Dissolved Gas Analysis of Alternative Fluids for Power Transformers

Transcription

1 F E A T U R E A R T I C L E Dissolved Gas Analysis of Alternative Fluids for Power Transformers Key Words: oil/paper insulation, DGA, esters, overheating, low and cold corona-type discharges Introduction onventionally, the insulation system of power transformers consists of mineral oil, cellulose paper, and pressboard. C In recent years, there has been an increase in the use of environmentally-friendly fluids such as synthetic esters and natural esters in place of mineral oil. This has been particularly prevalent at distribution voltage levels [1], but it now also is taking place at transmission voltage levels [2]. The main driver for the use of alternative fluids from the electrical supply utility perspective is the environmental benefits that result from their use with no compromise on safety or reliability [3]. Mineral oil is a mixture of hydrocarbons and is refined from crude oil. Mineral oil has a low biodegradability (20% of mineral oil will biodegrade within 28 days) resulting in the need to construct bund walls around large transformers preventing escape into the environment should a leak occur. In the case of an oil leak, significant financial penalties would be imposed by environmental enforcement agencies. In contrast, esters are very biodegradable (95% or more of esters will be biodegraded within 28 days [4],[5]), and they conform to the readily biodegradable definition according to the OECD 301 series of tests [6],[7]. In addition to their better environmental performance, esters have higher flash and fire points than mineral oil. This is desirable from a fire safety perspective, particularly for a transformer operating underground or offshore. Esters have been used in distribution transformers for several decades without fires being reported [2]. Esters also are far more hygroscopic than mineral oil as the ester group (COOR) in the molecular chain structure has a higher ability to participate in hydrogen bonding. The high moisture saturation level means that, for the same moisture content expressed in absolute parts per million, esters will have a lower relative humidity in comparison with mineral oil. This means that moisture has less of an impact on the dielectric strength of esters than mineral oil. When esters are used in conjunction with cellulose paper and pressboard, cellulose is kept in a drier Imad-U-Khan, Zhongdong Wang, and Ian Cotton Electrical Energy and Power System, University of Manchester, Manchester M60 1QD UK Susan Northcote TJ/H2b Analytical Services Ltd., Chester CH1 6ES UK Ester-based transformer fluids have the same DGA fingerprints as mineral oil. However, with lower volumes of gas produced, they will demand more precise dissolved gas measurements and modified or new ratio criteria to allow fault detection and diagnosis. condition and the rate of cellulose degradation consequently is slower than in mineral oil [8]. This article examines the impact of alternative fluids on dissolved gas analysis (DGA). DGA has been used for many years as an effective and reliable tool to detect incipient faults in mineral oil-filled transformers. The information provided by DGA analysis is extremely important to the asset managers with electricity supply companies. Therefore, it is essential to ensure that traditional DGA analysis techniques still can be used if alter- September/October 2007 Vol. 23, No /07/$25/ 2007IEEE 5

2 native oils are used in transformers. In order to clarify that DGA diagnostic techniques still yield the correct result when applied to ester filled transformers, it is necessary to determine if the same types of fault gases are generated, to identify the generation rate, and their concentration in the alternative fluids against a mineral oil benchmark. This article gives the results of experiments that have simulated a number of faults that can be found in power transformers and looks at the DGA analysis results in each case for a number of oil types. Table 1 gives the types of dissolved gases evolved during transformer faults and their indicative relationships with types of faults in mineral oil. A number of diagnostic methods are available to identify the types of faults and their intensities. These include the IEC, IEEE standard and Duval triangle diagnosis methods [9] [11]. Two broad categories of faults in a transformer can be detected by DGA: thermal faults and electrical faults. The various DGA standards then subdivide these basic types of fault in different ways. In IEC [9], thermal faults are represented as being in three temperature bands, <300ºC (T 1 ), >300ºC (T 2 ) and >700ºC (T 3 ). Electrical faults can be further classified