Insulation Interfaces - IEEE Xplore

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Polymeric HVDC Cable Design and Space Charge Accumulation. Part 2: Insulation Interfaces Key Words: HVDC, interfaces, space charge, polarization, charge injection, charge inversion Introduction

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onhomogeneous electrical insulating materials and in terfaces between different materials are frequently encountered in electric insulation systems. On one hand there are composite insulating materials obtained by a host material containing micro and/or nano fillers, which have been widely investigated with the aim of improving the properties of the base material. The presence of micro/nano scale interfaces may affect the physical and electrical properties of the composite material, improving or worsening them with respect to the base material. This depends on the nature and dispersion of micro/nano filler inside the matrix. On the other hand, interfaces can be found on a macro-scale in several HV insulation systems, e.g., in cables. In particular, interfaces between cable insulation and semiconductive layers and/or between different insulating materials in cable accessories (e.g., XLPE and EPR used for cable joints and/or terminations) are very common in transmission and distribution networks. It must be remembered that interfaces are often claimed to be critical within the complete insulation system, affecting electrical, mechanical, and thermal properties. Indeed, the effects of electro-thermal and mechanical stresses can be enhanced in the presence of interfaces that may, thus, become the weakest points of the insulation system, both in AC and DC [1]. Interfaces can act as a trigger for partial discharges (PD) when the contact between surfaces is not well made, and such activity should be strictly avoided for cable and accessories because organic insulation (e.g., XLPE and EPR) might not be able to withstand PD activity, even for short times [2], [3]. It is likely that PD activity will be a second-order problem under DC due to the smaller repetition rate with respect to AC. However, under a DC field, space charge may accumulate in the insulation bulk, especially if interfaces are present [4]–[8]. The latter, in fact, can behave as favored sites for charge build up, particularly for low-mobility materials (e.g., polymeric insulation). Space charge can modify the electric field in such a way that the actual field in the insulation can differ significantly from the geometric field, thus causing accelerated 14

S. Delpino, 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 & Systems, The Netherlands

L. A. Dissado University of Leicester, UK

The electric field distribution of cable insulation systems under HVDC can be affected significantly at interfaces due to space charge build-up. degradation and, premature breakdown of insulation systems [9]–[11]. These are the reasons for the concerns in the application of polymeric materials to HVDC insulation systems, even if the use of polymeric insulation in place of paper-oil (still used) would lead to great environmental and economic advantages. Therefore, the investigation of the behavior of interfaces between different insulating polymeric materials in the presence of thermal and electric stresses is a crucial research topic for the improvement of the design of polymeric cables and accessories, which could benefit by a more accurate knowledge of the actual field applied to the insulation system under design. In this article, the second part of a three-article series, inter-

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IEEE Electrical Insulation Magazine

face and space charge accumulation are analyzed first in terms of macroscopic physics, then through approximate mathematical models that will be used to fit experimental data obtained for model cables having two insulation layers and constituting cylindrical interfaces.

Theoretical Background It is well-known that charge tends to accumulate in solid insulating materials under DC or low-frequency fields, particularly in the presence of interfaces. In general, space charge accumulation can be associated with different phenomena: • injection from the electrodes; • gradient of the insulation conductivity (e.g., when a temperature gradient is present across the insulation) • nonhomogeneous dielectrics (e.g., interfaces). When the applied field is below the threshold for space charge accumulation [12], the injected charge is small and the insulation system behaves according to Ohm’s law. Thus, no net charge accumulates in a uniform insulating material. As temperature and/ or electric field increase (exceeding the space charge threshold), charge injection from the electrodes increases and accumulates in the insulation bulk (see Figure 1). A temperature gradient across the cable can play a nonnegligible role in charge accumulation, due to the dependence of insulation conductivity, γ, and permittivity, ε, on temperature. This phenomenon can affect the field distribution, particularly when charge injection does not have a dominant effect (e.g., at electric fields close to the threshold for space charge accumulation) [13], [14]. Moreover, in nonhomogeneous dielectrics, in which conductivity, γ, and permittivity, ε, are not constant, and/or in the presence of interfaces between insulation layers having different γ and ε, space charge also can accumulate. This is due to the so-called Maxwell-Wagner-Sillars (MWS) polarisation [15]. Again, the

Figure 1. Threshold characteristics for XLPE cable at 25°C and 70°C.

