Cellulose Insulation In Oil Filled Transformer-i

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Cellulose Insulation in Oil-Filled Power Transformers: Part I—History and Development Key Words: Transformer insulation, kraft paper, crepe paper, thermally upgraded paper, synthetic paper, pressboard. Overview

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ellulose insulation has been the preferred choice for the solid insulation in power transformers, but not because it is the best. In fact, it would never have been the preferred material if it were not available in plenty from natural renewable source—soft wood. In 1939, G. T. Kohan of Bell Telephone Laboratories in New York wrote an article in AIEE Transactions on cellulose as an insulating material for various electrical applications: paper capacitors, paper-insulated power cables, telephone cables [1]. He observed, “Although the use of cellulose does not go back to the Garden of Eden as do some of its applications, it is nearly as old as the electrical industry itself.” The same year, the annual consumption of cellulose insulation in the United States was estimated by Kohan at 18 million kg (40 million pounds). Since then, the annual consumption has increased many fold. Oil-filled HV power cables, condenser bushings, and power transformers consume gigantic quantities of cellulose insulation measured in millions of tons. The chief disadvantage of cellulosic material for electrical use is that it is hygroscopic and needs to be processed and maintained dry. For power transformers, the processing is elaborate and time-consuming. Once in the transformer, the insulation begins to age over many years. Its water content increases because of the degradation of the molecular chain by thermal stresses and oxidative processes. Depolymerization of the cellulose chain lowers the chain length and mechanical strength. The insulation finally becomes brittle and carbonaceous with no short circuit withstand capability. We may call this the absolute end of life. The dry-out is worth the effort because dry cellulosic insulation has excellent dielectric properties. It is instructive to review the use of cellulose in transformers from a historical perspective, the elaborate studies conducted

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Thomas A. Prevost EHV-Weidmann, Inc.

T. V. Oommen Consultant

Part I covers briefly the development of electrical grade paper/pressboard for transformer use, starting from the raw materials and including the improvements made, for example, thermally upgraded papers.

over the past 70+ yr to understand degradation processes, determine end-of-life criteria, and investigate methods to prolong insulation life. With a few hundred thousand power transformers in use, a large proportion of which is in a severely aged stage, the concern over transformer failures is real, and the condition assessment of these units has become a high priority. Therefore, despite our familiarity with cellulosic insulation for years and all of the understanding we have so far, new studies are launched every year. As far as improvements are concerned, the insulation papers we have now show acceptable thermal stability and mechanical strength for present-day applications. This has been achieved

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by the use of better pulps, optimized fiber length, and thermal upgrading. Blending cellulose fibers with synthetic fibers is yet another improvement, although not widely applicable.

Cellulosic Insulation in Transformers Cellulosic insulation is used primarily in oil-filled transformers from distribution to large power units covering a wide range from 10 kVA to 1500 MVA and from line voltage to 1000 kV. In terms of physical size, it ranges from the pole and pad-mounted units on our streets to large substation units that can have several tons of cellulosic insulation (paper and pressboard) immersed in 40,000 to 100,000 L (~10,000 to 30,000 gallons) of oil. Fig. 1 through 3 show views of the insulation structure in transformers [2]. Fig. 1 shows the cut-outs of a large shell form and a core form transformer, which clearly show the coil structures, ‘pancake,’ in shell and discs in the core forms. Fig. 2 gives a close-up view of a paper tape-wrapped HV transformer coil (core type). The LV insulation in distribution and small power transformers is a layer-wound coil as shown in Fig. 3. The total insulation would amount to tens to hundreds of kilograms in distribution and small power transformers. The diamond patterns are epoxy dots that enable bonding of the layers after the coil has gone through a curing process. The layer thickness is typically 10 mil (0.254 mm). The insulation structure consists of not only the HV and LV insulation but also support structures, winding tubes, spacer blocks, and formed items for end closing. These are illustrated in Fig. 4. These items are available as prefabricated items from pressboard suppliers [3].

