Resistance measurements on power transformers are offline diagnostic methods that are performed without opening the transformer main tank. In case the power transformer is equipped with an on-load tap changer (OLTC), this resistance measurement can be used to find degradation and defects inside the OLTC. Many OLTC defects as well as contact degradation can be measured by means of dynamic resistance measurements, which is a resistance test during OLTC operation. A low (DC) test current through the on-load tap changer is used during dynamic resistance measurements. This method is in particular sensitive to maintenance errors, contact degradation and contact timing problems. In general, distinction can be made between defects that interrupt the current through the OLTC, defects that affect the timing of the OLTC contacts and contact degradation resulting in abnormal contact resistance.

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.

This paper discusses the recently developed analysis method for service aged power transformers. This is a complete approach to assess the condition of a power transformer, both the on-load tap changer as well as the transformer windings. This paper also presents several case studies of tap changer defects that are discovered with the dynamic resistance measurement function.

Knowledge rules for paper/oil insulated high voltage (HV) power cables are necessary to support asset management decision processes. For development of these knowledge rules investigation has been done into a diagnostic method, for integral condition assessment of paper/oil insulation to DC voltage charging. This method is called the recovery voltage measurement and is based on the dielectric response of cable insulation. In the recovery voltage (RV), parameters have been investigated on possibility to indicate insulation degradation. In this paper single and periodic measurements with different temperatures have been performed. These measurements result in the conclusion that an increase in insulation degradation can be observed by selected recovery voltage parameters as well as dielectric losses behavior in function of test voltage. Measurement results of HV power cables performed at two temperatures can be used in line with dielectric losses measurements to support condition assessment.