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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
ASTM D6871 new fluid maximum
1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006
AcidNumber(mgKOH/g)
0.01
0.1
1
10
IEC 61203 continued service maximum
S/N 1429
S/N 1430
ASTM D6871 new fluid maximum
(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
KinematicViscosity(cSt)
0
10
20
30
40
50
minimum effortbest effort,
minimum effortbest effort,
40oC:
100oC:
ASTM D6871 new fluid 40oC maximum
ASTM D6871 new fluid 100oC 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
IEC 1203 continued service maximum
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
KinematicViscosity(cSt)
0
10
20
30
40
50ASTM D6874 new fluid 40
oC maximum
ASTM D6874 new fluid 100oC 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
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