Effect of Tree Channel Conductivity on Electrical Tree Shape and ...

IEEE Transactions on Dielectrics and Electrical Insulation

Vol. 18, No. 3; June 2011

847

Effect of Tree Channel Conductivity on Electrical Tree Shape and Breakdown in XLPE Cable Insulation Samples Xiangrong Chen, Yang Xu, Xiaolong Cao State Key Laboratory of Electrical Insulation and Power Equipment Xi’an Jiaotong University Xi’an 710049, China

S. J. Dodd and L. A. Dissado Department of Engineering University of Leicester Leicester LE1 7RH, UK

ABSTRACT The results of an investigation into electrical tree growth in XLPE cable insulation using an embedded needle electrode are reported for a range of voltages from 9 kV rms to 27 kV rms. The partial discharge (PD) activity and tree structures were measured simultaneously throughout the tree growth and the trees were recorded from initiation up to and including the final runaway stage. A multifractal analysis was also performed on the tree structures as they propagated, and it was found that their fractal dimension increased and the distribution of embedded structures changed as small side channels were added to the tree as it grew. At 11 kV rms only branch trees were found and only bush (bush-branch) trees at higher voltages, but at 9 kV rms trees of three different shapes were formed. Observation of the tree shapes at 9 kV rms under reflected light followed by a detailed analysis using con-focal Raman spectroscopy, showed that the stagnated and branch-pine (monkey puzzle) tree shapes were due to the formation of a conducting graphitic deposit upon the walls on tree branches in the region of the needle electrode. This was not present in the branch trees produced at 9 kV rms. A simple scheme is presented for the formation of branch-pine trees and their corresponding PD activity based on the concept of conducting branch generation. The trees produced at 13 kV rms and above have a bush shape, which converts into a bushbranch shape when a runaway branch grows from their periphery. This is shown to happen when the field at the bush tree periphery exceeded a voltage independent critical value, which was estimated to be 100 MV/m. The consequence of this result for the initiation of the runaway stage in branch trees is commented upon. Index Terms — XLPE cable insulation, electrical tree shape, fractal, multi-fractal, partial discharge, breakdown, conducting tree.

1 INTRODUCTION ELECTRICAL trees (see [1]) are one of the routes to failure in cable insulation [2]. In some cases they may be initiated from metallic protrusions, but in modern high voltage extruded cables for which the most widely used insulating material is cross-linked polyethylene (XLPE) they are most often initiated from discharging cavities [3] or water trees [1, 4]. It has long been known from needle-plane laboratory experiments that the speed at which an electrical tree extends towards the counter electrode depends upon its shape and hence upon its Manuscript received on 18 November 2010, in final form 16 March 2011.

fractal dimension (df), with bush trees for which df > 2 [5] often extending more slowly than branched trees (df < 2) [1, 6] that are produced at lower voltages [6]. In consequence the shape of an electrical tree is a major factor in the risk of insulation failure at a given voltage. In [7, 8] it was pointed out that the shape of an electrical tree and its discharge behavior was synergistically linked through space charge around the tree and on the tubule walls, as suggested in [9]. Both contributions to the space charge are produced by the discharges and control them. It was shown [8, 10] that fluctuations in the local space charge due to deterministic chaos lead to branching and hence the tree shape. Branched trees have long discharges restricted to a few branches at a

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X. Chen et al.: Effect of Tree Channel Conductivity on Electrical Tree Shape and Breakdown in XLPE Cable Insulation Samples

