An Overview FEM.pdf

8/9/2019 An Overview FEM.pdf

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An Overview of the Suitability of Vegetable Oil Dielectrics for Use inLarge Power Transformers

Daniel Martin, Imad Khan, Jie Dai & Zhongdong WangUniversity of Manchester

Abstract

It would seem a regular event that some sort of disruption affects the world’s oil supply.

Natural disasters such as hurricanes at sea and Alaskan oil spills impinge on supply while the

mere hint of conflict in certain parts of the world sends oil prices spiralling. Couple this with

ever growing world demand for oil and controversy whether oil production is past its peak

adds to the urgency to explore the use of non-fossil oils. Therefore, it can be considered of

the utmost importance to develop renewable technologies now to reduce future reliance on

fossil products. This paper presents some of the findings of the project to ascertain the

suitability of vegetable oil based dielectrics as alternatives to mineral oil in large powertransformers. So far, all of the evidence suggests that vegetable oils are indeed suitable

replacements.

Aims and Objectives of Research Project

This project has been initiated at the University of Manchester through the funding provided

by the UK Engineering and Physical Sciences Research Council and a variety of Utilities and

manufacturers involved in the power industry. These companies are AREVA T & D, EdF

Energy, National Grid, M & I Materials, Scottish Power, TJH2B and United Utilities. The

aim is to investigate whether ester oils are suitable replacements for mineral oil in large

power transformers above 132kV voltage levels. If esters are found not to be compatible withcurrent transformer designs, then suggest modifications to designs to permit the use of ester

oils.

Terminology

There are two types of esters available for use, one being a natural ester which has been

refined from plant materials and the second is the synthetic ester, manufactured industrially.

There are slight differences in natural and synthetic ester molecular structure which lead to

interesting physical and chemical differences.

Comparing Esters to Mineral Oil

Esters have higher flash and fire points than mineral oil making esters better suited to

transformers and essential in environments, such as underground or offshore, where fire

prevention is of high priority.

Esters are more biodegradable than mineral oil so if spillage occurs cleanup costs are

reduced. High biodegradabilities are demonstrated by esters when compared to regular

mineral oil1

. FR3 is covered by the U.S. Edible Oil Regulatory Act2

. Midel 7131 meets the

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Effect of Moisture on Oil Dielectric Strength

There are two mechanisms for oil to increase in water content in a transformer. The first is

through absorption from the atmosphere, unlikely if an air drier is used or the transformer is

sealed, and the second is ageing of the cellulose insulation creating water 11

. It has beenconcluded that it is the percentage saturation of water in the oil which affects breakdown

voltage rather than the absolute moisture content12

. As esters are far more hygroscopic than

mineral oil13

, moisture has less of an impact on the dielectric strength of esters than mineral

oil.

Table 1

Comparison of water solubility of transformer oils

Oil Solubility @ 20°C (ppm)

Mineral oil 55

Natural ester 1,100

Synthetic ester 2,700

In an investigation performed at the University of Manchester, glycerol solution was used to

vary air relative humidity in a desiccator. Oil was then allowed to uptake moisture from the

air over the duration of a week. It can be seen from figure 1 that esters retain high AC

dielectric strength with increasing moisture.

Figure 1

Oil breakdown voltages as functions of absolute moisture (left) and relative humidity (right)

performed to ASTM D1816 1mm gap

It can be seen from figure 2 that although the lightning impulse dielectric strengths of these

esters are less than mineral oil when dry, they are comparable when the moisture content of

the mineral oil is greater than around 10ppm.

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Figure 2

Lightning impulse voltage as functions of absolute moisture (left) and relative humidity(right) performed to ASTM D3300

Oil Quality and Insulation Ageing

The quality of the oil directly affects the condition of the cellulose insulation as both moisture

and acid content affect the rate of cellulose degradation. It has been proposed that natural

esters can extend the remaining life of a transformer by protecting the cellulose insulation14

.