January/February 2008 — Vol. 24, No. 1

effect of MWS polarization can be hidden by injected charge, if the electric field exceeds by a large amount the threshold and/or at high temperatures (injection is promoted by temperature). The extent of space charge accumulation and electric field alteration due to the MWS mechanism, neglecting the contribution of injected charge (i.e., valid only at fields close to the threshold) can be evaluated on the basis of Maxwell’s equations for cylindrical geometry (see Figure 2): (1) (2)

(3)

where E1(r) and E2(r) are the electric field at the generic radius, r, in the insulations 1 and 2, respectively; r1 and r2 represent the radius of internal conductor and external shield, respectively; ri is the radius of the boundary surface Σ between two coaxial dielectrics, having different permittivity, (ε1 and ε2, respectively) and conductivity (γ1 and γ2, respectively), V is the applied voltage; E1(ri) and E2(ri) are the components of electric field perpendicular to the boundary surface Σ, in the insulations 1 and 2, respectively, calculated at the interface radius r = ri; and σ is the surface “free” charge density at the interface (i.e. amount of charge per square metre). It should be noted that, if σ = 0 (no “free charge” accumulation at the interface), (1)–(3), give rise to the so-called geometric “Laplacian” field. Before calculating the electric fields E1 and E2, an evaluation of the interface charge, σ, can be carried out resorting to the following observations. At steady-state the current density [J(r) = γ · E(r)] in the two insulation layers will be the same at the interface (in cylindrical geometry, the steady state is defined by a current that is independent of radial position, i.e., divJ = 0) so that at the interface

Figure 2. Boundary surface, Σ, in a cylindrical geometry, with two insulating layers, 1 and 2. The inner conductor is indicated by C. 15

between material 1 and material 2 the following expression can be written: γ2E2(ri) – γ1E1(ri) = 0.

(4)

Once E2(ri) is derived from (4) and substituted in (3), the following expression for the interface charge can be achieved:

(5) Because J is continuous across the interface surface, we can write:

εiγk for i-th and k-th insulation layer. Because conductivity depends on temperature and electric field [15], and varies from one material to another, it may happen, for example, that e2g1 > e1g2 at room temperature and e2g1 < e1g2 at higher temperatures. This can cause the polarity of the interfacial charge, σ, to be inverted as a function of field and temperature. It can be observed that, when (11) is multiplied by the cylindrical surface area, 2πrl, a radially independent current is obtained, as required for the steady-state condition. Expression (11) allows us to deduce the electric field at a radial position r within each of the insulating materials from J(r) = g·E(r), that is: (13)

(6)

Assuming a uniform temperature distribution, the current density J can be calculated by the following equation: (7) where G is the conductance of the two insulation layers, V is the applied voltage, and S is the cross section of the J flux tube. The conductance of the two layers, G, is given by: (8) where G1 and G2 are the conductance of insulation layer 1 and 2, respectively. Their value is:

(9) where l is the cable length. Substituting (9) in (8), conductance G becomes: (10)

Therefore, it can be seen that in the DC steady state the field distribution is governed solely by the conductivity mismatch rather than the permittivity mismatch, even though the latter is present. This is due to the fact that the difference in conductivities causes the amount of charge transported to the interface to be different from the amount transported away from the interface, until, at the steady state, the free interface charge, σ of (12) becomes large enough to equalize the interface currents through an appropriate alteration of the field discontinuity at the interface. This can be demonstrated by evaluating the free interface charge, σ, and determining the alteration it introduces to the interface field discontinuity. For this purpose, the electric field at the interface and insulation bulk can be calculated by means of the superposition of effects:

