Insulation/Semicon Interface - IEEE Xplore

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Polymeric HVDC Cable Design and Space Charge Accumulation. Part 1: Insulation/Semicon Interface Key Words: HVDC cables, polymeric insulation, semicon shield, field inversion, space charge, charge accumulation, charge packets, conduction current, threshold

Introduction

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xtruded polymeric cables have almost entirely substituted paper-oil cables in extra-high-voltage AC applications since the 1990s. The wide use of polymeric cables is related to the high reliability, reduced cost, low maintenance and environmental impact of extruded insulation with respect to paper-oil cables. Polymeric HVDC cables have been used in several commercial projects with voltage source converters and now are available up to 300 kV and with dedicated (voltage-source) converter [1]. But systems with conventional converter technology working at voltage levels up to 500 kV still make use of paper-oil cables, because polymeric cables showed premature breakdown problems. The cause of the lack of reliability of HVDC polymeric cables with conventional converter technology has been associated with a deviation of real stresses withstood by cables in service from the insulation design stresses, limiting the typical average design field to 10-15 kV/mm to date. In particular, it must be recalled that the electric field applied to HVDC cable insulation depends mainly on electrical conductivity, which has an exponential relationship with temperature. A temperature gradient across the cable insulation, caused by heating due to Joule losses originating from the current in the conductor, generates a conductivity gradient. This provides an electric field profile that can be significantly different from that derived by considering only the cylindrical geometry of the cable insulation [2]. If the conductivity gradient is sufficiently large, a phenomenon known as “field inversion” can occur; that is, the electric field at the outer semicon can become even higher than the field at the inner semicon. Even if this aspect is taken into account in HVDC cable design, it is the effect of space charge generated by these factors, i.e. field inversion and temperature gradient, that cannot be predicted easily. Indeed, space charge accumulation generally is claimed to be the main factor accelerating degradation of polymeric insulation in HVDC conditions with respect to HVAC [3]–[6]. This is due to the local electric field

D. Fabiani and G. C Montanari University of Bologna, Italy

C. Laurent and G. Teyssedre Université Paul Sabatier, Toulouse, France

P. H. F. Morshuis TU Delft, The Netherlands

R. Bodega Prysmian Cables and Systems, The Netherlands

L. A. Dissado University of Leicester, UK

A. Campus and U. H. Nilsson Borealis AB Stenungsund, Sweden

Space charge measurements provide important information to cable material manufacturers enabling them to tailor an insulation and semicon specifically for HVDC application, and, thus, improving the reliability of polymeric cables.

November/December 2007 — Vol. 23, No. 6 0883-7554/07/$25/©2007IEEE

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amplification, produced by accumulated charge, which leads to an increase of degradation rate (for instance, 50% electrical field amplification means about a 60 times life reduction, considering an inverse-power life model with a voltage endurance coefficient (VEC) =10 [7]). However, space charge is also the effect of aging. The chemical-physical structure of an insulating material can be modified through aging processes, leading to a change of trap level distribution (as regards, depth and density) and, thus, charge injection effectiveness. All these considerations prompted research tasks within the fifth framework-program European Project HVDC, “Benefits of HVDC Links in the European Power Electrical System and Improved HVDC Technology”. The aim of this project was to show the potential technical, economic, and environmental benefits of using HVDC polymeric cables, and to improve the knowledge of the factors that affect design and reliability of polymeric HVDC cables. The outcomes of the project provide a better knowledge of the electrical properties at high DC field, through the development of improved test techniques for space charge and conduction current measurements. Moreover, evaluation of charge accumulation in insulation systems and models for space charge build-up and life performance aiming at providing reliability assessment were investigated in detail [8]–[10]. One of the most important topics dealt with in this project was the investigation of the effect of interfaces between electrode/semicon/insulation or different insulating materials on space charge injection and accumulation in insulation [11]–[13]. Interfaces, in fact, can constitute weak points of the insulation, in which space charge can be accumulated and/or injected, and electric fields can be enhanced. Indeed, if the electric field applied to an insulation system is below the threshold for space charge accumulation/ injection, which has been shown to coincide with the onset of the space charge limited current (SCLC) [14]–[16], the injected charge is small, and the insulation system behaves according to Ohm’s law [17]. As temperature and/or electric field increases, space charge injection from the electrodes becomes larger and larger, and the electric field distribution may depart from the Laplacian field. Often the Schottky law is used to describe the injection mechanism, i.e. [18]:

is to provide rationale and guidelines for an appropriate choice of semicon and insulation among materials candidates to be used for HVDC cables, as well as to prompt improvements of the technological processes for materials and cable manufacturing. The effect of temperature gradient on space charge accumulation and field profile modification is investigated in the third part [19].

Models Used Within the HVDC Project for Experimental Tests For the purpose of investigating the space charge behaviour of peroxide crosslinkable insulating/semiconducting materials considered for HVDC cable applications, experimental tests were performed on three kinds of specimens: • Plaques composed of a sandwich of an insulation layer and a semiconducting layer on one side. The specimens were obtained by press-moulding and crosslinked at 180°C, followed by cooling at a rate of 15°C/minute. • Reduced-scale cable models (mini-cables), having three layers (see Figure 1): inner semicon, insulation layer (thickness = 1.5 mm), and outer semicon. The conductor cross section is 6.1 mm2. • Full-scale cables, differing from the second specimen in the insulation thickness (4.5 mm) and conductor cross section (50 mm2). Plaques of three insulation systems (insulation/semicon) and five insulation systems of mini-cables and full-size cables were tested. Three insulating materials (INS1, INS2 and INS3) and two semiconductive compounds (SC1 and SC2) were tested in this study. INS1 and SC1 represent today’s state of the art of material technology used in AC EHV systems. INS3 is an example of a polar insulation with improved wet aging characteristics that has found wide acceptance in the MV distribution network. INS2 and SC2 are examples of material systems designed to respectively, minimize the concentration of peroxide reaction products and optimize the control of the semicon/insulation interface. The chosen combinations of insulation and semicon were INS1-SC1,

(1)

where J is the conduction current, A is a constant of the material, T is the absolute temperature, k is Boltzmann’s constant, ΔW is the activation energy, E is the electric field, and e is the electron charge. The properties of the electrode/insulation (and thus semicon/ insulation) interface will considerably affect ΔW and, thus, the extent of injected and accumulated space charge [8]. This article, the first of a series of three contributions, will discuss results of space charge measurements related to charge injection from the semicon/insulation interface and accumulation in the insulation bulk, considering different combinations of insulation/semicon. Space charge build-up at the dielectric/ dielectric interface is dealt with in the second part [15]. The aim

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Figure 1. Mini-cable specimen. Conductor diameter = 2.8 mm; Inner semicon thickness = 0.7 mm; Insulation thickness = 1.5 mm; Outer semicon thickness = 0.15 mm.

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INS2-SC1, INS2-SC2 for the plaques, and INS1-SC1, INS2-SC1, INS1-SC2, INS2-SC2, and INS3-SC1 for the mini-cables. Specimens of cables and plaques were thermally pre-treated for 5 days at 80°C and for 3 days at 70°C, respectively, in order to expel virtually all crosslinking by-products. However, one cable (INS3-SC1) also was evaluated without prior degassing. These different models were chosen in order to find the best compromise between the effort involved in manufacture and the capability of experimental tests to be representative of real conditions in power cables. Plaques are traditionally used for testing, but mini and full-scale cables are more appropriate to simulate actual stress conditions (e.g., electric field distribution, temperature gradient) and real manufacturing processes (e.g., expulsion of contaminants and impurities in cables is more difficult than in flat specimens). Nevertheless, it must be pointed out that comparable results, from space charge and conduction current measurements, were obtained on cables (mini and full-size) and plaques having the same combination of insulation/semicon materials under isothermal conditions [11]. Thus, for the sake of brevity, this article will focus mainly on results obtained on mini-cables. Results on full-scale cables are reported in [20], [21].

