2002 COOPER INDUSTRIES, INC.
BY C. PATRICK MCSHANE
IEEE INDUSTRY APPLICATIONS MAGAZINE • MAY|JUNE 2002 • WWW.IEEE.ORG/IAS
These new liquids possess such properties as LOWER FIRE AND
ENVIRONMENTAL RISKS while IMPROVING PAPER-INSULATION LIFE.
34
LECTRICAL TRANSFORMER INSU-
attention then focused on determining the ideal properties
LATION SYSTEMS are evaluated based
of mineral oil for dielectric application and developing pro-
on economic, safety, and environmental
cesses for producing a more consistent quality fluid. Key
standpoints using total life cycle analysis.
performance properties were identified, and by 1899 at
Because of the inherent high efficiency of liquid-cooled
least one mineral-oil refinery began to produce a cut of
transformer designs, new developments focus on improv-
mineral oil specifically designed for transformers.
E
ing the environmental and safety properties of fire resis-
Experimentation using natural ester fluids as dielectric
tant (less-flammable, high fire point) fluids. This article
coolant began around the same time as the early mineral oil
reports the latest findings on dielectric systems using nat-
trials. They proved less desirable than mineral oil due to in-
ural ester (vegetable oil) fluids. Because esters naturally
ferior oxygen stability and higher pour point, permittivity,
have lower oxidation resistance than mineral oils, a novel
and viscosity values [1]. To this day, liquid-filled trans-
blend of base oils and additives were developed to over-
formers primarily use mineral oils as the insulating fluid.
come this potential handicap. Single- and three-phase
Other alternatives to mineral oil-filled distribution trans-
prototype field installations using these new dielectric
formers, such as dry and essentially nonflammable liq-
coolants are discussed.
uid-filled types, were commercialized decades ago for use
in specialty applications.
Background
Nearly all nonflammable fluids used in transformers be-
In the United States, the first distribution-class trans-
long to a chemical group known as halogenated hydrocar-
former was built in 1885. It was a dry-type design, using
bons, typically with chlorine or fluorine. Halogenated
air as the dielectric coolant. Although the idea that trans-
dielectric fluids, chiefly Askarel fluids, once promoted for
formers using mineral oil as the dielectric coolant could be
their excellent fire-safety properties, are now undesirable
smaller and more efficient was patented by Elihu Thomson
due to their possible health hazards and proven environ-
in 1882, it took another decade before his idea was put into
mental persistence.
practice. In 1892, General Electric produced the first
Askarel is a halogenated hydrocarbon dielectric
known application of mineral oil in a transformer. Industry
fluid that is a mixture of halogenated hydrocarbons
1077-2618/02/$17.00©2002 IEEE
2002 IEEE. Reprinted, with permission from IEEE Industry Applications Magazine • May/June 2002 • www.IEEE.org/IAS
Volume 8, Issue 3, Pages 34-41
IEEE INDUSTRY APPLICATIONS MAGAZINE • MAY|JUNE 2002 • WWW.IEEE.ORG/IAS
[polychlorinated biphenyls (PCBs) and trichlorobenzene], approved less-flammable fluid [12]. Installation requiretypically containing 40-50% trichlorobenzene. Commer- ments based on specific heat-release rates were eliminated
cialization of Askarel-filled transformers began in the for FME&R-approved less-flammable dielectric fluids. In
1930s. Most applications were for installations that re- fact, FME&R no longer tests nor publishes heat-release rates
quired additional fire safety, primarily urban network for this class of material. This change is due to the proven
fire-safety history of listed less-flammable liquids (no
vaults and indoor locations.
In 1976, the Toxic Substance Control Act [2] targeted known pool fires) and extreme tests that indicate the near
PCBs, the key component of Askarel. Extensive EPA regu- impossibility of such an occurrence. Code listing companies’
latory limits soon followed and are periodically modified. requirements for listed HMWH are now equal to or more
The EPA published its most recent changes in 1998 [3], flexible than those for listed silicone oils [13], [14].
[4]. Although the industry appears resigned to living with these regulations,
Field Performance of
they are controversial.