E(r) = EL(r) + EP(r),

(14)

where EL(r) is the geometrical Laplace field, i.e., the field obtained considering only the permittivity mismatch between insulation layers and cable cylindrical geometry (σ = 0); EP(r) is the Poisson field associated with the free charge at the interface, σ, calculated by (12), under the assumption of zero space charge accumulation in the insulation bulk. The Laplacian electric field in the insulation layers 1 and 2, EL1 and EL2, respectively, easily can be calculated from (1), (2), and (3) under the assumption of σ = 0, providing:

The current density variation with the cable radius (r) can be calculated from (10) and (7), as follows: (11)

From (6) and (11) one obtains:

(12)

(15)

Because in a cylindrical geometry the Poisson field in the layer 1 and 2 is inversely proportional to the radius r, that is:

(16)

substituting (16) in (2) and (3) and considering V = 0, gives the following expressions for EP1(r) and EP2(r) after a few calculations [16]:

It should be noted that the sign of σ depends on the product 16


(17)

The electric field in insulation layer 1 and 2, E1 and E2, can be derived by inserting (15) and (17) in (14):

throughout the insulation by the space charge distribution. As a consequence, electric fields E1 and E2 may differ significantly from the quantities calculated by (18) and (19), i.e., (13). However, we can make use of the approach implied in (18) and (19) to obtain a general expression for the electric fields in the generic insulation layers 1, 2 (E1,2), which also takes into account the space charge accumulated in the bulk [17]: (20)

(18)

(19)

The first term of (18) and (19) is the field appropriate to a permittivity mismatch between the two materials. The second term gives the effect of the free interface charge, σ, upon the field discontinuity. When (12) is substituted for σ in (18) and (19) and the resulting expressions simplified, (13) is obtained for E1(r) and E2(r), thereby demonstrating that the free interface charge as given by (12) is consistent with the steady-state condition. It is noteworthy that, as temperature and/or electric field rise, injected space charge can increase significantly throughout the insulation bulk (see Figure 1), particularly at the interface at which the trap density is likely to be higher than in the bulk. Expressions (10) and (11) will no longer hold (rather, they become more and more approximate) due to the field distortion introduced

where ρ(r) is the space charge density profile obtained experimentally through space charge measurements. A first estimation of the electric fields E1,2 also can be made via (18) and (19) by substituting for σ the free interfacial charge estimated from the part of the measured space charge profile at the interface.

Test Procedures and Specimens The test specimens (designed and realized within the European Project HVDC) consisted of triple-extruded, dual-dielectric cables, giving rise to cylindrical chemical interfaces. Physical interfaces, obtained by pressing mechanically different materials, also were investigated. The results are reported elsewhere [18], [19]. Figure 3 shows a sketch of the test object cross section. Specimen #1 consists in practice of mini-cables having an inner semicon layer, a middle EPR layer (1.5-mm thick) and an outer XLPE layer (0.6-mm thick). Specimen #2 has the opposite interface, i.e., the middle layer (1.5 mm thick) is XLPE, and the

Figure 3. Sketch of test cables; dimensions in millimeters.


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outer layer is EPR (0.6-mm thick). An outer semicon layer was applied using semicon tape. Space charge measurements were performed by means of the pulsed electro acoustic (PEA) technique [9], [20], [21] at field values above and close to the threshold for space charge accumulation in XLPE. The poling time was generally 10,000 s (for tests close to the threshold and room temperature this time was set to 250,000 s (or about 3 days) due to the longer time needed for the charge to reach the steady state). Depolarization generally lasted 3600 s. Tests were performed at different temperatures, ranging from 25°C to 70°C. Because the purpose of the paper is not to analyze the effect of the temperature gradient across the insulation on space charge build-up (this will be dealt with in Part 3 of this article [22]), the tests presented here were carried out mostly under isothermal conditions, with the specimen placed inside an oven to keep it at a uniform temperature.

