DIAGNOSIS. TRANSFORMERS MAGAZINE Volume 3, Issue 2

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1 DIAGNOSIS 26 TRANSFORMERS MAGAZINE Volume 3, Issue 2

2 ADVERTORIAL A test instrument must effectively overcome measuring complexities that are present in a live power system test environment and must consistently deliver representative results Exploring multifunctional in the realm of transformer diagnostics Acquiring diagnostic insight into the condition of an asset through testing has several dimensions. Foremost, testing must be safe. Having diagnostic information should deliver positive net value when weighed against the cost of its acquisition. A test instrument must effectively overcome measuring complexities that are inherent byproducts of a live power system test environment and must consistently deliver representative results. Central to strengthening a test(s) s value proposition is to reduce its costs and maximize its value. The savings supplied by multifunctional instruments have spurred their popularity and are considered in this article. Megger s TRAX multifunctional instrument, specifically, not only delivers on cost reduction but delivers big on maximizing value too. True to the saying the devil is in the details, technically differentiating features of the TRAX strengthen its value proposition, particularly with regard to transformer diagnostics. These technical nuances and their importance will be summarized. Savings in multifunctional The resultant cost savings is a compelling reason to move away from separate instru ments in favour of a multifunctional test set. These include: 1. Easier usability. Less operator training is required; users of separate instruments need to familiarise themselves with the quirks of each, whereas users of well-designed multifunction test sets enjoy a consistent user interface across all functions, which means that the learning process is simplified. 2. Easier manageability and transportability. Users of multifunction instruments always have all of the test facilities they need readily at hand; there is never a need to go back to the van or, worse, back to base to fetch another instrument for the next test. Plus, a single multifunction tester is much easier to transport than several individual instruments. 3. Lower upfront side-by-side, test for test capability costs. Multi-functional test sets cost less than the individual instruments that would be needed to cover the same range of testing requirements. Four single-function test instruments = four displays, four user interface systems, four enclosures, etc. = higher total cost than one multifunction instrument with one display, one UI system, one enclosure, etc. 4. Time savings. Multifunction test sets provide on-going savings by reducing testing time. There is only one instrument to unpack, power up and configure; the same cable set is used for a whole range of meas- urements, so the connections only need to be made once; and, when carrying out a range of tests, users of multifunction instruments move quickly and easily from app to app, rather than having to go from instrument to instrument. Transformer diagnostics with Megger s TRAX multifunctional test set An extensive number of electrical field tests may be performed on a transformer with the Megger TRAX multifunctional test instrument, including: Single or three-phase (fully automated 3ф with TSX300) measurements of: Turns ratio Winding resistance Dynamic resistance measurements on on-load tap changers (OLTC) Adaptive demagnetization Exciting current Leakage reactance/short-circuit impedance Frequency response of stray losses Zero-sequence impedance HV tan delta/power factor (with the TDX) Dielectric frequency response (1 500 Hz) Individual temperature correction (ITC) of tan delta/power factor Automatic voltage dependence de- 27

3 DIAGNOSIS tection and power factor tip-up when indicated Can be used with external test transformer and standard capacitor for EHV testing Megger s TRAX multifunctional instrument, specifically, not only delivers on cost reduction but delivers big on maximizing value too Together these testing capabilities allow for the assessment of the transformer windings and winding leads, the insulation, the core, and accessories (tap changers, bushings, bushing CTs and surge arresters) the testing of all of which ultimately will verify the transformer s whole condition. One might imagine that combining this much functionality into one instrument risks an oversized instrument with unwelcome complexity. Navigating around potential multifunction pitfalls Attention to balance, in particular, is paramount when combining multiple tests into one instrument. Desirable outputs/ sources must be provided while avoiding excessive weight. Easy usability must be preserved while providing ample versatility. More specifically, a multifunctional test instrument design must: be capable of generating high currents and voltages yet remain easy to transport. For users with interests that cover a wide geographical area, it is highly desirable for the instrument to weigh less than the international maximum shipping weight of 32 kg for check-in luggage on passenger flights. alleviate the potential conflict between versatility and ease of use. There s little point in producing a multifunction instrument that can perform a wide range of tests if many of the tests are difficult to access and set up. This only leads to user frustration and ultimately dissatisfaction with the product, however impressive its claimed abilities may be. Megger s TRAX, the main section of which weighs just 26 kg, excels in attaining this balance. The outputs, discussed in part below, are pragmatic. The user interface of the TRAX, which utilizes the latest colour touch-screen technology, presents functions in the form of apps ( virtual instruments ). When the user has decided what to measure and has selected the app/instrument to work with from the start screen, the display shows only those elements that are appropriate to that function. Provision is also made for full manual testing with a generic instrument app that allows the user to freely select outputs, measurement inputs and the way in which the measured data should be processed. When it comes to transformer testing, as one example, the specific technical features of a multifunctional instrument also determine the magnitude of savings provided as well as the degree to which the value of the results are enhanced. Several technically differentiating features of the Megger TRAX are instrumental in strengthening its value proposition. 28 TRANSFORMERS MAGAZINE Volume 3, Issue 2

