IEEE PES Maine Chapter
Charles Sweetser - OMICRON
December 20, 2018
Freeport, ME
Understanding the Value of Electrical Testing for Power Transformers
1
4th Annual NEPEC Conference – March 19-21, 2019
Westin Hotel, Portland, ME
Transformers
Courtesy of ABB TRES - ABB Inc., Saint Louis, MO
Courtesy of Delta Star, San Carlos, CA
Courtesy of Delta Star, San Carlos, CA
Topics of Discussion
1. IEEE Transformer Committee
2. Life Expectancy
3. Moisture
4. DGA/Oil Screen (not covered)
5. Testing (Preparation/Execution/Analysis)
6. Summary/Conculsion
IEEE Societies (39) – There we are! • Aerospace and Electronic Systems Society
• Antennas and Propagation Society
• Broadcast Technology Society
• Circuits and Systems Society
• Communications Society
• Components, Packaging, and Manufacturing Technology Society
• Computational Intelligence Society
• Computer Society
• Consumer Electronics Society
• Control Systems Society
• Dielectrics and Electrical Insulation Society
• Education Society
• Electron Devices Society
• Electromagnetic Compatibility Society
• Engineering in Medicine and Biology Society
• Geoscience and Remote Sensing Society
• Industrial Electronics Society
• Industry Applications Society (IAS)• Information Theory Society
• Instrumentation and Measurement Society
• Intelligent Transportation Systems Society
• Magnetics Society
• Microwave Theory and Techniques Society
• Nuclear and Plasma Sciences Society
• Oceanic Engineering Society
• Photonics Society
• Power Electronics Society
• Power & Energy Society (PES)• Product Safety Engineering Society
• Professional Communication Society
• Reliability Society
• Robotics and Automation Society
• Signal Processing Society
• Society on Social Implications of Technology
• Solid-State Circuits Society
• Systems, Man, and Cybernetics Society
• Technology and Engineering Management Society
• Ultrasonics, Ferroelectrics, & Frequency Control
• Vehicular Technology Society
Information provided by Bruce Forsyth – Weidmann
(Vice Chair IEEE Transformer Committee)
PES TECHNICAL COMMITTEES
1. Analytics Methods for Power Systems
2. Electric Machinery
3. Energy Development & Power Generation
4. Energy Storage & Stationary Battery
5. Insulated Conductors
6. Nuclear Power Engineering
7. Power System Communications &
Cybersecurity
8. Power Systems Dynamic Performance
9. Power System Instrumentation and
Measurements
10. Power System Operations, Planning &
Economics
11. Power System Relaying and Control
12. Smart Buildings, Loads and Customer
Systems
13. Substations
14. Surge Protective Devices
15. Switchgear
16.Transformers
17. Transmission & Distribution
COORDINATING COMMITTEES
1. Intelligent Grid & Emerging
Technologies
2. Marine Systems
3. Wind & Solar Power
STANDING COMMITTEES
1. Awards
2. Organization & Procedures
3. Standards Coordination
4. Technical SessionsInformation provided by Bruce Forsyth – Weidmann
(Vice Chair IEEE Transformer Committee)
Transformer Committee Org ChartChair: Sue McNelly
Vice Chair: Bruce Forsyth Secretary: Ed teNyenhuis
Treasurer: Paul Boman Standards Coordinator: Jim Graham Past Chair: Stephen Antosz
RECOGNITION & AWARDS
Stephen Antosz
ADMINISTRATIVE
Sue McNelly
STANDARDS
Jerry Murphy
MEETINGS & PLANNING
Tammy Behrens
INSULATING FLUIDS
David Wallach
PERFORMANCE CHARACTERISTICS
Craig Stiegemeier
DIELECTRIC TESTS
Ajith Varghese
INSULATION LIFE
Sheldon Kennedy
DISTRIBUTION TRANSFORMERS
Steve Shull
POWER TRANSFORMERS
Bill Griesacker
DRY TYPETRANSFORMERS
Chuck Johnson
HVDC CONVERTERXFMRS & REACTORS
Mike Sharp
INSTRUMENT TRANSFORMERS
Ross McTaggart
BUSHINGS
Peter Zhao
SUBSURFACE XFMRS & NETWORK PROTECTORS
Dan Mulkey
Information provided by Bruce
