Comparative Investigation of Mineral and Synthetic Ester Oil on AC and DC Performances of Oil-paper Insulation

Against the backdrop of the energy transition towards hybrid AC/DC grids, and the pursuit of environmentally friendly insulation materials, a comparative understanding of alternative oil-paper composites has become increasingly critical. In this study, Weidmann insulating paper was selected via isothermal surface potential decay (ISPD) measurements and was separately compounded with 25^# Karamay mineral insulating oil and KI50EX synthetic ester oil, forming two oil-papers. Frequency dependent dielectric spectroscopy (FDS), thermally stimulated depolarization current (TSDC), and AC/DC breakdown strength tests were performed. Comparative analyses were conducted to elucidate differences between the two oil-papers in terms of charge transport, trap distribution, dielectric response, and breakdown performance. The results indicate that the synthetic ester oil-paper, owing to its high density of deep traps under an AC electric field, can effectively suppress dielectric loss, thereby exhibiting higher breakdown strength, as well as improved thermal stability. From a microscopic-mechanism perspective, this work clarifies the mechanism by which the two oil-paper insulation become applicable to different operating conditions (AC and DC), thereby providing a theoretical basis for the rational selection of transformer insulation materials and for environmentally friendly upgrades.