Before the 1920s, a variety of fibrous materials, both cellulosic and non-cellulosic, were used for electrical insulation: cotton rag, silk, jute, asbestos, etc. [4], [5]. Although varnished cambric cloth and other textiles were used in cables, varnished or ‘boiled-in-oil’ pressboard made up of cotton rags and paper clippings was used in transformers. In 1920, blends of kraft wood fibers and manila-hemp fibers began to be used for telephone insulation. In capacitors, linen was used until the late 1920s. The 1920s and 1930s were periods of much experimentation on how to improve the dielectric performance of the paper-oil system. A better understanding of fibers and impurities in the pulp resulted in better insulation. It seems that by the late 1920s and early 1930s, kraft paper insulation began to be used in combination with insulating oil in transformers. This combination was needed to satisfy the increasing insulation requirements as the voltage ratings escalated. In the 1940s, kraft paper in combination with oil was the dielectric material of choice for HV use as evidenced by the number of cellulose material studies done. Much more information on paper chemistry was generated in the 1950s and later at the Institute of Paper Chemistry. But interest in synthetic dielectric materials slowly developed in the late 1950s, and such materials began to replace cellulosic insulation in power cables and capacitors. Mixtures of cellulosic and synthetic materials are now used in many transformer insulation applications.

Paper and Pressboard from Wood Pulp Electrical grade paper and pressboard are mostly made from wood pulp processed by the kraft chemical process, hence, kraft

Figure 1. Cut out of large shell and core form transformers.

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Figure 2. Power transformer coil structure (core form).

paper and kraft board (kraft is German for strong). The starting material is wood, both soft wood and hard wood. Wood is a natural composite material that is made up of flexible tubes of cellulose bound together by lignin, a brownish aromatic polymer that is mostly removed during the pulping process. A schematic representation of the fine structure of wood pulp is shown in Fig. 5 [4]. Cellulose, the essential component of paper and pressboard, is a polymer of glucose units linked to one another in a special manner as shown in Fig. 6. It may be represented simply as [C5H10O5]n, ignoring the extra atoms on the end groups, where n

Figure 3. LV insulation, distribution transformer (ABB brochure; reprinted with permission).

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Figure 4. Formed items from pressboard.

is the degree of polymerization (DP). The repeating unit, however, is cellobiose, consisting of two glucose units. The DP values for paper samples can be estimated by specified methods such as ASTM D-4243. The DP of kraft pulps ranges from 1100 to 1200, but mixed pulp fibers can have much higher DP, e.g., 1400 to 1600. Soft woods (e.g., southern pine, Douglas fir, spruce, all conifers) contain typically 42% cellulose and 28% lignin; hardwoods (e.g., birch, aspen, red gum) contain 45% cellulose and 20% lignin [6]. In addition to cellulose, wood contains hemicellulose composed of mixed sugars, e.g., glucose and mannose (glucomannan) with smaller chain length than cellulose. Softwood may contain, on average, 27% hemicellulose. Both the lignin and the hemicellulose need to be removed from the finished pulp as much as possible. The most significant differences between soft and hard woods are fiber length (softwoods, 2 to 6 mm; hardwoods, 0.6 to 1.5 mm) and coarseness (soft woods, 15 to 35 mg/100 mm; hardwoods, 5 to 10 mg/100 mm). Softwood fibers provide strength; hardwood fibers provide more smoothness and evenness in machine and cross-machine directions. Therefore, mixed pulps are advised in making paper and pressboard. In the chemical process for making kraft pulp, wood chips are digested in a pressure cooker-type digester with ‘white liquor,’ a mixture of sodium hydroxide and sodium sulfide solutions Lignin is selectively dissolved and removed, although about 5% remains. For electrical grade pulp, no bleaching is done, but careful washing is required to remove ionic materials. The pulp is made into easily breakable sheets. The pulp sheets are fed into a large vat, where the fibers are separated and mixed to the right consistency; then, they are fed onto the moving mesh belts on revolving drums. Water is sucked off by suction pumps. For papermaking, a Fourdrinier press is

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large and medium power transformers. It may be pointed out that high strength fibers derived from hemp and cotton are sometimes mixed with the kraft pulp for improved mechanical strength. Examples are hemp-kraft paper (60:40) once used in conductor taping, and paper sheets made from a mixed pulp of kraft and cotton fibers from used clothing. The hemp-kraft paper has been found to degrade faster than pure kraft paper because of impurities, although the initial tensile strength is higher than for pure kraft paper. The percentage of pulp used for electrical grade material is only a tiny fraction of the total wood pulp produced annually. Some statistics on total U.S. pulp production in 1991 cites about 80 million tons produced annually. More than one-half (52%) goes for mostly unbleached packaging materials [8]. Electrical grade paper is perhaps only 1% of the total unbleached pulp. Although a significant amount of paper and board materials are recycled in the U.S. (40%), the pulp used for electrical grade material comes from virgin pulp.