Less-Flammable Liquid-Filled
The banning of further production and
Transformers
NATURAL ESTERS
commercialization of PCBs and increasToday, there are over 200,000 less-flamACHIEVE A
ingly restrictive Federal and state regulamable liquid-filled transformers in sertions led to the introduction of other
vice. In the 1980s, silicone oil-filled units
BALANCE OF
fire-resistant transformer types. Dry-type
were the primary choice for use in utility
transformer manufacturers responded by
network and rectifier transformers.
DESIRABLE
adding a more robust dry design, using
HMWH f l ui ds , one cl as s of
vacuum-pressure impregnation (VPI),
less-flammable
fluids, are also widely
TRANSFORMER
and increasing power and voltage ratings.
used, particularly in unit substations,
A few transformer manufacturers and
pad-mounted transformers, and oil
AND EXTERNAL
retrofilling service companies promoted
retrofills. To date, more than 120,000
other nonflammable dielectric coolants
ENVIRONMENTAL HMWH fluid-filled transformers have
that did not contain PCBs. These coolants
been installed. The promise of the 1984
PROPERTIES NOT
included perchloroethylene, trichloroUL Classification of HMWH fluid has
benzene, and chlorofluorocarbons. Howbeen achieved. There have been no reFOUND IN OTHER
ever, most replacements of the
ported explosions or fires involving
Askarel-filled transformers selected for reHMWH. All major U.S. transformer
DIELECTRIC
moval used less-flammable fluids [5], [6].
manufacturers now offer the option of
HMWH to their customers.
FLUIDS.
The operating performance record of
Code Recognition of
HMWH is equally positive. The stabilLess-Flammable Liquid-Filled
ity of key fluid properties (e.g., essenTransformers
Formal incorporation of less-flammable transformer fluids tially no sludging), even at high-temperature operation,
into the National Electrical Code (NEC) occurred in 1978. and their superior resistance to fast front impulse breakOriginally, and perhaps more appropriately, these fluids down, has resulted in very satisfactory field performance.
Due to their proven compatibility with aramid insulawere referred to as “high fire point liquids” until 1984,
when the NEC began referring to them as “less-flammable tion materials, HMWH are now used in high-temperature
fluids.” To qualify, the fluids needed a minimum open-cup transformers (HTTs) rated for 115 °C rise (175-185 °C hot
spot) [15]. Ideal applications for these HTTs are mobile
fire point of 300 °C.
The 1981 NEC edition added the requirement for substations (weight limits are critical) or double-ended
third-party certification of less-flammable fluids as “code substations (temporary double-load capability is desired).
However, HMWH, like conventional transformer oils,
listed.” Factory Mutual Engineering and Research
(FME&R) was the first nationally recognized testing labo- are mineral-oil based, thus, they are subject to expanding
ratory (NRTL) to list less-flammable transformer fluids environmental regulatory issues. For example, mineral oils
[7]. In 1984, the idea of combining the inherent fire resis- are essentially excluded from the classification of oils covtance of these fluids with other effective transformer pro- ered in the Edible Oil Regulatory Reform Act [19]. Most
tection methods was introduced by Underwriter’s states require even nonaquatic transformer oil spills to be
Laboratories (UL) in conjunction with transformer manu- reported, and many require removal and replacement of
facturers [8], [9]. Currently, there are ten listings of soil containing oil.
Silicone oil is also classified as nonedible oil. Tests prove
less-flammable dielectric coolants by the NRTLs covering
dimethylsiloxanes (silicones), high molecular weight hy- silicone is essentially nonbiodegradable. There are reports
drocarbons (HMWH), synthetic polyol esters (POE), and that under certain conditions degradation in the environment is possible [16], [17]. Reports showing relatively
polyalphaolefins (PAOs) [10], [11].
In 1994, FME&R adopted transformer installation quick “degradation” of silicone involve soils that have been
guidelines that emphasize built-in transformer protection oven-dried, an unlikely condition in the real world.
It is increasingly important that dielectric fluids proper their Approval Standard 3990. Transformer manufacturers now have the opportunity to offer a FME&R “ap- vide a better balance of functional performance inside the
proved and labeled” transformer along with a FME&R- transformer versus environmental impact in the event of
35
release. Inside the transformer, a stable, chemically inert
fluid having good thermal and dielectric properties is desired. Externally, the fluid should become environmentally
benign by being nontoxic and readily biodegradable.
