Life estimation of electrothermally stressed epoxy nanocomposites

1154 P. Preetha and M. Joy Thomas: Life Estimation of Electrothermally Stressed Epoxy Nanocomposites

DOI 10.1109/TDEI.2014.003545

Life Estimation of Electrothermally Stressed Epoxy Nanocomposites

P. Preetha and M. Joy Thomas

Nanodielectrics Laboratory Department of Electrical Engineering

Indian Institute of Science Bangalore, 560 012, India

ABSTRACT Accelerated electrothermal aging tests were conducted at a constant temperature of 60 oC and at different stress levels of 6 kV/mm, 7 kV/mm and 8 kV/mm on unfilled epoxy and epoxy filled with 5 wt% of nano alumina. The leakage current through the samples were continuously monitored and the variation in tan values with aging duration was monitored to predict the impending failure and the time to failure of the samples. It is observed that the time to failure of epoxy alumina nanocomposite samples is significantly higher as compared to the unfilled epoxy. Data from the experiments has been analyzed graphically by plotting the Weibull probability and theoretically by the linear least square regression analysis. The characteristic life obtained from the least square regression analysis has been used to plot the inverse power law curve. From the inverse power law curve, the life of the epoxy insulation with and without nanofiller loading at a stress level of 3 kV/mm, i.e. within the midrange of the design stress level of rotating machine insulation, has been obtained by extrapolation. It is observed that the life of epoxy alumina nanocomposite of 5 wt% filler loading is nine times higher than that of the unfilled epoxy.

Index Terms - Electrothermal aging, polymer nanocomposites, rotating machine insulation, epoxy, life estimation.

1 INTRODUCTION

HIGH voltage rotating machines play a significant role in generating electrical energy as the demand for power continues to increase. However, one of the main causes for down times in high voltage rotating machines is the problems related to the winding insulation. The utilities want to reduce the cost through longer maintenance intervals and at the same time expect a higher lifetime of the machines. So the main challenge for the utilities as well as the manufacturers of the winding insulation and the high voltage rotating machines is to develop new insulation materials which can improve the life of the equipment and simultaneously reduce its maintenance cost. The advent of nanotechnology in recent times has heralded a new era in materials technology by creating opportunities to significantly enhance the properties of existing conventional materials. Polymer nanocomposites belong to one such class of materials where the introduction of nanoparticles into the polymer material results in a unique morphology for the nanocomposite with a large interfacial area per unit volume

and inter-particle distances that are comparable to the particle size. This uniqueness creates an opportunity to design polymer nanocomposites with new combinations of properties that can circumvent the trade-offs in performances associated with traditional polymer composites apart from introducing additional functions which are nanoparticle specific. These materials are found to exhibit unique combinations of physical, mechanical and thermal properties that are advantageous as compared to the traditional polymers or their composites [1-4]. Even though they show tremendous promise for dielectric/electrical insulation applications, the understanding related to these systems is very premature [5] as there are only a few studies relating to the long term multistress aging performance as well as life estimation of the epoxy nanocomposites. To commercialize the use of polymer nanocomposites as electrical insulation for rotating machines, it is very much essential that the life of polymer nanocomposites is estimated under multistresses. Electrical insulation, especially those used in rotating machine winding insulation is expected to operate without failure for many years. It is not practical to determine the life of a new insulation based on direct operating experience. The commonly adopted practical method to determine the life of

Manuscript received on 7 March 2013, in final form 10 December 2013, accepted 23 December 2013.

