2006 Doble - Review of in-Service Tfmrs Using Natural Ester Fluid

7/24/2019 2006 Doble - Review of in-Service Tfmrs Using Natural Ester Fluid

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Presented at the 73rdInternational Conference of Doble Clients, April 9-14, 2006, Boston USA

REVIEW OF IN-SERVICE TRANSFORMERS USING

NATURAL (VEGETABLE OIL) ESTER DIELECTRIC FLUID

C. Patrick McShane Peter G. Stenborg John LuksichCooper Power Systems Cooper Power Systems Cooper Power Systems

ABSTRACT

After years of research and development, the first field trials of distribution transformers specifically designedfor and filled with natural ester dielectric fluid began in 1996. Since then, more than 15,000 natural ester

transformers are in service, the majority distribution class. However, due to the excellent performance of the

distribution class transformers, interest grew for the application of the fluid for power class transformers. Factorsdriving the use of natural ester fluid in power class applications include reduction in the aging rate of cellulose

insulation, endorsements by fire insurance companies, approvals by UL and FM Global, and improved

environmental profile. In 2001 the first field trial for generator step-up (GSU) power class transformers began. The

application of natural ester fluid in power class transformers is quickly growing, with 50 units now in service onfour continents. The demand has accelerated in recent years due in part to concerns about aging substationinfrastructure, increasing frequency of eventful failures, and a renewed focus on asset management. There is also

growing interest in alternatives to conventional mineral oil due to supply concerns and future availability of non-renewable resources, flammability, corrosive sulfur, sludge formation, and environmental regulations and incentive

programs for using bio-based resources.

This paper reviews the performance of two 225 kVA transformers, now in their 10thyear of operation, and GSU

transformers rated 200 MVA 161 kV 50 MVA 69 kV, retrofilled in 2004 and 2001, respectively. We discuss

applications of natural ester fluid in medium power transformers and review the in-service performance of two

retrofilled units. Assessments of in-service fluid condition using standard fluid properties and dissolved gases are

given. We present the current state of international standards for both new and in-service natural ester dielectricfluid.

INTRODUCTION

Natural ester (vegetable oil) dielectric fluids are the most recent addition to the category of alternative dielectric

coolants. Alternatives to mineral oil first appeared in the 1930s with the introduction of chlorinated aromatichydrocarbons such as polychlorinated biphenyls (PCB). With the banning of PCBs, less-flammable (very high fire

point) fluids entered the market and were offered in a variety of chemical types. First, in the 1970s, were the high

molecular weight hydrocarbons (HMWH) and silicones, followed in the 1980s by synthetic fluids such aspolyalphaolefins (PAO) and polyolesters (POE). Of these, the POEs easily have the most attractive environmental

and low temperature properties. Their high cost has limited them to niche applications and spurred the search for

more affordable alternatives.

The natural esters are chemically similar to the POEs and share many of their properties. Chemically, a natural

ester molecule is a triacylglycerol, or triglyceride, and consists of three long chain fatty acids attached to a glycerolbackbone. The natural esters have excellent environmental properties, provide high degree of fire safety due to very

high flash and fire points, and are shown to retard the degradation of paper insulation [1-5].

Development of a natural ester dielectric fluid began as a research project in 1991. Over the next five years,

more than two-dozen vegetable oils and blends were evaluated in a comprehensive and rigorous series of small-scaletrials. The most favorable natural ester formulation under went full-scale accelerated life testing throughout 1995,

per the IEEE C57.100 standard test procedure. The ester units lasted more than three times longer than required by

the standard procedure. The first prototypes, 225 kVA pad mount transformers, were installed in 1996. Theirperformance was discussed at the 66

thInternational Conference of Doble Clients [6]. Third party field trials began in

1997 in utility distribution systems throughout the USA. The first retrofills prototypes were done in 1998. Natural


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ester transformers went into standard production in 1999. Since then, natural ester power transformers have been

manufactured in four continents and currently in production in a fifth. Figure 1 shows the natural ester developmenttimeline.