It is hypothesized that natural esters reduce the rate of cellulose ageing by removing water

from the cellulose and the benignity of the compounds created during oil ageing. Acids

created during oil oxidation affect the rate of cellulosic degradation. ZTZ Service believesthat acids formed in mineral oil are detrimental

15

, while CPS considers that acids produced by

esters are beneficial8

. This is accounted for by differences in the chemical structures of acids

formed by esters and mineral oil.

Review of Dielectric Loss and Acidity

Dielectric loss (tan ) and acidity can be used to compare chemical stability. However, esters

are generally more polar than mineral oil and different by-products are formed during

chemical reactions, therefore, care must be taken when comparing measurements. It is

important to understand the effects of the by-products resulting from chemical instability onoverall transformer performance.

Esters degrade in a different manner to mineral oil. Mineral oil oxidises in the presence of

oxygen whereas an ester can hydrolyse in the presence of water as well as oxidise. Ester

hydrolysis is viewed as beneficial as this removes water, keeping the cellulose insulation dry.

Additionally, the transesterification reaction has been proposed, where acids formed from

natural esters bond with cellulose insulation and protect the cellulose from absorbing water 8

.

Ester and mineral oil degradation mechanisms create acids and polar compounds, which

increase tan and acidity, however such measurements will not provide any information on

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the nature or impact of the by-products.

Oil Degradation Through Biological Action

It is noted that although esters are biodegradable, they do not degrade inside the transformer

tank. One manufacturer has monitored free breathing transformers since 1996 and has

concluded, that in the case of FR3, no signs of micro-organisms have been found16

. The

explanation proposed is that transformer tanks are too dry to permit organisation growth, and

that if water is dissolved in the ester, it is locked away from the micro-organism. If a spillage

occurs, then there is an abundance of water in the environment to allow micro-organisms to

feed on the oil.

Chemical Stability Investigations

A critique of certain, previous, investigations is that researchers may have used accelerated

ageing test methods unrealistic of a transformer environment. Such tests may have employedeither high temperatures, oxygen atmospheres or pressure; conditions that would not

normally be encountered in a power transformer. Furthermore, it is difficult to extrapolate

reaction rates from an investigation to the transformer environment due to cyclic loading,

localised hotspots and temperature variations due to oil circulation.

The aim of the investigation performed at the University of Manchester was to ascertain how,

in the presence of air, esters degrade compared to mineral oil. Samples of oil were heated at

115°C for up to 28 days in an air circulating oven with copper catalyst. The AC and lightning

impulse dielectric strengths were found as well as tan and acidity. The dielectric strengths

of the esters show remarkable stability during 28 day ageing.

Figure 3

AC breakdown voltage (left) and Lightning impulse breakdown voltage (right) of aged oils

performed to ASTM D1816 1mm gap and ASTM 3300

The AC dielectric strength of mineral oil appears to increase first then decrease. The highest

point had the lowest moisture content of all samples of mineral oil and it is known that at the

start of thermal ageing, the dielectric capability of mineral oil can be influenced by driving

off impurities and lighter oil fractions. The mineral oil sludged, whereas no sludge was seen

in either ester, which may have affected the dielectric strength. These results infer that it may

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not be the ageing of the oil which affects transformer insulation operation, therefore, it is

important to show if the products created during oil degradation affect the rate of cellulose

ageing.

The rate of change in acidity and tan delta is an effective indication whether or not oil

degradation is occurring as these will be affected by the products of chemical reactions. It can be seen that the inclusion of copper increases the rate of acid production in mineral oil,

however decreased the rate of acid production in the natural ester. The copper has not

affected the synthetic ester, demonstrating good chemical stability.

Figure 4

Acidities due to ageing in air (left) and ageing in air + copper (right)

In the case of the tan delta, copper has increased the rate of polar compound production in the

natural ester. Thus, it can be concluded with the natural ester that in the presence of air and

copper, the resultant compounds are highly polar although have little or no acidity. This is

different to mineral oil as the resultant compounds are acidic, but not very polar.

