sfra training
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Knowledge Is Power SM Apparatus Maintenance and Power Management
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Transformer Properties
Dielectric Strength (Insulation)
Thermal Heating (Loss of Life)
Mechanical Strength
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Obvious Problem
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Good or Bad
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Current Techniques
Sweep Frequency Response Analysis (SFRA)Leakage Reactance
Capacitance*Excitation Current (Core Integrity)These independent diagnostic methods havetheir place in ascertaining transformer condition
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Diagnostic Tests
DIELECTRIC THERMAL MECHANICAL
DGA DGA SFRA
Oil Screen Oil Screen Leakage Reactance
Power Factor Thermal Imaging Capacitance
Capacitance DC Winding Resistance Exciting Current
Exciting Current DC Winding Resistance
TTR
Partial Discharge
Dielectric Spectroscopy
AC HipotDC Hipot
Insulation Resistance
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Purpose
Assess Mechanical Condition of Transformers(mechanical distortions)Detect Core and Winding Movement
Due to large electromagnetic forces from faultcurrentsWinding Shrinkage causing release of clamping
pressureTransformer Relocations or Shipping
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Purpose
Simply Stated: To measure the frequencyresponse of passive elements (RLC) for varioustypes of power apparatus. The result is a transferfunction which produces a fingerprint related tothe mechanical geometry for a given apparatus(i.e. transformers, reactors, generators, andmotors).
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FRA History
1960: Low Voltage Impulse Method was first proposed byW. Lech & L. Tyminski in Poland for detecting transformerwinding deformation.
1966: Results published as “Detecting Transformer Winding
Damage - The Low Voltage Impulse Method” by Lech &Tyminski in The Electric Review, ERA, UK
1976: Frequency Domain Analysis of Responses From L.V.I.Testing of Power Transformers by A.G. Richenbacherpresented at the 43rd Doble Int’l Client Conference
1978: Transformer Diagnostic Testing by FrequencyResponse Analysis, published by E.P. Dick & C.C. Erven,Ontario Hydro in IEEE Transactions of Power Delivery.
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FRA History Cont.
1978: Ontario Hydro develops the FRA test.
1988 - 1990: Proving trials by European utilities, thetechnology cascades internationally via EuroDoble andCIGRE.
1991 to Present: Results & Case Studies are publishedand presented validating the FRA method.
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FRA Methods
Sweep Frequency (SFRA)Apply a 10 volt peak to peak sinusoidal signal from anetwork analyzer (M5100) to a winding and measure thesignal input/output from other end of same winding, orfrom another winding as function of frequency
Low Voltage Impulse (LVI)Apply impulse to a winding and measure input &response signals using high speed digitizer. Perform FFTcalculation to convert time domain into frequency domain
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Sweep Frequency Method
out
in
out
in
V
V H
V
V dB H
1
10
tan)(
log20)(
Frequency Range10 Hz - 10 Mz
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Sweep Method Test Diagram
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Impulse Method
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SFRA Fundamentals and Concepts
RLC Circuits(Series, Parallel, S/P, Lumped, Distributive)Frequency Domain(Fourier(jw), Laplace(s), DFT, FFT)
Two Port Networks(input/output admittance, transferadmittance)Transfer Function
(Magnitude, Phase, Bode Plot)Vmeasure (jw) = H(jw)*V source (jw)-20*log 10 (Vsource /Vmeasure ), dB
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Transfer Function Principle
The RLC Network offers differentimpedance paths at different frequencies.
The transfer function at each frequency is ameasure of the effective impedance of theRLC network.
Changes in geometry due to deformationalter local RLC values and so modify thetransfer function at various frequencies.
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Define Parameters• Exciting Currents
• Leakage Reactance
•
Sweep Frequency Response Analysis
LOOK AT SIMPLE MODEL
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Tests Categories
•
Exciting Current - CORE
• Leakage Reactance - WINDINGS
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Test Results (Exciting Currents)
• 50 Hz or 60 Hz
• Influenced by core
• Current Magnitude, Power Loss (Eddy Currents) at10 kV.