INTERNATIONAL STANDARDS

Before 2003, international standards did not exist for natural ester dielectric fluids. Because the physical,

chemical, and electrical properties of synthetic ester dielectric fluids are comparable in many ways to natural esters,

the IEC standards for new and in-service synthetic fluids served as surrogate guides where the properties were

similar. In 2003, ASTM issued a standard for new natural ester fluids. An IEEE Working Group crafting amaintenance guide for natural ester dielectric fluids has completed several draft cycles, getting closer to issuing a

final version for ballot. IEC has accepted a proposal begin work on new and in-service natural ester dielectric fluidstandards as well, with the initial drafts in work.

TRANSFORMERS DESIGNED FOR NATURAL ESTER FLUID

Transformer engineers should take into account the thermal properties of natural ester fluid in the cooling

design of new transformers, which may differ a bit from the customary mineral oil design, typically in the size or

number of cooling ducts, radiator size, or number of fans. Free-breathing designs are not suitable for natural ester

fluid. Distribution transformers should be sealed tank type, and power transformers designed for natural ester fluidwill have nitrogen preservation systems or bladder conservators.

Extensive material compatibility testing has been performed. No material typically used in distribution andpower transformers has been shown to be less compatible with the natural ester than mineral oil. Standard electrical

clearance and BIL level standards suitable for mineral oil have been successfully applied in natural ester designs.

TABLE 1

International Standards for mineral oil, synthetic ester, and natural ester dielectric fluids

New In Service

Type ASTM IEC IEEE IEC

mineral oil D3487 60296 C57.106 60422synthetic ester none 61099 none 61203

natural ester D6871 proposed C57.147 (draft) proposed

1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

researchbegins

formulationselected

first d istributiontransformers

installed

first distributiontransformers

retrofilled

first GSU retrofill69 kV, 50 MVA

first 100% utilityconversion for

distribution

first newGSU

US EPA grantsEnvironmential

TechnologyVerification

transformersavailable

commercially

first newpower

transformer

first newmobile

substation

230 kVretrofills

161 kV,200 MVAGSU retrofill

availableto OEMs

full-scale acceleratedlife tests completed

utility fieldtrials begin

ASTMnatural ester

standard

Timeline of events in the development and application of natural ester dielectric fluid

FIGURE 1


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Cooling Performance

Natural ester fluid is slightly less efficient in natural convective cooling than mineral oil. The difference in

cooling efficiency between natural ester fluid and mineral oil is illustrated by comparative heat runs of mineral oiltransformers. A transformer designed for and filled with mineral oil is subjected to a heat run to determine the actual

winding rise at full rating. After removing the mineral oil, the transformer is filled with natural ester fluid and the

heat run repeated. For 65C rise designs, typical differences are about 1C higher average winding rise intransformers 1 MVA and less, 2-4C higher in 5 MVA, and 4-7C higher in 10 MVA distribution transformers.

However, because the thermal life of paper insulation is longer in natural ester fluid, the increased winding

temperature is more than offset by the decrease in thermal aging rate shown in Figure 2. In directed forced flow

cooling, the difference in cooling performance between natural ester and mineral oil is much less. Designers takethis into account when designing specifically for natural esters, especially when the transformer must meet mineral

oil winding rise limits.

Distribution

More than 15,000 distribution transformers designed for natural ester fluid are currently in service (Figure 3).

Often, mineral oil transformer designs are modified slightly for natural ester application in order to meet standardsfor mineral oil winding rise. The modifications most often involve the cooling ducts. The size of the duct can beincreased, the number of ducts increased, or both. Figure 4 shows fluid properties for the 225 kVA 3-phase padmount transformers installed in 1996 and first discussed in 1999 [6]. The properties show the natural ester fluid to be

in excellent condition. Increases in dissipation factor values in 3(b) from 2001 through 2004 appear to be scatterfrom inconsistent sampling or testing methods, since the most recent data are very close to the 1996 values.

Increasing viscosity is a definitive indicator of natural ester oxidation. The viscosities shown in 3(d) confirm the in-

service stability of natural ester fluid in sealed tank transformers.