time and thereby extended only from those branches, whereas discharging in bush trees was spread throughout the body of the bush and generated new tree tubules packed together in the bush form [7, 8]. Thus bush trees may extend more slowly than branch trees even though they are produced at higher voltages because they possess a higher damage density expressed by a higher fractal dimension [1, 6]. Recently trees possessing more complicated shapes that have been termed ‘stagnated’ (tree apparently ceases to grow) [11] and ‘monkey-puzzle’ [12, 13] or ‘branch-pine’ [14] (exhibiting many short side branches on each side of the long branches) have been noted to occur in both polyethylene and epoxy resins. It is possible that the more complex electrical tree shapes such as these may influence the relationship between tree extension rate and tree shape previously found for simple fractal trees. It was suggested in [13, 15] that the monkey-puzzle or branch-pine structures were due to the micro-morphology of XLPE produced by non-uniform crystallization. However, this is unlikely given the width of tree-tubules (around 0.5 m [16] to 10 m [1]) and evidence that tree tubules of 0.5 m width grow through lamella and spherulites [16]. Several years ago it was suggested [17] that variation of conductivity of the walls of the tree-tubule would best explain the differences in tree shape. The recent observation, in epoxy resins below their glass transition temperature [18] and certain polyethylene blends and copolymers [16, 19-21], of electrical trees with tubules possessing conducting walls brings that suggestion into prominence. In these trees discharges only occur at the tree tips. The tubules in the rest of the tree possess a conducting layer of disordered graphitic material about 4 nm thick [21] that shorts out the discharge in them. These trees have a branched structure composed of very thin ( 310 0 within this phase. This is followed by phase 2, where the conducting main channels are formed with the decrease of the length l’ of gasfilled tube that remains at the tip of the tree channel. In this phase the length of the whole channel l is almost unchanged giving a constant value for Cb in equation (8). The change in potential drop along the part of the tube l’ that is able to sustain partial discharges is given by ΔUc = (Einc –Eext)l’. and thus the change of Q with respect to time is proportional to dl’/dt and since dl’/dt < 0 in this phase the PD magnitude will decrease with time when phase 2 is entered. The growth of the tree is caused by local breakdown at the tips of the conducting sections. However, the growth direction of the pine branch structure seems to be affected by the morphology of the material. Some pine branches do not extend along the direction of the electric field (shown in Figure 4c). A similar result has previously been reported in [13]. One possible reason for this is that the lateral branches (the fur) of the branch pine tree are produced by a filamentary electromechanical mechanism [34], as the direction of such branches would certainly be influenced by local morphology. In phase 3, the pine branch with black (electrically conducting) tracks grows rapidly to bridge the counter electrode. Here, the carbonized section of the tubule now extends nearly the full length of the channel, with the carbonized section having a length l-l’. The length of the nonconducting (discharging) section at the growing tip, l’, can be regarded as a constant. The PD magnitude, Q, increases as the tree tip nears the counter electrode as

Q  Cb U c 

 0 r A (L  l)

.  Einc  Eext  l ' .

(10)

Assuming that the discharge characteristics of the gas do not change during this phase, then (Einc-Eext)l’ can be regarded as a constant and taking the differential with respect to time gives:

dQ K ' dl  dt ( L  l ) 2 dt

(11)

where K’=0rA(Einc – Eext)l’. As dl/dt>0 in this phase, the PD a mplitudes will show a clear increase as the tree tip grows near to the counter electrode. The final breakdown occurs when the tre e reaches the plane electrode with a large arc discharge star-ting from the counter electrode to end at the needle electrode [1].

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Table 2. Growth model for the branch-pine tree in XLPE insulation. The arrows in the model schematic indicate the growth of a non-conducting tree tubule from the needle tip in Phase A, the growth of carbonization starting at the needle tip and spreading along the existing tubule during Phase B and the growth of a conducting channel in which a small non-carbonized region exists at the growing tree tip. Phase

Propagation process

Model

1.Region A

2.Region B

3. Near the counter electrode in region C

4.4 RUNAWAY TO BREAKDOWN OF ELECTRICAL TREES IN SAMPLES The final conversion to an accelerated growth is a common feature of electrical tree propagation and the time at which it occurs essentially defines the breakdown time because the subsequent growth and arc generation are usually fast compared to the time for the tree to grow to this point [1]. In [13] it was suggested that this change in polyethylene was the result of local non-uniformity of the polymer morphology, whereas in [12] it was suggested that in epoxy resins it occurred because the field at the tree periphery exceeded a critical value as the tree approached the counter (planar) electrode. We have evaluated the maximum field at the periphery of a number of bush trees grown in a range of applied voltages from 13 kV to 27 kV to check the applicability to XLPE of the hypothesis of a critical field for the initiation of accelerated branch growth suggested in [12]. Expression (12), suggested in [12], was used to calculate the maximum field at the bush tree periphery,

Emax 

2V l  r  ln  a  1  a  1

(12)