Figure 5

Dielectric dissipation factors of aged oils



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CPS believes that the compounds formed by the natural ester are most likely to be ketones

and aldehydes. Without copper, which is acting as a pre-oxidation catalyst, the natural ester is

likely to be hydrolysing into weak acids. This explains why the acidity is increasing although

the tan delta is mostly constant. This would infer that if the natural ester were exposed to air,

it would not form the weak acids which are touted as beneficial to transformer cellulose

insulation. This demonstrates that esters degrade differently to mineral oil and that moreresearch is required to ascertain the impact of ageing by-products on the long term operation

of the transformer. The importance of this is that it adds credence to the belief that natural

esters should only be used in sealed systems. The synthetic ester appears to have been only

mildly affected, as neither acidity nor tan delta increases significantly.

At the operating maximum top oil temperature, normally 90o

C, both mineral oil and esters

should be stable for long periods of time without much change. Chemical stability can be

assessed by the gas emitted during ageing reactions.

Oil and paper samples were sealed in glass bottles and heated uniformly in an air circulating

oven at temperature of 90o

C for a period of time up to 14 days, and dissolved gas analysis

(DGA) was carried out. The mineral oil is Nynas Nitro 10GBN, the synthetic ester is Midel

7131 and the natural ester is FR3. The period of time is chosen arbitrarily. Table 2 compares

the concentration of fault gases of mineral oil, synthetic ester and natural ester with Kraft

paper at 90o

C. Of the three oils, although natural ester generated the smallest volume of fault

gases, it generated a significant amount of ethane and hydrogen. Paper inclusion at 90o

C has

caused the increase of carbon monoxide and carbon dioxide, CO and CO2, as compared to

that of oils only. These gases are key indicators for cellulose related faults, in both mineral oil

and esters. The concentrations of CO and CO2 are highest in mineral oil, lower in synthetic

ester and the least in natural esters.

Table 2

DGA results of mineral oil and esters at 90o

C (oil and paper)Oil type Mineral oil Synthetic ester Natural ester

Gas (ppm) /Duration Control 14 days Control 14 days Control 14 days

H2 8 46 7 13 8 244

CH4 1 10 1 3 1 6

C2H6 0 2 1 0 1 116

C2H4 1 2 1 1 1 2

C2H2 1 1 1 1 1 0

CO 6 590 5 307 6 88

CO2 108 3407 45 2212 82 1354



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Cellulose ageing can be detected by the reduction in degree of polymerisation (DP). The DP

values of paper samples were analysed and the results shown in Figure 6. The DP results of

paper aged in ester indicate that the paper integrity may be preserved.

Figure 6

DP values for mineral oil and esters impregnated paper at control, 90 and 150°C

Fault Detection Using DGA Results for Alternative Oils

DGA has been used as an effective tool to detect incipient faults in mineral oil filled

transformers. Two broad categories of faults, i.e. thermal and electrical, can be detected by

DGA. The electrical fault can be further classified into low energy partial discharges and high

energy arcing. Faults could occur in the bulk of oil as well as in cellulose/oil interface. In

order to apply the DGA diagnostic method on ester filled transformers, it is necessary to

determine if the same types of fault gases are generated, and then to identify the generation

rate and the concentration of the fault gases in alternative oils, as compared with mineral oil.

Thermal Tests

Oil samples, with and without paper inclusion, were subjected to 200o

C, the lower limit of

medium fault temperature range for one hour. The DGA results of mineral oil and esters were

obtained as shown in Table 3 on the following page.



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Table 3

DGA results of mineral oil and esters at 200o

C (oil only)Oil type Mineral oil Synthetic ester Natural ester

Gas (ppm) /Duration Control 1 hour Control 1 hour Control 1 hour

H2

5 21 7 8 8 17

CH4

1 95 0 16 1 7

C2H

60 48 0 4 2 177

C2H

41 9 1 3 1 4

C2H

2 1* 5 0 0 6

* 0

CO 18 148 9 74 6 68

CO2 73 1006 111 521 82 914

* C2H

2 in control samples is considered as affected by measurement accuracy

Figure 7 shows the relative percentages of dissolved combustible gases in mineral oil and

esters with and without paper inclusion. The relative percentages of methane, ethane and

ethylene become noticeable for mineral and synthetic ester at 200o

C, whereas the relative

percentage of ethane in natural ester is the highest of the six gases.

