Investigation of Thermal Aging Test Method for Transformer Oil-Paper Insulation under Over-Load Condition

Due to the temperature rise, the lifetime of insulation paper wrapped on the copper will be shortened. The accelerated thermal aging method is often adopted to do research on the thermal aging rate of oil-paper insulation materials under different temperature working points, to acquire the thermal lifetime model. According to existing aging test, both insulation paper and mineral oil were aged at the same temperature. In application, however, the temperature of mineral oil is lower than that of the hot spot due to oil circulation in transformer tank, and temperature gradients exist between oil and insulation paper. Serious deviation of the test results from the reality may appear for the fast degradation of the insulation oil's performance under high temperature condition. A new method is proposed in this paper for oil-paper insulation thermal aging test for windings. Current generator is adopted, a temperature sensor is fixed on the windings to get the temperature signal, and this signal is transmitted back to the voltage-controlling device. In this way, a relatively stable aging temperature is obtained. Based on this new method, oil-paper insulation accelerated thermal aging tests were finished, the variation rules of polymerization for the insulation paper in the aging process is analyzed. Test results with the proposed method in this paper is more tallies with the actual condition comparing to the traditional results. In addition, the accelerated thermal aging tests under different temperature points can be completed at the same time in only one tank, cost and time can also be substantially cut down.