6 IEEE Electrical Insulation Magazine

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Comprehensive data covering the first two years of service of a vege-table-oil-filled transformer are pre-sented. The characteristics that may be expected during normal operation are summarized.

Further Studies of a Vegetable-Oil-Filled Power TransformerKey words: transformer, oil insulation, dissolved gas analysis, natural ester

IntroductionSince its introduction in the late 1990s, the use of vegetable

oil as a transformer dielectric has become more common in the power industry. Some of the advantages of using vegetable oil rather than conventional mineral oil are that vegetable oil is non-toxic and more biodegradable and has a fire point that is approx-imately 200°C higher. Much research on the use of such fluids has been performed over the years, focusing on topics such as dielectric strength [1]–[3], aging [4], and dissolved gas content [5]–[8]. The next step is to assist the users of vegetable-oil-filled transformers in monitoring the condition of the transformers and to recognize oil test results that can be considered normal, although they differ from the corresponding results for mineral-oil dielectrics. In this article the test results for a new normally operating power transformer, covering the first two years of its service life, are presented. It is hoped that these results will as-sist power industry personnel in recognizing the characteristics of normal operation of vegetable-oil-filled transformers. Prelim-inary results were published in our previous article [9].

Two TransformersTwo vegetable-oil-filled transformers of the same design,

shown in Figure 1, were manufactured in 2008. They were in-tended as direct replacements for two of four existing mineral-oil-filled transformers, operating in pairs, in a substation in the center of Sydney, Australia. However, only one was installed, and the other was placed in storage. They are three-phase units with two secondary low-voltage windings per phase. A sum-mary of their design rating is given in Table 1.

The installed transformer was exceptional in that it was fit-ted with a comprehensive online monitoring system, which re-corded, at one-minute intervals, water content of the oil; cooling system status; and the temperatures of windings, oil, ambient air, and cooling water. The dissolved gas content of the oil was recorded every four hours. We began monitoring the transformer in the factory during manufacture and testing, and were con-fident that it was operating normally because it passed all ac-ceptance tests.

Both transformers used a soybean-based oil, Envirotemp FR3 (Cooper Power Systems, Waukesha, WI), which is more viscous than mineral oil, and were designed bearing this differ-

ence in mind. The temperature rise limits were the same as those given in IEC 60076 for a mineral-oil-filled transformer [10]. The transformer was water cooled, using two heat exchangers to increase the overall reliability of the cooling system. During normal operation only one of these heat exchangers was active at any given time. Water pumped in through the left-hand heat exchanger flowed to the right-hand heat exchanger (Figure 1), with each heat exchanger functioning only when its oil pump was active. The heat exchanger ratings were such as to ensure that adequate cooling would be provided, even if the exchangers were to be coated with salts deposited by water over many years.

Daniel Martin, Nick Lelekakis, and Wenyu GuoCentre for Power Transformer Monitoring, Diagnostics and Life Management (the transformerLIFE Centre), Department of Electrical and Computer Systems Engineering, Monash University, Clayton, VIC 3800, Australia

Yuriy OdarenkoWilson Transformer Company, Glen Waverley, VIC 3150, Australia

September/October — Vol. 27, No. 5 7

The transformers were fitted with a comprehensive monitor-ing system, which, as stated above, recorded data every minute from the various sensors mounted around the transformer, i.e., load current, winding temperatures measured by eight fiber-op-tic probes, oil temperature and wetness measured by four Vaisala probes, oil temperature and cooling water temperature recorded by resistance temperature detectors, and ambient temperature and relative humidity recorded by a Vaisala probe within the substation. The advantage of such a system is that the regular recording of data allows trends and relationships between the different properties to be observed.

The four Vaisala probes measuring the wetness of the oil were inserted into the top and bottom of the transformer tank, and top and bottom of one of the heat exchangers. They operate

by measuring the water activity of a thin strip of polymer [11]. Water activity is defined as the ratio of the partial pressure of water vapor to the partial pressure of water vapor above pure water at the same temperature [12]. Accurate measurement re-quires that the polymer strip be in thermodynamic equilibrium with the surrounding fluid. Despite the fact that the temperature within a transformer is usually changing continuously, due to the fluctuating load, thermodynamic equilibrium between the probe and the oil will be reached provided the time required for the probe strip to equilibrate with the oil is short compared with the time within which the transformer internal conditions change significantly.

