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November/December — Vol. 26, No. 6 7

F E A T U R E A R T I C L E

This article compares the thermal aging of insulating paper in mineral oil with that of paper in vegetable oil over the temperature range of 70 to 190°C and shows that the use of vegetable oil as an insulating liquid is a very promising option.

0883-7554/07/$25/©2010IEEE

Vegetable Oils, an Alternative to Mineral Oil for Power Transformers—Experimental Study of Paper Aging in Vegetable Oil Versus Mineral OilKeys words: vegetable oil, mineral oil, Kraft paper, degree of polymerization, thermal aging, furanic compounds

IntroductionTransformers are essential components of electric power gen-

eration, transmission, and distribution, and the majority of power transformers rely on liquid dielectrics as an insulating medium and for heat transfer. At present, the most common liquid di-electrics used are the mineral oils, which are produced from the middle range of petroleum-derived distillates. In recent years, concerns have been expressed regarding the presence of poly-nuclear aromatic hydrocarbons in mineral oils, because in the event of a transformer fire, or explosion, polynuclear aromatic hydrocarbons could readily be released into the environment. This situation has put mineral-oil-filled transformers in sensi-tive areas, such as inside buildings, in residential areas, and near schools or hospitals, for example, under scrutiny. Human health and environmental concerns have led to efforts to develop alter-native transformer insulating oils for use in these sensitive areas. In particular, vegetable oils have drawn the most attention, and research on the properties and performance of vegetable oils has been carried out in a few laboratories [1].

The main components of vegetable oils are triacylglycerols (triacyl esters of glycerol, i.e., glycerol esterified by three fatty acids), and depending on the source of the oil, the fatty acid composition of triglycerides varies considerably. The fatty acids vary in the length of the hydrocarbon chains and on the degree of unsaturation, and these compositional differences can lead to significant changes in the physicochemical properties of the oils. In addition, the presence of saturated fatty acids leads to higher viscosities and higher freezing and pour points. Furthermore, the presence of multiple double bonds makes the oils more prone to oxidation, which leads to an increase of polarity and a decrease in dielectric strength, both of which adversely affect the proper-ties of the oils, making them unsuitable for dielectric applica-tions.

Vegetable oils with low polyunsaturation show good oxida-tion stability. They should be a good alternative to mineral oil for low-power and low-voltage closed (sealed) type distribution transformers [2], and presently, a few vegetable-oil-based insu-lating fluid formulations are commercially available.

To date, the majority of research on vegetable-oil-based di-electric liquids has been directed at the electrical properties, flow characteristics, and oxidation stability of these liquids, with little attention paid to the effect of these dielectric fluids on other components of the transformers, particularly the cellulosic insulating paper. Cellulosic paper plays a vital role in the opera-tion of transformers by insulating the conductor windings. The normal service life of large transformers is approximately 40 to 50 years, and how well a transformer performs over its service life is often dependent on the physical integrity of the cellulosic

Maria Augusta G. MartinsLabelec – Grupo EDP, Rua Cidade de Goa, n°4, 2685-039 Sacavém, Portugal

8 IEEE Electrical Insulation Magazine

insulating paper, which is in part dependent on the interaction between the insulating paper and the insulating oil.

Considerable literature exists on aging of cellulosic paper in insulating mineral oils; however, aging in the presence of natural-ester-based insulating liquids has not yet been studied sufficiently. The present study was undertaken to examine the aging of cellulosic insulating paper in vegetable insulating oil, comparing it with the aging of paper in mineral insulating oil.

Because natural esters are inhibited, the correct practice is to compare them with inhibited mineral oil; however, as this type of uninhibited mineral oil is widely used, it is very useful to have some data on this comparison. In addition, the second part of this research, to be published later, will provide a comparison of the aging of insulating paper in inhibited mineral oil and in natural esters.

Experiment

Oil and Paper Sample PreparationThe following materials were used in this study:

(a) Vegetable insulating oil: Biotemp (a registered product of ABB T&D), a vegetable-oil-based dielectric fluid,

(b) Mineral insulating oil: Oil Nynas Nytro 11 EN, an un-inhibited naphthenic oil, and

(c) Kraft paper (electrical grade): Normal electrical grade Kraft paper (ABB Sécheron, Switzerland).