Test Results Figure 4 shows space charge profiles relevant to specimen #1 (EPR-XLPE mini-cable) at 25°C [Figure 4(A)] and 70°C [Figure 4(B)]. The value of the average applied electric field is 29 kV/mm. This corresponds to a poling voltage of 60 kV. At 25°C, a positive charge peak corresponding to the position of the interface, increasing with the poling time (up to about 10,000 s), can be noted clearly [Figure 4(A)]. A slight accumulation of homocharge (positive in EPR layer, negative in XLPE layer) can be observed as well (note that homocharge is a charge having the same polarity as the closest electrode). At 70°C, on the contrary, the net interface charge is negative, reaching a steady state after about 5000 s [Figure 4(B)]. A large amount of homocharge, likely charge injected from the electrode, can be observed again in the insulation bulk, especially during depolarization [volt off, Figure 4(B)]. To evaluate the amount of “free” space charge accumulated at the interface, σ, from the space charge distribution ρ(r), the following equation can be used [9], [16]:

(21)

where ri is the radius that corresponds to the EPR/XLPE interface, Δ1 and Δ2 are two parameters that define the integration spatial range in such a way that σ could represent the charge peak at the interface, ρ0(r) is the space charge profile obtained immediately after voltage application. It should be noted that the measured space charge profile ρ(r) includes the contribution of the dipole polarization mismatch due to the difference in relative permittivities of the two materials, which is responsible for the field mismatch in the Laplace fields of (15), (18), and (19). Because σ must account for the free charge only, the dipolar charge contribution at the interface should be removed by subtracting the space charge profile obtained immediately after voltage application [ρ0(r)]. The interface free charge density, σ, thus estimated by means

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Figure 4. Space charge profiles in specimen #1, poling voltage 60 kV at 25 °C (A) and 70 °C (B).

of (21) from the space charge profiles of Figure 4(A) and (B) at the end of the polarization time is +1.7×10-4 C/m2 and –2.0×10-4 C/m2, respectively. In order to better understand the reason for the polarity inversion of the interface charge, space charge measurements at the same poling voltage were carried out at an intermediate temperature, i.e. 45°C. Space charge profiles are shown in Figure 5. In this case, the free interface charge is positive, and its value is about +5.7×10-5 C/m2. However, a negative homocharge also is present in the XLPE layer close to the interface. Space charge tests also were performed at 25°C and 70°C for a lower voltage, i.e., a poling voltage of 21 kV that corresponds to an average electric field of 10 kV/mm. The resulting space charge profiles are reported in Figure 6. It should be noted that the poling time was increased from 10,000 to 250,000 s, as the interface charge needs a much longer time to reach a steady state, due to the small insulation conductivity at these temperatures and field. The results show that the interface charge density is positive


Figure 5. Space charge profile in specimen #1, poling voltage 60 kV at 45°C. both at 25°C and 70°C, with values of about +1.3×10-4 C/m2 and +1.0×10-4 C/m2, at 25°C and 70°C, respectively. Moreover, it can be seen that the amount of interface charge at 70°C decreases with poling time. This will be discussed later. Figure 7 reports space charge data for specimen #2, having the opposite chemical interface (XLPE-EPR) at a poling voltage of 60 kV. As expected, the polarity of interface charge is inverted with respect to the charge profiles reported in Figure 4. The interface charge at 25°C [Figure 7(A)] is almost the same (in absolute value) as that of specimen #1, i.e., –0.9×10-4 C/m2. At 70°C [Figure 7(B)] it is significantly larger, i.e., 8.0×10-4 C/m2. Figure 8 shows space charge profiles for specimen #2 carried out for an average applied field of 15 kV/mm [17], but not under isothermal conditions. Specifically, space charge measurements were carried out with a temperature drop of 10°C (conductor temperature of 40°C) and 18°C (conductor temperature of 64°C) applied to the cable. The results are shown in Figures 8(A) and (B), respectively. In this case, the effect of a temperature gradient on space charge build-up may be non-negligible, but mainly at fields below the threshold for space charge injection, that is, below 8 kV/mm (see Figure 1) [13], [14], [22]. Results show that, at both temperatures, negative charge accumulates at the dielectric interface. In addition, a significant accumulation of positive charge in the XLPE layer also can be observed at higher temperature. The experimental results presented here open up interesting questions. In particular, it is observed that the net interface charge changes polarity on raising the temperature from 25°C to 70°C, but that this occurs only for specimen #1 and for #2 at high field and isothermal conditions. It is important to understand how this occurs because it can affect the electric field distribution, and hence become a concern for the design of insulation systems.