Experimental A. Test Procedures Space charge measurements were performed by means of the pulsed electro acoustic (PEA) technique [22], [23] on mini-cables and plaques at different electric field values up to 60 kV/mm, with the aim of obtaining the space charge accumulation characteristics for each insulation/semicon combination. Tests were performed at different temperatures, ranging from 25°C to 70°C. They were carried out under isothermal conditions, i.e., by heating the specimen uniformly inside an oven, avoiding any temperature gradient across the insulation, which can have an influence on space charge build-up [19]. Polarization and depolarization lasted 10,000 s and 3600 s, respectively. To estimate space charge accumulation, the medium charge density, Qmed, which quantifies the absolute space charge density accumulated in the insulation bulk, can be calculated from the space charge profiles according to the following equation [9], [24]:

(2)

where x1 and x2 are the electrode positions, and ρ(x) is the space charge density profile measured at the beginning of the depolarization phase (2 seconds after voltage-off). Conduction current measurements also were carried out with the aim of supporting space charge results and to enable a better understanding of the behavior of the different combinations of insulation/semicon materials to be achieved.

B. Test Results Typical space charge patterns representing in a three-dimensional way the behavior of space charge as a function of time are

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reported in Figure 2. They were obtained using mini-cables at two fixed temperatures, i.e., 25°C (Figure 2A) and 70°C (Figure 2B), respectively, and different electric fields. To better understand the space charge patterns, Figure 3 shows an enlarged version of the three-dimensional plot relevant to INS3-SC1 (untreated) at 60 kV/mm and 25°C, where a packet of positive charge (warm colors) and negative charge (cold colors) are present. The white is due to saturation in the color scale, thus representing amount of space charge that exceeded the sensitivity scale (this is the case of electrode charge). The anode and cathode correspond to the inner and outer semicon, respectively. It can be seen from Figure 2 that more space charge accumulates in the insulation bulk on increasing electric field and/or temperature. In particular, charge packets in the insulation bulk i.e., injected charge travelling from one electrode to the other, can be observed, especially at low temperature (see, for example, Figure 2A, INS2-SC2 at 60 kV/mm). Moreover, a significant amount of heterocharge (i.e., charge having opposite polarity with respect to electrode charge) close to both electrodes, increasing with poling time, can be detected, particularly at higher temperature and electric field. In order to quantify the space charge accumulation, the values of Qmed, calculated by (2), as a function of maximum applied electric field (i.e., the field at the inner conductor semicon/insulation interface) are plotted in Figures 4A and B, for tests at 25°C and 70 °C, respectively. The following observations can be drawn: • The mini-cable model INS3-SC1, both thermally treated and untreated, shows the largest space charge accumulation at all electric fields and temperatures. • At low temperature (Figure 4A), INS2-SC1 displays the smallest amount of charge, followed by INS1-SC1, INS2SC2, and INS1-SC2. It can be noted, however, that estimated values of Qmed are about the same value for these last three specimens. • At higher temperatures (e.g., at 70°C, Figure 4B), space charge accumulation increases for all the insulation systems. INS2-SC2 seems to be the insulation system showing the smallest accumulated charge, followed by INS1-SC2, INS1SC1, and INS2-SC1 (in terms of space charge increase). • The specimens containing semicon SC2 accumulate more charge at room temperature when compared to materials associated with SC1, especially at higher fields. The opposite occurs at 70°C at which the materials associated with SC1 store a larger amount of charge. Similar results were obtained on plaque specimens. The plots of Figures 5A and B collect the space charge characteristics of the tested insulation systems, i.e., the accumulated charge amount as a function of the maximum geometric electric field, at 25°C and 70°C, respectively. The threshold field, below which space charge accumulation is negligible, decreases as temperature increases, probably because injection from the electrodes is promoted by high temperature. In particular, the threshold almost halves for all the tested insulation systems, between 25°C and 70°C. As already emphasized, tests performed on INS3-SC1 (treated and untreated) show a space charge accumulation that is much larger than the other materials.

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(A)

(B)

Figure 2. Summary of space charge patterns obtained with mini-cables at different electric fields (max value at the inner conductor semicon/insulation interface). Test temperature = 25°C (A) and 70°C (B).

