Industry System Testing for Liquid-Filled Transformers – An Update

Weidmann-ACTI Technical ConferenceSt. Petersburg, FL

November 14, 2007

Roger WicksDuPont804-383-3300

Industry System Testing for Liquid-Filled Transformers – An Update

• Background• Kraft-Mineral Oil Update• Extending this work to other insulation

systems

Background• DuPont and Weidmann have been conducting

test using a dual-temperature aging model to thermally age papers with different degrees of upgrading (nitrogen content).

• At last year’s conference two papers were presented related to this study.

• This presentation provides an update of the testing from this study – with a brief review of information from those two papers.

Transformer Systems – IEEE/IEC

• IEEE C57.100 - Standard Test Procedure for Thermal Evaluation of Liquid-Immersed Distribution and Power Transformers.

• IEC 62332 - IEC Technical Specification – Electrical Insulation Systems (EIS) – Thermal Evaluation of Combined Liquid and Solid Components. Part 1 –General Requirements.

Existing C57.100 MethodsDistribution Transformers – The Lockie Test was developed to evaluate insulation systems for distribution transformers by aging complete transformers.

Failure in the test is a dielectric failure after thermal degredation. Several OEM’s have facilities to do this testing, specific to their internal insulation systems.

Recent testing has been conducted to develop life data for combinations involving new fluid types.

Lockie Test Facility

Picture courtesy of Cooper Industries

Existing C57.100 MethodsPower Transformer - Sealed Tube Method (Annex) method is the basis for the historical life curves from C57.92.

Very easy to do this type of test - limited equipment required. A lot of historical data developed using this method, and still broadly used to screen materials for liquid-filled transformers.

Not very representative of the actual thermal profile within a power transformer.

Sealed Tubes

Same materials as in Dual temperature cells, at ½ the size, due to reduced amount of oil in the cell to allow for expansion due to the high temperatures of the oil. Insulation pictured is just the conductor loop.

Dual-Temperature Model Aging

• Sealed Tube designed to allow independent control of conductor temperature and fluid temperature.

• Simulates hot spots within transformer windings, with allowable top-oil temperatures.

• For Cellulose-Mineral Oil aging – have used hot spot temperatures ranging from 140°C to 200°C, and top oil temperatures of 115°C.

Dual Temperature Aging Cell

Insulation System - Breakdown

Test Cells

Dual temperature cells – independent control of conductor temperature and oil temperature

Dual-Temperature Aging Model Test

• Purpose – Utilize the IEC 62332 methodology to evaluate thermally upgraded kraft – comparing results to historical data presented in C57.100.

• Products to test: Range of kraft papers from 0% nitrogen (non-upgraded paper) to 3.17% nitrogen content (Insuldur) [0, 1.13, 1.72, 2.74 and 3.17% nitrogen]

• Evaluate materials (solid and liquid) under conditions specific to their use within the insulation system

Testing Protocol

• Conductor insulation: Mechanical testing (tensile strength), DP, nitrogen content, moisture content.

• Hot spacer material: Compressibility, DP, moisture content.

• Bulk cool insulation: Mechanical testing (tensile strength), DP, moisture content.

• Mineral oil: DGA, moisture, acid number, furans.

Arrhenius Life PlotEnd of Life 50% Tensile Retention

y = 93 54 .24 4 x - 17.79 7

y = 8 0 84 .559 x - 15.56 7

0

1

2

3

4

5

6

0 .0 02 0 5 0 .00 2 1 0 .0 02 15 0 .0 02 2 0 .0 0 22 5 0 .0 0 23 0 .0 0 23 5 0 .0 0 24 0 .0 0 24 5

1/T

Log

Life

3 .2 % N2 No n-Up g raded Linear (3 .2 % N2 ) Linear (No n-Up grad ed )

Arrhenius Life Plot of 50% Tensile Retention

Nov. 2006 Aging Plot

Arrhenius Life Plot End of Life DP = 200

y = 78 36 .8 x - 14 .3 6 1R2 = 0 .9 6 3 8

y = 78 15.9 x - 14 .9 0 8R2 = 0 .8 776

0

0 .5

1

1.5

2

2 .5

3

3 .5

4

4 .5

5

0 .0 0 2 0 5 0 .0 0 2 1 0 .0 0 2 15 0 .0 0 2 2 0 .0 0 2 2 5 0 .0 0 2 3 0 .0 0 2 3 5 0 .0 0 2 4 0 .0 0 2 4 5

1/T

Log

Life

3 .2 %N2 No n-Up g rad ed

Nov. 2006 Aging Plot

DP vs. Tensile Strength Retention

y = 0.002x3 - 0.1914x2 + 11.425x - 68.248R 2 = 0.96

0.00

200.00

400.00

600.00

800.00

1000.00

1200.00

1400.00

0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00 90.00 100.00

Tensile Strength (% Retention)

DP

No n-upgraded 3.2% N2 P o ly. (No n-upgraded)

DP vs. Tensile - Paper

Dual-Temperature Method compared to Sealed Tube Test

Conducting sealed tube tests with the same cell configuration (in an oven) – all materials at 148°C. Historical expectation would be for 264 hours life. Historical aging, however likely involved sealed with air and/or moisture content. Predicted life for the sealed tube is 888 hours with our test set up and oil processing.