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 21, No. 3; June 2014 1155

insulation is to go for the accelerated aging tests. In the long term aging test, one or more stresses are applied to the insulation at a stress level which is higher than that would occur during the normal operation of the equipment. The rate of aging will be faster at the higher stress level, and failure will occur more quickly leading to a reduced experimental time. Thus accelerated aging tests can be considered as a practical method for predicting insulation life for a new insulation [6] Electrical insulation in many electrical power equipments is epoxy based and it is subjected to multiple stresses when the equipment is in operation. The long term behavior of solid electrical insulation is influenced by the nature as well as the magnitude of the applied stresses. In order to get a reliable life estimation it is necessary to evaluate the performance of the electrical insulation system under realistic operating conditions. That means, the high voltage insulation should be evaluated under a combination of stresses which are normally present during the operation of the machine. It has already been reported by several investigators that a combination of the electrical, thermal and mechanical stresses has a more deleterious effect than when these stresses are applied individually [7]. The guidelines in conducting single as well as multistress ageing experiments on insulation under laboratory conditions to get a realistic value for service life by extrapolation is explained by Srinivas et al [8]. They have also highlighted the need for a careful statistical analysis of the life test data. Combined electrical and thermal stress aging studies have been conducted on scaled-down models of rotating machine insulation made of epoxy bonded mica and the life of the insulation at different stress levels have been calculated by developing a model for combined electrical and thermal aging [9]. Laghari and Cygan [10] have conducted accelerated life studies on a high temperature polyimide film under simultaneous electrical and thermal stresses at 60 Hz as well as at 400 Hz and has shown that electrical aging is the responsible failure mechanism under these conditions, and that thermal aging is not a major degradation factor in the applied temperature range particularly at the higher frequency of 400 Hz. Life tests were performed on unfilled epoxy at 30, 24 and 20 kV at three temperature levels of 30, 55 and 80 °C by Lorenzo et al [11, 12] and they have proposed a phenomenological life model for the epoxy material normally used in electrical machine insulation. The related effect was found to change the PD activity in such a way as to produce higher times to breakdown with temperature. Qikai Zhuang et al [13] have predicted a life of 37 years for epoxy resin insulated transformer winding by subjecting it to accelerated electrical aging. The above discussions show that several studies have been carried out by different authors on the combined stress accelerated aging of actual stator bars insulated with filled epoxy as well as conventional unfilled epoxy which is used as insulation in many high voltage applications. Reports are not yet available on the life estimation of polymer nanocomposites subjected to multiple stresses. So it is necessary that these materials should also be tested for the long time reliability before using them for practical applications. With this as the motivation, accelerated aging studies were conducted on

epoxy alumina nanocomposites containing 5 wt% of nanoalumina as well as unfilled epoxy, at an accelerated voltage of 6, 7 and 8 kV and at a constant accelerated temperature of 60 oC and arrived at the life of unfilled epoxy and epoxy alumina nanocomposite.

2 EXPERIMENTAL DETAILS

2.1 SAMPLE PREPARATION

Bisphenol-A epoxy resin (CY 1300, density 1.16 g/cm3) along with hardener (HY 956, density 1.02 g/cm3) supplied by Huntsman was used as the base polymer material. The fillers used in the present study are commercially available Al2O3

fillers supplied by Sigma-Aldrich, USA. The average particle size (APS) of the alumina nanofillers used in the study is 40 nm. The processing of nanoparticle is very important as it has been reported that careful control of the processing parameters are required to get a good dispersion and improved characteristics for the polymer nanocomposites [1, 4]. The epoxy nanocomposites are prepared in the laboratory by using direct dispersion method following a protocol developed by one of the authors to get the best possible dispersion [14]. The first step involved is mechanical mixing for 2 minutes after adding filler to epoxy and the second step is ultrasonication at a frequency of 24 kHz for 1 hour. The selection of the alumina filler loadings for the samples to be prepared for the present study was arrived at by calculating the interparticle distance and the surface area per unit volume using the equation given by Tanaka et al. [15], and by conducting the ac breakdown studies of the samples of different filler loadings. It is observed that the interparticle distance decreases and the surface area increases with increase in the filler loading. It is also observed that with the increase in the filler loading to 5 wt%, the interparticle distance becomes less than 100 nm and also the ac breakdown strength improved at this filler loading [16]. So in order to estimate the life of the nanocomposite insulation, epoxy alumina nanocomposite samples with filler loadings of 5 wt% and thickness of 1 mm were used.

