DRM OF OLTC

C I R E D 20th International Conference on Electricity Distribution Prague, 8-11 June 2009

Paper 0134

CIRED2009 Session 1 Paper No 0134

EXPERIMENTAL MODEL FOR DIAGNOSING ON-LOAD TAP CHANGER CONTACT AGING WITH DYNAMIC RESISTANCE MEASUREMENTS

Jur ERBRINK Edward GULSKI Johan SMIT Rory LEICH Seitz Instruments AG Delft University of Technology Delft University of Technology Liandon Switzerland The Netherlands The Netherlands The Netherlands [email protected] [email protected] [email protected] [email protected]

ABSTRACT It is known that many failures of power transformers are related to aging effects of the on-load tap changer. Especially the OLTC contacts are prone to several aging mechanisms. To understand the aging mechanisms and to develop knowledge rules to interpret different failure mechanisms a test model has been developed. The effect of the several aging stages of tap changer change-over selector contacts on dynamic resistance measurements (DRM) has been considered, by several experiments with aged tap changer contacts. These aging phases include clean contacts, contacts with an oil film layer and contacts with coking. 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. This change-over selector was accessible so the physical status of the tap changer contacts can be compared with the measurement results.

INTRODUCTION The most important cause of power transformer failures in the Netherlands is the tap changer, a transformer component used for voltage regulation. On-load tap changers are responsible for at least 50% of power transformer failures in the Netherlands [1,2]. By keeping the tap changer in good condition, by means of adequate maintenance, the transformer failure rate can be reduced. Tap changer diagnostics are important to determine when and which maintenance is necessary on the tap changer. Nowadays several on-line and off-line diagnostics are in use. This paper focuses on off-line diagnostic measurements on tap changers using dynamic resistance measurement (DRM).

FAILURES OF ON-LOAD TAP CHANGERS A tap changer is the only movable part of a power transformer and therefore suffers from various aging mechanisms. The insulation oil inside the tap changer compartment becomes dirty due to switching arcs, therefore

weakening its insulation properties. Electrical treeing along the supporting resin-bonded paper cylinder and insulating drive shaft can occur. Other aging effects are caused by the switching arcs that wear of the arcing contacts.

Fig. 1. The on-load tap changer model used for laboratory experiments. This hand-driven one-phase tap changer is connected in series with a transformer. An important aging mechanism is the so-called long term effect on the change-over selector and tap-selector, which occurs when the tap changer is motionless. The long term effect starts with the formation of a thin layer of oil. This organic film is a less conductive layer build from polymerized oil: organic components in the transformer oil bond to silver or copper oxide and sulphide that is formed on the stator blocks [3]. This oil film layer will not cause tap changer failures directly. The increased contact resistance due to the oil film layer will cause coking (creation of pyrolitic carbon) at places where the load current flows. Fig. 1 shows typical damage of coarse tap-selector contacts due to the long term aging effect. The long term effect is accelerated by high temperatures, high load current, infrequent movement and low contact pressure. Due to its infrequent movement, the change-over selector is prone to the long term effect. However, the change-over selector is not accessible during normal maintenance, so diagnostic measurements are necessary.

DYNAMIC RESISTANCE MEASUREMENT [4] describes an in 1993 developed off-line method, designed to diagnose the long term effect inside a tap changer. This tool is based on DRM, originally used for circuit breaker analysis. [5] describes another measurement device, the TPC analyzer, that is used for the experiments in


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this paper. Both devices use a fixed voltage source and a test current of 1ADC to measure the resistance. A short circuit at the secondary side of the transformer also allows a fast response of the test current. Dynamic resistance measurements use this fast response to detect increased contact resistances. When applied to a power transformer, DRM is able to detect deviant contact resistances when the on-load tap changer switches. These deviant resistances can be caused by the long term aging effect on the change-over selector contacts or by other defects inside the OLTC.

EXPERIMENTS WITH OLTC CONTACTS

Measurement setup Laboratory experiments on several aged tap changer contacts are performed to get more insight in the way aged contacts influence the dynamic resistance. Different stages of aging are considered. A test model is used for testing artificial tap changer aging. The test model is connected in series with the power transformer that is tested. The design of the test model is based on a common selector switch type tap changer in the Netherlands. The model consists of one phase of this tap changer and uses resistors to simulate the transformer windings. Fig. 2 shows a schematic representation of the test model.

