Study of Bubbles Evolution in Mineral Oil-paper Insulating System under Sudden Changes in Temperature

Transformers at nominal operating temperatures present low risk of bubble formation in mineral oil-paper insulating systems. However, in the case of overload with meaning temperature increase, bubble formation risk significantly increases. That is of concern since it can increase partial discharge risk, alter oil dielectric strength and increase insulation thermal degradation around the bubble. Therefore, this study aims to assess dynamics of bubble formation under sudden temperature changes, in an experimental model comprised of mineral insulating oil and thermally upgraded kraft paper. Results show that bubble inception temperature is dependent on paper water content. Water content values above 1% in insulating paper favored bubble formation. Temperature of bubble inception decreased as water content increased, being 55 °C at 2.5% of water and 120 °C at 0.3% of water. Interestingly, abrupt changes in temperature caused bubble appearance in mineral oil-paper insulation system, regardless of paper water content.