as partial discharges of the cold plasma (corona) type (PD), low energy discharges (D 1 ), and high energy discharges (D 2 ). The triangular graphical representation of Duval, which is used to visualize a DGA fault diagnosis, uses the same subdivisions as the IEC standard. However, instead of using the concentration of five gases and three ratios to define the type of fault, the relative percentage of three gases is used for the analysis in the Duval diagnosis technique [12]. The IEEE analysis method uses the concept of key gases [10]. The key gas for each type of fault is identified, and the dominating percentage of this gas to the others is used to diagnose the fault. For example, low intensity PD or corona produces mainly. Similarly, the key gas is for arcing, for overheating oil, and CO is for overheating of cellulose. The generation of various forms of fault within three dielectric fluids and the analysis of dissolved gas produced are under investigation in this article. The fluids used are a mineral oil Nynas Nytro 10GBN; a synthetic ester, Midel 7131; and a natural ester FR3. The synthetic ester, Midel 7131, consists of four ester groups with saturated chains as shown in Figure 1(a), i.e., there are no double bonds between the carbon atoms in the chain. FR3, the natural ester, is a natural triglyceride ester with a mixture of saturated and unsaturated fatty acids. The triglyceride ester molecule may be represented as in Figure 1(b), the glycerol backbone in blue and the fatty acid parts in red [13]. Fault Gases Evolved by Simulated Thermal Faults Thermal tests of both oil and oil/paper mixes have been carried out. For the oil/paper mix, the oil/paper ratio was 20:1 by weight. For the tests involving only oil, all the fluids were preprocessed by drying at 85 C under a vacuum for 72 hours. For the thermal tests involving both oil and paper, the Kraft papers were preprocessed by drying at 105 C in an air circulating oven for 24 hours followed by further drying at 85 C under vacuum for 24 hours. The fluids were preprocessed by drying at 85 C under a vacuum for 48 hours. Following the drying of both the oil and the paper, the paper was impregnated in the preprocessed fluid and dried under vacuum for an additional 24 hours. At the end of preprocessing, moisture contents in the samples were measured by Karl Fischer titration method using Metrohm KF 786 coulometer and KF Thermoprep 832 (Metrohm, The average moisture contents of mineral oil, Midel 7131 and FR3 were 6 ppm, 24 ppm, and 16 ppm, respectively. The paper samples had moisture contents of less than 0.6% by weight. All of the fluid and fluid/paper samples then were sealed in glass bottles and heated uniformly in an air circulating oven at temperatures of 90 C, 150 C or 200 C for periods up to 14 Table 1. Fault indicator gases. Fault gases Key indicator Secondary indicator (hydrogen) Corona Arcing, overheated oil C (methane) Corona, arcing, and overheated oil (ethane) Corona, overheated oil (ethylene) Overheated oil Corona, arcing (acetylene) Arcing Severely overheated oil CO (carbon monoxide) Overheated cellulose Arcing if the fault involves cellulose CO 2 (carbon dioxide) Overheated cellulose, arcing if the fault involves cellulose O 2 (oxygen) Indicator of system leaks, over-pressurization, or changes in pressure or temperature. N 2 (nitrogen) Indicator of system leaks, over-pressurization, or changes in pressure or temperature. TDGC: The total concentration of the six combustible gases (, C,,,, CO) in ppm. CO 2 /CO:Trending Ratio used to determine severity of cellulose degradation. O 2 / N 2 : Trending Ratio used to determine system leaks, over-pressurization, or changes in pressure or temperature. 6 IEEE Electrical Insulation Magazine

3 Figure 1. Chemical structure of synthetic and natural esters. days. This testing was intended to simulate the maximum top oil temperature found in a transformer during operating conditions (90 C) and two cases of low intensity thermal faults (150 C and 200 C). As with all of the other tests described in this article, a number of control samples of fluid and fluid/paper were kept for DGA tests to provide a benchmark. All the DGA results presented are the average of three samples to improve accuracy. A. DGA Results from Thermal Tests of Oil at 90 C, 150 C, and 200 C At the 90 C maximum top oil temperature found under operating conditions, both mineral oil and esters should be stable for a long period of time; therefore, no significant dissolved gas should be evolved. This should not be the case for the temperatures of 150 C and 200 C at which chemical decomposition will take place. Table 2 compares the concentration of fault gases found in the three oils under the different