Forsyth – Weidmann (Vice Chair
IEEE Transformer Committee)
IEEE Transformer Committee Docs
For Field Engineers:
– IEEE Std C57.152™ – IEEE Guide for Diagnostic Field Testing of Fluid-Filled Power Transformers, Regulators, and Reactors
– IEEE Std C57.149™ – IEEE Guide for the Application and Interpretation of Frequency Response Analysis for Oil-Immersed Transformers
– IEEE Std C57.104™ – IEEE Guide for the Interpretation of Gases Generated in Oil-Immersed Transformers
Prerequisite Terminology
• Transformer Types and Classifications
• Transformer Configurations
• Vector Groups
• Life Expectancy
• Oil Preservation Systems
• Insulating Materials and Fluids
• Construction Forms
• Core Steel
• Nameplates
• Ratings
• Cooling Schemes
• Tap Changers (OLTC, DETC)
Prerequisites for Testing
1. Burden
2. VA
3. Sources – V and I
4. Meters – V and I
5. KVL and KCL
6. Kelvin Connection
Kelvin Connection
• 4-Wire Technique
• Exclude the resistance from the measurement circuit
leads and any contact resistance at the connection
points of these leads
• Voltage sense leads (P3 and P4) "inside" the current
leads (P1 and P2)
Aging of Insulation Systems
The influence of oxygen
Oxidation
The influence of water
Hydrolysis
The influence of heat
Pyrolysis
Life Expectancy of Transformer Insulation
•180,000 hrs or 20.55 years
•Degree of Polymerization (200 -1200 DP)
•1200 DP - New Paper
•200 DP at 150,000 hrs (end of life)
Moisture Distribution
125/95°C
85/65°C
Temp.
1,4/2,1%
2.4/2.9%
Moisture
270/420
441/1105
DP
T+ T–
Oil 16 ppm 1,1 kg H2O
cellulose Cw = 3 %
210 kg water
Distribution example:150 MVA, 7 t Cellulose,
70 t Mineral oil,
Temperature 40°C
Important to know how wet
the paper/pressboard is,
not the oil!
Transformer Tests
Dielectric Thermal Mechanical
DGA DGA SFRAOil Screen Oil Screen Leakage ReactancePF/TD CAP IR PF/TD CAPExciting Ima DC Winding RES Exciting ImaTurns Ratio Tests DC Winding RESDFRInsulation Resistance
Transformer Test Protocol
1. Overall Power Factor and Capacitance
2. Bushings (C1, C2, Hot Collar)
3. Exciting Current
4. Surge Arresters
5. Insulating Fluids
6. Leakage Reactance
7. Turns Ratio Test
8. Insulation Resistance
9. IR
10. DFR
11. SFRA
12. DC Winding Resistance
Vector Groups
Testing Focus
1. Power Factor and Variable Frequency Power Factor
2. Exciting Current
3. Turns Ratio
4. Leakage Reactance
5. DC Winding Resistance
6. Sweep Frequency Response Analysis (SFRA)
• Natural aging and deterioration
• Overheating
• Moisture ingress
• Localized defects (such as partial discharge, voids, cracks, and partial or full short-circuits)
Overall Power Factor
Power Factor / Capacitance Measurement
R CV
IR IC
ITO
T
V
ITOTIC
IR
Insulation can be modeled through:
• Capacitance (Physical Geometry)
• Resistance (Losses)
Losses can be categorized as:
• Conductive
• Polarization (60 Hz Range)
Power Factor measures bulk degradation:
• Moisture
• Aging
• Contamination
Power Factor• 0.00% - 100%
• cos φ = IR/ITOT x 100%
1) Ensure that the transformer tank and core are solidly grounded, also connect both the test instrument and power source ground to this point. We will refer to this point as the “GROUND” node.
2) Ensure that all bushing surfaces are clean and dry.
3) Completely isolate the transformer terminals; remove external connections and buswork from H1, H2, H3, X1, X2, X3 and X0.
4) Bond/short the H1, H2, and H3, making sure that they are isolated. We will refer to this point as the “HV” node.
5) Bond/short the X1, X2, X3, and X0 making sure that they are isolated. We will refer to this point as the “LV” node.
6) Document tap-positions, temperatures, humidity, fluid levels, and pressures.