To determine how quickly the Vaisala probes responded to a change in fluid wetness, a probe was suspended in air and al-lowed to equilibrate with the ambient humidity and temperature. It was then inserted in a sample of FR3 fluid, with a water con-centration of 40 ppm at ambient temperature, and its output was observed. The fluid was stirred by a mechanical bar rotating at 352 rpm. To calculate the concentration of water dissolved in the fluid as a result of the probe measurement, the fluid solubil-ity at the relevant temperature must be known. Equation (1) was used to calculate solubility using the coefficients published by Lewand, i.e., A = 5.3318 and B = 684 [13]. T is the temperature in °C.

Solubility( )TA BT=−+

10 273 (1)

When the probe was inserted in the fluid, its output reading fell rapidly for a short time and then reached an almost constant value after approximately 100 minutes, as shown in Figure 2. It follows that there will be a time lag between a change in water content of the fluid within the transformer and the final response of the probe to this change. Such time lags must be taken into account when analyzing temperature and moisture dynamics. However, under normal operating conditions the water content of the fluid in the installed transformer would not be expected to vary significantly over a 100-minute period.

Table 1. Transformer Ratings.

Parameter Value

Year of manufacture 2008

Date first energized 12 October 2008

Cooling class KDWF

Rated power 50 MVA

Rated voltage and frequency 132/11/11 kV, 50 Hz

Rated current 218.7 A (HV winding)

2 × 1,312.2 A (LV windings)

Maximum temperature rise Top oil 60°C

Average winding 70°C

Insulation level HV lightning impulse 650 kV

HV ac 275 kV

LV lightning impulse 95 kV

LV ac 20 kV

Figure 1. Vegetable-oil-filled transformer and external equipment. DGA = dissolved gas analysis.

8 IEEE Electrical Insulation Magazine

The dissolved gas content of the fluid was measured and re-corded every four hours by the online automated system, using a gas chromatograph. Fluid was pumped from the transformer tank through the online system and returned to the tank. The dissolved gases were extracted from the fluid with the aid of he-lium carrier gas. The dissolved gas content of the fluid was also measured periodically by independent laboratories, as a check on the reliability of the automated system. A detailed analysis of our preliminary findings on gassing trends was given in [9]. Elevated ethane and hydrogen concentrations were found, as predicted by various researchers on the basis of laboratory tests [5], [8], [13], [14].

Transformer Condition DataThe temperatures of the oil and the paper insulation were

continually monitored, because such data could be useful in in-terpreting any observed changes in the condition of the insu-lation over long periods, or a phenomenon such as unusually heavy gassing. The highest temperatures recorded by the array of fiber-optic probes within the transformer are shown in Figure 3; they do not suggest overheating of paper or oil.

The water content of the oil was calculated from the output of one of the Vaisala probes. At room temperature the solubilities of water in mineral oil and in vegetable oil are approximately 50 and 1,100 ppm, respectively, so that a higher water content is to be expected in a transformer using vegetable oil. The water concentrations calculated from the Vaisala probe data (Figure 4) agree fairly well with the Karl Fischer titration measurements shown in Table 2. On energization of the transformer the water content of the oil was 30 ppm. It fell to 22 ppm after 5 months of operation, and the same value was observed after 25 months. The difference between oil water content measured by Karl Fischer titration, and that calculated using the Vaisala probe, may be due to absorption of water by the oil sample in transit from the transformer to the laboratory.

The insulation at the top of an energized transformer is usu-ally hotter than the insulation at the bottom, and therefore water released from the warmer cellulose at the top will tend to be adsorbed on the cooler cellulose at the bottom. Over time this

adsorbed water will slowly migrate toward the center of the cel-lulose. Du [15] calculated the time taken for water to diffuse through 1-mm-thick pressboard impregnated with mineral oil as 333 hours (≈14 days) at 20°C. Because much thicker blocks of cellulose had been used in the construction of the transformer, the corresponding diffusion times would be much longer. Con-sequently, the water content of the oil may change slowly over a long period, as water is exchanged between the center of the thick cellulose block and the bulk oil.

