Power Transformer Diagnostics: Novel Techniques and their Application

Charles SweetserTechnical Service Manager

Transformers

Topics of Discussion

• Introduction to Power Transformers • Life Expectancy• Failure Modes• Diagnostic Characteristics• Diagnostic Characteristics• Diagnostic Measurements• Standard Measurements• Advanced Measurements• SFRA - Sweep Frequency Response Analysis• DFR - Dielectric Frequency Response

Transformer Considerations

• Transformer Types and Classifications

• Transformer Configurations

• Vector Groups

• Oil Preservation Systems

• Insulating Materials and Fluids• Insulating Materials and Fluids

• Construction Forms

• Core Steel

• Ratings

• Cooling Schemes

• Tap Changers (OLTC, DETC)

• Bushings

• Surge Arresters

Transformer Types and Classifications

• Distribution• Power• Rectifier• Arc-Furnace• Arc-Furnace• Network• Regulating (Voltage Regulators)• Phase Shifting• Reactors*

Vector Groups

Winding Types

1. Disk Winding

2. Pancake Winding

3. Helical Winding

4. Cylindrical or Layer Winding4. Cylindrical or Layer Winding

Courtesy of Delta Star, San Carlos, CACourtesy of Delta Star, San Carlos, CA

Construction FormsCore Form• Concentric

• Less Iron

• More CU

Shell Form• Interleaved

• More Iron

• Less CU

Core Form Shell FormCore Form Shell Form

Life Expectancy

• 180,000 hrs or 20.55 years

• 110 °C Hottest Spot for 65 °C Temp Rise insulation

• Degree of Polymerization (200 -1200 DP)

• 1200 DP - New Paper• 1200 DP - New Paper

• 200 DP at 150,000 hrs (end of life)

¾ Heat¾ Moisture¾ Oxygen

Failure Modes

Failure Modes

Core Failure Modes

• Over-Heating

• Bulk Movement

• Multiple Core Grounding

• Lamination Gaps

• Shorted Laminations

• Ungrounded Core

Oil Analysis – DGA and Oil Screen

• Hydrogen (H2)

• Methane (CH4)

• Ethane (C2H6)

• Ethylene (C2H4)

• Rate of Gas Generation

• Partial Discharge

• Arcing

• Electrical Heating• Ethylene (C H )

• Acetylene (C2H2)

• Carbon Monoxide (CO)

• Carbon Dioxide (CO2)

• Oxygen (O2)

• Nitrogen (N2)

• Electrical Heating

• Metal Heating

• Decomposition of Paper

Oil Analysis – DGA and Oil Screen• Dielectric Breakdown• IFT• Color• Acidity• Power Factor• Power Factor• Moisture• Specific Gravity• Viscosity

• Degree of Polymerization DP (Paper)• Furans (Oil)

Transformer Tests

Dielectric Thermal Mechanical

DGA DGA SFRA

Oil Screen Oil Screen Leakage Reactance

PF/TD CAP IR PF/TD CAP

Exciting Ima DC Winding RES Exciting Ima

Turns Ratio Tests DC Winding RES

DFR

Insulation Resistance

Transformer Test Protocol

1. Overall Power Factor and Capacitance (Tip-Up, Variable Freq)

2. Bushings (C1, C2, Energized Collar) (Tip-Up, Variable Freq)

3. Exciting Current

4. Surge Arresters4. Surge Arresters

5. Insulating Fluids

6. Leakage Reactance (Frequency Response of Stray Losses)

7. Turns Ratio Test

8. DC Winding Resistance (Slope, Ripple)

9. Insulation Resistance

Overall PF and Capacitance

Bushings• Test Taps kV < 72 kV (C1 and C2)

• Potential Taps kV > 72 kV (C1 and C2)

• No Tap – Use Energized Collar at 10 kV

Bushings

Surge Arresters

• Analyzed on the basis of Watts

Advanced Diagnostics

1. Advanced Power Factor

Tip-Up: Voids in Insulation

Variable Frequency (15 Hz to 400 Hz): Moisture and Aging

2. Advanced DC Winding Resistance 2. Advanced DC Winding Resistance

Ripple and Slope

3. Advanced Leakage Reactance

FRSL (Frequency Response Stray Losses)

