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

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

Theory

© OMICRON Page 2

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

tank wall

windings

core

Theory

Page 3 © OMICRON

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

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

Mag

nitu

de in

dB

Frequency in Hz

0

-20

-40

-60

-80

101 103 105 107

Core influence

Interaction between windings

Winding structure influence

Earthing leads

influence

Theory

© OMICRON Page 4

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

Methods

Page 5 © OMICRON

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

Results Sine generator, variable frequency

Transformer (complex network)

Methods

Page 6 © OMICRON

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

Phase

Amplitude

© OMICRON

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

© OMICRON

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

© OMICRON

Standardization in the World

CHINA

DL 911/2004 C57.149 WG A2.26

IEC 60076-18

© OMICRON

Life Cycle

Delivery Port

Reception Port

Manufacturer Workshop

•Quality Assuring

•After Short Circuit Test

•Failure Investigation

•Transport Checking

•Transport Checking

•Routine Measurement

•After Transients/Overcurrents

•Failure Investigation (DGA)

Truck Transport 1

Truck Transport 2

Ship Transport

© OMICRON

The SFRA Measurement Principle

Input signal (sine wave of

variable frequency)

Output signal

Phase Magnitude

© OMICRON

Theoretical Background

Measurementcable

Measurementcable

CMC

CMC

CMC

CMC

RMC12 RMC34

Complex RLC Network

U1

Cables Grounding

50Ω U2 50Ω

50Ω

)sin()( φω += tYty

tXtx ωsin)( =

)/(log20 1210 UUk =

)/(tan 121 UU ∠∠= −ϕ

Magnitude (k) Phase

specimenm

m

ZRR

sUsUTF

+==

)()(

1

2

Presenter
Presentation Notes
This slide explains the so-called sweep frequency method. In contrast to the impulse method, here we work with a sinusoidal signal, which is swept between 10 Hz and 10/20 kHz (or more). Vin – represented in red – is the input signal, which is standardized to 1 or 100%. We sweep the signal over the whole frequency range and measure the signal Vout - represented in blue - at the other end of the winding. We see that the blue signal (output signal) compared to the red signal (input signal) shows both a damping as well as a phase shift. The transfer function (H) in dB is the result of the calculation according the shown formula: H(dB) = .... Advantages of the sweep FRA method: The input amplifiers of the equipment can be realized with a very small bandwidth; by that it can be avoided that other distortion signals superimpose the measurement signal Over the whole frequency range we have a constant amplitude of the oscillator signal and by that no limitation of / a constant energy at higher frequencies. This allows a more sensitive measurement of particularly at higher resonance frequencies.

© OMICRON

Passive Components

© OMICRON

RLC Characteristics

101

102

103

104

105

106

107

-150

-100

-50

0

Frequency (Hz)

Am

plitu

de [d

B]

101

102

103

104

105

106

107

-100

-50

0

Frequency (Hz)

Pha

se [°

]

L=200 mHL=2 mHL=20 H

L=200 mHL=2 mHL=20 H

101

102

103

104

105

106

107

-200

-150

-100

-50

0

Frequency (Hz)A

mpl

itude

[dB

]

C=1uFC=20nFC=1pF

101

102

103

104

105

106

107

0

50

100

Frequency (Hz)

Pha

se [°

]

C=1uFC=20nFC=1pF

© OMICRON

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

• Residual Magnetization

• Oil Status (With or Without)

• Grounding

© OMICRON

Radial Failure

Presenter
Presentation Notes
This slides shows the standard FRA measuring method: The yellow cable (generator output) is connected to the beginning of the winding, with the red cable the injected voltage is measured back to compensate wiring influences (reference channel). With the blue cable (output) the damped signal at the other end of the same winding is fed back to equipment to the measurement input. This is the standard FRA method which should be applied as a minimum for all FRA measurements. The measurement is performed for each phase: If a transformer has two voltage levels this means that six measurements are performed, if we have a three-winding transformer (upper, medium, and low voltage winding), nine such measurements are necessary.

© OMICRON

Axial Failure

Presenter
Presentation Notes
This slides shows the standard FRA measuring method: The yellow cable (generator output) is connected to the beginning of the winding, with the red cable the injected voltage is measured back to compensate wiring influences (reference channel). With the blue cable (output) the damped signal at the other end of the same winding is fed back to equipment to the measurement input. This is the standard FRA method which should be applied as a minimum for all FRA measurements. The measurement is performed for each phase: If a transformer has two voltage levels this means that six measurements are performed, if we have a three-winding transformer (upper, medium, and low voltage winding), nine such measurements are necessary.