Toward the end of its useful life mineral oil forms a sludge, whereas vegetable oil becomes more viscous. The thermal per-formance of a fluid is related to its viscosity [16], [17]. It may therefore be possible to assess the condition of the vegetable oil by monitoring the temperatures within the transformer and cor-relating them with the load, i.e., when the viscosity changes the temperature distribution may change sufficiently to be detected. If the temperatures deviate from those expected, the usual oxida-tion tests, which may include measurement of oxygen inhibitor content, dielectric dissipation factor, and acidity, should be car-ried out.

Figure 2. The output of the Vaisala probe measuring the wetness of the oil. The oil (in a beaker) was stirred by a magnetic stirrer rotating at 352 rpm.

Figure 3. The highest temperatures within the transformer as measured by the fiber-optic probe array.

Figure 4. The concentration of water dissolved in the oil at the top of the transformer tank, calculated from Vaisala probe data.

September/October — Vol. 27, No. 5 9

Fiber-optic probes were inserted into different sections of the windings to locate the hottest points within the transformer. Ex-cessive heat and oxygen can together degrade oil and cellulose. Consequently, it is necessary to ensure that the cooling of the transformer is adequate and the number of hot spots is mini-mized.

Analysis of Dissolved Gas Concentrations to Determine Transformer Condition

Oil FR3 is known to produce higher levels of ethane (C2H6) and hydrogen than mineral oil under nonfault conditions [5], [7], [8], [13]. Some of the components of soybean vegetable oil, such as linolenic acid, generate ethane by reacting with oxy-gen [18]. A catalyst, e.g., copper, is required for the ethane-pro-ducing reaction to proceed. The reaction noted by Schaich [18] may be the source of ethane production within the transformer. Atanasova-Hoehlein et al. [19] suggest that ethane is generated by the lipid peroxidation mechanism, which can occur in all omega-3 unsaturated fatty acids. They also suggest that ethane can be considered as the main gas involved in thermal-oxidative degradation of vegetable oils.

Mineral oil generates less ethane than does vegetable oil be-cause of differences in hydrocarbon molecular structure, i.e., ring structures in mineral oil but straight chains in vegetable oil triglycerides. Consequently, ethane generation within a trans-former may be related to the proportion of linolenic acid form-ing the triglyceride, the temperature, the availability of oxygen, and the copper surface area exposed to the oil.

A common measure of gas solubility is the Ostwald coef-ficient, which is the concentration of gas dissolved in the oil divided by the concentration of free gas in the headspace of a vessel, such as a sampling syringe [20]. Thus the concentration of a gas dissolved in the oil can be calculated from a measure-ment of the concentration of the same gas in the headspace of the syringe. The Ostwald solubility coefficients for various gases in FR3 and in mineral oil are given in Table 3 [21], [22].

The levels of dissolved gas in the transformer oil were moni-tored for nearly two years. The ethane level increased around the time of energization, plateaued at approximately 120 ppm, and remained at that level for nearly two years (Figure 5). It would

appear that the ethane-generating reactions slowed down and possibly ceased. The concentration of hydrogen fell, perhaps be-cause hydrogen was consumed in further reactions. The online dissolved gas analysis measurements agreed satisfactorily with the independent laboratory measurements (Table 4).

The dissolved gas content of the oil in the stored transformer was measured (Table 5) to compare its dissolved gas analysis signature with that of the operating transformer. No ethane was detected during factory acceptance tests conducted in July/Au-gust 2008. However, two years later ethane was found in concen-trations comparable with those in the operating transformer. The second transformer was energized only during carefully con-trolled factory testing two years prior to the sampling; therefore, it would appear that ethane can be produced in the absence of a fault, in agreement with Duval’s observations of stray gassing [7], [8]. High ambient temperatures may have been responsible.

Using Water Content to Verify Absence of Vegetable-Oil Biodegradation in Transformer Tank

Table 2. Properties of Vegetable Oil.

Test

In-service transformer Stored transformer

On energization of transformer

After 5 months of operation

After 25 months of operation

After 26 months of operation

Breakdown voltage (kV)75; IEC 60156; VDE 0370 electrodes

68.2; IEC 60156; VDE 0370 electrodes

67.2; IEC 60156; VDE 0370 electrodes

—

Moisture (ppm) Karl Fisher titration 30; ASTM D1533 22; IEC 60814 22; IEC 60814 7; IEC 60814

Acidity (mg of KOH/g of oil) 0.03; ASTM D974 0.03; AS 1767.2.1 0.03; AS 1767.1 0.03; ASTM D974

Interfacial tension (mN/m) 21.6; ASTM D971 21.6; ASTM D971 21.6; ASTM D971 24.5; ASTM D971

Dielectric dissipation factor0.00273 at 25°C; 50Hz; IEC 60247

0.0042 at 30°C; 0.0358 at 90°C; 50Hz; IEC 60247

0.0503 at 90°C; 50Hz; IEC 60247

—

Table 3. Ostwald Solubility Coefficients for FR3 and Mineral Oil.

