The character of electric charge in oil-paper insulation system-charge generation, recombination, trap formation dominated breakdown

Oil-paper insulation is the main insulation material in power transformers, and its insulation performance directly determines the operating life and reliability of the transformers. The lack of understanding of the electrical characteristics of oil-paper insulation materials leads to the misunderstanding of operating condition of the transformers. Extensive studies have found that the characteristics of generation, recombination, dissipation, trap formation of electric charge in oil-paper systems are the main factors affecting their insulation properties. In this paper, thermal stress coupling by AC and DC electrical stresses are found to have significant impact on ions migration properties. Under these stresses, transformer oil molecules are decomposed into carriers that upon recombining, various gases are generated. Cellulose-based paper in oil-paper systems adsorbs electric charge to form electric double layer at the oil-paper interface. The electric double layer are physically interfacial traps in nature, which leads to the superb dielectric and breakdown characteristics of the oil-paper, especially under DC conditions. As for insulating oil, the recombination rate of ions affects the concentration of conductive ions in oil under different electric field types. Further studies are still needed to confirm the correlation between the characteristics of charges and the insulation properties of oil-paper, which is expected to improve the accuracy of transformer condition monitoring to prolong its operating life.