Aliquots of mineral oil (MO) and vegetable Biotemp oil (Bio) were dehumidified under vacuum at 60°C, for 24 hours, prior to their use in the experiments. The oil samples were then saturated with clean (free of CO, CO

2, H

2O, and organics) dry

air at room temperature. The air-saturated mineral oil samples were fortified with di-tertiary-butyl-paracresol, a phenol anti-oxidant, at a concentration level of 0.3%. A copper passiva-tor, benzotriazole, was also added to the mineral oil, and its concentration was set at 25 mg·kg−1. Bio was used as supplied without additional fortification (only dried and air saturated, in the same way as for MO).

The paper was dried under vacuum, inside glass ampoules where the aging study was performed. Each ampoule was heated in an oven at 80°C until the moisture content of the paper was less than 0.5%.

Paper Aging in Insulating OilsThe aging experiments were carried out in 150-mL borosili-

cate glass ampoules with narrow necks. The ampoules were first washed with detergent, rinsed with water, rinsed with acetone to remove organic residues, dried overnight in an oven at 100°C, and then placed in desiccators to cool down to room tempera-ture. The ampoules were kept in desiccators until their use in the experiments. Two rectangular pieces (5 × 0.5 × 0.1 cm) of unvarnished copper conductors wrapped with three layers of Kraft paper were placed inside half of the ampoules, which were then filled with 100-mL aliquots of the oils. Ampoules without paper-wrapped copper conductors were also filled with 100-mL aliquots of the vegetable and mineral oils as controls.

After filling, the ampoules were sealed by a flame torch and transferred to a forced air oven set at 70°C and heated for 24 hours. After this time, a set of ampoules was removed and the temperature was then increased to 80°C for another 24 hours. After this time, a second set of ampoules was removed from the oven and the temperature was increased to 90°C for another 24 hours. This procedure was repeated at each temperature, reach-ing a final equilibration temperature for the last set of ampoules of 190°C [3]. Upon removal from the oven, the ampoules were placed in a darkened fume hood to cool down to room tempera-ture.

Aliquots of the aged oil samples were then analyzed for water content by the test method described in IEC 60814 [4] and for dissolved gases according to the test method described in IEC 60567 [5]. The furanic content of the oils was determined ac-cording to the method outlined in IEC 61198 [6], and the degree of polymerization (DP) of the paper was assessed by the method described in IEC 60450 [7].

Each ampoule was duplicated so all the analyses were per-formed twice. The difference between each duplicate was within the repeatability range of the test methods, so the average of both samples was used in the following graphs.

Results and Discussion

Water ContentThe water content is an important parameter of insulating oil

because it affects the dielectric properties of the oil as well as the integrity of the cellulosic insulating paper. It has been well established that the water content of insulating mineral oils in-creases with age. High water content can, over time, severely degrade the dielectric properties and lead to transformer failure.

In this article, “MO blank” and “Bio blank” refer to mineral oil and vegetable oil, respectively, sealed in ampoules without paper-wrapped copper inside; “MO” and “Bio” refer to mineral oil and vegetable oil, respectively, heated over the same tem-perature range (70–190°C) in the presence of insulating-paper-wrapped copper, as described in the previous section.

Plots showing the change in water content of MO, Bio, MO blank, and Bio blank, after aging at different temperatures, are shown in Figure 1. These plots show that the initial water con-tent of Bio, after drying, was markedly higher than that of MO (≈60 mg·kg−1 vs. ≈8 mg·kg−1), and the water content of both oils showed an increase with aging. This increase is related to the aging temperature and the type of oil. A significant increase in water content of Bio was observed after aging at 100°C, whereas the increase in water content of MO did not occur until around 135°C. The water content of Bio rose from an initial value of ~60 mg·kg−1 to 184 mg·kg−1 after equilibration at 190°C; how-ever, the overall increase in water content of MO was signifi-cantly lower.

