Abstract: Biaxially oriented polypropylene (BOPP) is the main dielectric material for metallized film capacitors, and its dielectric and physical and chemical properties directly affect the operation and service life of capacitors. At present, there is insufficient research on the aging mechanism of BOPP in the complex multi-physical field environment inside the capacitor, and the degradation mechanism of the capacitor is not clear. In this paper, an electro-thermal combined aging experiment of BOPP under pulsed electric fields was carried out. The breakdown field strength, charges and traps, surface composition and morphology of BOPP samples were tested under different aging conditions, and the impact of pulse parameters and environmental temperature on the aging process of the samples was analyzed. The results show that with the pulse frequency of 500 Hz and the temperature of 60 °C, the DC breakdown field strength of BOPP samples falls from 610 kV/mm to 515 kV/mm when the amplitude of pulse voltage increases from 1 kV to 5 kV. With the fixed conditions of pulse voltage, the breakdown field strength falls from 630 kV/mm to 490 kV/mm when the temperature increases from 40 °C to 100 °C. Under the synergistic effect of pulse voltage and ambient temperature, BOPP simultaneously undergoes two aging processes: molecular chain scission and thermal oxygen reaction, which leads to a decrease in the electrical strength of the samples. In all the aging parameters, the amplitude of pulse voltage and ambient temperature have the most significant impact on the electrical performance of BOPP.

Key words: polypropylene film, breakdown field strength, pulsed electric field, thermal-oxygen reaction, degradation mechanism