Transformer Insulation Development

Figure 5. Cellulose macro and micro structure (reprinted from [4], with permission).

commonly used. For pressboard, a cylinder press is preferred. Both types of presses use cylinders, but the Fourdrinier press uses several of these. Fig. 7 shows a cylinder press for pressboard making [7]. Pressboard sheets may be calendered or multiple sheets compressed to form denser and stronger (precompressed) boards that find application in making strong support items, such as spacer blocks and laminated blocks. The calendered board is used for washers, tubes, and formed items in transformers. Needless to say, several tons of pressboard and paper insulation are used in

It may be noted that transformer insulation had to be developed almost concurrently with transformer development, but it took a few decades before the paper-oil combination became reliable and well accepted. The transformer had been invented as far back as 1885 by a team of Austrian engineers and further developed by other inventors, especially George Westinghouse and his team [9]–[12]. The one built by George Westinghouse in 1885 based on the work of his team of experts in the U.S. was, in principle, similar to theirs, and was a dry-type distribution transformer with 500-V primary and 100-V secondary. It used air as coolant. Cellulose-oil insulation was critical for all transformers developed since the 1920s. Transformer oil itself had been introduced for transformer use in 1892 by GE and underwent improvement from paraffinic to naphthenic by 1925 [13]. Vacuum filling of oil was introduced in 1932.

A. Kraft Paper and Board It is difficult to pinpoint the time when electrical grade paper was introduced, but it is known that such papers were used for capacitors and cables extensively before becoming the primary solid insulation in transformers. The use of resin-impregnated paper for transformer insulation was introduced at the turn of the 20th century. The introduction of oil impregnation of paper led to the discontinuation of resin-impregnated paper.

Figure 6. Cellulose polymer.

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Figure 7. Cylinder press for pressboard manufacture. Although resin-impregnated cylinders functioned remarkably well in the earlier days, they were not desirable in high-stressed areas such as angles and corners (boundary areas) as the voltage rating increased. By the late 1920s in Switzerland, Weidmann had developed transformerboard (now called pressboard) from kraft pulp, which could be easily fabricated into formed items, and these were ideal for high-stressed areas. The wet sheets, built up from a number of required plies, pass through compressing and drying cylinders and emerge as dry sheets. The calendered pressboard is ideal also for washers and tubes used in power transformers. The calender press was already shown in Fig. 7. Another European manufacturer of calendered board, based in Sweden, is Figeholm, which started its operation in 1931. Other companies once in production have been acquired by other companies or shut down. Figeholm itsef is now owned by ASEA in Sweden. Mention had also been made of the special transformerboard known as precompressed board, another Weidmann development in the early 1950s, to meet the needs for strong support blocks and spacers to replace more conducting materials, such as lebanite (derived from saw dust, etc.) and even micrata. The precompressed board is tougher and denser than the regular calendered board, with a specific gravity about 1.2 [14]. It is tougher to impregnate with oil for the same reason. Laminates are made in a press with special adhesives.

B. Creped Paper Turn Insulation Although plain kraft paper is widely used for conductor insulation in transformers in many countries, creped kraft paper is used for such purposes in the U.S. Crepe paper for turn insulation was introduced by Dennison Paper Company in Framingham, MA in the 1970s with the blessing of the Westinghouse Large Power Transformer Plant in Muncie, IN, which was interested in

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a tear-free paper for taping. The tough hemp-kraft paper used for taping at the time had very little stretch. The crepe paper has as much as 20% stretch (elongation). The creping is done on the regular sheet of paper, as a drum of it unrolls and is picked up by another drum revolving at a slower speed; the paper goes through an aqueous bath containing a creping compound. The crepe paper described here should not be confused with the 100% stretch lead tape used in transformers that was available earlier. The introduction of the crepe paper was a few years after thermal upgrading agents were put into paper (see subsequent), so the crepe paper could be thermally upgraded at the same time from a non-upgraded paper.

