Abstract：Metallized film capacitors (MFCs) are key DC-link support components in modular multilevel converter (MMC) submodules, and their condition directly affects submodule voltage stability and long-term reliability in flexible direct current transmission systems. Existing condition assessment methods usually determine capacitor degradation using parameters measured at a single frequency, such as capacitance, equivalent series resistance, or dielectric loss tangent. These methods are limited in distinguishing defect sources and degradation stages. To address the insufficient diagnostic information provided by a single parameter, a multi-type defect diagnosis method for MFCs based on multi-frequency dielectric-loss curves is proposed. A humidity-corrected series-parallel equivalent model of the MFC is first established. In this model, equivalent series resistance is used to characterize conductive-path loss, equivalent parallel resistance is used to characterize dielectric leakage, effective capacitance is used to characterize energy-storage capability variation, and the dielectric dispersion coefficient is used to characterize moisture-induced frequency response. Based on the diagnostic frequency band of 50~1 000 Hz, the effects of different equivalent parameters on dielectric-loss curves are analyzed. Theoretical analysis shows that when capacitance C decreases from 100%C0 to 95%C0, the dielectric-loss curve remains nearly linear, and its slope changes by approximately 5%. Therefore, capacitance variation alone is insufficient for identifying defect sources. A decrease in equivalent parallel resistance Rp0 enhances low-frequency leakage loss and raises the low-frequency part of the curve. An increase in equivalent series resistance Rs increases the high-frequency slope, indicating enhanced conductive-path loss. Moisture ingress strengthens interfacial polarization and dielectric dispersion, leading to concave bending in the middle- and high-frequency regions. Poor end sprayed-metallization contact causes nonuniform current paths and contact resistance distribution, producing a more obvious nonlinear residual relative to linear fitting. According to the parameter-variation analysis, four features are extracted from the multi-frequency dielectric-loss curve: low-frequency integral area A, high-frequency slope ratio r, high-frequency curvature c, and weighted sum of squared residuals (WSSR). These features are used as characteristic parameters for multi-type defect recognition, and corresponding diagnosis criteria are established. Normal insulation failure satisfies WSSR≤WSSRth, A≥ALF, r≥rHF, c<c0; moisture-induced insulation failure satisfies WSSR≤WSSRth, A≥ALF, r≥rHF, c≥cALL; internal breakdown defect satisfied WSSR≤WSSRth, A<ALF, r≥rHF, c<c0; poor sprayed-metallization contact satisfies WSSR>WSSRth. These criteria establish the correspondence among dielectric-loss curve morphology, equivalent-parameter variation, and typical defect types. A soft-gating multi-type defect recognition model is further constructed. The four characteristic parameters are normalized and converted into continuous category scores, and the specific MFC defect type is identified according to the maximum score. This strategy reduces the possibility of recognition jumps between adjacent categories for samples close to the diagnostic thresholds. An accelerated aging platform for MFCs under DC conditions is built to verify the proposed method. The validation results show that different defects exhibit distinguishable morphological response characteristics in multi-frequency dielectric-loss curves. The recognition confidence of all samples is higher than 97%, with the lowest confidence being 97.48%. In subsequent tests, the sample with abnormal sprayed-metallization contact, MFC7, exhibits end ejection after 6 h of voltage application, while the internal-defect sample, MFC8, breaks down after 4 h of voltage application. The dissection results are consistent with the diagnosis results of the recognition model. The results indicate that the method addresses the limitation of conventional single-parameter diagnosis in identifying MFC defect sources and degradation stages, and provides criteria for multi-type defect diagnosis and condition-based maintenance of MFCs.
刘艺雯, 汲胜昌, 许馨愉, 祝令瑜. 基于多频介质损耗曲线的金属化膜电容器缺陷诊断识别方法[J]. 电工技术学报, 2026, 41(15): 5218-5236.
Liu Yiwen, Ji Shengchang, Xu Xinyu, Zhu Lingyu. Defect Diagnosis and Identification Method for Metallized Film Capacitors Based on Multi-Frequency Dielectric-Loss Curves. Transactions of China Electrotechnical Society, 2026, 41(15): 5218-5236.
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