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Here r is the curvature radius of the needle electrode, V is the peak voltage, l is the tree length, and

a 1

lr Ll

(13)

with L the needle-plane distance. This equation treats the periphery of the bush tree as an equi-potential surface, with a maximum value equal to the potential of needle electrode, and is equivalent to that given by Mason [35] for the needle-plane geometry. Emax is a function of l, so its value changes with the tree propagation. In using equation (12) we assume that any charge injection and net space charge build-up is negligible limiting its use to alternating applied voltages, and also that any local charge injection during each half cycle of the applied voltage acts to homogenize the electric potential at the tree periphery. Figure 17 shows the electric field Emax as a function of the tree length l for three bush trees grown at 15 kV. The plot shows that the initial rapid formation of a bush shape results in strongly diminishing maximum field. This is followed by a slower decelerating period of growth (see Figure 11) that is associated with a slightly diminishing maximum peripheral field. Finally a branch begins to accelerate towards the counter electrode once a critical field is exceeded. The value of this field has been estimated as the average over three bush-trees for a range of voltages and the results are given in Table 3.The values obtained for the critical field are remarkably constant at 100 MV/m, particularly given the fact that the needle voltage doubles over the range of bush trees measured. It is therefore clear that in XLPE acceleration of the tree propagation requires the tree to grow to a position where the maximum field at its periphery exceeds 100 MV/m. This result suggests that the similar acceleration found for branch trees also occurs when the field at the tip of its leading branch exceeds 100 MV/m. The verification of this contention is more difficult than for bush trees because the tree periphery is not smooth and the spatial variation of space charge around the tree tips will inevitably play a role in determining the maximum field at the tree tips. This analysis will be left to a later paper.

Figure 17. Local field at the tree tip as a function of tree length for the bush trees grown with 15 kV applied to the needle.

Table 3. Critical field for the accelerating growth of bush trees. Needle voltage

13 kV

15 kV

18 kV

23 kV

27 kV

Emax 1 (MV/m)

96.8 98.3 97

98.1 101.1 103.9

106.9 98.3 105

103.6 95.8 102.6

103 100 103

97.4

101

103.4

100.7

102

Emax 2 (MV/m) Emax 3 (MV/m) Average Emax (MV/m)

One important point to note is that the approach of the bush tree to the counter electrode is the main factor in determining the increase of the maximum periphery field towards the critical value, and hence in initiating the accelerating branch (see Figure 10). Thus the higher the needle voltage the smaller the bush tree from which an accelerating branch emerges. The time to the onset of the accelerating runaway branch in electrical trees will therefore be determined by the time to grow close enough to the counter electrode for the critical field to be exceeded, and this will depend upon its shape (i.e. fractal dimension), and also the needle-plane separation. The latter will assume an importance in this final stage of tree failure and very large needle plane separations may not allow a bush tree to reach the required critical field, leading to stagnation, cavitations in the bush interior, and possibly crack generation [17]. For non-conducting branch trees runaway will occur when the leading branch approaches the counter electrode. In this case the field between the leading branch and the counter electrode is the important factor. In contrast the final stage for a conducting tree may be determined by the field generated at the branch tips as shielded by the other branches and may not be so strongly affected by the close approach to the counter electrode.

5 CONCLUSIONS Electrical trees with some branches that have conducting walls due to a layer of graphitic carbon have been identified for the first time in XLPE cable insulation. Such trees were found to be predominant (80%) at low (9 kV) needle voltage. It is the generation of conducting branches near to the needle electrode that is responsible for the formation of branch-pine (monkey puzzle) shapes and may even cause the tree to stop its extension (stagnated tree). Branch trees are only produced at this voltage when branch walls near the needle electrode do not become conducting (20%). This result may provide a means of early detection of pre-breakdown phenomena in cable insulation. At higher voltages all the trees had nonconducting branches and showed the usual cross-over from branch to bush shape at a needle voltage of 13 kV. Multifractal analysis shows that trees shapes evolve slowly as the tree grows, with a slowly increasing fractal dimension and a distribution of embedded shapes that shifts steadily as smaller lateral micro-channels are added to the main branches. Bush trees were found to possess a smaller range of embedded fractal structures than branches.