Discussion The first observation regards the polarity and the value of the interfacial charge at room temperature for specimens #1 and #2. It can be seen from Figures 4(A), 6(A), and 7(A) that the charge


Figure 6. Charge profiles in specimen #1, poling voltage 21 kV at 25°C (A) and 70°C (B). Polarization time = 250,000 s.

accumulated at the interface is positive in specimen #1 and negative in #2. An attempt to calculate the value of interface charge can be carried out by means of the model expression, (12). Considering the conditions of Figures 4(A) and 6(A), (i.e., γ1 = γEPR = 1.5×10-16 S/m, γ2 = γXLPE = 5.0×10-17 S/m; ε1 = εEPR = 2.9·ε0, ε2 = εXLPE = 2.3·ε0, which are values derived experimentally), the value of interface charge is calculated to be σ = 2.0×10-4 and 6.0×10-4 C/m2 for V = 21 kV (average field = 10 kV/mm) and 60 kV (average field = 29 kV/mm), respectively, for specimen #1. At the same voltage levels, the values obtained by measurements are σ = +1.2×10-4 C/m2 and +1.7×10-4 C/m2, respectively. On the basis of these findings, it can be speculated that, at low electric fields close to the threshold for space charge accumulation, the values of charge derived by the macroscopic model agree quite well with those measured experimentally, confirming that the charge at the interface is mainly due to MWS polarization. On increasing the applied field, a considerable difference between the calculated and measured charge is found due to injected charge from the electrodes accumulating at the interface.

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Figure 7. Space charge profiles for specimen #2. Voltage = +60 kV (average applied field = 29 kV/mm). (A) Temperature = 25°C. (B) Temperature = 70 °C.

Another relevant point is the change in the interface charge polarity with temperature. The interfacial charge for specimen #1 and high field [Figure 4(B)] becomes negative at 70°C. This can be explained by one of the following hypotheses: • γXLPE(70°C) > (2.3/2.9) × γEPR(70°C) (while γXLPE(25°C) < (2.3/2.9) × γEPR(25°C)) so that σ < 0 in (12), • injected (negative) charge from one electrode accumulates at the interface, covering the (positive) MWS interfacial charge. The space charge profiles of Figure 4(B) show that a certain amount of homocharge is accumulated in the XLPE layer close to the cathode. This is indicative of significant charge injection and would support the second hypothesis. Space charge tests at an intermediate temperature (45°C) were performed at the same poling field to further clarify this point. Figure 5 shows that a significant amount of negative injected charge is present in the XLPE layer close to the interface, and at the interface a positive charge is still present due to MWS polarization. Indeed, the average electric field (29 kV/mm) greatly exceeds the threshold for space charge

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Figure 8. Space charge profiles obtained from specimen #2. Voltage = +30 kV (average applied field = 15 kV/mm). (A) Temperature inner semicon = 40°C, temperature outer semicon = 30°C. (B) Temperature inner semicon = 64°C, temperature outer semicon = 42°C.

accumulation, which for XLPE is close to 8 kV/mm and 3 kV/ mm (average value) at 25°C and 70°C, respectively, with a guess of about 6 kV/mm at 45°C (see Figure 1). These observations seem to support the second hypothesis above, which is that injected negative charge increasing with temperature and applied field is overwhelming the MWS positive charge. As a consequence, one could expect that in service the interface charge could be positive or negative depending on the load and environmental conditions. This can affect significantly the electric field. A further confirmation of these results is provided by Figure 6, in which space charge profiles obtained for tests at lower fields (average field, 10 kV/mm) are shown. At 25°C [Figure 6(A)], space charge accumulation due to injection is negligible, so the charge at the interface is still positive (according to the MWS polarization). At 70°C a positive charge is still present, but its amplitude decreases with time, probably due to injection of negative charge from the cathode (the applied field is larger than the threshold at 70°C).