On the other hand, the water content in paper aged inside the vegetable oil was much lower than that in paper aged inside mineral oil, confirming the results obtained in a previous experi-ment [8]. This is also in agreement with the results obtained by other researchers [9], [10] who have found that paper aged in natural ester fluids remains drier than paper aged in mineral oil


under the same experimental conditions. It is known that the hy-groscopicity of esters is greater than the hygroscopicity of min-eral oil, and this is due to the greater ability of the ester group (COOR) in the molecular chain structure of esters to participate in hydrogen bonding.

Dissolved OxygenThe change in the dissolved oxygen content of both mineral

and vegetable oils with temperature is shown in Figure 2. The results show that during aging, the dissolved oxygen content de-creased in both oils. The decrease started around 90°C in Bio and around 130°C in MO. The decrease in Bio at a lower tem-perature can be attributed to a higher susceptibility to oxidation

of the vegetable oil, relative to mineral oil, due to the unsatura-tion in the fatty acid chains.

It is also apparent that the consumption of oxygen, in the deg-radation of paper, is not very meaningful in Bio. This means that oxidation plays a minor role in the degradation of paper, as hydrolysis is the main cause of paper degradation.

Carbon Monoxide and Carbon DioxideConcentrations of dissolved carbon monoxide and carbon

dioxide in both oils increased with aging temperature. The in-crease in carbon monoxide concentration was more pronounced in mineral oil than in vegetable oil, which is in agreement with the results obtained by other researchers [11]. Carbon monoxide

Figure 1. Change in water content in mineral oil and in vegetable oil with paper, MO and Bio, respectively, and in mineral oil and vegetable oil without paper, MO Blank and Bio Blank, respectively, as a function of aging temperature.

Figure 2. Change in dissolved oxygen content in mineral oil and in vegetable oil with paper, MO and Bio, respectively, and in mineral oil and vegetable oil without paper, MO Blank and Bio Blank, respectively, as a function of aging temperature.


and carbon dioxide concentrations in both oils aged in the pres-ence of paper were much higher than those in oils without paper, indicating that thermal degradation of paper is largely respon-sible for the production of these gases.

Light Hydrocarbons and HydrogenThe oils were also analyzed for CH

4, C

2H

4, C

2H

6, and C

2H

2,

and the results showed that the methane concentrations were higher in MO than in Bio. The concentrations of ethylene were nearly the same in both oils.

However, ethane concentration was higher in Bio than in MO, which is in agreement with the findings of other researchers for natural esters [11]–[13]. At temperatures higher than 130°C, methane, ethane, and ethylene concentrations increased in both oils. It was also observed that the concentration of light hydro-carbons in MO increased in the presence of paper, whereas these gases were not significantly different in Bio.

Acetylene was also not detected in either oil, over the range 70 to 90°C, as indicated elsewhere [8]. Below 110°C, the hy-drogen concentration was very low (<5 μL·L−1) in MO; how-ever, in Bio, the hydrogen concentration was significantly higher (about 30 μL·L−1) at 70°C. Hydrogen concentrations in both oils increased significantly at temperatures above 110°C, and con-centrations as high as 200 μL·L−1 were detected in both oils at 180 to 190°C.

Furanic CompoundsBoth mineral and vegetable oils were analyzed by HPLC [6]

for five furanic compounds: 2-furfuraldehyde (2FAL), furfu-ril alcohol, 2-acetylfuran, 5-metil-furfuraldehyde, and 5-hy-droxymethyl-2-furfuraldehyde (5HMF). However, only 2FAL

and 5HMF were found above quantification levels, even in the oils aged at 190°C (Figure 3).

The presence of these compounds in oil is attributable to the degradation of cellulosic paper and the subsequent migration of these degradation products into the oil. The concentrations of these compounds are often used as a diagnostic tool for the physical state assessment of the insulating paper.

