Estimation of Insulation Life Based on a Dual Temperature Aging Model
Weidmann-ACTI Inc. Fifth Annual Technical Conference
Albuquerque, NM Nov. 13-15, 2006
Thomas A. Prevost, Hans Peter Gasser
Weidmann Electrical Technology
Roger Wicks, Brian Glenn, Rick Marek
DuPont
Presenter – Thomas A. Prevost, Weidmann-ACTI
ABSTRACT
In order to predict the life of oil filled power transformers it is vital to understand the
insulation system. Loading guides as well as diagnostics for power transformers are
based on insulation models which predict loss of life based on material characteristics,
temperature and time. Up to now the IEEE has defined full-scale test models for
distribution transformers but has avoided full scale tests of power transformers due
primarily to cost. As an alternative, IEEE is considering a standard model that can be
used to analyze insulation systems for power transformers. This model, known as the
dual temperature aging model, separately controls the temperature of the conductor with
its associated insulation material and the bulk oil. This paper presents preliminary results
from a dual temperature aging model study that is currently in progress. Data includes
typical diagnostic parameters such as the tensile strength and degree of polymerization
(DP) of the cellulose insulation and the furan content present in the oil. This study is also
investigating non-thermally upgraded and thermally upgraded conductor insulation to
determine the effects of thermal upgrading on insulation aging. The by-products of aging
that are the key to diagnostic analysis are also presented.
INTRODUCTION
It is a common belief that liquid filled power transformers are designed with a life
expectancy of approximately 20 years, although there are many transformers in operation
today whose life exceeds fifty years. The life of a transformer depends on the life of the
insulation system. This life is defined by a chemical process which depends primarily on
temperature and time. Other factors such as the presence of oxygen and moisture will
accelerate the aging process.
With the aging global infrastructure that we have in place today it is becoming
increasingly important to be able to accurately estimate the remaining life expectancy of
power transformers. IEEE and IEC have refined the loading guides which give the user a
basis for estimating life consumption of the transformer based on time and temperature.
However, the foundation for these loading guides are insulation life curves which are
based on both models and full-scale devices. Such life curves have been published for
many years.
Two significant changes were made during the last revision of the IEEE guide for loading
transformers. (1) The first significant change was that the three loading guides for
transformers based on size were combined into one guide. The following three guides:
1
C57.91-1981 IEEE Guide for Loading Mineral-Oil-Immersed Overhead and PadMounted Distribution Transformers Rated 500kVA and Less with 65 °C or 55 °C
Average Winding Rise
C57.92-1981 IEEE Guide for Loading Mineral-Oil-Immersed Power Transformers Up to
and Including 100 MVA with 55 °C or 65 °C Average Winding Rise
C57.115-1991 IEEE Guide for Loading Mineral-Oil-Immersed Power Transformers
Rated in Excess of 100 MVA (65°C Winding Rise) (Folded into C57.91-1995) (65°C
Winding Rise)
Were combined into one guide:
C57.91-1995 IEEE Guide for Loading Mineral-Oil-Immersed Transformers
The second major change was the insulation life criteria suggested for power
transformers. In the previous guide, C57.92-1991, the insulation life was based on sealed
tube aging tests of insulation models with the end of life criteria being 50% tensile
retention. The present loading guide, C57-91-1995, permits the user to select the criteria
most applicable to their needs. The end-of-life criteria can be based on % strength,
reduction, degree of polymerization, dielectric strength, etc. and uses an insulation-aging
factor.
While the IEEE loading guide was being revised the standard test procedure for thermal
evaluation of liquid-immersed Distribution and Power Transformers IEEE C57.100 was
also revised in 1999.(2) C57.100 defined a minimum life expectancy curve for liquidimmersed distribution, power and regulating transformers with a 65°C average winding
rise and a 80°C hottest-spot rise. This curve gives a minimum life expectancy of the
insulation system of 180,000 hours (20.5 years) at rated load as defined in C57.12.001993. This curve was taken from the loading guide for distribution transformers, C57-911981 based on full-scale models.
Figure 1
Minimum Life Expectancy Curve from IEEE C57.91-1995
2
C57.100-1999 defines test procedures for distribution transformers based on full-scale
models. For power transformers it is recognized that full-scale models are impracticable.