Key Environmental and Health Issues
IEEE INDUSTRY APPLICATIONS MAGAZINE • MAY|JUNE 2002 • WWW.IEEE.ORG/IAS
Today, not only are performance and value key criteria in
material selection, but overall environmental and total life
cycle costs are becoming part of the analysis. Materials to
be applied as dielectric fluids should meet suggested minimum health and environmental related requirements. For
example, they should
■ be essentially nontoxic
■ be biodegradable
■ produce by-products with acceptable low risk thermal degredation
■ be recyclable, reconditionable, and readily disposable
■ not be listed as a hazardous material by the EPA or
OSHA.
Highly refined uncontaminated conventional transformer
and HMWH mineral oils meet the above environmental cri-
Ester-Based Fluid Alternatives
Esters are a broad class of organic compounds. They are
available as natural agricultural products or chemically
synthesized from organic precursors.
Synthetic Esters
1
Structure of a typical polyol ester.
Synthetic ester dielectric fluids, most commonly POEs,
have suitable dielectric properties [18] and are significantly more biodegradable than mineral oil or HMWHs.
Their high cost compared to other less-flammable fluids
generally limits their use to traction and mobile transformers and other specialty applications.
Since 1984, synthetic ester fluids have been used as an
askeral substitute in compact railroad traction transformers as well as in scientific apparatus such as klystron modulators. These applications require low viscosity, high
lubricity, and very low pour-point properties to justify the
higher cost. Failure rates of traction transformers significantly decreased after replacing the Askarels with POEs.
Natural Esters
2
36
teria. However, perhaps due to occurrences of conventional
mineral oils contaminated with PCBs, there is a trend of increasingly strict regulations and potential liabilities associated with petroleum oil spills. Their potential for containing
some polynuclear aromatics also adds to the issue.
Petroleum-based oils are complex mixtures of hundreds of
different organic compounds. Although organic compounds
will eventually biodegrade, their respective rates of
biodegradation differ significantly. They consist of saturated
and unsaturated straight-chain, cyclic, and aromatic compounds containing 1-40 carbon atoms. Modern petroleum-refining techniques minimize the unsaturated and
aromatic content of new dielectric oils. These oils are obtained
using higher distillation temperatures and result in a predominance of naphthenic compounds—also known as cyclo-paraffins—with higher melting points [18]. Mineral oils, such as
HMWH, are classed as paraffinic oil, consisting mainly of saturated compounds with long straight-chain structures.
With these trends and concerns in mind, the potential
of nonpetroleum, nonhazardous alternative materials with
environmental characteristics better than even highly refined mineral oils have been studied. Additional minimum
requirement goals for improved health and environmental
safety include
■ a magnitude increase in rate and degree of biodegradation
■ essential nontoxicity when consumed
■ derivation from renewable resources.
One class of material with the potential to function as a
dielectric coolant that appears to meet these health and environmental criteria is organic esters.
Structure of a natural ester (vegetable oil) triglyceride.
Natural seed-oil esters were considered unsuitable for use
in transformers, although past applications of rapeseed oil
in capacitor applications hint at considerable potential.
Their susceptibility to oxidation was the primary obstacle
to their application as a dielectric fluid. However, modern
transformer design practices, along with suitable fluid additives and minor design modifications, compensate for
this characteristic.
TABLE 1. COMPARISON OF TRANSFORMER DIELECTRIC FLUIDS—TYPICAL VALUES
Mineral
Oil
Silicone
Oil
HMWH
Synthetic
Ester
Natural
Ester
Test
Method
New
42
40
52
43
47
D-877
After 50 switch
operations [22]
41
<4
43
36
47
D-877
40 °C
9.2
37
121
29
33
D-445
100 °C
2.3
15.5
12.5
5.6
7.9
D-88
Flash Point (°C)
147
300
276
270
328
D-92
Fire Point (°C)
165
343
312
306
357
D-92
Specific Heat (cal/gm/ °C) @ 25 °C
0.39
0.36
0.45
0.45
0.45
D-2766
Pour Point (°C)
−50
−55
−21
−50
−21
D-97
Specific Gravity
0.87
0.96
0.87
0.97
0.92
D-1298
Biochemical Oxygen Demand (ppm)
6
0
6
24
250
Five-day
SM5210B
BOD/COD Ratio (%)
7
0
17
−
45
−
Trout Fingerling Toxicity Mortality
N/A
N/A
N/A
N/A
0
OECD 203
Dielectric Breakdown
(kV)
In 1892, experiments with liquids other than mineral oils
included ester oils extracted from seeds. None made operational improvements over mineral oil and were not commercially successful. A particular problem with
seed-oil-based coolants was their high pour point and inferior resistance to oxidation relative to mineral oil.