ALL CABLES “IN THE CLEAR”
Overall Power Factor - Test Preparation
Two-Winding Transformer Model
• Windings are short-circuited to remove unwanted inductance• CH, CL and CHL insulation systems• CH includes H-C1• CL includes X-C1
Overall Test Data
2-WINDING TRANSFORMER – OVERALL
Measurement TypeRef@10 kV
Test # Energize Ground Guard UST Test kV I mA Cap pF
Watt
Loss
PF [%]
Measured
PF [%]
Corrected
Correction
Factor Mode
Insulation
Condition
ICH+ICHL H (prim) L (sec) 10.013 33.241 8814.88 0.746 1.00 GST
ICH H (prim) L (sec) 10.010 7.889 2089.50 0.217 0.28 0.28 1.00 GST gA PASS
ICHL H (prim) L (sec) 10.013 25.355 6725.82 0.526 0.21 0.21 1.00 UST A PASS
Calculated ICHL 25.353 6725.38 0.529 0.21 0.21 1.00 PASS
ICH-C1 = ICH minus H (prim) bushings; HV C1 ONLY 5.206 1377.91 0.156 0.30 0.30 1.00 PASS
ICL+ICHL L (sec) H (prim) 7.500 94.449 25051.64 2.375 1.00 GST
ICL L (sec) H (prim) 7.501 69.096 18325.39 1.864 0.27 0.27 1.00 GST gA PASS
ICHL L (sec) H (prim) 7.500 25.356 6725.70 0.519 0.20 0.20 1.00 UST A PASS
Calculated ICHL 25.353 6726.25 0.511 0.27 0.27 1.00 PASS
ICL-C1 = ICL minus L (sec) bushings; LV C1 ONLY 58.678 15562.15 1.619 0.37 0.37 1.00 PASS
IEEE C57.152
• PF < 0.5% at 20 °C for “new” liquid filled power transformers rated under 230kV• PF < 0.4% at 20 °C for “new” liquid filled power transformers rated over 230kV • PF < 1.0% at 20 °C for “service aged” liquid filled power transformers• PFs between 0.5% and 1.0% at 20 °C warrant additional testing and investigation
NETA MTS
• PF < 1.0% for liquid filled power transformers• PF < 2.0% for liquid field distribution transformers• PF < 2.0% for dry-type power transformers (CHL insulation)• PF < 5.0% for dry-type distribution transformers (CHL insulation)• PF Tip-Up for dry-type insulation should be < 1.0%
Note: Measured values should also be compared to the manufacturer’s published data.
Overall Power Factor - Expected Results
Bushing Power Factor
Condenser Bushing with Potential Tap
Condensers Bushing with TestTap
Non Condenser
Visual Inspection Visual Inspection Visual Inspection
C1 Power Factor (60 Hz) C1 Power Factor (60 Hz) Hot Collar Test
C1 Capacitance (60 Hz) C1 Capacitance (60 Hz) Infrared Test
C2 Power Factor (2.0 kV) C2 Power Factor (0.5 kV)
C2 Capacitance (2.0 kV) C2 Capacitance (0.5kV)
Advance Power Factor Measurements
Advance Power Factor Measurements
Power Factor Tip Up Test Power Factor Tip Up Test
Infrared Test Infrared Test
Bushing Power Factor – Test Connections
C2
C1
Hot Collar
Bushing Standard Limits
Insulation%PF IEEE (C57.19.01)
%DF(IEC 60137)
Typical %PF New values
Oil Impregnated Paper 1.5X to 2.0X <0.7% 0.2%-0.4%
Resin Impregnated Paper
<0.85% <0.7% 0.3% - 0.4%
Resin Bonded Paper <2.0% <1.5% 0.5%-0.6%
Power Factory limits at power frequency and
corrected to 20°C
• Bushings shall remain shorted, similar to the overall power factor test. Failure to short the bushing terminals, may result in compromised measurements.
• Hot Collar tests are optional; they will not be performed if test taps or potential taps are available.
• Test taps and potential taps can be identified, based on the bushing rating, as follows:
– Test Taps <= 350 kV BIL
– Potential Taps > 350 kV BIL
• C2 tests must be performed carefully, ensuring that the “hook” is in the clear, completely.
