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Page 1 of 6 TAP-CHANGER TESTING USING DRM Matz Ohlen1*, Nils Wäcklen2, Rene Yvrard2 1Megger Sweden AB, Stockholm, Sweden 2TSV, Lyon, France *Email: <[email protected]> Abstract: Dynamic Resistance Measurement (DRM) has been used for circuit-breaker diagnostics for over 20 years. It is an interesting technique that can be used also to verify the switching operation of load tap changers (LTC). Existing methods and techniques for dynamic measurements on load tap-changers are based on measuring current and/or voltage on the primary side of the transformer and short-circuiting the secondary side to minimize the inductance in the circuit. Static resistance measurements per tap are performed in a separate test sequence with the secondary side open. A new technique (patent pending) is to combine current measurement with voltage measurements on both primary and secondary side of the transformer and then use the transformer parameters to calculate inductive and resistive voltages to be able to calculate the dynamic resistance during a tap change. Measurements have been performed on non-mounted tap-changers (no-oil condition) and after being mounted inside a transformer (with oil). Different test setups have also been evaluated including the new technique where current measurement is combined with voltage measurements on both primary and secondary side of the transformer. 1 INTRODUCTION The power transformer is an integral and expensive part of all electric power networks at all levels from generation and transmission down to distribution. The on-load tap changer is the only moving part connected to the transformer windings. The importance of its reliability cannot be overemphasised. Taking a transformer off the system to investigate an internal problem with a tap changer is an expensive exercise; therefore it is in every utility’s interest to carry out condition assessments of their tap changers to help detect developing faults at an early stage. The tap changer is the only moving part of a transformer, and as such is the most susceptible to failure. 2 TAP CHANGER TYPES Tap changers can be divided into two main types; On-load (OLTC, On-load Tap Changer) and Offload/de-energized (DETC, De-Energized Tap Changer). The OLTC allows selection of voltage change while the transformer is in service. This type of tap changer allows voltage output of a transformer to be changed while power (current) is still passing through it. In Europe and internationally, the most common configuration is to have the tap-changer on the HV side of the transformer. Figure 1 depicts a typical on-load tap changer with tap selector and diverter switch with transition resistors. Figure 2 depicts a linear-type OLTC with diverter resistors. Figure 1. A typical diverter switch type OLTC showing both tap selector switch and diverter switch (MR) The resistors in the diverter switch are typically a few ohms. Total operation time of an LTC is between 3 and 10 sec pending design. Contact switching time is usually in the order of 30-100 ms (resistor types) Reactance type LTC’s, common in US and mostly mounted on the LV side of the transformer, use a preventive auto transformer instead of the two resistors in the standard diverter switch which means that the additional resistance in the diverter device is very low. Measurements on reactance type load tap-changers are not covered in this paper. Page 2 of 6 4 DYNAMIC MEASUREMENTS ON TAPCHANGERS There are several methods developed for dynamic testing of tap changers but common for all is that a current is injected in the tap changer and during the operation of the tap changer, the current and/or the voltage is measured as a function of time. Test currents vary from about 0.1 A to standard test current for winding resistance measurements (typically 1% of rated current for the transformer winding). The measurements are sometimes performed at the same time as measuring winding resistance and sometimes as a separate test. The most common tests are; Figure 2. A typical OLTC with selector switch (17 taps linear) (ABB) 3 STATIC MEASUREMENTS ON TAPCHANGERS The most common diagnostics test for tapchangers is to perform winding resistance measurements [1]. WRM is normally performed for each tap in the same way as an individual winding without taps. A suitable test current is injected through the winding and the resistances for each tap are measured sequentially as the tap changer is stepped through its positions. Results are typically presented as a graph or table with resistance values for each tap. Resistance changes between taps should be consistent with only small deviations between different tap position changes. An