(*)jose-carlos.mendes@br.abb.com

Advanced Application of a Natural Ester Vegetable Oil in a HV Power Transformer

Dr.J.C.Mendes*, A.S.G.Reis, E.C.Nogawa, C.Ferra

ABB – Divisão de Transformadores de Potência São Paulo, SP, Brazil

A.J.A.L.Martins, A.C.Passos

CEMIG – Companhia Energética de Minas Gerais Belo Horizonte, MG, Brazil

SUMMARY This paper presents an overall view and results of a development project, fully developed by ABB Brazil, dealing with the application of BIOTEMP® in a high voltage power transformer. BIOTEMP® is the brand name of the natural ester high oleic vegetable oil developed by ABB in USA some years ago. General aspects associated with the customer motivation for this project, overall transformer characteristics, and transformer design details having advanced vegetable oil as an insulating fluid, manufacturing process as well as tests results are presented. Additionally, relevant information regarding this technology is presented. KEYWORDS Transformer, Power Transformer, Refurbishment, Hybrid Insulation, Natural Ester, Vegetable Oil, BIOTEMP®.

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1. INTRODUCTION In 1890, five years after transformer invention, BBC Brown Bovery & Cie in Germany was the first one to apply mineral oil as a transformer insulating and cooling media. Transformer industry and technology has developed further to very large sizes and functionalities in the nowadays very complex Electric Power Systems. Despite of that mineral insulating oil still being used extensively. However, in the last decades, due to oil crisis and demanding for environment friendly combustibles, several initiatives have been in place searching for alternative fuels for the automotive industry. This also has established the foundation for the insulating vegetable oil development. Since the crude oil worldwide crisis in early Seventies, Brazil has promoted extensive technology development in the ethanol application as an automotive combustible and from 1979 ethanol has been used extensively in Brazil. Later on, biodiesel has been also introduced and is being used in a mixture with mineral diesel in truck motors. Renewable energy is one efficient ways to achieve sustainable development [1]. Nowadays most of the Brazilian cars are using a flexible fuel automatic control system that allows cars run on a natural gas, gasoline, sugarcane ethanol and a mixture of gasoline and ethanol in any percentage of each combustible, while biodiesel is being used to move Brazilian trucks. Demand for such alternative is growing up due to concerns on oil prices and greenhouse-gas emissions legislations [2]. In this environment Brazilian electric power utilities have dedicated in to develop insulating vegetable oil based on Castor oil [3]. However, later on, Dr. TVOommen and Dr. Clair Claibornes both from ABB USA have succeed to develop BIOTEMP®, an advanced high performance insulating vegetable oil to be applied in high voltage power transformers and apparatus [4]. In this context, this paper describes the first worldwide application project of BIOTEMP® as the insulating and cooling fluid in a 138kV high voltage 25MVA power transformer main tank, its high voltage bushings and its on load tap-changer. 2. CUSTOMER PERSPECTIVE A transformer originally rated 138/13.8kV, 10/15MVA, ONAN/ONAF and delivered by ASEA Brazil in 1974, has been refurbished and uprated to 20/25MVA, ONAN/ONAF by ABB Brazil in 2006 to CEMIG - Companhia Energética de Minas Gerais - one of the largest Brazilian electric power utilities, in a joint development project. CEMIG is the electrical power utility of the State of Minas Gerais, which is located in the Southeastern region of Brazil. CEMIG is working in the power generation, transmission and distribution segments, with an installed capacity of 6113 MW, a transmission system comprising more 21000 km of lines (4912km at extra-high voltage level from 230 kV to 500 kV) and a distribution system extending over more than 379400km. CEMIG supplies electricity to 774 cities and to a population of 17 million of inhabitants. The area supplied is 568 thousand square kilometers and is equivalent to a country the size of France. Figure 1 shows location of Minas Gerais state in Brazil and its capital, Belo Horizonte, a city 3 million inhabitants.