4. SFRA - Sweep Frequency Response Analysis

5. DFR - Dielectric Frequency Response

Variable Frequency Losses

1.50E-03

2.00E-03

2.50E-03SerialParallelSum

0.00E+00

5.00E-04

1.00E-03

1.50E-03

0 50 100 150 200 250 300 350 400

Normal Power Factor

Power Factor Influenced by Moisture

Slope and Ripple

Ripple

Slope D

Ripple

Advanced DC Resistance - Slope

Slope

-0.2A/s

-0.1A/s

0.0A/s

A UPA DOWN

-0.6A/s

-0.5A/s

-0.4A/s

-0.3A/s

-0.2A/s

000 005 010 015 020 025 030 Taps

A DOWNB UPB DOWNC UPC DOWN

Advanced DC Resistance - Slope

Slope

-0.2A/s

0.0A/s0 2 4 6 8 10 12

A UP

-1.2A/s

-1.0A/s

-0.8A/s

-0.6A/s

-0.4A/s

Taps

A UPA DOWNB UPB DOWNC UPC DOWN

Advanced DC Resistance - Ripple

Ripple

35.0%40.0%45.0%50.0%

A UPA DOWN

0.0%5.0%

10.0%15.0%20.0%25.0%30.0%35.0%

0 2 4 6 8 10 12Taps

A DOWNB UPB DOWNC UPC DOWN

FRSL - Good

R(f)

3.0 Ohm3.5 Ohm4.0 Ohm4.5 Ohm

0.0 Ohm0.5 Ohm1.0 Ohm1.5 Ohm2.0 Ohm2.5 Ohm3.0 Ohm

0 100 200 300 400 500

Frequency (Hz)

ABC

FRSL – Poor Result Short-Circuit Between Parallel Strands

R(f)

250

300

50

100

150

200

0 50 100 150 200 250 300 350 400

Frequency [Hz]

ABC

Dielectric Frequency Response (DFR)

HV-winding

Voltage source

~Current m eter

1

10

Pow

er F

acto

r

Power Factor in Frequency Domain

?

Tank

Guard

LV-winding

Main insulat ion 0,001

0,01

0,1

0,0001 0,001 0,01 0,1 1 10 100 1000Frequency (Hz)

Pow

er ?

Moisture in Transformers

1. Reduces Dielectric Strength

2. Promotes the Formation of Bubble2. Promotes the Formation of Bubble

3. Ages Insulation with Heat and Oxygen

Moisture Terminology – Paper, Pressboard, and Oil

1. Water Content

• Pressboard & Paper (percentage of total mass)

• Oil (PPM)

2. Moisture Saturation – P,P,O (Relative, Humidity)

Moisture Fact in Transformers

1. If mass of oil equals paper-pressboard,

then at equilibrium the water content is

(2000:1)

2. In transformers there is 10X more oil

mass than paper and pressboard, so

the water content is (200:1)

Oil Ratio to Paper : Pressboard

Mass of the oil:100,000 kg = 220,000 Lbs

Water content at 60 °C:40 ppm

Mass of the solid insulation:13,000 kg = 20,000 Lbs

Water content at 60 °C:4 %

Mass of the water, dissolved in the oil:

4 kg = 8.8 Lbs

Mass of the water contained in the paper:

520 kg = 1200 Lbs

.DUO�)LVFKHU�7LWUDWLRQ�DQG�(TXLOLEULXP�&XUYHV

9

[%]

11

20 °C 30 °C 40 °C

50 °C

• Curves are only valid for new oil and new paper, for aged oil/paper different curves are necessary

• Balance between water

0

1

2

3

4

5

6

7

8

0 10 20 30 40 50 60 70 [ppm] 90

Wat

er C

onte

nt in

the

Pap

er

Water Content in the Oil

60 °C

80 °C

100°C

Temperature

low

high

• Balance between water content in the paper and in oil needs constant temperatures over a long period

• Only average measurement

Water Content Recommendations

Category Moisture content in %

• Dry below 2.2

• Moderately wet 2.2-3.7• Moderately wet 2.2-3.7

• Wet 3.7-4.8

• Extremely wet above 4.8

Measurement Characteristics

¨ Current in wide frequency range, e.g. 1 mHz – 1 kHz

¨ Display as dissipation factor or complex capacitance or complex permittivity

)()()()()(0

00 ωωχ

ωεσωχεωω UjCjI

′′+−′+∞=

)()(

)(

)()(

)()(

)(tan 0

0

ωχε

ωχωε

σ

ωεωε

ωωωδ

′+∞

′′+=

′′′

=′′′

=CC

Dis

sipa

tion

fact

or permittivity

Interpretation¨ Slope - oil conductivity¨ Hump - insulation geometry¨ Low frequencies

- moisture and aging

- long test duration�0.001

0.01

0.1

1

10

0.0001 0.001 0.01 0.1 1 10 100 1000Frequency (Hz)