example showing deviations between taps is shown in figure 3. Red phase – OK, Blue phase – “Questionable” Figure 3. WRM/tap, diverter design – Old/aged condition 4.1 Continuity/break-before-make testing o Shut-off if current path is interrupted Dynamic measurements o Dynamic current/”ripple” o Dynamic voltage o Dynamic resistance, DRM Continuity testing This test should detect if there is a break-beforemake condition in the tap changer by monitoring current change. The measurement is typically performed at the same time as winding resistance measurements and the instrument detects if the contact switching is continuous or if there is an interruption in the current path. Open contact detection can be made by a variety of methods/detection principles but common for all of them is that if the current change (dI/dt) exceeds a certain value, the instrument identifies this state as “open circuit” and sets an alarm or stops test current injection. 4.2 Dynamic current measurements Dynamic current measurements are pending test current. If the test is performed at a current level below saturation level, the inductance in the transformer winding is high and smoothes the current change. If the test is performed at a current level at or above saturation level, the inductance is low and current level change will be higher [1]. A method to reduce transformer inductance when performing TC measurements is to short-circuit the not tested corresponding LV (or HV) windings. This action is principally “replacing” the inductance of the winding with the short-circuit impedance. Inductance is greatly reduced and changes in current can be measured more precisely. Figures 4-7 below show dynamic current measurements on a 30 MVA, 130/46 kV, YNyn0 transformer using different test currents and with LV windings open and shorted [2] Page 3 of 6 As seen in the figures, dynamic current measurements are pending the inductance of the circuit. If the test is performed at high current or with LV shorted (low inductance), timing of the tap switch can be identified and estimated. Resistor values cannot be estimated. 4.3 Figure 4. 10 A test current, shorted LV Dynamic Resistance Measurement with shorted LV Dynamic resistance measurement (DRM) is a standard method for circuit-breaker testing and can also be applied to tap changer contacts [3]. A relatively small test current (0.1 to 1A) is injected through the tap changer using a high-impedance current source and the LV windings of the transformer are short-circuited. The voltage over the test circuit is measured and resistance versus time can be calculated. Contact timing as well as diverter resistor values can be measured. The test setup is described in figure 8 and an example from a 20MVA, 22/10 kV transformer is shown in figure 9 and 10. Figure 5. 1 A test current, shorted LV Figure 8. Dynamic Resistance Measurements on a load tap-changer (direct method) Figure 6. 10 A test current, open LV Figure 9. DRM on OLTC, 0.1 A test current, constant current source, shorted LV. Red; Current, Blue; Resistance As seen in the figure, contact timing and resistor values are easily recognized. Figure 8 provides a summary of measured switch times for this linear type tap-changer with 17 taps. No malfunctions detected. Figure 7. 1 A test current, open LV Page 4 of 6 5 OLTC MEASUREMENTS AT TSV, LYON, FRANCE A mutual project between Megger and TSV (Transformer Service Venissieux) to study different test methods for DRM on tap-changers is ongoing at TSV, Lyon, France. Several tap-changers are measured with and without simulated faults, as separate units as well as mounted in transformers [4]. 5.1 Figure 10. H1 OLTC transition times (ms). 1-2 to 16-17 tap change 4.4 Dynamic resistance measurement with open LV A new method (patent pending) is to measure dynamic resistance in the tap-changer by simultaneously measuring the test current together with voltages on both HV and LV windings and combine the results with transformer modeling. The test setup is the same as in figure 6 but with the difference that the LV winding is left open. An example of a measurement is shown in figure 11. The source impedance in this example is about 10 ohm and we can see a small current change during tap change. Due to the inductance in the circuit, the voltage change on the primary (HV) side is rather high (open LV). This voltage is a sum of inductive and resistive voltage and cannot be used for directly calculating the resistance in the circuit. However the voltage on the secondary (LV) side is purely inductive and if we use transformer model parameters to calculate the inductive voltage on the primary, we can deduct this value from the measured primary (HV) winding voltage and calculate