C. Thermal Upgrading of Paper As the rating of transformers climbed in the 1950s and 1960s and as transformers were occasionally overloaded, the concern for transformer life, or rather, paper insulation life, was raised. Thermal upgrading of the paper insulation was considered one remedy and was attempted by several research groups associated with transformer or paper manufacturing in the late 1950s through the 1970s; upgraded paper began to be used in the U.S. since the mid 1960s. An EPRI Report on thermal upgrading agents released in 1987 gives both historical and ongoing studies [15]. The upgrading systems developed include Insuldur (Westinghouse), Cyanoethylate (GE), Thermacel (McGraw Edison), Celloflex (Allis Chalmers), Mannitherm (Manning Paper Co.), HAS (McGraw Edison), Hovotherm (Hollingsworth & Vose), and Rigel 65oC Rise (Rigel Products). The superiority of the upgraded papers was demonstrated by both short-term and long-term aging. The purpose of upgrading is to increase the insulation life. Accelerated aging studies confirmed that cellulose degradation is considerably slowed by upgrading agents [16]. Transformers

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rated at 55°C oil rise could be upgraded to 65°C oil rise, which meant the insulation life was extended by at least three times. Although several upgrading agents were tested by different researchers, the most successful formulas used amine compounds, particularly dicyandiamide (“dicy”). Westinghouse Electric researchers introduced Insuldur in 1958, which was improved further in 1960 [16]–[18]. The major component of this is “dicy” (>60%); the others are melamine (>30%) and polyacrylamide. The Insuldur content should be in the 2.75 to 4.0% range, corresponding to a nitrogen content of 1.74 to 2.54% in paper. However, manufacturers allow a 2% lower limit. This combination of the chemicals has been found to be more effective than just the “dicy.” However, some transformer papers are made with only “dicy” upgrading. The nitrogen content of the various upgrading systems ranged from 0.3 to 2.7%. Insuldur is at the high end regarding nitrogen content. With the widespread use of upgrading agents containing “dicy,” processes such as cyanoethylation (developed by GE) were discontinued [19]. Long-term aging studies were conducted on several upgraded papers in the 1960s to generate life plots. Morrison’s life tests showed that long-term life varied considerably among the upgraded papers, and the better ones lasted 10 times longer than regular kraft paper based on an end point of 60% tensile strength and /25% bursting strength retention [20]. Westinghouse studies on Insuldur paper showed that the upgraded paper could take a temperature rise of 20 to 30°C greater as compared with nonupgraded paper. It is now generally accepted that upgraded paper designated as 65°C rise insulation should have at least a 12oC thermal improvement. End-of-life criteria will be discussed in more detail in Part II.

D. Cellulose Modification Attempts Another attempted improvement of cellulosic paper was to reduce its hygroscopicity. Dry paper can absorb (a better term is ‘adsorb’ because of surface absorption) a considerable amount of water, which is a function of relative humidity and temperature, as shown in Fig. 8 [21]. Paper and pressboard, if left exposed to air at room temperature, can pick up anywhere from 4 to 10% water depending on the humidity of the environment. Therefore, transformer insulation requires rigorous dry out before oil impregnation. Oil-impregnated paper slows down moisture absorption considerably, but it is not desirable to leave impregnated insulation exposed for long periods. Once moisture is adsorbed, it is not easy to dry out by simple heating because of the oil presence. Chemical modification, such as cyanoethylation mentioned previously, not only improves thermal performance but also reduces hygroscopicity. Fresh attempts made in the mid 1990s by Oommen and Andrady [22] to reduce hygroscopicity included by graft polymerization, polymer deposition, and alkylation, yielded a less hygroscopic material. The grafted material had the lowest moisture absorption. For example, grafted cellulose, which had a polymer chain of acrylonitrile attached to the cellulose polymer, picked up only 2.5% moisture at room temperature and 50% relative humidity, whereas cellulosic paper picked up 6.5% mois-

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Figure 8. Moisture absorption curves for paper.

ture. However, the reduced moisture absorption came with a price: the paper was more brittle, had higher thickness, and the dissipation factor was higher than for paper. Economic factors also have to be considered in developing such material. Another attempted modification was to develop a low permittivity pressboard to bring it closer to that of transformer oil with a permittivity of 2.2 [23]. The best results were achieved by blending cellulose fibers with various polymer fibers, including Aramid (Nomex®), achieving permittivity of about 3.2 (cf. permittivity of pressboard = 4.5) The higher cost of the blends makes these products uneconomical.

