Transformer Frequency Response Analysis

8/9/2019 Transformer Frequency Response Analysis

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Spring 2005 1

IntroductionFrequency response analysis (FRA) is a well-understood technique inelectrical testing. It is the ratio of an input voltage or current to an outputvoltage or current. Since the pioneering work of Dick and Erven at OntarioHydro in the late 1970s, FRA has been applied to power transformersto investigate mechanical integrity. Experience has shown how to makemeasurements successfully in the eld and how to interpret results. Thisarticle shows some typical results and how utilities are gaining bene tfrom what is, in fact, a simple test.

Frequency Response Analysis — FRA and SFRAA standard de nition of frequency response analysis (FRA) is the

ratio of a steady sinusoidal output from a test object subject to a steadysinusoidal input. Sweeping through the frequency range of interest givesrise to the S in SFRA to distinguish it from impulse methods where theresponse is estimated rather than measured.

Figure 1 shows a simple two-coil arrangement subject to windingmovement. The before and after SFRA traces are different, as shown inFigure 2.

The peaks and valleys of the

traces are called resonances andcorrespond to combinations of ca-pacitance and inductance withinthe coils. A change in resonancemust be linked to a physicalchange in inductance or capaci-tance. Hence, we can deduce therehas been a mechanical change inthe windings.

But how do we use SFRA resultsin practice? The following exam-ples are from real transformers. Itis not always necessary to have ref-erence results, although this is theideal situation. Comparison withsister units or between phases isalso possible. Phase-by-phaseanalysis is more complex andless reliable as windings are oftenasymmetrical through design andconstruction.

Figure 1 — Coil Movement

Transformer Frequency Response AnalysisAn Introduction

Feature

by Tony McGrailDoble Engineering



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Practical Results — Axial Collapse of WindingAxial collapse is also known as telescoping, where

one winding shifts relative to the other like an expand-able telescope. In this case a transformer tripped outon a fault pressure relay during a storm and threedays after a previous dissolved gas analysis (DGA)

test. The DGA showed a rise in acetylene from zeroparts per million to 84 parts per million, indicatinga major arc within the transformer. Other dissolvedgases supported this diagnosis. Electrical testing in-cluded power factor, capacitance, winding resistance,excitation, and turns ratio. All gave acceptable results.SFRA, however, gave an indication of a partial collapseon one winding. The high voltage results gave no sig-ni cant indication of a problem while the low voltageresults were suspect, as shown in Figure 3.

Figure 2 — Change in SFRA Figure 3 — LV Results for Partial Axial Collapse

Figure 4 — Results for a Good Phase (left) and Bad Phase (right)

Clearly, in Figure 3, the X3-X0 results (in red) showa shift to the right at a number of resonant frequen-cies. Natural asymmetry of the windings was ruledout as the high voltage results were acceptable andvery similar. This result was similar to a case where

another transformer had suffered a close-in fault, andsubsequent SFRA traces revealed shifts to the rightfor just one phase. This would provide a referenceset of results. Figure 4 shows SFRA results taken in1994 and 2001. The good phases are very repeatableand indicate no signi cant problem. The bad phaseshows clear and consistent shifts of resonances tohigher frequencies.



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Note that all results are between 300 kilohertzand one megahertz. A tear down of the transformershowed the damage in the B-phase, as shown in Figure5, where the blocking can be seen to have collapsed

under the winding.

Practical Results — Hoop Buckling of WindingHoop buckling, or winding compressive failure,

is a common cause of deformation in transformers.The winding is bent but not broken. There are casesof transformers with severe hoop buckling remain-ing in service for several years. They have elevatedDGA levels but otherwise perform almost normally.

Figure 5 — Damage

The problem lies with their reduced ability to with-stand further faults and the increased likelihood ofcatastrophic failure while in service. Figure 6 showshoop buckling on one phase of a mobile transformer,as found during an inspection.

The SFRA response for this transformer showeda clear shift to lower frequencies for that phase, asshown in Figure 7. Previous results were available fora sister unit that showed the variation was not due to

design or construction.The diagnosis was made before the inspection, andthe transformer returned to service during a peak loadperiod with advice that it would be more likely to failif it saw another nearby fault. The bad phase was re-wound some months later when the picture in Figure6 was taken.

Figure 6 — Hoop Buckling in Winding

Figure 7 — Effect of Hoop Buckling on SFRA Results



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Practical Results — No ChangeOne of the strengths of SFRA is in the power of a

null result. When using a good repeatable test system,a null result, which shows no variation in the traces,is very strong evidence that the windings have notmoved or deformed. Figure 8 shows ve results forone phase of a generator step up (GSU) transformertaken over the course of eight years from July 1994to July 2002. Clearly there has been no change in re-sponse, and the transformer is in unchanged condi-tion. Small variations at low frequency relate to thestate of core magnetization when the transformer wasswitched out of service, but it is clear that the mainresonances have not changed.

Key Features of SFRA in the FieldPractical experience has shown that SFRA measure-

ments are simple to make, and a consistent measure-ment may be made in the eld up to two megahertz(see McGrail and Lapworth). Above that frequencythe value of the measurement is greatly limited by thevariability of the test equipment and set up, especiallywith impulse systems, where it has been found thatsmall movements in the leads will show great varia-tion in responses (see Vandermaar).

The value in SFRA is the repeatability of results.Experience over many years has shown that a sweepsystem is the only way to extract that value from theresults. The null result showing no movement inFigure 8 would be lost if the measurement were notrepeatable to within +/- one dB. However, SFRA isonly one tool and should be used in conjunction withother electrical and diagnostic tests to paint a completepicture of transformer health.

To investigate further, please visit the Doble website(www.doble.com) where further information may be

found and examples and case studies are given.

References1. Horowitz, P., The Art of Electronics, Cambridge

University Press, Boston, 1989.2. Heathcote, M., The J & P Transformer Handbook

Newnes, London, 1998.3. McGrail, T. and Lapworth, J., “Transformer Wind-

ing Movement Detection,” proceeding of the 1999Doble Client Conference, Boston, MA.

4. McGrail, T. and Sweetser, C., “Substation Diag-nostics with SFRA,” proceedings of the 9 th EPRSubstation Diagnostics Conference, New Orleans,LA, March, 2004.

5. Vandermaar, J., and Wang, M., “Key Factors Affect-ing FRA Measurements,” IEEE Electrical InsulationMagazine, v. 20, n. 5, September/October, 2004.

For the last two years, Tony McGrail has been the M5100SFRA Product Series Manager with the Doble EngineeringCompany, based in Watertown, Massachusetts, USA. Priorto this, Tony was a transformer engineer with the NationalGrid Company in the UK. He is a Chartered Engineer in theUK, holding a Ph.D. in Electrical Engineering and a Mastersin Instrumentation. He may be contacted at Doble or [email protected].

Figure 8 — Five SFRA Results ShowingNo Signi cant Variation

InterpretationExperience has shown that the simplest form of in-

terpretation of SFRA traces is with respect to a baseline.However, it is unlikely that we will ever get a baselinefor all transformers on the electric supply network.Consequently, interpretation strategies which involvesister units or short circuit results are used. The leastreliable method is phase-by-phase analysis, due toasymmetries brought in through construction and,for example, end mounted tap-changers. Automatedapproaches have been tried, but these produce bothfalse positive results and false negative results. Simpledifference analysis and correlation are sometimes use-ful as indicators but tend to show where the engineershould investigate the original traces (see McGrail andSweetser). Doble Engineering is involved in the fore-front of research into automated analyses that bringtogether differences, correlation, and manufacturer-speci c experiences.

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Transformer Frequency Response Analysis