End-of-life modelling for power transformers in aged power system networks

SUMMARY Although the design lifetime of transmission transf ormers was 40 years or less, operational experience has been very good with transformers up to this age, as the majority of transformers on National Grid’s UK system were installed between 1955 and 1975 it is i mportant to understand and predict the future relia bility of ageing transformers. This paper presents a statistical analysis of the f ailure history of the transformer fleet and its age profile in order to understand the trend and whethe r/how the failure rate is associated with age. Data from failures to date only show random failure modes and no correlation with transformer age. As we know th at transformers are subject to ageing mechanisms it is not reasonable to base a prediction of future reli ability on this historical data. A further problem arises b ecause with the asset condition assessment techniqu es available, transformers are being replaced somewhat ahead of actual failure, so a method of predicting the ‘age of failure’ of the replaced transformers has h ad to be developed using DP analysis. For in-servic e transformers a calculation of life based on the phy sical ageing process of paper insulation using desi gn data, loading condition and ambient temperature, un der the normal IEC assumptions, has been made. This analysis shows that for a typical network transform er, the ageing rate depends more on transformer des ign and cooler operation temperature settings than on l oading. If the cooler on temperature setting is low and the transformer thermal behaviour is as designed, t hen the expected lifetime would be in excess of 100 years. Despite this there have been examples of age related failures indicating that operating tempera tures for at least some transformers are significantly hi gher than designed. Further statistical and thermal modelling is requir ed to fully understand how the range of thermal parameters of transformers will affect the ageing r ate of the transformers, their future reliability a nd hence the replacement plans required to maintain the futu re reliability of the power system.