The DC-link capacitor is a critical component of the modular multilevel converter (MMC) in flexible HVDC transmission systems

and its operational state directly impacts transmission quality and system reliability. In this paper

the evolution trend of discharge characteristics of metallized film capacitors is investigated. An acoustic-electrical joint monitoring method for metallized film capacitors is proposed by utilizing the statistical characteristics of discharge time series and the characteristics of acoustic signal energy distribution. First

the discharge time series characteristics of aging capacitors in operation are investigated

and the aging degrees of capacitors are reflected by the skewness and kurtosis of discharge characteristic spectra. Based on this

a preliminary classification can be made between capacitors in good health and different aging states. Further

for the capacitors identified to be in moderate aging or near the end-of-life

the discharge acoustic signals are analyzed and clustered based on the signal energy distribution. The detection of signals with the highest energy in the lowest frequency band among all discharge signals indicates that the capacitor is faulty and requires replacement. This study enables the identification of capacitor discharge types and the condition monitoring and diagnosis of in-service capacitors.