Effect of long-term electro-thermal-mechanical stresses on insulation degradation of biaxially oriented polypropylene films for dry direct-current capacitors application

• This study investigates the effects of long-term electro-thermal-mechanical multi-physics field aging on the electrical and physico-chemical properties of biaxially oriented polypropylene (BOPP) films used in dry direct current capacitors. To achieve the aging conditions, a multi-stress aging platform and thermal pulse method (TPM) space charge testing platform were developed indigenously. The physico-chemical properties, surface morphology, crystallization behavior, electrical breakdown performance, space charge distribution, and trap characteristics of 5.8 μm thick biaxially oriented polypropylene films were examined after aging at 150 kV/mm, 80°C, and 10 N for 0 h, 168 h, 360 h, and 720 h. The findings demonstrate that the breakdown performance of the films is closely related to changes in charge injection and the internal structure of polypropylene. Increased space charge injection at the nanoscale leads to greater defect sizes and trap densities at the microscale, resulting in electric field distortion that frequently triggers the partial discharges and reduces breakdown strength at the macroscale. This study provides theoretical insights and experimental data to support the desired improvements in the manufacturing process of biaxially oriented polypropylene films for dry direct current capacitors to sustain the multi-stress induced aging conditions. As a key insulating medium in dry direct-current (DC) capacitors, biaxially oriented polypropylene (BOPP) film undergoes performance degradation under the long-term electro-thermal-mechanical multi-physics field, leading to failure of capacitor. To investigate the degradation mechanism of BOPP film’s insulation properties under long-term multi-physics field stress, a multi-stress aging platform and thermal pulse method (TPM) space charge testing platform were developed indigenously. The physicochemical properties, electrical properties, and space charge characteristics of 5.8 μm thick BOPP films were examined after aging at 150 kV/mm, 80°C, and 10 N for 0 h, 168 h, 360 h, and 720 h. The results indicate that under prolonged multi-stress aging conditions, polypropylene molecular chains undergo scission, generating low molecular weight products. The size of surface defects, surface roughness, degree of crystallinity, and lamellar thickness gradually increased. The breakdown strength and inception voltage of internal discharge in the film decreased, whereas the discharge repetition rate increased. Successive injection of both positive and negative charges was observed within the films during the aging. The findings demonstrate that the breakdown performance is closely related to changes in charge injection and the internal structure of polypropylene. Increased space charge injection at the nanoscale leads to greater defect sizes and trap densities at the microscale, resulting in electric field distortion that frequently triggers the partial discharges and reduces breakdown strength at the macroscale.