Gas

FR3 Mineral oil

25°C [19] 70°C [20] 25°C [19] 70°C [20]

H2 0.05 0.097 0.05 0.092

O2 0.15 0.255 0.17 0.208

N2 0.07 0.141 0.09 0.127

CH4 0.30 0.387 0.43 0.432

CO 0.09 0.148 0.12 0.143

CO2 1.33 1.187 1.08 0.921

C2H2 1.63 1.763 1.20 0.992

C2H4 1.19 1.389 1.70 1.419

C2H6 1.45 1.677 2.40 2.022

10 IEEE Electrical Insulation Magazine

A major advantage of using a vegetable oil is that, if a leak occurs, the oil will be consumed by microorganisms. The manu-facturers of FR3 noted some speculation that natural ester insu-lation fluid may support microbiological growth in transformers; however, their eight-year study did not produce any supporting evidence [23].

The food industry has carried out much research on spoilage prevention [22]–[26]. One method is to limit access to water, thus preventing the growth of microorganisms. The term “water activity” was first used by the food industry to determine the ef-fect of the water content of a food on its spoilage [25] and is now used in connection with loss or gain of water by a food in a given environment [26]. It is a ratio, based on water vapor pressure,

and covers the range 0 to 1, where 0 = dry and 1 = saturation. The minimum water activity levels required to sustain various organisms are given in Table 6 [24].

It is assumed in the water activity approach that the system is in thermodynamic equilibrium, contrary to the usual situa-tion in transformers. However, it may be reasonable to assume that, provided the ratio (instantaneous water vapor pressure/maximum water vapor pressure at the same temperature) is kept below the relevant water activity, organisms will not survive within the transformer tank. The solubility of vegetable oil in water is around 1,000 ppm at room temperature and increases with increasing temperature. The standard ASTM D6871 Stan-dard Specification for Natural Ester Fluids Used in Electrical Apparatus [27] specifies a maximum oil water content of 200 ppm (the breakdown voltage of FR3 falls at around 300 ppm [1]). Thus, if the oil water content is kept below the level speci-fied by the ASTM standard, the oil would be expected to be too dry for microorganisms to survive within the transformer tank and degrade the oil.

Using Changes in Dielectric Dissipation Factor to Monitor Oil Condition

Regular monitoring of the condition of an oil allows a vari-ety of problems to be detected and rectified before the overall operation of the transformer is affected. However, without full lifetime data it can be difficult to establish the significance of a given parameter value for the condition of a transformer. Few data are available for vegetable-oil-filled transformers.

The dielectric dissipation factor (DDF) of an oil is a function of its relative permittivity and conductivity, both of which are normally higher for a vegetable oil than for a mineral oil. It is ex-pected that the DDF and acidity of an oil (vegetable or mineral) will increase as the oil ages. Work is continuing to predict the likely effect on the insulation of a transformer of the compounds that cause its DDF to increase, e.g., acids [28].

Figure 5. Dissolved gas analysis results from the online monitor, showing gas levels recorded over two years following energiza-tion. The dashed lines indicate when the monitor was offline.

Table 4. Laboratory Measurements of Dissolved Gas in the Oil of the Operating Transformer.1

Gas (ppm)

11 March 2009 17 November 2010

Lab 1 Lab 3 Online May 2009 Lab 3 Online July 2010

H2 47 52 33 11 7

O2 321 448 330 846 368

N2 40,510 32,100 30,236 33,100 30,683

CH4 3 2.2 0 2.2 0

CO 77 60 55 53 55

CO2 313 200 361 308 479

C2H2 0 0 0 0 0

C2H4 2 1.5 0 0.9 0

C2H6 113 61 129 63 112

1The laboratory measurement in March 2009 is compared with the closest online measurement in May 2009, and the laboratory measurement in November 2010 is compared with the closest online measurement in July 2010.

