Electrical transformer with vegetable oil dielectric fluid

Abstract

High oleic acid triglyceride compositions that comprise fatty acid components of at least 75% oleic acid, less than 10% diunsaturated fatty acid component; less than 3% triunsaturated fatty acid component; and less than 8% saturated fatty acid component; and having the properties of a dielectric strength of at least 35 KV/100 mil gap, a dissipation factor of less than 0.05% at 25 NC, acidity of less than 0.03 mg KOH/g, electrical conductivity of less than 1 pS/m at 25 NC, a flash point of at least 250 NC and a pour point of at least −15 NC are disclosed. Electrical insulation fluids comprising the triglyceride composition are disclosed. Electrical insulation fluids that comprise the triglyceride composition and a combination of additives are disclosed. Electrical apparatuses comprising the electrical insulation fluids and the use of electrical insulation fluids to provide insulation in electrical apparatuses are disclosed. A process for preparing the high oleic acid triglyceride composition is disclosed.

Description

BACKGROUND OF THE INVENTION

Although eclipsed by mineral oils and later polychlorinated biphenyl (PCB) fluids, vegetable oils have regularly been used as dielectric fluids since the late 1880's. Prior art patents routinely describe vegetable oil as being a conventional dielectric fluid. For example, U.S. Pat. No. 4,355,346 to Gauger et al., when discussing a biodegradable aromatic dielectric fluid, states (with emphasis added): “While this dielectric fluid has a burn point, it will not burn as readily as other conventional dielectrics, such as mineral oil and vegetable oil . . . ”Vegetable oil dielectrics, however, have received increased attention lately due to the banning of PCBs and a more general heightened ecological and physiological sensitivity. Developers then and now have recognized that vegetable oils inherently possess good dielectric properties and flow properties that are suitable for electrical devices. For example, rapeseed oil has a relative dielectric constant of 3.1 at 20° C., a viscosity of 50 centistokes at 25° C. and a pour point of −20° C. (see Japanese Patent 61-260,503). Developers, however, have also recognized that vegetable oils are susceptible to oxidation, despite the fact that vegetable oils inherently contain Vitamin E, which is an antioxidant. In order to improve the oxidation stability of vegetable oil dielectric fluids, additional quantities of antioxidants have been added to vegetable oil dielectrics. For example, British Patent 835,078, which published in 1960, discloses a capacitor having a dielectric fluid consisting of castor oil and one or more antioxidants, namely hydroquinone and Vitamin E (which is inherently contained in castor oil). In addition, U.S. Pat. No. 4,388,669 to Cichanowski, which issued in 1983, discloses a capacitor having a dielectric fluid consisting of cottonseed oil and one or more antioxidants, namely 2,6 di tert-butyl-p-cresol (butylated hydroxytoluene) and Vitamin E (which is inherently contained in cottonseed oil).

In addition to antioxidants, other additives have been added to vegetable oil dielectrics to improve the functional characteristics thereof. For example, U.S. Pat. No. 4,538,208 to Shedigan discloses a capacitor having a dielectric fluid consisting of soybean oil, a gas absorber (an olefin) and one or more antioxidants, namely butylated hydroxyanisole and Vitamin E (which is inherently contained in soybean oil). Specifically with regard to transformers, Japanese Patent 61-260,503 (published Nov. 18, 1986) discloses a dielectric fluid for a transformer consisting of a vegetable oil (e.g. soybean oil, cottonseed oil), a low temperature additive (alkyl methacrylate) and an antioxidant (Vitamin E which is inherently contained in vegetable oil). In fact, Japanese Patent 61-260,503 discloses that the alkyl methacrylate can be present in as little as 0.01%

None of the references discussed above, however, discloses a transformer with a vegetable oil-based dielectric fluid having the oxidation stability and electrical properties of the dielectric fluid of the present invention.

SUMMARY OF THE INVENTION

In accordance with the present invention, a transformer is provided having a core-coil assembly, a housing containing the core and coil assembly and a dielectric fluid disposed in the housing. The dielectric fluid includes one or more vegetable oils and one or more antioxidant compounds and has an oxidative stability of 100 or more AOM hours.

