An on-load tap changer (OLTC) is prone to many different defects and degradation mechanisms due to its continuous operation under load condition. A high load current and high operation temperature accelerate the process of contact wear. Contacts erode due to the fact that under certain conditions the contacts develop a resistive layer due to hot insulation oil. As a major cause of OLTC failure, these defects might take place after some years of operation and their frequency increases with the operation time. Regular maintenance is applied due to degradation of the insulation oil. Dynamic resistance measurement (DRM) can be conducted during this maintenance to measure the resistance of the OLTC when it operates. The measurement results provide important information for assessing the actual condition of the OLTC. This paper aims to analyze the effect of test parameters on DRM results from service-aged OLTCs and describes the implementation of DRM in OLTC condition assessment. Two test parameters were selected for observation: the test current amplitude and the circuit resistance. Their amplitude was varied during laboratory experiments as well as on-site after regular maintenance. To quantify the various measurement results, an analytical tool using 4 performance parameters has been developed. Finally for interpretation, with the help of a three dimensional classification system which is presented in this paper, the actual condition of OLTCs can be identified during maintenance activities.@en

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.@en

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.@en

Static resistance measurements on power transformers give an indication about increased contact resistances of the OLTC. Increased contact resistances can also be measured by means of a dynamic resistance measurement (DRM). This paper elaborates on condition diagnosis of electrical contacts of in-tank high-speed resistor tap changers using DRM. DRM uses a low test current through the on-load tap changer to detect increased contact resistances and current interruptions. The difference with static resistance measurements is that the tap changer is switched through all its tap positions during the test. The test current gets no change to fully stabilize due to the inductance of the transformer windings. DRM is therefore less accurate then static resistance measurements but contains more information about the type and location of tap changer defect. This contribution will show that more defects on OLTC's can be found with DRM compared to static resistance tests: the additional value of DRM compared to static winding resistance measurements is discussed. Secondly, important topics that should be considered when performing dynamic resistance measurements are discussed, like the effect of the test current amplitude, the secondary short-circuit and circuit resistance. Simulation results and experimental data are presented to make the reader familiar with the pros and cons of dynamic resistance measurements.@en

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.@en

It is known that many failures of power transformers are related to aging effects of the on-load tap changer contacts. This paper discusses the recently developed analysis method for service aged power transformers. In particular, the effect of several aging stages of the tap changer change-over selector contacts on dynamic resistance have been considered. As a result, to assess on-site the condition of a power transformer, the following approaches are combined: - The dynamic resistance measurement, to detect deviant contact resistances inside an on-load tap changer; - The velocity measurement to the driving axis of the tap changer drive mechanism and the motor power measurement on the drive motor of the tap changer, to detect mechanical wear or damage inside the drive mechanism; - Transformer turns ratio measurement and DC winding resistance measurement, to diagnose the transformer windings. This paper also presents decision support rules for dynamic resistance measurements to make an unambiguous decision about the status of the tap changer contacts.@en

It is known that most failure of power transformers are related to aging effects of the on-load tap changer contacts. To understand the aging mechanisms and to develop knowledge rules to interpret different failure mechanisms a test model has been developed. In particular, the effect of the several aging stages of tap changer change-over selector contacts on dynamic resistance measurements (DRM) has been considered, by several measurements with aged tap changer contacts. These aging phases include clean contacts, contacts with an oil film layer, contacts with coking and contacts with pitting. Based on these experiments it is shown that the DRM method is very sensitive to aged change-over selector contacts of an on-load tap changer. In addition to laboratory experiments, the same measurements are performed on naturally aged change-over selector contacts at a Dutch utility.@en