Mobile Substations

Several mobile substation transformers designed for and built using natural ester fluid are in service. The first

unit was constructed as a free-breathing transformer, a design not recommended for long-term natural ester fluid

application. While the fluid properties were acceptable after a year in service, a bladder conservator, as

Hottest Spot Temperature (oC)

60 80 100 120 140 160 180

PerUnitofNormalLife

0.001

0.01

0.1

1

10

100

1000

10000

mineral oil: A = 9.80 x10-18

natural ester fluid: A = 7.82 x10-17

273

1500

)(T

eATlife

Transformer per unit insulation life versuswinding hottest spot temperature used for both distribution and power

transformers. Mineral oil curve from IEEE Loading Guide C57.91-1995; natural ester curve determined using

results from [1,4] and other experiments. A per unit life of 1 normal life, as discussed in the Loading Guide, occurs

at 110C for thermally upgraded insulation in mineral oil. The curve for natural ester fluid shows the slower rate ofthermal degradation of thermally upgraded insulation. The mechanisms responsible for the slower aging rate are

discussed in [5].

FIGURE 2


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recommended by the fluid manufacturer, was retrofit to prevent free exchange of air in the headspace of theconservator.

Power Substations

Medium power transformer designs using natural convective cooling require similar cooling design

modifications as those used for distribution transformers. In addition to winding design changes, additional fans orradiators are often used. Designs using directed forced oil flow are less sensitive to the cooling efficiency

differences between mineral oil and natural ester fluid, but still benefit from designing specifically for natural ester

fluid. Examples of new transformers designed for natural ester fluid are given in Table 2.

TRANSFORMERS RETROFILLED WITH NATURAL ESTER FLUID

Retrofilling existing transformers with natural ester fluid is considered for environmental reasons, to extend the

remaining thermal life of the transformer, and/or to upgrade the fire safety of the installation. Retrofill procedures

are straightforward, as natural ester fluid is miscible with mineral oil and compatible with standard transformer

construction materials. However, transformers with free-breathing conservators must be retrofitted with bladders oruse other means to seal the headspace from the atmosphere.

Because of the high fire point of natural ester fluid, transformers can often be upgraded to high fire pointcondition (fire point > 300C) by replacing the mineral oil with natural ester fluid. There is no reduction in fire point

up to 7% residual mineral oil. Reduced power ratings are not required, as any increase in temperature due to fluid

differences is more than adequately compensated for by the reduction in insulation thermal aging rate. In essence,this results in an extension of the remaining thermal life of the transformer. At least one utility has driven their

power unit loading 10% harder after retrofilling with natural ester fluid.

Distribution

Distribution transformers are retrofilled with natural ester fluid typically to upgrade the fire safety of thetransformer for units installed adjacent to or in buildings. The first retrofills were done in 1998 on two 225 kVA

transformers manufactured in the early and mid 1970s. Fluid samples have been drawn periodically to monitor

changes in fluid properties and levels of dissolved gases. Figure 5 compares the fluid properties of these 3-phase padmount transformers. The retrofill of one transformer employed a "minimum effort" procedure: the transformer was

simply drained of mineral oil and refilled with natural fluid. No effort was made to flush residual mineral oil or

contaminants from the unit. The other followed stricter "best effort" procedures. The benefits of better flushing are

clearly observed in dissipation factor (b), flash point (c), and acid number (f), and are due to less residual mineral oiland sludge in the oil, indicating that contaminants from more than 25 years of mineral oil and paper aging were

0

5,000

10,000

15,000

20,000

25,000

1999 2000 2001 2002 2003 2004 200 5 2006

Transformers

Number of natural ester distribution transformers in serviceFIGURE 3


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removed by conscientious flushing before refilling. Both levels of effort yielded fire points >300C (c), incompliance with fire safety codes, after the residual mineral oil mixed with the natural ester.

The minimum effort retrofill is interesting for two reasons. First, the tap changer switch, noted during the

retrofill to have considerable contact coking, started to generate very high levels of acetylene mid-year 1999. The

decrease in dielectric strength (a) and increases in dissipation factor (b) and acid number (f) are attributed to arcingproducts and oil contamination resulting from the bad contacts. An outage was taken to replace the switch, and the

unit was topped off with new natural ester fluid. This resulted in the dissolved gases returned to normal, an increasein dielectric strength and flash point and a slight increase in viscosity, and decreases in dissipation factor and acidnumber.