2.2 EXPERIMENTAL SET UP

A multistress-aging chamber of 0.56 m x 0.56 m x 0.71 m was designed and developed for the study. Nine samples for breakdown under electrothermal aging were arranged inside the aging chamber with equal distance between them. A PD-free high voltage source was used for the electrical aging. A corona free lead was taken through a PD-free bushing and connected to all the samples where the temperature of the samples was maintained at the desired level of 60 oC. The electrodes used for the aging experiments are epoxy embedded Rogowski profile electrodes. The photographs of the epoxy embedded Rogowski profile electrodes (both top and bottom) are shown in Figure 1a and a schematic showing the details of the electrodes is given in Figure 1b. The sides of the test samples in contact with the electrode surface were coated with a fine layer of silver paint to avoid partial discharges in the gap between the sample and the electrode surface [17]. The silver coating was made on the


top and bottom surfaces of the sample corresponding to the area of the exposed region of the electrodes used. The experimental set up for the aging studies is shown in Figure 2. The ac leakage current through the samples was measured by measuring the ac voltage drop across a resistor connected in series with the sample which in turn was acquired using a data acquisition card of National Instruments (NI) make in Labview environment. Tan values were also calculated from the voltage and the current values obtained in order to get an idea about the impending failure of the samples.

Figure 1(b). Details of the Rogowski profile electrodes and the sample used in the study.

2.3 AGING METHODOLOGY

All the nine samples of thickness 1 mm were continuously energized at 6 kV and subjected to an ageing chamber ambient temperature of 60 oC which is approximately equal to the stator winding temperature of a 450 kW, 6.6 kV machine, until the samples failed [18]. The experiments were repeated for 7 kV and 8 kV also. The aging chamber was preheated and brought to the required temperature before the application of the voltage. To arrive at the stress levels at which aging has been carried out, ac breakdown studies on unfilled epoxy and epoxy filled with 5 wt% epoxy alumina nanocomposites were conducted. It was observed that the power frequency ac breakdown strength of unfilled epoxy sample of 1 mm thickness was 40 kV and that of epoxy alumina nanocomposite of 5 wt% filler loading was 43 kV with a ramp type of voltage application as per ASTM D149 [16]. The aging stresses are selected based on the acceleration factor, g, which is the ratio of the stress applied during the aging to the breakdown stress. The values of g used in this series of experiments were 0.15, 0.175 and 0.2, respectively. The acceleration values are so chosen that the aging mechanism is not altered at these stress levels and that all the specimens subjected to ageing at a particular stress, are likely to breakdown in a duration of about 3000 hours to give the complete data set.

3 RESULTS AND DISCUSSIONS The time to failure of the samples subjected to multistress ageing has been noted. The experimental data obtained on a large number of nominally identical epoxy based specimens show a characteristic scatter, which are very

(a) Unfilled epoxy before ageing (b) Unfilled epoxy after ageing

(c) Epoxy alumina nanocomposite (d) Epoxy alumina nanocomposite before ageing after ageing

Figure 3. Unfilled epoxy and epoxy alumina nanocomposite samples before and after multistress ageing.

Guard ring (i) (ii)

Figure 1(a). Unfilled epoxy embedded Rogowski profile electrodes. (i) top electrode and (ii) bottom electrode (with guard ring).

Figure 2. Experimental set up for the electrothermal aging of the samples.

All dimensions are in mm

High voltage electrode

Epoxy embedded Rogowski profile electrodes

1 mm thick sample

Guard ring

Grounded electrode

307 kΩ

HV Transformer 375V/50 kV, 2 kVA HV Bushing

DAQ System

200 Ω

fuse link

SCXI


large compared to the normal or the Gaussian data. Such data are called as extremal data. The failure data has been processed in two different ways. One method is the graphical method in which Weibull probability plot has been done and the characteristic time () has been obtained. Another method is the analytical method for which the least square regression (LSR) method has been adopted. The photograph of the samples before and after the failure is shown in Figure 3. The puncture region has been encircled in red for both the cases.

3.1 LIFE ESTIMATION

3.1.1 WEIBULL GRAPHICAL METHOD

The distribution of times to failure is assumed to be a two parameter Weibull. The probability density function of a two parameter Weibull in the random variable ti indicating the times to failure is given by

f t |β, τβτ

t e 1

The relevant parameters of the distribution are the shape parameter β that represents the inverse of data scatter and the scale parameter that represents the breakdown strength at the

cumulative failure probability of 63.2 % and is called the Weibull breakdown strength.

