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