Vegetable Oils for Liquid-FilledTransformers

Key Words: High oleic oils, biodegradable fluids, ester fluids, insulating fluids, transformers, environmental,high-temperature fluids.

For over one-hundred years, petroleum-based mineraloils purified to “transformer oil grade” have been usedin liquid-filled transformers. Synthetic hydrocarbon

fluids, silicone, and ester fluids were introduced in the latterhalf of the twentieth century, but their use is limited to distri-bution transformers. Several billion liters of transformer oilare used in transformers worldwide.

The popularity of mineral transformer oil is due to avail-ability and low cost, as well as being an excellent dielectric andcooling medium. Ever since the world oil reserves weretapped in the 1940s, petroleum products have become widelyavailable. Petroleum-based products are so vital in today’sworld that we cannot imagine a time we may not have themeasily available. Transformers and other oil-filled electricalequipment use only a tiny fraction of the total petroleum con-sumption, yet even this fraction is almost irreplaceable.

There are two reasons why we should be seriously think-ing of alternate natural sources of insulating fluids:

1. Transformer oil is poorly biodegradable. It could con-taminate our soil and waterways if serious spills occur. Gov-ernment regulatory agents are already looking into thisproblem and are imposing stiff penalties for spills. Manythousands of transformers are located in populated areas,shopping centers, and near waterways. Figure 1 shows somepole-mounted transformers near a coastal region, andpad-mounted transformers in a public area.

2. Petroleum products are eventually going to run out,and there could be serious shortages even by themid-twenty-first century. Conserving the petroleum reservesand recycling are vital for petroleum-based products—plas-tics, pharmaceuticals, organic chemicals, and so on. Until wedevelop economically viable alternate energy sources, thereis no easy replacement for gasoline, jet fuel, and heating oil.Vegetable oils are natural products available in plenty. Theyare used mostly for edible purposes, but special oils are usedfor drying and cutting oils. The only significant electrical useof vegetable oils suggested until the late 1990s were forpower capacitors. Even there, the use is more experimentalthan commercial.

Prior Use of Vegetable Oils in CapacitorsCapacitors were the only type of electrical equipment se-

riously considered for the use of vegetable oils. Clark, for ex-ample, mentions castor and cotton seed oils for use incapacitors (with cellulose insulation) as early as 1962 [1].The higher dielectric constants of these fluids provide abetter match with cellulose than mineral oil. In 1971, Indianresearchers reported testing of coconut oil and hydroge-nated castor and groundnut oils for electrical use [2]. Lateron, in 1974, these authors reported their work on processedcastor and cottonseed oils and noted that castor oil was thebetter choice for capacitors [3]. Further technical papers ap-peared in 1979 and 1983 by other Indian researchers [4],[5]. A U.S. patent issued in 1985 describes possible use ofsoybean oil with additives in capacitors [6]. Interest in castoroil was shown by Brazilian researchers, who reported theirwork in a CIGRE paper in 1987 [7]. Castor oil is mainly (80percent) a hydroxy-acid ester, unlike other vegetable oils,which are fatty acid esters. The acidic part is ricinoleic acidand has the molecular formula

CH3 (CH2)5 CHOH CH2 CH=CH (CH2)7 COOH.

It is more viscous than most vegetable oils and has a higherdielectric constant than most vegetable oils (4.7 versus 3.2).

The above-referenced papers reveal that castor oil, alongwith polypropylene films in power capacitors, was seriouslyconsidered. Yet, synthetic aromatic hydrocarbons are stillthe fluid of choice for power capacitors.

In the 1990s rapeseed oil became the center of interest, asshown by technical papers published in 1995 [8]. Rapeseedoil, while not edible, was available in some countries and

6 0883-7554/02/$17.00©2002IEEE IEEE Electrical Insulation Magazine

F E A T U R E A R T I C L E

T.V. OommenConsultant, Raleigh, SC, U.S.A.

Several billion liters of transformer oilare used in transformers worldwide.

needed commercial exploitation. Its main acidic part of thefatty acid ester is erucic acid (50 percent or more), a C-22acid with one double bond (the most common fatty acid invegetable oils is oleic acid, a C-18 acid with one doublebond). A methyl ester of rapeseed oil (MRSO) has also beentested for possible use in power capacitors [9].