Dis

sipa

tion

fact

or

high

low

high

high

low

low

moi

stur

e of

ce

llulo

se

and

agin

g

insu

latio

nge

omet

ry

oil conductivity

moisture of cellulose, aging

Measurement Examples

0.05

0.1

0.2

0.5

1.0

2.0

5.0

New Moderate Aged 0.12

60H

z

Freq/Hz0.0001 0.010 0.10 1.0 10.0

0.005

0.01

0.02

0.05

10000.001

0.0024 0.0036

New, dry, cold: 1kHz - 0.1 mHz

Moderate: 1kHz - 1 mHz

Very aged: 1kHz - 10 mHz

3.1 % at 9 qC – Oil Sample Yielded 5.0%

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

Diagnostic Category

• Dielectric

• Thermal

• “Mechanical”• “Mechanical”

• Use SFRA:

1. Transportation

2. Post Fault

Life Cycle

Delivery Port

Manufacturer Workshop

•Quality Assuring

•After Short Circuit Test

•Transport CheckingTruck Transport 1

Reception Port

•After Short Circuit Test

•Failure Investigation

•Transport Checking

•Routine Measurement

•After Transients/Overcurrents

•Failure Investigation (DGA)Truck Transport 2

Ship Transport

> 17/03/2010

The SFRA Measurement Principle

Transformator

Erregungssignal (variable Frequenz)

Antwortsignal

-3

-2

-1

0

1

2

3

0 50 100

Zeit t in µs

Sp

ann

ung

U1/

U1

in V

/V

1

2

3

Sp

ann

ung

U/U

1 in

V/V

U2^U1^

1-|TF(f1)|

-3

-2

-1

0

1

2

3

0 50 100

Zeit t in µs

Sp

ann

ung

U2/

U1

in V

/V

Input signal(sine wave of

variable frequency)

Output signal

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

f in MHz

|TF U

2/U

1(f)

| in

V/V

|

-200.0

-150.0

-100.0

-50.0

0.0

50.0

100.0

150.0

200.0

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

f in MHz

arc(

TFU

2/U

1(f)

) in

deg

Betragsfunktion |TFU2/U1(f)|

Phasenfunktion arc(TFU2/U1(f))

-3

-2

-1

00 50 100

Zeit t in µs

Sp

ann

ung

U/U

1 in

V/V

U2^

U1^|TF(f1)| =

M(f1)/2 �f

arc(TF(f1)) = M(f1)

N V

f/Hz1.000e+002 1.000e+003 1.000e+004 1.000e+005

dB

-90

-80

-70

-60

-50

-40

-30

-20

-10

N V

f/Hz1.000e+002 1.000e+003 1.000e+004 1.000e+005

°

-150

-100

-50

0

50

100

150

PhaseMagnitude

Theoretical Background

Measurementcable

Measurementcable

CMC

CMC

CMC

CMC

RMC12 RMC34

Complex RLC Network

Cables Grounding

)sin()( φω += tYty

tXtx ωsin)( =

)/(log20 1210 UUk =

specimenm

m

ZRR

sUsU

TF+

==)()(

1

2

Complex RLC Network

U1 50 U2 50

50

)/(log20 1210 UUk =

)/(tan 121 UU ∠∠= −ϕ

H1 H2 H2 H3 H3 H1

f/Hz1.000e+002 1.000e+003 1.000e+004 1.000e+005 1.000e+006

dB

-120

-110

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

10

H1 H2 H2 H3 H3 H1

f/Hz1.000e+002 1.000e+003 1.000e+004 1.000e+005 1.000e+006

°

-200

-150

-100

-50

0

50

100

150Magnitude (k) Phase

Passive Components

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

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

-40

-30

-20

dB

-70

-60

-50

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

Failure Modes

• Radial “Hoop Buckling” Deformation

• Axial Winding Elongation “Telescoping”

• Overall- Bulk & Localized Movement

• Winding Turn-to-Turn Short Circuit

• Open Circuited Winding

Radial Failure

Axial Failure

Pag

Conductor Tilting

Core Faults

Pag

Measurements Types

• Open Circuit - Exciting Ima

• Short Circuit - Leakage Reac

• Interwinding - CAP• Interwinding - CAP

• Transfer Voltage - TTR

HV and LV Open Circuit

Open Circuit Tests

Open Circuit vs. Short Circuit

Short Circuit Tests

Analysis Strategies

• Baseline

• Similar Unit• Similar Unit

• Phase Comparison

FRA Industry Groups

• CIGRE WG A2.26 (Guide)

• DL 911/2004 (Standard)

• IEC 60076-18 (Draft)• IEC 60076-18 (Draft)

• IEEE WG PC57.149 (Guide) D8

Standardization in the World

CHINA

DL 911/2004DL 911/2004PC57.149/D8PC57.149/D8WG A2.26WG A2.26

IEC 60076IEC 60076--1818

Available Documents

Cigré Brochure 342 DL 911/2004

Available Documents

IEC 60076-18 IEEE PC57.149

Thank You for Your Attention