E. Synthetic Materials Special synthetic formulations such as Aramid (an aromatic polyamide) developed by DuPont under the trade name Nomex® are being used for making paper sheets and pressboard for limited transformer use. Nomex® has a considerably higher thermal rating (220 vs. 105°C for cellulosic paper). Moisture absorption by Nomex® paper is significantly lower than for cellulosic paper, e.g., for 0.075-mm (3-mil) thick papers, saturation values at

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Summary Table 1. Paper/pressboard characteristics in specifications. Physical and Mechanical

Electrical

Thickness

Dielectric breakdown strength, 60 Hz

Apparent density

Impulse strength

Tensile strength

Hold strength, 60 Hz (pressboard)

Edge tear strength (paper)

Same, Impulse (pressboard)

Shrinkage (pressboard)

Dissipation

Stretch under tension (paper)

Factor at 25°C

Air resistance (porosity)

room temperature at 50% humidity are as follows: Nomex®, 4%; cellulose paper, 6.5%. Hybrid insulation structures in distribution, mobile, and small power transformers containing both Nomex® turn insulation and cellulosic structural parts are in commercial use. The higher cost of Nomex® insulation prevents its widespread use in medium and large power units. The use of enamel-coated conductors in transformers, especially high voltage conductors, has reduced the consumption of cellulosic turn insulation significantly. However, older transformers in the field contain mostly cellulose insulation. Several types of wire enamels are currently in use for transformer HV windings: 1) Fromvar enamels, which have a thermal index of 105 to 120oC and 2) high temperature enamels containing epoxies and polyamide/imide. Epoxy coating by electrostatic powder coating is a fast and reliable coating method. In power transformer HV windings, CTC (continuously transposed conductors), which are epoxy-coated to reduce eddy losses, are common.

F. Paper/Pressboard Specifications Material specifications stipulate all of the essential properties of these materials for transformer use. These may be divided into 1) pulp composition and characteristics and 2) mechanical strength properties, physical properties, and electrical behavior. Detailed specifications may be found in standards related to the use of these materials. The paper/pressboard manufacturer strives to meet the requirements. The transformer manufacturer also keeps a set of specifications. A typical set of specifications may contain the items shown in Table 1. In addition to these properties, certain critical properties are sometimes specified by the material’s buyer (transformer manufacturer), depending on the type of unit and the type of insulation paper/pressboard used. Thus, for paper, the initial DP value of humidity conditioned and dried paper may be specified. The thermal upgrading agent and its content will be specified for upgraded papers. For pressboard, bend and ply adhesion tests may be included.

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Part I has covered briefly the development of electrical grade paper/pressboard for transformer use from the raw materials, improvements made, and particularly the use of thermal upgrading agents to extend the useful life of transformers. Part II will cover practical measures to preserve insulation integrity by maintaining the insulation dryness level and by efforts to minimize aging by controlling the factors that influence it. Aging mechanisms and load-related life will be discussed.