4 Further increasing the value proposition with Megger s TRAX multifunction unit Technical features that augment savings: To increase test efficiency, Megger s TRAX switchbox maximizes the number of transformer diagnostic tests that can be performed with one time lead connection to each bushing terminal. Rather than multiple ladder climbs to rotate lead connections after each single-phase measurement, the switching of connections between the TRAX and each successive phase is automated within the switchbox. After each of the three phases has been tested, it s onward to the next test with no lead changes required. Not only does minimizing ladder climbs reduce overall testing time, it also reduces exposure to fall hazards. Turns ratio, winding resistance, dynamic winding resistance, exciting current, leakage reactance, frequency response of stray losses and zero sequence impedance measurements can all be performed through the use of Megger s TRAX switchbox. The efficiencies of a switchbox are verifiable but here, too, balance is important. Switchbox cables to each bushing terminal must be manageable, safe and affordable while the ratings of these leads remain practical for the intended tasks at hand. This is particularly important for winding resistance measurements. Megger s TRAX switchbox maximizes the number of transformer diagnostic tests that can be performed with one time lead connection to each bushing terminal Winding resistance tests are used to assess the integrity of the current carrying path between bushing terminals, detecting problems such as loose or defective connections, broken strands or high contact resistance(s) in tap changers. While simple in theory, the true resistance of the energized winding path is not immediately attainable. It is necessary to first saturate the transformer core in order to reach stability and obtain the actual winding resistance. The speed at which core saturation is reached is the cumulative effect of both applied current and voltage. There is no universal magic current threshold with which to perform a winding resistance measurement. A particular source will be sufficient to saturate the core quickly for one transformer while, for another, the same source may not be adequate. In addition, the magnitude of test current should not exceed 15 % of the current rating of the winding so that heating of the winding is avoided. When choosing a current source, the compliance voltage is also important. The compliance voltage is the maximum voltage a current source will go in its at tempt to source the programmed current. A high compliance voltage is desirable. The Megger TRAX provides up to 100 A true DC at up to 50 V compliance voltage, thus 29

5 DIAGNOSIS Improved test measurements and methods, such as true dynamic resistance measurements, adaptive core demagnetization and elevated insulation diagnostic capabilities, strengthen the value proposition of the Megger TRAX securing fast and stable winding resistance readings for any size transformer. To the point of balance in the design of a switchbox, the TRAX switchbox provides up to 16 A DC per phase winding which is sufficient to saturate most cores while delivering all of the conveniences of automated switching between tests. TRAX has multiple built-in safety features to protect the end-user, the test object and the unit itself, including auto-discharge in the event of input power loss. Technical features that boost value: The value proposition of the Megger TRAX is strengthened by improved test measurements and methods. One such test measurement (available with the TRAX) is true dynamic resistance meas- urements on OLTC s (on-load tap changers), whereby current, voltage and calculated resistance are plotted as functions of time during the switching operation. Resistance [e.g., of the winding + diverter resistors] is calculated using a Megger patent pending technique. In addition to the dynamic measurement (including timing) of the switching sequence, TRAX is also automatically monitoring the continuity during the operation sequence and immediately reacts if the test current is interrupted by a malfunction in the OLTC. A noteworthy example of a value-boosting method introduced in the TRAX is its adaptive algorithm for fast and efficient demagnetization of the transformer core. After completing DC winding resistance testing, it is recommended to perform demagnetization before the transformer is put back into service, thus avoiding unnecessary high in-rush currents. Often, when a transformer trips off-line, or after applying DC test signals in, for example, a winding resistance test, the transformer core remains magnetized. All open-circuit AC tests on transformers (e.g. exciting current, SFRA, and to some extent even turns ratio), wherein a transformer is being excited and transformer action occurs, may be influenced by a saturated transformer. Demagnetization of the transformer is necessary to get representative results but not all demagnetization methods are created the same. The Megge r TRAX method, which adapts a demagnetization cycle unique for the specific transformer design and size, is effective and fast, minimizing the time needed for a successful demagnetization. Finally, from an insulation diagnostic perspective, the TRAX elevates power factor/ tan δ measurements to a far more insightful platform, with the same differentiating features employed by the Megger s Delta 4000 series dedicated insulation power factor/dissipation factor test set. It does 30 TRANSFORMERS MAGAZINE Volume 3, Issue 2

