FINDINGS AND REPAIR OF WESTINGHOUSE UTT AND McGRAW 550 TAP CHANGERS

Transcription

1 FINDINGS AND REPAIR OF WESTINGHOUSE UTT AND McGRAW 550 TAP CHANGERS ABSTRACT Chad Tremaine, First Energy Corp. Goran Milojevic, DV Power Raka Levi, DV Power Toledo Edison had several issues with tap changers indicated by DGA results. Toledo Edison opened the tap changer looking for the source of the gasses, this eventually lead to performing a dynamic DC current test. The results of the tests indicated a problem with contacts and transfer switches. Resistance values were not consistent, while dynamic graphs showed significant ripples, which pointed to bad contacts. Following a detailed investigation of the UTT Westinghouse transfer switches, the laminates were found to be cracked and contact surfaces were worn to a significant degree. After the transfer switches were replaced, a retest was performed to verify the quality of the maintenance work. The McGraw 550 tap changer had an issue with the burned surface of stationary contacts, this was also detected through the dynamic test. INTRODUCTION FirstEnergy Corporation proudly serves 6 million customers in 10 different regulated companies across 6 states. In total we have 14,270 substations with 6,413 transformers providing reliable transmission and distribution services. Of those transformers only 2,014 transformers have an OLTC. FirstEnergy Corporations is focused on being a forward-thinking electric utility powered by a diverse team of employees committed to making customers lives brighter, the environment better, and our communities stronger. PROBLEM DESCRIPTION The Westinghouse tap changer model UTT had increasing DGA gas results, pointing to a problem with overheating in the tap changer compartment. Toledo Edison had the OLTC opened several times looking for the source of the gassing and the investigation was not conclusive. Toledo Edison decided to perform a dynamic test and try to pinpoint the cause of the increased dissolved gases. A continuous test and step by step tests were performed on all phases in April The McGraw tap changer model 550 had a history of high gas content. It operates about 10 times per day. Over the past year the ratio of Ethylene to Acetylene was very high in the order of Toledo Edison replaced the oil in February of 2016 and the gas sampling pointed to increased ratio Ethylene/Acetylene of 8.4 after only 30 days of operation. We have decided to perform the dynamic recording test also known as a DVtest, and in October of 2016 Toledo Edison preformed a complete battery of dynamic tests obtaining resistance values and dynamic graphs.

2 TAP CHANGERS The two types of On Load Tap Changers (OLTC) we are dealing in this paper are two different designs. One is an arcing tap switch the other with selector and transfer switch. Let us provide a brief description of various constructions. OLTC s can be divided by their construction into several groups: PLACEMENT: on-tank or in-tank ARRANGEMENT: diverter switch or selector switch SWITCHING PRINCIPLE: reactor or resistor As far as placement, most of USA s OLTCs are placed on-tank where a separate compartment is placed on the main transformer tank. A barrier separates the OLTC oil from the main tank oil. Both selector and switching elements are in the same oil. The in-tank design usually has a selector in the main tank oil while the diverter switch is in a separate sealed compartment placed inside the main tank, but with its own oil where it does not mix. Arrangement of the OLTC is based on several principles. Either the same contact performs the function of selecting the tap and switching the current, or there are separate contacts for each function. In the first case it is called an arcing tap switch or a selector switch, as it does both functions. The other arrangement has separate contacts where the selector only connects with the taps, while the diverter switch or a transfer switch performs all the current interruption and switching. The difference should be made between the selector switch and selector, as this terminology causes confusion. For that reason we will use the selector and arcing tap switch terms. There are modern vacuum type tap changers where a new term is introduced: the bypass switch. In that particular case all the switching is performed inside the vacuum bottle while the bypass switch opens and closes the circuit without current interruption and no arcing, further the selector is selecting the taps while disconnected by these bypass switches, thus not carrying any current and not arcing. The switching principle divides tap changers into two philosophical groups: resistor, fast tap changers, and reactor or slow tap changers. The principle of changing taps while the transformer is fully loaded was patented by Dr. Jansen in Today the principle is still applicable: make before break! The connection to the next tap should be made before the connection from the previous tap position is open (broken). During this period of time when both taps are connected to the transformer output, we have a portion of the regulating winding shorted. To minimize the short circuit current through this portion of the transformer, winding resistors are used in the resistor types, and a reactor or a preventive autotransformer (PA) for reactor types. One benefit of reactor construction is possibility to operate indefinitely in the bridging position, while the resistor tap changer slides through this state within few milliseconds. TAP CHANGERS IN QUESTION Both of the tap changers we will be dealing with in this paper are of reactor type, and both are mounted in the on-tank construction. The difference is that the McGraw 550 is an arcing tap switch design, while the Westinghouse UTT is a selector with transfer switches. Also, the McGraw has two stationary contacts per tap, while the Westinghouse unit has one fixed contact per tap. Two principles of motion are applied here: with one set of contacts per tap the one single moving arm with two separate contacts connect the taps. On the contrary, with two fixed contacts per tap, there are two moving arms that move alternatively controlled by a Geneva gear. All this is of big importance for analysis of the DVtest graphs that will be explained in the next section. We will concentrate in this paper on reactor type tap changers only, and will not deal with the resistor types as this was explained in other presentations [1,2]. 2 of 15

