1. Power Transformer Diagnostics:
Novel Techniques and their Application
Charles Sweetser
Technical Service Manager

2. Transformers

3. 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

4. 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

5. Transformer Types and Classifications
• Distribution
• Power
• Rectifier
• Arc-Furnace• Arc-Furnace
• Network
• Regulating (Voltage Regulators)
• Phase Shifting
• Reactors*

6. Vector Groups

7. 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, CA
Courtesy of Delta Star, San Carlos, CA

8. Construction Forms
Core Form
• Concentric
• Less Iron
• More CU
Shell Form
• Interleaved
• More Iron
• Less CU
Core Form Shell FormCore Form Shell Form

9. 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 Heat
¾ Moisture
¾ Oxygen

10. Failure Modes

11. Failure Modes

12. Core Failure Modes
• Over-Heating
• Bulk Movement
• Multiple Core Grounding
• Lamination Gaps
• Shorted Laminations
• Ungrounded Core

13. 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

14. 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)

15. 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

16. 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

17. Overall PF and Capacitance

18. 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

19. Bushings

20. Surge Arresters
• Analyzed on the basis of Watts

21. 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 Resistance2. 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

22. Variable Frequency Losses
1.50E-03
2.00E-03
2.50E-03
Serial
Parallel
Sum
0.00E+00
5.00E-04
1.00E-03
1.50E-03
0 50 100 150 200 250 300 350 400

23. Normal Power Factor

24. Power Factor Influenced by Moisture

25. Slope and Ripple
Ripple
Slope D
Ripple

26. Advanced DC Resistance - Slope
Slope
-0.2A/s
-0.1A/s
0.0A/s
A UP
A 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 DOWN
B UP
B DOWN
C UP
C DOWN

27. Advanced DC Resistance - Slope
Slope
-0.2A/s
0.0A/s
0 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 UP
A DOWN
B UP
B DOWN
C UP
C DOWN

28. Advanced DC Resistance - Ripple
Ripple
35.0%
40.0%
45.0%
50.0%
A UP
A DOWN
0.0%
5.0%
10.0%
15.0%
20.0%
25.0%
30.0%
35.0%
0 2 4 6 8 10 12
Taps
A DOWN
B UP
B DOWN
C UP
C DOWN

29. FRSL - Good
R(f)
3.0 Ohm
3.5 Ohm
4.0 Ohm
4.5 Ohm
0.0 Ohm
0.5 Ohm
1.0 Ohm
1.5 Ohm
2.0 Ohm
2.5 Ohm
3.0 Ohm
0 100 200 300 400 500
Frequency (Hz)
A
B
C

30. 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]
A
B
C

31. Dielectric Frequency Response (DFR)
HV-winding
Voltage source
~
Current meter
1
10
PowerFactor
Power Factor in Frequency Domain
?
Tank
Guard
LV-winding
Main insulation 0,001
0,01
0,1
0,0001 0,001 0,01 0,1 1 10 100 1000
Frequency (Hz)
Power
?

32. 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

33. 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)

34. 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)

35. 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

36. .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
WaterContentinthePaper
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

37. 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

38. Measurement Characteristics
¨ Current in wide frequency
range,
e.g. 1 mHz – 1 kHz
¨ Display as dissipation
factor or complex
capacitance or complex
permittivity
)()()()()(
0
0
0 ωωχ
ωε
σ
ωχεωω UjCjI












′′+−′+∞=
)()(
)(
)(
)(
)(
)(
)(tan 0
0
ωχε
ωχ
ωε
σ
ωε
ωε
ω
ω
ωδ
′+∞
′′+
=
′
′′
=
′
′′
=
C
C
Dissipationfactor
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 1000
Frequency (Hz)
Dissipationfactor
high
low
high
high
low
low
moistureof
cellulose
andaging
insulation
geometry
oil
conductivity
moisture of
cellulose,
aging

39. Measurement Examples
0.05
0.1
0.2
0.5
1.0
2.0
5.0
New Moderate Aged
0.12
60Hz
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

