Initial temperature of bubble formation at Oil-Paper interface based on bubble dynamics

Abstract

The initiation of bubbles at the surface of insulating paper under high temperature may result in partial discharge and ultimately lead to the insulation failure of oil-immersed transformers. To study the formation mechanism of bubbles, the process of bubble growth at the oil-paper interface is established by using Rayleigh-Plesset equation. The Ideal Gas Law as well as the Hertz-Knudsen relation is used to quantify the flux of water vapor entering the bubble. The growth curve of bubbles in the oil-paper during temperature rise is calculated. And the Initial Temperature of Bubble Escape (ITBE) is then obtained based on the force analysis of the bubble growth. The results show that the bubble detaches from the surface of paper when its radius reach 198 μm, which is consistent with experimental observations. And the minimum average relative error between the predicted ITBE and experimental results is 1.11%. The effect of influence factors on the ITBE of oil/paper are then studied in detail. It is shown that the increased moisture content in paper accelerates the growth rate of bubbles, then reduces the ITBE. And the higher pressure inside oil tank restrains the bubble growth through the overall process. Additionally, as the cavity size of paper increase, the surface tension of bubble increases. Consequently, the bubbles require a larger size to achieve sufficient buoyancy for their escape from the aged paper.