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A2-201
CIGRE 2012
POWER TRANSFORMERS
with environmentally friendly and low flammability ester liquids
G. J. PUKEL1, R. SCHWARZ1, F. BAUMANN1,
H. M. MUHR2, R. EBERHARDT2, B. WIESER2, D. CHU3
1
Siemens Transformers Austria GmbH & Co KG,
Elingasse 3, A-8160 Weiz, Austria
2
Department of High Voltage Engineering and System Management,
Graz University of Technology, Inffeldgasse 18, A-8010 Graz, Austria
3
Consolidated Edison Co. of New York, Inc.,
Irving Place NY 10003-3598
SUMMARY
To improve personal safety and reduce the environmental impact of electrical power supply,
alternative insulation fluids are demanded for large power transformers. With their
biodegradability, the fact that they can be produced from renewable resources, and the high
fire point properties, ester fluids are becoming potential substitutes for mineral oil. A number
of companies offer such alternative insulation fluids, and in many distribution transformers
they are already used satisfactorily. So far however there is little experience for use of these
liquids in large power transformers. Even in proven high-voltage applications with mineral
oil, every change in transformer design or materials needs to be evaluated. Changing such a
crucial component as the insulating liquid itself, the suitability must be proven most
thoroughly. This paper illustrates various issues which must be clarified in order to
successfully operate power transformers filled with these environmentally friendly, renewable
and barely inflammable insulating liquids.
KEYWORDS
Power transformer, insulating liquid, synthetic ester, natural ester, vegetable oil
georg.pukel@siemens.com
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INTRODUCTION
Many investigations have been carried out and are still in progress concerning alternative insulating
fluids [1]. These investigations are driven by research facilities at universities, but also by liquid
suppliers, utility companies and transformer manufacturers. Over the past 15 years the performance
demanded and the standards for transmission equipment – such as transformers and their insulation
systems – have increased dramatically [2]. Areas of high population density are growing more and
more. To assure the rising energy demand even large power transformers have to be placed in these
areas. When power transformers of several hundred MVA must be accommodated in residential tower
blocks, terms like fire point and environmental effects become increasingly important.
Figure 1 shows typical values for flash and fire points of different insulating fluids. This graph shows
that ester based fluids are advantageous compared to mineral oil. Figure 2 demonstrates the
biodegradability of insulation fluids where natural and synthetic esters meet the criteria for
classification as “readily biodegradable”.
Flash point [°C]
100
Fire point [°C]
Biodegradation in %
Natural ester
Synthetic ester
Silicon oil
Mineral oil
0
100
200
300
400
Temperature in ° C
Figure 1: Typical flash and fire points of
various types of insulating fluids [3-9]
80
60
Natural ester
Synthetic ester
Mineral oil
Silicone oil
40
20
0
0
5
10
15
20
Time in days
25
30
Figure 2: Biodegradability of
various types of insulation fluids [22]
To heat up 1 liter of liquid from for example 70 °C to the fire point, about 170 kJ is necessary for
mineral oil and about 500 kJ for synthetic ester (see formula below). Thus approximately 2,9 times the
energy is needed to heat up synthetic ester to fire point in comparison to mineral oil.
W   V  c  (Tfirepoint Toil temperature)
W
ρ
V
c
Tfire point
Toil temperature
= Energy in J or Ws
= Density of fluid in kg·m-3
= Volume of fluid in l
= Specific heat in Ws·kg-1·K-1
= Fire point of the fluid in °C (fluid in air)
= Oil temperature in °C
TYPES OF INSULATING LIQUIDS
Mineral oil
Mineral oil is made from fossil oil and consists of hydrocarbon compounds with various bonds. These
molecule structures can be divided into paraffinic, naphthenic, aromatic and olefin bounds. These
components are contained in varying ratios in all mineral oils [10]. The main disadvantages of
transformer oil are the low fire point (Figure 1) and the very limited biodegradability characteristic
(Figure 2).
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Silicone fluids
Silicone fluids specially developed for transformer applications are fully synthetic coolants and
insulation fluids. Due to the high ignition temperature and the self extinguishing behavior these liquids
present a lower fire hazard than mineral oil [4].
The thermal stability, even under the presence of air, is better than that of the other liquids.