TABLE 2

Medium power transformers using natural ester fluid

Base VoltageQty MVA (kV) Type Notes (original commission date)

4 8 230 1transmission *RF March 2005 (1980)1 200 161 GSU RF March 2004 (1966)1 25 161 distribution RF July 2005 (1980)

2 25 161 distribution Built February 2006

1 36 138 GSU RF August 2005 (1990)

1 24 138 distribution RF June 2004 (1965)

1 12 138 distribution Built December 2005

1 120 115 GSU RF May 2004 (1994)

1 55 115 GSU RF March 2006 (1955)

1 30 115 distribution Rebuilt June 2004

2 15 115 distribution Built October 2004

1 50 69 GSU RF October 2001 (1957)

1 20 69 GSU RF October 2003 (1990)

1 20 69 GSU Built December 20052 18 69 distribution Built October 2003, March 2004

2 14 69 GSU RF October 2003, May 2004 (1973)

1 10 69 distribution Built March 20042 7.5 69 distribution RF August 2005 (1965)

1 37.5 67 distribution RF December 2005 (1988)

2 20 46 distribution RF September 2004 (1981)

1 20 46 distribution RF May 2005 (1995)2 15 44 distribution RF August 2005 (1989)

2 7.5 44 distribution RF August 2005 (1989)

1 27 37 rectifier RF May 2004

2 27 37 rectifier RF April 20051 26 37 rectifier RF April 2005

2 30 34.5 rectifier RF April 2005

1 30 34.5 rectifier RF May 20041 10 34.5 distribution Built November 2002

1 10 33 mobile Built November 2002

2 20 23 distribution Built February 20062 11 20 aux. power RF March & May 2004 (1966)

1 25 19 aux. power RF April 2004 (2000)

1 10 18 aux. power RF May 2004 (1965)1 11.5 13.8 distribution RF August 2005 (1990)

*RF = retrofilled


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The water content (e) steadily increased over time. As a result, the dissipation (b) increased and finally thedielectric strength (a) decreased as the water content climbed above 40% relative saturation. A bushing leak turned

out to be the culprit, allowing water into the transformer. After replacing the bushing and topping off with newnatural ester fluid, the dielectric strength and water content returned to acceptable levels.

Power

The first power unit to be retrofilled with natural ester fluid was a 50 MVA 69 kV GSU transformer. In

continuous service since 1957, it was retrofilled in October 2001 and remains in service today. Several dozen

additional power transformers have been retrofilled since then, including a 200 MVA, 161 kV GSU transformer andfour 8 MVA, 230 kV single-phase substation transformers. Examples are given in Table 2.

Figure 6 summarizes oil sample data from the October 2001 GSU retrofill. Dielectric strength (a), viscosity (g),dissipation factor (b) and acid number (h) have been observed to be quite stable. Absolute water content of the oil

(e) has shown an increase over time confirming natural ester's greater affinity for water and, importantly, its ability

to remove water from paper insulation. However, despite the higher absolute water content, the relative saturation (f)

at 20C remains below 7%, well below the level at which the dielectric performance is compromised.

The retrofill procedure did not attempt to minimize the amount of mineral oil remaining in the transformer afterretrofill by flushing or removal of the dregs from the tank bottom. The consequence is seen in the flash and fire

points of Figure 7 (a). The residual mineral oil was estimated, using the change in flash point, to be about 4% shown

in Figure 7 (b). The residual oil held in the paper insulation slowing mixed with the natural ester fluid. The fire

point, initially steady at 350C, dropped below 300C to 245C as the residual mineral oil rose above 7%, reachingabout 9%. Figure 8 shows the dissolved gas levels remaining relatively stable.

