Multi-Method Transformer Diagnostics for Root Cause Analysis of Tap Changer Faults

Abstract

Measuring a transformer tap changer is an important part of transformer diagnostic testing, as it enables the assessment of contact transition resistance, the condition of contact elements, and the potential impact of contamination on switching reliability. The paper presents practical diagnostic cases in which an unusual, distorted current waveform was observed during tap changing, deviating from the expected waveform. Because oxidation or other contaminants can cause unstable contact conditions and a temporary increase in transition resistance, the measurements were repeated with a higher test current across all tap positions to reduce the influence of surface contamination. If the current waveform remained similarly distorted, this indicated that contact wear or degradation of the contact material was the more likely cause. The field measurement findings are normally further supported by laboratory results of dissolved gas analysis (DGA) of the transformer oil, that in case of overheating or local electrical discharges (arcing) reveal the presence of gases characteristic of specific faults. In the case of abnormal results indicating a serious issue, an inspection of the switch is usually performed. This includes a visual inspection of the tap changer and replacement of the problematic parts.