DETAILED DESCRIPTION OF THE INVENTION

This present invention provides a novel application for high oleic vegetable oils as electrical insulation fluids. Vegetable oils usually have a high percent of triglyceride esters of saturated and unsaturated organic acids. When the acid is saturated, the triglyceride is either a semi-solid or a liquid with high freezing point. Unsaturated acids produce oils with low freezing points. However, monounsaturated acids are preferred over diunsaturated and triunsaturated acids because the latter tend to dry fast in air due to cross-linking with oxygen. Increasing the amount of diunsaturates and triunsaturates makes the oil more vulnerable to oxidation; increasing the saturates raises the pour point. Ideally, the higher the monosaturate content, the better the oil as an electrical fluid.

Oleic acid is a monounsaturated acid found as triglyceride ester in many natural oils such as sunflower, olive oil and safflower in relatively high proportions (above 60%). High oleic acid content is usually above 75% of the total acid content. Oleic acid content above 80% is achieved by genetic manipulation and breeding: Two oils that are currently available in the United States with high oleic acid content and low saturates are sunflower oil and canola oil. These oils are of value in producing high quality lubricating oils but have not been used in the production of electrical insulation fluids.

High oleic oils may be derived from plant seeds such as sunflower and canola which have been genetically modified to yield high oleic content. The pure oils are triglycerides of certain fatty acids with a carbon chain ranging from 16 to 22 carbon atoms. If the carbon chain has no double bonds, it is a saturated oil, and is designated Cn:0 where n is the number of carbon atoms. Chains with one double bond are monounsaturated and are designated Cn:1; with two double bonds, it will be Cn:2 and with three double bonds Cn:3. Oleic acid is a C18:1 acid while erucic acid is a C22:1 acid. The acids are in the combined state as triglycerides, and when the oils are hydrolyzed they are separated into the acid and glycerol components. High oleic oils contain more than 75% oleic acid (in combined state with glycerol), the remaining being composed mainly of C18:0, C18:2 and C18:3 acids (also in combined state with glycerol). These acids are known as stearic, linoleic and linolenic. Oils with a high percentage of double and triple unsaturated molecules are unsuitable for electrical application because they react with air and produce oxidation products. Monounsaturated oils such as oleic acid esters may also react with air, but much slower, and can be stabilized with oxidation inhibitors.

A typical 85% high oleic oil has the following approximate composition:

Saturates:       3-5%
monounsaturates: 84-85%
diunsaturates:   3-7%
triunsaturates:  1-3%

While the present invention provides for the use of vegetable oils, the invention may use synthetic oil having the same compositional characteristics of those oils isolated from plants. While plant derived material is suitable for almost all applications, synthetic material may provide a desirable alternative in some applications.

According to the present invention, high oleic acid content oils are used as starting materials for the production of an oil composition which has physical properties useful for electrical insulation fluids. The present invention provides the processed compositions having specific structural and physical characteristics and properties, methods of making such composition, electrical insulation fluids which comprise the composition, electrical apparatuses which comprise the electrical insulation fluids and methods of insulating electrical apparatuses using such fluids.

The present invention provides a high oleic acid triglyceride composition useful as an electrical insulation fluid and more particularly as a component material of an electrical insulation fluid. A triglyceride composition is a glycerol backbone linked to three fatty acid molecules. The triglyceride compositions of the invention comprise fatty acid components of at least 75% oleic acid. The remaining fatty acid components include less than 10% diunsaturated fatty acid component, less than 3% triunsaturated fatty acid component; and less than 8% saturated fatty acid component.

The triglyceride compositions of the invention preferably comprise fatty acid components of at least 80% oleic acid. The triglyceride compositions of the invention more preferably comprise fatty acid components of at least 85% oleic acid. In some embodiments, the triglyceride compositions of the invention comprise fatty acid components of 90% oleic acid. In some embodiments, the triglyceride compositions of the invention comprise fatty acid components of greater than 90% oleic acid.

Di-unsaturated, triunsaturated and saturated fatty acid components present in the triglyceride are preferably C16-C22. It is preferred that 80% or more of the remaining fatty acid components are C18 diunsaturated, triunsaturated and saturated fatty acids, i.e. linoleic, linolenic and stearic acids, respectively. In some embodiments, the diunsaturated, triunsaturated and saturated fatty acid components of the triglyceride comprise at least 75% oleic acid, less than 3% linoleic acid, less than 4% stearic acid and less than 4% palmitic acid (saturated C16).