© OMICRON

Conductor Tilting

Presenter
Presentation Notes
This slides shows the standard FRA measuring method: The yellow cable (generator output) is connected to the beginning of the winding, with the red cable the injected voltage is measured back to compensate wiring influences (reference channel). With the blue cable (output) the damped signal at the other end of the same winding is fed back to equipment to the measurement input. This is the standard FRA method which should be applied as a minimum for all FRA measurements. The measurement is performed for each phase: If a transformer has two voltage levels this means that six measurements are performed, if we have a three-winding transformer (upper, medium, and low voltage winding), nine such measurements are necessary.

© OMICRON

Core Failure Modes

• Over-Heating

• Bulk Movement

• Multiple Core Grounding

• Lamination Gaps

• Shorted Laminations

• Ungrounded Core

© OMICRON

Typical Results

f/Hz5.000e+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

dB

-70

-60

-50

-40

-30

-20

N W sec N V sec N U

f/Hz5.000e+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

°

-100

-50

100

150

© OMICRON

RLC Basics

• Parallel RLC - VALLEY

• Series RLC – PEAK

• 0 dB = 0 Ohms = Short • -100 dB = ∞ = Open

© OMICRON

Typical Results f/Hz

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

dB

-70

-60

-50

-40

-30

-20

N W sec N V sec N U

f/Hz5.000e+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

°

-100

-50

100

150

© OMICRON

Measurement Setup – OPEN CIRCUIT

Presenter
Presentation Notes
This slides shows the standard FRA measuring method: The yellow cable (generator output) is connected to the beginning of the winding, with the red cable the injected voltage is measured back to compensate wiring influences (reference channel). With the blue cable (output) the damped signal at the other end of the same winding is fed back to equipment to the measurement input. This is the standard FRA method which should be applied as a minimum for all FRA measurements. The measurement is performed for each phase: If a transformer has two voltage levels this means that six measurements are performed, if we have a three-winding transformer (upper, medium, and low voltage winding), nine such measurements are necessary.

© OMICRON

HV vs. LV Winding Responses

Presenter
Presentation Notes
FRAnalyzer is completely PC controlled comprising a convenient, versatile software. This slide shows the screen which appears after the FRAnalyzer software has been started up. The upper part contains a list of all transformers where measurements so far have been made. The lower part contains a corresponding list of all measurements available for the transformer currently selected in the upper list.

© OMICRON

Open Circuit Tests

© OMICRON

Measurement Setup – SHORT CIRCUIT

Short Circuit Test

Presenter
Presentation Notes
This slides shows the standard FRA measuring method: The yellow cable (generator output) is connected to the beginning of the winding, with the red cable the injected voltage is measured back to compensate wiring influences (reference channel). With the blue cable (output) the damped signal at the other end of the same winding is fed back to equipment to the measurement input. This is the standard FRA method which should be applied as a minimum for all FRA measurements. The measurement is performed for each phase: If a transformer has two voltage levels this means that six measurements are performed, if we have a three-winding transformer (upper, medium, and low voltage winding), nine such measurements are necessary.

© OMICRON

Open vs. Shorted tests

© OMICRON

Short Circuit Tests

© OMICRON

Usable Frequency Ranges

© OMICRON

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

© OMICRON

Test Connections

© OMICRON

Test Recommendations (IEEE)

• LTC Extreme Raise

• DETC as Found

• Open Circuit Test

• Short Circuit Test

© OMICRON

Series Winding Open Circuit Test

H1-X1 (A) H2-X2 (B) H3-X3(C)

© OMICRON

Common Winding Open Circuit Test

X1-X0 (A) X2-X0 (B) X3-X0 (C)

© OMICRON

Short Circuit Test

H1-H0X0 (A) H2-H0X0 (B) H3-H0X0(C)

© OMICRON

Overview of B Phase

H1-X2 (B) X2-X0 (B) H2-H0X0 (B)

© OMICRON

Analysis Strategies

1. Baseline 2. Similar Unit

3. Phase Comparison

© OMICRON

SFRA Interpretation

Fingerprint

Date X Date Y

Tim

e ba

sed

com

paris

on

Pha

se b

ased

com

paris

on

f/Hz1.000e+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

dB

-80

-70

-60

-50

-40

-30

-20

-10

f/Hz1.000e+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

dB

-80

-70

-60

-50

-40

-30

-20

-10

f/Hz1.000e+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

dB

-80

-70

-60

-50

-40

-30

-20

-10

A B C A B C

f/Hz1.000e+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

dB

-80

-70

-60

-50

-40

-30

-20

-10

A vs B vs C

A B C A B C

f/Hz1.000e+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

dB

-80

-70

-60

-50

-40

-30

-20

-10

f/Hz1.000e+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

dB

-80

-70

-60

-50

-40

-30

-20

-10

Construction based comparison

© OMICRON

Radial Deformation (IEEE)

IEEE WG PC57.149 (Guide) D8

© OMICRON

Axial Deformation (IEEE)

