Technical challenges linked to HVDC cable development

26/10/2014
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Publication REE REE 2014-4 Dossier Jicable HVDC'13
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Technical challenges linked to HVDC cable development

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Jicable HVDC'13DOSSIER 2 III REE N°4/2014 Technical challenges linked to HVDC cable development Marc JEROENSE, Markus SALTZER, Hossein GHORBANI ABB AB, (Sweden), marc.jeroense@se.abb.com, markus.saltzer@se.abb.com, hossein.ghorbani@se.abb.com ABSTRACT HVDC as a large scale concept has been in commercial use since the 50’s. Nowadays paper cables impregnated with a high viscous compound (mass impregnated – MI), oil pressurized cables and HVDC extruded cables coexist. The paper will concentrate on MI and extruded cables, and their principal strong and weak points. For MI cables the fundamental link between the pressure dynamics, cavity volume and partial discharges is highlighted, while for the polymer insulation system, the importance of conductivity as one of the basic properties is stressed. In the paper also some principles of field control in accessories of extruded cable systems are summarized. Challenges towards development of these systems as well as the testing situation are mentioned. KEYWORDS HVDC Cable Systems; HVDC Cables; XLPE Cables; Mass Impregnated Cables; MI-cables; HVDC Cable Accessories INTRODUCTION There is a world-wide increased demand for electrical en- ergy. Consequently, generation, transmission, and distri- bution capabilities and concepts must be increased and be made more efficient. The two basic alternatives are alternating current (AC) and direct current (DC) transmission systems. Due to losses DC is nowadays in many cases the preferred technology for large powers and long distances. On land, power can be transmitted by overhead lines or underground cables, while for sub-sea transmission only cables are available. Current commercial voltage levels are e.g. 800 kV for overhead line systems [1], whereby 1100 kV are discussed [2], and 500 kV for land and sea cable systems [3]. Politically, there is an underlying driving force, which is the European climate and energy 20/20/20 targets, defined by the European Commission [4], requiring an adaption of current network structures. As a result, the vision of a pan-European energy market is exemplified in the generation of the Ten-Year Network Development Plan as drafted by ENTSO-E. A strong growth in the number of wind energy projects in the North Sea of Germany is seen, and on the midterm the number of wind energy projects around the UK and along the French and Spanish coasts is also expected to increase. Additionally, initiatives on the interconnection of the energy networks of countries and continents around the Mediterranean, e.g. DESERTEC, Transgreen and Dii, have been started. Overall a strong demand on sub-sea, long distance, high power transmission is expected, leading to increased activities in HVDC cable system development. Mainly two cable technologies are commercially available for HVDC at the moment: mass impregnated (MI) cables with a combination of kraft paper and oil based compound as insulation system and extruded cables with cross linked polyethylene (XLPE) as insulation material. As depicted in Figure 1. The MI-cable is a lapped technology, where the kraft paper is lapped around the conductor including semiconducting and insulation layers and afterwards impregnated as a whole cable in an impregnation vessel. For XLPE the polymeric layers, i.e. the semiconducting layers and the insulation layer, are extruded in one extrusion step. Figure 1. Cable models of a MI-cable (left) and an XLPE cable (right). In the following some challenges with respect to the demand of increased power transmission and higher voltages in HVDC cable systems will be described. The main focus is hereby on extruded XLPE cables, MI cables, as well as stress grading in extruded cable systems. FUNDAMENTALS OF EXTRUDED CABLES A typical extruded HVDC land cable structure is shown in Figure 2. An aluminum or copper conductor is covered with a thin semiconducting layer to have a smooth interface to the following insulation layer. A second semiconducting layer covers the insulation. Usually these three layers are extruded in the same step. Depending on the application and design other layers are added to the cable. For example, in case of damage to the cable, swelling tape absorbs water and expands, blocking the water from moving axially along the cable. The screen wires are grounded. Aluminum laminate provides a diffusion barrier against foreign substances, especially humidity. And finally the covering sheath jackets all layers. Beside the mentioned layers, sea cables usually include an extruded lead sheath as water barrier and armoring wires provide mechanical protection to the cable and take the burden of the cable weight during installation. REE N°4/2014 IV Technical challenges linked to HVDC cable development As for electrical insulation of HVDC cables different options exist. Since its introduction, HVDC grade XLPE has been the main material used for insulation of HVDC cables. Currently the highest operation voltage for HVDC XLPE cables is 200 kV and cables up to 320 kV are produced and being installed. Thermoplastics such as polyethylene and polypropylene and filled