Figure 7

Relative percentages of dissolved combustible gases for mineral oil and esters at 200o

C with

and without paper inclusion



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Electrical Tests

Arcing Tests. The needle to plate electrode configuration, with an oil gap distance of15mm, was adopted. Table 4 shows the normalized DGA results of 20 breakdowns of mineral

oil and esters. A total of 20 breakdowns were executed having a one minute interval betweensuccessive two breakdowns. Acetylene is a key gas produced during arcing, and is a primary

indicator for this type of high energy fault. Hydrogen and ethylene are also evident in

significant amounts. Although the same energy arcing occurred in the three oils, acetylene

concentration in mineral oil is about 5 to 10 times higher than that found in esters. synthetic

ester has the lowest concentration of gases. Acetylene, hydrogen and ethylene are in high

concentrations in mineral oil and are in the lowest concentrations in natural ester.

Table 4

Normalized DGA results for arcing in mineral oil and esters

Gas (ppm) / Oil type Mineral oil Synthetic ester Natural ester

H2 901 97 191

CH4 145 9 14

C2H6 24 2 10

C2H4 270 26 63

C2H2 1540 126 280

CO 6 37 51

Figure 8

Relative percentages of dissolved combustible gases for mineral oil and esters as a result of

20 breakdowns



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Partial Discharge (PD) Tests . The PD test circuit differs from the arcing circuit only bythe adding of a water resistor between the high voltage sources to the oil test vessel.

Table 5 shows the normalised DGA results of three types of fluid under the conditions of PD

activity for 1 hour. Hydrogen is the key indicator for low energy discharge. Hydrogen is

found in the highest concentration in mineral oil and is the lowest in synthetic ester.

Table 5

Normalized DGA results for 1 hour PD activity in mineral oil and esters

Gas/Oils Mineral Synthetic ester Natural ester

H2

20 5 23

CH4

2 2 2

C2H

60 0 1

C2H

42 0 2

C2H2 2 0 2CO 2 2 8

Although the molecular structures of esters differ from that of mineral oil, the profile of

thermal fault indicating gases is the same for both types of fluid. The cleavage and

recombination of molecular fragments split from esters seems to give rise to lower fault gas

concentrations as compared to those found in mineral oil. Esters seem to be more stable under

medium temperature range thermal fault conditions.

However, natural ester generates a significant amount of ethane under thermal faults, and this

may identify ethane as a key indicator of thermal faults in natural ester.

In electrical faults, Acetylene is the key gas for indicating high energy arcing and hydrogen is

the key gas for indicating low energy partial discharges. Under the same electric faults, esters

generate faults gases 5 to 10 times less than mineral oil.

Interaction Between Esters and Cellulose

At the voltage level of 132kV and above, the transformer insulation system consists of oil and

oil-impregnated cellulose, and the life of transformer is mainly dominated by the cellulose

insulation. To apply esters in large transformers, ester impregnated solid insulation should be

proven to have comparable dielectric strength to mineral oil impregnated solid insulation.

Impregnation of Solid Insulation with Ester Fluids

Solid insulations including paper, pressboard, and blocks, need to be impregnated by

insulation oil to have better dielectric properties. There are three variables that would affect

the impregnation result: the pressure, the impregnation time under vacuum and the viscosity

of the fluid. Ester based fluids have higher viscosities than conventional mineral oil, which

raises an issue whether the impregnation by the ester would take much longer time than the

mineral oil.



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Impregnation of both paper and pressboard brings no technical problem due to their thin

thickness. Experimental tests in laboratory show that paper with thickness less than 0.5mm

can be fully impregnated by an ester under 15mbar vacuum at 60o

C within 12 hours, whereas

3 mm thick pressboard would need 48 hours.

Impregnation of laminated blocks, either pressboard blocks or wood blocks, brings more

engineering challenges because of their greater thickness, the more viscous fluid and

consequently a longer impregnation time. Some comparative laboratory experiments were

carried out in the laboratory to study the impregnation process of blocks by mineral oil and

ester fluid. Increasing the temperature of oil can reduce its viscosity and thereby shorten the

impregnation time, still the exact temperature and time need to be determined for transformer

manufacture.

