It is known that most failure of power transformers are related to aging effects of the on-load tap changer contacts. To understand the aging mechanisms and to develop knowledge rules to interpret different failure mechanisms a test model has been developed. In particular, the effect of the several aging stages of tap changer change-over selector contacts on dynamic resistance measurements (DRM) has been considered, by several measurements with aged tap changer contacts. These aging phases include clean contacts, contacts with an oil film layer, contacts with coking and contacts with pitting. Based on these experiments it is shown that the DRM method is very sensitive to aged change-over selector contacts of an on-load tap changer.In addition to laboratory experiments, the same measurements are performed on naturally aged change-over selector contacts at a Dutch utility.
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