The main insulation for common transformer is Class A insulation, it is specified that the maximum application temperature is 105℃, the winding temperature of the transformer in operation is higher than the upper layer oil by 10~15℃. If during operation the upper layer temperature of the transformer is always around 80~90℃, that is to say, the winding is often under 95~105℃, its insulation will age seriously due to over high temperature, and the insulation oil degradation will be quicken, thus affecting the service life.