Non-upgraded Kraft Aging

y = -9.156479E-10x3 + 1.135695E-05x2 - 4.050719E-02x + 1.000000E+02

0.0020.0040.0060.0080.00

100.00

0 1000 2000 3000 4000 5000 6000 7000 8000

Hours at 148C

Perc

ent T

ensi

le

Rete

ntio

n

Model Testing Sealed Tube Testing

Kraft-Mineral Oil Study - Update

• Completion of initial aging, including 4 paper types

• Start the study of the affect of moisture• Comparison of the data to industry life

curves

4 Paper Arrhenius

y = 8357.4x - 16.176

y = 9630.5x - 18.395

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0.00205 0.00210 0.00215 0.00220 0.00225 0.00230 0.00235 0.00240 0.00245

1/T

Log

Life

3.18%N2 0%N2 1.72%N2 1.13%N2

200C 180C 160C 140C

4 Different Nitrogen content papers

100,000 hours

Middle two paper types

Percent Nitrogen vs. Aging

0.00.51.01.52.02.53.03.5

0.00205 0.00210 0.00215 0.00220 0.00225 0.00230 0.00235

I/T

Log

Life

1.72%N2 1.13%N2

200C 184C 160C

Sealed Tube Testing – Upgraded paperSealed Tube Testing

0

20

40

60

80

100

0 500 1000 1500 2000

Hours at 148C

Perc

ent T

ensi

le

Ret

entio

n

0% Nitrogen 3.18% Nitrogen

All samples dry when put into ovens. 50% tensile life for non-upgraded paper = 888 hours, for 3.18% Nitrogen papers = 1282 hours. C57.92 (sealed tube) predicted life for non-upgraded papers (with 0.25 to 0.50% moisture) = 267 hours, for upgraded paper = 1479 hours.

Comparison to Industry CurvesAging Curves - Non-Upgraded Kraft

y = 8357.4x - 16.176

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

0.00215 0.00220 0.00225 0.00230 0.00235 0.00240 0.00245

1/T

Log

Life

New Aging Cells -Dry Lockie Test Sealed Tubes

180C 164C 140C

Life “better” than predicted by industry curves

Effect of Moisture

One data point (green triangle) from second aging program at 1.2% moisture

Effect of Moisture on Kraft

y = -2.118653E-07x3 + 4.027244E-04x2 - 2.681548E-01x + 1.000000E+02

y = -2.523494E-05x3 + 1.130767E-02x2 - 1.604166E+00x + 1.000000E+02

020

4060

80100

0 200 400 600 800 1000Hours at 172C

Perc

ent T

ensi

le R

eten

tion

Dry Insulation 1.8% Moisture 1.2% Moisture

Effect of moisture – at temperature

Effect of Moisture on Kraft

y = -1.869676E-06x3 + 1.928611E-03x2 - 6.170323E-01x + 1.000000E+02

020406080

100120

0 100 200 300 400 500 600Hours at 180C

Perc

ent T

ensi

le

Rete

ntio

n

Dry Insulation 1.2% Moisture

Effect of Moisture on Kraft

y = -6.668867E-08x3 + 1.841667E-04x2 - 1.637252E-01x + 1.000000E+02

020406080

100120

0 500 1000 1500 2000Hours at 164C

Perc

ent T

ensi

le

Ret

entio

n

Dry Insulation 1.2% Moisture

Effect of Moisture on Kraft

020406080

100120

0 1000 2000 3000 4000 5000

Hours at 156C

Perc

ent T

ensi

le

Ret

entio

n

Dry Insulation 1.2% Moisture

These three sets of tests, along with the point from the previous slide at 172°C, can then used to develop a life curve.

Aging Curves - Non-Upgraded Kraft

y = 7461.2x - 15.052

y = 8357.4x - 16.176

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

0.00215 0.00220 0.00225 0.00230 0.00235 0.00240 0.00245

1/T

Log

Life

New Aging Cells -Dry Lockie Test Sealed Tubes New Aging Cells - Wet

180C 164C 140C

Comparative Curves

•Data “estimated” based on VERY limited data for the “wet” aging – in most cases only one cell. Also combined data for both 1.2 and 1.8% Moisture.•Need to get data for 0.5% Moisture – should fall close to C57.91 data.

