Jur J. Erbrink

Advanced on-site diagnosis of transformer on-load tap changer

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.

Experimental model of aging mechanisms of on-load tap changer contacts

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.

Diagnosis of Onload Tap Changer Contact Degradation by Dynamic Resistance Measurements

Due to different degradation mechanisms, onload tap changers (OLTCs) may contribute to power transformer failures. If offline diagnosis is undertaken during functional checks performed during the service life of OLTC devices, some of the degradation effects can be assessed. This paper addresses an OLTC diagnostic method based on the measurement of dynamic resistance. An experimental investigation of the degraded OLTC contacts was used to identify a number of defects revealed by dynamic resistance measurements.

Advanced on-site diagnosis of power transformers

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.

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 OLTCs in good condition, OLTCs with increased contact resistance, OLTCs with open contacts and OLTCs 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 OLTCs will be discussed by showing results of DRM tests. Most graphs presented in this paper are recorded on service-aged OLTCs in the Dutch utility grid. Some measurement results are performed during experiments in a high voltage laboratory.

Effect of test parameters on dynamic resistance measurement results from on-load tap changers

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 OLTCs 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.

Condition assessment of on load tap changers using dynamic resistance measurements

Resistance measurements on power transformers are a widely accepted method for finding winding defects. Continuity of the tap changer contacts can also be checked when the on-load tap changer (OLTC) is operated during the resistance measurement. Resistance measurements that also record the resistance during OLTC operation are called dynamic resistance measurements. This off-line diagnostic method can be used during regular maintenance to assess the condition of OLTC parts not accessible for inspection. Besides condition assessment of inaccessible parts of the OLTC, this method can also be used to detect maintenance errors that could lead to failure. In particular, these measurements are very helpful in finding contact discontinuity, mechanical defects that influence the operation of the tap changer contacts, problems with the switch time and the transition resistors and can find contact degradation in an early stage.

On-load tap changer diagnosis - an off-line method for detecting degradation and defects: Part 1

The following conclusions may be drawn from the material presented above: 1. Dynamic resistance measurements on OLTCs provide early warning of developing faults. They may be used to assign condition indices based on dynamic resistance boundary values. 2. The boundary values can be derived from field measurements, laboratory experiments, and failure investigations. 3. Using a flowchart in conjunction with the dynamic resistance data greatly simplifies the condition index assignment.

Diagnosis of degradation effects of on-load tap changer in power transformers

By performing diagnostics on on-load tap changers by means of dynamic resistance measurements a condition diagnosis of high-speed resistor-type tap changers can result in pre-failure detection of contact degradation. It provides the measurement parameters that are most suitable for detecting these defects, such as the presence of a contact film, contact wear and pitting. The resistance curves of OLTCs are interpreted, to determine the occurring degradation mechanisms, which can be used to support strategic decisions about the maintenance or overhaul of the tap changer.

Test procedure and test circuit considerations for on load tap changer dynamic resistance measurement

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.