The triglyceride compositions of the invention are of an electric grade. That is, they have specific physical properties which make them particularly suited for use as an electrical insulation fluid. The dielectric strength of a triglyceride composition of the invention is at least 35 KV/100 mil (2.5 mm) gap, the dissipation factor is less than 0.05% at 25 NC, the acidity is less than 0.03 mg KOH/g, the electrical conductivity is less than 1 pS/m at 25 NC, the flash point is at least 250 NC and the pour point is at least −15 NC.

The dielectric strength, dissipation factor, acidity, electrical conductivity, flash point and pour point are each measured using the published standards set forth in the Annual Book of ASTM Standards (in Volumes 5 and 10) published by the American Society for Testing Materials (ASTM), 100 Barr Harbor Drive West Conshohocken Pa. 19428, which is incorporated herein by reference. The dielectric strength is determined using ASTM test method D 877. The dissipation factor is determined using ASTM test method D 924. The acidity is determined using ASTM test method D 974. The electrical conductivity is determined using ASTM test method D 2624. The flash point is determined using ASTM test method D 92. The pour point is determined using ASTM test method D 97.

The dielectric strength is measured by taking 100-150 ml oil sample in a test cell and applying a voltage between test electrodes separated by a specified gap. The breakdown voltage is noted. The test is preferably run five times and the average value is calculated. The dielectric strength of a triglyceride composition of the invention is at least 35 KV/100 mil (2.5 mm) gap. In some preferred embodiments, it is 40 KV/100 mil (2.5 mm) gap.

The dissipation factor is a measure of the electrical loss due to conducting species and is tested by measuring the capacitance of fluids in a test cell using a capacitance bridge. The dissipation factor of a triglyceride composition of the invention is less than 0.05% at 25C. In some preferred embodiments, it is less than 0.02%. In some preferred embodiments, it is less than 0.01%.

The acidity is measured by titrating a known volume of oil with a solution of alcoholic KOH to neutralization point. The weight of the oil in grams per mg KOH is referred to interchangeably as the acidity number or the neutralization number. The acidity of a triglyceride composition of the invention is less than 0.03 mg KOH/g. In some preferred embodiments, it is less than 0.02 mg KOH/g.

The electrical conductivity is measured using a conductivity meter such as an Emcee meter. The electrical conductivity of a triglyceride composition of the invention is less than 1 pS/m at 25 NC. In some preferred embodiments, it is less than 0.25 pS/m.

The flash point is determined by placing an oil sample in a flashpoint tester and determining the temperature at which it ignites. The flash point of a triglyceride composition of the invention is at least 250 NC. In some preferred embodiments, it is at least 300 NC.

The pour point is determined by cooling an oil sample with dry ice/acetone and determining the temperature at which the liquid becomes a semi-solid. The pour point of a triglyceride composition of the invention is not greater than −15 NC. In some preferred embodiments, it is not greater than −20 NC. In some preferred embodiments, it is not greater than −40 NC.

In some preferred embodiments, the triglyceride composition of the invention is characterized by the properties of a dielectric strength of at least 40 KV/100 mil (2.5 mm) gap, a dissipation factor of less than 0.02% at 25 NC, acidity of less than 0.02 mg KOH/g, electrical conductivity of less than 0.25 pS/m at 25 NC, a flash point of at least 300 NC and a pour point of not greater than −20 NC. In some preferred embodiments, the pour point is not greater than −40 NC.

In some preferred embodiments, the triglyceride composition of the invention comprises fatty acid components of at least 75% oleic acid, linoleic acid at a proportion of less than 10%, linoleic acid at a proportion of less than 3%, stearic acid in a proportion of less than 4%, and palmitic acid in a proportion of less than 4%, and is characterized by the properties of a dielectric strength of at least 40 KV/100 mil (2.5 mm) gap, a dissipation factor of less than 0.02% at 25 NC, acidity of less than 0.02 mg KOH/g, electrical conductivity of less than 0.25 pS/m at 25 NC, a flash point of at least 300 NC and a pour point of not greater than −20 NC. In some preferred embodiments, the pour point is not greater than −40 NC.

