Degradation mechanism of oil–pressboard insulation by multiphysical field aging characteristics and cellulose crytallinity

Oil–pressboard insulation in converter transformer is subjected to the combined effects of AC and DC electric fields, high temperature, and mechanical vibration in actual operation, with a serious impact on its deterioration. In this study, comparative tests are performed of coupled electrical–thermal–mechanical aging, and several characteristic parameters that characterize the degree of aging of oil–pressboard are measured. Relationships between aging parameters are established, with the degree of polymerization (DP) of the pressboard being adopted as the criterion to characterize the degree of aging. Scanning electron microscopy and X-ray diffraction are used to investigate the surface morphology and crystal structure of the pressboard at different aging stages. Then, the influences of temperature, electric field and mechanical field on the cracking mechanism of the oil–pressboard insulation in terms of the aging characteristic parameters and the changes in cellulose crystallinity were analyzed. It is finally found that thermal stress is the most important factor involved in insulation pressboard cracking. The addition of electric and mechanical fields will reduce the crystallite size of the cellulose pressboard material, increase the acid value and furfural content of the oil, and increase the rate of decrease in the degree of polymerization of the pressboard. All of which accelerate the aging of the oil–pressboard insulation.