Fig. 2 illustrates that the smaller MVA rated
transformers contribute to the larger number of
failures. This is expected, as this group represents the
majority of the installed base. An average failure rate
of 0.55% is calculated based on the assumption of 1%
growth per annum for transformers [4]. Conducting a
sensitivity analysis for this figure, and assuming a 2%
growth per annum, raises the average failure rate to
0.56%.
Therefore, the transformer reliability for Eskom
Distribution compares favourably with the figures
produced in the EPRI report 1001707 [2]. Here, 2%
was determined as a low failure rate with 7% being a
high failure rate.