The aging rate of the insulating paper was also assessed by determining the change in the DP with time of aging. A number of mechanisms for the production of 2FAL and 5HMF during degradation of Kraft paper have been suggested in the literature. The most commonly cited mechanism involves levoglucosan as an intermediate [14]. The relative concentrations of 2FAL and 5HMF were found to be about 10:1. It was also observed that concentrations of 2FAL and 5HMF, in both oils, increased with aging temperature. However, 2FAL and 5HMF concentrations were approximately three to four times higher in MO than in Bio. The concentrations of both products were detected, only in the oil samples containing paper, when the samples were aged at 130°C or higher. Concentrations of 2FAL and 5HMF dissolved in oil increased with aging temperature beyond 130°C.

The concentration of 2FAL in mineral oil was nearly an or-der of magnitude higher (about 2 mg·kg−1) than the correspond-ing concentration in vegetable oil when both oils were aged at 180°C. In oils aged at 180°C, the concentrations of 5HMF showed a similar trend, i.e., 5HMF concentrations were found to be higher in mineral oil than in vegetable oil. The difference in concentrations was approximately three times, and as for 2FAL, this is attributable to a lower formation in vegetable oil of furan-ic compounds by hydrolysis of paper.

Figure 3. Concentrations of 2-furfuraldehyde (2FAL) and 5-hydroxymethyl-2-furfuraldehyde (5HMF) in mineral oil (MO) and vegetable oil (Bio), both aged with paper, as a function of aging temperature.


DP of PaperThe DP represents the average number of repeating units of

glucose in the cellulose polymer molecule. It also provides a measure of the mechanical strength of the paper. New (unused) Kraft paper has a DP of around 1,200. As the paper undergoes aging, the polymer chains degrade into smaller units, and the shorter length chains lower the average DP. Thus, this method allows one to assess the degradation of the paper. As the paper degrades, furanic compounds are formed.

However, the initial degradation of paper in oil consists of some scissions of weak bonds, without the production of 2FAL (lower than the detection limit), which is in agreement with the results shown in Figure 3 and also with the results of other re-searchers [15]. Measured DP values indicate that initial degra-dation of paper occurs at about 90°C in Biotemp and at around 110°C in mineral oil. Degradation of paper in vegetable oil is slightly higher than in mineral oil, although not significantly dif-ferent, at a temperature below 130°C. However, the opposite oc-curs above 130°C (Figure 4).

DP of Paper Versus 2FAL Concentration in OilAn inverse correlation between the concentration of 2FAL in

oil and the DP of Kraft paper was observed in both oils (Figure 5). Regression analysis of log

10 [2FAL] in mineral oil versus the

DP yielded a straight line with a regression coefficient of 0.994. A similar correlation of log

10 [2FAL] in vegetable oil versus DP

also yielded a straight line relationship but with a lower regres-sion coefficient of 0.847. The slower degradation of paper at a temperature above about 130°C in vegetable oil can be attributed to the fact that water solubility in Bio is higher than in MO.

The higher water solubility in Bio (as in other triglyceride-based insulating liquids) can prevent moisture build-up on paper, thus retarding hydrolytic degradation of cellulose. This is impor-tant because the most significant step in degradation of cellulose is the hydrolytic scission of glycosidic linkages through intramo-

lecular transglycolysation to acid hydrolysis. The scission leads to shorter chains, one of which has a short-lived glycosyl cation that results in the formation of levoglucosan, which serves as starting material for the formation of 2FAL and 5HMF.

The formation of these molecules is accompanied by the re-lease of water and formaldehyde [14]. Water released during the process, if not removed, can lead to further degradation of the paper, resulting in higher concentrations of furanic compounds in mineral oils [8], [16]. The present results are in agreement with the results reported by Rapp and coworkers [9], who have suggested that, at high temperatures, the main path for degrada-tion of triglycerides (the main components of vegetable oils) is hydrolysis, not oxidation.

The hydrolysis provides the fatty acids necessary for chemi-cal modification of the cellulose. The reactive OH groups of the cellulose molecule react with the fatty acid via transesterifica-tion. This reaction hinders cellulose degradation mechanisms using these sites. Because this reaction is expected to take place at faster rates at higher temperatures [9], the aging rate of pa-per in vegetable oil is lower than in mineral oil at temperatures higher than 130°C.

