THE USE OF NATURAL ESTER FLUIDS IN TRANSFORMERS Alan DARWIN, AREVA T&D (UK), alan.darwin@areva-td.com Christophe PERRIER, AREVA T&D (France), christophe.perrier@areva-td.com Philippe FOLLIOT, AREVA T&D (France), philippe.folliot@areva-td.com ABSTRACT In this paper, the use of natural esters is discussed for both power transformers and distribution transformers. A comparison is given between fundamental characteristics (breakdown voltage, viscosity and ageing stability) of different vegetable oils and other types of oil. This comparison is established using experimental results. Next, issues related to technical, manufacturing and monitoring aspects are described based on the experience from two recent power transformer projects involving natural ester fluid, one rated at 132kV, 90MVA for the UK, the other rated at 242kV for Brazil (a shunt reactor). Finally, temperature rise tests are presented that have been carried out on distribution transformers filled with either mineral or vegetable oil. differences in characteristic that affect both the design and manufacturing processes. To go forward it is necessary to understand these differences that principally occur with respect to the following topics: dielectric issues; thermal issues; processing issues; diagnostic issues; cost issues. KEYWORDS This paper will firstly compare different insulating liquids, namely mineral oil, synthetic oil and vegetable oil, with respect to some of their fundamental characteristics. Secondly, the paper will discuss technical and manufacturing issues connected to the use of natural esters. This is based on experience obtained from recent projects using natural ester fluids, namely a power transformer for the UK, (rated at 132kV and 90MVA) and a Shunt Reactor for Brazil, (rated at 242kV and 22 MVAr). Finally, the paper will talk about temperature rise tests carried out on duplicate distribution transformers filled with either mineral oil or vegetable oil. Insulating oils, natural ester, breakdown voltage, ageing stability, heat transfer, manufacturing, monitoring, temperature rise test, hot spot, distribution transformer, power transformer. COMPARISON OF FUNDAMENTAL CHARACTERISTICS OF TRANSFORMER OILS INTRODUCTION In the face of increasing demand for the use of environmentally friendly products in the industry, AREVA T&D has been working to develop the use of vegetable oils in distribution transformers and to extend its use to HV power transformers and reactors. In a liquid-filled transformer, the insulating liquid plays an important function by providing both the electrical insulation (in combination with a solid such as cellulose) and the means of transferring the thermal losses to the cooling system. The insulating liquid can also provide important and easily obtainable information for use in diagnosing the health of a transformer. For more than one hundred years, the majority of liquidimmersed transformers have been filled with mineral oil. The significant use of this petroleum-based product has been justified until now by its wide availability, its good properties, its good combination with cellulose and its low cost. However, with environmental issues now becoming extremely important, the use of a product with a high fire point temperature and high biodegradability is becoming extremely attractive. Thus, the recent availability of natural ester fluids based on “renewably sourced” vegetable oils has provided a new insulating liquid for use with transformers. To make the change from mineral oil to a natural ester vegetable oil is an interesting challenge, as there are many Different oil types Nowadays, transformers can be filled with 3 basic types of insulating liquids: (i) mineral oils, (ii) synthetic oils or (iii) natural esters. The use of each type is justified by the application. However, in the face of increasing demand for the use of environmentally friendly products in the industry, more and more companies are working towards developing the use of esters and specifically natural esters for use in the majority of their products. (i) Transformers have been filled with mineral oils for more than one century. This type of oil is a petroleum-based product, essentially composed of hydrogen and carbon atoms. Carbon and hydrogen are assembled in different structures: napthenic (CnH2n), paraffinic (CnH2n+2) and aromatic (CnH2n-6). The distribution of carbons in napthenic and paraffinic structures defines the type of mineral oil. This distribution is controlled by the crude oil and the refining processes used. Because of its wide availability, good properties and low cost, mineral oil is the fluid most used in the transformer industry and at present is