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Figure 3. Enlarged view of the space charge pattern relevant to INS3-SC1 (untreated) at 60 kV/mm and 25°C as a function of time. Cathode (outer semicon), anode (inner semicon), and charge packets also are highlighted.

charge. The opposite holds for INS1-SC2, which shows the lowest threshold (10 kV/mm) and an amount of charge almost double with respect to INS2-SC1 at any electric field. At higher temperature (70°C) the situation looks different. In this case, the best combination regarding space charge amount is INS2 (as at 25°C) and SC2, because in INS2-SC2 the threshold is about 5 kV/mm. The second best combination contains again semicon SC2 (i.e., INS1-SC2), and the one providing the largest amount of space charge involves SC1 (INS2-SC1). Conduction current measurements confirm the differences in the thresholds for space charge accumulation (see Figure 6) among the materials tested. Indeed, at 25°C (Figure 6A) materials having the largest space charge amount (e.g. INS2-SC2) also have the highest conductivity. However, on increasing the temperature, the thresholds and current levels of all insulation systems inves-

Among the different combinations, INS2-SC1 and INS2-SC2 show the largest threshold at 25 °C and 70°C, respectively (i.e., 12 kV/mm and 5 kV/mm). This confirms the previously commented results in which the semicon SC1 seems to be much more sensitive to temperature in terms of charge injection into the insulation bulk than SC2. This aspect will be clarified in the next Section.

Discussion The experimental data show that the insulating material INS3 accumulates the most charge of all materials at both 25°C and 70°C. This behavior probably can be attributed to its slightly polar nature as compared to the other materials tested. It should be clearly repeated here that the ranking is based on studies on almost completely degassed specimens. The most important aspect to be raised, however, deals with the other kinds of insulation and semiconductive combinations, which play a significant role in space charge injection/accumulation. In particular, the semiconductive material seems to have a large influence on charge injection and removal at the electrodes. It can be seen that the presence of charge packets in the insulation bulk at high fields (see Figure 2) gives rise to heterocharge formation close to the electrodes. Indeed, the charge injected from one semicon electrode travels across the bulk and stops at the other semicon interface which seems to act as a partially-blocking electrode. The mobility of this charge is significant, especially at high field and temperature, as heterocharge starts accumulating after just a few seconds from voltage application. The assumption of fast charge packets traveling in the insulation bulk has been proven by an ultra-fast acquisition system and results reported in [25]. It can be highlighted that the amount of heterocharge at one semicon/insulation interface increases as the temperature rises, because charge injection from the other electrode is promoted by temperature (as shown by experimental results, see Figure 2). Comparing space charge features (e.g., accumulation threshold and space charge amount) in the different combinations of insulating and semiconducting materials tested (see Figures 4 and 5), it can be seen that, at room temperature, the best combination of insulating and semicon materials for DC applications are INS2 and SC1, as INS2-SC1 shows the highest space charge accumulation threshold (12 kV/mm) and the smallest amount of trapped


Figure 4. Charge density, Qmed, for mini-cables at 25°C (A) and 70°C (B) as a function of max applied field, i.e., at the inner conductor semicon/insulation interface.

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Figure 5: Space charge characteristics for mini-cables at 25°C (A) and 70°C (B). The thresholds for space charge accumulation are indicated by arrows.

Figure 6: Conduction current characteristics for mini-cables at 25°C (A) and 70°C (B). The thresholds for SCLC conduction are indicated by arrows.

tigated come closer together (Figure 6B). It should be noted, as mentioned in the introduction, that, at any temperature and for every insulation system (plaques or mini cables), space charge thresholds coincide with the bend in the conduction current characteristic, corresponding to the transition from ohmic to SCLC conduction [17]. In summary, the experimental tests show that the insulating material INS2 accumulates less charge than INS1, especially in combination with semicon SC1. However, at high temperature, the semicon seems to play the major role regarding space charge accumulation, because all the combinations involving SC1 show the larger amount of charge. Thus, it can be speculated that SC1 and SC2 will show a different dependence on temperature as regards charge injection, with SC1 injecting more charge than SC2

