Study on bubble evolution in oil-paper insulation during dynamic rating in power transformers

Solid insulation tended to absorb moisture during the operation and maintenance of oil-immersed transformers, which could be dangerous to the apparatus especially under rated conditions. As had been researched, wet insulation could lead to bubble effect in turn-to-turn insulation when the temperature in transformer increased rapidly. Primarily, this paper investigated the degradation of dielectric strength caused by gas bubbles generated from oil-paper insulation. This paper mainly focused on clarifying the evolution of thermal bubble formation. The experimental platform consisted of an oil-paper insulation system, and an adjustable heating system was established to study the influence of water and gas content on bubble evolution temperature. Results showed that the gas content and moisture content in the paper could significantly affect inception temperature of bubble formation, which could well explain the high probability of bubble evolution in old and wet oil-impregnated transformers. Then, a mathematical model was founded to calculate the bubble evolution temperature considering the solubility of gas and moisture in transformer oil at a certain temperature. Finally, based on the above results, this paper provided a strategy for managing the risk of insulation failures in oil-immersed transformers caused by thermal bubbles in dynamic rating conditions.