IEEE Transactions on Dielectrics and Electrical Insulation

Vol. 18, No. 3; June 2011

The time to breakdown due to the bush electrical trees is dominated by the propagation of the tree until the field at its periphery reaches a critical value of ~ 100 MV/m irrespective of the needle voltage. At this time a rapidly accelerating branch forms and the tree takes a bush-branch shape, with breakdown taking place soon after the leading branch reaches the counter electrode. The propagation time of the electrical trees is influenced by their shape as well as the needle voltage thereby leading to breakdown times in different ranges when there are different shapes at the same voltage, and a non-monotonic dependence of the breakdown time on the applied needle voltage.

ACKNOWLEDGMENT The authors wish to thank the National Science Foundation of China under Projects No.50877057. They also acknowledge Prof. Kai Wu and Prof. Xiaoquan Zheng of Xi’an Jiaotong University (XJTU), China, for their help during the course of the work. Xiangrong Chen also thanks Mr. Jun Wang of the School of Science at XJTU for his help with the test of Raman spectroscopy.

REFERENCES [1] [2] [3] [4]

[5] [6]

[7] [8] [9] [10]

[11]

[12]

[13]

L. A. Dissado and J. C. Fothergill, Electrical Degradation and Breakdown in Polymers. Peter Peregrinus for IEE: London, 1992. D. W. Kitchin and O. S. Pratt, "Treeing on polyethylene as a prelude to breakdown", AIEE Trans. Power App. Syst., Vol. 77 (Part III), pp.180-186, 1958. J. H. Mason, “The deterioration and breakdown of dielectric resulting from internal discharges”, Proc. IEE, Vol. 98, No.1, pp. 44-59, 1951. Y. Hayashi, Y. Suzuoki, T. Kato, F. Komori, and H. Mashima, “Temperature dependence of electrical tree inception from water tree degradation”, IEEE Conf. Electr. Insul. Dielectr. Phenomena (CEIDP), pp. 498-501, 2009. K. Kudo, “Fractal analysis of electrical trees”, IEEE Trans. Dielec. Electr. Insul., Vol.5, pp.713-727, 1998. J.C. Fothergill, L.A. Dissado and P.J.J. Sweeney, “A dischargeavalanche theory for the propagation of electrical tree: a physical basis for their voltage dependence”, IEEE Trans. Dielectr. Electr. Insul., Vol. 1, pp. 474-486, 1994. J.V. Champion and S.J. Dodd, “Systematic and reproducible partial discharge patterns during electrical tree growth in an epoxy resin”, J. Phys. D: Appl. Phys., Vol. 29, pp. 862-868, 1996. L.A. Dissado, “Understanding electrical trees in solids: from experiment to theory”, IEEE Trans. Dielectr. Electr. Insul., Vol. 9, pp.483-497, 2002. R. J. Densley, “An investigation into the growth of electrical trees in XLPE cable insulation”, IEEE Trans. Electr. Insul., Vol. 14, pp. 148158, 1979. L.A. Dissado, J.C. Fothergill, N. Wise, A. Willby, and J. Cooper, “A deterministic model for branched structures in the electrical breakdown of solid polymeric dielectrics”, J. Phys. D: Appl. Phys., Vol. 33, pp. L99-L112, 2000. X.R. Chen, Y. Xu, J. Xu, W. Shi, W.H. Yang, Y. Liu and X. L. Cao, “Propagation and partial discharge characteristics of electrical trees in 110 kV XLPE cable insulation at power frequency applied voltage”, High Voltage Engineering, Vol. 36, pp. 2436-2443, 2010 (In Chinese). J.V. Champion, S.J. Dodd and G. C. Stevens, “Analysis and modeling of electrical tree growth in synthetic resins over a wide range of stressing voltage”, J. Phys. D: Appl. Phys., Vol. 27, pp.1020-1030, 1994. X.Q. Zheng and G. Chen, “Propagation mechanism of electrical Tree in XLPE cable insulation by investigating a double electrical tree structure”, IEEE Trans. Dielectr. Electr. Insul., Vol.15, pp.800-807, 2008.