Except for occasional applications in capacitors and other
specialty applications, renewed interest in ester-based dielectric fluids did not occur until after the infamous PCB issue arose in the 1970s. By then, there was a mature synthetic
organic ester industry serving other markets.
Depending on the types of acid and alcohol precursors, a
wide variety of synthetic esters are possible. This allows the
industry to produce “designer” ester molecules. Synthetic
aliphatic polyol esters were selected for Askarel substitution in transformers because of their favorable viscosity/fire
point ratios and excellent environmental and dielectric
properties. They are members of the same family of esters
used for decades as jet engine lubricants.
In 1984, the first transformer applications of these synthetic esters in the United States were in rolling stock
transformers with very high duty requirements. Due to
their compact dimensions, such transformers have forced
circulation flow to remote heat exchangers. Therefore, excellent lubricity, very low pour-point temperatures, and a
high fire point were important fluid characteristics for this
80
"100%
"100% biodegradable"
biodegradable" above
above 60%
60% of
of the
the
theoretical
theoretical maximum
maximum CO
CO22 evolution
evolution
60
Envirotemp FR3 Fluid
Conventional Transformer Oil
Sodium Citrate Reference Material
(EPA "Ultimate Biodegradability")
40
100
75
50
20
0
0
Biodegradation (%)
History of Ester Fluids as Dielectric Coolant
100
25
5
10
15
20
25
30
35
40
Elapsed Time (Days)
0
45
3
Aerobic biodegradation of natural ester and conventional
transformer oil.
IEEE INDUSTRY APPLICATIONS MAGAZINE • MAY|JUNE 2002 • WWW.IEEE.ORG/IAS
The application of natural esters in transformers
achieves a balance of desirable transformer and external environmental properties not found in other dielectric fluids.
An attractive source of natural esters is edible seed oils.
Used mainly in foodstuffs, these agricultural commodity
oils are widely available and, unlike mineral oil, are derived
from renewable resources.
CO2 Evolution (% of Theoretical Max)
Viscosity (cSt)
4
Lockie accelerated-aging transformer test facility.
37
15000
75
10000
50
5000
25
0
0
1000
2000
3000
75
800
600
50
400
25
200
0
0
4000
100
Mineral Oil
Natural Ester
1000
0
IEEE INDUSTRY APPLICATIONS MAGAZINE • MAY|JUNE 2002 • WWW.IEEE.ORG/IAS
5
38
1000
2000
3000
Aging Time (hours)
0
4000
Retained DoP (% of Unaged)
100
Retained Tensile Strength
(% of Unaged)
Tensile Strength (lb/in2)
Mineral Oil
Natural Ester
(Error Bars = 1σ)
Degree of Plymerization
1200
20000
6
Aging rates (as tensile strength) of thermally upgraded pa-
Aging rates (as degree of polymerization) of thermally up-
per in natural ester and conventional transformer oil.
graded paper in natural ester and conventional transformer oil.
application. Market acceptance of synthetic esters has been
limited to specialty applications, primarily due to their
high cost compared to other dielectric fluids. It has been reported that the electric utility of Berlin bought several distribution transformers with such synthetic esters.
Because of environmental regulations and liability
risks involving nonedible oils, an extensive R&D effort,
begun in the early 1990s, revisited the natural esters.
They share many of the excellent dielectric and fire safety
properties of synthetic polyol esters and are classified as
edible oils. Importantly, they are much more economical
than synthetic esters.
The remainder of this article offers a summary of development work on a natural edible seed-oil-based dielectric
fluid. It includes a background discussion of esters, key
property comparisons with other major dielectric fluid
types, and details of field trials.
