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