Chemical Composition of Vegetable OilsCrude vegetable oils extracted from oil seeds have a dark

color and contain solid constituents such as proteins and fi-bers, and liquid (fats and oil). Both fats and oil aretriglyceride esters of fatty acids, but fats contain a relativelyhigh percentage of saturated triglycerides and would freezeto solid below room temperature. The oily part usually re-mains as liquid above 0 oC; oils with high unsaturation mayremain as liquid at -15 to -30 oC. The triglyceride ester mole-cule may be represented as

CH2-OOCR1

CH-OOCR2

CH2-OOCR3

where R1, R2, and R3 are fatty acid chains of same or differenttypes. Saturated fatty acids with eight to 22 carbon atoms arefound in oils. Fatty acids with one unsaturated bond havecarbon atoms ranging from 10 to 22. Fatty acids with di-andtri-unsaturation mostly contain 18 carbon atoms (these arenamed Linoleic and Linolenic acids). The fatty acid composi-tion of some vegetable oils is shown in Table I.

Development of Vegetable Oilsfor Transformer Use

Liquid-filled transformers use billions of liters of insulat-ing fluid. They come in various sizes: large, medium, andsmall power as well as distribution, each one using as muchas forty thousand liters in each phase of a large power trans-former to as small as eighty liters for a small distributiontransformer. The smaller units are more numerous than thelarger units because distribution is more widespread by defi-nition, and hence the smaller units hold, in total, much morefluid than the larger units. Mineral oil purified to trans-former grade oil is the most commonly used transformerfluid and has been in use for more than a century. Small unitsused in confined areas like shopping centers may use fire-re-sistant fluids such as silicone, high-temperature mineral oil,and synthetic ester fluids.

In recent years, environmental concerns have been raisedon the use of poorly biodegradable fluids in electrical appa-ratus in regions where spills from leaks and equipment fail-ure could contaminate the surroundings. Contamination ofthe water supply is considered much more serious than con-tamination of the soil.

Due to the utility interest in biodegradable insulating flu-ids, research efforts were started in the mid 1990s to developa fully biodegradable insulating fluid. This effort was started

by R&D labs that initiated oil development work. Vegetableoil was considered the most likely candidate for a fully bio-degradable insulating fluid. Vegetable oil is a natural re-source available in plenty; it is a fairly good insulator, and isfully biodegradable.

The researchers soon recognized that vegetable oils neededfurther improvement to be used as a transformer fluid. Thefluid in a sealed transformer remains in the unit for many years(as many as 30 to 40 years, unless the oil is changed in be-tween). Only in the larger units is the fluid periodically re-freshed. Long-term stability is of critical importance. Vegetableoils inherently have components that degrade in a relativelyshort time. The degree of unsaturation is an indicator of ther-mal instability, becoming more unstable as the degree ofunsaturation progresses from mono- to tri-unsaturation. Therelative instability to oxidation is roughly 1:10:100:200 forsaturated, mono-, di-, and tri unsaturated C-18 triglycerides[11]. In transformers, the presence of copper (as a conductor)enhances tendency for oxidation. Powerful oxidation inhibi-tors are needed for the oils used in transformers. Another factor

January/February 2002 — Vol. 18, No. 1 7

Distribution Transformers

Pad-mountedPole-mounted

Fig. 1.

Table I. Typical Fatty Acid Composition of Some Vegetable Oils [10].

Vegetable Oil Saturated Fatty Acids, % Unsaturated Fatty Acids, %

Mono- Di- Tri-

Canola oil* 7.9 55.9 22.1 11.1

Corn oil 12.7 24.2 58 0.7

Cottonseed oil 25.8 17.8 51.8 0.2

Peanut oil 13.6 17.8 51.8 0.2

Olive oil 13.2 73.3 7.9 0.6

Safflower oil 8.5 12.1 74.1 0.4

Safflower oil, high oleic 6.1 75.3 14.2 -

Soybean oil 14.2 22.5 51 6.8

Sunflower oil 10.5 19.6 65.7 -

Sunflower oil, high oleic 9.2 80.8 8.4 0.2

*Low erucic acid variety of rapeseed oil; more recently canola oil containing over 75%monounsaturate content has been developed.

is the purity of the oil. The oil has to be free of conducting ionicimpurities to acceptable levels, and commercial-grade oils arenot of this purity.