September/October — Vol. 27, No. 5 11

The DDF of the FR3 in the two transformers was measured in our laboratory, at various temperatures between ambient and 90°C, following IEC 61620 [29], and in a commercial labora-tory, at ambient temperature and at 90°C, following IEC 60247 [30]. Figure 6 shows the changes in DDF over the first two years of operation. In Table 7 our measurements are compared with those made by a commercial laboratory. It can be seen that there is reasonable agreement between the two.

Some differences between the DDF values for the in-service transformer and the stored transformer can be seen in Figure 6, but they are small relative to the maximum value 0.005 sug-gested for new vegetable oil in the IEEE Guide for Acceptance and Maintenance of Natural Ester Fluids in Transformers [31]. In this standard the suggested DDF limit (0.005) is applicable only to the natural ester in new equipment; at the time of writing, insufficient data were available to allow specification of limits for service-aged oil. However, prompt investigation is recom-mended in the IEEE guide if the dissipation factor exceeds 0.03 at 25°C. Another standard, developed for synthetic organic es-ters, recommends a maximum value of 0.01 at ambient tempera-ture [32].

Although the DDF values for the two transformers differ, the differences are small compared with the maximum level proposed in the IEEE guide (0.005). An increasing DDF may indicate that chemical reactions, initiated by the reactions that created ethane, are occurring.

Conclusions and RecommendationsIn this article, data obtained from a transformer fitted with

an online monitoring system, and using FR3 vegetable oil, were presented. The concentration of water dissolved in the oil was initially around 15 ppm, increased to 25 ppm, and then decreased to 15 ppm over a period of two years. These levels are higher than the levels expected in mineral oil because of the higher hygroscopy of vegetable oil.

Ethane was found in a nominally identical transformer that had not been used for two years. This observation supports the hypothesis proposed in previous work that ethane can be gen-

Table 5. Laboratory Measurements of Dissolved Gas in the Oil of the Stored Transformer.

Gas (ppm)

Factory testing August 2008

17 November 2010

Online DGA record during heat run test

Lab 2

H2 12 37

O2 156 5,833

N2 4,211 58,742

CH4 0 <1

CO 10 32

CO2 65 146

C2H2 0 <1

C2H4 0 <1

C2H6 0 62

Table 6. Minimum Levels of Water Activity Required to Sustain Various Organisms [24].1

MicroorganismRange of water activity required

Bacteria 0.8–1.0

Yeasts 0.7–0.9

Molds 0.6–0.8

1Water activity is a ratio, based on water vapor pressure, covering the range 0 to 1, where 0 = dry and 1 = saturation.

Figure 6. Vegetable oil dielectric dissipation factors for the in-service transformer (Unit A) and the stored transformer (Unit B) over a two-year period. The relevant IEEE guide [29] recom-mends a maximum value of 0.03 at 25°C.

Table 7. Comparison of Dielectric Dissipation Factors Measured by the Authors and by a Commercial Laboratory.1

Authors; IEC 61620

Commercial laboratory; IEC 60247

On energization 0.002 at 25°C 0.00273 at 25°C

After 25 months of operation

0.065 at 90°C 0.0503 at 90°C

1The authors’ measurements were performed at several temperatures, and the data interpolated to the commercial laboratory measurement temperatures.

12 IEEE Electrical Insulation Magazine

erated in FR3 fluid even under no-fault conditions [6]. In the operating transformer the ethane concentration increased during the first month of energization and then decreased to a constant value.

The DDF did not vary significantly between oil samples taken from the operating transformer. In the IEEE Guide for Ac-ceptance and Maintenance of Natural Ester Fluids in Transform-ers, a maximum value of 0.005 at 25°C is suggested for unused vegetable oil in new equipment. The corresponding value in the operating transformer was around 0.003 at ambient temperature.

AcknowledgmentsThe authors thank Ausgrid (Sydney, Australia) for funding

this project and Wilson Transformer Company (Glen Waverley, Australia), Dynamic Ratings (Glen Waverley, Australia), and TJ|H2b Australia (Glen Waverley, Australia) for their in-kind support. They also thank Peter Cole and Matthew Gibson of Ausgrid and Robert Wilson and Ken Budin of Wilson Trans-former Company. Many technical discussions held over the years with John Luksich, Kevin Rapp, and Patrick McShane, of Cooper Power Systems (Waukesha, WI), were greatly appreci-ated. Finally the authors wish to thank Dr. Valery Davydov for his advice while he was working at Monash University.

