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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.
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.
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.
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.
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