859

[14] G. Chen and C.H. Tham, “Electrical treeing characteristics in XLPE power cable insulation in frequency range between 20 and 500 Hz”, IEEE Trans. Dielectr. Electr. Insul., Vol. 16, pp.179-188, 2009. [15] A.S. Xie, X.Q. Zheng, S.T. Li and G. Chen, “Investigation of electrical trees in the inner layer of XLPE cable insulation using computer-aided image recording monitoring”, IEEE Trans. Dielectr. Electr. Insul., Vol.17, pp. 685-693, 2010. [16] Y.Zhao, A.S.Vaughan, J.V.Champion, S.J.Dodd, and S.J.Sutton, “The structure of Electrical Trees in Semi-Crystalline Polymers”, Proc. 8th Int’i. Conf. Dielec. Materials Measurements and Applications (DMMA), IEE Conf. Pub. 473, pp. 314-319, 2000. [17] C. Laurent and C. Mayoux, “Analysis of the propagation of electrical treeing using optical and electrical methods”, IEEE Trans. Electr. Insul., Vol.15, pp. 33-42, 1980. [18] J.V. Champion and S. J. Dodd, “Simulation of partial discharges in conducting and non-conducting electrical tree structures”, J. Phys. D: Appl. Phys., Vol. 34, pp.1235-1242, 2001. [19] A.S. Vaughan, S.J. Dodd, and A.M. Macdonald, “Tree growth in a Propylene/Ethylene Copolymer: Relationships between Electrical Activity during Growth and the Structure, Chemistry and Properties of the Trees that Result”, IEEE Conf. Electr. Insul. Dielectr. Phenomena (CEIDP), pp. 548-551, 2005. [20] S.J.Dodd, J.V.Champion, Y.Zhao, A.S.Vaughan, S.J.Sutton and S.G.Swingler, “Influence of morphology on electrical treeing in polyethylene blends”, IEE Proc.-Sci. Meas. Technol., Vol. 150, pp. 58-64, 2003. [21] A.S. Vaughan, I.L. Hosier, S.J. Dodd and S.J. Sutton, “On the structure and chemistry of electrical trees in polyethylene”, J. Phys. D: Appl. Phys., Vol. 39, pp.962-978, 2006. [22] M. Fuji, T. Saito, H. Ohnishi, K. Arii and Y. Yoshino, "Three dimensional measurement of tree in polymer applied successive impulse voltage and their multifractal", IEEE 5th Int’l. Conf. Cond. Breakdown Solid Dielectr. (ICSD), pp.304-308, 1995. [23] L.A. Dissado, “Predicting electrical breakdown in polymeric insulators”, IEEE Trans. Dielec. Electr. Insul., Vol. 9, 860-875, 2002. [24] IEC 60270, Partial Discharge Measurement, 3rd ed., 2000. [25] P. Grassberg, “Generalized dimensions of strange attractors”, Phys. Lett. A, Vol. 97, pp. 227-230, 1983. [26] J. Feder, Fractals, Plenum Press: New York, 1988. [27] L.A. Dissado, J.C. Fothergill, A. See, G.C. Stevens, L. Markey, C. Laurent, G. Teyssedre, U.H. Nilsson, G.Platbrood and G.C. Montanari, “Characterising HV XLPE cables by electrical, chemical, and microstructural measurements on cable peeling: effects of surface roughness, thermal treatment and peeling location”, IEEE Conf. Electr. Insul. Dielectr. Phenomena (CEIDP), pp. 136-140, 2000. [28] N. Hozumi, T. Okamoto and H. Fukugawa, “Simultaneous measurement of microscopic image and discharge pulses at the moment of Electrical Tree Initiation”, Jap. Jour. Appl. Phys., Vol. 27, pp. 572-576, 1988. [29] C. Laurent and C. Mayoux, “From initiation to propagation in electrical treeing”, Proc. IEE Int. Conf. Partial Discharges, pp.7-8, 1993. [30] H.Kaniewa, Y.Suzuoki and T.Mizutani, “Partial discharge characteristics and tree inception in artificial simulated channels”, IEEE Trans. Dielectr. Electr. Insul., vol.7, pp. 834-838, 2000. [31] J.V. Champion, S.J. Dodd, J. Alison and L. Askew, “Electrical tree growth and space charge behaviour of epoxy resin blends”, IEEE 5th Int’l. Conf. Cond. Breakdown Solid Dielec. (ICSD), pp.321-324, 1998. [32] K. Wu, Y. Suzuoki, T. Mizutani and H.K. Xie, “Model for partial discharges associated with treeing breakdown: III. PD extinction and re-growth of tree”, J. Phys. D: Appl. Phys., Vol. 33, pp. 1209-1218, 2000. [33] R.Joctuer, E.Favrie, and H.Auclair, “Influence of surface and internal defects on polyethylene electrical routine tests on VHV Cables”, IEEE Trans. Power App. Syst., Vol.96, pp. 513-523, 1977. [34] J.C.Fothergill, “Filamentary electromechanical breakdown”, IEEE Trans. Electr. Insul., Vol. 26, pp.1124-1129, 1991. [35] J. H. Mason, “Breakdown of solid dielectrics in divergent fields”, Proc. IEE-Part C: Monographs, Vol.102, No.2, pp. 254-263, 1955.

