Study on Bubble Formation of Different Moisture Content Oil-immersed Cardboards in Natural Ester Oil

Oil immersed transformers are widely applied in electric power systems. In view of the excellent insulation performance and environmental friendliness of plant dielectric oil, it has been gradually applied to power transformers in recent years. In oil-impregnated transformers, the paper insulation between turns in the winding often endures high temperature. As the rapid increase of temperature of the insulation paper over the conductor, the moisture in paper can be vaporized. It turns out a rapid rise in vapour and gas pressure within the inner layers of the paper. Finally, water vapour escapes from the surface of insulation paper in the form of gaseous bubbles, which constitutes a risk for transformer operation, especially when bubbles move into an area with high dielectric stress. This study experimentally investigated the evolution of gaseous bubbles in a wet oil-paper insulation system with FR3. The difference in inception temperature and heating rate between plant insulation oil and mineral insulation oil is emphatically compared. The effects of surface tension of oil, gradient of temperature rise and moisture content of dielectric oil on the inception temperature are analyzed. Suggestions on the application and maintenance of plant insulation oil in power transformers are put forward.