test conditions. In terms of gas volume, Midel 7131 generated the smallest amount of fault gases. In contrast, FR3 generated a significant amount of ethane and hydrogen, particularly in the case of the 90 C test. At 90 C, none of the three types of fluid produced ethylene, which usually taken is to be a characteristic of high energy thermal faults (see Table 1). This is a positive result as the oils are expected to be stable at this temperature. For the test carried out at 200 C, the gases ethylene, ethane, and methane were generated with approximately the same ratios seen at 150 C. Figure 2 shows the relative percentages of fault gases in the total dissolved combustible gases (TDCG) at 90 C and at 150 C. Ethylene, the primary indicator for high energy thermal faults, is negligible in concentration at 90 C. It is more dominant in the results relating to the 150 C test (see Figure 3 for clarity, which is plotted without H2 and CO). In FR3, Ethane is generated in significant quantities. In Midel 7131 and mineral oil this is not the case. In all cases, methane is not present in significant quantities. B. DGA Results from Thermal Tests of Oil/Paper at 90 C, 150 C, and 200 C Table 3 compares the concentration of fault gases of mineral oil, Midel 7131, and FR3 in the presence of Kraft paper (these tests were carried out for 14 days at 90 C/150 C and 1 hour for 200 C). The inclusion of paper has caused an increase in the concentration of carbon monoxide and carbon dioxide for the 90 C temperature. These gases are key indicators for cellulose degradation, in both mineral oil and esters. The concentrations of CO and CO 2 are the highest in mineral oil, lower in Midel 7131, and the least in FR3 at this temperature. At 150 C, the inclusion of paper also increases the level of carbon monoxide and carbon dioxide significantly. This is expected as Kraft paper generally starts to be thermally degraded at temperatures above 105 C. The generation of CO is less in esters than in mineral oil suggesting that they may be protecting the paper in some way. Figure 4 shows the relative percentage of fault gases in the TDCG for the oil/paper mix at 90 C and at 150 C. In the case of mineral oil and Midel 7131, the dominant gas is carbon monoxide; however, for FR3, the concentration of carbon monoxide is similar to that without paper, indicating paper integrity may be preserved [14]. Figure 5 shows the same results with and CO excluded to allow viewing of the other fault gases more prominently. Fault Gases Evolved by Simulated Electrical Faults A. DGA Results from Low-Energy Arc Tests A 220 V/40 kv, 8 kva test transformer was used to generate a breakdown across needle to plane electrode configuration with an oil gap distance of 15 mm. When the breakdown occurs, it will degrade the oil locally as the energy being dissipated causes the molecular structure of the oil to be disrupted. This allows the formation of fault gases like acetylene. The fault gases then dif- September/October 2007 Vol. 23, No. 5 7

4 Table 2. Dissolved gas content in parts per million (ppm) for thermal tests of oil only at 90 C aand 150 C for 3 and 14 days and at 200 C for 1 hour. The values highlighted in bold italics are considered to be significant shifts in dissolved gas values (only combustible gases examined). Oil type Mineral oil Synthetic ester (Midel) Natural ester (FR3) Test temp 90 C C 90 C C 90 C C Test time C 3d 14d 3d 14d 1h C 3d 14d 3d 14d 1h C 3d 14d 3d 14d 1h C CO CO TDCG fuse from the local fault location to the bulk volume. To ensure a sufficient concentration of fault gases, a total of 20 breakdowns was produced in oil that had previously been preprocessed in a similar way to that described for the thermal tests. There was at least a 1-minute interval between each breakdown. For each breakdown test, the voltage was steadily ramped up until the oil gap broke down. The current was interrupted by the operation of an over-current relay. This relay, on the low voltage side of the power supply, was set at a 3-A limit to ensure rapid interruption of the current following formation of the arc. It normally operated within 20 ms after the formation of the breakdown, but it could reach 100 ms in certain cases. Oil samples for DGA testing were taken from the bottom valve of a sealed test vessel. The oil was naturally forced into glass syringes according to the BS EN standard [15]. As the test vessel is sealed, a homogeneous distribution of fault gases can be expected when enough time is left after the tests for the fault gases to diffuse into the bulk of the oil. Table 4 shows the results of these tests. Acetylene should be one of the key gases produced during lowenergy arc discharge faults; and it therefore, is a primary indicator for this type of fault. This is found in the largest concentration in all samples. Hydrogen and ethylene also usually are evident in significant amounts. Although the same level of low energy Figure 2. Relative percentages of dissolved combustible gases for mineral oil and esters at 90 C and 150 C (oil only). 8 IEEE Electrical Insulation Magazine

