Tap-changer diagnostics: Present state and new developments

The primary function of power transformers is to transform voltage levels for transmission and distribution. Often, transformers are equipped with a device to adjust the voltage ratio by changing the turns ratio: the tap-changer. About 30% of all European substation transformer failures are attributed to tap-changer failure, as well as about 22% of the failures in which a fire or explosion has occurred. Although the contribution of tap-changer failures has decreased over the years, it justifies appropriate measures to timely identify tap-changer defects and mitigate failure risks. In this paper, we present an overview of tap-changer diagnostic options, and discuss recent developments, in particular concerning improved techniques for dynamic resistance measurement