References[1] D. Martin, “Evaluation of the dielectric capability of ester based oils for

power transformers,” Doctoral thesis, University of Manchester, UK, 2008.

[2] D. Martin and Z. D. Wang, “Statistical analysis of the ac breakdown volt-ages of ester based transformer oils,” IEEE Trans. Dielectr. Electr. Insul., vol. 15, no. 4, pp. 1044–1050, 2008.

[3] K. J. Rapp, J. Corkran, C. P. McShane, and T. A Prevost, “Lightning impulse testing of natural ester fluid gaps and insulation interfaces,” IEEE Trans. Dielectr. Electr. Insul., vol. 16, no. 6, pp. 1595–1603, 2009.

[4] D. Martin, Z. D. Wang, A. W. Darwin, and I. James, “A comparative study of the chemical stability of esters for use in large power transform-ers,” in IEEE Annual Report Conference on Electrical Insulation and Dielectric Phenomena, 2006, pp. 493–496.

[5] I. Khan, Z. D. 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, 2007.

[6] M. Duval, “The Duval Triangle for load tap changers, non-mineral oils and low temperature faults in transformers,” IEEE Electr. Insul. Mag., vol. 24, no. 6, pp. 22–29, 2008.

[7] M. Duval and R. Baldygam, “Stray gassing of FR3 oils in transformers in service,” presented at the 76th Doble International Client Conference, Boston, MA, 2009.

[8] M. Duval, “The Duval Triangle for LTCs, alternative fluids and other ap-plications,” presented at the 76th Doble International Client Conference, Boston, MA, 2009.

[9] D. Martin, N. Lelekakis, V. Davydov, and Y. Odarenko, “Preliminary re-sults for dissolved gas levels in a vegetable oil filled power transformer,” IEEE Electr. Insul. Mag., vol. 26, no. 5, pp. 41–48, 2010.

[10] Power Transformers, IEC 60076, 2000.[11] Vaisala, HMP228 Moisture and Temperature Transmitter for Oil User’s

Guide, Helsinki, Finland: Vaisala Oyj, 2002.[12] International Food Information Service, Dictionary of Food Science and

Technology, 2nd ed. Wiley-Blackwell, West Sussex, UK, 2009.[13] L. Lewand, “Laboratory evaluation of several synthetic and agricultural-

based dielectric liquids,” presented at the Doble International Client Conference, Boston, MA, 2001.

[14] Cooper Power Systems, Envirotemp FR3 Fluid Testing Guide, Waukesha, WI: Cooper Industries Inc., 2004.

[15] Y. Du, M. Zahn, B. C. Lesieutre, A. V. Mamishev, and S. R. Lindgren,

“Moisture equilibrium in transformer paper-oil systems,” IEEE Electr. Insul. Mag., vol. 15, pp. 11–20, Jan./Feb. 1999.

[16] J. Aubin and Y. Langhame, “Effect of oil viscosity on transformer loading capability at low ambient temperature,” IEEE Trans. Power Del., vol. 7, no. 2, pp. 516–524, Apr. 1992.

[17] O. Martynenko and P. Khramtsov, Free-Convection Heat Transfer with many Photographs of Flows and Heat Exchange, Berlin, Germany: Springer-Verlag, 2005.

[18] K. M. Schaich, “Lipid oxidation: Theoretical aspects,” in Bailey’s Indus-trial Oil and Fat Products, 6th ed., F. Shahidi, Ed. New York, NY: John Wiley and Sons Inc., 2005, pp. 269–355.

[19] I. Atanasova-Hoehlein, Th. Hammer, and M. Schaeffer, “Diagnostic markers for oxidation condition of mineral oil and ester insulating fluids,” Cigre Session, paper D1_231, Paris, France, 2010.

[20] R. Battino, “The Ostwald coefficient of gas solubility,” Fluid Phase Equi-libria, vol. 15, no. 3, pp. 231–240, 1984.

[21] J. Jalbert, R. Gilbert, P. Tétreault, and M. A. El Khakani, “Matrix effects affecting the indirect calibration of the static headspace-gas chromato-graphic method used for dissolved gas analysis in dielectric liquids,” Analytical Chem., vol. 75, no. 19, pp. 5230–5239, 2003.