into the insulation bulk at high temperatures, while the opposite occurs at room temperature. To better investigate the dependence of space charge on temperature, the activation energy can be estimated by making Arrhenius plots (Figure 7) of the conduction current values measured at three different temperatures (25, 45, and 70°C) for a given electric field (20 kV/mm in Figure 7) and for three combinations involving two insulations and two semicons [26]. It can be observed that the larger the activation energy, the greater the influence of temperature on conduction current. The results of Figure 7 show that the combinations of semicon/insulating materials having SC1 exhibit a larger activation energy than those containing SC2, even when the insulation is the same. This demonstrates that injection is an important factor in the steady state current and may explain why the semicon,

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loring insulation and semicon specifically for HVDC application and, thus, improving the reliability of polymeric cables.

Acknowledgments This research was performed within the European Project (of the fifth framework-program) HVDC, “Benefits of HVDC Links in the European Power Electrical System and Improved HVDC Technology,” Contract No. ENK6-CT-2002-00670.

References

Figure 7: Arrhenius plot relevant to conduction current values at different temperatures on mini-cable specimens. The value of activation energy also is indicated.

particularly SC1, plays an increasing role with temperature and the different dependence of charge injection on temperature for the two semicon materials. It is noteworthy that, because INS2-SC2 shows the lowest activation energy, the electrical conductivity and, thus, the electric field profile of a DC cable manufactured using this material combination, would be less dependent on temperature, which can affect in a positive way the cable reliability. Cable systems in operation witness a temperature gradient across the insulation that is dependent on the load of the cable. This is the cause of the well-known field inversion in DC cables. A low activation energy would mitigate the change of the electric field during load changes, resulting in a more uniform electric field.

Conclusions From theory and experiments, it can be deduced that materials for DC applications should not accumulate a large amount of space charge if accelerated degradation of the insulation system is to be avoided. Therefore, the characterization of DC insulation must take into account the evaluation of space charge accumulation. This cannot be done exhaustively without taking a system approach considering both the semiconductive material and the insulation, in particular, the properties of the semicon/insulation interface. The latter interface, in fact, plays a major role in space charge injection/accumulation in the insulation bulk. Having analyzed different semiconductive and insulating materials candidate for HVDC cable applications, the best solution to be exploited for HVDC cable design would be the combination showing a high threshold for space charge accumulation, a small rate of charge accumulation as a function of electric field and a small activation energy, i.e., a space charge amount less dependent on temperature. Therefore, space charge measurements will provide important information to cable material manufacturers with the aim of tai-


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[15] S. Delpino, D. Fabiani, G. C. Montanari, C. Laurent, G. Teyssedre, P. H. F. Morshuis, R. Bodega, and L. A. Dissado, “Polymeric HVDC cable design and space charge accumulation. Part 2: Insulation interfaces,” IEEE Elect. Insul. Mag., to be published. [16] G. C. Montanari, “The electrical degradation threshold of polyethylene investigated by space charge and conduction current measurements,” IEEE Trans. Dielectr. Elect. Insul., vol. 7, pp. 309–315, June 2000. [17] L. A. Dissado, C. Laurent, G. C. Montanari, and P. H. F. Morshuis, “Demonstrating a threshold for trapped space charge accumulation in solid dielectrics under DC field,” IEEE Trans. Dielectr. Elect. Insul., vol. 12, pp. 612–620, June 2005. [18] D. A. Seanor, Electrical Properties of Polymers, London: Academic, 1982. [19] D. Fabiani, G. C. Montanari, C. Laurent, G. Teyssedre, P. H. F. Morshuis, R. Bodega, and L. A. Dissado, “Polymeric HVDC cable design and space charge accumulation. Part 3: Effect of temperature gradient,” IEEE Elect. Insul. Mag., to be published. [20] R. Bodega, P. H. F. Morshuis, D. Fabiani, G. C. Montanari, and J. J. Smit, “Calculations and measurements of space charge in loaded MV-size extruded cables systems,” IEEE CSC, Tours, France, July 2006. [21] R. Bodega, P. H. F. Morshuis, G. C. Montanari, D. Fabiani, and J. J. Smit, “The use of cable system models for the assessment of space charge behaviour in full-size DC cable systems”, 2006 Ann. Rep. CEIDP, Kansas City, Oct. 2006, pp. 412–415. [22] G. C. Montanari, G. Mazzanti, E. Boni, and G. De Robertis, “Investigating ac space charge accumulation in polymers by PEA measurements,” 2000 Ann. Rep. CEIDP, Victoria, Canada, Oct. 2000, pp. 113–116. [23] T. Maeno, T. Futami, H. Kushibe, T. Takada, and C. M. Cooke, “Measurement of spatial charge distribution in thick dielectrics using the pulsed electroacoustic method,” IEEE Trans. Elect. Insul., vol. 23 no. 3, pp. 433–439, 1988. [24] G. C. Montanari, “Extraction of information from space charge measurements and correlation with insulation ageing,” Proc. IEEE CSC, Tours, France, July 2001, pp. 178–184. [25] S. Delpino, D. Fabiani, G. C. Montanari, L. A. Dissado, C. Laurent, and G. Teyssedre, “Fast charge packet dynamics in XLPE insulated cable models,” 2007 Ann. Rep. CEIDP, Vancouver, Canada, pp. 421–424, Oct. 2007. [26] K. R. Bambery and R. J. Fleming, “Activation energy and electron transport in LDPE”, 2003 Ann. Rep. CEIDP, Albuquerque, Oct. 2003, pp. 28–31.

