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Fig. 3: Test system for winding resistance measurement.

The purpose of a tapchanger is to boost (or buck) the voltage on the LV side of a transformer for a low (or high) voltage condition on the HV side, or to allow for increased losses in the transformer and the surrounding power system due to loading which results in a decreased voltage overall.

Both offload and onload tapchangers reduce a specific number of windings from the HV side of the transformer to boost the LV voltage and vice versa. Offload tapchangers may only be operated with the transformer being switched off. Onload tapchangers the tapchanger may be operated while the transformer is in operation and supplying a load.

A typical onload tapchanger is shown in Fig. 1. To change from tap 1 to tap 2, the tap selector switch on the right needs to be moved to tap 2. The diverter switch then starts moving from position A towards position B. Firstly a diverter resistor is switched in series with tap 1, then tap 1 and tap 2 are shorted via two diverter resistors in series, then only the diverter resistor of tap 2 is in series with tap 2 and ultimately tap 2 is directly connected. The transition from tap 1 to tap 2 must be completely continuous without any interruptions, as this causes arcing, which apart from resulting in a voltage interruption on the LV side will contaminate the transformer oil. Furthermore the contacts of the tap selector switch as well as the diverter switch have to be as perfect as possible, i.e. any carbon or similar build up will result in additional resistance to the circuit resulting in additional losses and heat generation. A typical diverter switch is shown in Fig. 2. To maintain such an onload tapchanger, it is important that the contacts of both the tap selector switch as well as the diverter switch are tested, and that the continuity of the tapchange process is verified for tapping both upwards and downwards.

Static and dynamic testing of onload tapchangers of power transformers

by Dr. Michael Krüger, Omicron Electronics and Alexander Dierks, Alectrix

The failure of onload tapchangers (OLTC) contributes considerably to the failure statistics of power transformers as a whole. Static winding resistance tests yield a good measure of the state of the selector as well as diverter switches. Dynamic winding resistance tests indicate if the tap transition process from one tap to the next is smooth and continuous

Fig. 1: Typical onload tapchanger.

Fig. 2: Typical diverter switch.

Static winding resistance measurement

Winding resistances are measured in the field to check for loose connections, broken strands as well as high contact resistance in tap changers.A winding resistance test is carried out by injecting a DC current of approximately 1% of the rated current and measuring the DC voltage drop across the winding after the measurement has stabilized. Special care must be taken when conducting this test to not interrupt the test leads at any time during the test, to allow the core to be magnetized by the DC current injected resulting in unstable voltage measurements as well as allowing the core to discharge all the energy at the end of a test by discharging such energy to ground before touching any of the test leads. Note that opening any of the test lead connections during a DC winding resistance test may result in very high voltages appearing across the open leads which can be dangerous, if not lethal, to both the operator as well as the test equipment. A typical test system with connections is shown in Fig. 3.

The winding resistance measurement described above needs to be performed for each tap of the tapchanger. Plotting the winding resistance against the tap number, should give a linear relationship as shown in Fig. 4 for a healthy normal tapchanger, and Fig. 5 for a tapchanger with tap winding reversal.

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Fig. 6: New voltage selector contact.

Fig. 7: Faulty voltage selector contact.

Fig. 6 shows a new silver plated voltage selector contact. Fig. 7 shows a faulty voltage selector contact with a lot of carbon build up, resulting in additional contact resistance between these contacts.

Results

A transformer under test (220 kV/110 kV, 100 MVA) was found to have conspicuously high quantities of gas in the oil, from which the conclusion was drawn of inner overheating. Except for the middle tap, all taps showed a significant increase compared to the original measured values. The differences were more than 10% or, in absolute values, up to 70 mΩ (Fig. 8).

The deviations between switching upwards and switching downwards are likewise clearly visible. This indicates high contact resistances caused by the contacts of the tap selector switches. No silver plated contacts were used and the copper contact surfaces were coated with oil carbon. After a full maintenance of the tap selector, no significant difference to the values originally measured at the factory in 1954 could be observed. To examine the results in more detail, it is recommended to view the difference between "up" and "down" values (Fig. 9). The difference before contact maintenance was up to 30 mΩ (or 5%) and after it was below 1 mΩ (or 0,18%).

Dynamic behaviour of the diverter switch

To date, only the static behaviour of the contact resistances has been taken into account in maintenance testing. With a dynamic resistance measurement, the dynamic behaviour of the diverter switch can be analyzed (Fig. 10).

Process of tapchanging from Tap A to Tap B:

• Diverter switch commutes from the first tap to the first commutation resistor

• The second commutation resistor is switched in parallel

• Commutation to the second tap (direct contact)

• Regulation back to the set current value

Fig. 4: Healthy tapchanger.

Fig. 5: Healthy tapchanger with tap winding reversal

Comparison to "fingerprint" results, which were taken when the item was in a known (good) condition and to the other phases, allows an efficient analysis. A glitch detector measures the peak of the ripple (Imax - Imin) and the slope (di/dt) of the measuring current, as these are important criteria for correct switching. If the switching process is interrupted, even for less than 500 μs, the ripple and the slope of the current change dramatically.

For tap changers in good condition the ripple and slope measurements for all three phases tapping up should be similar as well as those for tapping down should be similar. Fig. 11 shows the ripple measurement, and Fig. 12 the slope measurement for a diverter switch in a good condition.

Fig. 13 shows the ripple measurements for the three phases of an aged diverter switch. The differences of the ripple values were due to the advanced aging of the switch contacts (Fig. 14), which proves the sensitivity of the measurement principle to changes of the contact surface.

Fig. 8: C Phase winding resistance measurement: Absolute [mΩ]

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Fig. 11: Ripple measurement of a good diverter switch

Fig. 12: Slope measurement of a good diverter switch

Fig. 10: Dynamic resistance measurement for analysis of the diverter switch

Fig. 9: A phase winding resistance measurement: Difference ("Up" "Down") [mΩ].

the selector as well as diverter contacts can be analyzed. The dynamic winding resistance test from one tap to the next yields a ripple and slope measurement, which will highlight any unwanted interruptions in the switching process of one tap to the next.

References

[1] Seitz, V: Vorbeugende Instandhaltung an Leistungstransformatoren Betriebsbegleitende Messungen an Stufenschaltern and Durchfuhrungen, OMICRON Anwendertagung 2003, Friedrichshafen

[2] Cigre WG 12 05: An international survey on failures in large power transformers in service, Electra No. 88 1983, S. 21 48

[3] Hensler Th, Kaufmann R, Klapper U, Kruger M, Schreiner S, 2003, “Portable testing device”, US Patent 6608493

[4] Omicron CPC 100 Reference Manual, Omicron Electronics Gmbh, 2007

Acknowledgements

This paper was presented at the ABB University of South Africa and ABB School of Maintenance Power Transformer Health Monitoring and Maintenance Symposium 2008, and is published with permission.

Contact: Alexander Dierks, Alectrix, Tel 021 790-1665,

alexander.dierks@alectrix.co.za v

Conclusion

With advancing age, transformers require regular checks. Hence it becomes important to perform meaningful maintenance to avoid sudden and/or total failure. The state and health of onload tapchangers can successfully be assessed by performing static and dynamic winding resistance tests for each tap. Comparing the static winding resistance for each tap against the values measured at commissioning, the state of

Fig. 13: Ripple measurement of an aged diverter switch

Fig. 14: Aged diverter switch contacts