• Turn to Turn, Core Grounding, Core Faults
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Test Results (Leakage Reactance)
• 50 Hz or 60 Hz
• Influenced by windings (Impedance)
• Impedance (Per Unit)
• Winding Movement
• 3 Phase Equivalent or Per Phase
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SFRA
HOW DOES IT FIT !
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SFRA•
Can be applied in the same wayas exciting currents and leakagereactance.
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Parallel RLC Circuit
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Response of the RLC Circuit
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M5100 SFRA Measures R Circuit
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M5100 SFRA Measures L Circuit
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M5100 SFRA Measures C Circuit
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Capacitor & Inductor
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Transformers are R-L-C Circuits
C H C H C H C T C T
R H L H R H
R L L L
C HL C HL C HL
C L C L C L C T C T
HV
Winding
LV
Winding
Inter
Winding
L H
R L L L
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Frequency Band Sensitivity
Experience to date shows that certain bandsindicate different problem conditionsLow Frequencies (<5 kHz) - scan sensitive to coredeformation, open circuits, shorted turns & residual
magnetismMid Frequencies (10 kHz > freq < 600 kHz) - scan issensitive mainly to bulk winding movementsHigh Frequencies (> 750 kHz) - scan is sensitive to
movement of inter-connections, winding leads, LTCand DETC connections.Special note for short circuit tests
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HV Winding Responses
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Testing Preparations
Transformer should be disconnected fromsystemTransformer should be in normal service
condition (assembled, oil-filled)Make connections as determined bywinding configuration. Phases not under
test are left floating.
Typical Test Connections for a Delta-
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Typical Test Connections for a Delta-Wye unit
H1-H3 } HV Winding TestsH2-H1H3-H2
X1-X0 } LV Winding TestsX2-X0X3-X0H1-X1 } Cross-Winding TestsH2-X2H3-X3
Typical Test Connections for a Delta-
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Typical Test Connections for a Delta-Wye unit Cont.
H1-H3 } HV Winding Tests withH2-H1 X1-X2-X3 ShortedH3-H2
Typical Test Connections
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Typical Test ConnectionsAuto Transformer w/Tert
H1-X1 } Series Winding TestsH2-X2H3-X3X1-H0X0 } Common Winding TestsX2-H0X0X3-H0X0Y1-Y3 } Tert Winding TestsY2-Y1Y3-Y2
Auto Transformer w/Tert
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Auto Transformer w/TertCont.
H1-H0X0 } Short Circuit TestsH2-H0X0 X1-X2-X3 ShortedH3-H0X0 H0X0 Floating]
H1-H0X0 } Short Circuit TestsH2-H0X0 Y1-Y2-Y3 ShortedH3-H0X0 H0X0 Floating
Auto Transformer w/Tert
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Auto Transformer w/TertCont.
H1-Y1 } H to Y Inter-Winding TestsH2-Y2H3-Y3
X1-Y1 } X to Y Inter-Winding TestsX2-Y2X3-Y3
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Simple XFMR Model
L m
C UST
R m
RL- 1
L1
L2
RL- 2
RDC-1
RDC- 2
E2E1
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Typical SFRA Response
l
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Typical SFRA Response
d
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HV Winding Responses
Sh Ci i R
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Short Circuit Responses
Ph A All R
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Phase A All Responses
A l i S i
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Trace Comparison • Baseline Data• Sister Unit Data• Phase
Case Study Comparison• Identify Known Failure Mechanisms
Expected Characteristic Comparison• Identify Any Unexpected Behavior
Analysis Strategies
T C i
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Baseline Data The comparison of baseline data provides the most effectivemethod for determining change. SFRA traces for a givenmeasurement should overlay. Magnetization of the core mayaffect the comparison at low frequencies.
Sister Unit Data Sister data should overlay well. Some variation may exist athigher frequencies (> 500 kHz), however the respective tracesshould be similar. Low frequencies are also influenced bymagnetization.