Triglycerides with high oleic acid oil content are described in U.S. Pat. No. 4,627,192 issued Dec. 4, 1986 to Fick and U.S. Pat. No. 4,743,402 issued May 10, 1988 to Fick, which are incorporated herein by reference. These oils or those with similar fatty acid component content according to the present invention may be processed to yield an oil with the desired physical properties. High oleic vegetable oils may be obtained from commercial suppliers as RBD oils (refined, bleached and deodorized) which are further processed according to the present invention to yield high oleic oils useful in electrical insulation fluid compositions. There are several suppliers of high oleic RBD oils in the USA and overseas. RBD oil useful as a starting material for further processing may be obtained from SVO Specialty Products, Eastlake Ohio and Cargill Corp., Minneapolis Minn. The oil manufacturer goes through an elaborate process to obtain RBD oil during which all nonoily components (gums, phospholipids, pigments etc.) are removed. Further steps may involve winterization (chilling) to remove saturates, and stabilization using nontoxic additives. The processes for converting oil to RBD oil are described in Bailey=s Industrial Oil and Fat Products, Vols. 1, 2 & 3, Fourth Edition 1979 John Wiley & Sons and in Bleaching and Purifying Fats and Oils by H. B. W. Patterson, AOCC Press, 1992, which are incorporated herein by reference.

RBD oils are further processed according to the present invention in order to yield an oil with the physical properties as defined herein. The purification of the as received oil designated RBD oil is necessary because trace polar compounds and acidic materials still remain in the oil, making it unfit as an electrical fluid. The purification process of the present invention uses clay treatment which involves essentially a bleaching process using neutral clay. RBD oil is combined with 10% by weight clay and mixed for at least about 20 minutes. It is preferred if the oil is heated to about 60-80 NC. It is preferred if the mixture is agitated. The clay particles are removed subsequently by a filter press. Vacuum conditions or a neutral atmosphere (by nitrogen) during this process prevent oxidation. Slightly stabilized oil is preferable. More stabilizer is added at the end of the process. The purity is monitored by electrical conductivity, acidity and dissipation factor measurement. Further treatment by deodorization techniques is possible but not essential. The polar compounds that interfere most with electrical properties are organometallic compounds such as metallic soaps, chlorophyll pigments and so on. The level of purification needed is determined by the measured properties and the limits used. An alternative embodiment provides passing RBD oil through a clay column. However, stirring with clay removes trace polar impurities better than passing through a clay column. In preferred embodiments, neutral Attapulgite clay, typically 30/60 mesh size, is used in a ratio of 1-10% clay by weight. In some embodiments, clay particles are removed using filters, preferably paper filters with a pore size of 1-5 μm. The clay is preferably mixed with hot oil and agitated for several minutes, after which the clay is filtered off using filters. Paper or synthetic filter sheets may be used if a filter separator is used. The filter sheets are periodically replaced.

Electrical insulation fluids of the invention comprise the triglyceride composition of the invention and may further comprise one or more additives. Additives include oxidation inhibitors, copper deactivators and pour point depressors.

Oxidation inhibitors may be added to the oils. Oxidation stability is desirable but in sealed units where there is no oxygen, it should not be critical. Commonly used oxidation inhibitors include butylated hydroxy toluene (BHT), butylated hydroxy anisole (BHA) and mono-tertiary butyl hydro quinone (TBHQ). In some embodiments, oxidation inhibitors are used in combinations such as BHA and BHT. Oxidation inhibitors may be present at levels of 0.1-3.0%. In some preferred embodiments, 0.2% TBHQ is used. Oxidation stability of the oil is determined by AOM or OSI methods well known to those skilled in the art. In the AOM method, the oil is oxidized by air at 100 NC and the formation of peroxide is monitored. The time to reach 100 milliequivalents (meq) or any other limit is determined. The higher the value, the more stable the oil is. In the OSI method, the time to reach an induction period is determined by the measurement of conductivity.

Since copper is always present in the electrical environment, another type of additive is copper deactivators. Copper deactivators such as benzotriazole derivatives are commercially available. The use of these in small, such as below 1%, may be beneficial in reducing the catalytic activity of copper in electrical apparatus. In some embodiments, the electrical insulation fluid contains less than 1% of a copper deactivator. In some embodiments, the copper deactivator is a benzotriazole derivative.

According to some preferred embodiments the present invention, a combination of additives set forth herein particularly is effective when used in combination with high oleic acid triglyceride compositions to form electrical insulation fluids. The additives include a combination of combination of. The combination of additives included in the electrical insulation fluid of the invention include three additives: IRGANOX L-57 antioxidant, IRGANOX L-109 antioxidant and IRGAMET-30 metal deactivator which are each commercially available from CIBA-GEIGY, Inc. (Tarrytown, N.Y.). The combination of additives is present in a combined total in the fluid at between 0.2 and 2.0%, preferably between 0.5-1.0%. In some preferred embodiments, the combination of additives is present at about 0.5%.