Only recently have transformer-grade vegetable oils be-come available. The first commercial product wasBIOTEMP®, patented in the U.S. in September 1999 by ABBand developed at its Raleigh, NC-based transformer lab[12]. The base fluid was high oleic oil with over 80 percentoleic content. These oils are produced mostly from seedsthat have been developed by selective breeding; more re-cently, gene manipulation techniques have been used. Partialhydrogenation is an added step that may be used to minimizethe very unstable tri-unsaturates. The high mono- unsaturateoils are in demand in the food and lubrication industries.The BIOTEMP® fluid, also from high oleic oils, is now usedin some distribution and network transformers in critical ar-eas. Another U.S. patent was issued later, in September1999, for a transformer oil from regular soybean oil, ob-tained by Waverly Light & Power in Iowa, though this prod-uct is not yet commercially used [13]. It is not a high oleic oil.

In March 2000, another U.S. patent was granted to CooperIndustries, Inc in Milwaukee, WI under the trademarkEnvirotemp FR3® [14]. This fluid also is from stan-dard-grade oleic base oils, and is used commercially in somedistribution transformers. A second patent was issued to theABB inventors on the BIOTEMP® fluid in August 2001 [12].Figure 2 shows typical oil seeds used from which oils are ex-tracted and processed for transformer use.

Fluid development details are not available except for theBIOTEMP® fluid, for which a dozen technical papers havebeen published. Commercial brochures are available for theBIOTEMP® and Envirotemp FR3® fluids. For theBIOTEMP® fluid, the starting oil is a high oleic oil, such assunflower oil, containing 80 percent or more oleic content.Canola oil upgraded to this level of oleic content has alsobeen tested for use. The commercially available RBD grade isthe starting material, where RBD stands for Refined,Bleached, and Deodorized. These processes are well knownin the seed oil industry.

After separation of solid matter, the oil is treated with spe-cial solvents to remove many unwanted components.Bleaching is usually done by clay filter presses, which furtherpurify the oil. Deodorization by steam removes volatiles thatproduce odor. The RBD oil varies in electrical purity over awide range from marginal to impure, with conductivitiesranging from 5 to 50 pS/m. For transformer use, it is desir-able to have a conductivity of 1 pS/m or below. To achievethis, special clays are used with improved adsorbing power.A conductivity meter, such as the Emcee meter described inASTM D4308, may be used to monitor the purity of the oil.

The final stage is the degassification and dehumidifying ofthe oil. Vegetable oils are hygroscopic; hence, they may ab-sorb water at as much as 1200 ppm or more, at saturation andat room temperature. It is desirable to lower this to 100 ppm.

To stabilize the oil, it is necessary to add suitable antioxi-dants. Commonly used inhibitors such as DBPC andfood-grade antioxidants are not powerful enough to pro-duce an oil that will pass the ASTM oxidation tests, such asD-2440 and D-2112. A special antioxidant package that usescomplex phenols and amines is used in the BIOTEMP® fluid.Care should be taken not to add too much because the con-ductivity would rise to unacceptable levels. It is desirable tokeep the level of the additive component to below 1%. Theapproach used for the Envirotemp FR3® fluid is to avoidcontact with air by careful sealing of the transformer and us-ing an oxygen-scavenging powder above the oil level. TheFR3 fluid does not pass the ASTM oxidation test because ofits lower monounsaturate content, even with reasonableamount of inhibitors. The oxidation stability of vegetableoils is greatly dependent on the monounsaturate content,which should be over 80% for long-term transformer use.Proper inhibitors are still needed. The percentage oftri-unsaturates should be negligible in these oils. Figure 3shows how a poorly stabilized and inferior oil (3B) will gelduring a standard oxidation test, ASTM D 2440, while awell-stabilized superior fluid (3A) will not [15]. The gel test

8 IEEE Electrical Insulation Magazine

Sunflower Canola/Rapeseed Soybean

Oil Seeds

Fig. 2.