Moreover, water solubility (moisture saturation levels) for Bio and other natural-esters-based insulating oils is higher (about 20–40 times) than for mineral oil. For the same concentration of water in mineral oil and vegetable oil, the relative humidity (the ratio of water content in oil to the saturation level) is lower in vegetable oil than in mineral oil.

Finally, because the effects of water on the dielectric break-down voltage of insulating oils are inversely proportional to the relative humidity and not to the absolute water concentration in the oil, the breakdown voltage of vegetable oil is higher than the breakdown voltage of mineral oil for the same absolute water content. This is one more advantage of vegetable oil in compari-son with mineral oil.

Figure 4. Degree of polymerization (DP) of Kraft paper in mineral (MO) and vegetable (Bio) oils as a function of aging tempera-ture.


ConclusionsThe experimental results have shown that under the test con-

ditions described in this article, the vegetable-oil-based dielec-tric fluid Biotemp exhibited thermal stability similar to that of the oil Nynas Nytro 11 EN, an uninhibited naphthenic mineral oil, over the temperature range 70 to 190°C. Noticeable differ-ences were observed in the water content and the dissolved CO and CO

2 contents between the two oils. The water concentra-

tions were found to be higher in the aged vegetable oil than in the aged mineral oil; however, the CO and CO

2 concentrations

were higher in the aged mineral oil than in the vegetable oil.Below 130°C, the aging rate of paper in vegetable oil is high-

er than in mineral oil, which is consistent with the measured values of DP of paper being lower in the vegetable oil than in the mineral oil. However, above 130°C, the concentrations of 2FAL and 5HMF were found to be lower in vegetable oil than in mineral oil. Also, DP values were higher for paper dipped in vegetable oil, indicating that thermal degradation of Kraft paper occurs at a lower rate in vegetable oil than in mineral oil. These findings are in agreement with those of other researchers [9] and can have the following possible explanations:

1. Under aging, there is a movement of water produced by the degradation of paper into the vegetable oil, followed by hydrolysis of triglycerides of the vegetable oil with the production of long-chain fatty acids. In this reac-tion, there is a consumption of dissolved water, which is evident from the graph of water content versus tem-perature, in Figure 1. The decrease in the water content of the paper has a consequence of decreased degrada-

tion of the paper by thermo-hydrolytic degradation, as reported by several other researchers [9], [10].

2. The long-chain fatty acids, produced by hydrolysis, are transesterified by the reaction of the hydroxyl groups (OH) in the cellulose molecule, consequently prevent-ing the degradation of cellulose. This reaction hinders cellulose degradation mechanisms using these sites and occurs at a higher rate at higher temperatures [9]. The transesterification reaction was confirmed by Fourier transform infrared spectroscopy as the main reaction that takes place in paper immersed in vegetable oil, for example at 170°C [9].

Another possible explanation for the lower degradation of paper immersed in vegetable oil, at higher temperatures, is the deposition on paper of a gel-like substance produced by the deg-radation of vegetable oil, which shields the paper from further degradation.

The present results on the degradation of Kraft paper in veg-etable oil are quite promising for the application of these oils as dielectric fluids in transformers. However, some improvements are still needed, mainly in oxidation stability, in order for veg-etable oils to become suitable for use in free-breathing trans-formers.

AcknowledgmentsABB T&D, USA, and ABB Sécheron, Switzerland,

respectively, are acknowledged for kindly providing the vegetable-oil-based dielectric fluid Biotemp, and the Kraft paper.

Figure 5. The 2-furfuraldehyde (2FAL) concentrations in oil versus degree of polymerization (DP) of Kraft paper observed in mineral (MO) and vegetable (Bio) insulating oils.


References[1] Z. Wang, A. Darwin, and R. Martin, “New insulation fluids: Use of envi-

ronmentally friendly fluids in power transformers,” presented at CIGRÉ Symposium 29, Brügge, Belgium, 2007.

[2] T. V. Oommen and C. Clairbone, “Biodegradable insulations fluid from high oleic vegetable oils,” presented at CIGRÉ 15-302, Paris, France, 1998.

