Experimental model for diagnosing on-load tap changer contact aging with dynamic resistance measurements

It is known that many failures of power transformers are related to aging effects of the on-load tap changer. Especially the OLTC contacts are prone to several aging mechanisms. To understand the aging mechanisms and to develop knowledge rules to interpret different failure mechanisms a test model has been developed. The effect of the several aging stages of tap changer change-over selector contacts on dynamic resistance measurements (DRM) has been considered, by several experiments with aged tap changer contacts. These aging phases include clean contacts, contacts with an oil film layer and contacts with coking. 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 changeover selector contacts at a Dutch utility. This change-over selector was accessible so the physical status of the tap changer contacts can be compared with the measurement results.