7-Prevost FRA.pdf

Diagnosis of Winding Faults with Frequency Response Analysis in Power Transformers Conference on Electrical Power Equipment Diagnostics Bali, Indonesia Thomas Prevost

Theory Frequency Response Analysis (FRA) > Powerful and sensitive method to evaluate mechanical integrity of core, windings, and clamping structures within power transformers > Power transformers are complex electrical networks of capacitances, inductances and resistors > Geometrical changes in this network cause deviations of frequency response

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windings

core

tank wall

Theory Frequency Response Analysis (FRA) > FRAnalyzer performs Sweep Frequency Response Analysis (SFRA) > Measurement of electrical transfer functions over a wide frequency range > Worldwide proven method for measurements in frequency domain > Evaluation of transformer condition by comparing SFRA results to reference results 0 Magnitude in dB

> Different failures are directly related to different sections of the frequency range and can usually be discerned from each other

-20 -40 -60 -80 101

Interaction between windings

Core influence

103

Winding structure influence

105

Earthing leads influence

107

Frequency in Hz

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Theory When to perform a Frequency Response Analysis > After short-circuit testing > Before and after transport > After the occurrence of high transient fault currents > For diagnostic routine measurements > After significant changes of monitored values > After the observation of unusual routine test results

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Methods How FRAnalyzer analyzes frequency response > Injection of sinusoidal excitation voltage with continuously increasing frequency into one end of the transformer winding > Measurement of signal returning from the other end

Sine generator, variable frequency

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Transformer (complex network)

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Results

Methods How FRAnalyzer analyzes frequency response > Comparison of signals generates unique frequency response which can be compared to reference data > Deviations indicate geometrical and/or electrical changes within the transformer > No additional data processing required due to direct measurement in the frequency domain

Results

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Phase

Amplitude

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What is SFRA?

• Powerful and sensitive tool to assess the mechanical and electrical integrity of power transformers active part • Measurement of the transfer function over a wide frequency range

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SFRA Discussion Outline

1. Basic SFRA Theory, History, and Evolution 2. SFRA Measurement Characteristics 3. Failure Modes 4. Test Procedures 5. Analysis of Results 6. Case Sudies

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Standardization in the World WG A2.26 C57.149

IEC 60076-18

DL 911/2004

CHINA

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Life Cycle Manufacturer Workshop Delivery Port

•Quality Assuring

Truck Transport 1

•Transport Checking

•After Short Circuit Test •Failure Investigation Ship Transport

Reception Port

•Routine Measurement •After Transients/Overcurrents Truck Transport 2 © OMICRON •Failure

Investigation (DGA)

•Transport Checking

The SFRA Measurement Principle

Input signal (sine wave of variable frequency)

Magnitude

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Output signal

Phase

Theoretical Background Cables Grounding

Measurement cable

x(t ) = X sin ωt

y (t ) = Y sin(ωt + φ )

Measurement cable

RMC12

RMC34

CMC

TF =

Rm U 2 (s) = U1 ( s ) Rm + Z specimen

CMC

CMC

CMC

Complex RLC Network

k = 20 log10 (U 2 / U1 ) ϕ = tan −1 (∠U 2 / ∠U1 )

50Ω

Magnitude (k)

U1

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50Ω

U2

50Ω

Phase

Passive Components

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RLC Characteristics

0

Amplitude [dB]

Amplitude [dB]

0

-50 L=200 mH L=2 mH L=20 H

-100

-150 1 10

2

10

3

10

4

10 Frequency (Hz)

5

10

6

10

2

10

3

10

4

10 Frequency (Hz)

5

10

6

10

Phase [°]

Phase [°]

C=1uF C=20nF C=1pF

-150 2

10

3

10

4

10 Frequency (Hz)

5

10

6

10

7

10

100

L=200 mH L=2 mH L=20 H

-50

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-100

-200 1 10

7

10

0

-100 1 10

-50

7

10

C=1uF C=20nF C=1pF

50

0 1 10

2

10

3

10

4

10 Frequency (Hz)

5

10

6

10

7

10

Failure Mode Identified with SFRA 1.

Radial “Hoop Buckling” Deformation of Winding

2.

Axial Winding Elongation “Telescoping”

3.

Overall- Bulk & Localized Movement

4.

Core Defects

5.

Contact Resistance

6.

Winding Turn-to-Turn Short Circuit

7.