Figure 9 shows the dissolved gases and their proportions in a 200 MVA 161 kV GSU transformer retrofilled in

2004. The gases levels remain stable. Not shown are the fluid properties, which indicate good condition and stableoperation before and after the retrofill. The mineral oil content immediately after retrofilling was below 1%, and

reached about 4.5% after the mineral oil held in the solid insulation mixed with the tank fluid.

SUMMARY

Natural ester dielectric coolant now has seen a decade of application in distribution transformers and a half adecade in power transformers. Data collected from the earliest distribution and power transformers indicate good

stability of natural ester fluid key performance properties. There have been no reported operating problems with any

medium and large power transformer with the natural ester. Based on the experienced to date, key fluid properties

and DGA testing guides have been developed.

REFERENCES

[1] McShane, C.P., Rapp, K.J., Corkran, J.L. Gauger, G.A., and Luksich, J., Aging of paper insulation in natural

ester dielectric fluid, IEEE/PES Transmission & Distribution Conf., 2001, Atlanta, USA[2] McShane, C.P., Rapp, K.J. Corkran, J.L. Gauger, G.A., and Luksich, J., Aging of plain Kraft paper in natural

ester dielectric fluid, IEEE/DEIS Conf. on Dielectric Fluids, 2002, Graz, Austria

[3] McShane, C.P., Rapp, K.J. Corkran, J.L., and Luksich, J., Aging of cotton/Kraft blend insulation paper in

natural ester dielectric fluid, TechCon Asia-Pacific, 2003, Sidney, Australia

[4] McShane, C.P., Corkran, J.L., Rapp, K.J., and Luksich, J., Aging of paper insulation retrofilled with naturalester dielectric fluid, IEEE Conf. on Electrical Insulation and Dielectric Phenomena, 2003, Albuquerque, USA

[5] Rapp, K.J., McShane, C.P., and Luksich, J., Interaction Mechanisms of Natural Ester Dielectric Fluid andKraft Paper, 15th International Conf. on Dielectric Liquids, 2005, Coimbra, Portugal

[6] McShane, Gauger, G.A., and Luksich, J., Determining and Interpreting Key Properties of Ester-BasedDielectric Fluids, 66thInternational Conf. of Doble Clients, April 12-16, 1999, Boston, USA


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BIOGRAPHIES

Patrick McShanereceived his BS in Electrical Engineering from Marquette University in 1970, and an MS in

Engineering Management from the Milwaukee School of Engineering in 1998. Currently the Product Line Manager

for Dielectric Fluids at Cooper Power Systems Transformer Products, his employment experience includes

International Area Manager for RTE Corp. and Regional Technical Director for the State of Sao Paulo (Brazil)Rural Electrification Program. His professional activities include USA Delegate IEC TC99, IEC TC99 Liaison to

TC64, IEC TC89 Expert Delegate, Chair ASTM W.G. D-5222, and W.G. Chair IEEE TC Dielectric Fluids

Subcommittee C57.121. Several of his proposals have been adopted as USA Codes and Standards (NEC, NESC,FMRC). He has presented papers at domestic and international engineering conferences including IEEE, EPRI,

Doble, and CIRED. He is the principal inventor of several US patents relating to dielectric fluids.

Peter Stenborg earned a BS in Mechanical Engineering and MBA from the University of Wisconsin Madison. In 23 years with Cooper Power Systems (RTE Corp.) his experience includes the application and testing ofdistribution and small power transformers and their overcurrent and overvoltage protective accessories, as well as

automatic step voltage regulators and their electronic controls. Peter has worked extensively with the development,testing, processing and fire safety code compliance of dielectric coolants and their use for new and retrofill

applications in all sizes of distribution and power transformers. He is a licensed Professional Engineer in the State of

Wisconsin.

John Luksichreceived his BS in Chemistry in 1980 and MS in Materials Engineering in 1990, both from the

University of Wisconsin. His engineering career includes thin film materials development at the McDonnell Douglas

Space & Physics Laboratory and sensor development for Johnson Controls. John has been an engineer in the

Dielectric Fluids group of Cooper Power Systems since 1998. He is a member of the ASTM D27 Dielectric Liquidsand Gases Committee.