3 DYNAMIC RECORDING MEASUREMENT (DRM) This test method, also incorrectly known as a dynamic resistance measurement (same DRM acronym) is an off-line, non-destructive test in which a DC current is injected through the winding and tap changer as it moves through all tap positions. Results from the current signatures recorded with high sampling rate of 10 khz are examined and compared against previous tests or similar unit test results. This test may be used to detect tap changer problems such as slow transition time, mechanical or contact problems, and open circuits, among many others. The advantage of this method, also known as a DVtest is an advanced algorithm incorporated to allow testing and evaluation of tap changers connected through a series transformer [5]. The simplest application would be performing a continuous recording. A DC current of Amps is injected through the winding from the bushing through the tap changer to the neutral bushing. Connection is identical to a winding resistance measurement. The current measurement is recorded continuously with high resolution as the tap changer moves through all tap positions. Motor current is also monitored and recorded during the test, and this signature is very useful for the mechanism and motor control diagnostics. Typical DRM Graph of a Reactor Tap Changer Going Through 2 Positions Figure 1 The DRM current trace in the graph of Figure 1 shows key points of the tap changer operation identified by the sudden current change we call ripples. These ripples should be consistent and clean straight lines without bouncing marks. Length of ripples depends on the transition: is it bridging to non-bridging or reverse, and if the tap changer is moving from position 16L to 16R or in the opposite direction. Figure 1 shows that the ripple of bridging to non bridging transition is longer while the other ripple is shorter in length, due to the different switching phenomena with inclusion in one, or shorting the PA in the other case. The initial small ripple is a current change due to the start of the contact motion. The third ripple on the graph of a single transition is the moment when the moving contact is connecting with the next tap. As the graph shows, bridging to non-bridging and non-bridging to bridging transitions are similar but not identical. The next procedure is a little more complex and it is known as a step-by-step test. The resistance readings and the transition graphs are recorded alternately as the tap changes from one position to the next. The plot in Figure 1 shows the current signature, the recording is for a specified period of time (usually 10 seconds) once a tap change is detected. The next step is a measurement of the winding resistance at the new tap position. The recording picks-up again after the user is satisfied that the current has again reached stabilization and the winding resistance is measured and recorded. The black vertical dashed line in Figure 1 is where the recording ends and resumes after the unspecified period of time during which current re- 3 of 15

4 stabilizes. The DRM data collected should serve as a benchmark for future tests, as signatures are very repeatable. Winding resistance of all tap positions can be plotted as well, this shows a characteristic pattern of values alternating (bridging to non bridging) and changing direction following a neutral position, as it is a case with plus minus regulation. As most OLTC in the USA have 33 positions, the middle or neutral one is the position 17 (or in case 16R to 16L, the neutral is marked N). The graph below in Figure 2 shows a sample of winding resistance values for all three phases Ran [mω] Rbn [mω] Rcn [mω] Winding Resistance Values for All Tap Positions Figure 2 Several typical problems were found using this DVtest method and reported at various presentations. Contact coking is a very common problem in hot oil-copper or even silver contact-plating environments, detected by small ripples in the trace [3]. The mechanism is also a problematic assembly of this component - the only moving part under voltage in a power transformer. DVtest has shown several defects in the mechanism or motor control [4] of OLTC by analysis of motor current trace in conjunction with the test current DRM trace. FIRST ENERGY TAP CHANGER CASES DIAGNOSED USING DRM CASE 1 Westinghouse Type UTT Tap Changer Clyde Substation averages 7.18 MVA over a 12 month span to part of the city of Clyde, Ohio. TR #1 is rated for 14 MVA and tested regularly by the use of a DGA test. The OLTC type UTT is manufactured by Westinghouse, a reactor type tap changer with transfer switches that arc in oil. The results of the DGA pointed to overheating contacts. Investigation included lowering the oil, opening the LTC and visually inspecting the tap changer. The DRM test was performed at 25A, testing each tap on each phase of TR #1. The graphs of winding resistance measured, and dynamic trace of phase A are given in the graphs of Figures3, 4 and 5 below. 4 of 15

