Modeling thermal life expectancy of the UK transmission power transformers

According to the current operating experiences, the dominating failure mechanism of the UK transmission power transformer population has been verified by the forensic tear-down investigations to be of a random manner. In long term consideration, it is widely acknowledged that due to the thermal aging, the mechanical breakdown of the insulating paper is responsible to the ultimate failure of the transformer. Conventionally the insulating paper´s thermal usage shall only be reliably learnt via measuring paper´s degree of polymerization (DP) after transformer scrapping, which is infeasible in assessing the active units. In dealing with this problem, a systematic model, based on the thermal model in IEC transformer loading guide 60076-7, is presented in this paper to estimate individual active transformer´s thermal life via evaluating its operating condition and thermal design characteristics. The model features the incorporation of the practical aging mechanisms of the insulating paper, and the derivation of the conventionally unknown hot-spot factor (HSF) via utilizing the information gained from scrapped units. As the modeling result, the thermal life expectancy of 185 transformers has been estimated as 83 years. A thermal life matrix is also presented as an attempt to reveal the general trend of the transformer´s thermal life under different combinations of load and thermal design characteristic.