitoring Method for Submodule Capacitor in Modular Multilevel Converter

As a key component in modular multilevel converter (MMC), the reliability of submodule (SM) capacitor is a major concern. To get the health state of SM capacitor and take proactive action, this paper develops a condition monitoring method without any extra sensors or complicated algorithms. In the proposed method, one SM in each arm is bypassed symmetrically at a time, and the energy stored in each capacitor dissipates through the internal bleeding resistor. The discharging time proves to be an effective indicator of capacitor degradation. The information of all SMs are acquired with a proper rotating sequence. This condition monitoring process could be enabled regularly and demonstrate the degradation trend accurately. Besides, it does not affect the normal operation of MMC, and works well in different control or modulation strategies. Experimental results verify the feasibility of the proposed method.