The cumulative distribution function for the Weibull distribution is given by

F t 1 e 2

The Weibull probability plot is as shown in Figure 4a and 4b for unfilled epoxy and epoxy alumina nanocomposite of 5 wt% filler loading. The values of and obtained from the Weibull probability plot is given in Table 1. It is observed that the variation in the scale parameter is less for unfilled epoxy for a stress level of 6 kV/mm and 7 kV/mm whereas the difference is highly pronounced in the case of epoxy alumina nanocomposites.

3.1.2 LEAST SQUARE REGRESSION (LSR) METHOD

A real time experimental data acquisition results in a set of data represented by an independent variable and a dependent variable with its characteristic uncertainty. The parameters controlling any statistical model to the data fitted will always contain a certain degree of uncertainty. All parametric estimates therefore can be derived, to a reasonable

Table 2. Time to failure data for unfilled epoxy at 6 kV/mm.

xi = ln ln(1/(1-F(ti))

ti

(h) yi = ln(ti)

xi2 xiyi

-2.8619 348 5.85220 8.19047 -16.74841

-1.7019 410 6.01615 2.89646 -10.23889

-1.1226 479 6.17170 1.26023 -6.928351

-0.7083 480 6.17378 0.50168 -4.372892

-0.3665 481 6.17586 0.13432 -2.263455

-0.0571 577 6.35784 0.00326 -0.363032

0.2475 653 6.48157 0.06125 1.604190

0.5832 723 6.58340 0.34012 3.839444

1.0613 812 6.69950 1.12635 7.110179

Table 1. and at different stress levels for unfilled epoxy and epoxy alumina nanocomposite using graphical method.

Stress level 6 kV/mm 7 kV/mm 8 kV/mm

Material

(h)

(h)

(h)

Unfilled Epoxy 607 4.16 526 3.95 240 2.27

Epoxy Al2O3 (5 wt%)

2200 4.6 646 1.6 421 3.1

(a) Unfilled epoxy

(b) Epoxy alumina nanocomposite (5 wt%)

Figure 4. Weibull probability plot of (a) unfilled epoxy and (b) epoxy alumina nanocomposite (5 wt%).


degree of certainty, if the equations describing the models can be reformulated as a linear relationship. The procedure of estimating the model parameters and the associated uncertainties using a set of exhaustive data using linear or approximations there of is called Simple Linear Regression or Least Square Regression.

Consider an ordered pair of i observations. The conditional expectation of y given x is then a linear function of (xi, yi) given by

Where

)5()ln(

)4(1

22

2

22

ii

iiiii

ii

iiii

xxn

xyyxxa

andxxn

yxyxn

The time to failure data obtained for unfilled epoxy as well as epoxy alumina nanocomposite at a stress level of 6 kV/mm are shown in Tables 2 and 3 respectively. The value of and are estimated using the equations (4) and (5). The parameters and which represent the scatter of the data and the average life were estimated using the least square regression method and are tabulated in table 4 for different stress levels.

It is observed from Table 4 that there is not much variation in the value of obtained at different stress levels. Also it can be clearly observed that the characteristic time to failure of epoxy alumina nanocomposite samples at all stress levels are higher than that of unfilled epoxy. In order to obtain the life of the rotating machine insulation at the design stress level of 3 kV/mm, the inverse power law is used. The inverse power law connecting the life and applied stress is written as

L = KE-n (6)

where

L represents the at the corresponding stress level

n the endurance coefficient and

K the constant of power law

Figure 5. ln E vs ln .

By plotting ‘ln E’ vs. ‘ln ’ and estimating the slope, the endurance coefficient can be obtained. The value of endurance coefficient obtained is 3.85 for unfilled epoxy and 5.28 for epoxy Al2O3 nanocomposite. The value of K is 5.94x105 for unfilled epoxy and 250.9x105 for epoxy alumina nanocomposite of filler loading 5 wt%. By extrapolation, the nominal life of the unfilled epoxy at a design stress level of 3 kV/mm is estimated to be about 8652 hours and that of epoxy nanocomposites is 75,921 hours.

Table 4. and at different stress levels for unfilled epoxy and epoxy alumina nanocomposite using LSR method.