[22] IEEE Guide for the Interpretation of Gases Generated in Oil-Immersed Transformers, IEEE Std. C57.104-1991, 1991.

[23] Cooper Power Systems Field Analysis of Envirotemp FR3 Fluid Filled Transformers For Microbiological Growth, Certified Test Report, Cooper Power Systems, 2005.

[24] F. J. Francis, Wiley Encyclopedia of Food Science and Technology, 2nd ed., vol. 1–4. John Wiley & Sons, New York, NY, 1999.

[25] G. V. Barbosa-Cánovas, A. J. Fontana, S. J. Schmidt, and T. P. Labuza, Water Activity in Foods—Fundamentals and Applications. Blackwell Publishing Ltd, Oxford, UK, 2007.

[26] J. M. deMan, Principles of Food Chemistry, 3rd ed., New York, NY: Springer Science+Business Media, 1999.

[27] Standard Specification for Natural (Vegetable Oil) Ester Fluids Used in Electrical Apparatus, ASTM D6871 - 03, 2008.

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

[29] Insulating Liquids—Determination of the Dielectric Dissipation Factor by Measurement of the Conductance and Capacitance—Test Method, IEC 61620 Ed. 1.0, 1998.

[30] Insulating Liquids—Measurement of Relative Permittivity, Dielectric Dissipation Factor (tan δ) and d.c. Resistivity, IEC 60247 Ed 3.0, 2004.

[31] IEEE Guide for Acceptance and Maintenance of Natural Ester Fluids in Transformers, IEEE C57.147, 2008.

[32] Synthetic Organic Esters for Electrical Purposes—Guide for Maintenance of Transformer Esters in Equipment, IEC 61203, 1992.

Daniel Martin received the BEng degree in electrical and electronic engineering from the University of Brighton, UK, in 2000 (with study abroad in Germany). He then joined Racal Electronics, which be-came the international electronics compa-ny Thales, working on communication and aircraft systems. He left Thales in 2004 to pursue his PhD degree in electrical insula-

tion at the University of Manchester, UK. He investigated the possibility of using vegetable oils and synthetic esters as substi-tutes for mineral oil within large power transformers and gradu-ated in 2008. In his current appointment as a research fellow at Monash University he is project leader of studies investigating the suitability of using vegetable oils as transformer dielectrics. He provides technical expertise to industry on this topic.

September/October — Vol. 27, No. 5 13

Nick Lelekakis holds a bachelor of science with honors in chemistry from Monash University. He has worked at Monash on transformer-related projects since his grad-uation in 1995. He has 14 years of experi-ence in sampling, measuring, and monitor-ing gases dissolved in electrical insulating oil, using gas chromatography. He has compared vacuum extraction with head-space methods for dissolved gas analysis

and has made comparative tests with many other laboratories. He also has experience with online gas chromatograph instru-mentation.

Wenyu Guo received his PhD in computer science from the University of Manchester in 2007. He joined Monash University in 2007, initially engaged in computer vision research. He later transferred to the Centre for Power Transformer Monitoring, Diag-nostics and Life Management (the trans-formerLIFE Centre) to carry out research in the area of computational modeling. He

has also been involved in dynamic thermal modeling and insula-tion aging studies in power transformers.

Yuriy Odarenko graduated with an MEng degree in power engineering from Zaporizhzhya State Engineering Academy, Ukraine, in 2002. This program included participation in a re-search project at the Institute of Polymer Technology of the Uni-versity of Erlangen–Nuremberg, Erlangen, Germany. Between

2002 and 2008 he was a research fellow at the Thermal Laboratory of the Ukrainian Transformer Institute (VIT), Zaporizhzhya, investigating fluid dynamics and heat and mass transfer phenomena in transformers incorporating various types of insulation, e.g., dielectric liquid and SF6 gas. At VIT he also modeled the thermal performance of transformer windings as part of a PhD program, graduating in 2007. In 2008 he moved to Melbourne to work with the Cen-

tre for Power Transformer Monitoring, Diagnostics and Life Management within Monash University. Currently he is a de-sign development engineer with Wilson Transformer Company. He participates in the CIGRE working groups A2-24 Thermal Performances of Power Transformers and IEC TC-14 MT-06 Thermal Performance of Transformers.