generally the only one used for power transformers. New mineral oils have to be in accordance with the IEC 60296 or ASTM D3487. As mineral oil has been used for such a long time, a large data base of information is available to enable interpretation of changes to its characteristics and thus predict the possible malfunction of a transformer. IEC 60422 is a good tool to evaluate the quality of insulating oils in operational transformers. (ii) There are two main synthetic liquids that can be used in Experimental investigations for comparison This part presents a comparison between different types of fresh insulating oil described briefly in Table 1. Name Data Vegetable oil 1 Mixture of mono and tri-ester Vegetable oil 2 Tri-ester Vegetable oil 3 Tri-ester Synthetic ester Tetra-ester largely used Mineral oil Napthenic/uninhibited, largely used Silicone oil Largely used Table 1: Tested fluids The comparison study is based on experimental work realized in a laboratory and is focused primarily on the three main properties of transformer fluids, i.e. the insulating capability with the breakdown voltage; the heat transfer with the viscosity; the ageing stability with oxygen and temperature influences. Breakdown voltage (BDV) A major function of oil is to ensure the electrical insulation in a transformer. Specifically, this insulation capability is controlled by the complex paper/oil structures. Oil impregnates the cellulose (paper, pressboard and wood) and thus drives away the air which presents a lower dielectric strength than the oil. Breakdown voltage is one of the main properties used to define the efficiency of oil as an insulator. This characteristic is very sensitive to the quality of the oil, which can be influenced by the presence of impurities like humidity, particles and gases. Even though the breakdown voltage is more associated to the oil quality than the oil chemistry, it forms a good characteristic for comparing different oils when the impurity content is well controlled. One big difference between ester oils and mineral oils is the water solubility level. Indeed, ester oils can absorb around 20 to 30 times more moisture than mineral oil at 25°C before saturation. This better water solubility decreases the effect of the humidity influence on insulation strength, but also dries the paper. This could possibly increase the life of a transformer, as its life is controlled by the state of the paper. On the other hand, due to this higher solubility, ester oils recapture moisture rapidly and special care has to be taken during handling compared to mineral oil. Experimental results are represented on Figure 1. As BDV measurement is not an accurate method, the test was repeated 18 times instead of the 6 times required by the IEC 60156 standard. 120 100 Breakdown Voltage (kV) transformers: silicone oil and ester of pentaerythritol (synthetic ester). These oils were developed in the seventies to replace askarels (PCB), which became outlawed because of their toxicity. As PCB was a nonflammable liquid (no fire point), the main particularity of these new oils was their high fire resistance (higher fire point than mineral oil). Until now, their use was essentially restricted to distribution and traction transformers, partly because of their price (three to eight times more expensive than mineral oil), but also because a better fire resistance was required for these applications. Their better thermal stability was also a positive point for their use and especially for traction transformers. Esters of pentaerythritol, also called tetra-ester, are obtained by an esterification reaction between a tetraalcohol (pentaerythritol) and mono-carboxylic acids. These oils are composed of hydrogen, carbon and oxygen. New synthetic esters have to be in accordance with the IEC 61099 and a maintenance guide that is published in the IEC 61203. Silicone oil is a polymer based on silicon, which also includes carbon, oxygen and hydrogen atoms. Specifically, the final product is obtained by the polymerization of polydimethylsiloxane (PDMS). This oil presents the advantage to have a high thermal stability, but on the other hand is not biodegradable at all. New silicone oils have to be in accordance with IEC 60836 and a maintenance guide that is published in IEC 60944. (iii) Finally, there are vegetable oils, also known as natural esters (tri-ester) as opposed to synthetic esters. These oils are naturally synthesized from living organisms and come in particular from soya, sunflower, rapeseed, etc. Specifically, natural esters are created from an esterification reaction between a tri-alcohol and fatty acids. Other processes allow the final product to be obtained