These new liquids possess such properties as LOWER FIRE AND ENVIRONMENTAL RISKS while IMPROVING PAPER-INSULATION LIFE.

BY C. PATRICK MCSHANE 22 COOPER INDUSTRIES, INC. 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 E LATION SYSTEMS are evaluated based of mineral oil for dielectric application and developing processes for producing a more consistent quality fluid. Key on economic, safety, and environmental 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 improving the environmental and safety properties of fire resis- Experimentation using natural ester fluids as dielectric mineral oil specifically designed for transformers. 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 natural ester (vegetable oil) fluids. Because esters naturally ferior oxygen stability and higher pour point, permittivity, trials. They proved less desirable than mineral oil due to in- have lower oxidation resistance than mineral oils, a novel and viscosity values [1]. To this day, liquid-filled transformers primarily use mineral oils as the insulating fluid. blend of base oils and additives were developed to overcome this potential handicap. Single- and three-phase Other alternatives to mineral oil-filled distribution transformers, such as dry and essentially nonflammable liq- prototype field installations using these new dielectric coolants are discussed. uid-filled types, were commercialized decades ago for use in specialty applications. Background Nearly all nonflammable fluids used in transformers belong to a chemical group known as halogenated hydrocar- In the United States, the first distribution-class transformer 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 transformers using mineral oil as the dielectric coolant could be their excellent fire-safety properties, are now undesirable dielectric fluids, chiefly Askarel fluids, once promoted for smaller and more efficient was patented by Elihu Thomson due to their possible health hazards and proven environmental persistence. in 1882, it took another decade before his idea was put into 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 177-2618/2/$17. 22 IEEE

[polychlorinated biphenyls (PCBs) and trichlorobenzene], typically containing 4-5% trichlorobenzene. Commercialization of Askarel-filled transformers began in the 193s. Most applications were for installations that required additional fire safety, primarily urban network vaults and indoor locations. In 1976, the Toxic Substance Control Act [2] targeted PCBs, the key component of Askarel. Extensive EPA regulatory limits soon followed and are periodically modified. The EPA published its most recent changes in 1998 [3], [4]. Although the industry appears resigned to living with these regulations, they are controversial. The banning of further production and commercialization of PCBs and increasingly restrictive Federal and state regulations led to the introduction of other fire-resistant transformer types. Dry-type transformer manufacturers responded by adding a more robust dry design, using vacuum-pressure impregnation (VPI), and increasing power and voltage ratings. A few transformer manufacturers and retrofilling service companies promoted other nonflammable dielectric coolants that did not contain PCBs. These coolants included perchloroethylene, trichlorobenzene, and chlorofluorocarbons. However, most replacements of the Askarel-filled transformers selected for removal used less-flammable fluids [5], [6]. NATURAL ESTERS ACHIEVE A BALANCE OF DESIRABLE TRANSFORMER AND EXTERNAL ENVIRONMENTAL PROPERTIES NOT FOUND IN OTHER DIELECTRIC FLUIDS. Code Recognition of Less-Flammable Liquid-Filled Transformers Formal incorporation of less-flammable transformer fluids into the National Electrical Code (NEC) occurred in 1978. Originally, and perhaps more appropriately, these fluids were referred to as high fire point liquids until 1984, when the NEC began referring to them as less-flammable fluids. To qualify, the fluids needed a minimum open-cup fire point of 3 C. The 1981 NEC edition added the requirement for third-party certification of less-flammable fluids as code listed. Factory Mutual Engineering and Research (FME&R) was the first nationally recognized testing laboratory (NRTL) to list less-flammable transformer fluids [7]. In 1984, the idea of combining the inherent fire resistance of these fluids with other effective transformer protection methods was introduced by Underwriter s Laboratories (UL) in conjunction with transformer manufacturers [8], [9]. Currently, there are ten listings of less-flammable dielectric coolants by the NRTLs covering dimethylsiloxanes (silicones), high molecular weight hydrocarbons (HMWH), synthetic polyol esters (POE), and polyalphaolefins (PAOs) [1], [11]. In 1994, FME&R adopted transformer installation guidelines that emphasize built-in transformer protection per their Approval Standard 399. Transformer manufacturers now have the opportunity to offer a FME&R approved and labeled transformer along with a FME&Rapproved less-flammable fluid [12]. Installation requirements based on specific heat-release rates were eliminated for FME&R-approved less-flammable dielectric fluids. In fact, FME&R no longer tests nor publishes heat-release rates for this class of material. This change is due to the proven fire-safety history of listed less-flammable liquids (no known pool fires) and extreme tests that indicate the near impossibility of such an occurrence. Code listing companies requirements for listed HMWH are now equal to or more flexible than those for listed silicone oils [13], [14]. Field Performance of Less-Flammable Liquid-Filled Transformers Today, there are over 2, less-flammable liquid-filled transformers in service. In the 198s, silicone oil-filled units were the primary choice for use in utility network and rectifier transformers. HMWH fluids, one class of less-flammable fluids, are also widely used, particularly in unit substations, pad-mounted transformers, and oil retrofills. To date, more than 12, HMWH fluid-filled transformers have been installed. The promise of the 1984 UL Classification of HMWH fluid has been achieved. There have been no reported explosions or fires involving HMWH. All major U.S. transformer manufacturers now offer the option of HMWH to their customers. The operating performance record of HMWH is equally positive. The stability of key fluid properties (e.g., essentially no sludging), even at high-temperature operation, and their superior resistance to fast front impulse breakdown, has resulted in very satisfactory field performance. Due to their proven compatibility with aramid insulation materials, HMWH are now used in high-temperature transformers (HTTs) rated for 115 C rise (175-185 C hot spot) [15]. Ideal applications for these HTTs are mobile substations (weight limits are critical) or double-ended substations (temporary double-load capability is desired). However, HMWH, like conventional transformer oils, are mineral-oil based, thus, they are subject to expanding environmental regulatory issues. For example, mineral oils are essentially excluded from the classification of oils covered in the Edible Oil Regulatory Reform Act [19]. Most states require even nonaquatic transformer oil spills to be reported, and many require removal and replacement of soil containing oil. Silicone oil is also classified as nonedible oil. Tests prove silicone is essentially nonbiodegradable. There are reports that under certain conditions degradation in the environment is possible [16], [17]. Reports showing relatively quick degradation of silicone involve soils that have been oven-dried, an unlikely condition in the real world. It is increasingly important that dielectric fluids provide a better balance of functional performance inside the transformer versus environmental impact in the event of 35

