On the development of power transformer failure models: An Australian case study

Power transformers are expected to operate reliably for decades. Loading guides and models exist to estimate the functional life remaining of transformer cellulosic insulation. However, failure is essentially probabilistic and dependent on many factors, and so statistical models to ascertain the expected number of failures for a power transformer fleet are highly desirable when planning future investment. Given their low failure rate, approximately 1% per transformer year, a fleet size much larger than an individual Utility owns is required for statistically significant results. Therefore, a large study was performed on the Utilities in Australia who together operate around 6,000 power transformers. For this, data on 626 failures and retirements, over a fifteen year period, were collected. The Weibull distribution was used to determine failure rate for age, voltage class of transformer, and failure mode. Finally, the development of a tool to model future replacements is discussed.