TABLE 2. TEST PARAMETERS FOR
TRANSFORMER ACCELERATED AGING
EVALUATION AND RESULTS TO DATE
Test Cell
A
B
C
Target Hottest-Spot Temperature (°C)
167
175
Synthetic Esters
Synthetic esters have excellent thermal stability and good
low-temperature properties. There are seven main types of
synthetic esters: diester, phthalate, trimellitate,
pyromellitate, dimer acid ester, polyols, and polyoleates.
An example of a commercially available polyolester-based dielectric coolant is made from a branched
mono-acid (C5-C18) and the alcohol pentaerythritol. The
structure is C(CH2CO2R)4, where the R groups are
branched as shown in Fig. 1.
183
Natural Esters
Standard Required Expected Life*
Hours
1,302
721
407
Years
Equivalent
≈ 21
≈ 21
≈ 21
Standard Test Method Required Life**
Hours
6,510
3,604
2,036
Years
Equivalent
≈ 105
≈ 105
≈ 105
#
Natural and Synthetic Esters:
Key Composition Similarities and Differences
Actual Times to Failure:
Hours
11,400 +
1,190
10,186
6,623
Years
Equivalent
464
328
271
* Per Fig. 1, ANSI/IEEE C57.91-1981
** Test method per ANSI/IEEE C57.100-1986
#
Years equivalent calculations include correction
to actual hot spot temperature from target hot
spot temperature.
Seed-based esters, including liquid fats and oils, are derived from glycerol and are known as tryglycerides. The
fatty acid segments are composed of straight chains having
an even number of carbon atoms. This is the natural result
of the biosynthesis of fats, where molecules are built up two
carbons at a time. The structure in Fig. 2 is a triglyceride
where the (R, R′, R″) groups consist of C8-C22 chains.
The natural esters tested for potential transformer application are fatty acid ester triglycerides. The fatty acid
components are linear chains 14-22 carbons long containing zero to three double bonds.
Natural Ester and Additive Screening
Based on Key Characteristics
In 1993, two dozen food-grade-base oil and blend candidates were evaluated. Some oils contained a high percentage
of unsaturated fatty acids, resulting in lower viscosity and
better low-temperature properties. Others had a higher percentage of saturated types, improving oxidative stability.
The ratio between the two types of fatty-acid oils requires a
careful balance. An optimal balance was selected, and the
next step, improving its oxidation and pour point, began.
Once the natural ester-based formula was determined to
possess acceptable key properties, small and large scale accelerated thermal system life testing began. Small-scale
testing focused on the compatibility of natural ester and
conventional transformer construction materials. No material incompatibilities were found.
Based on the very satisfactory results of the small-scale
test, the decision was made to proceed with a large-scale
Relative Water Content (% Saturation)
Absolute Water Content (mg/kg)
40
400
(IEC 1203 Continued Service Maximum)
30
300
S/N 1429
S/N 1430
20
200
10
100
0
0
0
1
2
3
Years in Service
4
5
7
Moisture content of prototype transformers. The IEC limit for
in-service synthetic esters is shown as a dashed line.
Fluid Dissapation at 25 °C (%)
Accelerated Insulation Life Tests
thermal life test. Per ANSI/IEEE, when a new insulation
system is developed, it is recommended to test the system
following its C57.100 standard. The standard is known in
the industry as the “Lockie” method and is entitled Standard Test Procedure for Thermal Evaluation of Oil-Immersed
Distribution Transformers Life Test [20].
The method uses actual transformers (Fig. 4). Units are
placed in one of three cells, with each cell set to run at a particular high hot-spot temperature at normal primary voltages. The test requirements were successfully met. Details
of the Lockie method on the new insulation system and its
results were presented at the 1999 IEEE/PES Conference
[21]. A condensed data summary is shown in Table 2. The
very positive Lockie test results led to beta-site field testing
of prototype transformers using the natural ester-based dielectric coolant.
The Lockie test results were so favorable that they compelled us to further quantify the relative improvement of
paper-aging rates between natural esters and conventional
transformer oil impregnation [24]. Figs. 5 and 6 show the
significantly slower aging rate of thermally upgraded
Kraft paper in natural ester. We estimate that thermally
upgraded paper in natural ester will have the same life in a
85 °C rise transformer as identical paper in a 65 °C rise
mineral-oil transformer.