The standard recommends that the transformer manufacturer construct a model which
utilizes typical conductors, insulation, and supporting structures that can provide
performance which is representative of power transformer thermal endurance. The
models are based on the assumption that the conductor turn insulation is subjected to the
most critical thermal degradation and is the limiting material. The test procedure for the
power transformer models should be similar to that of distribution transformers which
involves circulating current through the windings to elevate the temperature to the desired
test condition and time and then applying end-point tests on the model. The end-point test
for power transformer models, as with full-scale distribution transformer models, is
comprised of a short circuit test followed by dielectric test.
The IEEE PES transformer committee currently has a project to revise C57.100-1999.
There is concern that the requirement for model testing of the insulation system for power
transformers is not well defined and could be costly to perform because each power
transformer is unique from a design and construction standpoint. The working group is
considering standardization of an insulation system model which includes independently
controlled conductor and bulk oil insulation which can economically model insulation
systems for power transformers. A significant task for the working group will be to
establish end-of-life criteria for the insulation system for this model.
END-OF-LIFE CRITERIA
In order to properly measure and then predict insulation life it is critical to understand the
basis from which any reference values are based. Over the years there have been many
studies on insulation aging which have generated significant and often conflicting results.
(3),(4),(5)
It is important to understand the insulation system model which served as the basis
for these studies as well as the criteria for determining end-of life. Early work depended
on sealed tube models in which insulation materials, oil and other construction materials
were placed in a sealed tube and then heated to the desired aging temperature. These
models were relatively inexpensive to manufacture so that it was practical to obtain
considerable amounts of data. This test, however simple though, was never standardized
in regard to material volumes and ratios so that comparing the results of these test is
difficult. The early consensus of end-of-life for sealed tube aging models was based on
50% retention of tensile strength. In 1948 Dakin published a paper(6) in which he
premised that insulation aging was a chemical rate phenomenon and that deteriation of
insulation strength could be modeled by the Arrhenius reaction rate theory with the
following form:
B


)
Per Unit Life = AExp (
 Θ + 273 
Where:
Θ = Winding hottest spot temperature, °C
A = Constant
B = Constant
Equation 1
Arrhenius Reaction Rate
3
The Arrhenius rate theory is still used today to describe the aging process of insulation in
transformers.
The criteria for end-of-life for insulation models has continued to be debated. It is well
known that many transformers that have conductor insulation with less than 50% tensile
strength retention continue to operate with no problems. Accordingly, a value of 25%
retention has been suggested as end-of life. Another argument has been to use a
measurement of the degree of polymerization of the paper rather than the relative
mechanical strength.(7) This parameter is more practical when estimating remaining life
on equipment in service because it is almost impossible to determine a baseline value for
the mechanical strength. As mentioned earlier, another approach for end-of-life is to base
failure criteria on dielectric tests following short-circuit tests on transformers or
insulation models. Each of these end-of-life criteria give different expected life values for
standard 65 °C rise, insulation systems. Table 2 in C57.91-1995 lists these values for
various end-of-life criteria.
Table 2 - Normal insulation life of a well-dried, oxygen-free 65ºC average winding
temperature rise insulation system at the reference temperature of 110ºC
Normal insulation life
Basis
Hours
Years
50% retained tensile strength of insulation (former IEEE Std.
C57.92-1981 criterion)
65,000
7.42
25% retained tensile strength of insulation
135,000
15.41
200 retained degree of polymerization in insulation
150,000
17.12
Interpretation of distribution transformer functional life test
data (former IEEE Std. C57.91-1981 criterion)
180,000
20.55
NOTES
1 - Tensile strength or degree of polymerization (D.P.) retention values were determined by sealed
tube aging tests on well-dried insulation samples in oxygen-free oil.
2 - Refer to I.2 in annex I for discussion of the effect of higher values of water and oxygen and also for
the discussion on the basis given above.
