Accurately monitoring the capacitance of all sub-modules is crucial to maintain the reliability of modular multilevel converters (MMCs). This paper proposes a novel capacitance monitoring method based on the inserted window recognition scheme without installing additional sensors and interrupting system operations. The principle of using the inserted window and internal voltage/current information to obtain the SM capacitance is introduced. Then

a selected time-slicing algorithm based on the inserted duration is proposed to enhance the monitoring precision and efficiency. A straightforward implementation method for the full MMC system is developed. Validated by extensive simulated and experimental results

the method demonstrates exceptional accuracy with errors below 0.5%. The method also shows strong robustness in dynamics and is not complicated for implementation for all SMs in an MMC.