Abstract Metallized film capacitors (MFCs) are essential components in modular multilevel converters (MMCs) used for flexible direct current (DC) transmission. The assessment of their aging state is critical to ensuring the reliability of MMCs. This study aims to establish a non-intrusive aging evaluation method for MFCs based on the analysis of phase-resolved partial discharge (PRPD) spectra derived from ultrasonic signals, thereby providing real-time condition monitoring without interrupting system operation. An experimental setup was constructed to facilitate accelerated aging of MFCs and to capture discharge events through ultrasonic sensing. Metallized polypropylene film capacitor samples, each with a rated capacitance of 18 µF and rated voltage of 2 kV, were subjected to an elevated voltage of 1.5 times their nominal rating under a controlled temperature of 65℃. The ultrasonic signals generated during discharge events were recorded using high-sensitivity ultrasonic sensors placed at the grounded terminals of the capacitors. The recorded ultrasonic data were processed to obtain PRPD spectra, which characterize the discharge event phase distribution and amplitude. Several key metrics were extracted, including phase entropy, amplitude entropy, and their respective standard deviations. These metrics were used to construct a comprehensive health index (HI) that was combined with the observed capacitance decline data. A linear regression model was subsequently developed to relate the HI to the aging state, with optimal weights for the HI parameters determined through nonlinear optimization. The experimental results revealed distinct changes in PRPD characteristics as aging progressed. Initially, the self-healing discharges dominated, but as the capacitor aged, partial discharges became more significant. As the capacitance degraded from 100% to 94% of its nominal value (C0), the proportion of self-healing discharges decreased from 47.6% to 18.5%, while the proportion of partial discharges increased from 52.4% to 81.5%. The PRPD spectra showed that the phase distribution of discharges became more dispersed, while discharge amplitudes concentrated at lower levels during the aging process. The health index effectively captured the trends in PRPD spectra, with the linear regression model demonstrating an error of less than 1 percentage point compared to offline capacitance measurements conducted using a high-precision bridge. This finding indicates that the health index can reliably represent the MFC aging state, providing a basis for real-time evaluation without invasive electrical testing. The proposed method presents a novel, non-intrusive approach for assessing the aging of metallized polypropylene film capacitors, particularly suitable for use in MMCs within flexible DC transmission systems. The ultrasonic-based analysis of PRPD spectra not only avoids interference with electrical circuitry but also provides strong anti-interference capability and high precision. The health index, constructed from phase entropy, amplitude entropy, and their standard deviations, correlates effectively with the decline in capacitance, enabling a quantitative assessment of the aging state. This contributes to the development of non-electrical parameter-based monitoring techniques for power electronic capacitors. Future work will focus on further refining the health index model and exploring its application to other capacitor types or components in high-voltage power systems. Expanding the use of non-electrical monitoring parameters could enhance the reliability of aging assessment methods in critical power electronics applications.
Xu Xinyu,Ji Shengchang,Zheng Linzi等. Aging State Evaluation Method of Metallized Film Capacitors Based on Ultrasonic Signals[J]. Transactions of China Electrotechnical Society, 2025, 40(5): 1652-1661.
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