Its high viscosity at higher temperatures (Figure 7) and poor lubrication properties are disadvantages
in transformer application. The very low biodegradability and the formation of jelly-like bridges of
silicone-oxide under arcing are further disadvantages.
Synthetic esters
Synthetic esters are derived from chemicals. They are usually the product of a polyol with synthetic or
natural carboxylic acids to give structures where several acid groups are bonded to a central polyol
structure. Polyol is a molecule with more than one alcohol functional group. The acids used are
usually saturated in the chain, giving the synthetic esters a very stable chemical structure [12].
The viscosity of synthetic ester fluid is about four times higher than the viscosity of mineral oil at
room temperature (Figure 7). Their flash and fire points are higher than those of mineral oil (Figure 1)
[3, 5, 6].
Natural esters
Natural ester fluids can be broken down into saturated, single-, double and triple unsaturated fatty
acids. Saturated fatty acids are chemically stable, but have a high viscosity. Triple unsaturated fatty
acids have a lower viscosity but they are very unstable in oxidation. To reach an acceptable value of
oxidation stability of natural esters, it is necessary to add suitable antioxidants. In addition to DBPC
(2,6-di-tert-butyl-p-cresol) specific antioxidants that use complex phenols, amines are in operation. To
avoid an unacceptable increase in the conductivity, the total amount of antioxidant is limited to 1 %
and less. Fluids with a high percentage of single unsaturated fatty acids have proven to be suitable.
The viscosity of natural esters is about four times higher than the viscosity of mineral oil. Their flash
and fire points are significantly higher compared to mineral oil [3, 6, 7, 8, and 9].
COMPARISON OF ESSENTIAL PROPERTIES
The basic purposes of transformer oil are electrical insulation, cooling, and lubrication of sliding
components. The different properties of each fluid have to be investigated to make sure that the
transformer meets customer expectation. The following issues have to be clarified before successfully
using environmentally friendly insulating liquids in power transformers:







Verification of the dielectric strength compared to mineral oil, for characteristic configurations
Impregnation of insulating material
Influence on the cooling system
Material compatibility
Oxidation behavior and interaction with cellulose
Components (bushings, tap changers, pumps)
Limits for DGA and other fluid parameters
DIELECTRIC DESIGN
The comparison of insulating fluids for the application in power transformers does not only involve
testing the oil characteristics itself. It is also important to investigate the whole insulation system.
Standards like IEC 60156 [14] test only the oil breakdown. To design a power transformer the
interaction between oil and solid insulation material has to be considered. Very important properties
georg.pukel@siemens.com
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are partial discharge, creep and breakdown behavior. Characteristic arrangements have to be
developed and tested in the different insulating fluids. Verification of the dielectric strength for the
different voltage stresses AC, LI (Lightning Impulse) and SI (Switching Impulse) is essential as
criteria for the insulation design of power transformers. These tests are required in IEC 60076-3 [15]
as routine tests for all transformers with voltage higher than 72,5 kV.
Investigations
Various test arrangements were created which represent the actual situation in power transformers
(Figure 3). These tests must cover breakdown in free oil space, through boards and papers, and surface
creep. In addition to the arrangements related to power transformers, very inhomogeneous
configurations were investigated to get broader knowledge of the fluids properties.
Figure 3: Characteristic test arrangements in relation to transformer situation
Flat wire model
This model aims to represent creepage behavior on medium length paths with considerable
inhomogeneity.
Flat copper conductors were bent into a U-shape and placed on the edge of upright pieces of
transformerboard, thus forming a gap with creepage path towards the grounded disc electrode, as
shown in figure 4. The flat conductors with 0,5 mm edge radius were either bare copper, or with a
machine-wrapped paper insulation of 0,75 mm. The gap length ranged from 10 mm to 100 mm. The
field inhomogeneity was calculated with a numeric field simulation program. These arrangements
were tested with AC, lightning impulse and switching impulse in mineral oil, synthetic and natural
ester.
Figure 4: Electric field pattern and physical flat wire model
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The procedures for drying and impregnation were carried out as follows: Samples were dried for one
week at 65 °C under a vacuum of less than 3 mbar. The fluids were dried, degassed and filtered with a
treatment device before filling. The samples were then installed in a specific test vessel, placed under
vacuum and impregnated with fluid at elevated temperature. The impregnation time was about three
days until tests were performed.
