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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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/
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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?