5 Winding Resistance of All 3 Phases Figure 3 As it can be observed from the results plotted in the graph of Figure 3, certain contacts measured higher resistance in the order of 3% above the expected values for that particular tap position. Phase A DRM Trace Figure 4 In Figure 4 shown above is of a continuous test recorded from phase A. The arrows are pointing to the ripples that were not what a clean and proper transition ripple should be. The Figure 5 below zooms into the transition and ripple shapes. 5 of 15

6 The 6 Transitions: 3 B>NB and 3 NB>B Figure 5 We have used the overlay functionality of the DV Win software to compare graphs in Figure 5 of transitions associated with the same contacts. Although these tap positions are different, they use the same contacts as in one case the regulating winding is added and in the other it is subtracted from the secondary winding, to provide for the double number of tap positions. It can be observed that the graphs are almost identical, which is expected as the contacts leave the characteristic fingerprint each time they switch the test current. Following the repair, when the transfer switches and stationary contacts were replaced, attached are a few photographs showing the issue of the worn out parts. Moving Transfer Switch Contacts Comparison Figure 6 6 of 15

7 Moving Transfer Switch Contacts Comparison Figure 7 The photos of Figures 6 and 7, show worn out contacts of the moving parts. The difference is clear when compared to the new contact. Stationary Transfer Switch Comparison Figure 8 7 of 15

8 Broken Down Laminates Figure 9 The photos of Figures 8 and 9, show a delaminated transfer switch fixed parts old and new. The gap difference is noticeable due to laminates breaking down. Stationary Transfer Switch and Movable Contact (Old) Figure 10 8 of 15

9 Stationary Transfer Switch and Movable Contact (New) Figure 11 The photos of Figures 10 and 11, show the difference between the old stationary and movable contacts compared to the new stationary and movable contacts. The gaps between the two contacts caused arcing which lead to the increase in gassing in the LTC tank. Through testing it could be determined what was wrong before opening the LTC which saved time and effort. A retest was performed to confirm the repair was successful. The trace of the graph is now smooth and when compared to the trace from before the difference is clear. Figure 12 shows two DRM graphs overlaid of the same phase, the smooth one taken after the repair. It is obvious that the trace before the repair with all the undesired ripples, points to a bad condition of contacts that operate on all transitions where these are the transfer switch contacts. Comparing DRM Traces Before and After Repair Figure 12 9 of 15

10 CASE 2 McGraw 550 tap changer Maumee Substation averages MVA over a 12 month span to part of the city of Maumee, Ohio. TR #1 is rated for 28 MVA. The OLTC on this transformer operates about 10 times per day, this transformer is also tested regularly by the use of a DGA test. Laboratory DGA results are presented in the table 1 below, only a portion of the data is shown: the ratio of ethylene to acetylene, and the total heat generated gas. The oil from the OLTC was replaced in February and the data associated with the sampling on February the 3rd 2016 shows no data for the C2H4/C2H2 ratio, as the tap changer did not operate in the new oil before sampling. Table 1 Laboratory Analysis of Gasses from the Tap Changer Gas analysis C2H4/C2H2 ratio Total heat gas The OLTC type 550 is manufactured by McGraw Edison, it is a reactor type, with an arcing tap switch, and double stationary contacts. The DRM test was performed at a 20A test current. Both the test current and motor current were recorded. The graph of phase B (X2) is shown in the Figure 13. Phase B DRM Graph Recorded in Continuous Mode Figure 13 During the step by step testing method, a measurement of winding resistance on each of 33 taps for all three phases are recorded and these values are represented in the Figure of 15