Stress 6 kV/mm 7 kV/mm 8 kV/mm

Material

(h)

(h)

(h)

Unfilled Epoxy 600 4.59 502 4.5 198 1.41

Epoxy Al2O3 (5 wt%)

1954 14.65 612 2.5 428 2.69

Table 3. Time to failure data for epoxy-Al2O3 nanocomposite at 6 kV/mm.

xi = lnln(1/(1-F(ti))

ti

(h) yi =ln(ti) xi

2 xiyi

-2.8619 2415 7.78945 8.19047 22.2926

-1.7019 1787 7.48829 2.89646 12.7443

-1.1226 1500 7.31322 1.26023 8.20982

-0.7083 661 6.49375 0.50168 4.59952

-0.3665 2156 7.67600 0.13432 2.81325

-0.0571 2325 7.75147 0.00326 0.44260

0.2475 2420 7.79152 0.06125 1.92840

0.5832 2201 7.69666 0.34012 4.48869

1.0613 2600 7.86326 1.12635 8.34528

)3(1

/ axxy ii


3.2 LEAKAGE CURRENT ANALYSIS

In order to get an idea about the impending failure and to understand the reason for the failure, the tan values were continuously monitored by measuring the leakage current. A PC based eight-channel data acquisition system of National Instruments make, with 16 bit resolution and 250 kS/sec sampling rate for each channel is employed to acquire the leakage current. LabVIEW software and the associated driver software has been used for the DAQ system. Leakage current is passed through 200 Ω shunt resistors and drop across the resistors is fed to the DAQ system.

Figure 6 shows the voltage and leakage current waveforms of the sample at the start of the experiment and Figure 7 shows the voltage and current oscillograms at the onset of breakdown. The leakage current values are shown enlarged in Figures 6 and 7 as the values are very low to identify the phase shift between the voltage and the current. For tan calculation, the voltage as well as the current was acquired at a very high sampling rate in order to get a more accurate value. The leakage current values were monitored every hour during the aging experiments. The tan value with the aging duration is shown in Figure 8 for unfilled epoxy and epoxy alumina nanocomposite sample.

It is observed from Figure 8 that the tan values are lower for epoxy alumina nanocomposites as compared to the unfilled epoxy during the entire duration of aging except at few points. The occurrence of a lower tan value at these filler loadings in the epoxy alumina nanocomposites systems can be due to the reduction in their electrical conductivities at these filler loadings. The electrical conductivity in nanocomposites can decrease due to the hindrances in charge transport caused by the nanoparticles [2]. It is also observed that for unfilled epoxy, tan started to increase after 50 hours of aging duration and then there is a steep increase for unfilled epoxy. In the case of epoxy alumina nanocomposites of 5 wt% filler loading, the tan values are lower than unfilled epoxy initially. The tan values then increases to a value above that of unfilled epoxy up to 50 hours and then decreases and remains less than the unfilled epoxy for the entire duration of aging considered in the present study. On application of the electrical stress, partial discharges initiate in defects and erode the insulating material leading to an initial increase in the tan . In the case of unfilled epoxy these discharges can easily penetrate through the polymer leading to the breakdown. In nano alumina filled epoxy insulation, however, this erosion is often suppressed by high partial discharge resistance of the alumina fillers [19,20]. Therefore the erosion rate due to partial discharge is much slower for epoxy nanocomposites. This could be one of the reasons for the increase in the time to failure of epoxy nanocomposites. An enhancement in the life of epoxy-mica insulation filled with silica nanofiller has been reported by Jurgen et al [21] and this was attributed to the enhanced partial discharge resistance of the nanofillers. Another reason that can be attributed for the increase in time to breakdown of epoxy alumina nanocomposite is the increase in the value of thermal conductivity for epoxy alumina nanocomposites. The thermal conductivity of epoxy alumina nanocomposite samples are almost 1.5 times that of unfilled epoxy[16]. So better heat dissipation in the epoxy alumina nanocomposites can also lead to an increased time to failure of epoxy nanocomposites. A better insight in to the reason for the enhanced life of epoxy alumina nanocomposites can be obtained by estimating the life of samples by thermal aging alone and also by subjecting the samples to different electrical and thermal stresses.

Figure 6. Representative voltage and current oscillograms measured at the start of the experiment.

Figure 7. Representative voltage and current oscillograms measured at the onset of breakdown of unfilled epoxy.