5 Figure 3. Relative percentages of dissolved combustible gases (without and CO) for mineral oil and esters at 90 C and 150 C (oil only). discharge took place in the three oils, the acetylene concentration in mineral oil is about 5 to 10 times higher than that seen in the esters. Midel 7131 has the lowest amount of dissolved gas as a result of this test; this is the same result as that seen in the thermal tests. Figure 6 shows the relative percentages of fault gases in the total dissolved combustible gases (TDCG). B. DGA Results from Partial Discharge Tests The fluids used for the partial discharge test were preprocessed as earlier described. The electrical circuit and test electrodes used were the same as the ones in the arcing test with the addition of a water resistor to limit the current in case of inadvertent breakdown. The partial discharge tests used a standard PD detection circuit. The partial discharge inception voltages were 27.9 kv for mineral oil, 15.5 kv for Midel 7131, and 12 kv for FR3. The PD level measured during the tests was less than 100 pc for all of the types of oil. Table 4 shows the DGA results of three types of fluid for PD activity with a normalized duration of 1 hour (owing to the different generation rates, the test on mineral oil was carried out for half an hour, on Midel 7131 for 4 hours, and on FR3 for 1 hour). As shown in Figure 5, hydrogen is the key indicator for low-energy discharges, and this was found significantly in all of the oils. Mineral oil had the highest generation rate, and Midel had the lowest. Table 3. Dissolved gas content in parts per million (ppm) for thermal tests of oil and paper at 90 C and 150 C for 14 days and at 200 C for 1 hour. The values highlighted in bold italics are considered to be significant shifts in dissolved gas values (only combustible gases examined). Oil type Mineral oil Synthetic ester (Midel) Natural ester (FR3) Test temp Control 90 C C Control 90 C C Control 90 C C C CO CO TDCG September/October 2007 Vol. 23, No. 5 9

6 Figure 4. Relative percentages of dissolved combustible gases for mineral oil and esters at 90 C and 150 C for 14 days (oil and paper). DGA Diagnosis The results obtained were taken as inputs into DGA diagnosis based on the various standards. In doing this, it has to be noted that the laboratory measurement error expected during the DGA measurements is ±10% for the 50 ml (minimum) oil samples used in these tests. Figure 7 shows the results of the first analysis technique to be assessed, the Duval triangle DGA fault diagnosis method. Results from the thermal tests and the low-energy, arc discharge test were analyzed. The results of the corona test were not analyzed for reasons that will be detailed later. In terms of the results for mineral oil, use of the Duval triangle method diagnosed all faults correctly as being either in the T 1 region (thermal fault of less than 300 C) or in region D 1 (lowenergy discharges). This result is to be expected if the laboratory tests are assumed appropriate. For Midel 7131, the Duval triangle places the oil used in the 200 C thermal test into the T 1 region (i.e., overheating < 300 C). The oil subjected to the 150 C thermal tests was placed incorrectly into the T 2 region (i.e. overheating temperature T, 300 C< T <700 C). The low-energy, arc discharge test results in the allocation of the data points into the D 1 region, the correct diagnosis as per test conditions. For FR3, all thermal test results ended up as being identified as being in the T 2 region, thermal overheating temperature, therefore, is over-estimated in this case as was the case for Midel being tested at 150 C. Again, low-energy, arc discharge is correctly allocated into the D 1 region. In the case of cold corona type discharge (PD) the fault gases concentrations are not sufficient to allow a diagnosis. There is some evidence of generation of methane, ethylene, and acetylene in Table 4, but the values are very low. Other authors have suggested that the use of the Duval triangle is indeterminate, unless the PD activity is intense and/or occurs over a long period of time [11]. To allow the correct diagnosis of this form of discharge using the Duval technique, results need to have dissolved gas that is >98% C and <2%. The next form of analysis carried out uses the IEC standard [9]. The results of this analysis