Sweep Frequency Response Analysis (SFRA) and Transformer Diagnostics

M5100 Product Series Doble Engineering

Knowledge Is PowerSM

Apparatus Maintenance and Power Management for Energy Delivery

2004 SFRA Presentation2

Introduction

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

Signal & Reference Leads

M5100

Shield Ground

Test lead

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Bushing failure….

This bushing failed catastrophically

SFRA showed that the transformer windings were in good shape.

The transformer did not need to be scrapped.

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Bridge Impact

Paint 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...

Overview

Individual measurement made at each frequency of interest to produce a fingerprint

Frequency in MHz

Some individual resonances

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If we can measure:

changes in the frequency response

of a winding caused by winding movement then we can

detect winding movement.

Overview of test

A good repeatable SFRA measurement will detect

this problem

Bulge in winding caused by electrical fault

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Experience: Certain Frequency Bands Indicate Different Problem Conditions

Interpretation

400kHz to 2MHz: Movement of main and tap winding leads

2kHz to 20kHz: Bulk Winding Movement Relative to Each Other, clamping structure

<2kHz: Core Deformation, Open Circuits, Shorted Turns & Residual Magnetism

20 kHz to 400kHz: Deformation Within the main and tap windings

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Transformer: Normal Response

Three phases respond differently at low frequencies

Three normal open circuit responses

Results as expected and acceptable

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One year later

Variation is clear indicator of shorted turn on A phase

After an incident - one year later...

One phase has clear inductive roll off associated with short circuit measurement

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Null result has valueDrax GT4 GEC A phase HV Tap 1

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

0 50 100 150 200

Frequency in kHz

Am

pli

tud

e in

dB

21/07/94 N to HV, #1 LV terminals earthed 20/05/97 N to HV, #1 LV terminals earthed 6/4/99 N to HV, #1 LV terminals earthed G0322 9/7/02 a1 earthed. All others floating

Four results overlaid on this graph from four different years

Clearly - the transformer has not changed

1994, 1997, 1999, 2002

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

Bushing Failure

Sister unit has exactly the same shape

The two traces are clearly very similar - can just see one trace behind the other

No variation between traces implies this phase is OK, despite bushing failure

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One winding tested twice with each method

Method Comparison

The impulse responses are ‘roughly’ OK but the huge variability loses the value of the test

Two 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 making

The Range and Resolution, from 10 Hz to 2 MHz, and +/- 1dB, means we can make confident diagnoses

Used as a decision making tool, SFRA can bring a lot of value in practice

10. Conclusions

Decision 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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Doble’s Application Support

SFRA User Group meets twice a year On-line User Forum On-line Client Reference Library On-line resource center Telephone application support Interpretation of all results during first year Client 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 ask

Difficult Technical Questions?

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Does your test set guarantee repeatability to +/- 1 dB? Yes - it means we can trust the results

Does 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 measurements

Does your test set measure up to 2 MHz? Yes - so we can see variation caused by tap and

winding lead movement

Difficult 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 measure

Does your test set give a response measured down to -80 dB within 1 dB? Yes - so we can see details of large HV windings

Does your test set use three leads? Yes - to remove effects of leads on the measurements

Difficult Technical Questions?

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Does your test set use coax all the way to the bushing terminals Yes - so we can remove the effect of incorrect

terminations

Do different M5100’s give the same results? Yes - so we don’t have to use the same test set on each

transformer each time

Do results change if you move the test set or rearrange test leads? No - the results are independent test set location and

lead disposition

Difficult Technical Questions?