Analysis on Extending Service Life of Aged Transformers by Oil Replacement with Novel 3-Element Mixed Insulation Oil

Oil-paper composite insulation is the key parameter affecting the overall performance and service life of transformers, and it gradually ages with the operation time. In order to extend the aging lifespan of transformers, scholars all over the world have spared no effort to delay the aging rate of the insulation paper. Recently, our group has proposed a novel 3-element mixed insulation oil, whose main parameters meet the current IEC 60296-2012 standard for mineral oil, indicating the novel 3-element mixed insulation oil can be used for replacing mineral oil directly. In this paper, different oilpaper systems which consist of 3-element mixed insulation oil, mineral oil and Kraft paper, were used for accelerating thermal aging test at $130^{\circ}C$. In the middle aging period, aged mineral oil in some samples was replaced with new mineral oil and 3-element mixed insulation oil respectively, and then the accelerating thermal aging test continued. Comparison on thermal aging rate and AC breakdown of oil shows that the 3element mixed insulation oil could not only keep excellent insulation property in aging process after oil replacement, but also indeed decrease the thermal aging rate of aged insulation paper. Oil replacement with 3-element mixed insulation oil increases 1.5 times aging lifespan of insulation paper in the thermal aging experiment, which suggests that the service life of aged transformer may be extended by oil replacement with the novel 3-element mixed insulation oil.