‐2.5

‐2

‐1.5

‐1

‐0.5

0

0.5

1

1.5

2

2.5

‐8

‐6

‐4

‐2

0

2

4

6

8

7.98 8 8.02 8.04 8.06 8.08 8.1

Voltage

Current

Vol

tage

(kV

)

Time (s)

Cur

rent

(m

A)

Figure 8. Variation of tan with aging duration for unfilled epoxy and epoxy alumina nanocomposite (5 wt%).


CONCLUSIONS Under long term multistress aging epoxy

nanocomposite is found to perform better than unfilled epoxy.

The tan of unfilled epoxy was found to be greater than that of epoxy alumina nanocomposite after a prolonged period of multistress aging.

The estimated life of unfilled epoxy corresponding to a stress level of 3 kV/mm is 8652 hours and that of epoxy alumina nanocomposites with a filler loading of 5 wt% is 75,921 hours. The increased life of epoxy nanocomposites could be due to their higher thermal conductivity as well as their improved partial discharge resistance [16, 19].

ACKNOWLEDGMENTS The authors would like to thank Mr. Riaz Ahmed for the help in setting up of the experimental facility. The help rendered in collecting experimental data by Mr. Joseph Vimal Vas, Mr. Sridhar Alapati, Mr. Vamsi Kaushik and Ms. Sunitha K. are greatly acknowledged.

REFERENCES [1] T. Tanaka, G. C. Montanari and R. Mulhaupt, “Polymer

Nanocomposites as Dielectrics and Electrical Insulation–Perspectives for Processing Technologies, Material Characterisation and Future Applications”, IEEE Trans. Dielectr. Electr. Insul., Vol.11, No.5, pp. 763-784, 2004.

[2] S. Singha and M. Joy Thomas, “Dielectric Properties of Epoxy Nanocomposites”, IEEE Trans. Dielectr. Electr. Insul., Vol.15, No. 1, pp. 12-23, 2008.

[3] Yuta Okasaki, M. Kozako, M. Hikita and T. Tanaka, “Effects of Addition of Nano-scale Alumina and Silica Fillers on Thermal Conductivity and Dielectric Strength of Epoxy/Alumina Microcomposites”, IEEE Int’l. Conf. Solid Dielectrics, Potsdam, Germany, pp. 1-4, 2010.

[4] C. Calebrese, L. Hui, L. S. Schadler and J. K. Nelson, “A Review on the Importance of Nanocomposite Processing to Enhance Electrical Insulation”, IEEE Trans. Dielectr. Electr. Insul., Vol. 18, No. 4, pp. 938-945, 2011.

[5] J. Samuel, M. Fu and F. Perrot, “Epoxy Alumina Nano-dielectrics: A Promising Material for High Voltage Insulation?”, Int’l. Sympos. High Voltage Eng., Hannover, Germany, paper No. E067, 2011.

[6] G. Stone, Electrical Insulation for Rotating Machines: Design, Evaluation, Aging, Testing, and Repair, Wiley-IEEE Press, 2000.

[7] G.C. Stone, “The Statistics of Aging Models and Practical Reality”, IEEE Trans. Electr. Insul., Vol. 28, No. 5, pp. 716-728, 1993.

[8] M. B. Srinivas, M. Nagao and M. Kosaki, “Experimental and Statistical Considerations in Ageing Studies on Electrical Insulation”, IEEE 4th Int’l. Conf. Properties and Applications of Dielectric Materials, Brisbane, Australia, pp.819-822, 1994.

[9] T. S. Ramu, “On the Estimation of Life of Power Apparatus Insulation under Combined Electrical and Thermal Stress”, IEEE Trans. Electr. Insul., Vol. 20 No.1, pp. 70-78, 1985.

[10] J. R. Laghari and P. J. Cygan, “Accelerated Life Studies of Polyimide Film Under Electrical and Thermal Multistress”, IEEE Int’l. Sympos. Electr. Insul., Baltimore, MD USA, pp. 66-69,1992.

[11] M. Di L. del Casale, “On Multistress Aging of Epoxy Resins: PD and Temperature”, IEEE Trans. Dielectr. Electr. Insul., Vol. 8, No. 2, pp. 299-303, 2001.