Figure 1 - Brazil and Minas Gerais State CEMIG Covered Area

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CEMIG is also one key customer to ABB and it has long tradition in to support development and application of advanced ABB new technologies. As an example, over the Seventies, CEMIG has been the first utility to introduce 525kV long transmission systems in Brazil supported by ABB (formers BBC Brown Boveri and ASEA), including large autotransformers and shunt reactors. Later on, in 1992, CEMIG also has been the first utility in Brazil to support ABB in the development and application of site-repair of large power transformers [5]. More recently, CEMIG has also worked out a complete solution for 525kV shunt reactor diagnostic and complete refurbishment [6]. Cost effective operation of an electric power generation, transmission and distribution systems is related to reliability, availability, loading capability, extended useful life and reduced maintenance of their power transformers in connection with environmental preservation requirements. This also has influenced power utilities to search and select advanced new technologies for both new and refurbishment transformers. In this context, the refurbished transformer delivered by ABB Brazil to CEMIG includes the following advanced characteristics: � up-to-date ABB TrafoStar™ technology; � hybrid high temperature insulation system; � filled with superior performance ABB BIOTEMP® vegetable oil as an insulating and cooling

transformer fluid; � extreme high overload capability, up to 70% above rated power, without loss of useful life; � advanced ABB TEC transformer electronic control and on-line monitoring system; � low voltage on-load regulation using ABB UZ type on-load tap changer filled with superior

performance ABB BIOTEMP® vegetable oil; � high voltage ABB GOB type bushings filled with superior performance ABB BIOTEMP®

vegetable oil; and � reduced maintenance requirements. The application of advanced technologies involving hybrid solid insulation (DuPont Nomex® + Cellulose) combined with superior performance insulating vegetable oil (ABB BIOTEMP®) has allowed to uprated the transformer rated power from originally 15MVA to 25MVA (ONAF) with a extreme high and reliable overload capacity without loss of useful life. 3. VEGETABLE OIL FILLED TRANSFORMERS Nowadays in Brazil there is an increasing interest from utilities and industries to move for vegetable oil filled equipments due to a combination of facts: � environment reasons as greenhouse-gas emissions legislations; � government effort to move for vegetable fuels as much as possible (ethanol from sugar-cane is a

reality for cars and 25% also added to gasoline, bio-diesel production has been started by Petrobras - the largest oil producer in Brazil - etc);

� replace as much as possible imported mineral insulation oil due to economic reasons and corrosive sulphur issue in mineral oil delivered by foreigners oil supply; andBrazilian Standard Committee has already published a Standard on insulating vegetable oil specification based on the corresponding ASTM.

In addition, the drive factors for CEMIG to move a step further in this development project with ABB were: � increase power delivery reliability to its customers, taking into account a extreme and safe high

transformer overload capability; � take the lead in the segment of environment clean and reliable energy delivery; � use of a fully reprocessed, renewable and easily dismissible insulating oil; and � installation safety reasons. As benefits CEMIG are foreseen the following aspects: � high transformer reliability;

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� high transformer availability due to reduced maintenance needs; � reduced installation costs; and � vegetable oil filled transformer has much less risk of a transformer explosion, fire and consequent

ground and underground waters contaminations when compared to a mineral oil filled transformer. Even in case of a such accidental case, the vegetable oil will generate much less hazardous non-toxics by-products.

4. BIOTEMP ®: A SUPERIOR VEGETABLE OIL BIOTEMP® is an advanced biodegradable electrical insulating fluid from high oleic vegetable oil renewable natural agricultural sources developed by ABB to be friendly to the environment. The fluid has excellent dielectric characteristics with high temperature stability and superior flash and fire resistance. BIOTEMP® has excellent compatibility with solid insulating materials and is biodegradable in a short time period. It is an inhibited oil approved in an Oxidation Stability Test according to Test Method ASTM D2440 (72h at 110Cdeg) for mineral insulating oil type II acc. ASTM D3487 [7]. Table I shows some BIOTEMP® properties in comparison with others insulating fluids. Table I - Typical Properties of Insulating Fluids