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1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

DielectricBreakdownVoltage

(kV)

0

20

40

60

80

100

IEC 1203 continued service minimum using IEC method 60156

D1816, 2mm gap,

D1816, 2mm gap,

D877

D877

S/N 1429:

S/N 1430:

ASTM D6871 new fluid minimum using D877

1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

DissipationFactorat25oC

(%)

0.0

0.2

0.4

0.6

0.8

S/N 1429

S/N 1430

IEC 1203 continued service maximum (60 Hz)

ASTM D6871 new fluid maximum

(a) (b)

1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

Temperature(oC)

300

320

340

360

IEC 61203 continued service fire point minimum

ASTM D6871 new fluid fire point minimum

S/N 1429 S/N 1430

S/N 1429 S/N 1430Fire Point:

Flash Point:

1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

KinematicViscosity(cSt)

0

10

20

30

40

50

ASTM D6871 100oC new fluid maximum

ASTM D6871 40oC new fluid maximum

S/N 1429 S/N 1430

S/N 1429 S/N 1430

40oC:

100oC:

(c) (d)

1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

AbsoluteWaterContent(mg/kg)

0

100

200

300

400RelativeWaterContent(%satura

tion)

0

10

20

30

40

IEC 61203 continued service maximum

S/N 1429

S/N 1430


1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

AcidNumber(mgKOH/g)

0.01

0.1

1

10


S/N 1429

S/N 1430


(e) (f)

Fluid properties of the oldest operating transformers designed for and built using natural ester fluid. Two 225 kVA

pad mount transformers, installed in Waukesha, WI in 1996, have been operating 50 weeks per year continuously

loaded at 85-90% of nameplate rating. The temperature used for water content as relative saturation is 20C.

FIGURE 4


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1998 1999 2000 2001 2002 2003 2004 2005 2006

DielectricBreakdownVoltage(kV)

0

20

40

60

80

100

D1816 (2mm),

D1816 (2mm),

D877

D877best effort:

minimum effort:

IEC 1203 continued serviceminimum using IEC method 60156;

ASTM D6871 new fluid minimumusing D877

1998 1999 2000 2001 2002 2003 2004 2005 2006

DissipationFactorat25oC(%)

0

1

2

3

4

IEC 1203 continued service maximum (60 Hz)

minimal effort

best effort

(a) (b)

1998 1999 2000 2001 2002 2003 2004 2005 2006

Temperature(oC)

225

250

275

300

325

350

375

IEC 1203 continued service fire point minimumASTM D6871 new fluid fire point minimum

minimum effortbest effort,

minimum effortbest effort,Fire Point:

Flash Point:

1998 1999 2000 2001 2002 2003 2004 2005 2006


0

10

20

30

40

50



40oC:

100oC:

ASTM D6871 new fluid 40oC maximum


(c) (d)

1998 1999 2000 2001 2002 2003 2004 2005 2006

Ab

soluteWaterContent(mg/kg)

0

100

200

300

400

500

600

700

800

RelativeWaterContent(%satu

ration)

0

10

20

30

40

50

60

70

(IEC 1203 continued service maximum)

minimum effort

best effort

1998 1999 2000 2001 2002 2003 2004 2005 2006

AcidNumber(mgKOH/g)

0.01

0.1

1

10


minimum effort

best effort

(e) (f)

Fluid properties of the longest operating transformers designed for mineral oil and retrofilled with natural ester fluid.

Two 225 kVA pad mount transformers, installed in Waukesha, WI in the 1970s and retrofilled in 1998, have been

operating 50 weeks per year continuously loaded at 85-90% of nameplate rating. The temperature used for water

content as relative saturation is 20C. No data prior to retrofilling were available.The minimal effort unit required atap changer switch and new fluid top-off in mid-1999. The continuously increasing water content triggered a leakrepair and fluid replacement in 2002.