[12] M. Di L. del Casale, P. Romano and R. Schifani, “A Life Model For Epoxy Resins Subjected to PD Activity at Different Temperatures”, IEEE Conf. Electr. Insul. Dielectr. Phenomena, Victoria, BC, Canada, pp. 564-568, 2000.

[13] Q. Zhuang, P. H.F. Morshuis, X. Chen, S. Meijer, J. J. Smit and Z. Xu, “Life Prediction for Epoxy Resin Insulated Transformer Windings through Accelerated Aging Tests”, Int’l. Conf. Solid Dielectr., Potsdam, Germany, pp.1-4, 2010.

[14] S. Singha and M. J. Thomas, “Polymer Composite/ Nanocomposite Processing and its Effect on the Electrical Properties”, IEEE Conf. Electr. Insul. Dielectr. Phenomena, Kansas city, Missouri, USA, pp. 557-560, 2006.

[15] T. Tanaka, M. Kozako, N. Fuse and Y. Ohki, “Proposal of a Multi-core Model for Polymer Nanocomposite Dielectrics”, IEEE Trans. Dielectr. Electr. Insul., Vol.12, No.4, pp. 669-681, 2005.

[16] P. Preetha and M. J. Thomas, “AC Breakdown Characteristics of Epoxy Nanocomposites”, IEEE Trans. Dielectr. Electr. Insul., Vol. 18, No. 5, pp. 1526-1534, 2011.

[17] Y. Li, J. Unsworth and B. Gao, “The Effect of Electrical Ageing on a Cast Epoxy Insulation”, IEEE Electrical Electronics Insulation Conf. Electrical Manufacturing and coil Winding Conference, Chicago, pp. 1-5, 1993.

[18] R. Brutsch, J. A. Alliso, R. Scollay and F. Wolf, “New Trends in the Insulation Technology of Rotating High Voltage Machines”, Coil Winding, Insulation and Electrical Manufacturing Conf., Berlin, Germany, pp. 1-5, 1999.

[19] P. Preetha and M. J. Thomas, “Partial Discharge Resistant Characteristics of Epoxy Nanocomposites”, IEEE Trans. Dielectr. Electr. Insul., Vol. 18, No. 1, pp. 264-274, 2011.

[20] Z. Li, K. Okamoto, Y. Ohki and T. Tanaka, “The Role of Nano and Micro Particles on Partial Discharge and Breakdown Strength in Epoxy Composites”, IEEE Trans. Dielectr. Electr. Insul., Vol. 18, No. 3, pp. 675-681, 2011.

[21] J. R. Weidner, F. Pohlmann, P. Groppel and T. Hildinger, “Nanotechnology in High Voltage Insulation Systems for Turbine Generators – First Results”, XVII Int’l. Sympos. High Voltage Eng., Hannover, Germany, paper No. E088, 2011.

P. Preetha was born in Kerala, India in 1973. She received the B.Tech. degree in electrical engineering from N S S College of Engineering, Palakkad, India in 1995, PG diploma in thermal power plant engineering from RPTI Neyveli in 1997 and the M.E. degree in high voltage engineering from the Indian Institute of Science, Bangalore, India in 2003. Presently she is a research scholar at the Department of Electrical Engineering, Indian Institute of Science,

Bangalore, India. Her areas of interest are aging and discharge resistant characteristics of nanocomposites, and GIS.

M. Joy Thomas (S’85-M’95) was born in Kerala, India in 1961. He received the B.Tech. degree in electrical engineering from the Indian Institute of Technology, BHU, Varanasi, India, the M.S. and the Ph.D. degrees in electrical engineering from the Indian Institute of Science, Bangalore, India. Presently, he is working as an Assistant Professor at the Department of Electrical Engineering, Indian Institute of Science, Bangalore, India. He is also a member of CIGRE. His areas of interest are

EHV/UHV power transmission, high voltage engineering, dielectrics and electrical insulation, condition monitoring and asset management of high voltage power apparatus, nanodielectrics, biodielectrics, pulsed power engineering, plasma science and technology, EMC, lightning, high power electromagnetics and engineering pedagogy.

Documents

Life estimation of electrothermally stressed epoxy nanocomposites