An Investigation on the Evolution of Insulation Performance of BOPP Films for Power Capacitors Under Multi Physical Fields Aging

As an important part of flexible HVDC transmission, the polypropylene film of power capacitor will undergo irreversible insulation aging and breakdown under the long-term effect of electricity-heat-stress. In order to study the effect of electro-thermal-stress multi-physical field aging on the insulation performance evolution of biaxially oriented polypropylene film for power capacitors, the BOPP film with a thickness of 5.8 μm was aged for 168 h at 235 kV / mm, 60°C and 10 N. The infrared spectroscopy (FTIR), X-ray diffraction (XRD), dielectric properties, DC breakdown and conductivity tests of the samples were carried out. The results show that with the application of multi-physical fields, the sample did not undergo crystal transformation, but caused molecular chain breakage and free radical formation, and produced some low molecular products. The application of electric and thermal fields leads to the decline in breakdown field strength of BOPP film, and the maximum decrease of breakdown field strength can reach 17 %, and promotes the increase of dielectric loss and conductivity. Additionally, given the long-term influence of stress field and thermal field, the aged sample's dielectric constant decreases significantly, while the effect of electric field is opposite. The above results show that the aging effect of multi-physical field leads to the decrease of electrical properties of BOPP film for capacitor, which should be paid more attention and studied.