The breakdown voltage for impulse was measured by the rising voltage procedure on the basis of
IEC 60243-3 [16]. The starting voltage was set at about 70 % of the expected breakdown voltage. The
voltage was increased in steps of 10 kV/1 min, applying one shot per step. At least six samples per gap
distance and model type were tested. At least 5 minute break was given after each breakdown to let the
discharge by-products and gas bubbles diffuse.
The following example considers the results for the negative full wave lightning impulse behavior.
Figure 5 shows the average breakdown voltage for different creepage distances. To clearly compare
the difference between the model variations and the fluid types, the graphs are scaled in percent of the
highest average breakdown voltage that has occurred in the test.
100
Voltage level in %
80
60
40
20
mineral oil b
mineral oil iso
synthetic ester b
synthetic ester iso
natural ester b
natural ester iso
b... bare electrode
iso... isolated electrode
0
0
10
20
30
40
50
60
70
80
90
100
Distance in mm
Figure 5: Average breakdown voltage at negative full wave lightning impulse
The highest breakdown voltage that could be applied in the test vessel limited the isolated electrode
tests to a distance of 50 mm. With bare electrodes the 100 mm distance could be broken easily. The
basic shape of the graphs is well-known and shows the non-linear increase in strength with increased
distance. With bare electrodes the esters are somewhat weaker than mineral oil at all distances. Optical
detection of pre-discharge indicate faster streamer propagation in esters: this was previously
investigated and reported in [17, 18, 19].
Paper insulation performance is equally good or even better in esters. This could be attributed to the
closer epsilon-match; especially effective for shorter distances.
To derive design criteria, the one percent withstand voltage has to be calculated from the average
breakdown voltage, based on the standard deviation (not shown in this diagram). In many tests the
deviation in esters was wider than in mineral oil. With the high number of individual breakdown
values, the distribution shape can be considered. It shows a deviation from the symmetric Gaussian
towards the skewed Weibull distribution. The negative skew with its fatter tail at low voltages
demands more conservative 1 % limits.
georg.pukel@siemens.com
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THERMAL DESIGN
Temperature models are generally used in the calculation of the temperature distribution in
transformers. These empirical models work well as long as the parameters are constant. If fluids with
different characteristics are used, such models will no longer work correctly. When we use alternative
liquids in transformers, the whole cooling cycle (Figure 6) and the physical properties of the insulating
fluid have to be considered.
Characteristic temperatures:
1) Bottom oil winding
2) Top oil winding
3) Top oil cooler
4) Bottom oil cooler
Figure 6: Schematic cooling cycle
The most important physical parameters of liquids are the kinematic viscosity, the thermal capacity
and the thermal conductivity. The hydraulic resistance of the whole cooling cycle is proportional to the
kinematic viscosity.
Figure 7 shows the kinematic viscosity and the specific heat capacity of various insulating fluids.
Natural and synthetic esters have viscosity values approximately 4 times higher than mineral oil at
typical transformer operating temperatures.
10000
3000
mineral oil
natural ester
synthetic ester
Specific Heat in Ws/kg/K
Kinematic Viscosity in mm 2/s
1000
silicone fluid
100
10
2000
mineral oil
natural ester
1000
synthetic ester
0
1
-20
0
20
40
60
80
Temperature in °C
100
120
140
-20
0
20
40
60
80
100
120
140
Temperature in °C
Figure 7: Typical viscosity and specific heat capacity values of insulating fluids [3-8]
The calculation of the temperature distribution based on the physical properties in a disc winding for a
power transformer with different fluids is shown in Figure 8 for natural oil flow. The different liquid
properties influence the hot spot temperature; therefore slight modifications of the winding design are
required.
georg.pukel@siemens.com
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Mineral oil
Hot spot
70 K
Synthetic ester Natural ester
77 K
80 K
Figure 8: Calculated temperature distribution in a disc winding with different insulating fluids
TRANSFORMER MANUFACTURE
Impregnation
Due to the higher viscosity and different interfacial tension values of ester liquids the impregnation
process had to be reviewed. Comparative studies of pressboard in mineral oil, synthetic and natural
ester were performed (Figure 9).
Figure 9: Pressboard and laminated wood samples
In order to be able to make a direct comparison of the immersed samples, it is necessary to take into
account the different individual density values by adjusting the measured increase in weight, due to oil
absorption. Figure 10 shows the deviation from final value of liquid content in transformer board in %
(with density correction).