The combination of additives may be present in a ratio of about 1 part IRGANOX L-57 antioxidant to about 2-4 parts IRGANOX L-109 antioxidant to about 1 part IRGAMET-30 metal deactivator. In some preferred embodiment, the combination of additives is present in a ratio of about 1 part IRGANOX L-57 antioxidant to about 3 parts IRGANOX L-109 antioxidant to about 1 part IRGAMET-30 metal deactivator.

IRGANOX L-57 antioxidant is commercially available from CIBA/GEIGY and is a liquid mixture of alkylated diphenylamines; specifically the reaction products of reacting N-Phenylbenzenamine with 2,4,4-trimethlypentane.

IRGANOX L-109 antioxidant is commercially available from CIBA/GEIGY and is a high molecular weight phenolic antioxidant, bis(3,5-di-tert-butyl-4-hydroxyhydrocinnamate. IRGANOX L-109 antioxidant is a bis(2,6-di-tert-butylphenol derivative.

IRGAMET-30 metal deactivator metal deactivator is commercially available from CIBA/GEIGY and is a triazole derivative, N,N-bis(2-Ethylhexyl)-1H-1,2,4-triazole-1 methanamine.

IRGANOX L-57 antioxidant and IRGANOX L-109 antioxidant are antioxidants, and IRGAMET-30 metal deactivator is a copper pasivator. In electrical apparatuses, copper is widely used as conductor and copper has a catalytic effect in the oxidation of oil. The antioxidants react with free oxygen thereby preventing the latter from attacking the oil.

Pour points depressants may also be added if low pour points are needed. Commercially available products can be used which are compatible with vegetable-based oils. Only low percentages, such as 2% or below, are needed normally to bring down the pour point by 10 to 15 NC. In some embodiments, the pour point depressant is polymethacrylate (PMA).

In some embodiments, the pour point may be further reduced by winterizing processed oil. Essentially, the oils are winterized by lowering the temperature to near or below 0 NC and removing solidified components. The winterization process may be performed as a series of temperature reductions followed by removal of solids at the various temperature. In some embodiments, winterization is performed by reducing the temperature serially to 5 N, 0 N and −12 NC for several hours, and filtering the solids with diatomaceous earth.

In some embodiments, the electrical insulation fluid of the invention that comprises at least 75 percent triglyceride composition of the invention as described above further comprises about 0.1-5% additives and then up to about 25% other insulating fluids such as mineral oil, synthetic esters, and synthetic hydrocarbons. In some embodiments, the electrical insulation fluid comprises 1-24% of insulating fluids selected from the group consisting of mineral oil, synthetic esters, synthetic hydrocarbons and combination of two or more of such materials. In some embodiments, the electrical insultion fluid comprises 5-15% of insulating fluids selected from the group consisiting of mineral oil, synthetic esters, synthetic hydrocarbons and combinantion of two or more of such materials. Examples of mineral oils include poly alpha olefins. An example of a mineral oil which may be used as part of the present invention is RTEemp, Cooper Power Fluid Systems. Examples of synthetic esters include polyol esters. Commercially available synthetic esters which can be used as part of the invention include those sold under the trade names MIDEL 7131 (The Micanite and Insulators Co., Manchester UK), REOLEC 138 (FMC, Manchester, UK) and ENVIROTEMP 200 (Cooper Power Fluid Systems). In some preferred embodiments, the electrical insulation fluid comprises at least 85% of the triglyceride composition of the invention. In some preferred embodiments, the electrical insulation fluid comprises at least 95% of the triglyceride composition of the invention.

According to some preferred embodiments of the present invention, high oleic acid content oils are used as starting materials for the production of an oil composition which has physical properties useful for electrical insulation fluids. The high oleic acid content oils are combined with a preferred combination of antioxidant and metal deactivating additives to provide electrical insulation fluids. Some preferred embodiments of the present invention relates to such electrical insulation fluids, to electrical apparatuses which comprise the electrical insulation fluids and methods of insulating electrical apparatuses using such fluids.