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Xiangrong Chen (S’08) was born in Hunan, China in 1982. He received the B.S. degree in electrical engineering from the Lanzhou Jiaotong University, Lanzhou, China in 2005, the M.S. degree in electrical engineering from XJTU, Xi’an, China in 2008. He is currently a Ph.D. student at the State Key Laboratory of Electrical Insulation and Power Equipment (SKLEIPE) of XJTU, Xi’an, China. He was also the winner of the OUTSTANDING AWARD of 3M innovation scholarship in 2008 and the excellent prize winner of the first HYOSUNG thesis competition in 2010.

Yang Xu (M’05) was born in Shaanxi, China in 1969. He received the B.S., M.S. and Ph.D. degree in electrical engineering from XJTU, Xi’an, China, in 1991, 1994 and 2005, respectively. He has been an associate professor of XJTU. His research interests lie in PD measurements, on-line monitoring of power equipments. He is a member of the National Standardization Technical Committee of insulating materials, China, and also a member of the CIGRE SC B1 WG-28.

Xiaolong Cao (SM’98) was born in Shaanxi, China in 1945. He received the B.S., M.S. and Ph.D. degree in electrical engineering from XJTU, Xi’an, China, in 1968, 1983 and 1986, respectively. He became a Professor of XJTU in 1996. His research interests include power cable, electrical insulation test, nanodielectrics. He is the Vice Chairman of the National Standardization Technical Committee of insulating materials, China, and also a Member of the CIGRE SC B1.

S. J. Dodd was born in Harlow, Essex in 1960. He received the B.Sc. (Hons) Physics degree in 1987 and the Ph.D. degree in physics in 1992, both from London Guildhall University, UK and remained at the University until 2002 as a Research Fellow. He joined the University of Southampton in 2002 as a Lecturer in the Electrical Power Engineering Group in the School of Electronics and Computer Science and then the University of Leicester in the Electrical Power and Power Electronics Research Group in the Department of Engineering in 2007 as a Senior Lecturer. His research interests lie in the areas of light scattering techniques for the characterization of polymer morphology, electrical treeing breakdown process in polymeric materials and composite insulation materials, electroluminescence and its relationship with electrical and thermal ageing of polymers, characterization of liquid and solid dielectrics and condition monitoring and assessment of high voltage engineering plant. His research has attracted funding from the electrical generation and transmission industries. He has published 21 papers, 37 conference papers and contributed to two books. L. A. Dissado (F’06) was born in St. Helens, Lancashire, U.K on 29 August 1942. He was educated in Thomas Linacre Technical School, Wigan, Lancashire, 19531960, gaining a State Scholarship for University Entry in 1959. He graduated from University College London with a 1st Class degree in chemistry in 1963 and was awarded the Ph.D. degree in theoretical chemistry in 1966 and the D.Sc. degree in 1990. After rotating between Australia and England twice he settled in at Chelsea College in 1977 to carry out research into dielectrics. His interest in breakdown and associated topics started with a consultancy with STL that begun in 1981. Since then he has published many papers and one book, together with John Fothergill, in this area. In 1995 he moved to The University of Leicester, and was promoted to Professor in 1998. He has been a visiting Professor at The University Pierre and Marie Curie in Paris, Paul Sabatier University in Toulouse, Nagoya University, and NIST at Boulder Colorado. He has given numerous invited lectures, the most prestigious of which was the Whitehead lecture at CEIDP 2002 in Cancun, Mexico. Currently he is an Associate Editor of IEEE Transactions on Dielectrics and Electrical Insulation and Chair of the DEIS Publications Committee. He was awarded the degree of Doctuer Honoris Causa by the Universite Paul Sabatier, Toulouse, France, in 2007, and was made an Honorary Professor of XJTU, Xi’an, China in 2008. He was also made an international academic committee of SKLEIPE of XJTU in 2010.