Quatek Co.,Ltd-On Load Tap Changer Dynamic Resistance Measurement

Quatek Co.,Ltd-On Load Tap Changer Dynamic Resistance Measurement

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On Load Tap Changer Dynamic Resistance Measurement

Abst ract This paper presents a concept of the analysis of power transformer On Load Tap Changer (OLTC) operating conditions. Tap Changer failures represent over 40 % of total failures of power transformers. Analysis of power transformer OLTC operating conditions encourages a big interest in utilities and industry. Reasons for increased interest in this analysis are at first big desire for good utilization of transformers, then increase of transformer reliability caused by prevention of failures, and last but not least, the improvement of operator safety and environment conditions. Results of analysis are illustrated on a power transformer. A PC software DV-Win together with Winding Resistance Ohmmeter and Tap Changer Analyzer RMO60TC is used for measurement and drawing of the graphs. Introduction Power tr ansformers ansformers are critical, capital-intensive capital-intensive assets assets f or utilities as well as for industry. industry. Transformers are extremely reliable devices. However, many of the transformers in use today have already exceeded their design life. Today, transformers are not automatically replaced if they have reached their life span; instead they are left in service as long as possible. In contradiction to the past, power transformers are operated nowadays at or above rated power. Transformers are subject to vibration. Problems or faults may occur due to poor design, assembly, handing, poor environment, overloading, or poor maintenance. Failures Failures appear on all transformer tr ansformer parts wi th a certain frequency (Figure 1.) In an international analysis there are stat istically presented data on a sample of 47000 transformer-years. The failures are classified by the part of a transformer that is believed to have initially started the failure. The tap changer failures represent more than 40 % of all failures on power transformers. Measuring the resistance of the windings assures that the connections are good and that there are no severe mismatches or open circuits. Many transformers have taps built into them. These taps allow ratio to be increased or decreased by fractions of a percent. Any ratio change involves a mechanical movement of a contact from one position to another. These tap changes will be checked during the winding resistance test. The OLTC consists of two parts; a diverter switch that carries the load during transition, and tap selector that chooses the tap positions. Both of these two parts are placed in transformer oil. Figure 2 shows the scheme of the OLTC.



Figure 1 Allocation of failures for power transformers with the OLTC The tap changer is intended to regulate the voltage of a transformer by increasing or decreasing the certain number of turns. At the European type of transformers the OLTC is placed on primary side.

Figure 2 OLTC scheme TRANSFORMER STATIC WINDING RESISTANCE MEASUREMENT Static winding resistance consists of all internal contact resistances increased by the resistance of the windings in certain tap position. The result is calculated using Ohm's law: voltage drop divided by the test current R=U/I. Winding resistance measurements in transformers are of quintessential for the following purposes: Calculation of the I R component of conductor losses, Calculation of winding temperature at the end of a temperature test cycle, As a base for assessing possible damage in the field. Regardless of the configuration, either star or delta, the measurements are made phase to phase and comparisons are made to determine if the readings are comparable. If all readings are within one percent of each other, then they are acceptable. Keep in mind that the purpose of the test is to check for gross differences between the windings and for open connections. The scheme of measuring method is shown on figure 3.



Figure 3 Method for winding resistance measurement of power transformer

Figure 4 bellow shows damaged tap selector contacts of power transformer’s OLTC.

Figure 4 Damaged tap selector contacts TRANSFORMER DYNAMIC RESISTANCE MEASUREMENT Up to date, only the static behavior of the contact resistance has been taken into account in maintenance testing and diagnosis of power transformers. Dynamic resistance mentioned above is actually test current change, which could be detected with RMO60TC device in combination with the DV-Win software, during the change from one tap position to the next one. That current is an inverse to the total resistance of the circuit. The scheme of measuring method for the dynamic resistance measurement is shown in Figure 5.

Figure 5 Method for the OLTC analysis of power transformers Switching problems of the diverter switch of the OLTC can be found through dynamic resistance measurements method which is shown on the Figure 6.



Figure 6 Dynamic behavior of the diverter switch For the dynamic resistance measurement the test current should be as low as possible, otherwise short interruptions or bouncing of the diverter switch contacts cannot be detected. In this case the initiated arc has the effect of shortening the open contacts internally. During the test of the OLTC, test instrument continuously records while positions of the OLTC are changed. The measurement is performed very fast without necessity to discharge the transformer and then to charge it again for all the tap changes. The test instrument stabilizes the test current after each tap position change. If the tap changer is faulty (open) or if there is a short period of time when the circuit is open, the winding resistance ohmmeter continues to generate the current. The operator can then chose either to continue the test with changing the tap changer to the next position or to stop the test. If the operator stops the test, the instrument automatically discharges the energy stored into the transformer. The case of the open circuit in the tap changer is clearly indicated to the user of the device by message „Discontinuity“ on the display of the instrument. If the circuit is open, measuring instrument will not damage the transformer at all. Figure 7 shows aged diverter switch contacts of the OLTC.

Figure 7 Aged contacts of the diverter switch Dynamic Resistance Measurement graph analysis Specialized PC soware DV-Win is used for data acquision, storing, and the analysis of the transformer stac and dynamic resistance measurements. The soware operates on a standard PC configuraon using Windows operang system. Connecon between PC and test instrument is accomplished using USB interface. The soware enables controlling the test instrument during measurement and downloading measuring results stored in the internal instrument memory. Stac resistance measurements are saved in standard file formats like XLS, CSV and TXT. These Figures 8 and 9 show stac resistance measurements on two different transformers T1 and T2 while changing the OLTC posion from minimum to maximum posion. Figure 8 shows good condion of OLTC on transformer T1, while figure 9 shows bad condion of OLTC on transformer T2.



