Experimental Study on Bubble Effect of Oil–Paper Insulation in Natural Ester

The rapid temperature rise brought by short-term overload may cause the oil–paper insulation of transformer to produce bubbles. The existence of bubbles will reduce the electrical strength of transformer insulation and may lead to partial discharge or even breakdown of the oil–paper insulation in the transformer. In this article, the bubble effect of oil–paper insulation in natural ester is studied experimentally. A bubble effect test platform is designed and built according to the temperature conditions of the transformer in operation, and bubbling tests are carried out for natural ester-cellulose insulation. Initial temperature of bubble escape (ITBE) is obtained for different winding insulation paper layers, temperature rise rate, and pressboard aging time, respectively. The experimental results show that the bubbles aggregate in the gap between the paper layers before bubbles escape into oil, and ITBE is affected by winding insulation paper layers. Besides, a higher temperature rise rate corresponds to a lower ITBE. With the thermal aging of pressboard, ITBE increases, because the number and diameter of pores decrease in fibers of pressboard.