the resistance in the circuit. The result is shown in figure 11. Summary of tests In the initial tests, a separate (not mounted in a transformer) unit V-type OLTC was used as a test object for verifying the methodology. Test equipment was a constant current supply and a data recording device, test setup as in figure 8. The initial measurement results for verifying the methodology on a tap-changer without any known defects are summarized in Figure 12 and 13. Figure 12. Contact timing for the load tap-changer Figure 13. Resistance values during switching of the tap-changer Figure 11. DRM on OLTC, 5 A test current, source impedance 10 Ω, Green; Test current, Red; Primary voltage, Blue; Secondary voltage, Black; Resistance As seen in Fig. 12, contact timing is rather stable except for the very first two operations 17-16 and 16-15. The reason was that the unit had not been operated in a quite long time and this is an example of the importance to “exercise” a switching device before stable timing results can be achieved. Measured resistance values (figure 13) are as expected almost constant with R1 slightly higher than R2. Page 5 of 6 5.2 Measurement details Switching times in the tap-changers are critical for correct operation. Figure 14 describes the operation of a typical design (MR) 4. R1 released 5. R2 released 6. Load contact makes Contact bouncing can be seen, especially related to R2 making. Figure 15 below presents results for a simulated fault. Spring damage, one of the two springs removed. Figure 14. Switching sequence in a typical OLTC with diverter switch [5] Figure 16. DRM on V-type OLTC, test current 5A, simulated spring defect (one spring removed) As seen in the figure, total switching time is about 60 ms and contains a number of switching sequences (+ static positions before and after the tap change). With one spring removed, switching time is about twice as long as when two springs are engaged. Note that contact bouncing almost disappeared due to the lower contact speed. A DRM measurement on a similar design is shown in Figure 15. 6 Several failures/defects have been simulated in attempts to characterize faults based on DRM results and determine the viability of this diagnostic method. The investigated tap-changer is an OLTC type VIII 350A 60kV 17/19 3W. Two examples of individual DRM results are presented in Figure 14 and 15. Nominal load resistor value is 1.9 Ω Figure 15. DRM on V-type OLTC, test current 5A, normal condition. Note that this tap-changer type has 6 distinctive states for each tap switch. Each of them is recognized in the DRM test. 1. Load contact releases 2. R1 inserted 3. R2 inserted in parallel with R1 CONCLUSION A CB analyzer/signal recorder with analog inputs together with suitable power supply is a good tool for performing various dynamic measurements e.g. on load tap-changers. To measure dynamic properties in a tap changer (in this paper assumed to be mounted on the HV side of the transformer), several methods are possible: Measure dynamic current/”ripple” on primary with a constant voltage source and secondary open o Current drop value is strongly pending test current/winding inductance and can only be used as fingerprint o Contact timing may be measured (difficult) o Diverter resistor values cannot be measured Measure dynamic current/”ripple” on primary with a constant voltage source and secondary short-circuited o Current drop value is slightly pending test current/winding inductance o Contact timing can be measured o Diverter resistor values cannot be measured Page 6 of 6 7 Measure dynamic resistance on primary with a constant current source and secondary short-circuited o Contact timing (including loadcontacts) can be measured o Actual diverter resistor values can be measured (high impedance source) Measure dynamic voltage on secondary winding (secondary open), dynamic voltage on primary and test current o Contact timing (including loadcontacts) can be measured o Resistance can be calculated REFERENCES [1] D. Chajer and Matz Ohlen, “Preventive Maintenance of Transformer Accessories Bushings & Tap Changers”, 2011 Weidman conference, USA [2] J. J. Erbrink et al, “Reproducibility of Dynamic Resistance Measurement Results of On-Load Tap Changers – Effect of Test Parameters”, Proceedings of the 2010 International Conference on Condition Monitoring and Diagnosis, September 6-11, 2010, Tokyo, Japan [3] N. Wacklen, “Timing of Tap Changers”, Programma Electric Application Guide, 1996 [4] P. Illuzzi, “Abstract about measures on V-type OLTC Model”, TSV Lyon-Megger Sweden internal report, 2012 [5] “Switching Sequence of OLTC Type OILTAP M Diverter Switch”, Presentation by Maschinenfabrik Reinhausen GmbH, 2002 (used with permission)