36 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. Structure of a typical polyol ester. Structure of a natural ester (vegetable oil) triglyceride. 1 2 Key Environmental and Health Issues 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 criteria. 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 1-4 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. 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 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 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 Dielectric Breakdown (kv) Viscosity (cst) Mineral Oil 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. Silicone Oil HMWH Synthetic Ester Natural Ester Test Method New 42 4 52 43 47 D-877 After 5 switch operations [22] 41 <4 43 36 47 D-877 4 C 9.2 37 121 29 33 D-445 1 C 2.3 15.5 12.5 5.6 7.9 D-88 Flash Point ( C) 147 3 276 27 328 D-92 Fire Point ( C) 165 343 312 36 357 D-92 Specific Heat (cal/gm/ C) @ 25 C.39.36.45.45.45 D-2766 Pour Point ( C) 5 55 21 5 21 D-97 Specific Gravity.87.96.87.97.92 D-1298 Biochemical Oxygen Demand (ppm) 6 6 24 25 BOD/COD Ratio (%) 7 17 45 Five-day SM521B Trout Fingerling Toxicity Mortality N/A N/A N/A N/A OECD 23 CO Evolution (% of Theoretical Max) History of Ester Fluids as Dielectric Coolant 5 2 In 1892, experiments with liquids other than mineral oils 25 included ester oils extracted from seeds. None made operational improvements over mineral oil and were not com- 5 1 15 2 25 3 35 4 45 mercially successful. A particular problem with Elapsed Time (Days) seed-oil-based coolants was their high pour point and inferior resistance to oxidation relative to mineral oil. 3 Aerobic biodegradation of natural ester and conventional Except for occasional applications in capacitors and other transformer oil. specialty applications, renewed interest in ester-based dielectric fluids did not occur until after the infamous PCB issue arose in the 197s. 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 4 high fire point were important fluid characteristics for this Lockie accelerated-aging transformer test facility. 37 2 1 8 6 4 "1% biodegradable" above 6% of the theoretical maximum CO 2 evolution Envirotemp FR3 Fluid Conventional Transformer Oil Sodium Citrate Reference Material (EPA "Ultimate Biodegradability") 1 75 Biodegradation (%)

Tensile Strength (lb/in ) 2 2 15 1 5 Mineral Oil Natural Ester (Error Bars = 1 σ) 1 2 3 4 1 75 5 25 Retained Tensile Strength (% of Unaged) 5 Degree of Plymerization 12 1 8 6 4 2 Mineral Oil Natural Ester 1 2 3 4 Aging Time (hours) 1 75 5 25 Retained DoP (% of Unaged) 6 Aging rates (as tensile strength) of thermally upgraded paper in natural ester and conventional transformer oil. Aging rates (as degree of polymerization) of thermally upgraded paper in natural ester and conventional transformer oil. 38 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 199s, 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. TABLE 2. TEST PARAMETERS FOR TRANSFORMER ACCELERATED AGING EVALUATION AND RESULTS TO DATE Test Cell A B C Target Hottest-Spot Temperature ( C) 167 175 183 Standard Required Expected Life* Hours 1,32 721 47 Years Equivalent 21 21 21 Standard Test Method Required Life** Hours 6,51 3,64 2,36 Years Equivalent # Actual Times to Failure: Hours Years Equivalent 15 15 15 11,4 + 1,19 1,186 6,623 464 328 271 * Per Fig. 1, ANSI/IEEE C57.91-1981 ** Test method per ANSI/IEEE C57.1-1986 # Years equivalent calculations include correction to actual hot spot temperature from target hot spot temperature. 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. Natural and Synthetic Esters: Key Composition Similarities and Differences 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 (C 5 -C 18 ) and the alcohol pentaerythritol. The structure is C(CH 2 CO 2 R) 4, where the R groups are branched as shown in Fig. 1. Natural Esters 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 C 8 -C 22 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.