Davide Fabiani was born in Forlì, Italy, on January 7, 1972. He received the M.Sc. (honors) and Ph.D. degrees in electrical engineering from the University of Bologna in 1997 and 2002, respectively. He is currently a research associate at the Department of Electrical Engineering of the University of Bologna. His research interests deal with the effects of harmonics on accelerating insulation degradation, characterization of insulating, magnetic, superconducting, nanocomposite and electret materials, aging

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investigation and diagnostics on power system insulation, and, particularly, motor windings subjected to fast repetitive pulses. To date he is the author or co-author of more than 70 papers. He is a member of IEEE DEIS, IEEE PES, and AEI.

Gian Carlo Montanari was born on August 11, 1955. In 1979, he took the Master degree in electrical engineering at the University of Bologna. He is currently a full professor of electrical technology at the Department of Electrical Engineering of the University of Bologna, and he teaches courses of technology and reliability. He has worked since 1979 in the field of aging and endurance of solid insulating materials and systems, diagnostics of electrical systems, and innovative electrical materials (magnetics, nanomaterials, electrets, superconductors). He also has been engaged in the fields of power quality and energy market, power electronics, reliability, and statistics of electrical systems. He is an IEEE Fellow and member of AEI and Institute of Physics. He is a member of the AdCom of the IEEE DEIS. Since 1996 he has been President of the Italian Chapter of the IEEE DEIS. He is convener of the Statistics Committee and member of the Space Charge, Multifactor Stress, and Meetings Committees of IEEE DEIS. He is Associate Editor of IEEE Transactions on Dielectrics and Electrical Insulation. He is founder and President of the spin-off TechImp, established on 1999. He is the author or co-author of about 500 scientific papers.

Christian Laurent was born in Limoges, France, in 1953. He studied solid state physics at the National Institute for Applied Sciences in Toulouse and received his Eng. degree in physics in 1976. He joined the Electrical Engineering Laboratory at Paul Sabatier University in 1977 to study electrical treeing and partial discharge phenomena, which were the topics of his Dr. Eng. Degree (1979). He joined CNRS (National Centre for Scientific Research) in 1981 and received his Doc-ès Sc. Phys. in 1984. In 1985, he spent 1 year as a post-doctoral fellow with the IBM Almaden Research Center, where he studied plasma-polymerized thin films. Back in Toulouse he developed an approach to electrical aging in polymeric materials based on luminescence analysis. He is now dealing with experimental and modeling activity relating to charge transport and aging. He is currently research director at CNRS and director of the Laboratory of Plasma and Energy Conversion -LAPLACE- in Toulouse.