FIGURE 5


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2001 2002 2003 2004 2005 2006DielectricBreakdownStrengthD1816,2mm(kV)

0

20

40

60

80

proposed continued service limit for natural ester fluid (69kV and less)

after retrofill

before retrofill

2001 2002 2003 2004 2005 2006

DissipationFactor(%)

0.01

0.1

1

10

25oC before retrofill

25oC after retrofill

100oC after retrofill

(a) (b)

2001 2002 2003 2004 2005 2006

WaterContent(%20oCsaturation)

0

5

10

15

20

after retrofill

before retrofill

2001 2002 2003 2004 2005 2006

WaterContent(mg/kg)

0

20

40

60

80

100

after retrofill

before retrofill

(c) (d)

2001 2002 2003 2004 2005 2006


0

10

20

30

40

50ASTM D6874 new fluid 40

oC maximum


before retrofi ll , after retrofi ll

before retrofi ll , after retrofi ll

40oC:

100oC:

2001 2002 2003 2004 2005 2006

AcidNumber(mgKOH/g)

0.00

0.02

0.04

0.06proposed limit for natural ester fluid in new equipment (69kV and less)

before retrofill

after retrofill

(e) (f)

Fluid properties of a 50 MVA 69 kV generator step-up transformer in service since 1957 and retrofilled in 2001. This

is the longest operating transformers designed for mineral oil and retrofilled with natural ester fluid. The watercontents are given in absolute amount and relative saturation, and illustrate the higher saturation limit of natural ester

fluid.

FIGURE 6


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2001 2002 2003 2004 2005 2006

Temperature(oC)

150

200

250

300

350

after retrofill

after retrofill

before retrofill

before retrofillfire point

flash point

IEC 1203 continued service fire point m inimumASTM D6874 new fluid fire point minimum

2001 2002 2003 2004 2005 2006

ResidualMineralOil(%)

0

2

4

6

8

10

residual mineral oil maximum

to retain 300oC fire point

estimated from flash point

measured by GC

(a) (b)

Flash and fire points of a 50 MVA 69 kV generator step-up transformer retrofilled with natural ester fluid. Thetransformer was drained and refilled without removing the mineral oil dregs. The result is a high initial mineral oilcontent in the natural ester fluid. As the mineral oil held in the solid insulation mixed with the tank fluid, the overall

residual mineral oil content climbed above the threshold needed to reduce the fire point. Typical retrofills use flush

and drain techniques that minimize the residual mineral oil, usually below 1%.

FIGURE 7


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0

1

10

100

1,000

10,000

hydrogen carbon

monoxide

methane ethane ethylene acetylene Total

Combustible

carbon

dioxide

DissolvedGas(pp

m)

mineral oil 0.01 yr 0.04 yr 0.1 yr 0.2 yr 0.4 yr 0.7 yr 0.9 yr

1 yr 1.3 yr 2.1 yr 2.5 yr 3.9 yr 4.1 yr 4.3 yr (a)

0

20

40

60

80

100

hydrogen carbon

monoxide

methane ethane ethylene acetylene

Re

lativeProportions(%)

(b)

Chart (a) shows the amounts of dissolved gases in the 50 MVA retrofilled transformer slowly returning to pre-

retrofill levels. The exception is ethane, which shows the proportionally higher values typical of natural ester fluid(b). The dark blue bars show gases in mineral oil immediately prior to retrofilling. The light green and red striped

bars show natural ester values and represent data from two different laboratories.

FIGURE 8


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0

1

10

100

1,000

10,000

hydrogen carbon

monoxide

methane ethane ethylene acetylene Total

Combustible

carbon

dioxide

DissolvedGas(ppm)

July '98 Nov '00 Sept '01 May '02 Aug '02 May '03 April '04 May '04 Oct '04

(a)

0

20

40

60

80

100

hydrogen carbon

monoxide

methane ethane ethylene acetylene

RelativeProportions(%)

(b)

Amounts of dissolved gases (a) and their proportions (b) before (blue striped bars) and after retrofilling withnatural ester fluid (light green bars) in a 200 MVA 161 kV generator step-up transformer. The transformer, in

service since 1966, was retrofilled in the spring of 2004. Ethane is beginning to show the higher proportional

level typically seen in natural ester fluid.

FIGURE 9

Documents

2006 Doble - Review of in-Service Tfmrs Using Natural Ester Fluid