On-load tap changer diagnosis: Interpretation of dynamic resistance deviations

This paper discusses the recently developed analysis method for service aged power transformers equipped with an OLTC: dynamic resistance measurements (DRM). Because the on-load tap changer contributes to many power transformer failures the emphasis is on OLTC diagnosis. Off-line diagnosis of on-load tap changers during regular maintenance can detect maintenance errors and assess the condition of OLTC parts not accessible for inspection. DRM can be valuable to on-load tap changer (OLTC) condition assessment because of the large variety of defects and OLTC degradation mechanisms that can be found. This paper deals with the results that can be measured with DRM. Besides undisturbed DRM-data, most common deviations originating from degraded and defect tap changers will be discussed. It is explained how these different types of defects appear in the DRM results. The location and shape of the deviation can be used to find the cause and importance of the defect. A subdivision is made between OLTC's in good condition, OLTC's with increased contact resistance, OLTC's with open contacts and OLTC's with deviating switch times. By doing so, an overview of possible DRM results is gained. The seriousness of the defects is also discussed in this paper. Differences between the DRM results of different types of OLTC's will be discussed by showing results of DRM tests. Most graphs presented in this paper are recorded on service-aged OLTC's in the Dutch utility grid. Some measurement results are performed during experiments in a high voltage laboratory.