by the transesterification reaction (mono-ester) or mixture of mono and tri-esters [1]. Since the middle of the nineteen nineties and because of environmental concerns, a lot of studies were launched for the development of vegetable oils. Nowadays, its use is starting to be widespread in the distribution transformer market and the challenge of these next years is to extend its use to the power transformer market. The most difficult step will be to define new evaluation criteria to diagnose the status of a transformer. At this time, no international standards exist for vegetable oils, although an IEEE Guide, will be published soon (“C57.147 - IEEE Guide for Acceptance and Maintenance of Natural Ester Fluids in Transformers”). The only standard available at present is the ASTM D6871. A Project Team (PT-5) was launched within TC 10 (“Fluids for Electrotechnical Application”) to develop an IEC standard for vegetable oils as soon as possible. 80 60 Vegetable oil 1 (67ppm) Vegetable oil 2 (55ppm) 40 Vegetable oil 3 (51ppm) Synthetic ester oil (64ppm) 20 Mineral oil (9ppm) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Series Figure 1: Breakdown voltage measurements in accordance with IEC 60156/spherical electrodes with 2.5mm gap (water content in ppm) Results showed that after the same laboratory treatment (sintered glass filtration by vacuum) for each new oil, natural and synthetic esters presented a BDV relatively close to mineral oil and above the limits of IEC 60296 for mineral oil (BDV > 70kV on the mean of 6 measurements). Note that after the laboratory treatment, the water content of the mineral oil was around 13% of its saturation value (at 25°C), whereas ester oils were all largely below 10% of their saturation level. Insulating oil in a transformer must ensure the transfer of heat. This function is realized both by thermal conductivity and convection [2]. The convection represents all of the properties which lead to the heat transfer by fluid displacement (viscosity, specific heat, thermal expansion coefficient), whereas the conduction is realized within the fluid. In a previous paper [3], it was shown that the kinematic viscosity is the most influential parameter for the heat transfer. Viscosity measurements were realized in accordance with the ISO 3104 specification. 40 5 4.5 Before ageing 4 After IEC 61125C ageing 3.5 3 2.5 2 1.5 1 30 25 Vegetable oil 1 0.5 Vegetable oil 2 0 Vegetable oil 3 20 Veg. Oil 1 Synthetic ester 15 Veg. Oil 2 Mineral oil Veg. Oil 3 Synth. Ester Mineral oil Oil type Silicone oil 10 5 0 20 40 60 80 100 Temperature (°C) Figure 2: Viscosity evolution versus temperature for different oil types At the normal operating temperature of a power transformer, the viscosity of ester oils is higher than that of mineral oil, but lower than that of silicone oil (see figure 2). These results show that the transfer of heat in the transformer by convection will be less efficient with ester oils and the worst with silicone oil. This is not necessarily a critical issue, but care has to be taken especially when designing the cooling system of power transformers. It should be noted that some vegetable oils can have a viscosity closer to mineral oil (e.g. vegetable oil 1), so minimising this effect. Ageing stability In addition to the BDV and the heat transfer characteristics, insulating oil must have a good ageing stability. Oxygen, which is present in the oil and can also ingress from the environment, constitutes one of the more influential factors, ageing the oil by oxidation. Temperature acts as a catalyst as well as some metals such as copper. The ageing stability of oil, therefore, is even more important if the transformer is free breathing. Sulphur compounds are present in mineral oils and act as natural inhibitors, but also some synthetic inhibitors like DBPC can be added to reduce the ageing process. As esters oils are more biodegradable than mineral oil, they tend to have lower oxidation stability. This lower oxidation stability can enforce transformer manufacturers to use a sealed design (not free breathing), although vegetable oil suppliers could also add an inhibitor (which should be as “green” as possible). For studying the oxidation stability of the different oil types, the ageing was realized in accordance with IEC 61125 C. This is the reference test to evaluate a new mineral oil. It consists in ageing oil in the presence of a copper catalyst at 120°C for 164 hours (for non inhibited oil) with a defined oxygen flow rate. These tests were realized on new oils, as Figure 3: Evolution of acidity after IEC 61125 C ageing (120°C / 164h) Figure 3 shows that synthetic ester keeps a very low acidity after the