• The C1 results should compare well with the nameplate data. C1 Power Factor values should not exceed 1.5X to 2.0X nameplate data. C1 capacitance should not exceed +/- 5% of nameplate data.
• C2 values should compare well with the nameplate or amongst similar bushings.
• The hot collar results are analyzed from watts loss. We expect less than 100 mW loss.
Bushing Power Factor - Expected Results
Transformer Exciting Current Test
Vs
Exciting Current Failure Modes
• Compromised/shorted Insulation (turn-to-turn,
inter-winding, and/or winding-to-ground
insulation)
• Core and core ground defects, including
magnetization
• Poor Connections and/or open circuits
Exciting Currents - Analysis Strategy
• Confirm Expected Phase Pattern
• Confirm Expected LTC Pattern
(For load tap changing
transformers)
• Compare to Previous Results
Exciting Current - Analyzing ResultsConfirming the Expected Phase Pattern:
1. High – Low – High (HLH) Pattern➢ Expected for a 3-legged core type transformer.
➢ Expected for a 5-legged core (or shell) type transformer with a
Delta connected secondary winding.
2. Low – High – Low (LHL) Pattern➢ Will be obtained on a 3-legged core type transformer if the
traditional test protocals are not followed.
Neutral on high side Wye-configured transformer is
inaccessible
Forget to ground 3rd terminal on a Delta-connected
transformer
➢Expected for a 4-legged core type transformer.
3. All 3 Similar Pattern➢ Expected for a 5-legged core (or shell) type transformer with a
non-delta secondary winding.
Exciting Current Test Results
Transformer: Delta – Wye (Dyn1)
X1
X2
X3
X0
H1 H3
H2
Test HV Lead LV Lead Ground Float Mode Measure Result
1 H1 H3 H2, X0 X1,X2,X3 UST H1-H3 63.8 mA
2 H2 H1 H3, X0 X1,X2,X3 UST H2-H1 48.6 mA
3 H3 H2 H1, X0 X1,X2,X3 UST H3-H2 64.2 mA
Exciting Current LTC Pattern – Reactor Type
0.00
100.00
200.00
300.00
400.00
500.00
600.00
16
L
15
L
14
L
13
L
12
L
11
L
10
L
9L
8L
7L
6L
5L
4L
3L
2L
1L N 1R
2R
3R
4R
5R
6R
7R
8R
9R
10
R
11
R
12
R
13
R
14
R
15
R
16
R
Exci
tin
g C
urr
en
t
LTC Position
Exciting Current
A
B
C
Turns Ratio
Turn Ratio - Expected Results
Turn Ratio - Expected Results
The turn ratio measurement results should be
within 0.5% of nameplate markings.
Leakage Reactance
Leakage Reactance - Example
Phase V I Z R X L
H1-H3 55.22 1.05 51.59 4.38 51.41 136.4
H2-H1 54.68 1.05 51.15 4.37 50.96 135.2
H3-H2 54.46 1.05 50.96 4.46 50.76 134.2
Nameplate: 6.85% 69 kV 12.5 MVA
DC Winding Resistance - Failure Modes
A change greater than the criteria mentioned can be
indicative of the following:
1. Shorted Circuited Turns
2. Open Turns
3. Defective DETC or LTC (contacts)
4. A Poor Connection Between Terminals Measured
DC Winding Resistance -Test Procedure
1. By performing DC Winding Resistance test, this will
magnetize your core. A magnetized core will affect
your Exciting Current and SFRA Test Results.
2. Recommended to perform DC Winding Resistance
last.
3. Important to let the measurement stabilize.
Depending on the size of the transformer could take
up to several minutes
DC Winding Resistance - Case Study
SFRA - Diagnostic Category
• Dielectric• Thermal• “Mechanical”
• Use SFRA:
1. Transportation2. Post Fault
Winding Movement
Passive Components
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
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
Conclusion
• When performed properly, electrical diagnostic testing can provide useful and in depth information regarding the condition of the power transformer. Dielectric, thermal, and mechanical incipient failure modes can be identified.
• Care should be taken to ensure useful results. The test data is only as good as the technician performing the tests. The technician should always know what to expect; utilizing invalid test data can lead to an undesired result in the decision-making process.
• NETA and IEEE standards and guides provide comprehensive information regarding test plans test procedures test preparations, and analysis of the results.
Contact Information