[3] B. Pahlavanpour, M. A. G. Martins, and A. De Pablo, “Experimental investigation into the thermal ageing of Kraft paper and mineral insulat-ing oil,” presented at the IEEE International Symposium on Electrical Insulation (ISEI), Boston, MA, 2002.

[4] IEC 60814 Standard - “Insulating liquids – Oil-impregnated paper and pressboard. Determination of water by automatic coulometric Karl Fis-cher titration”, Second edition, August 1997.

[5] Oil-Filled Electrical Equipment. Sampling of Gases and of Oil for Analy-sis of Free and Dissolved Gases—Guidance, IEC 60567 Standard, 3rd ed., June 2005.

[6] “Mineral Insulating Oils - Methods for the Determination of 2-Furfural and Related Compounds” - First edition, September 1993

[7] Measurement of the Average Viscometric Degree of Polymerization of New and Aged Cellulosic Electrically Insulating Materials, IEC 60450 Standard, 2nd ed., April 2004. Amendment 1 - May 2007.

[8] M. A. G. Martins, “É o óleo vegetal, uma alternativa ao óleo mineral para uso em transformadores? Estudo da degradação térmica do sistema óleo vegetal/papel Kraft, versus óleo mineral/papel Kraft,” presented at XII ERIAC (Encontro Regional Ibero-Americano da CIGRÉ, Foz do Iguaçu-Pr, Brazil, 2007.

[9] K. J. Rapp, C. P. Mcshane, and J. Luksich, “Interaction mechanisms of natural ester dielectric fluid and Kraft paper,” in Proceedings of IEEE International Conference on Dielectric Liquids, 2005, pp 393–396.

[10] Y. Bertrand and D. Laurichesse, “Comparison of the oxidation stabilities of vegetable based and mineral insulating oils,” presented at MatPost, Lyon, France, 2007.

[11] I.-U-Khan, Z. Wang, I. Cotton, and S. Northcote, “Dissolved gas analysis of alternative fluids for power transformers,” IEEE Electr. Insul. Mag., vol. 23, no. 5, pp. 5–14, Sept./Oct. 2007.

[12] I. Atanasova Höhlein, C. Rehorek, and T. Hammer, “Gassing and oxida-tion behaviour of insulating fluids under thermal stress,” presented at CIGRÉ, 6th Southern Africa Regional Conference, Paper C107, 2009.

[13] I. Atanasova Höhlein, “Gassing and oxidation behaviour of insulating fluids under thermal stress—Performance of conventional and new mate-rials for high voltage apparatus,” presented at CIGRÉ SC D1 Colloquium, Hungary, Budapest, 2009.

[14] M. A. G. Martins, “Furfuraldeído – um indicador prático da degradação térmica do papel Kraft de transformadores,” Ciência e Tecnologia de Materiais, vol. 19, no. 1/2, pp. 25–33, 2007.

[15] M. Mulej, A. Varl, and M. Končan-Gradnik, “Up-to-date experience on furans for transformer diagnostics,” Internal report, 2004.

[16] M. A. G. Martins, “Será o óleo vegetal um possível substituto do óleo mineral para transformadores? Comparação da degradação térmica do sistema óleo vegetal/papel Kraft com a do óleo mineral/papel Kraft,” Ciência e Tecnologia dos Materiais, vol. 29, no. 3/4, pp 15–20, 2008.

M. Augusta G. Martins received her de-gree in chemical engineering from the Uni-versity of Lisbon in 1975. She lectured and was a researcher at the University of Lis-bon from 1975 to 1983. In 1978 she joined EDP, and today she is the head of the In-sulating Materials Department of Labelec–EDP Group, where she is responsible for the supervision of all transformers for the

Portuguese electrical system. She is a member of several IEC Technical Committees and CIGRÉ Working Groups, the Portu-guese representative in CIGRÉ Study Committee D1, secretary of the Portuguese IEC TC 10 mirror Committee, and member of IEC TC 15 and IEC TC 112 Portuguese mirror Committees. She has published several scientific papers and technical reports in journals, has presented seminars, and was the 2006 IEC 1906 Award winner.

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