Fig. 3.

is perhaps much more meaningful than the acidity values forvegetable oil after the oxidation test.

Properties of Transformer-GradeVegetable Oil

Table II lists several physical, chemical, and electricalproperties of vegetable oil specifically devel-oped for transformer use. Comparison dataare given for high-temperature mineral oiland silicone fluid used in transformers [15].

BiodegradabilityThe most accepted test is the CEC-L-33

test developed to test biodegradability of lu-bricating oils in an aquatic environment. Thetest sample and a reference sample containing“poisoned” (with mercury) material are bothinoculated with bacterium. After 21 days thepercentage of C-H hydrocarbon part left ineach set is determined by IR spectroscopy.The percentage biodegradability is computedas 100 (P-T)/P, where P is the residual contentof the poisoned flask and T is the residualcontent of the test flask.

Figure 4 shows comparison chart ofbiodegradability of vegetable oil and othertransformer fluids [16].

Another test for biodegradability is the bi-ological oxygen demand (BOD) test. Pub-lished values for HMW mineral oil, siliconefluid, and polyol ester for a 20-day period are122, 3.6, and 377, respectively [11].

Decomposition ProductsWhen used in transformers, the above-

mentioned fluids experience thermal andelectrical stress; hence it is important to de-termine the effect of these stresses. Gas gener-ation is the most easily measured property,and it is meaningful to study gas generationafter ageing in presence of copper for specificperiods. Figure 5 shows the percentage ofgases generated in a test conducted by theDoble Engineering lab on the BIOTEMP®

and the Envirotemp FR3® fluids for 22 daysat 250 oC [17].

The notable difference in the decomposi-tion products, as compared to hydrocarbonfluids, is in the large amount of CO and CO2

generated. This is because, unlike hydrocar-bon fluids, ester fluids contain a carbonylgroup –COO, which breaks down to give COand CO2. Hydrogen should not normally re-sult from thermal decomposition, but certaincomponents or additives in the oil could pro-duce hydrogen, as seen in the FR3 fluid.

Under partial discharge (PD) conditions, the main prod-ucts are hydrogen, methane, CO, and CO2. Again, the CO andCO2 result from the breakdown of the carbonyl group. Thegeneration of methane and hydrogen are similar to their pro-duction from mineral oils, and result from extraction of hy-drogen atoms from the molecular framework in the electric

January/February 2002 — Vol. 18, No. 1 9

Table II: Properties of Transformer Fluids: Typical Values/Limits.

Veg. Oil High Temp.Mineral Oil

Silicone 561 Fluid

Physical

Appearance Light yellowa

Light yellow Colorless

Specific Gravity at 25 °C 0.91 - 0.92 0.89 0.96

Kinematic viscosity, cSt

0 °C 170 – 250 2200 95

25 °C 55 – 75 300 50

40 °C 33 – 45 125 38

100 °C 8 – 10 13 16

Pour point, °C –15 to –25 –20 max. –50 max.

Interfacial Tension (IFT), dynes/cm 25 40 – 45 25

Flash point, °C 310 – 325 275 min. 300 min.

Fire point, °C 354 – 360 160 – 180 340

Moisture content, ppm dry oil 50 – 100 10 – 25b

50

(Water solubility at 25 °C) 1200 60 200

Thermal constants

Heat capacity, cal/g. °C 0.50 – 0.57 0.488 0.363

Thermal conductivity, W/m.K 0.17a

0.13 0.15

Coefficient of expansion, / °C 0.0007 0.00073 0.00104

Chemical

Chemical type Ester Hydrocarbon Organo-silicon

Acidity, mg KOH/g 0.06a

0.01 0.01

Oxidation stability - ASTM D 2440 Passa

Pass Pass

Electrical

Dielectric constant at 25 °C 3.1 2.2 2.71

Volume resistivity at 25 °C, Ohm.cm 1014

1014

– 1015

1014

Breakdown voltage, kV

ASTM D 1816, 2 mm gap electrodes 74a

60 –

Impulse breakdown voltage, kV (needle negative) 116a

145 136

Dissipation factor, %

25 °C 0.25a

0.05 max. -0.01

100 °C 1.00a

0.3 max –

Gassing tendency – ASTM D 2300 –50a

–10 to +20 N/A

Biodegradabilityc

CEC-L-33 (21 days) 97 – 99 30 Very low

Notes: a: For BIOTEMP fluid,b: Varies with transformer rating,c: See below.

field. Figure 6 shows the gases produced for the BIOTEMP®

fluid and compares with transformer oil degradation [18].Under arcing conditions, the gases produced are mainly

hydrogen and acetylene for mineral oil-based transformeroil; but for vegetable oil, in addition, CO and CO2 are also

produced in large quantities due to the ester group present.Figure 7 shows comparison of gas generation from vegetableoil (BIOTEMP®) and from regular transformer oil [18].