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.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 835.31. 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]. Accelerated Insulation Life Tests 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 thermal life test. Per ANSI/IEEE, when a new insulation system is developed, it is recommended to test the system following its C57.1 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 [2]. 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. Absolute Water Content (mg/kg) 4 3 2 1 (IEC 123 Continued Service Maximum) S/N 1429 S/N 143 1 2 3 4 5 Years in Service Moisture content of prototype transformers. The IEC limit for in-service synthetic esters is shown as a dashed line. Fluid Dissapation at 25 C (%).8.6.4.2 (IEC 123 Continued Service Maximum) 4 3 2 1 S/N 1429 S/N 143 Relative Water Content (% Saturation). 1 2 3 4 5 Time in Service (Years) 8 Dissipation factor of prototype transformers. The IEC limit for in-service synthetic esters is shown as a dashed line. 7 39

1 1 Dielectric Breakdown (kv) 8 6 4 2 (IEC 123 Continued Service Minimum) D-1816: S/N 1429 S/N 143 D-877: S/N 1429 S/N 143 1 2 3 4 5 Years in Service 9 Acid Number (mg KOH/g) 1.1 (IEC 123 Continued Service Maximum) S/N 1429 S/N 143.1 1 2 3 4 5 Years in Service 1 Dielectric breakdown strength of prototype transformers. The IEC limit for in-service synthetic esters is shown as a dashed line. Neutralization (acid) number of prototype transformers. The IEC limit for in-service synthetic esters is shown as a dashed line. 4 Transformer Beta Site Performance Status 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. 7-1. Since there are neither ASTM nor IEEE standards for ester fluids, nor IEC standards for natural esters, all data are compared to IEC Standard 199, Specifications for Unused Synthetic Organic Esters for Electrical Purposes and IEC Standard 123, 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 Rise Over Mineral Oil ( C) 1 8 6 4 2 2 4 6 Top Oil Average Winding 15 225 3 1, 4, 1, Transformer Size (kva) 11 Comparative heat runs of mineral oil design transformers. The average winding rise and top oil differentials between mineral oil and natural ester are shown. 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 (2 ppm) and continued use of aged fluid (4 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. 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 7-56 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 (3 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.3 mg KOH/g, just at the.3-limit set in IEC 199. The latest service-aged fluid sample measured less than.5 mg KOH/g, well below the 2. limit set in IEC 123 (Fig.1). All the data, representing one year of typical transformer life, are very positive, showing no significant performance changes in fluid properties.

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 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, kva, the average winding rise is within 1 C of mineral oil. A 4-kVA transformer showed 3.5 C higher average winding rise with natural ester. Comparative heat runs using a 1-MVA mineral-oil design transformer are planned. Code and Approval Status for Natural Esters 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 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. 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, 4 CFR, 1979. [4] Disposal of Polychlorinated Biphenyls (PCBs), EPA, Parts 75-761, 4 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. 5-55, Jan./Feb. 1999. [7] Factory Mutual Approval Guide. Norwood, MA: Factory Mutual Research Corporation, 1979. [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 rupture, explosion, and fire, presented at the Edison Electric Inst. Transmission and Distribution Conf., Fort Worth, TX, 1985. [1] Factory Mutual Approval Guide. Norwood, MA: Factory Mutual Research Corporation, 1999. [11] Gas & Oil Equipment Directory. Underwriters Laboratories, 1984. [12] Less and Nonflammable Liquid-Insulated Transformers, Factory Mutual Research Corporation Approval Standard Class Number 399. [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. 29, 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. [18] A.C.M. Wilson, Insulating Liquids: Their Uses, Manufacture, and Properties. New York: Institution of Electrical Engineers; New York: Peter Peregrinus Ltd., 198. [19] Edible Oil Regulatory Reform Act, Public Law 14-55, 1995. [2] Standard for Thermal Evaluation of Oil-Immersed Distribution Transformers, ANSI/IEEE C57.1-1986. [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. 89-894. [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. 2 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. 21 IEEE/PES Transmission and Distribution Conf., Atlanta, GA, 21, 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 21 IEEE/PCA Cement Industry Conference. 41