0.8
(IEC 1203 Continued Service Maximum)
0.6
S/N 1429
S/N 1430
0.4
0.2
0.0
0
1
2
3
4
Time in Service (Years)
5
8
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The selection of additives for enhancing performance
and oxidation stability began in 1994. Included in the additive study were several food-grade materials. After completing small scale accelerated aging and other tests, the
combination and quantities of additives were determined.
In the end, it was possible to use food-grade materials exclusively for the additives and base esters selected.
Candidate formulation key property testing was the
next step. A summary of the results is shown in Table 1.
It can be seen that the candidate formulation of food-grade
materials possesses superior key characteristics in just about
all categories. Test data shows natural ester fluid to have the
highest fire resistance. Its dielectric breakdown is superior,
both in new condition and after multiple-load break-switching operations. Its viscosity is closer to that of conventional
mineral oil than either silicone or HMWH less-flammable
fluid at operating temperatures. On the negative side, the natural ester formula has a relatively high pour point, although
no higher than the HMWH, which has a very good service record in transformers installed in cold and hot climates.
Synthetic ester has a pour point close to that of conventional mineral oil. Its dielectric strength and viscosity are
similar to natural ester formulation, but it possesses a lower
fire point and slower biodegradation rate. Another unfavorable characteristic is its specific gravity of 0.97, between the specific gravity of water and ice. This could
promote forced migration of any free water between the
bottom and top oil level of the transformer in cases where
supersaturation has occurred.
Biodegradation rate is a popular measure to quantify environmental-persistence impact. Biodegradation rate,
measured by the biochemical oxygen demand (BOD)
method, is significantly faster for natural ester formulation
than other dielectric fluids. It also possesses the most favorable BOD/COD (chemical oxygen demand) ratio.
Another measure of relative biodegradability can be made
using EPA Method 835.3100. As seen in Fig. 3,
food-grade-based dielectric coolant degrades at a rate and degree similar to that of the EPA “ultimate biodegradability”
reference material, sodium citrate. Conventional transformer
oil’s biodegradation rate is much slower.
Organic natural esters and food-grade additives making
up the formulation meet or exceed all minimum environmental and health safety targets. The formulation has essentially no human toxicity (all base oils and
performance-enhancing additives are food grade) and a
magnitude higher biodegradation rate than mineral oil. Its
complete combustion products are carbon dioxide and water. The fluid can be rejuvenated, recycled, and readily disposed. It conforms to the classification of oils covered in the
Edible Oil Regulatory Reform Act [19].
Dissipation factor of prototype transformers. The IEC limit for
in-service synthetic esters is shown as a dashed line.
39
10
Acid Number (mg KOH/g)
Dielectric Breakdown (kV)
100
80
60
40
(IEC 1203 Continued Service Minimum)
20
D-1816:
D-877:
0
0
1
2
3
S/N 1429
S/N 1430
S/N 1429
S/N 1430
4
0.1
0
1
2
3
4
5
Years in Service
9
10
Dielectric breakdown strength of prototype transformers.
Neutralization (acid) number of prototype transformers.
The IEC limit for in-service synthetic esters is shown as a
The IEC limit for in-service synthetic esters is shown as a
dashed line.
dashed line.
Transformer Beta Site Performance Status
strength. Typical sources of moisture in dielectric fluid are
breakdown of cellulose insulation and contact with moist
air in the transformer headspace. Moisture levels rose only
slightly and remain well below the IEC synthetic-ester acceptance limits for new fluid (200 ppm) and continued use
of aged fluid (400 ppm). This is an especially positive result
since natural esters typically have a magnitude higher
moisture saturation level than mineral oils.
Dissipation factor is a measure of the dielectric losses, or
energy dissipated as heat, in an insulating fluid when exposed to an alternating electric field. It is a significant indicator of contamination or deterioration. Results using
ASTM D924 (essentially the same as IEC 247) showed a
small increase in dissipation factor, remaining well below
the limit of 1.0 for service-aged synthetic esters.
Dielectric-breakdown voltage defines the ability of the
fluid to withstand dielectric stresses. Degradation of dielectric strength typically indicates the presence of moisture and polar particle contamination from external sources
and/or insulation aging. The dielectric breakdown voltage
test traditionally used for service aged fluids, ASTM D877,
is less sensitive to small amounts of contamination than
ASTM D1816. ASTM Method D1816, similar to IEC 156,
is the favored method for new and service-aged fluids due
to improved repeatability.