A significant finding has been that the total gas producedwas only one-fourth of the gas produced from regulartransformer oil. This shows the arc-quenching ability ofvegetable oils.

Functional Life TestAny new transformer fluid developed is subjected to a

functional life test in which the fluid is tested in the actualtransformer, under full load and voltage, but at an elevatedtemperature to produce accelerated ageing, so that the age-ing in many years’ lifetime can be tested in a limited time pe-riod of weeks. For distribution transformers, the life testprotocol specified in ANSI/IEEE C 57.100-1986 providesthe following life expectancy formula

Log10Life (h) = [6328.8/(273 + θ)] - 11.269

where θ is the hot spot temperature of the winding, oC.This gives the following minimum periods at accelerated

hot spot temperatures.

Temperature °C Time, h

160 2224

180 503

200 129

Since multiple tests are too expensive, either a single or aduplicate test is carried out. To compensate for the statisticaluncertainty, it is specified that the life testing be conductedfive times the minimum period shown above. It is customaryto round off the final numbers to 10,000, 2500, and 720 h,and this interval is divided into ten equal periods for the tenend point tests.

For vegetable oils with high fire points, the 200 oC hot spottemperature is acceptable, and this reduces the life test timeconsiderably. Sufficient cooling should be provided to keepthe oil temperature below 140 oC because the gaskets maysoften at high temperatures.

The vegetable oils mentioned above surpassed the nor-mal life test period without the unit failing under shortcircuit tests. Separate long-term ageing has shown that

these fluids prolong the life of the paper insula-tion considerably more than mineral oil whenused in transformers. It is possible that the greataffinity of vegetable oils for moisture will keepthe paper drier.

Fire Hazard TestsBased on publicity literature from the oil devel-

opers, the new fluids have passed both the stan-dard UL and Factory Mutual tests for certificationas a less flammable fluid for use in transformers.

10 IEEE Electrical Insulation Magazine

100

50

0

Veg. Oils

Mineral Oil

Silicone Fluid

Per

cent

Bio

degr

adab

ility

Fig. 4.

0

10

20

30

40

50

60CO CO2

Biotemp

FR3

H2

CH4

C H2 6 C H2 4

% G

as G

ener

ated

Fig. 5.

0

2000

4000

6000

8000

10000

Hydrogen CO Methane CO2 Ethane

Gas

Con

tent

(pp

m)

Veg. Oil Trans. Oil

Fig. 6.

010203040506070

Hydrogen

COMethane Ethylene

Acetylene

Veg. Oil

Trans. Oil

Per

cent

Gas

in G

as S

pace

CO2

Fig. 7.

Special ChallengesCold Weather

The use of vegetable oils in transformers that are exposedto cold weather has been an issue. The pour point of vegeta-ble oils does not go below –30 oC, even after adding pourpoint depressants. Without additives, the fluid could freezeat subzero temperatures.

To address this issue, vegetable oil-filled transformerswere frozen to –50 oC or below in lab cooling chambers andthen energized. There have been no failures. Since vegetableoil is a mixture of esters that freeze at different temperatures,there is no sudden freezing or thawing. This helps preventthe formation of cracks and air spaces, which could triggerPD. Under operating conditions, the oil in the units would bein the liquid state even if the ambient outside temperatureswere very low.

Exposure to AirVegetable oil-filled units should be sealed well to prevent

air and moisture from entering the unit. Sufficient antioxi-dant should be present even in sealed units because of possi-ble entry of air and moisture during the life of the unit.