Figure 2
Normal Insulation Life Values from IEEE C57.91-1995
DUAL TEMPERATURE AGING MODEL
While IEEE C57.100-1999, Standard Test Procedure for Thermal Evaluation of LiquidImmersed Distribution and Power Transformers, has a practical, cost-effective procedure
for thermal evaluation of insulation systems for distribution transformers, many feel that
it is not practical for larger power transformers. Furthermore, there is concern that the
sealed tube test method is an unrealistic model. For these reasons, a Dual Temperature
4
Aging Model is being developed. In this model the insulated conductor and associated
turn spacer insulation can be controlled at one temperature while the bulk oil and its
associated barrier type insulation can be controlled at a second temperature. This model
has been incorporated into IEC TS 62332, IEC Technical Specification – Electrical
Insulation Systems (EIS) – Thermal Evaluation of Combined Liquid and Solid
Components. Part 1 – General Requirements. (2005).
A study is currently underway to measure several potential end-of-life criteria during
aging experiments in order to establish appropriate values for this model. When selecting
end-of-life criteria it is important not only to be able to correlate these with actual service
conditions but also to be able to measure these properties with established laboratory
methods which give repeatable results and are cost effective. The use of retained tensile
strength has remained a useful life criterion for this reason. The degree of polymerization
has become widely recognized as well. Other diagnostics methods such as furanic
compound measurement and DGA are being determined in order to provide correlation
information to support in-service diagnostics for estimation of remaining insulation life.
Test Program:
To verify a base model and then to demonstrate that this model can respond to different
insulation materials the following configurations were tested:
1) Non-Thermally upgraded crepe conductor insulation together with high density
precompressed kraft pressboard spacer material and low-density pressboard
barrier (bulk oil) insulation material.
2) Insuldur thermally upgraded conductor insulation with spacer and bulk oil
insulation materials as in (1).
These models were aged at different temperatures and at least three different times in
order to establish aging curves for each of the models. The results will be compared to
existing curves in the IEEE loading guide that establish the baseline for conventional
insulation systems.
Aging criteria:
The criteria for end of life will be dependent on the performance criteria for specific
insulation materials. These are:
Conductor Insulation:
Tensile retention
DP (Degree of Polymerization)
Other properties that were measured in order to understand the aging mechanisms:
Fluid:
Gasses
Moisture
Acid
Furans
RESULTS:
For each paper type and aging temperature a polynomial regression curve was plotted in
order to determine the time at which the end-of-life parameter was reached. These values
5
were then plotted on the Arrhenius plots to develop life curves. As an example two of the
plots for obtaining 50% tensile retention values are shown below as figures 3 and 4.
176C Upgraded 3.2% N2
100.00
90.00
Percent Tensile
80.00
70.00
60.00
50.00
40.00
30.00
20.00
10.00
0.00
0
500
1000
1500
2000
2500
Aging Hours
Figure 3
Plot of tensile retention vs aging time at 176 C
50% Tensile Retention = 692 hours
192 Upgraded 3.2% N2
Percent Tensile
100.00
80.00
60.00
40.00
20.00
0.00
0
100
200
300
400
500
Aging Hours
Figure 4
Plot of tensile retention vs aging time at 192 C
50% Tensile Retention = 176 hours
The corresponding end-of-life time for each aging temperature was then plotted
following the Arrhenius relationship to develop life curves based on the results of the
dual temperature aging model. The two properties, which were used to determine end-oflife, were tensile strength retention and degree of polymerization.
6
% Tensile Retention
Arrhenius Life Plot
End of Life 50% Tensile Retention
3.2% N2
Non-Upgraded
Linear (3.2% N2)
Linear (Non-Upgraded)
6
5
y = 9354.244x - 17.797
Log Life
4
y = 8084.559x - 15.567
3
2
1
0
0.00205
0.0021
0.00215
0.0022
0.00225
0.0023
0.00235
0.0024
0.00245
1/T
Figure 5
Arrhenius Life Plot of 50% Tensile Retention
Degree of Polymerization
Arrhenius Life Plot
End of Life DP = 200
3.2%N2
Non-Upgraded
5
4.5
y = 7836.8x - 14.361
4
2
R = 0.9638
Log Life
3.5
3
2.5
y = 7815.9x - 14.908
2
2
R = 0.8776
1.5
1
0.5
0
0.00205
0.0021
0.00215
0.0022
0.00225
0.0023
1/T
Figure 6
Arrhenius Life Plot of 200 DP
7
0.00235
0.0024
0.00245
Arrhenius Life Plot
Thermally Upgraded Paper N2=3.2%
50% Tensile
C57.92
C57.91
4
DP 200
y = 9354.2x - 17.797
R2 = 0.9295
3.5
y = 6328.8x - 11.269
3
y = 7836.752x - 14.361
2
Log Life
R = 0.964
2.5
y = 6972.2x - 13.391
2
1.5
1
0.5
0
0.0021
0.00215
0.0022
0.00225
0.0023
0.00235
1/T
Figure 7
Plot of Tensile Retention and DP versus standard aging curves
Relationship of Tensile Strength Retention versus DP
In this study we found a strong relationship between the tensile strength retention and the
degree of polymerization. The influence of thermal upgrading did not affect this
relationship.