40. 3.1 % at 9 qC – Oil Sample Yielded 5.0%

41. 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

42. Diagnostic Category
• Dielectric
• Thermal
• “Mechanical”• “Mechanical”
• Use SFRA:
1. Transportation
2. Post Fault

43. 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

44. > 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
SpannungU1/U1inV/V
1
2
3
SpannungU/U1inV/V
U2^
U1
^
1-|TF(f1)|
-3
-2
-1
0
1
2
3
0 50 100
Zeit t in µs
SpannungU2/U1inV/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
|TFU2/U1(f)|inV/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(TFU2/U1(f))indeg
Betragsfunktion
|TFU2/U1(f)|
Phasenfunktion
arc(TFU2/U1(f))
-3
-2
-1
0
0 50 100
Zeit t in µs
SpannungU/U1inV/V
U2^
U1^
|TF(f1)| =
M(f1)/2
f
arc(TF(f1)) =M(f1)
N V
f/Hz
1.000e+002 1.000e+003 1.000e+004 1.000e+005
dB
-90
-80
-70
-60
-50
-40
-30
-20
-10
NV
f/Hz1.000e+002 1.000e+003 1.000e+004 1.000e+005
°
-150
-100
-50
0
50
100
150
PhaseMagnitude

45. Theoretical Background
Measurement
cable
Measurement
cable
CMC
CMC
CMC
CMC
RMC12 RMC34
Complex RLC Network
Cables Grounding
)sin()( φω += tYty
tXtx ωsin)( =
)/(log20 1210 UUk =
specimenm
m
ZR
R
sU
sU
TF
+
==
)(
)(
1
2
Complex RLC Network
U1 50 U2 50
50
)/(log20 1210 UUk =
)/(tan 12
1
UU ∠∠= −
ϕ
H1 H2 H2 H3 H3 H1
f/Hz
1.000e+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
H1H2 H2H3 H3H1
f/Hz
1.000e+002 1.000e+003 1.000e+004 1.000e+005 1.000e+006
°
-200
-150
-100
-50
0
50
100
150Magnitude (k) Phase

46. Passive Components

47. RLC Characteristics
10
1
10
2
10
3
10
4
10
5
10
6
10
7
-150
-100
-50
0
Frequency (Hz)
Amplitude[dB]
10
1
10
2
10
3
10
4
10
5
10
6
10
7
-100
-50
0
Frequency (Hz)
Phase[°]
L=200 mH
L=2 mH
L=20 H
L=200 mH
L=2 mH
L=20 H
10
1
10
2
10
3
10
4
10
5
10
6
10
7
-200
-150
-100
-50
0
Frequency (Hz)Amplitude[dB]
C=1uF
C=20nF
C=1pF
10
1
10
2
10
3
10
4
10
5
10
6
10
7
0
50
100
Frequency (Hz)
Phase[°]
C=1uF
C=20nF
C=1pF

48. 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
-40
-30
-20
dB
-70
-60
-50
N W sec N V sec N U
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
°
-100
-50
100
150

49. Failure Modes
• Radial “Hoop Buckling” Deformation
• Axial Winding Elongation “Telescoping”
• Overall- Bulk & Localized Movement
• Winding Turn-to-Turn Short Circuit
• Open Circuited Winding

50. Radial Failure

51. Axial Failure
Pag

52. Conductor Tilting

53. Core Faults
Pag

54. Measurements Types
• Open Circuit - Exciting Ima
• Short Circuit - Leakage Reac
• Interwinding - CAP• Interwinding - CAP
• Transfer Voltage - TTR

55. HV and LV Open Circuit

56. Open Circuit Tests

57. Open Circuit vs. Short Circuit

58. Short Circuit Tests

59. Analysis Strategies
• Baseline
• Similar Unit• Similar Unit
• Phase Comparison

60. 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

61. Standardization in the World
CHINA
DL 911/2004DL 911/2004PC57.149/D8PC57.149/D8
WG A2.26WG A2.26
IEC 60076IEC 60076--1818

62. Available Documents
Cigré Brochure 342 DL 911/2004

63. Available Documents
IEC 60076-18 IEEE PC57.149

64. Thank You for Your Attention