Open Circuited Winding

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•

Residual Magnetization

•

Oil Status (With or Without)

•

Grounding

Radial Failure

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Axial Failure

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Conductor Tilting

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Core Failure Modes •

Over-Heating

•

Lamination Gaps

•

Bulk Movement

•

Shorted Laminations

•

Multiple Core Grounding •

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Ungrounded Core

Typical Results 5.000e+001

1.000e+002

5.000e+002

1.000e+003

5.000e+003

1.000e+004

5.000e+004

1.000e+005

5.000e+005

1.000e+006

5.000e+001

1.000e+002

5.000e+002

1.000e+003

5.000e+003

1.000e+004

5.000e+004

1.000e+005

5.000e+005

1.000e+006

f/Hz

-20

-30

-40

-50

-60

-70

dB

150

100

-50

-100

° N W sec

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N V sec

NU

f/Hz

RLC Basics

•

Parallel RLC - VALLEY

• Series RLC – PEAK • 0 dB = 0 Ohms = Short • -100 dB = ∞ = Open

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Typical Results 5.000e+001

1.000e+002

5.000e+002

1.000e+003

5.000e+003

1.000e+004

5.000e+004

1.000e+005

5.000e+005

1.000e+006

5.000e+001

1.000e+002

5.000e+002

1.000e+003

5.000e+003

1.000e+004

5.000e+004

1.000e+005

5.000e+005

1.000e+006

f/Hz

-20

-30

-40

-50

-60

-70

dB

150

100

-50

-100

° N W sec

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N V sec

NU

f/Hz

Measurement Setup – OPEN CIRCUIT

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HV vs. LV Winding Responses

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Open Circuit Tests

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Measurement Setup – SHORT CIRCUIT

Short Circuit Test

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Open vs. Shorted tests

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Short Circuit Tests

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Usable Frequency Ranges

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Transformer Types • 2 Winding (H, X)  3-H OC  3-X OC  3-HX SC

• 3 Winding (H, X, Y)  3-H OC  3-X OC  3-Y OC  3-HX SC  3-HY SC

• Auto Transformer (Series, Common, Tert)  3-H Series OC  3-X Common OC  3-Y Tert OC  3-HX SC  3-HY SC

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Test Connections

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Test Recommendations (IEEE)

• LTC Extreme Raise • DETC as Found • Open Circuit Test • Short Circuit Test

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Series Winding Open Circuit Test

H1-X1 (A)

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H2-X2 (B)

H3-X3(C)

Common Winding Open Circuit Test

X1-X0 (A)

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X2-X0 (B)

X3-X0 (C)

Short Circuit Test

H1-H0X0 (A)

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H2-H0X0 (B)

H3-H0X0(C)

Overview of B Phase

H1-X2 (B)

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X2-X0 (B)

H2-H0X0 (B)

Analysis Strategies

1. Baseline 2. Similar Unit 3. Phase Comparison

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A

Date X

1.000e+002

5.000e+002

1.000e+003

5.000e+003

C

B

Phase based comparison

Time based comparison

SFRA Interpretation

Date Y

1.000e+004

5.000e+004

1.000e+005

5.000e+005

1.000e+006

f/Hz

1.000e+002

5.000e+002

1.000e+003

5.000e+003

1.000e+004

5.000e+004

1.000e+005

5.000e+005

1.000e+006

f/Hz

-10 -10

-20 -20

-30 -30

-40 -40

-50 -50

-60

-60

-70

-70

-80

-80

dB

dB

Fingerprint

A

C

B

1.000e+002

5.000e+002

1.000e+003

5.000e+003

1.000e+004

5.000e+004

1.000e+005

5.000e+005

1.000e+006

f/Hz

-10

-20

-30

-40

-50

-60

1.000e+002

5.000e+002

1.000e+003

5.000e+003

1.000e+004

5.000e+004

1.000e+005

5.000e+005

1.000e+006

-70

f/Hz

-10

-80 -20

dB

-30

-40

-50

-60

-70

-80

dB

A vs B vs C

Construction based comparison A

B

C

A

B

C

1.000e+002

1.000e+002

5.000e+002

1.000e+003

5.000e+003

1.000e+004

5.000e+004

1.000e+005

5.000e+005

1.000e+006

f/Hz

-10

-10 -20

-20 -30

-30 -40

-40 -50

-50 -60

-60 -70

-70 -80

dB -80

dB

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5.000e+002

1.000e+003

5.000e+003

1.000e+004

5.000e+004

1.000e+005

5.000e+005

1.000e+006

f/Hz

Radial Deformation (IEEE)

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IEEE WG PC57.149 (Guide) D8

Axial Deformation (IEEE)

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IEEE WG PC57.149 (Guide) D8

Core Defects (IEEE)

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IEEE WG PC57.149 (Guide) D8

CASE STUDY

1969 Transformer

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Initial Problem

Phase 1: Trip out of Service, Differential Phase 2: DGA

Initial Problem

Phase 1: Trip out of Service, Differential Phase 2: DGA

Phase 3: Test -Visual Inspection -Power Factor -Exciting Current -Transformer Turns Ratio -SFRA -Second DGA – 19 PPM of Acetylne

Phase 4: Reviewed SFRA data

HV Open Circut

LV Open Circut

Failure Modes due to Radial Forces Shift to the right

IEEE PC57.149 © OMICRON

HV Short Circut

HV Short Circut – Zoom In

~0.1db difference.. Not bad!

