Abstract：Dry-type DC capacitors using biaxially oriented polypropylene (BOPP) films as dielectric media serve as core components in flexible DC converter valves, playing critical roles in maintaining voltage stability and filtering harmonics. However, under actual operating conditions, prolonged exposure to combined electrical-thermal-mechanical stresses leads to performance degradation and even insulation breakdown of the polypropylene film, seriously threatening the stability and reliability of the conversion valves. To investigate the degradation law of the insulation properties of BOPP films under long-term electro-thermal-mechanical multi-physical field aging, this study independently constructed a multi-physical field aging test platform. BOPP films with a thickness of 5.8 μm, produced by Ningbo Great Southeast Wanxiang Science & Technology Co.Ltd, were selected and subjected to aging treatments under varying electric fields, temperatures, and mechanical tensile force for durations of 7, 15, and 30 days. differential scanning calorimetry (DSC), gel permeation chromatography (GPC), Fourier transform infrared spectroscopy (FTIR), and DC breakdown tests were conducted to analyze the effects of electric field gradient, temperature gradient, mechanical tension gradient, and aging time on the degradation law of the insulation properties of BOPP films. The results show that BOPP films undergo recrystallization under thermal field and the aging temperature shows a positive correlation with crystallinity and lamellar thickness, while showing a negative correlation with molecular weight and DC breakdown strength. With the increase of aging electric field, the crystallinity of the film increases, the molecular weight and the DC breakdown strength decrease. The effect of mechanical tension on the breakdown strength of BOPP films exhibits a threshold effect. Specifically, when the mechanical tension is lower than the threshold level, the polypropylene molecular chains orient and arrange along the direction of tension, leading to increased crystallinity and molecular weight, thus enhancing DC breakdown strength. However, excessively high tension will cause damage to the molecular chain structure of the films, resulting in reduced molecular weight and decreased breakdown strength. With the extension of aging time, the crystallinity and lamellar thickness of the film increase, while the molecular weight and breakdown strength decrease. Among all samples, the 150-80-10-30 sample exhibits the lowest DC breakdown strength of 466.09 kV/mm, which is 27.9% and 11.5% lower than that of the Ref sample and 150-80-10-7 sample, respectively. Finally, based on the above test results, a degradation model for the breakdown strength of BOPP films considering the long-term action of electro-thermal-mechanical multi-physical fields was proposed. Due to the large number of parameters to be solved, a two-stage parameter estimation algorithm was employed for iterative optimization. Firstly, the Levenberg-Marquardt algorithm was adopted for preliminary fitting to obtain reasonable initial values of core parameters. Next, the algorithm was extended to the complete model: inheriting the initial parameter values from the previous step, adding tension, coupling terms, and time terms, and using the particle swarm optimization (PSO) algorithm for global optimization to obtain the optimal model parameter values. The model results indicate that the calculated results are in good agreement with the experimental results, with deviations all within 5%. This degradation model can provide a reference for predicting the breakdown strength of BOPP films under different aging electric fields, temperatures, mechanical tensions and aging times.
李卓函, 陈向荣, 宋家乐, 徐双, 李晓军. 电-热-力长期作用下干式直流电容器用BOPP薄膜的绝缘劣化特性[J]. 电工技术学报, 2026, 41(15): 5307-5317.
Li Zhuohan, Chen Xiangrong, Song Jiale, Xu Shuang, Li Xiaojun. Insulation Degradation Properties of BOPP Films for Dry-Type DC Capacitors under Long-Term Electrical-Thermal-Force Action. Transactions of China Electrotechnical Society, 2026, 41(15): 5307-5317.
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2026, Vol. 41 