DP vs. Tensile Strength Retention
Non-upgraded
3.2% N2
Poly. (Non-upgraded)
1400.00
1200.00
y = 0.002x3 - 0.1914x2 + 11.425x - 68.248
R2 = 0.96
1000.00
DP
800.00
600.00
400.00
200.00
0.00
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)
Figure 8
Relationship between DP and % Tensile Retention
(Note: this includes thermally upgraded and non-upgraded paper)
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FURANS
Background
Because it is very difficult to obtain paper samples from in service transformers to
determine insulation life, an alternative method has been developed. In this method the
insulating oil is analyzed for furan content. Furanic compounds are produced during the
breakdown of the cellulose insulation in transformers. There have been many studies
which correlate the furan content in the oil to the degree of aging of the cellulose
insulation. (8),(9)
As the cellulose insulation ages the polymer chain breaks down. Each splitting of the
chain liberates a glucose monomer which undergoes further chemical reaction and
becomes one of several furanic compounds. Measurement of the furanic compounds,
which are partially soluble in oil, can then give an estimate of the DP. Chendong
developed a widely accepted relationship between the amount of 2-furaldehyde and
DP.(10) Based on field data and laboratory studies he developed the following equation:
Log10[2-FALPPM] = 1.51 - .0035 * DP
Equation 2
Chendong’s Relationship between 2-Furaldehyde and DP
The correlation between furan content and DP has been developed by the analysis of
actual data in thousands of transformers. It needs to be noted that most of these studies
were done on European transformers which in most cases do not contain thermallyupgraded paper. Subsequent studies done looking at the transformer database based on
thermally upgraded insulation and non-upgraded insulation supported a theory that the
thermal upgrading chemicals react with the furans breaking them down. (11) Stebbins
proposed a modified Chendong equation to be used for transformers with thermally
upgraded paper.
DP= [Log10(2-FALPPB * 0.88) – 4.51]/-.0035
Equation 3
Stebbin’s Modified Chendong Equation for Thermally Upgraded Kraft
When Stebbin’s equation is put into the same form as Chendong’s equation it becomes:
Log10[2-FALPPM] = 1.5655 - .0035 * DP
Equation 4
Stebbin’s Modified Chendong Equation for Thermally Upgraded Kraft
9
Results of test:
Log 2-furaldehyde vs. Paper DP
Kraft DP
TU Kraft DP
Linear (TU Kraft DP)
Linear (Kraft DP)
5
4.5
Log (2-FAL - ppb)
4
3.5
3
2.5
2
1.5
1
0.5
0
0.0
100.0
200.0
300.0
400.0
500.0
600.0
700.0
Paper DP
Figure 9
Log 2-FAL versus DP for Thermally upgraded and Non-Thermally Upgraded Paper
Log 2-furaldehyde vs. Paper DP
DuPont-Weidmann Aging
Chengdong Aging
Stebbins (mod. Chendong)
Linear (DuPont-Weidmann Aging)
5
4.5
Log(2-FAL)-ppb
4
3.5
3
2.5
2
1.5
1
0.5
0
0
100
200
300
400
500
600
700
800
900
1000
DP
Figure 10
Dual Temperature Aging Study Results versus Chendong and Stebbins
Based on the results of this study we found that there was minimal difference between the
formation of 2-FAL between non-thermally upgraded kraft paper and thermally upgraded
kraft paper.
