Presented by Charles Sweetser, OMICRON Electronics Corp USA

The exciting current test has long been applied to power transformers. This test has been used to accepted as a powerful tool for identifying insulation faults, core problems, and tap-changer failures. The exciting current measurement produces unique test results in the form of a pattern, which differ based on the transformer’s core construction, test technique, and tap-changer design. The industry standards do not provide limits, but only guidance, and are often difficult to analyze. This paper will present techniques for identifying and analyzing exciting current patterns on power transformers. The main focus will be recognizing patterns associated with phase comparison, variances in test technique, and tap-changer design. Also, this paper is going to address the diagnostic differences between power factor / CAP testing and exciting currents. This will help the audience easily identify expected pattern and failure modes when performing the exciting current measurements. Case studies will be presented focusing on the importance of the exciting current measurement highlighting various expected patterns and failure modes.