Figure 10. Electrical conductivity of two insulating materials (#A and #B) with different temperature dependence, versus reciprocal of absolute temperature for applied field of 20 kV/ mm (after [23]).

Figure 9. Electric field profiles evaluated from (18) and (19) at 25°C (A) and 70°C (B). The geometric field also is reported. The interface charge used for calculation is obtained from space charge measurements, i.e., σ = -0.9×10-4 C/m2 and 8×10-4 C/m2. Applied voltage = +60 kV (average field = 29 kV/mm).

When the order of the insulating layers is changed (specimen #2), the polarity of interface charge is inverted [Figure 7(A)], as would be expected from the macroscopic approach (12). At a higher temperature [Figure 7(B)] the polarity of the interface charge changes as occurs in specimen #1. Again, this can be due to injection of positive charge from XLPE, favored by the temperature rise. The interface charge at 70°C is significantly larger for the XLPE-EPR mini-cable with respect to that of EPR-XLPE. Indeed, the XLPE/electrode interface seems to be more active in charge injection, particularly when the XLPE layer is subjected to the highest field, i.e., it is located near to the high voltage electrode. It should be noted that space charge tests in nonisothermal conditions showed a different behavior (Figure 8). In this case, a significant amount of injected charge can be observed on increasing the temperature as before. But the polarity of the interface


charge is not seen to change, as was the case previously. A tentative explanation is that, because the average applied field is not very high (15 kV/mm) and the temperature at the interface is lower than the maximum value due to the temperature gradient, the injected space charge does not advance sufficiently to hide the MWS interface charge, unlike the situation observed in specimens at a uniform temperature of 70°C. The electric field profiles (Figure 9), obtained from (18) and (19) taking the measured space charge at the interface for σ, show an inversion at a temperature between 25°C [Figure 7(A)] and 70°C [Figure 7(B)]. At 25°C, the actual field is, in fact, close to the geometric Laplacian field (i.e., larger in the innermost XLPE layer and smaller in the outermost EPR layer). At 70°C the field is more than doubled in the EPR layer. This behavior can be explained by the polarity inversion of the charge at the interface, due to injected charge from the electrode. It must be pointed out again that this can cause reliability loss in polymeric cable systems because the design field may differ significantly from the real applied field if space charge build-up is not taken into account at the design stage. Moreover, it should be noted that the same charge accumulation and inversion effects discussed above may occur also if the conductivity of the two insulation layers shows a different activation energy, that is, a different temperature dependence as shown in Figure 10. Depending on the difference between γ1ε2 and γ2ε1 the interface charge can invert its polarity and so may the electric field when the temperature is increased from room value to higher temperatures.

Conclusions The electric field distribution of cable insulation systems under HVDC can be affected significantly at interfaces due to space charge build-up. Optimization of cable and accessory design should, thus, take into account a complex scenario. On one hand the nonhomogeneities in conductivity and permittivity can cause 21

interface charge modifying the insulation field distribution that can be evaluated roughly using macroscopic models. Interface charge and field are affected largely by material electrical properties (e.g., conductivity) that are strongly correlated with electrical and thermal stresses and temperature gradient across the insulation. This could be solved partially by manufacturing insulating materials with properties showing small temperature dependence. On the other hand, the increase in HV cable design fields implies that microscopic effects, such as injection and extraction processes at the electrodes, have to be considered when the electric field exceeds the threshold for space charge accumulation. To avoid significant underestimation in calculations of interfacial charge and field, space charge measurements must be performed and models considered that also should take into account the space charge accumulation features of the insulating material. In this way models can support efficiently cable and accessory design improving the reliability of whole insulation systems.