Fig. 2. The on-load tap changer model used for laboratory experiments. This hand-driven one-phase tap changer is connected in series with a transformer. A fixed DC test voltage is applied to the test model (and the in series connected power transformer) with such amplitude that the initial test current is 1ADC. After stabilizing the current, the test model is switched through all its tap positions and the current and voltage are recorded. Fig. 3 shows an example of such a current profile in case no aging is put on the contacts (the reference measurements). The transition resistors cause a short decrease of the test current each time the tap changer selects another tap. The slope of the test current is caused by resistors that simulate the winding resistance of the transformer’s tapped windings.

Oil film layer As discussed above, the long term effect starts with the formation of an oil film layer. For these experiments an artificial grown oil film layer is used, grown from used tap changer oil at higher temperatures to accelerate the growth rate. This growth rate of oil film under Shell Diala

insulation oil on copper contacts is according to [6]:

( )6 3.862 0.35591.883*10 10s tθ−=

Where s is the thickness of the oil film layer in Å, θ0 is the contact surface temperature in ˚C and t the duration of the aging test in hours. To form a representative artificial oil film layer, simulation parameters are selected based on the transformer population of a Dutch utility. The average age of power transformers is 30 years. The long-term effect is measured (using DRM) at transformers of at least 25-35 years old. The population of transformers with disruptions in the DRM current pattern has an average age of 40 years.

Fig. 3. An undisrupted current pattern as measured with dynamic resistance measurement on the test model. The slope of the current profile is caused by resistors that simulate the winding resistance of the transformer's regulating windings. Each time the tap changer selects another tap; its transition resistors shortly change the test current. An oil layer corresponding to 40 years is formed in the high voltage laboratory, based on an operation temperature of 60ºC, using oil of 165ºC. The selected oil film layer therefore has a calculated thickness of 1300Å. Several dynamic resistance measurements show that this layer of oil film is not measurable with this method. All measured DRM current profiles are without any disruptions and look like fig. 3. Additional measurements with test currents of 100ADC show an average increase of contact resistance of 300µΩ, which are outside the measurable range of the test setup. Because the local contact temperature of tap changer contacts can be higher, the experiments have been extended to thicker oil film layers (up to 15h at 200ºC). Fig 4 shows that an advanced oil film layer is indeed able to disrupt the current pattern. The disruptions decrease during the experiment due to the wiping effect of the contacts: the oil film layer is damaged and a good contact is restored again.

Coking When the oil film layer grows, coking will occur. Coking accelerates itself because the carbon will increase the resistance and act as heat insulator. Local thermal runaway is likely to occur [7]. Two levels of coking are artificially applied to the tap changer model, using semi conductive paint used for field


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Fig. 4. A typical dynamic resistance current pattern as recorded on the test model. A thick oil film layer is applied to all contacts. The tap changer constantly moves over this layer, resulting in a decreasing spiky current pattern. control in power cable terminations. This artificial aging showed to have the same resistive behavior as naturally grown coking when a test current from 0 to 100ADC and back to 0 is applied. A thin layer of artificial coking results in an increased contact resistance, but this layer is damaged by the tap changer contacts in most of the cases, see fig. 5 (left). The conductive path is fully restored, only one of the six aged tap positions shows a deviant resistance. A thick layer of coking is always measurable, see fig. 5 (right). The amplitude is equal.

Fig. 5. A typical dynamic resistance current pattern as recorded on the test model. A thin (top) and thick (bottom) layer of artificial coking is applied to some tap positions. These defects mostly produce quite stable and reproducible current patterns because the model is motionless between the tap change operations. A third test concerns the constant movement of tap changer contacts over a layer of artificial coking. Fig. 6 shows a spiky current pattern as measured with DRM. These current spikes are quite common at field

measurements on tap changers which suffer from the long term effect.

Fig. 6. A typical dynamic resistance current pattern as recorded on the test model. A thick layer of artificial coking is applied to all contacts. The tap changer constantly moves over this layer, resulting in a spiky current pattern.

