Online Degradation of Biaxial-Orientated Polypropylene Film from HVDC Filter Capacitors

A comparative study of biaxial-orientated polypropylene (BOPP) films extracted from one failed HVDC filter capacitor (serving for 10 years) and one fresh capacitor (spare capacitor from the same batch) is reported in this paper. Experimental results show that the 10-year online operation does not bring about molecular changes; however, molecule reorganization and recrystallization was detected using differential scanning calorimetry and dynamic mechanical analysis tests. Electrical measurements including dielectric spectra, DC conductivity measurement, DC breakdown strength and were also performed. The experimental results show that the aging process causes an increase in dielectric constant, dissipation factor, DC conductivity, and the DC breakdown strength. Thermal stimulated current measurements indicate that traps were introduced by aging and are responsible for such abnormal changes. The nano-cavitivies generated by molecule reorganization could possibly act as charge traps. Therefore, homo-charges can accumulate in the vicinity of the electrodes, leading to the degradation of long-term withstand strength but an enhancement of short-term breakdown strength.