Figure 9 Figure 10

Viscosities vs. temperature Laminated block impregnation by ester

As shown in figure 9, the viscosity of natural ester is much higher than that of mineral oil and

the viscosity of oil decreases quickly as the temperature increases. The viscosity of natural

ester at 60o

C is approximately the same as the viscosity of mineral oil at 20o

C. In order to

reveal the effect of temperature upon the impregnation of blocks in esters, two Weidmann

pressboard blocks were impregnated under 20o

C and 80o

C respectively. The dimensions of

the laminated block used in this test are 101.6×101.6×34.3 mm (4×4×1.35 inch) having a hole

with diameter of 12.7 mm (0.5 inch) in the centre, and four side faces of block were sealed by

epoxy resin. The reason of fabricating blocks in this way is to simulate a real impregnation

condition of laminated blocks. In transformers, the supporting blocks are normally drilled

with holes to help impregnation, at the same time the mechanical strength of laminated

blocks would not be compromised. The distance between two adjacent holes is 101.6 mm (4

inches), which means that the oil need to travel 2 inches to achieve full impregnation.

As shown in figure 10, there was remarkable difference between the impregnation under

20o

C and 80o

C. At first 48 hours, both block samples have similar impregnation

speedHowever the impact of viscosity on impregnation appeared later. Ester has

approximately 6 times greater viscosity under 20o

C than under 80o

C, yet the impregnation

volume at 80o

C is only doubled the volume at 20o

C. Increasing the impregnation temperaturehelps to facilitate the impregnation but not proportionally.



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Figure 11

Pressboard blocks impregnated by mineral oil and ester fluid (10mbar, 65o

C)

Figure 11 shows the comparative impregnation speed of pressboard block by nature ester and

mineral oil. The pressboard blocks with dimension of 248×110×45 mm were used. Although

natural ester at 65o

C is 3-4 times more viscous than mineral oil, the difference ofimpregnation volume after 72 hours is negligible between mineral oil and nature ester.

High viscosity of ester based fluids requires longer time for impregnation than mineral oil.

Fortunately, it was found that the impregnation time was not proportion to viscosity as stated

before, and ester impregnates solid insulation faster than expectation.

Dielectric Capability of Ester-Impregnated Cellulose

Paper insulation in a transformer normally takes the electrical stress between turn to turn

under ac operating voltage and impulse surges. The designed electric stresses onto the paper

insulation need to be lower than the dielectric strength and a proper safety margin should be

maintained.

Figure 12

Dielectric strength comparison of different oil impregnated paper

(Paper density= 0.93 g/cm3

; moisture< 0.2%)

As shown in figure 12, the dielectric strengths of oil impregnated paper decrease as the



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thickness of paper increases; and within controlled moisture level, ester impregnated paper

shows comparable dielectric strength to mineral oil impregnated paper. Similar to pressboard,

paper with less density would have lower ac breakdown voltages18

. Laboratory test results

show that layer insulation paper with density of 0.75g/cm3

has lower dielectric strength than

paper with density of 0.93 g/cm3

. Nevertheless, the test results also indicate that the ester impregnated layer insulation paper has comparable dielectric strength to mineral oil

impregnated paper.

Ester Impregnated Pressboard

Oil impregnated pressboard is widely used between transformer windings as oil barriers for

breaking up large oil gaps and acting as mechanical support. Withstand voltage tests on

mineral oil or ester impregnated pressboards were carried out with different electrode

geometries. It was found that direct breakdown of pressboard itself rarely happened; it is the

failure of the weaker component of oil/pressboard interface, that gradually damages the

cellulose surface, known as creep discharge17

, and finally causes ‘breakdown’ of pressboard.

Considering this, the breakdown field strength calculated as breakdown voltage divided by

thickness of pressboard ‘cannot be regarded as the true strength but only as a mean apparent

strength of the transformer board’18

.

AC Stress Test (ester-pressboard vs. mineral-pressboard)

Mineral oil impregnated pressboard and natural ester impregnated pressboard were tested

using partial sphere electrodes (see figure 13) under ac voltages. During tests, the ac voltage

was raised up to 75kV with increasing speed of 0.5kV/s.

