Thermal aging test for transformer oil–paper insulation under over‐load condition temperature

The accelerated thermal aging test is an effective method to simulate insulation material degradation in oil-immersed transformers. Traditionally, cellulose paper and mineral oil are heated up together at the same temperature when a thermal aging test is conducted. However, variation occurs between laboratory results and field conditions. Temperature gradients exist in real transformers, and each part of the winding is aged under various conditions. In order to rebuild temperature gradients and conduct a thermal aging test under this condition. First, 3D models with designed U-shape wingding which can guarantee different parts aged simultaneously under different temperatures are calculated for temperature field analysis. Then a thermal aging platform with a large current generator is proposed to construct temperature gradients in a thermal aging test. The large current generator helps to heat up the copper winding. Temperature sensors are installed in the aging tank to monitoring winding and oil temperature and control temperature. The aging temperature is selected according to the standard IEC 60076-7 for the over-load condition. The degree of polymerisation of the insulation paper is measured and properties of each layer of the insulation paper are analysed. Other aging products under both traditional test and method in this study are compared.