References [1] G.T. Kohman, “Cellulose as an insulating material,” Ind. Eng. Chem., vol. 31, no. 7, pp. 807–817, 1939. [2] ABB Brochures on Transformers, used with permission. [3] EHV-Weidmann brochure, The Complete Insulation Package. [4] M. Schaible, “Electrical insulation papers—An overview,” IEEE Elect. Insul. Mag., vol. 3, no. 1, Jan. 1987, pp. 8–12. [5] H.R. Sheppard, “A century of progress in electrical insulation,” IEEE Elect. Insul. Mag., vol. 2, no. 5, Sep. 1986, pp. 20–30. [6] L. Blum, ‘The production of bleached kraft pulp,” Environmental Defense Fund, 1996, www.rfu.org/KraftPulp.htm. [7] EHV-Weidmann Industries, St. Johnsbury, VT. [8] The U.S. Pulp, Paper & Pressboard Industry Publication, “Paper: Linking people and nature.” [9] A.A. Halacsy and G.H. Von Fuchs, “Transformer invented 75 Years Ago,” A.I.E.E. Trans, Part III, Power Apparatus & Systems, Jun. 1961, pp. 121–128. [10] S. Jeszensky, ‘History of transformers,” IEEE Power Eng. Rev., Dec. 1996, pp. 9–12. [11] J.R. Lucas, “Historical development of the transformer,” Chairman’s Lecture, The Institution of Electrical Engineers, Sri Lanka Centre, Nov. 14, 2000. [12] J.W. Coltman, “The electric power transformer,” Westinghouse R&D Report. 87-1RO-RPLAN-R1, Feb. 1988, used with permission. [13] A Guide to Transformer Maintenance. S.D. Myers Publication, 1981, pp. 139–142. [14] Transformerboard. EHV-Weidmann Publication, 1979 [15] “Improved cellulosic insulation for distribution and power transformers,” EPRI Report EL-4935, Mar. 1987, McGraw Edison Company. [16] J.G. Ford, A.M. Lockie, and M.G. Leonard, “A new and improved heat-stabilized insulation,” Conf. Paper 60-936, Summer meeting of the A.I.E.E., Atlantic City, NJ, June 22, 1960. [17] L.E. Feather, “A new Insuldur turn insulation for large power transformers,” Proc. Elect./Electron. Insul. Conf., 1969, pp. 155–158. [18] “Insuldur,” Westinghouse/ABB brochure. [19] W.A. Wink, K. Ward, Jr., and H.A. Swensen, “Improving the thermal stability of paper by chemical modification,” Tappi, vol. 51, no. 10, pp. 155–158. [20] E.L. Morrison, “Evaluation of the thermal stability of electrical insulating paper,” IEEE Trans. Elect. Insul. vol. EI-13, no. 3, Aug. 1968, pp. 76–82. [21] T.V. Oommen, “Moisture equilibrium curves—Use and misuse,” Doble Conf. Paper, Apr. 2003. [22] T.V. Oommen and T.L. Andrady, “Graft polymerization and other methods to reduce the hygroscopicity of cellulose insulation,” Conf. Rec. Int. Symp. Elect. Insul., Montreal, Quebec, Canada, Jun. 16-19, 1996, pp. 538–541.

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[23] “Advanced concepts for transformers. Pressboard dielectric constant and mechanical strength,” Report by EHV-Weidmann Industries Inc., St. Johnsbury, VT, for U.S. Dep. of Energy, DOE/ET/29307-1, Mar. 1982.

Thomas A. Prevost is an active member of IEEE. He is currently the secretary of the IEEE PES Transformers Committee. He is Past Chair of the IEEE PES Standards Coordinating Committee and served on the IEEE-SA Board of Governors from 2002 to 2004. Thomas is the Vice President of Technical Service at EHV Weidmann Industries in St. Johnsbury, VT, where he has been employed since 1985. Prior to that, he worked at Tampa Electric Company as an engineer in distribution and production. Thomas received his BSEE from Virginia Polytechnic Institute. Thomas is also active in ASTM D-9 Committee on Solid Insulating Materials. He has written several technical papers on the subject of Electrical Insulation Materials. E-mail: [email protected].

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T. V. Oommen is a consultant mainly with ABB Power Technology Division of ABB Inc. He worked as a R&D engineer/ scientist for 24 yr at Westinghouse Electric and the ABB Power Transformer Division and retired in October 2000. During this period, he led research projects related to insulation degradation, gas generation, moisture equilibrium in transformers, bubble generation from overload, and biodegradable natural ester fluids; some of these were funded by EPRI. He earned his Bachelor’s and Master’s degrees in chemistry in India and his Doctorate degree in chemistry at the University of Washington in the U.S. in 1970. His did postdoctoral work in spectroscopy and electrochemical synthesis at the University of Washington and Southern Illinois University in Carbondale before he joined Westinghouse in 1977. Dr. Oommen has published over 70 technical papers in IEEE and CIGRE electrical journals and magazines and has given symposia and seminars on electrical insulation and transformer diagnostics. He is a Senior Member of IEEE, member of PES and DEIS, and also a member of the Insulation Life and Insulating Fluids Subcommittees. He and his wife reside in Raleigh, NC. Email: [email protected].

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