ageing, lower even than that of mineral oil. The three vegetable oils tested each reacted differently with respect to ageing: one (veg. oil 2) stayed below the IEC limit required for mineral oil (red dashed line), whereas another (veg. oil 1) showed a large increase of acidity. These results could be correlated to the presence or lack of an additive, as well as to the nature of the seed used to produce the vegetable oil. At present, specifications for natural esters are not well established, and it is thus difficult for the user to know if a vegetable oil contains additives or not. before ageing 1.2 after IEC 61125C ageing 1 Tan delta (90°C) Viscosity (mm2/s) 35 received from the supplier. Acidity and tan delta were measured, as they constitute the more pertinent properties for evaluating the oxidation [4, 5]. These measurements were realized in accordance with the IEC 62021-1 and IEC 60247 specifications respectively. Acidity (mg KOH/g) Heat transfer 0.8 0.6 0.4 0.2 0 Veg. Oil 1 Veg. Oil 2 Veg. Oil 3 Synth. Ester Mineral oil Oil type Figure 4: Evolution of tan delta after IEC 61125 C ageing (120°C / 164h) Figure 4 shows that tan delta of synthetic ester increases significantly after ageing in comparison to mineral oil and some vegetable oils. One vegetable oil (veg. oil 2) stays below the IEC limits for mineral oil. As a summary, ageing test results show that some vegetable oils can pass the IEC 61125C test for non inhibited mineral oil, whereas others do not. Synthetic ester shows good acidity stability, but a large increase in tan delta. One of the challenges for the industry is to understand if the different ageing behaviour of ester oils is significant for the transformer life expectancy or whether a new reference level could be used? TECHNICAL AND MANUFACTURING ISSUES FOR POWER TRANSFORMERS Technical issues For all new designs it is essential to use the design review process to identify where differences to previously proven designs need validation and/or new development. This was especially true for the transformer (132kV, 90MVA) and reactor (242kV, 22MVAr) designs that were to be filled with natural ester fluid. Although the basic design and construction was similar to previous designs, a key component was being replaced, i.e. the insulating fluid. “Oil-immersed” transformers and reactors rely on the “oil” to provide part of the insulation structure as well as allow the thermal losses to be cooled. If this insulating fluid is to be changed, it is necessary to validate its use with respect to both design and manufacture. In the above cases, the design review process highlighted differences between the two fluids that needed to be understood in case their impact was significant during design, manufacture and operation. These issues were investigated and their impact is discussed below. Dielectric issues The natural ester has a relative permittivity that is closer to the solid insulation used in oil-immersed transformers and reactors (such as paper and pressboard). This has several effects on the dielectric design. The capacitances of the insulation structure change, causing different voltage distributions under transient conditions such as impulse application (lightning strike). This change was not found to be significant. The distribution of voltage stress within the insulation structure also changed, such that for a given voltage distribution the voltage stress in the fluid was less for the natural ester than for the mineral oil. This was beneficial and allowed higher levels of voltage withstand for certain electrode and insulation configurations. Much work was carried out to investigate the comparative voltage withstand of the different “oils” under various test and service voltage conditions such as power frequency and impulse application. This enabled the design of the windings and insulation structures to be carried out with maximum confidence. Information about modified acceptance criteria for the condition of the oil was made available for use during manufacture and service. The ester-filled transformer and shunt reactor both successfully passed all their dielectric tests. Thermal issues As discussed above, the esters have higher viscosities than mineral oil for the same temperature. This reduces the fluid flow rate for a given dynamic head causing a higher temperature difference between top and bottom of the cooler. This is significant for natural cooled transformers, as, although the mean oil temperature rise is controlled by the cooler, the top oil is controlled by the natural thermosyphonic fluid flow and is effectively the summation of mean oil rise and half the top to bottom fluid rise. Where the cooling system uses