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8/12/2019 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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Knowledge Is Power SM Apparatus Maintenance and Power Managementfor Energy Delivery

Tests Categories

•

Exciting Current - CORE

• Leakage Reactance - WINDINGS



Knowledge Is Power SM

Apparatus Maintenance and Power Managementfor Energy Delivery

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





Test Results (Leakage Reactance)

• 50 Hz or 60 Hz

• Influenced by windings (Impedance)

• Impedance (Per Unit)

• Winding Movement

• 3 Phase Equivalent or Per Phase





SFRA

HOW DOES IT FIT !





SFRA•

Can be applied in the same wayas exciting currents and leakagereactance.





Parallel RLC Circuit





Response of the RLC Circuit





M5100 SFRA Measures R Circuit





M5100 SFRA Measures L Circuit





M5100 SFRA Measures C Circuit





Capacitor & Inductor





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





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





HV Winding Responses





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-





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-





Typical Test Connections for a Delta-Wye unit Cont.

H1-H3 } HV Winding Tests withH2-H1 X1-X2-X3 ShortedH3-H2

Typical Test Connections





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





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





Auto Transformer w/TertCont.

H1-Y1 } H to Y Inter-Winding TestsH2-Y2H3-Y3

X1-Y1 } X to Y Inter-Winding TestsX2-Y2X3-Y3





Simple XFMR Model

L m

C UST

R m

RL- 1

L1

L2

RL- 2

RDC-1

RDC- 2

E2E1





Typical SFRA Response

l





Typical SFRA Response

d





HV Winding Responses

Sh Ci i R





Short Circuit Responses

Ph A All R





Phase A All Responses

A l i S i





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





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



l ( k k )





105

-80

-70

-60

-50

-40

-30

-20

Frequency - Hz

d B

Baseline Data (Zoom 50 kHz - 500 kHz)





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 )





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 )





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 )





104

105

106

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

Frequency - Hz

d B

Phase Data (Poor Comparison)

Ph C i D i i





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





105

-80

-70

-60

-50

-40

-30

-20

Frequency - Hz

d B

Sister Units and Poor Phase Comparison






105

-30

-25

-20

-15

-10

-5

0

Frequency - Hz

d B


Design and Configuration





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.


D ig d C fig ti





Delta Wye


Delta and W e Characteristics





102

103

104

105

106

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

Frequency - Hz

d B

Delta

Wye

Delta and Wye Characteristics

Influence of Testing Error





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





• 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





102

103

104

105

106

-120

-110

-100

-90

-80

-70

-60

-50

-40

Frequency - Hz

d B

Open Circuit Characteristics

Poor Grounding





• 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





105

106

-40

-35

-30

-25

-20

-15

-10

-5

0

Frequency - Hz

d B

Poor Grounding - Staged

Poor Grounding Field (4 Cases)





104

105

106

-60

-50

-40

-30

-20

-10

0

Frequency - Hz

d B

Poor Grounding - Field (4 Cases)

Conclusion



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.

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