Figure 13

Partial sphere electrodes

Mineral oil impregnated pressboard Ester impregnated pressboard

Figure 14

Pressboard sample after test (moisture


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Considerable surface discharges occurred in the oil wedge of mineral oil impregnated

pressboard and caused breakdown after 5 test cycles, while the natural ester impregnated

pressboard withstood the maximum voltage of 75kV with out surface discharges. Figure 14

shows the pressboard samples after test.

Although the results are in favour of the ester impregnated pressboard, the above-mentioned pressboard breakdown test is not comparable in different oil medium. The lower permittivity

ratio of ester-impregnated pressboard to ester fluid results in less stress taken by oil wedge,

which prevented creep discharge initiation on ester impregnated pressboard. Lower

permittivity ratio of oil-impregnated pressboard to oil is beneficial in oil-pressboard-oil

system, since less stress will be distributed in oil duct or oil wedge, which is helpful to

prevent the occurrence of the creep discharge.

Summary

This paper has presented an overview of the research carried out at the University of

Manchester on alternative oils, and so far based on the evidence in this paper has found thatesters are likely to be suitable replacements for mineral oil in large power transformers.

However, it is noted that this is an ongoing activity with further research being required to

study the behaviour of esters in areas such as large oil gaps.



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Biography

Daniel Martin was awarded the degree of Electronic and Electrical Engineering with study

abroad in Germany by the University of Brighton, UK, in 2000. In 2000 he joined Racal

Electronics, which went on to form the international electronics company Thales, working on

communication and aircraft systems. In 2004 he left Thales to pursue his PhD in high voltagetechnologies at the University of Manchester, UK, and is a recipient of a EPSRC CASE

scholarship.

Imad Khan received a BEng (Hons) degree in Electrical Engineering from the National

University of Science and Technology, Pakistan in 2004. He is currently a PhD student at the

Electrical Energy and Power Systems Group at the University of Manchester. His research

interests include Alternative transformer insulation, Electric stress analysis using FEM and

Dissolved gas analysis. He is a Student Member of the Institution of Engineering and

Technology.

Jie Dai received a BSc degree in Electronic Engineering from the University of ElectronicScience and Technology of China (UESTC) in 2003 and an MSc degree in Electrical Power

Engineering from the University of Manchester in 2005. He is currently a PhD student at the

Electrical Energy and Power Systems Group at the University of Manchester and carrying out

research on transformer solid insulation.

Zhongdong Wang received a BEng and a MEng degree in High Voltage Engineering from

Tsinghua University of Beijing in 1991 and 1993, and a PhD. in Electrical Engineering from

UMIST in 1999. Since 2000 Dr. Wang has been a Lecturer at the Electrical Energy and

Power Systems Group at the University of Manchester. Her current research interests include

condition monitoring, transients’ simulation, transformer insulation ageing and alternative

insulation materials.



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References

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proceedings of the 1997 Electrical Insulation Conference, Pages 465 – 468

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Sealed Versus Free-Breathing Transformers”, CP0414, Cooper Power Systems, 2004

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WORLD, pages 52 – 57,Autumn 2004

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[13] T.V Oommen, C. C. Claiborne, C.T. Mullen, “Biodegradable Electrical Insulation

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[14] K. Rapp, C. Patrick McShane, J. Luksich, “Interaction Mechanisms of Natural Ester

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Proceedings, pp. 393 – 396, Coimbra, IEEE, 2005.



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[15] V. Sokolov, D. Hanson, “Impact of Oil Properties and Characteristics on Transformer

Reliability”, in TechCon Conference Proceedings, Nashville, TJH2B, 2006.

[16] K. Rapp, P. Stenborg, “Cooper Power Systems field analysis of envirotemp® fr3™ fluid

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[17] Bedoui, N.K., Beroual, A., Chappuis, F. “Creeping discharge on solid/liquid insulating

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[18] H.P.Moser, V.Dahinden, Transformerboard II,1987, Page.23-25

[19] www.midel.com

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