concepts with XLPE or thermoplastics are other alternatives for extruded HVDC cable insulation. Figure 2. Typical HVDC land cable. From insulation material development, to modeling, design, production, quality control to testing, there are many challenges which need to be overcome in order to assure the quality of the final product. a) Material development What are the properties of an ideal extruded HVDC insulation material? Beside the properties of a good HVAC electrical insulation such as high electrical withstand, chemical stability and good aging properties the DC conduction behavior of the material also needs to be considered. Conductivity and space charge behavior have been the focus of study on many different insulation materials. But even simple properties such as conductivity can be challenging to measure. Figure 3 shows an example of typical leakage current curve from a plaque sample measurement. The leakage current continues decreasing even after a rather long period of time. This makes it hard to assign a stable conductivity value to a material based on such measurements. The same is valid for other DC measurements such as space charge measurements. Starting from plaque samples studies, different materials need to be compared and the best alternatives would be candidates for model cables or experimental cables. There are challenges in the interpretation of plaque samples studies. Cables have much thicker insulation than the typical plaque samples, therefore the effects related to the volume and thickness of insulation are inherently ignored in plaque sample studies. Besides, the cables undergo a different production process (i.e. temperature, time and atmosphere) than plaques so the same material can behave differently in plaque and cable. The electrode material also may be different in plaque studies than cables. On top of all the differences mentioned above, in cables, temperature gradient through the insulation has an effect on the conduction phenomena in HVDC cables. This effect can be challenging to be simulated by plaque samples. Although plaque samples are very convenient and simple way of comparing different materials, one needs to pay careful attention to the differences between the plaques and the cables to avoid wrong conclusions from plaque sample results. Figure 3. An example of leakage current curve measured on an insulation plaque sample under DC voltage b) Modeling and design There are two main approaches to conduction modeling of HVDC insulation materials. In the charge carrier transport model [5] [6], a number of charge carriers (mainly electron and hole) are considered being transported through the insulation. A charge carrier with a density of ( kn ) leads to a partial current density kj : )( lk kk nR x j t n [1] )( lk nR represents phenomena such as recombination, ionization, etc. The drift diffusion equation will then govern the movement of each charge carrier: x n Dnj k kkkk [2] where kD is the diffusion constant of the carrier and the Jicable HVDC'13DOSSIER 2 V REE N°4/2014 drift speed is a function of electric field and carrier mobility: Ekk [3] The total apparent current density is the sum of all partial current densities from different charge carriers plus the polarization current: k kk t D jqj [4] In addition to the equations above, one needs to define boundary conditions for each charge carrier. The boundary conditions depend on electric field, temperature and the chemical composition of the insulation/electrode interface. To be more accurate one needs to rethink the polarization equations and take into account the temperature and polarization time constants as well. Obviously, such an approach to modeling leads to a complicated model with large number of constants which the result can be sensitive to the defined constants. Therefore, good knowledge of all charge carriers is needed to reach reasonable results. On the other end of spectrum, being pragmatic, one can assign a conductivity function to the material which is a function of electric field, temperature [7] and chemical composition or location: EfTfrf ETR )( [5] Together with other self-explanatory equations: VE [6] Ere 0 [7] t j e [8] Ej [9] the space charge e can be written as: rer e j t 00 [10] The model is then complete with simple heat transfer equation: heatpm STk t T c [11] The electric losses due to conduction are provided from the electric field and conductivity as: 2 ESheat [12] The task left is then to find the functionality of conductivity on temperature, electric field and location in the insulation. This is usually done by conductivity measurements on plaques, or cables together with space charge measurements. Different proposed models can be found in literature [8]. Using this model, one has to remember the simplifications involved in the formulation of the model, being the assumption that an inherent property of the insulation material as conductivity can indeed be defined, and it can be defined as a function of field, temperature and location. The advantage of this model is that it is rather robust and converges to reasonable results, but as it should be expected it does not explain all of observed phenomena. c) Production and quality In order to achieve higher voltages, it is not feasible to simply scale the insulation thickness. Instead higher voltages have been introduced by increasing the insulation thickness and average