Contact wear Contact wear of the arcing contacts results in a small difference of the contact timing. The worn contact will break the current earlier and the transition resistor is used a little longer. This difference can easily be detected by DRM. Fig. 7 shows the time the transition resistor is used divided by the total switch time at the test model. Contact 3 has worn arcing contacts.

Fig. 7. The time that the transition resistor is used for each tap change operation. The arcing contact of contact 3 is worn. This results in longer use of the transition resistor.

SERVICE AGED OLTC CONTACTS Measurement data from more then hundred different tap changers have been examined within this project. The selector or diverter switch of all these tap changers was taken out of service for maintenance. However, only a limited number of change-over selectors were available for visual inspection because of their accessibility. One of these change-over selectors is measured extensively during an overhaul.


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Fig. 8. Left: a stator contact of the tested change-over selector suffered from the long term effect. Deep pitting and coking is visible at the top, the oil film layer is visible in the middle. Right: silver plated stator contacts are measured after the overhaul. The measured tap changer is a common selector switch type tap changer in the Netherlands and is almost equal to the laboratory model used for the experiments described above. However, this tap changer has a change-over selector which doubles the number of tap positions. DRM data from this tap changer showed deteriorated contacts due to the long term effect. Based on these diagnostic measurements, it was decided to overhaul the transformer and to plate the contacts of the change-over selector with silver. All aging and damage from the long term effect was removed during this overhaul. The tap changer has been tested extensively before and after this process. Fig. 8 shows two representative stator contacts of this tap changer. Typical DRM data before (collapsed current pattern) and after cleaning (undisrupted current pattern) are shown in fig. 9. It can be clearly seen that the dynamic resistance measurement is very sensitive for the long term effect on the aged change-over selector contacts.

Fig. 9. A typical dynamic resistance current pattern as recorded before (top) and after (bottom) the tap changer was cleaned. The tap changer suffered from the long term effect.

CONCLUSION This paper focused on the contact diagnosis of on-load tap changers for high voltage power transformers using DRM. In particular the following can be concluded:

1. The contact deterioration of OLTC’s can be described by different aging mechanisms.

2. Modelling and laboratory investigation of the deterioration processes is helpful to gain more systematic knowledge about the applicability of DRM.

3. Dynamic resistance measurements provide more information about the contact status compared to static resistance measurements.

4. Laboratory investigation has shown that light aging (the thin oil film layer as presented above) is not measured with DRM while advanced aging (coking) is measurable. Thick artificial coking is measured at every measurement while a thin layer of artificial coking tends to break due to contact pressure and is therefore not always measured.

5. Transitions resistors are used longer when the arcing contacts are worn and this can be measured by DRM.

6. Laboratory experiments and a test case showed that DRM is indeed effective for assessing the long term effect on the tap changer contacts.

REFERENCES [1] R. Jongen, et. al., 2007, “A statistical approach to

processing power transformer failure data”, Cired 19th International Conference on Electricity Distribution, Paper 546.

[2] H.F.A.Verhaart, 1994, “Tussenrapport over faaloorzaken van distributie- en koppeltransformatoren op basis van de onvoorziene niet-beschikbaarheid.”, KEMA rapport 43613-T&D 94-102278.

[3] EPRI, December 2006, “Evaluation of load tap changer oil for susceptibility to coking.”, from www.epriweb.com.

[4] H. F. A. Verhaart, 1995, “A diagnostic to determine the condition of the contacts of the tap changer in a power transformer,” in Proc. CIRED, Brussels, Belgium, paper 1.13.

[5] P. Seitz, et al., 2008, “Advanced on-site diagnosis of power transformers”, International conference on condition monitoring and diagnosis, paper 148.

[6] K. Lemelson, 1973, “About the failure of closed heavy current contact pieces in insulating oil at high temperature.”, IEEE transactions on parts, hybrids, and packaging, vol. Php-9, no.1.

[7] Z. Wang, Y. Liu, P.J. Griffin, 2000, “Artificial Intelligence in OLTC Fault Diagnosis Using Dissolved Gas-In-Oil Information.”, IEEE Power Engineering Society Summer Meeting, Volume 4, 2422-2427.

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DRM OF OLTC