Figure 8 Static resistance measurement on T1

Figure 9 Static resistance measurement on T2 Measuring results acquired during the change of OLTC positions are saved in special format that enables graphing the current values while changing from one position to another. Based on this graph, it is possible to diagnose the operating condition of OLTC switching elements. Positions are usually changed starting from the first or the last position. After each position change, the software displays a graph showing current value change during the transition from two subsequent positions. Measured values can be saved after each transition or after completing the measurement by reaching the final OLTC position. The analysis of OLTC operating condition is realized in two separate modes. The first one is on- line mode, while using the test instrument on real test object. The other one is off-line mode, which enables analysis of saved data. Figures 10 and 11 show current values while changing the OLTC positions from minimum to maximum on transformers T1 and T2. The values shown on abscissa denote the transition (e.g. 3>4 for transition from position 3 to position 4), while values on the y axis denote current value during the transition. Figure 10 displays a graph which shows good operating condition of T1 OLTC; figure 11 displays a graph which shows bad operating condition of T2 OLTC.

Figure 10 Current value during transitions on T1



Figure 11 Current value during transitions on T2 It is possible to show more detailed diagram by zooming the interesting parts of the graph. The most interesting part of each transition is the part which shows current value during the change from maximum to minimum value. Typical current value diagram is shown on figure 12. This diagram shows a good condition of OLTC. Separate control window provides useful tools, like markers on the graph measuring haracteristic values, time and amplitude. Markers can be moved to desired position on the graph. The values shown on the control window are the time interval between two markers, current difference and percent current difference between the markers.

Figure 12 Current change during one transition Figure 13 shows the current value during the transition with a short discontinuity. This current diagram is related to bad operating condition of OLTC switching elements.

Figure 13 Current interruption during the transition



Three phase turns ratio measurement In addition to the transformer condition assessment using the dynamic resistance recording, a very useful tool was developed to provide a true three phase turns ratio measurement for power transformers. This technique allows measuring the phase shifting transformers and zig zag units that could not be otherwise measured using a single phase or a three-single phase instruments. An interesting addition to the diagnostic tool box, this instrument provides detection of vector group on a three phase units and measures excitation current, phase angle deviation and the turns ratio. One interesting example of a failed transformer data is shown in the table below, indicating a defect of one phase of the power transformer detected using this powerful tool.

Table 1 Test results CONCLUSION Transformers are very reliable devices that are able to work correctly for a long time under regular maintenance and service. Transformer failures are very serious and usually require expensive repair and long out-of-service time. The most important thing to do in order to prevent failures is to provide correct installation and regular maintenance. Transformer winding resistance measurements are required each time the transformer testing is done. Three-phase turns ratio measurements provide a good diagnostic tool for transformer condition assessment. Modern measuring devices provide reliable, accurate and easy measurement. Comparison to „fingerprint“ results which were taken when the transformer OLTC was in good condition, and to the other phases, allows for an efficient analysis. Defective contacts (e.g. tap selector contacts) could be detected by measuring static resistance in all OLTC positions. It is recommended to perform the measurement starting from the first position to final position, and vice versa, and then to compare the measurement results in the same positions. Switching problems of diverter switch could be detected by measuring dynamic resistance. Dynamic resistance measurement is a new method still unknown worldwide. A big dvantage of this method is that there is no necessity to open the OLTC, or to remove the oil from the tank, to see what the problem inside the OLTC is. In such a way maintenance and service costs are significantly decreased. References 1. Transformer failure causes in Germany, IEH Leibnitz University Hanover -Shering Institute, Institute of electric power systems, Division of high voltage engineering 2. MR OLTC instruction manual 3. IBEKO Power AB “Manual of Winding Resistance Ohmmeter RMO60TC, February 2008. 4. Michael Krüger “Fault location on Power Transformers with electrical measuring methods” Klaus, Austria 2007 5. Dynamic resistance measurements in LTC, Luis Miguel Perea, Angel Ramos Gómez Unión Fenosa Distribución, Spain, José Luis Lozano Gómez Norcontrol Soluziona, Spain - Proceedings of the EuroDoble Colloquium 2000



Biography Raka Levi, Dr.Eng., has over 25 years of asset performance and condition assessment experience, specializing in apparatus test, monitoring and diagnostics. His education includes Ph.D. in the field of HV diagnostics for circuit breakers, and ME from the RPI, New York. Raka Levi, Dr.Eng.. ， During his career, Dr. Levi has provided consulting service to utility clients in Europe and USA in the field of diagnostic testing, monitoring and investigations as a part of comprehensive substation condition assessment programs. He has performed and evaluated numerous diagnostic HV tests on transmission equipment and power generators, including large HV power transformers rated up to 400kV and breakers up to 550kV. He is also specialist for dynamic testing of circuit breakers, and on-line condition monitoring and evaluation for large HV apparatus. Raka Levi started a European technical committee on HV diagnostic and maintenance, and coordinated its activities as the convener. He has been running committees that assemble asset managers and operations specialists of major European utilities since 1995. is a senior test engineer at IBEKO Power AB, Sweden, working as technical support engineer responsible for power transformers test equipment. Since he received his BSEE, he has worked in the field of HV diagnostics and testing in substations. Keith Bensley is managing director of Pacific Test Equipment Pty Ltd. He has more than 30 years experience in electrical testing, product development & marketing of electrical test equipment. * On Load Tap Changer Dynamic Resistance Measurement By Raka Levi / Al ex Cincar / Keith Bensley

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Quatek Co.,Ltd-On Load Tap Changer Dynamic Resistance Measurement

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