TYPICAL PROPERTIES OF INSULATING FLUIDS BIOTEMP Mineral Oil H.T.H Silicone Electrical Dielectric Strengh, kV (ASTM D877) 45 30 40 43 Physical Viscosity, cSt. 100°C 10 3 11.5 16 (ASTM D445) 40°C 45 12 110 38 0°C 300 76 2200 90 Flash Point °C (ASTM D92) 330 145 285 300 Fire Point.°C (ASTM D92) 360 160 308 330 Specific Heat (cal/g/°C) (ASTM D2766) 0.47 0.43 0.45 0.36 Coefficient of Expansion, per °C (ASTM D1903)

6.88x10-4 7.55x10-4 7.3x10-4 1.04x10-3

Sp. Gravity (ASTM D1298) 0.91 0.91 0.87 0.96 Color (ASTM D1500) <0.5 0.5 0.5 – 2.0 <0.5 Environmental Aquatic Biodegradation Rate (%) 21days test using CEC-L-33-A-93/94

97.0 25.2 27.1 0.0

note: HTH = High Temperature Hydrocarbons Fluid (ASTM D5222) While mineral oil has flash and fire point temperatures of 145°C and 160°C, BIOTEMP® has 330°C and 360°C respectivelly. It must be noted that BIOTEMP® is 97% biodegradable in 21 days. Figure 2 shows in detail a comparison of flash and fire temperatures of mineral and BIOTEMP® vegetable oils.

Figure 2 - Flash and Fire Points Temperatures Insulating Mineral and BIOTEMP® Vegetable Oils

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Due to the great affinity of BIOTEMP® to absorb water, the life of insulation paper can be increased further in comparison with insulation immersed in transformer oil from petroleum sources. The study showed that BIOTEMP®/Kraft paper has double the life of mineral oil/Kraft paper with tensile strength and degree of polymerization measurements validating this conclusion. The great affinity in to absorb water in combination with its superior thermal properties, BIOTEMP® allows a higher hot spot temperature in the winding. Then ABB BIOTEMP® has been the natural choice of CEMIG of the insulating vegetable oil to be used to fill the transformer after refurbishment and uprated. Further advantages of the BIOTEMP® application in power transformers are related to the transformer installation requirements: � eliminate requirement to build fire walls in the substation bay; � reduced fire liability and insurance costs; � optimization of requirements on fire extinction systems in the substation bay; � reduction of distances clearances between the transformer and adjacent equipments and/or

buildings in the installation place; and � possible future optimized requirements to build expensive oil basin containment, narrow channels,

etc. 5. ENGINEERING AND MANUFACTURING Table II shows the transformer main data before and after complete refurbishment and uprated. Table II - Transformer Data

Original Refurbished Serial Number 54381 Manufacture ASEA ABB Year 1974 2006 Frequency , Hz 60 60 Phases 3 3 Voltages 138 ± 2 x 2.5% / 13.8 ± 16 x 0.625 kV Rated Power, MVA 15 (ONAF 2) 25 (ONAF 2) Overload, MVA - 37.5 (6h, 150%)

42 (4h, 170%) OLTC UZERN 250 UZF 250 Insulation Cellulose Hybrid (Nomex + Cellulose) Oil Type Mineral BIOTEMP®

The transformer electrical and mechanical design has been fully developed by ABB Brazil. Engineering efforts have been dedicated specially to: � winding insulation dimensioning having Nomex® as paper and board insulation. In fact ABB Brazil

has also long time experience in Nomex® aplicattion as a transformer solid insulation. Several industrial and network transformers have been designed, tested and are under reliable operation using this technology, including a large size 3phase, 300MVA, 245kV transformer;

� winding insulation dimensioning as Nomex® impregnated with insulating vegetable oil has a dielectric permissivity value different than the standard cellulose paper impregnated with insulating mineral oil. In consequence, electric potential distribution will get a specific distribution in the winding mix insulation;

� internal windings connection leads electrical and thermal dimensioning for overload conditions; and

� external cooling system dimensioning and temperature rises evaluations, taking into account differences in the viscosity of the vegetable and mineral oils and the extreme high overload requirement.

