Sweep Frequency Response Analysis (SFRA) and Transformer Diagnostics

M5100 Product Series Doble Engineering

Knowledge Is PowerSM Apparatus Maintenance and Power Management for Energy Delivery

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IntroductionThis is an Sweep Frequency Response Analysis presentation:A tool to investigate mechanical integrity of transformersWhen: after relocation, after an incident, or to get a baseline

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Bushing failure.SFRA showed that the transformer windings were in good shape.The transformer did not need to be scrapped.

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Bridge ImpactPaint scrapes on delivery are suspicious? How did the scrapes get there, and what does the impact recorder say?Do we know if anything moved inside the transformer? SFRA helps you decide...

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SFRA is really a lot of measurements...OverviewIndividual measurement made at each frequency of interest to produce a fingerprintFrequency in MHz

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If we can measure:changes in the frequency responseof a winding caused by winding movement then we candetect winding movement.Overview of testA good repeatable SFRA measurement will detect this problemBulge in winding caused by electrical fault

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Experience: Certain Frequency Bands Indicate Different Problem ConditionsInterpretation

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Transformer: Normal ResponseThree normal open circuit responses

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One year laterAfter an incident - one year later...

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Null result has valueFour results overlaid on this graph from four different yearsClearly - the transformer has not changed1994, 1997, 1999, 2002

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Bushing FailureThere are two traces here - suspect phase of suspect unit and same phase of sister transformer

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One winding tested twice with each methodMethod ComparisonThe impulse responses are roughly OK but the huge variability loses the value of the testTwo SFRA responsesTwo impulse responses

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The VALUE of an SFRA measurement is in the Repeatability: we can avoid false positives and false negatives in decision makingThe Range and Resolution, from 10 Hz to 2 MHz, and +/- 1dB, means we can make confident diagnosesUsed as a decision making tool, SFRA can bring a lot of value in practice10. ConclusionsDecision making with respect to transformers is not an easy job: SFRA helps

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Thank You.

Any questions?

http://www.doble.com/products/SFRA_Center/

m5100support@doble.com

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Dobles Application SupportSFRA User Group meets twice a yearOn-line User ForumOn-line Client Reference LibraryOn-line resource centerTelephone application supportInterpretation of all results during first yearClient training

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Support - User Forum

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Support - on line results library

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Here are some technical questions which informed clients may askDifficult Technical Questions?

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Does your test set guarantee repeatability to +/- 1 dB?Yes - it means we can trust the resultsDoes your test set give a resolution of less than 10 Hz below 1 kHz?Yes - so we can see shorted turns and see the detail in short circuit measurementsDoes your test set measure up to 2 MHz?Yes - so we can see variation caused by tap and winding lead movementDifficult Technical Questions?

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Does your test set have an interference band width of less than 1/5 of frequency?Yes - so we minimize effects of noise at each frequency we measureDoes your test set give a response measured down to -80 dB within 1 dB?Yes - so we can see details of large HV windingsDoes your test set use three leads?Yes - to remove effects of leads on the measurementsDifficult Technical Questions?

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Does your test set use coax all the way to the bushing terminalsYes - so we can remove the effect of incorrect terminationsDo different M5100s give the same results?Yes - so we dont have to use the same test set on each transformer each timeDo results change if you move the test set or rearrange test leads?No - the results are independent test set location and lead dispositionDifficult Technical Questions?

This graph covers the 20 kHz range, with tests performed by three different test teams on three different dates with three different HP test sets.

Excellent repeatability is the conclusion

This was a 660 MVA GSUThis graph covers the 20 kHz range, with tests performed by three different test teams on three different dates with three different HP test sets.

Excellent repeatability is the conclusion

This was a 660 MVA GSU