forced directed fluid flow, the effect is minimal, providing that the correct rating of pump is used to take into account the higher impedance to oil flow caused by the higher viscosity. The better thermal conduction of the esters means that the temperature gradient between winding conductor and local oil is slightly reduced, depending on its oil velocity component. The net effect is that for naturally cooled transformers, the top oil and thus hot spot temperature rises will be higher with esters than with mineral oil. Temperature rise tests carried out on the two 90 MVA transformers confirmed this effect, there being an increase of 5 degrees for the top oil rise (top to bottom rise increased by 20%), whilst the winding gradients were very similar. Similar results were seen for the 242 kV reactors. The vegetable oil used for these tests was a tri-ester. Similar effects are seen for distribution transformers later in the paper. Recent publications [6, 7] demonstrate that using a natural ester with cellulose insulating material could have a beneficial effect on the life time of the solid insulation. By decreasing the water content in the cellulose paper, the natural ester reduces the hydrolysis phenomenon and the rate of ageing is reduced for a given winding hot spot temperature compared to that for normal mineral oil impregnated cellulose. Table 2 shows the relative ageing rate of cellulose paper impregnated with mineral oil or natural ester with respect to hot spot temperature. The relative ageing rate V = 1 occurs at 98°C for a standard mineral oil /paper system, but this temperature is increased to 114/116°C for a natural ester/paper system. This effect could allow a natural ester filled transformer to run at a higher overload for the same loss of life, or alternatively, the higher hot spot temperature observed for ester-filled naturally cooled transformers could be accepted without extra ageing. Thus, it would be worthwhile studying new applications for conventional mineral oil design transformers immersed in natural ester. There are possibilities of using high temperature cellulose paper impregnated with natural ester to allow the design of transformers withstanding overload without increased life consumption or to allow a more compact design of the transformer ,or more specific applications such as wind farms, converters and transformers installed in sensitive environments. Hot spot (°C) Mineral oil Natural ester 98 104 110 114 116 128 140 1 2 4 6.35 8 32 128 0.16 0.32 0.63 1 1.26 5 20 Table 2: Relative ageing rate due to hot spot temperature Manufacturing issues Although the electrical design issues were critical to the success of the transformer design, it was equally important that the transformer could be manufactured, processed and tested satisfactorily. A major issue was the effect of higher viscosity on the processing time. High viscosity of the insulation fluid increases the time to fully impregnate the solid insulation, or even prevent full impregnation. This is particularly relevant for thick laminated insulation components. Tests showed that by increasing the temperature of the fluid during the impregnation process and by increasing the standing time before voltage application, satisfactory impregnation and air release were achieved. Careful control of component sizes and thickness allowed this process to be optimised. To maintain control of the processing operation it was necessary to redefine the procedural instructions and processing acceptance criteria. This included impregnation temperature, standing time, moisture content and gas content. Another important issue was the characteristic of a natural ester to “gel” with exposure to air (especially if hot). Although this effect was not instantaneous and was found not to have a deleterious effect on the dielectric withstand capability, if the “gel” reduced the size of oil ducts, then the cooling could be put at risk. For this reason, the “breathing” system of the transformers and reactors was effectively of a sealed type, using a nitrile rubber bag to isolate the ester from the atmosphere during operation. It was also necessary to ensure that any exposure after impregnation (such as for repack and making connections) was eliminated or reduced to an absolute minimum. Future research should investigate this "gel" effect on the dielectric withstand capability, as well as on the thermal implications, as it is an important issue for both manufacturing and operational maintenance. All components used for the transformer needed to be confirmed for use with ester fluids with respect to both technical and chemical issues (including health and safety). As with any factory