electric field at the same time. This is possible by developing new insulation materials and production techniques and increasing the quality of the produced cables. In HVDC cables, besides the typical HV cable quality control techniques, such as PD measurement, AC voltage withstand and in-line geometrical measurements, new techniques need to be applied. Since the DC conductivity of insulation materials can vary by the amount of unwanted chemicals, new requirements on the so called “Chemical Cleanliness” apply. To do so, the effect of different chemicals on the conduction in the insulation material needs to be understood and controlled. In case of nano-composites, the concentration and distribution of the particles will be added to the quality control list. Therefore new methods for quality assurance and quality control of the cables with filled insulation need to be developed. FUNDAMENTALS OF MI CABLES What happens inside a MI cable? The insulation system of a MI cable is built up of many thin layers of high density paper impregnated with a high viscous insulating compound concealed in a metal sheath. This insulation type has been used for over a hundred years, first starting with MVAC cables and since the 1950s in HVDC cables. The insulation system is in a different state when the cable is loaded compared to when the conductor carries no current. In the former case the conductor and the compound are expanded due to their positive temperature expansion coefficient. The compound is also in a low viscous state. The insulation underneath the metal sheath is now well-filled. When the current is switched off, the conductor and compound cool down. This results in contraction of these components; the pressure close to the conductor falls. While the compound tries to flow back to the regions of low pressure (close to the conductor) this becomes more difficult as viscosity increases. In the end some regions will not be completely back-filled and due to the under-pressure voids may arise. These voids will typically be present in the butt-gaps. In this unloaded cold state, the MI cable is in its weakest state, contrary to the extruded cable. Although this sounds dangerous this does not need to be the case as the compound is to a certain extent self-healing. Small carbon traces in the compound can be “washed” away. Partial discharges, n and q When voids and an electric field are present in the insulation, partial discharges may occur. A classic way to start a description of partial discharges is using the abc REE N°4/2014 VI Technical challenges linked to HVDC cable development scheme [12]. With the aid of this scheme the repetition rate n can be derived. It is given by rr r rrr rs r r E E K E E n 1 1 1 1 min, 1 min, , [13] in which stands for the time constant of the void, Emin for the minimum breakdown field of the void, Es for the asymptotic field of the void, E for the actual electric field in the void, relates the residual field Eres in the void to Emin by min=Ures, the suffix r relates all the quantities to the location with radius r and K1 relates the asymptotic field to the field in the void by Es=K1E. Typical values of K1 are estimated between 1 and 5. The magnitude q of a single discharge when approximated to a flat void is given by 11 min0min0 AEU h A q rr [14] where A is the surface area of the void, the voltage U is related to the field strength E by Emin=Umin/h and h stands for the height of the void. For a circular void we can state that A=1/4 h 2 . And when we introduce the radial dependence the equation becomes the following rrrrr Ehq 1min, 2 04 1 . [15] One should remember that this charge magnitude q is the displacement of charge at the site of the discharge, not the measured one. Measuring the discharges is based on the measurement of the charge displacement in the leads to the cable. One can calculate the measured charge qM,r by using the following equation rc rb r rcrb rb rrM C C q CC C qq , , ,, , , [16] in which Cc is the capacitance of the void and Cb is the capacitance of the healthy insulation in series with the void (see he Figure) and Cb,r<http://eurlex.europa.eu/LexUriServ/LexUriServ.do?uri =COM:2007:0002:FIN:EN:PDF [5] T. Christen and E. Logakis, “The generic conduction model for solid HVDC insulation material”, paper 258 ICSD, Bologna, Italy, 2013 [6] T. J. Lewis, “Polyethylene under electrical stress”, IEEE Trans. Dielectr. Electr. Insul., Vol. 9, pp. 717- 729, 2002. [7] C. O. Olsson and M. Jeroense, “Evolution of the distributions of electric field and of space charge in an extruded HVDC cable”, Jicable, paper 271, Versailles, France, 2011. [8] S. Boggs, D. H. Damon, J. Hjerrild, J. T. Holboll, and M. Henriksen, “Effect of insulation properties on field grading of solid dielectric DC cables”, IEEE Trans. Power Delivery, vol. 16, pp. 456-461, 2001. [9] F.H. Kreuger, 1995, “Industrial High DC Voltage: Fields, Breakdowns, Tests”, Delft University Press, Delft, Netherland. [10] T. Christen, L. Donzel, F. Greuter, 2010, ”Nonlinear resistive electric field grading Part 1: Theory and simulation”, IEEE Electr. Insul. Mag., Vol. 26, pp. 47- 59. [11] L. Donzel, F. Greuter, T. Christen, 2010, “Nonlinear resistive electric field grading Part 2: Materials and Applications”, IEEE Electr. Insul. Mag., Vol. 27, pp. 18-29. [12] U.Fromm, F. Kreuger, “Partial Discharges in Gaseous Voids for DC Voltage”, Jpn. J. Appl. Phys. 33 (1994) pp. 1079-1084