Condition Monitoring Method of Metallized Film Capacitor in MMC Based on ESR Frequency Characteristic Curve

In the modular multilevel converter of flexible DC transmission systems, metallized film capacitors are often used as DC-link capacitors. Under long-term exposure to high electric field strength, the performance of metallized film capacitors gradually deteriorates until failure occurs. Most condition monitoring methods for metallized film capacitors using only one characteristic parameter: the capacitance value. However, the degradation of metallized film capacitors often involves multiple characteristic parameters. This paper proposes a condition monitoring method for capacitor condition based on the equivalent series resistance (ESR) frequency characteristic curve. By using frequency-domain analysis of voltage and current signals, the capacitance value is calculated while also obtaining the ESR frequency characteristic curve. This enables an analysis of the causes of capacitor degradation and provides a more comprehensive monitoring of the capacitor's condition.