DC Breakdown and Space Charge Characteristics of Mineral Oil Impregnated Thermally Upgraded Paper With Different Ageing Conditons

Oil-paper insulation is widely used as the insulating material in transformers. Temperature is one of the major factors causing mineral oil-paper insulation system ageing. In order to relieve mineral oil-paper insulation's thermal ageing problem, the thermally upgraded paper is used as the insulation system. In this paper, a thermal ageing experiment had been conducted on the thermally upgraded paper impregnated in mineral oil. Samples were collected on different days, then the space charge characteristic test was performed using pulsed electro acoustic method (PEA method). DC breakdown tests were conducted with and without pre stressing. A ramp test was conducted to determine the space charge injection threshold voltage. From the experiment result, the space charge injection threshold voltages for mineral oil-thermally upgraded paper at each thermal ageing state are attained. Thermal ageing of the insulation will not influence the space charge injection threshold voltage, but with further deterioration, the higher the corresponding voltage will be. The space charge injection for the samples is homo charge injection. The more the deterioration, the easier it becomes for the injection. Thermal ageing does not influence the DC breakdown voltage significantly. Pre stressing will increase samples' DC breakdown field strength. The more the deterioration of the sample, the more apparent the increase will be.