Acknowledgments This research was performed within the European Project (of the 5th framework-program) HVDC, “Benefits of HVDC links in the European Power Electrical System and improved HVDC Technology” (Contract No. ENK6-CT-2002-00670). The authors are grateful to all the people involved in the project, particularly to Borealis and Prysmian Cables and Systems for providing the specimens used for the experiments.

References

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[9] G. C. Montanari and D. Fabiani, “Evaluation of dc insulation performance based on space charge measurements and accelerated life tests,” IEEE Trans. Diel. Elect. Insul., vol. 7, no. 4, pp.322–328, Aug. 2000. [10] L. A. Dissado, G. Mazzanti, and G. C. Montanari, “The role of trapped space charges in the electrical ageing of insulation materials,” IEEE Trans. Diel. Elect. Insul., vol. 4, pp. 496–506, Oct. 1997. [11] Y. Zhang, J. Lewiner, C. Alquie, and N. Hampton, “Evidence of strong correlation between space charge buildup and breakdown in cable insulation,” IEEE Trans. Diel. Elect. Insul., vol. 4, pp. 778–783, Dec. 1997. [12] G. C. Montanari, “The electrical degradation threshold of polyethylene investigated by space charge and conduction current measurements,” IEEE Trans. Diel. Elect. Insul., vol. 7, pp. 309–315, June 2000. [13] 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,” in Proc. IEEE CSC, Tours, France, July 2006. [14] D. Fabiani, G. C. Montanari, R. Bodega, P. H. F. Morshuis, C. Laurent, and L. A. Dissado, “The effect of temperature gradient on space charge and electric field distribution of HVDC cable models,” in Proc. IEEE ICPADM, Bali, Indonesia, pp. 65–68, June 2006. [15] D. A. Seanor, Electrical Properties of Polymers, London: Academic, 1982. [16] R. Bodega, Space Charge Accumulation in Polymeric High Voltage Cable Systems, Ph.D. Thesis, Delft, The Netherlands, 2006, Technical University of Delft. [17] R. Bodega, P. H. F. Morshuis, U. H. Nilsson, G. Perego, and J.J Smit, “Charging and polarization phenomena in coaxial XLPE-EPR interfaces,” in Proc. IEEE ISEIM, Kitakyushu, Japan, pp. 107-110, June 2005. [18] 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. Rpt. IEEE CEIDP, Kansas City, pp. 732–735, Oct. 2006. [19] D. Fabiani, G. C. Montanari, S. Delpino, R. Bodega, and P. H. F. Morshuis, “The effect of temperature on space charge accumulated at chemical and physical interfaces of HVDC polymeric insulation systems,” in Proc. IEEE ICSD, Winchester, UK, July 2007. [20] 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. Insulation, vol. 23, no. 3, pp. 433–439, 1988. [21] G. C. Montanari, “Extraction of information from space charge measurements and correlation with insulation ageing”, in Proc. IEEE CSC, Tours, France, pp. 178–184, July 2001. [22] 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.”, to be published. [23] R. Bodega, G. C. Montanari, and P. H. F. Morshuis, “Conduction Current measurements on XLPE and EPR insulation,” 2004 Ann. Rpt. IEEE CEIDP, Boulder, pp. 101–105, Oct. 2004.


Saverio Delpino, was born in Bari, Italy, on May 31, 1975 He received the Master Degree in electrical engineering (2005) from the University of Bologna, where he is currently a researcher. He is studying the phenomenon of space charge in cable systems and partial discharges in insulation cavities, collaborating within the HVDC European project. He also has been working with TechImp S.p.A since 2006.

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 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 spinoff TechImp, established in 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.

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degree in electrical engineering in 1993 on the topic of ultrawideband 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 M.Sc. 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 polymeric-type 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.

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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.