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SUMMARY AND CONCLUSION:
End-of-Life Criteria
The results of the Dual Temperature Aging Model tests to date suggest that the end-oflife for thermally upgraded kraft paper insulation systems are close to the values
suggested in the IEEE Loading Guide C57.91-1995 for insulation models. This gives an
insulation life of 180,000 hours at a hot spot temperature of 110 °C. This is in contrast to
an end-of-life value of 65,000 hours for thermally upgraded kraft paper when aged in a
sealed tube at 110 °C.
A DP value of 200 correlated very closely with 50% tensile retention. This contradicts
earlier studies in which a DP value of 200 was found in samples with % tensile retention
values in the range of 25%.(7) Although one study by Schoff in 1985 stated that the DP
dropped to a value of 250 when the tensile strength was reduced to 50% of its original
value.(5)
The authors are collecting more data to support this aging study. The conclusion to date is
that end-of-life value for both 50% tensile retention and a DP value of 200 is close to the
life curve given in the IEEE Loading guide of 180,000 hours (20.5 years) when operated
continuously at rated load.
Furans
The results of this study supported the theory that there is a relationship between the
concentration of 2-Furaldehyde and the DP of the paper. There was no difference in this
relationship between thermally upgraded and non-thermally upgraded paper.
Measurement of Furans in transformer oil should enable one to predict remaining
insulation life.
Prediction of Insulation Life
The results of the Dual Temperature Aging Study demonstrate that the criteria given in
the IEEE Loading Guide, IEEE C57.91-1995 for distribution transformer models is
applicable to power transformers as well. The challenge to the asset manager is how to
apply this information when attempting to measure remaining life of a transformer in
service. Ideally one should take samples of the insulation periodically for measurement of
either tensile strength or DP. However, this is nearly impossible since the critical paper is
located within the transformer windings at the hot spot. Periodic measurement of furan
content in the oil is therefore the most cost-effective diagnostic for prediction of
remaining insulation life. The data from this study supports a strong correlation of furans
to DP.
11
REFERENCES
1. IEEE C57.92-1995, Guide for Loading Mineral-Oil-Immersed Transformers
2. IEEE C57.100-1999, IEEE Standard Test Procedure for Thermal Evaluation of
Liquid-Immersed Distribution and Power Transformers.
3. Montsinger, V.M., “Loading Transformers by Temperature” AIEE Transactions,
vol. 49, 1930 pp. 1151-1162
4. McNutt, W.J. “Insulation Thermal Life Considerations for Transformer Loading
Guides”, IEEE Transactions on Power Delivery, vol. 7, no. 1 pp. 392-401, Jan.
1992
5. Shroff, D.H. and Stannet, A.W., “A Review of Paper Aging in Transformers”,
IEE Proceedings, vol. 132, pt. C no. 6, Nov. 1985
6. Dakin, T., “Electrical Insulation Deteriation Treated as a Chemical Rate
Phenomenon”, AIEE Transactions, vol. 67, 1948 pp. 113-122
7. Oommen, T.V. and Arnold, L.N., “Cellulose Insulation Materials Evaluated by
Degree Of Polymerization Measurements”, CH1717-8/81/0000-0257 pp. 257261, 1981
8. De Pablo A., “Recent Research Relating to the Usefulness of Furanic Analysis to
Transformer Condition Assessment:,CIGRE, paris, 1998, WG 15-01.
9. Griffin, P.J., Lewand, L.R., Finnan, E., Barry, J., “Measurement of Cellulosic
Insulation Degradation”, Minutes of the Sixtieth Annual International Conference
of Doble Clients, Doble Engineering Company, 1993, Section 10-3.
10. Chendong, I. , “Monitoring Paper Insulation Aging by Measuring Furfural
Contents in Oil”, Seventh International Symposium on High Voltage Engineering,
Dresden, August 1991.
11. Stebbins, R.D., Myers, D.S., Shkolnik, A.B., “Furanic Compounds in Dielectric
Liquid Samples: Review and Update of Diagnostic Interpretation and Estimation
of Insulation Ageing”, Proceedings of the 7th International Conference on
Properties and Applications of Dielectric Materials, 2003. Volume 3, 1-5 June
2003 Page(s):921 - 926 vol.3
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