Remaining Lifetime Assessment Method Based on On-Line Condition Monitoring of Metallized Film Capacitors
Wu Yunjie1, Zhang Jinbao2, Lai Wei1, Li Hui1, Wang Hao1
1. State Key Laboratory of Power Transmission Equipment Technology Chongqing University Chongqing 400044 China; 2. China Southern Power Grid Power Generation Co. Ltd Operation Management Company Guangzhou 511493 China
Abstract：Modular multilevel converter (MMC) is the core equipment in flexible direct current transmission systems, where the sub-module (SM) metalized film capacitors represent one of the weak links within MMCs. Thus, state monitoring and lifetime assessment of metalized film capacitors are of significant importance for enhancing the operational reliability of MMCs and formulating system maintenance plans. Current lifetime assessment methods for metalized film capacitors do not consider the impact of capacitor aging on capacitance value and equivalent series resistance (ESR). It is difficult to accurately assess the remaining lifetime of capacitors under different service conditions. Therefore, this paper proposes a method to assess the remaining lifetime of metalized film capacitors, considering the current capacitance state for online monitoring. The main research contents include the following aspects. (1) A method based on capacitance value and hotspot temperature is proposed. An analytical lifetime model is established according to the failure mechanism of metalized film capacitors. Subsequently, considering the impact of aging on ESR and capacitance value, a time-varying degradation model for the capacitance and ESR is developed. The nonlinear cumulative damage and degradation process are obtained under actual operational conditions. Based on the coupling relationship between the state of the capacitor, ESR, capacitance value, aging rate, and temperature, the change in failure rate is analyzed at different aging levels. (2) A non-invasive method for monitoring capacitance value based on sliding window cumulative sum detection is proposed. Firstly, the coupling relationship between changes in capacitor voltage and switching signals is established based on the basic working principle of MMC. Then, the principle of the bilateral cumulative sum algorithm is described, and a sliding window is introduced to process the capacitor voltage, allowing for accurate determination of charging and discharging times within the sliding window. Thus, capacitance values can be calculated online. The effectiveness of the proposed method is validated through simulation and experiment, and the maximum error under different power levels is less than 0.6%. This method uses the existing capacitor voltage and arm current without affecting the original operating state of the MMC system, achieving non-invasive monitoring. (3) An electrothermal co-simulation model based on field-circuit coupling is proposed to precisely simulate capacitors' electrical and thermal characteristics under actual MMC conditions. The circuit model of the sub-module capacitor is constructed according to the circuit structure of the MMC system and the equivalent circuit of the capacitor. The temperature field model of the capacitor group is established on a finite element simulation platform of the actual physical structure of the capacitor components. Subsequently, a metalized film capacitor group's electrothermal coupling model is established through the mutual transfer of power loss and temperature parameters in Matlab. Finally, the proposed modeling method is analyzed and validated.
吴云杰, 张金保, 赖伟, 李辉, 王浩. 基于金属化薄膜电容器状态在线监测的剩余寿命评估方法[J]. 电工技术学报, 2024, 39(22): 7239-7255.
Wu Yunjie, Zhang Jinbao, Lai Wei, Li Hui, Wang Hao. Remaining Lifetime Assessment Method Based on On-Line Condition Monitoring of Metallized Film Capacitors. Transactions of China Electrotechnical Society, 2024, 39(22): 7239-7255.
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