ConclusionTo meet the challenges posed by environmental concerns,

fully biodegradable vegetable oils have been developed foruse in electrical equipment, particularly in transformers.Further exploitation of these fluids for use in capacitors andcables need further study and tests.

AcknowledgmentThe author wishes to thank Mr. Lance Lewand of Doble

Engineering Company, USA, referenced above, for the prep-aration of Figure 5.

T.V. Oommen worked for 24 years in thetransformer industry as an R&D scientist andengineer. In October 2000, he became a con-sultant to the industry. Dr. Oommen is one ofthe pioneers in developing an award-winningbiodegradable vegetable oil-based transformerfluid from high oleic oils. He has been a regu-lar presenter of technical papers for the E/EIC

for 20 years and he has taught short courses at the E/EIC onTransformer Insulation Fundamentals and Insulating Fluids.He may be contacted at tv.oomen@ieee.org.

References1. F.M. Clark, Insulation Materials for Design and Engineering Practice.

New York: McGraw-Hill, 1962.

2. K.M. Kamath, et al., “Variation of dielectric properties of some vegetable

oils in the liquid-solid transition phase,” Indian J. of Technology, pp.

312-313, August 1971.

3. K.M. Kamath, et al., “Vegetable oils for electrical use—Processing and

application,” Industrial Engineering (I) Journal-EL, vol. 56, pp.64-70,October 1975.

4. T.S. Ramu, “On the high frequency dielectric behavior of castor oil,”

IEEE Trans. Elec. Insul., vol. EI-14, no. 3, pp. 136-141, 1979.

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all-polypropylene and polypropylene-paper capacitors,” IEEE Trans.

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6. U.S. Patent 4,536,331, issued August 20, 1985, “Non-toxic impregnat

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7. A. Marinho, et al., “Castor oil as an insulating fluid,” International

Council on Large Electric Systems (CIGRE) Symposium Paper 500.06,

Symposium 05-87, Vienna, 1987.

8. I. Moumine, et al., “Vegetable oil as an impregnant in HV ac capacitors,”

in Proc. IEEE 5th Int’l Conf. on Brkdn. in Sol. Dielec., pp. 611-615,

1995.

9. H.C. Keshavamurthy, et al., “Rape seed oil derivative as a new capacitorimpregnant,” Conf. Record of the 1994 IEEE International Symposium

on Electrical Insulation, Pittsburgh, PA, 1994, pp. 418-421.

10. E.W. Lucas and K.C. Rhee, “Animal and vegetable fats, oils and waxes,”

in Riegel’s Handbook if Industrial Chemistry, 9th ed., J.A. Kent, Ed. New

York: Van Nostrand-Reinhold, 1992, Ch. 8.

11. S.S. Lin, “Introduction to fats and oils technology,” AOCS Publication,

pp. 211-231, 1997.

12. U.S. Patent Nos. 5,949,017, September 7, 1999, 6,274,067, August 14,

2001, and 6,312,623, November 6, 2001, “Electrical transformers

containing electrical insulation fluids comprising high oleic acid oil

compositions” (Inventors: Oommen and Claiborne, ABB Power T&D

Company, Inc., Raleigh, NC).

13. U.S. Patent No. 5,958,851, September 28, 1999, “Soybean based

transformer oil and transmission line fluid” (Inventors: Cannon and

Honary, Waverly Light & Power, Waverly, IA).

14. U.S. Patent No. 6,037,537, March 14, 2000, “Vegetable oil based

dielectric coolant” (Inventors: McShane et al., Cooper Industries, Inc.,

Houston, TX).

15. T.V. Oommen, et al., “A New vegetable oil based transformer fluid:

Development and verification,” in Proc. CEIDP, Victoria, British

Columbia, Canada, 2000, pp. 308-312.

16. T.V. Oommen, et al., “Biodegradable transformer fluid from high oleic

vegetable oils,” Doble Conf. Paper, April 1999.

17. L.R. Lewand, “Laboratory evaluation of several synthetic and

agricultural-based dielectric liquids,” presented at the Doble Spring

Conference, April 2000.

18. T.V. Oommen and C.C. Claiborne, “Biodegradable insulating fluid from

high oleic vegetable oils,” presented at the International Council on

Large Electric Systems (CIGRE) Paris Session, 1998.

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