Dielectric-breakdown voltage decreased with use from
59-45 kV per ASTM D 877 and from 70-56 kV per ASTM
D 1816. These in-service values are well above acceptance
limits for new synthetic ester fluid (45 kV) and continued
use of aged fluid (30 kV) per IEC standards (Fig. 9).
The neutralization number is the measure of the acidic
components of chemical breakdown in service-aged fluids.
The neutralization number for new fluid is 0.03 mg
KOH/g, just at the 0.03-limit set in IEC 1099. The latest
service-aged fluid sample measured less than 0.50 mg
KOH/g, well below the 2.0 limit set in IEC 1203 (Fig.10).
All the data, representing one year of typical transformer life, are very positive, showing no significant performance changes in fluid properties.
In 1996, a single-phase pole and several three-phase
pad-mount units were put into service at an in-house manufacturing facility. Beginning in 1997, units were installed at utility and industrial sites throughout the United
States.
Fluid samples taken periodically from beta units were
analyzed for key fluid properties and dissolved gases.
In-service and time-of-manufacture test results for a typical unit are detailed in Figs. 7-10.
Since there are neither ASTM nor IEEE standards for ester fluids, nor IEC standards for natural esters, all data are
compared to IEC Standard 1099, Specifications for Unused
Synthetic Organic Esters for Electrical Purposes and IEC Standard 1203, Synthetic Organic Esters for Electrical
Purposes—Guide for Maintenance of Transformer Esters in
Equipment. ASTM and IEEE standards specifically tailored
to both natural and synthetic esters are needed, as well as an
IEC standard for natural ester-based dielectric coolants.
Moisture content of a dielectric fluid must remain well
below saturation to prevent a decrease in dielectric
10
Rise Over Mineral Oil (°C)
IEEE INDUSTRY APPLICATIONS MAGAZINE • MAY|JUNE 2002 • WWW.IEEE.ORG/IAS
S/N 1429
S/N 1430
0.01
5
Years in Service
8
Top Oil
Average Winding
6
4
2
0
–2
–4
–6
15
225
300
1,000
4,000
Transformer Size (kVA)
10,000
11
Comparative heat runs of mineral oil design transformers.
The average winding rise and top oil differentials between
40
(IEC 1203 Continued Service Maximum)
1
mineral oil and natural ester are shown.
Dozens of additional beta installations include singleand three-phase overhead, pad-mount, and substation
transformers located indoors and outdoors at industrial,
commercial, and utility facilities across the United States.
They include various voltage classes with all-aluminum
and all-copper designs. All beta field units have also performed flawlessly.
Thermal Performance
[4] Disposal of Polychlorinated Biphenyls (PCBs), EPA, Parts 750-761,
40 CFR, 1998.
[5] Mark Earley, “Minimizing the hazards of transformer fires,” Fire J.,
pp 73-74, Jan./Feb. 1988.
[6] D.A. Hallerberg, “Less-flammable liquids used in transformers,”
IEEE Ind. Applicat. Mag., vol. 5, pp. 50-55, Jan./Feb. 1999.
[7] Factory Mutual Approval Guide. Norwood, MA: Factory Mutual Research Corporation, 1979.
The thermal performance of natural ester is comparable to
that of conventional mineral oil, despite its higher viscosity. The differences between average winding rises using
mineral oil and natural ester in transformers designed for
mineral oil are shown in Fig. 11. Through 1,000 kVA, the
average winding rise is within 1 °C of mineral oil. A
4000-kVA transformer showed 3.5 °C higher average
winding rise with natural ester. Comparative heat runs using a 10-MVA mineral-oil design transformer are planned.
[8] “R-temp fluid UL classification marking,” in Gas & Oil Equipment
Directory. Underwriters Laboratories, 1984.
[9] S.D. Northrup, “Protection of transformers for prevention of
Code and Approval Status for Natural Esters
Mutual Research Corporation Approval Standard Class Number
3990.
[13] “Loss prevention data sheet 5-4/14-8,” Factory Mutual Research
Corporation, 1997, revised 1998.
[14] NEC Requirement Guidelines, 1999 Code Options for the Installation of Listed Less-Flammable Liquid-Filled Transformers.
Waukesha, Wisconsin: Cooper Power Systems, 1999.