The transformer has been manufactured in the ABB Brazil medium and large power transformer plant. The manufacturing process has been in accordance with the well-established TrafoStar manufacturing process. The final oil filling process has been specially adapted to meet the requirements established for the vegetable oil filling:

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� completely separate temporary oil system; � dedicated oil processing machines (thermovacuum plant, filters, heaters, hoses, etc); � specifics before filling oil conditioning, vacuum time, oil filling under vacuum process, oil

circulation time and final standing time period before tests have been defined and applied to this case; and

� specific oil quality controls, with several tests in all processing steps have been established in order to assure the insulating vegetable oil filled transformer overall quality.

The quality controls of all manufacturing processes have been recorded according to internal ABB Six-Sigma procedures certified in compliance with ISO 9001 and ISO14001 standards. 6. FINAL TESTS AND HIGH PERFORMANCE Figure 3 shows the transformer, completed assembled, sitting in the test bay of the ABB Brazil High Voltage Laboratories.

Figure 3 - BIOTEMP® Vegetable Oil Filled Transformer at HV Laboratory Test Bay

Before assembly in this transformer, ABB high voltage GOB bushings have been also filled with BIOTEMP® vegetable oil. Later on the bushing were submitted to all routine tests and to a set of special tests with large number of impulses (lightning and switching) as well as applied AC voltage with Partial Discharge measurement. All bushings have been successfully approved with superior performance proving the excellence of BIOTEMP® also for transformer apparatus as HV bushings. Then, the complete assembled vegetable oil filled transformer has been fully tested. All standard [8; 9] routine tests as well as a set of special and type tests have been performed in this unit, including dielectric, thermal and operational tests as: � lightning (full and chopped waves) and switching impulse tests in all windings terminals; � short duration AC applied tests; � long duration induced voltage tests, before and after heat run and overload thermal tests, including

measurement of Partial Discharges; � no-load and load sound level measurements tests, including octave-band noise spectrum

measurement; � complete heat run test with oil and winding temperature rises measurements; � long duration overload test (initial load 100% or 25MVA and peak load of 170% or 42MVA over 4

hours) with oil temperature measurement; � power frequency long duration over-excitation tests; and � frequency response (FRA) test. All dielectric and thermal tests have been monitored by oil dissolved gas analysis (DGA). The transformer has been successfully approved in all electrical and thermal tests. The oil DGA tests results has not shown any significant variation in gases concentrations before and after electrical and thermal tests, given a very good as demonstration of the transformer superior and reliable test performance, [10]. 7. OPERATION AND PERFORMANCE After being approved in all acceptance tests, the transformer has been shipped by road for about 550km from ABB factory to CEMIG Cidade Industrial Substation located nearby Belo Horizonte downtown.

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While under shipment, the transformer tank was kept filled with BIOTEMP® vegetable oil, means a further advantage and cost reduction associated to transportation and related licenses. In case of mineral oil instead, the oil should be shipped in a separate container. Before transformer site installation, when the transformer has arrived in Belo Horizonte at company headquarters area, CEMIG has promoted a media and press event to present the first high voltage power transformer 100% filled with vegetable oil. Several press releases have been published since then including declarations of CEMIG directors about the benefits of this technology to the company and to the energy consumers as well as the importance of the partnership established with ABB. In addition several technical papers and seminary presentations have also provided information about the good results of this project. After site erection, oil processing and commissioning, the transformer entered into commercial operation in July, 31st-2006. Figure 4 shows an overall view of the transformer at substation bay after energization and loading.

Figure 4 - BIOTEMP® Vegetable Oil Filled Transformer in Operation at Substation Bay

Since then, the transformer is operating properly and reliable, taking peak overload up to 42MVA (170%) almost every weekday. As an example, figure 5 shows a heavy duty loading cycle supplied by this transformer with peak load above 35MVA.