IEEE WG PC57.149 (Guide) D8

© OMICRON

Core Defects (IEEE)

IEEE WG PC57.149 (Guide) D8

© OMICRON

1969 Transformer

CASE STUDY

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

© OMICRON

Failure Modes due to Radial Forces

IEEE PC57.149

Shift to the right

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

From Left side of Buldge

B phase Zoom In

Right Side of Buldge

© OMICRON

Fault on a furnace 25 MVA transformer

© OMICRON

Overpressure valve was spitting out 200l of oil

© OMICRON

DC insulation resistance

© OMICRON

Ratio error [%]

Ratio Deviation (Tap)

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

00.1

000 005 010 015 020Taps

UVW

© OMICRON

No-Load Current

Excitation Current

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

000 005 010 015 020Taps

Io UIo VIo W

Angle of Excitation Current

-60.0°

-50.0°

-40.0°

-30.0°

-20.0°

-10.0°

0.0°

000 005 010 015 020Taps

Phase (I) UPhase (I) VPhase (I) W

Taps

Taps

© OMICRON

Z0 (f) = R0 (f) + j X0 (f)

0.0Ω

1000.0Ω

2000.0Ω

3000.0Ω

4000.0Ω

5000.0Ω

6000.0Ω

0.0Hz 100.0Hz 200.0Hz 300.0Hz 400.0Hz 500.0Hz

R0 W17R0 V17R0 U17

0.0Ω

1000.0Ω

2000.0Ω

3000.0Ω

4000.0Ω

5000.0Ω

6000.0Ω

0.0Hz 100.0Hz 200.0Hz 300.0Hz 400.0Hz 500.0Hz

X0 W17X0 V17X0 U17

© OMICRON

FRA (log view)

© OMICRON

Comparison to known cases

Tested transformer

Faulty B phase

Transformer with shorted tertiary winding

© OMICRON

FRA (linear view)

Faulty B phase

Faulty B phase

© OMICRON

FRA (linear view zoomed)

Faulty B phase

© OMICRON

Opened transformer

© OMICRON

Opened transformer

© OMICRON

Melted screw

© OMICRON

Melted screw

© OMICRON

Melted Steel with copper marks

© OMICRON

Interrupted screen connection

© OMICRON

Interrupted screen connection

© OMICRON

FRA measurement 220 kV – 110 kV Autotransformer

Presenter
Presentation Notes
This slide shows a practical measurement at a 220 kV transformer. In the area of the manlift –allowing an easy connection to the high voltage winding – we see the 220 kV bushing; in the front – towards the yellow transport case / measuring equipment – we see the 110 kV bushings. Not visible in this picture are the tertiary terminals, in this case for the 10 kV windings.

© OMICRON

Measurement (2)

Presenter
Presentation Notes
This picture shows the measuring arrangement in front of the transformer.

© OMICRON

Results 110 kV

Presenter
Presentation Notes
This picture shows a comparison of the FRA measurement results of the three 110 kV phases.

© OMICRON

Results 220 kV

Presenter
Presentation Notes
This picture shows the equivalent results on the 220 kV side.

© OMICRON

Chinese standard DL/T 911-2004

Standard variance of two compared sequences 21

0

1

0)(

N1-)(1 ∑ ∑

=

=

=

N

K

N

Kx kXkX

ND

21

0

1

0)(

N1-)(1 ∑ ∑

=

=

=

N

K

N

Ky kYkY

ND

Covariance of two compared sequences 21N

0K

1N

0Kxy X(k)

N1-X(k)

N1C ∑ ∑

=

=

= ×

21

0)(

N1-)(

∑−

=

N

KkYkY

Normalized covariance factor LRxy=Cxy / yx DD

Relative factor Rxy =

−−<− −

othersLRgLR

XY

xy

)1(110110 10

Presenter
Presentation Notes
This picture shows the equivalent results on the 220 kV side.

© OMICRON

Chinese standard DL/T 911-2004

Winding Deformation degree Relative Factors R

Severe Deformation RLF < 0.6

Obvious Deformation 1.0> RLF ≥ 0.6 or RMF < 0.6

Slight Deformation 2.0> RLF ≥ 1.0 or 0.6 ≤ RMF < 1.0

Normal Winding RLF ≥ 2.0, RMF ≥ 1.0 and RHF ≥ 0.6

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

Presenter
Presentation Notes
This picture shows the equivalent results on the 220 kV side.

© OMICRON

Good winding according to DL/T 911-2004

Presenter
Presentation Notes
This picture shows the equivalent results on the 220 kV side.

© OMICRON

Defective winding according to DL/T 911-2004

Presenter
Presentation Notes
This picture shows the equivalent results on the 220 kV side.

Thank You for Your Attention