In some embodiments, the electrical insulation fluid of the invention that comprises at least 75 percent triglyceride composition of the invention as described above further comprises about 0.1-5% additives, including preferably 0.5-2.0% combination of IRGANOX L-57 antioxidant, IRGANOX L-109 antioxidant and IRGAMET-30 metal deactivator, and then up to about 24.5% other insulating fluids such as mineral oil, synthetic esters, and synthetic hydrocarbons. In some embodiments, the electrical insulation fluid comprises 1-24% of insulating fluids selected from the group consisting of mineral oil, synthetic esters, synthetic hydrocarbons and combination of two or more of such materials. In some embodiments, the electrical insulation fluid comprises 3-20% of insulating fluids selected from the group consisting of mineral oil, synthetic esters, synthetic hydrocarbons and combination of two or more of such materials. In some embodiments, the electrical insulation fluid comprises 5-15% of insulating fluids selected from the group consisting of mineral oil, synthetic esters, synthetic hydrocarbons and combination of two or more of such materials.

The present invention relates to an electrical apparatus which comprises the electrical insulation fluid of the invention. The electrical apparatus may be an electrical transformer, an electrical capacitor or an electrical power cable. U.S. Pat. No. 4,082,866, U.S. Pat. No. 4,206,066, U.S. Pat. No. 4,621,302, U.S. Pat. No. 5,017,733, U.S. Pat. No. 5,250,750, and U.S. Pat. No. 5,336,847, which are referred to above and incorporated herein by reference describe various applications of electrical insulation fluids for which the electrical insulation fluid of the invention may be used. In addition, U.S. Pat. No. 4,993,141 issued Feb. 19, 1991 to Grimes et al., U.S. Pat. No. 4,890,086 issued Dec. 26, 1989 to Hill, U.S. Pat. No. 5,025,949 issued Jun. 25, 1991 to Adkins et al., U.S. Pat. No. 4,972,168 issued Nov. 20, 1990 to Grimes et al., U.S. Pat. No. 4,126,844, and U.S. Pat. No. 4,307,364 issued Dec. 22, 1981 to Lanoue et al., which are each hereby incorporated herein by reference contain descriptions of various electrical apparatuses in which the electrical insulation fluid of the invention may be used. In some preferred embodiments, the electrical apparatus of the invention is a transformer, in particular, a power transformer or a distribution transformer.

EXAMPLES

Example 1

Several high oleic oils were further purified and stabilized according to the present invention to make them electrically suitable. Electrical tests showed that such purified oils had properties similar to currently used high temperature fluids in distribution transformers. Table 1 compares the properties of the purified oils of the present invention with currently used fluids.

TABLE 1
Comparison of Purified Vegetable Oils
with High Temperature Fluids Used in
Transformers
	High Oleic	High Temp. Mineral	Synthetic Ester
	Veg. Oil	Oila	Fluidb
Dielectric Strength,	42.4	40-45	50
KV/100 mil gap
Dissipation Factor,	0.02	0.01	0.1
% at 25NC
Neutr. No. mg	0.05	—	0.03
KOH/g
Electrical	0.25-1.0	(0.1 o 10)*	(5.0)*
Conductivity pS/m,
25NC
Flash Point	328NC	275-300NC	257NC
Pour Point	−28NC	−24NC	−48N
aRTEemp, Cooper Power Fluid Systems
bPolyol Esters (such as MIDEL 7131 and REOLEC 138)
*deduced from resistivity
The properties listed for the high oleic oil are for purified oils with no additives.

Example 2

The purification of the as received oil designated RBD oil (refined, bleached and deodorized) is necessary because trace polar compounds and acidic materials still remain in the oil, making it unfit as an electrical fluid. The purification we attempted involved clay treatment as follows: approximately 1 gal. of the RBD oil was treated with 10% Attapulgite clay. Oil was produced with electrical conductivity of less than 1 pS/m. The attapulgite treated oil showed conductivities as low as 0.25 pS/m. Commercial grade oils had conductivities in the range of 1.5 to 125 pS/m. Conductivity below 1 pS/m (or resistivity above 10¹⁴ ohm.cm) is desired for electrical grade oil. Other indicators of purity are dissipation factor and neutralization number (acid number). Dissipation factor is a measure of electrical losses due to conduction caused by conducting species, usually organometallic trace components, and should be below 0.05% at room temperature. The clay treated oils had dissipation factor of 0.02%. Untreated RBD oils had DF ranging from 0.06% to 2.0%. With a finer grade of clay, the same results could be achieved with only 2% of clay. A filter separator was preferred to a filter column.

