Failure analysis of 188 power transformers in the voltage range of 88 kV to 765 kV and MVA rating from 20 to 800 is presented. The analysis of the data shows that in smaller transformers ageing related failures are dominant. In the medium MVA rating class, tap-changer failures constitute the highest failure rate. In the large transformers, insulation coordination failure is the most common cause in the early service life of the transformer. This study is an update of earlier investigations. Appropriate monitoring techniques have been suggested with a view to transformer size and voltage rating.
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