involved in the use of different insulation fluids, it is necessary to isolate the plant used to fill and process the ester fluid to avoid cross-contamination of factory bulk mineral oil supplies. Also, pipe work used for filling and processing can become “contaminated” by gelled fluid if not properly cleaned after use. For this ester-filled transformer contract, an outside contractor with specialist equipment was used. Monitoring issues Physical characteristics of mineral oil are universally used to check the condition of oil-immersed transformers. Such diagnostic interpretation for ester fluids cannot use the same criteria as is presently available for mineral oil. Esters have a higher dissipation factor, which will mean that values of tan delta will generally be higher for an ester-filled transformer of comparable condition (figure 4). For mineral oil, a higher value of tan delta generally indicates ageing of the transformer. The existing acceptance criteria will need to be re-assessed. A similar problem exists for volume resistivity, as insulation resistance levels will be generally lower for ester-filled transformers. Dissolved Gas Analysis (DGA) criteria will need to be reassessed, as different amounts of gases are evolved and the ratios of gases may not be the same. Initial investigations suggest that present gas ratios may be used to show major faults, but time will be needed to obtain a sufficient library of data to be able to match the information presently available for mineral oil. Esters effectively contain more moisture than mineral oil when measured in ppm (relative humidity values are similar). This means that different acceptance criteria are required compared to mineral oil. Also the voltage breakdown acceptance criterion needs to be reviewed. THERMAL ISSUES TRANSFORMERS IN DISTRIBUTION Description of Temperature rise tests For transformers, the top oil and mean winding temperature rises are guaranteed values. The hot spot temperature rise is usually estimated for distribution and power transformers in accordance with the overload guide IEC 60076-7, and is often confirmed for large power transformer by direct measurement using optical fibre sensors. The winding hot spot temperature allows the evaluation of relative ageing and transformer life, and is based on the power loading and its duration, as well as ambient and cooling conditions. Life time expectancy of the transformer is generally considered to be 20 to 30 years for normal operating conditions at constant load and ambient temperature. In order to evaluate the ageing for distribution transformers, the following values are measured during the temperature rise tests: -Top oil temperature (ΘO) under cover with thermometer, -Mean winding temperature (ΘW ) by measurement of winding resistance variation during temperature rise test, - Ambient temperature (ΘA), By using the recommendations of the loading guide (IEC 60076-7), the data presented in Table 2 can be calculated. Meaning Top oil temperature rise Mean winding temperature rise Mean oil temperature Mean winding/ oil gradient Top winding hot spot gradient Hot spot winding temperature Hot spot winding temperature rise Symbol ∆ΘOr ∆ΘW calculation ΘO - ΘA ΘW - ΘA ∆ΘOM gr Θhr 0.8 x ∆ΘOr ∆ΘW – ∆ΘOM H x gr with H = 1.1 ΘA + ∆Θhr ∆Θhr ∆ΘOr + ∆Θhgr ∆Θhgr Table 3: Symbols for temperature rise Temperature rise tests comparing vegetable oil and mineral oil Table 4 shows a comparison between temperature rise tests for different fluid systems. Symbol guarantee TR1 TR2 TR3 ∆ΘOr 60 K 50 51.1 52.8 ∆ΘW HT 65 K 56.2 57.7 59.3 Gr HT 16.2 16.8 17.1 ∆Θhr HT 67.8 69.6 71.6 ∆ΘW BT 65 K 59.5 60.7 62.3 Gr BT 19.5 19.8 20.1 ∆Θhr BT 71.4 72.9 74.9 Table 4: Comparative measurements for temperature rise tests The transformers used for the temperature rise tests were manufactured according to the same technical specification, except that TR1 and TR2 were filled with mineral oil and TR3 was filled with vegetable oil. Temperature rise tests carried out with a natural ester did not show significant variations in value compared to the ones for identical transformers filled with mineral oil However, they do show the same effect of the increased viscosity of the ester as seen for the power transformers, with the top oil and hot spot rises being about 2-3 degrees higher than for mineral oil. Although these differences are small and could be interpreted as measurement uncertainty, they do confirm that the top to bottom oil rise will be greater for the esters than for mineral oil because of their increased viscosity. It is interesting to note that the natural ester used in these tests was a mixture of mono and tri-ester, which had the lowest viscosity