Status assessment of polymeric materials in mineral oil under electro-thermal aging by frequency-domain dielectric spectroscopy

Transformer development is limited to smaller sizes and larger capacities due to two major shortcomings: first, the solid insulation that is used in an oil-immersed transformer, which is generally cellulosic paper, cannot resist high temperatures, and second, the significant difference in the dielectric constants between the cellulose paper and the transformer oil will lead to an uneven electric field. In light of this situation, in combination with the requirements of oil-immersed transformers for the dielectric, physicochemical, and mechanical properties of solid insulations, in this study two kinds of polymeric materials are chosen as the research objects - Polycarbonate (PC) and Polyester (PET) - that can resist high temperature, and then, a comparative analysis is made with cellulose paper. They are put in the transformer oil for electro-thermal aging for 300 days at 90, 110, and 130°C, respectively. The variation characteristics of permittivity and the dielectric dissipation factor with frequency were examined in stages (30 days, 140 days, 180 days, 250 days, and 300 days). This paper studies the characteristics of polymer materials so as to analyze whether they can be used in an oil-immersed transformer by considering the materials' physical properties, electron microscope scanning (SEM), and degree of polymerization (DP) at different stages during aging. This valuable exploratory work has never been studied before. If these two new materials can be applied, it will be of great significance to the innovation of the insulation structure of a power transformer.