A new accelerated thermal aging test for over-loading condition transformer

Transformer life span is severely affected by hot spot insulation failure. In operational transformers, heat mainly generated by windings and iron cores. Temperature of winding hot spot under over-load condition can reach a dangerous level. Due to oil circulation and heat convection, there is temperature gap between oil and windings. In addition, temperature gradients exist among windings and oil at axis direction, respectively. However, according to current method, both windings and oil are put into heating cabinet together to conduct accelerated thermal aging and no temperature gradients exist. For a better and more accurate oil-paper insulation aging model, one new method has been developed and initial testing has been made to conduct thermal aging test. In this procedure, winding sample is made into a `U' type three-layer structure. The winding is fixed at the copper electrodes and then is immersed with oil into a sealed tank. Current produced by large current generator and flows through windings to heat up windings. On account of heat convection and oil circulation, there are temperature gradients at axis direction for both windings and insulating oil, respectively. Top layer windings' temperature is precisely controlled within the preset temperature by adjusting current to simulate hot spot temperature status. Temperature distribution of oil and windings at axis direction in tank are recorded, respectively. Then 21-days thermal aging testing is conducted under 148°C. Degree of polymerization (DP) values of winding paper of each layer at different height during whole aging period are obtained.