of the three vegetable oils tested in the laboratory (see figure 2). Hence a smaller difference would be expected than with the other types of vegetable oil, such as used in the power transformer and shunt reactor. When compared to power transformers, the effect of using vegetable oil in distribution transformers is generally less significant, because the top to bottom temperature rise of the fluid is usually smaller. Thus there is normally no need to modify the design of the cooling system. Synthetic esters have been used as the liquid cooling medium of distribution transformers successfully for 25 years. The characteristics of thermal conductivity, kinematic viscosity and specific heat are very similar for both natural and synthetic esters. Thus, with respect to the design of the distribution transformers cooling system, natural esters have similar thermal cooling effects to the synthetic esters that have a proven cooling capability for distribution transformers. With respect to cooling properties for a transformer, silicone oil has an inferior thermal performance to the ester oils. This fluid type has also been used for a long period of time as the dielectric liquid and cooling medium for distribution transformers, but requires some specific modifications to the active part and the tank cooling systems. Due to its higher kinematic viscosity at normal transformer temperature condition work (see Figure 2), the number and thickness of cooling ducts would need to be increased and the cooling fins or radiators of the tank would need to be increased by up to 20%. CONCLUSIONS Laboratory investigations show that: o Ester oils are similar to mineral oil with respect to the dielectric strength. o Ester oils are seen to be more viscous than mineral oil for a given temperature and are thus less efficient for the transfer of heat by convection. Different types of vegetable oil have different levels of viscosity effect. o Ester oils are less stable with respect to oxidation than mineral oil. However, depending on the type and the presence or lack of inhibitor, the oxidation behaviour of vegetable oils can be very different. The understanding of chemical, physical, dielectric and thermal properties of these fluids allows transformers to be designed to encompass the differences compared to using mineral oils, such that dielectric tests are not an issue. Thermal tests have shown that for naturally cooled power transformers, the top oil and hot spot temperature rises are higher due to the higher viscosity of esters. This effect can be mitigated if necessary with additional coolers or the use of forced directed cooling. Natural ester oils can increase the thermal stability of paper, as they remove moisture from solid insulation more effectively than mineral oil, thus allowing either higher hot spot temperatures, or increased component life. Processing and factory issues have been successfully addressed and useful experience has been gained to enable larger and higher voltage power transformers to be designed and built in the future, although monitoring issues need to be investigated in order to set the reference criteria for ester oils. Finally, temperature rise tests have shown that the use of natural esters for distribution transformers is not a problem for the cooling system. This has been confirmed by the successful use of synthetic ester for this type of transformer for the last 25 years. REFERENCES [1] Y. Bertrand, L. C. Hoang, 2004, “Vegetable oils as substitute for mineral insulating oils in medium voltage equipment”, Proceedings CIGRE conference, Paris, No. D1-202 [2] J. Perret, M. Paris, 1987, “Silicone oils for transformers (in french)”, E.D.F. Bulletin des études et recherches – Série B, Réseaux électriques, matériels électriques, No. 2, pp. 5-13 [3] C. Perrier, A. Beroual, J-L. Bessede, 2006, “Improvement of power transformers by using mixtures of mineral oil with synthetic esters”, IEEE TDEI, No. 3, Vol. 13, pp. 556-564 [4] V. Prabhashankar, D. J. Badkas, 1961, “Mechanism of oxidation of transformers oils”, J. of Inst. of Petr., No. 450, Vol. 47, pp. 201-211 [5] M. Duval, J. P. Crine, 1985, “Dielectric behaviour and stabilization of insulating oils in EHV current transformers”, IEEE Trans. Elec. Insul., No. 2, Vol. EI-20, pp.437-441 . [6] C.P McShane, K.J Rapp , J.L Corkran , G.A Gauger , J Luksich , 2002, “Aging of Kraft paper in natural ester th dielectric fluid”, proceeding of 14 international conference on dielectric liquids, Austria [7] R Berti, F Barberis, 2007, “Experimental characterization of ester based oils for the transformer insulation“, CIRED th 19 international conference on electricity distribution paper 0555, Austria