Data from 0.5% moisture samples

Testing conducted on materials already wrapped (1.72% nitrogen papers).

50% point lower when wet (307 hours vs. 423 hours).

Data falls between expected life from historical C57.91 for upgraded and non-upgraded papers: non-upgraded – 133 hours, upgraded 664 hours. Not sure if this is because the paper used falls somewhere in between .

176C Aging - Moisture - 1.72% nitrogen paper

020406080

100

0 500 1000 1500 2000 2500

Hours at 176C

Perc

ent T

ensi

le

Stre

ngth

Dry Paper 0.5% Moisture Added

Further work• Re-evaluate non-upgraded and upgraded papers with

0.5% moisture content.• Evaluate effect of higher moisture contents.• Look at the effect of oil temperature.

- Single data point from related study at 150°C hot spot/130°C oil – 20% remaining tensile strength for an upgraded kraft paper after 2000 hours.

- By comparison – in our kraft study, non-upgraded paper @156°C has 50% tensile strength at over 3000 hours.

Extending this work to other insulation systems

• Aging of NOMEX® paper in mineral oil• Comparison of sealed tube testing to dual-

temperature model testing.• Learnings to date from both kraft study and

high-temperature aging with NOMEX®

paper.• IEC 60076-Part 14 provides options using a

variety of new materials.

IEEE 1276 Predicted Life

• IEEE 1276 - IEEE Guide for the Application of High-Temperature Insulation Materials in Liquid-Immersed Power Transformers

• Based on Study by GE for ESEERCO published in 1987.

• Used data points from aging which did not reach end of life

• Samples were based on sealed tube testing in mineral oil sealed with air.

Material Life Curves – IEEE 1276

Comparison Aging

Predicted Life Dry Aging Sealed Tube Initial Model Test240°C 36,000 2,100 ~7,950270°C 3,860 365 ~1,280

NOMEX® Aging Results

y = 9009.5390x - 13.0005

y = 7059.0231x - 10.4366

0123456

0.0016 0.0017 0.0018 0.0019 0.002 0.0021

1/T

Log

Life

(hou

rs)

ESEERCO Dry Aging Model Test

320 265 250 220

100,000 hours

Issues Encountered in Model Aging

• Long, high-temperature operation causes leaks from sampling ports

• Original concept of multiple oil sampling from individual cells caused issues

• Temperatures above 240°C require a new cell design

• Aged “conductor wrap” acquires shape of coil

Modified cross section for future tests

Modified cross section has helped reduce the thermal input at hotspot temperatures – have scouted operation at 320°C hotspot. Still may need additional external cooling (heat sink) to gain control at 115°C oil temperature.

Paper shape after agingWire wrap acquires shape of the conductor –leading to high variability in tensile testing

Solution is to use pre-slit tensile strips as the test papers for the aging.

IEC TS60076-14 Proposes FourHigh-Temperature Insulation Systems

• Homogeneous– High-temperature materials for all solid insulation– High-temperature insulating liquid

• Hybrid– High-temperature solid insulation only when contacting conductor– Conventional insulating liquid

• Semi-Hybrid– High temperature solid insulation on conductor only– Conventional insulating liquid

• Mixed– High-temperature insulation only at localized hot regions– Conventional insulating liquid

Temperature Limits For Insulation Systems with Mineral Oil

13090110N/AHigh Temperature insulation

hot spot temperature rise(K)

78Conventional insulation

hot spot temperature rise(K)

95756565Average windingtemperature rise

(K)

100Top oil

temperature(°C)

60Top oil

temperature rise(K)

HybridSemi-hybridMixedConventional

Temperature Limits For High-Temperature Homogeneous Insulation Systems

180150Winding hot spot temperature rise

(K)

130115Average windingtemperature rise

(K)

155130Top liquid

temperature(°C)

11590Top liquid

temperature rise(K)

Silicone liquidor

equivalent

Ester liquidor

equivalent

Typical Applications for the FourHigh-Temperature Insulation Systems

• Homogeneous– Wind turbine step-up – Compact pole-type distribution

• Hybrid– High power density mobile substation– High overload / high reliability applications

• Semi-Hybrid– Generator step-up– High ambient applications

• Mixed– Large power rectifier– Furnace application

Summary• Significant progress has been made in validating

the dual-temperature model test by evaluating the life of kraft papers in mineral oil.

• Current work involves the addition of moisture to match existing life curves that serve as the basis for the IEEE loading guides.

• Testing of higher temperature materials, as described in IEC 60076 Part 14, will allow a wide variety of insulation options for manufacturers and users of liquid filled transformers.