Gilbert Teyssedre was born in May 1966 in Rodez, France. He received his engineering degree in materials physics in 1989 from the National Institute for Applied Science (INSA) and graduated in solid state physics the same year. Then he joined the Solid State Physics Lab in Toulouse and obtained a Ph.D. degree in 1993 for a work on transition phenomena and electro-active properties of fluorinated ferroelectric polymers. He entered the CNRS in 1995 and has been working since then at the Electrical Engineering Lab (now Laplace) in Toulouse. His research activities concern the development of luminescence techniques in insulating polymers with focus on chemical and physical structure, degradation phenomena, space charge and transport properties. He is currently research Director at CNRS and is leading a team working on the reliability of dielectrics in electrical equipment.

Peter H. F. Morshuis was born in The Hague, The Netherlands, on December 23, 1959. In 1986, he received the master degree in electrical engineering from Delft University of Technology. Between 1986 and 1988, he was involved in studies for NKF Kabel on the effects of defects on the lifetime of high voltage cables and in studies on new cable concepts. Since 1988, he has been a staff member of the High Voltage Group at Delft University of Technology where he was awarded the Ph.D. degree in electrical engineering in 1993 on the topic of ultrawide-band electrical and optical studies of partial discharges in solid dielectrics. In 1998, he was a visiting professor at the University of Bologna. Since 1999, he has been an associate professor in high voltage engineering at Delft University of Technology. He is involved in teaching first-year students in the field of electricity and magnetism and M.Sc. students in the field of high voltage DC. His most important fields of interest are HVDC (materials and systems), space charge, partial discharge, and aging of electrical insulation. He is involved in a number of CIGRÉ activities and is an associate editor of the IEEE Transactions on Dielectrics and Electrical Insulation.

Riccardo Bodega was born in 1976 in Lecco, Italy. He received his MSc in electrical engineering at the Politecnico di Milano, Milan, Italy in 2002. In the same year, he joined the HV Technology and Management Department at the Delft University of Technology, Delft, The Netherlands, where he performed research on HVDC polymerictype cable systems, leading to a Ph.D. thesis in 2006. Riccardo Bodega is now with the cable manufacturer Prysmian Cables and Systems, in The Netherlands, as a system engineer.


Leonard A. Dissado graduated from University College London with a degree in chemistry in 1963, obtained a Ph.D. degree in theoretical chemistry in 1966 and a D.Sc. degree in 1990 from the same university. After rotating between Australia and England twice, he settled in Chelsea College in 1977 to carry out research in dielectrics. Since then he has published many papers and one book, together with John Fothergill, on breakdown and associated topics. 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, and Nagoya University. He also has given numerous invited lectures, including the Whitehead Memorial Lecture in 2002. Currently he an associate editor of the IEEE Transactions on DEI, co-chair of the Multifactor Aging Committee of DEIS, and a member of DEIS AdCom.

Alfred Campus was born in Belgium in 1941. He studied polymer chemistry at the University of Louvain, Belgium, where he received a Ph.D. degree in 1967. After a 2-year post doctoral research at the University of Florida, Gainesville, FL, he joined Union Carbide Europe in 1969. With the restructuring of the petrochemical industry, he eventually became part of BP Chemicals in 1979, Neste in 1993, and Borealis in 1994. During his 35 years of industrial experience, he has held different technical and management positions in different fields, of which 22 years were spent in developing and promoting the use of polyethylene in cables, particularly XLPE in the power cable sector. He retired from Borealis in 2006, but he remains active in the field.

Ulf Nilsson was born in 1961. He received his Master of Science degree in engineering physics in 1987 at Lund Instutute of Technology, Sweden. The focus was solid state physics, especially silicon-based semiconductive technology. He joined Neste Polyeten AB in Stenungsund north of Gothenburg in 1987. He is responsible for electrical testing of polyethylene compounds for wire and cable applications. This company later became part of the Borealis group. He is currently engaged in product development of power cable compounds and activities related to improved understanding of the performance of insulating and semiconductive materials under high AC and DC electric stress. http://www.borealisgroup.com/wireandcable

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