With reference to the final value: the last measured weight is taken as the 0 % level. Three test
samples for each liquid were used. To highlight the worst case, the fastest increase of weight of
mineral oil samples was plotted (blue curve) and the slowest for the two other liquids (brown and
green curve).
The average values show only slight difference to Figure 10.
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Deviation from final value of
liquid content in board in %
60
Mineral oil
50
Synthetic ester
40
Natural ester
30
20
10
0
0
12
24
36
48
Impregnation time in h
Figure 10: Impregnation of pressboard
The test results show that an extension of the impregnation time is necessary. An additional measure
would be the increase of the impregnation temperature.
Material compatibility
A further important topic is the material compatibility. A large range of materials (e.g. gaskets, rubber
bag, enamel, etc.) were tested for compatibility with ester fluids by performing accelerated aging at
120 °C and 140 °C for two weeks (figure 11).
Figure 11: Materials tested for compatibility
300
100
Tensile strength in N
200
after drying
mineral oil
150
synthetic ester 1
100
synthetic ester 2
natural ester 1
50
natural ester 2
Tensile strength in N
NBR
250
80
FPM
after drying
60
40
20
mineral oil
synthetic ester 1
synthetic ester 2
natural ester 1
natural ester 2
0
0
Figure 12: tensile strength for NBR and FPM gaskets
The samples were cut into pieces or put together, ensuring that there is a defined surface area of each
sample present for testing. Each test sample, with the exception of some specified samples (gaskets)
was dried at 105 °C. Each test sample was then weighed, measured, labeled and placed into separate,
clean glass jars, containing the test fluids. A portion of each sample was kept at ambient as a
reference.
The brittleness or the large change of the fluid parameters (color, acid value, etc.) observed in a
number of samples reveal that these materials are not compatible with ester fluids at high temperatures
(e.g. NBR gaskets).
georg.pukel@siemens.com
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In addition to the above mentioned parameters the degree of polymerization (DP) of the paper
samples, which have been aged at 140 °C in different insulating liquids, has been analyzed. The
degree of polymerization gives the number of chemical bonded glucose rings per cellulose molecule
and is an indicator of the mechanical strength of the paper. New paper has a DP of about 1000; a DP
of 200 is normally used as a limiting value representing end of life of the insulation. Note however
that an unfortunate decrease in the DP number of new papers has been observed in the past few years:
new paper in the "as received" condition in former times had a DP over 1100.
Figure 13 shows the results of the investigation into aging behavior for different insulating liquids,
with and without contact with air. The test clarifies that esters fluids have better behavior concerning
paper aging than mineral oil. In comparison to new paper, the degree of polymerization of the aged
samples clearly decrease, but the decrease of DP for the ester aged samples is not as high as for the
mineral oil aged samples. There are differences among the various types of natural ester. These results
correspond to the outcome of [20, 21].
1200
1052
closed
Degree of polymerization
1000
open
800
600
578 to 718
600
495
398 to 532
400
345
320
200
0
new
natural ester
synthetic ester
mineral oil
Figure 13: Degree of polymerization of the paper after aging
CONCLUSION
When alternative liquids are used in power transformers, many parameters have to be evaluated.
Simply exchanging the insulating fluid from mineral oil to ester could lead to a dielectric problem or
to reduced life time of the transformer due to overheating of the windings.
The comparison between different insulating liquids requires the collation of many different
parameters. Many tests have already been done and valuable data has been collected.





In many tests the deviation in esters was wider than in mineral oil. More conservative 1 %
limits are demanded. Design modifications of the windings and barrier system are required.
Due to the much higher moisture capacity of esters, the interaction with cellulose has to be
considered carefully.
Physical parameters of the fluids have to be considered for the calculation of the temperature
distribution.
Manufacturing processes have to be adapted to make sure that all insulation parts are fully
impregnated and only materials are used which are fully compatible.
Esters fluids have a better behavior concerning cellulose aging than mineral oil (degree of
polymerization).
Medium power transformers up to 135 MVA and 245 kV with ester liquids have already been tested
successfully by Siemens.
georg.pukel@siemens.com
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Acknowledgments
The authors would like to thank the members of the High Voltage Test Laboratory at Graz University
for their assistance during the testing.
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