[15] “High-temp fluid, insulation protect mobile transformer,” Elec.
World, vol. 209, No. 6, p. 32, June, 1995.
[16] “Anthropogenic compounds,” in The Handbook of Environmental
Chemistry, G. Chandra, Ed. New York: Springer-Verlag, 1997, vol.
3, part H.
[17] R.E. Pellenbarg and D.E. Trevault, “Evidence for a sedimentrary
siloxane horizon,” Environ. Sci. Technol., pp. 743-744, July 1986.
At this time, two commercially available seed-oil-based dielectric coolants are listed by NRTLs. One manufacturer is
offering transformers, listed and labeled by a NRTL, filled
with seed-based oils [12].
Conclusion
Acknowledgments
The author gratefully acknowledges the technical contributions of Jerry Corkran, John Luksich, and Kevin Rapp,
of Cooper Power Systems and Hark Huber, IEEE/PCA
Working Group Chair-Power Generation and Distribution, for helping make this article a reality.
References
[1] F.M. Clark, Insulating Materials for Design and Engineering Practice. New York: Wiley, 1962.
[2] Toxic Substance Control Act, U.S. Public Law 94-469.
[3] PCB Regulations, EPA, Part 761, 40 CFR, 1979.
Research Corporation, 1999.
[11] Gas & Oil Equipment Directory. Underwriters Laboratories, 1984.
[12] Less and Nonflammable Liquid-Insulated Transformers, Factory
[18] A.C.M. Wilson, Insulating Liquids: Their Uses, Manufacture, and
Properties. New York: Institution of Electrical Engineers; New
York: Peter Peregrinus Ltd., 1980.
[19] Edible Oil Regulatory Reform Act, Public Law 104-55, 1995.
[20] Standard for Thermal Evaluation of Oil-Immersed Distribution
Transformers, ANSI/IEEE C57.100-1986.
[21] C.P. McShane, G.A. Gauger, and J. Lukich, “Fire resistant natural
ester dielectric fluid and novel insulation system for its use, in Proc.
1999 IEEE/IAS Transmission and Distribution Conf., 1999, vol. 2,
pp. 890-894.
[22] G.P. McCormick and E. Howells, Arcing Resistance of High Fire
Point Dielectric Liquids. Piscataway, NJ: IEEE Press, 1996.
[23] W.J. Chatterton and J.L. Goudie, “An update on silicone transformer fluid,” in Conf. Rec. 2000 IEEE Int. Symp. on Electrical Insulation, pp. 412-416.
[24] C.P. McShane, K.J. Rapp, J.L. Corkran, G.A. Gauger, and J.
Luksich, “Aging of paper insulation in natural ester dielectric
fluid,” in Proc. 2001 IEEE/PES Transmission and Distribution
Conf., Atlanta, GA, 2001, vol. 2, pp. 675-679.
C. Patrick McShane (PMcshane@cooperpower.com) is with Cooper Power Systems in Waukesha, Wisconsin, USA. This article
first appeared in its original format at the 2001 IEEE/PCA Cement Industry Conference.
IEEE INDUSTRY APPLICATIONS MAGAZINE • MAY|JUNE 2002 • WWW.IEEE.ORG/IAS
Market and regulatory pressures to reduce liability-risk exposure of mineral-oil-filled distribution and power transformers are increasing. In addition, there are demands to
improve equipment efficiencies and adopt more
earth-friendly options in our power systems. Considering
these paradigm shifts, the industry has been developing
new transformer concepts.
Based on data obtained from laboratory and field trials,
a practical, edible-oil-based dielectric coolant using
food-grade additives can be successfully incorporated into
transformer insulation systems, with minimal modifications. Testing indicates that they can offer a significant reduction in fire and environmental risks compared to
conventional mineral oil. Compared to all commercially
available fire-resistant nonester dielectric coolants, testing
indicates a most favorable environmental profile. Preliminary studies indicate that the concept is a competitive alternative to all existing transformer types based on total
life-cycle cost and, with several fire-resistant types, on both
a first and total life-cycle cost basis.
rupture, explosion, and fire,” presented at the Edison Electric Inst.
Transmission and Distribution Conf., Fort Worth, TX, 1985.
[10] Factory Mutual Approval Guide. Norwood, MA: Factory Mutual
41