6 Not only is Megger s TRAX an extraordinarily balanced multifunctional instrument, it is packed with technically differentiating features that boost asset condition awareness this in several important ways. With the capability of generating an AC voltage output ranging from Hz, dielectric frequency response (DFR) measurements can be made. Pub lications are available that describe this diagnostic test in more detail. Essentially, since it is now known that a line frequency (50/60 Hz) power factor/tan δ measurement is not sufficiently sensitive to changing levels of contamination, DFR testing from Hz confirms when seemingly good power factor values actually are good, and reveal when they are not, thereby enabling earlier detection of a problem(s) in the dielectric and providing planning opportunities. A DFR test measurement with the TRAX allows for the determination of an insulation system s unique, or individual temperature correction (ITC). ITC is a patented technique of using frequency data for estimating the actual temperature dependence of the test object. The temperature dependence of the power factor/tan δ of an insulating material needs to be considered when comparing measurement results with previous tests or factory value s. Temperature can influence the power factor/tan δ measurement significantly and the conventional methods to normalise every test result to a common base for comparison are inaccurate. Testing has revealed that not only does every transformer exhibit unique sensitivity to temperature and require individual temperature compensation, but, over its life, the temperature dependency of a transformer can change. Generally, as insulation deteriorates an increase in temperature causes power factor to increase dramatically. Megger s ITC provides the tester with the means to accurately correct power factor/ tan δ results to a 20 C base given any of a wide range of temperatures that the test may be most conveniently performed at, and also gives provisions to make a correct comparison to previously measured, noncorrected power factor/tan δ data at other insulation temperatures. Another noteworthy example of a valueboosting method included in the TRAX and pertaining to insulation diagnostics is a patented technique known as (automatic) voltage dependence detection (VDD). Since some problems are revealed by their changing responses as the applied test voltage varies, repeating power factor/ tan δ tests at an additional test voltage(s), known as power factor tip-up tests, may add value to a test program. However, unless the subject asset is at higher risk for developing voltage dependent problems, the industry largely foregoes this test because of the additional time it requires. VDD is a solution, alerting users when power factor/tan δ results indicate that the test object may have a voltage dependence, and prompting users to then perform power factor tip-up tests. Summary The value proposition of off-line electrical testing, such as for transformer diagnostics, is strengthened as the costs associated with testing are reduced and the diagnostic reach of the informa tion grows. Multifunctional instruments, where by the testing capability of multiple instruments is included in one, are increasing in popularity because of costs savings, including those delivered by easier usability, man ageability and transportability, lower equivalent upfront costs, and reduction in test time. Balance is important with these instruments. The drive to include more testing capability (i.e. multiple power sources et al.) should not come at the expense of instrument weight and size, and its ease of use. While the power source s should not be overbuilt, they must be adequate for the test at hand. A switchbox associated with a multifunctional instrument provides time savings in testing by minimizing the overall number of ladder climbs to complete testing. A switchbox is a junction point between the test instrument and the test specimen that determines which track the train is routed so that connections to bushing terminals do not have to be moved after completion of each single phase test. Balance is important here too. Switchbox cables to each bushing terminal must be manageable, safe and affordable while the ratings of these leads remain practical for the intended tasks at hand. This is particularly important for winding resistance measurements. It is desirable, from a time perspective, to have a switchbox facilitate as many tests as possible. Not only is Megger s TRAX an extraordinarily balanced multifunctional instrument, it is packed with technically differentiating features that boost asset condition awareness. True dynamic (winding) resistance measurements, TRAX s adaptive demagnetization procedure, and elevated insulation diagnostic capabilities through DFR, ITC and VDD are some examples. For more information, visit megger.com/trax-tm Author Jill Duplessis is the Global Technical Marketing Manager for Megger. She was previously Director of Power Programs for SmartSenseCom, Inc. where she was responsible for developing, testing and deploying new applications of SmartSenseCom s optical monitoring systems for transmission and distribution utilities. She has over 17 years of experience in the condition assessment of substation assets, first as a Principal Engineer at Doble Engineering Company and following as a Primary Manager and Regional Application Specialist, Transformers, for OMICRON electronics USA. Jill is considered a specialist in power transformer diagnostics, including emerging test technologies, and in addition to a number of papers, is the author of a book titled Electrical Field Tests for the Life Management of Transformers. She has prior electric utility work experience, with exposure to a wide breadth of engineering functions. Jill received a BSEE degree from Georgia Institute of Technology in 1991 and has completed extensive studies in electrical power engineering at Rensselaer Polytechnic Institute, Troy, NY. 31