Example 3

Oxidation stability tests were conducted on treated and untreated oil samples using ASTM and AOCS methods. The untreated and treated RBD oils failed the tests. Oxidation inhibitors were added to the oils and the tests were repeated. Several oxidation inhibitors were tested: BHT (Butylated Hydroxy Toluene, BHA (Butylated Hydroxy Anisole) and TBHQ (mono-Tertiary Butyl Hydro Quinone) in 0.2% by weight in oil. In the AOCS method used (Cd 12.57) 100 ml samples are bubbled with air at 100C, and the peroxide formation was measured at several time intervals. Hours to reach 100 meq of peroxide were noted. Since copper is always present in the electrical environment, all oil samples had copper wire placed in them. With no additive, the time to reach the limit was 18 hours; with additive (0.2%), the times were 100 hours for BHT+BHA. With TBHQ, even after 400 hours, the peroxide value reached only 8.4 meq. TBHQ proved to be the best antioxidant of the three. Without an oxidation inhibitor the oils upon oxidation would produce hydroperoxide which is then converted to acids, alcohols, esters, aldehydes, ketones and polymer structures. Most electrical apparatus that use a fluid insulation operate in low oxygen or oxygen-free environment, so the concern over oxidation is not great.

Example 4

The pour point of the treated oil was typically −25 NC. To lower the pour point further, the treated oils were winterized at 5 N, 0 N and −12 NC for several hours, and the solids that separated were filtered with diatomaceous earth. The lowest pour point reached so far was −38 NC, close to the specified value of −40 NC for transformer oil. Further lowering is possible by extended winterization. Another approach is by the use of pour point depressants such as PMA (polymethacrylate) which has been used for mineral oil.

Example 5

A laboratory oxidation stability test was conducted using the OSI (Oil Stability Index) Method, AOCS Cd 12b-92. The additives were used in a 1:3:1 ratio at several concentrations in both the high oleic vegetable oil and in regular mineral oil used in transformers. In the OSI method, 50 ml of the oil is taken in a conductivity cell, and is placed in a bath kept at 100^NC. Air is bubbled through it at 2.5 ml/min. The effluent air containing the volatile fatty acids is passed through a vessel containing deionized water. The conductivity of the water is monitored as a function of time. When the antioxidant is consumed, a sudden rise in conductivity is observed. This taken as the end point. The number of hours is noted as the OSI value at 110^NC. It is usual to convert these values to a 97.8^NC. OSI value to correspond to the temperature used in another oil stability test, the AOM (Active Oxygen Method), A.O.C.S Cd 12-57.

Table 2 summarizes the test results:

TABLE 2
OSI Values in Hours for Various Oils
	OSI,		AOM,
	110NC	OSI, 97.8NC	97.8NC
High Oleic Veg. oil with Cu	1.3	3.0	3.1
Same, with 0.2% TBHQ	13.5	31.3	32.6
Same, with 0.2% CIBA	79.7	185.2	192.8
Same, with 0.5% CIBA	226	526	548
Transformer oil (mineral	162	377	392
oil) + Cu
High Temp. Mineral Oil +	137	315	328
Cu

Compositions which comprise the additives at 0.5% concentration in oil is as effective as regular transformer oil, and more effective that the high temperature mineral oil used in some transformers. Another superiority of the combination of additives is that the oil conductivity at 0.5% concentration below 2 pS/m, compared to 4.5 pS/m for oil with 0.2% TBHQ.

Example 6

Mixing the composition with other fluids can result in the lowering of pour point. For example, the electrical insulation fluid was mixed with regular mineral oil (pour point of −50^NC. or below) and at a 5% concentration in the mixture (i.e. final electrical insulator fluid includes 5% mineral oil), the pour point was reduced to −40^NC. In another embodiment, the electrical insulation fluid was mixed with the synthetic ester Reolec 138 and at a 10% concentration in the mixture (i.e. final electrical insulator fluid includes 10% synthetic ester), the pour point was lowered to −42^NC. The above fluid may, for example, be mixed with regular mineral oil.

Claims

1. An electrical transformer comprising: a core-coil assembly; a housing containing the core and coil assembly; and a vegetable-based dielectric fluid disposed in the housing, said dielectric fluid consisting essentially of one or more vegetable oils and 0.1% to 3.0% of one or more antioxidant compounds; and wherein said dielectric fluid has an oxidative stability of 100 or more AOM hours.

