Abstract:Transformer failure was divided into four types of failure process: thermal

electrical

mechanical and random. With the hypothesis of independence

service life model under combined stresses was established. In the model

the impact of overload was considered for thermal life

and for electrical and mechanical life

relative aging rate was introduced to consider the variability of various parameters. Results showed there was a probability of 77.7% that the failure would occur between 20 to 30 years

and the failure rate was consistent with the median and late periods of bathtub curve. The failures in the first 15 years were attributed to random failures

and after that a rapid growth trend appeared in which electrical or mechanical failure played a dominant role

but the role of thermal fault was less obvious. Parameter sensitivity analysis showed that the sensitivity of electrical stress was greater than thermal or mechanical stress

which indicated the differences in ambient temperature might be less important than the differences in operating voltage for transformer life.