11 Winding Resistance Values of All Three Phases in All 33 Positions Figure 14 The phase C graph showed an interesting discrepancy at transitions associated with positions 9 to 10 and 28 to 29 both associated with the same fixed tap changer contact number 6. This is shown in the Figure 15, the arrows point to the two outstanding ripples. Phase C DRM Graph with Suspicious Ripples Figure 15 Three distinctions were observed in this graph associated with transitions of contact 6. A zoomed in view of these transitions is shown in the Figure 16: 1. Current is lower on that tap indicating higher resistance 2. The ripple is longer than others 3. There is a small peak when moving starts again 11 of 15

12 Comparison of Two Transitions at the Same Contact Figure 16 Analysis of the graph points to a problem with this particular fixed contact as the other transitions are consistent, where this would eliminate a possible moving contact problem. It was decided to look into contact 6 further by removing the oil and opening the LTC. Upon opening the LTC the stationary contacts on 6 showed sever arcing along with arcing to the movable contacts. This is shown in Figure 17 and 18 below. Burned Contacts on Stationary and Movable Contacts Figure of 15

13 Burned Stationary Contacts Figure 18 The contacts were removed and replaced with new contacts both on the stationary and movable contacts. Below in Figure 19 and 20 the difference between the burned contacts and new contacts is shown. Stationary Contact Comparison as Replaced from the OLTC Figure of 15

14 Movable Contact Comparison as Replaced from the OLTC Figure 20 There was no time to run a retest due to the need for the transformer to be put back in service. The latest DGA results show improved results after maintenance was complete. CONCLUSION This is a very useful test for OLTC diagnostics as it does not require opening the tap changer, which involves draining the oil and long term delays. This allows us the advantage of knowing the source of problem which allows us the possibility of bringing replacement parts before the OLTC is opened for repair and knowing where to look for a troublesome part once the tap changer is open. REFERENCES [1] Kellie Robinson, BPA, Steve Larson, Snohomish County PUD, Marcos Ferreira, Reinhausen Manufacturing, Results from Experiences of Performing Dynamic resistance Measurements on Resistance and Reactance LTC Types, Doble Client Conference, Boston 2012 [2] R. Levi, M. Ferreira, Dynamic Resistance Measurement Applied to On-Load Tap-Changers, Weidmann Annual Transformer Conference, Las Vegas, 2011 [3] G. Andersson, R. Levi, E. Osmanbasic, Dynamic tap changer testing, reactors and reactance, CIRED, 22nd International Conference on Electricity Distribution Stockholm, June 2013, Paper [4] R.Levi, G. Milojevic, From the AMforum Knowledge-base: Case studies of OLTC problems detected by DVtest, TechCon Training Track presentation, Sacramento CA, February 2015 [5] R. Levi, V. Mrdic, Advanced Dynamic Testing of Substation Apparatus, TechCon Albuquerque, NM, USA, of 15

15 BIOGRAPHY Chad Tremaine, EE, is a commissioning engineer and substation maintenance engineer at FirstEnergy Corp., USA. He has been employed by FirstEnergy Corp. since December 2015 Goran Milojevic, MSEE, is an application engineer at DV Power, Sweden, working as a technical support engineer for North America and Western Europe. After receiving his MSc, he worked as a product manager for the high-current transformer testing instruments, including the TWA40D, the most advanced threephase winding ohmmeter in the market. He is a member of the AMforum On-Load Tap Changer Dynamic Resistance Measurement working group. He has led a DRM research project in cooperation with ESBI, Ireland. Raka Levi, Dr.Eng., is an application expert at DV-Power Sweden. He has 30 years of asset performance and condition assessment experience, specializing in apparatus testing, monitoring and diagnostics. During his career, Dr. Levi has provided consulting service to utility clients in Europe and USA in the field of diagnostic testing, and investigations as a part of comprehensive substation condition assessment programs. Seven years ago he started within the AMforum organization a working group on DRM test methodology for tap changers. For 20 years he has been running committees that assemble asset managers and operations specialists of major European utilities, organizing conferences in Europe, TC Universities in USA, and TC Colleges in Asia. He has written over 25 technical papers on the subject of electrical testing, transformers, OLTCs, and breaker diagnostics and condition monitoring. His education includes Ph.D. in the field of HV diagnostics for circuit breakers and Diploma of electrical engineering, both at the U. of Belgrade, and ME in electric power from the RPI, New York. 15 of 15