are given in Table 5. Some results produced during the tests lead to a no valid result being placed into the table. These are cases in which the gas concentration produced during the tests was not sufficient to fall into the code ranges within the standard. This was the case for all tests at 90 C. The temperature was low enough for the oils or oil/paper being stable enough for insufficient fault gases to be generated. At 150 C, tests using all of the oil types with and without paper correctly diagnose thermal faults in the low temperature range (<300 C). For the tests at 200 C, the generation of methane in the natural ester is too low to diagnose the fault correctly. For the synthetic ester, the magnitude of the thermal fault is overstated for the case when it was tested with paper. For the electrical faults, the IEC diagnosis method [9] recognizes all of the low-energy, arc discharge faults correctly as is shown in Table 6. For partial discharge, the result from the test involving the synthetic ester is correct, the results for mineral oil and the natural ester showing an incorrect diagnosis of a highenergy discharge. The reason for this is that a low concentration of was generated in mineral oil, and a large amount of was generated in the synthetic ester. To confirm the diagnosis of PD, the C / ratio also should be approximately between 0.02 and Table 7 shows the results from the final method of analysis that was examined, the IEEE method [10] that is based on the identification of key gases. For faults involving overheating of oil, the key gas is ethylene. This is expected to be 63% of the 10 IEEE Electrical Insulation Magazine

7 Figure 5. Relative percentages of dissolved combustible gases (without and CO) for mineral oil and esters at 90 C and 150 C for 14 days (oil and paper). total dissolved combustible gases with 20% of ethane also being present. None of the samples exhibit this gas concentration. In all cases, no fault would be diagnosed by using the key gas method (KGM). This is the case for both mineral oil and both of the alternative fluids. Table 8 shows the analysis of the KGM for thermal tests involving both oil and paper. Overheating of cellulose is indicated by having a relative proportion of CO above or equal to 92%. Therefore, the results suggest that the paper degradation is occurring in all cases for the test sample that uses the synthetic ester; only the 150 C test indicates the correct fault for the mineral oil and natural ester tests. Table 9 shows the results of the KGM on the fault gases generated as a result of electrical faults. For the low-energy, arc discharge, is the key indicator of that form of fault and the relative percentage according to KGM should be greater than or equal to 30%. This was the case for all three oils, suggesting that the KGM is applicable in this case. In case of a cold corona-type discharge, is the primary indicator and the relative percentage should be greater than or equal to 85%. The test results show that hydrogen is the dominant gas that is generated, its percentage of the TDCG is not at that level; however, its percentage of the TDCG is less than the specified level of 85% by the KGM method. These results lead to doubts on the applicability of setting exact percentages of key gases for fault diagnosis (both for mineral oils and for alternative fluids). It is felt that defining a range of percentage would be more appropriate, a view also expressed in [16] In mineral oil, the ratio of the and gas concentrations usually reflects the intensity of electrical discharges. When / is close to or more than 1; the discharges are of the high Figure 6. Relative percentages of dissolved combustible gases for mineral oil and esters. September/October 2007 Vol. 23, No. 5 11

8 Table 4. DGA results for low energy arc discharge and cold corona type discharge in mineral oil and esters. Oil type Mineral oil Synthetic ester Natural ester Sample C* LEDT a CDT b C LEDT CDT C LEDT CDT C CO TDCG *: Control samples. a : Low energy arc-discharge test. b : Corona type discharge test. Figure 7. Duval triangle diagnostic results for three oils (mineral oil black, Midel 7131 red and FR3 green) under thermal and electrical faults. energy type. When / is smaller than one, the discharges are weaker and belong to the low energy discharge category (i.e., PD). Table 5. IEC diagnosis results for thermal tests. Diagnosis result Diagnosis result Test method Oil type (oil only) (oil and paper) 14 days at 150 C Mineral oil Thermal fault Thermal fault (t< 300 C) (t< 300 C) Synthetic ester Thermal fault Thermal fault (t< 300 C) (t< 300 C) Natural ester Thermal fault Thermal fault (t< 300 C) (t< 300 C) 1 hour at 200 C Mineral oil Thermal fault Thermal