Figure 5 - BIOTEMP® Vegetable Oil Filled Transformer in Operation - Example Loading Cycle

Transformer operation is being monitored by ABB TEC on-line monitoring system as well as by conventional oil tests, oil and windings temperatures monitoring and periodic infrared scan. The aim is to close follow performance of the transformer and of the BIOTEMP® vegetable oil. Updated monitoring results are show that the transformer performance is operating with reliability even under severe overload conditions. 8. CONCLUSION The development of advanced new materials is a significant step for transformer technology upgrade. Also well-established long run cooperation development projects between ABB and customers as CEMIG create the conditions for the real cases application of advanced transformer technologies resulting benefits to both companies, to the power industry and society. The overall results of the described development project are:

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� high thermal properties of modern transformer new materials allows to increase rated transformer power and optimize substation required space;

� combined high level application of advanced materials allows to increase transformer reliable overload capacity from 150% up to 170% of its rated power that also means an important power delivery additional reserve;

� by application of BIOTEMP®, a superior performance renewable and biodegradable vegetable oil from ABB, brings several benefits to the utilities as: increased installation safety, reduction in the installations cost, reduced risk of explosions and fire, less maintenance and transformer useful life extension, as well as to the society: in accordance with greenhouse-gas emissions legislations and promotion of sustainable development [1].

� further demonstration that by qualified application, using ABB status-of-the-art transformer technology, utilities can refurbish and uprate their existing old transformers close to their end of useful life at optimized cost and with additional benefits;

� it also foreseen that, by reviewing today's transformers installations standards and local legislations, utilities can further reach simplification and optimization of the future substation design; and

� a reliable step to move further this technology to applications in transformers at higher voltages levels than 145kV.

The above results are a demonstration of how a strong cooperation with a customer as CEMIG leads ABB to offer advanced new environment friendly safety products based on its superior technology and quality with further benefits to all: customer, industry and society. 9. ACKNOWLEDGEMENTS The authors would like to thank several colleagues of ABB Brazil, Power Transformer Division, whose dedication and competence have contributed to the development of technology and made high quality high voltage vegetable oil filled transformer possible. Special thanks to CEMIG and DuPont Brazil by the friendly cooperation dedicated to this joint development project with ABB. BIBLIOGRAPHY [1] GOLDEMBERG, J. Ethanol for a Sustainable Energy Future. Science Magazine. Vol 315, No.

5813, pp. 808-810, Feb 2007. [2] IEEE. The Omnivorous Engine. IEEE Spectrum Magazine, pp30-33. NYork, January 2007. [3] MARINHO JR., SAMPAIO, E.G. and MONTEIRO, M.M. Castor Oil as an Insulating Fluid.

CIGRÉ , 500-06, Symposium 05-87. Vienna, 1987. [4] U.S. Patent No. 4.627.1992 of Dec. 9, 1986. Sunflower Products and Methods of Their

Protection. [5] MENDES, J.C. et alli. On Site Repair of HV Power Transformers. CIGRÉ , 12-114, Paris,

Session 2002. [6] ROCHA, A.C.O. and MENDES, J.C. Assessment of An EHV Shunt Reactor Insulating and

Mechanical Performance by Switching Surge Analysis. CIGRÉ , A2-301, Paris, Session 2006. [7] OOMMEN, T.V. and CLAIBORNE, C.C. Biodegradable Insulating Fluid from High Oleic

Vegetable Oils. CIGRÉ , 15-302. Paris, 1998. [8] ABNT. Transformador de Potência: Especificação. Norma NBR 5356, Rio de Janeiro, BR, Ago

1993. [9] ABNT. Transformador de Potência: Método de Ensaio. Norma NBR 5380, Rio de Janeiro, BR,

Maio 1993. [10] ABB. Transformador No. de Série 54831 - CEMIG. Relatório de Ensaio 1ZBR 06-0150, São

Paulo, 2006-05-29, 112 pag.