Advanced on-site diagnosis of transformer on-load tap changer

It is known that many failures of power transformers are related to aging effects of the on-load tap changer contacts. This paper discusses the recently developed analysis method for service aged power transformers. In particular, the effect of several aging stages of the tap changer change-over selector contacts on dynamic resistance have been considered. As a result, to assess on-site the condition of a power transformer, the following approaches are combined: - The dynamic resistance measurement, to detect deviant contact resistances inside an on-load tap changer; - The velocity measurement to the driving axis of the tap changer drive mechanism and the motor power measurement on the drive motor of the tap changer, to detect mechanical wear or damage inside the drive mechanism; - Transformer turns ratio measurement and DC winding resistance measurement, to diagnose the transformer windings. This paper also presents decision support rules for dynamic resistance measurements to make an unambiguous decision about the status of the tap changer contacts.