Volume resistivity and PDIV characteristics of polymer materials under electro-thermal aging

Traditionally, oil-filled transformers have utilized cellulose paper as primary insulation. However, cellulose insulation is a relatively low temperature material and thermal degradation due to overload conditions and long-term aging can lead to transformer failures, Therefore, it restraints the development of high density power transformers. What´s more, since the transformer oil has a relative dielectric constant of 2.2, a material with a high dielectric constant can increase the stress in the oil and cause a breakdown in the transformer. In an effort to find a more suitable insulating material than the cellulose paper for use in an oil-filled transformer, on the basis of fundamental properties of polymer materials, we choose 4 polymer materials with low permittivity and dielectric loss, high electric strength, partial discharge inception voltage and insulation resistance, including polyphenylene sulfide (PPS), polycarbonate (PC), Polytetrafluoroethylene (PTFE) and polyester film (PET). In our experiment, the kraft paper, PC, PET, PPS, PTFE with 0.5mm thick are used. The kraft paper and these four polymer materials are aged in mineral oil for a period of 270 days under 90°C, 110°C and 130°C respectively, simultaneously an electric pre-stressing of 4kV/mm is performed, Then analyze the SEM, the PD inception voltage and volume resistivity of the kraft paper and four polymer materials before and after electro-thermal aging.