fault (t< 300 C) (t< 300 C) Synthetic ester Thermal fault Thermal fault t< 300 C) (300 C< t <700 C) Natural ester No valid result No valid result (C too low) (C too low) Conclusions Although the molecular structures of esters are different from those of mineral oil, the thermal test results show that the gases generated by a selection of thermal and electrical faults are not different from those generated by mineral oil. The rate of generation of dissolved gases in esters is less in comparison with mineral oil. Esters are particularly stable under medium temperature range thermal faults. However, FR3 is shown to generate a significant amount of ethane for thermal faults, suggesting that this also should be used as a key indicator of thermal faults in combination with ethylene in equipment using this type of fluid. The thermal tests also confirm that carbon monoxide and carbon dioxide are the key indicators of cellulose degradation in both mineral oil and esters. For electrical faults, acetylene is the key fault gas observed for low energy discharges, and hydrogen is the key fault indicating gas for partial discharges. Again, less gas is evolved by esters in comparison to that evolved by mineral oil. For both forms of fault, the reduction in the total gas generation will make it more difficult to identify faults at an early stage in practical applications unless the analysis process sensitivity can be improved. In terms of the diagnosis of the DGA samples using three different methods, thermal faults in all three oils are correctly diagnosed by the Duval triangle method. However, the temperature range of the fault is overestimated for the tests involving the Table 6. IEC diagnosis results for electrical tests. Test method Oil type Diagnosis result Low energy discharge Mineral oil Discharge of low energy Synthetic ester Discharge of low energy Natural ester Discharge of low energy Cold corona-type discharge Mineral oil Discharge of high energy Synthetic ester Partial discharge Natural ester Discharge of high energy 12 IEEE Electrical Insulation Magazine

9 Table 7. Key gas method analysis for thermal test (oil only). Diagnostic Oil/gas (%) Time (days) C CO result Mineral oil 14 days at N/A 1 hour at 200 C N/A Synthetic ester 14 days at N/A 1 hour at 200 C N/A Natural ester 14 days at 150 C N/A 1 hour at 200 C N/A Table 9. Key gas method for electrical tests. Diagnostic Oil/gas (%) Test type C CO result Mineral oil LED Arcing in oil CDT N/A Synthetic ester LED Arcing in oil CDT N/A Natural ester LED Arcing in oil CDT N/A esters. Correct identification of thermal faults also takes place for the majority of samples assessed using the IEC technique, the only exceptions being natural esters tested with paper at 200 C in which no result could be produced (not enough methane was generated) and synthetic esters tested with paper at 200 C in which case the temperature of the fault was overestimated. For electrical discharges, all methods correctly identify the low-energy arcing fault. For corona discharge, significant hydrogen is generated in all cases, but only the test involving the synthetic ester that was then diagnosed using the IEC technique yielded the correct results. When comparing the three diagnostic methods on different types of oil under thermal and electrical tests, it is concluded that Duval triangle and IEC methods are more applicable than the IEEE KGM (see Table 10). Esters have the same DGA fingerprints as mineral oil but to yield the correct fault diagnosis result, the criteria of fault gas ratio or percentage needs to be redefined. Table 8. Key gas method analysis for thermal test (oil and paper). Diagnostic Oil/gas (%) Time (days) C CO result Mineral oil 14 days at 150 C Overheated cellulose 1 hour at N/A Synthetic ester 14 days at 150 C Overheated cellulose 1 hour at Overheated cellulose Natural ester 14 days at 150 C Overheated cellulose 1 hour at 200 C N/A Table 10. Correctly identified faults for different types of analysis. Standard diagnosis methods Duval IEC IEEE Mineral Oil 6 of 7 5 of 6 2 of 6 Synthetic Ester 5 of 7 6 of 6 3 of 6 Natural Ester 4 of 7 3 of 6 2 of 6 Acknowledgements The authors wish to thank AREVA T & D, EdF Energy, M & I Materials, National Grid, Scottish Power, TJ H2b analytical services and United Utilities for their financial and technical support to form the research consortium Alternative fluids for large power transformers at The University of Manchester. The authors particularly appreciate the help given from Ian James, Alan Darwin, Paul Dyer, Russell Martin, James O Neil, Paul Jarman, Peter Docherty, Dave Walker, John Noakes, and Tony Byrne for providing invaluable expertise and technical guidance toward the project. References [1] K. Rapp, and P. Stenborg, Cooper Power Systems field analysis of Envirotemp FR3 fluid in sealed versus free-breathing transformers, CP0414, Cooper Power Systems, Waukesha, WI, [2] D. Martin, I. U. Khan, J. Dai, and Z. D. Wang, An overview of the suitability of vegetable oil dielectrics for use in large power transformers, in Proc. 5th Annual Euro TechCon, Chester, United Kingdom, November 28 30, [3] EPRI Report : Environmentally acceptable transformer fluids; Phase 1 state of the art review; Phase 2 Laboratory testing of fluids, Palo Alto, CA, Nov [4] T. V Oommen, C. C. Claiborne, and C. T. Mullen, Biodegradable electrical insulation fluids, in Proc. IEEE Electrical Insulation Conference, Chicago, 1997, pp [5] T. V. Oommen, C. C. Claiborne, and E. J. Walsh, A new vegetable oil based transformer fluid: Development and verification, in Proc. Conf. IEEE Elect. Insul. Dielect. Phenomena, 2000, pp September/October 2007 Vol. 23, No. 5 13

10 [6] OECD 301 test series OECD guideline for testing of chemicals, Adopted by the Organisation for Economic Co-operation and Development Council, Paris, France, on July 17, [7] C. P. McShane, Vegetable oil based dielectric coolant, IEEE Industry Appl. Mag., vol. 8, pp , May/June [8] K. Rapp, C. McShane, and J. Luksich, Interaction mechanisms of natural ester dielectric fluid and Kraft paper, in Proc. IEEE Int. Conf. Dielect. Liquids, 2005, pp [9] IEC60599, Mineral oil-impregnated electrical equipment in service: Guide to the interpretation of dissolved and free gases analysis, IEC Publication ( ), Mar [10] IEEE Guide for the Interpretation of Gases Generated in Oil- Immersed Transformers, IEEE Standard C , June/July [11] M. Duval, A review of faults detectable by gas-in-oil analysis in transformers, IEEE Elect. Insul. Mag., vol. 18, pp. 8 17, May/ June [12] M. Duval and A. depablo, Interpretation of gas-in-oil analysis using new IEC Publication and IECTC 10 Databases, IEEE Elect. Insul. Mag., vol. 17, pp , Mar./Apr [13] T. V. Oommen Vegetable oils for liquid filled transformers, IEEE Elect. Insul. Mag., vol. 18, pp. 6 11, Jan./Feb [14] C. McShane, K. Rapp, J. Corkran, G. Gauger, and J. Luksich, Aging of paper insulation in natural ester dielectric fluid, in Proc. IEEE/PES Transmission Distribution Conf. Exposition, Oct. 28 Nov. 2, [15] BS EN60567 Oil-filled electrical equipment Sampling of gases and of oil for analysis of free and dissolved gases Guidance, pp 19 39, Dec [16] X. Chen and W. Chen, Research of relationship between partial discharge and dissolved gases concentration in oil, in Proc. XIVth Int. Symp. High Voltage Eng., Tsinghua University, Beijing, China, August 25 29, Imad-Ullah-Khan received a BEng. (Hons.) degree in electrical engineering from the National University of Science and Technology, Pakistan, in He is currently a Ph.D. student at the Electrical Energy and Power Systems Group at the University of Manchester. His research interests include alternative transformer insulation, electric stress analysis using FEM and dissolved gas analysis. He is a student Member of the Institution of Engineering and Technology (IET). Zhongdong Wang received the BEng. and MEng. degrees in high voltage engineering from Tsinghua University of Beijing in 1991 and 1993, and a Ph.D. in electrical engineering from UMIST in Dr. Wang joined the Electrical Energy and Power Systems Group of the School of Electrical and Electronic Engineering at the University of Manchester in 2000, where she now is a senior lecturer. Her current research interests include condition monitoring, transformer modeling and FRA and transients simulation, insulation aging, and alternative insulation materials for transformers. Ian Cotton (M 98, SM 07) received a Class I BEng. (Hons.) degree in electrical engineering from the University of Sheffield in 1995 and a Ph.D. degree in electrical engineering from UMIST in He is currently a Senior Lecturer at the Electrical Energy and Power Systems Group of the School Of Electrical and Electronic Engineering at the University of Manchester. His current research interests include power systems transients, the use of higher voltage systems in aerospace applications, and power-system induced corrosion. He is a member of the Institution of Electrical Engineers and a Chartered Engineer. Susan Northcote is a Chartered Chemist and a Designated European Chemist. She is the Operational Director for TJ H2b Analytical Services European Centre. Ms. Northcote represents the Royal Society of Chemistry on the British Standards Gel 10 committee and represents the UK on a number of IEC maintenance teams, whose roles are to evaluate and update methods for the analysis of insulating media. 14 IEEE Electrical Insulation Magazine