Phase 4: Reviewed SFRA data Phase 6: Perform Addition Test -Leakage Reactance +FRSL -Winding Resistance

Leakage Reactance – 3 Phase Equivalent and Per Phase Test

9.62% difference compared to average!

Leakage Reactance – FRSL

Winding Resistance

Phase 4: Reviewed SFRA data Phase 5: Perform Addition Test -Leakage Reactance +FRSL -Winding Resistance

Phase 6: Tear down

During Tear Down, Transformer caught on fire

Tear Down B Phase

Take a closer look

B phase Zoom In

From Left side of Buldge

Right Side of Buldge

Fault on a furnace 25 MVA transformer

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Overpressure valve was spitting out 200l of oil

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DC insulation resistance

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Ratio error [%] Ratio Deviation (Tap)

0.1 0 -0.1 -0.2 -0.3 -0.4 -0.5 -0.6 -0.7 -0.8 -0.9 000

U V W

005

010 Taps

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015

020

Excitation Current

No-Load Current

0.7 0.6 0.5 Io U Io V Io W

0.4 0.3 0.2 0.1 0 000

005

010 Taps

Angle of Excitation Current

0.0° -10.0° -20.0°

Phase (I) U Phase (I) V Phase (I) W

-30.0° -40.0° -50.0° -60.0° 000

005

010 Taps

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015

Taps

020

015

Taps

020

Z0 (f) = R0 (f) + j X0 (f) 6000.0Ω 5000.0Ω 4000.0Ω R0 W17 R0 V17 R0 U17

3000.0Ω 2000.0Ω 1000.0Ω 0.0Ω 0.0Hz

100.0Hz

200.0Hz

300.0Hz

400.0Hz

500.0Hz

6000.0Ω 5000.0Ω 4000.0Ω X0 W17 X0 V17 X0 U17

3000.0Ω 2000.0Ω 1000.0Ω 0.0Ω 0.0Hz

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100.0Hz

200.0Hz

300.0Hz

400.0Hz

500.0Hz

FRA (log view)

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Comparison to known cases Faulty B phase

Tested transformer

Transformer with shorted tertiary winding

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FRA (linear view) Faulty B phase

Faulty B phase © OMICRON

FRA (linear view zoomed)

Faulty B phase

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Opened transformer

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Opened transformer

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Melted screw

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Melted screw

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Melted Steel with copper marks

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Interrupted screen connection

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Interrupted screen connection

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FRA measurement 220 kV – 110 kV Autotransformer

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Measurement (2)

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Results 110 kV

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Results 220 kV

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Chinese standard DL/T 911-2004 Standard variance of two compared sequences

Dx =

1 N

  1 X k X k ( ) ( ) ∑ ∑   N K =0 K =0  N −1

N −1

2

1 Dy = N

  1 N −1 Y ( k ) Y ( k ) ∑ ∑   N K =0 K =0  N −1

Covariance of two compared sequences

C xy =

1 1   X(k) X(k) ∑ ∑  N K =0 N K =0  N −1

N −1

2



× Y ( k ) -



1  Y k ( ) ∑  N K =0  N −1

Normalized covariance factor LRxy=Cxy /

Dx D y

 10 1 − LR xy < 10 −10 Relative factor Rxy =  − 1g (1 − LR XY ) others © OMICRON

2

2

Chinese standard DL/T 911-2004 Winding Deformation degree Severe Deformation Obvious Deformation Slight Deformation Normal Winding

RLF in the range 1kHz∼100kHz RMF in the range 100kHz∼600kHz RHF in the range 600kHz∼1000kHz

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Relative Factors R RLF < 0.6 1.0> RLF ≥ 0.6 or RMF < 0.6 2.0> RLF ≥ 1.0 or 0.6 ≤ RMF < 1.0 RLF ≥ 2.0, RMF ≥ 1.0 and RHF ≥ 0.6

Good winding according to DL/T 911-2004

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Defective winding according to DL/T 911-2004

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Thank You for Your Attention