Phase DataSubjective - Is not always effective in some cases.
Trace Comparison
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l ( k k )
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105
-80
-70
-60
-50
-40
-30
-20
Frequency - Hz
d B
Baseline Data (Zoom 50 kHz - 500 kHz)
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102
103
104
105
106
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
Frequency - Hz
d B
Sister Unit Comparison
Ph D (G d C i )
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102
103
104
105
106
-80
-70
-60
-50
-40
-30
-20
-10
0
Frequency - Hz
d B
Phase Data (Good Comparison)
Ph D (F i C i )
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104
105
106
-100
-90
-80
-70
-60
-50
-40
-30
-20
Frequency - Hz
d B
Phase Data (Fair Comparison)
Ph D (P C i )
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104
105
106
-100
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-50
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0
Frequency - Hz
d B
Phase Data (Poor Comparison)
Ph C i D i i
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How do w e know that poor phase com pari son m aybe norm al?
By assuming that new two sister units are in good
condition. If two sister units overlay well, but poorphase comparison exists, then poor phase comparisonis normal.
General Observation: The center phase has the mostdiscrepancy.
Phase Comparison Determination
Si t U it d P Ph C i
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105
-80
-70
-60
-50
-40
-30
-20
Frequency - Hz
d B
Sister Units and Poor Phase Comparison
Sister Units and Poor Phase Comparison
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105
-30
-25
-20
-15
-10
-5
0
Frequency - Hz
d B
Sister Units and Poor Phase Comparison
Design and Configuration
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Delta - The transfer function includes all phases for agiven winding. The measurement includes all phases ina series parallel combination. Example (H1 - H3)
Za||(Zb + Zc)
Wy e - Transfer impedance is isolated to the phase ofinterest.
Observation - Characteristics for each configurationcan be observed only on HV windings, and they areisolated to the low to mid frequency range.
Design and Configuration
D ig d C fig ti
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Delta Wye
Design and Configuration
Delta and W e Characteristics
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102
103
104
105
106
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-90
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-50
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-30
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0
Frequency - Hz
d B
Delta
Wye
Delta and Wye Characteristics
Influence of Testing Error
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Open Circuits• Test Leads (Obvious)
• Internal to Specimen* (Appears Capacitive)
Short Circuits• Rarely Occurs
Poor Grounding• Test Leads (Poor or No Connection - Paint)• Bushing Flange* (Floating)
* Not a testing error
Influence of Testing Error
Open Circuits
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• Open circuits caused by faulty test leads produce anobvious sign, which is best described as an unstableresistive trace.
• Open circuits that occur within the test specimen have atendency to be more stable than open circuits within testleads, because additional distributive RLC elementswithin the test specimen provide alternative distributivereturn paths, thus creating a different stable transferfunction.
Open Circuits
Open Circuit Characteristics
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102
103
104
105
106
-120
-110
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-90
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-50
-40
Frequency - Hz
d B
Open Circuit Characteristics
Poor Grounding
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• The trend of poor grounding causes the SFRA scan toshift in the 300 kHz – 500 kHz range.
•
Since winding scans are collected in groups of three,poor grounding is easily identified if one of the tracesmisbehaves as describe above.
• Check test lead ground connections first, theninvestigate the bushing flange if necessary.
Poor Grounding
Poor Grounding Staged
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105
106
-40
-35
-30
-25
-20
-15
-10
-5
0
Frequency - Hz
d B
Poor Grounding - Staged
Poor Grounding Field (4 Cases)
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104
105
106
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-50
-40
-30
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-10
0
Frequency - Hz
d B
Poor Grounding - Field (4 Cases)
Conclusion
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Conclusion
Interpretation of SFRA results is often subjective.Comparing traces, identifying faults, and verifying to knownexpectations are methods for analyzing SFRA data.
Various transfer function characteristics can be associatedwith transformer design and test technique.
It is important to understand proper test application. Opencircuits, short circuits, and poor grounding should berecognized and the source identified.