2. The electrical transformer of claim 1, wherein the one or more vegetable oils have a flash point of at least 300° C.

3. The electrical transformer of claim 2, wherein the one or more vegetable oils have a dielectric strength of at least 35 KV/100.

4. The electrical transformer or claim 3, wherein the one or more vegetable oils have an electrical conductivity of less than 1 ps/m at 25° C.

5. The electrical transformer of claim 1, wherein the one or more vegetable oils comprise triglycerides having fatty acid components of less than 3% linolenic acid.

6. The electrical transformer of claim 1, wherein the one or more vegetable oils comprise triglycerides having fatty acid components of at least 75% oleic acid.

7. The electrical transformer of claim 1, wherein the one or more antioxidant compounds comprise one or more antioxidants selected from the group consisting of butylated hydroxy toluene (BHT), butylated hydroxy anisole (BHA), mono-tertiary butyl hydroquinone (TBHQ) and combinations thereof.

8. The electrical transformer of claim 7, wherein the transformer is a three phase transformer and the core-coil assembly comprises three coils.

9. The electrical transformer of claim 1, wherein the one or more vegetable oils is winterized by lowering the temperature of the one or more vegetable oils to 0° C. or below and filtering the one or more vegetable oils, thereby providing the one or more vegetable oils with a pour point below −25° C.

10. An electrical transformer comprising: a core-coil assembly; a housing containing the core and coil assembly; and a dielectric fluid disposed in the housing, said dielectric fluid consisting essentially of 0.1% to 3.0% of one or more antioxidant compounds, at least 75% of a vegetable oil-based triglyceride composition and 1% to 24% of an insulating fluid selected from the group consisting of mineral oil, synthetic esters, synthetic hydrocarbons and combinations thereof.

11. The electrical transformer of claim 10, wherein the vegetable oil-based triglyceride composition is purified by a method comprising: providing the vegetable oil-based triglyceride composition; heating the vegetable oil-based triglyceride composition; contacting the heated vegetable oil-based triglyceride composition with clay; and filtering the vegetable oil-based triglyceride composition to remove clay particles therefrom.

12. The electrical transformer of claim 11, wherein the vegetable oil-based triglyceride composition is winterized by lowering the temperature of the purified vegetable oil-based triglyceride composition to 0° C. or below and filtering the vegetable oil-based triglyceride composition, thereby providing the vegetable oil-based triglyceride composition with a pour point below −25° C.

13. (canceled)

14. (canceled)

15. The electrical transformer of claim 10, wherein the one or more antioxidant compounds comprise one or more antioxidants selected from the group consisting of butylated hydroxy toluene (BHT), butylated hydroxy anisole (BHA), mono-tertiary butyl hydroquinone (TBHQ) and combinations thereof.

16. The electrical transformer of claim 10, wherein the vegetable oil-based triglyceride composition comprises triglycerides having fatty acid components of at least 75% oleic acid.

17. A three phase electrical transformer comprising: a core-coil assembly comprising three coils; a housing containing the core and coil assembly; and a dielectric fluid disposed in the housing, said dielectric fluid consisting essentially of: 0.1 to 3.0% antioxidant additives comprising: one or more antioxidant compounds selected from the group consisting of butylated hydroxy toluene (BHT), butylated hydroxy anisole (BHA), mono-tertiary butyl hydroquinone (TBHQ) and combinations thereof; and at least 75% of a vegetable oil-based triglyceride composition; and wherein the dielectric fluid has an oxidative stability of 100 or more AOM hours.

18. (canceled)

19. (canceled)

20. (canceled)

21. The three phase electrical transformer of claim 20, wherein the antioxidant additives comprise TBHQ.

22. The three phase electrical transformer of claim 17, wherein the vegetable oil-based triglyceride composition comprises triglycerides having fatty acid components of less than 3% linolenic acid.

23. The three phase electrical transformer of claim 17, wherein the vegetable oil-based triglyceride composition comprises triglycerides having fatty acid components of at least 75% oleic acid.

24. The electrical transformer of claim 1, wherein the one or more vegetable oils comprise sunflower oil or canola oil.

25. The electrical transformer of claim 7, wherein the one or more antioxidant compounds further comprise an alkylated diphenylamine.

26. The three phase electrical transformer of claim 17, wherein the vegetable oil-based triglyceride composition comprises sunflower oil or canola oil.