The resonance of transmission MacNeill, El-Hawary, and Molloy highlight the unique electrical characteristics of submarine cables that need to be taken into account when specifying the transmission of power from marine energy converters. Aaron MacNeill Mohamed E. El-Hawary Sue Molloy Who should read this paper? All those with an interest in or responsibility for the design and development of marine energy power systems. Why is it important? There is a fundamental difference between transmitting electrical power through overhead conductors and submarine cables. For overhead lines, a bare conductor can be used because, under normal conditions, air is a good natural insulator. Salt water, on the other hand, is electrically conductive and, therefore, the cable must be isolated from its surroundings. In simple terms, this means that the conductor must be wrapped in three different layers – one to electrically isolate the conductor from the seawater, one to keep out moisture, and a third to protect the cable from abrasion. The thickness and composition of these layers of wrapping, together with the design of the conductor (single or triple core), impart unique characteristics to the cable in terms of resistance (the opposite of conductance), capacitance (the ability to store electrical energy), and inductance (a unique characteristic of AC circuits whereby current flowing through a conductor produces a magnetic field, which in turn induces a current, both in the conductor itself and the surrounding area). The authors simulate the performance of various designs of submarine cables in terms of these three parameters. Their findings will assist in tailoring cable geometry for specific marine energy applications such that power quantity and quality is optimized. About the authors Aaron MacNeill is a PhD candidate in electrical engineering at Dalhousie University. His research interests include power system network modelling, transient analysis and operation/control. Mohamed El-Hawary is a Professor of electrical and computer engineering at Dalhousie University and has pioneered many computational and artificial intelligence solutions to problems in economic/ environmental operation of power systems. Sue Molloy is the engineer in residence for tidal power research in the Faculty of Engineering at Dalhousie University, where she specializes in marine renewable energy and eco-ships. 104 The Journal of Ocean Technology, Vol. 8, No. 1, 2013 Copyright Journal of Ocean Technology 2013 SUBMARINE POWER CABLE ELECTRICAL PERFORMANCE EVALUATION FOR MARINE ENERGY APPLICATIONS Aaron M. MacNeill, Mohamed E. El-Hawary, Sue Molloy Dalhousie University, Halifax, NS, Canada ABSTRACT This paper reviews the constructional details of single-core and three-core AC power submarine cables. The inductance and capacitance electric circuit parameters are then evaluated in terms of the geometry, electromagnetic, and electrostatic (dielectric) properties of the conductors and insulation of the cable. Given the electric circuit parameters, two-port network models are then evaluated which allows the determination of steady state electrical performance characteristics of the cables. The study examines the effect of the cable length on the receiving end (cable output) voltage, active, reactive power and performance indices such as transmission efficiency and voltage regulation. KEY WORDS Submarine power cable; Underwater cable; Cable performance; Line parameters; Marine energy NOMENCLATURE σ ω ε μ L Z a b = = = = = = = = = conductivity radial frequency permittivity permeability inductance impedance inner radius of the sheath radius of the core conductor length of transmission line Copyright Journal of Ocean Technology 2013 The Journal of Ocean Technology, Vol. 8, No. 1, 2013 105 INTRODUCTION Ocean current power generation systems use the power of wave and tidal currents to generate renewable energy. Bringing electric power to shore requires electrical cables. Submarine power cables have been used for offshore wind transmission, bulk power transmission or to link power to offshore structures and islands. Submarine power cables are also used in applications such as offshore wind farms. The choice of cable will affect both the quality of received power and the security of the power link. There are various cable design types that could be used to transmit power to and from shore. A cable’s line parameters must be evaluated in order to determine how the cable performs at transmitting electrical energy under various operational conditions. The selection of cable is site-specific and there are many other criteria other than cable electrical characteristics that will inform the cable selection. The steady state electrical performance of a cable under normal operating conditions is defined by the following variables: the receiving end voltage, active power, reactive power, voltage regulation, and the transmission efficiency. Line parameters are evaluated based on studying the magnetic and electric fields established by power flow in the cable. They depend solely on the materials used and geometry of the cable. This implies that, by varying the materials and geometry used for the different cable layers, the quality of the power at the receiving end of the transmission line could be controlled. There are specific challenges associated with transmitting marine energy ashore since the power generated varies 106 The Journal of Ocean Technology, Vol. 8, No. 1, 2013 depending on the height of the wave or velocity of tidal current that vary with time. The results of our work indicate that when practically feasible, and for the same power carrying capacity and transmission length, overhead lines outperform an equivalent submarine power cable. Cables provide positive reactive power compensation for the system. As the transmission distance increases, the active power delivered is reduced and DC power transmission is required. SUBMARINE CABLES An electrically conductive substance, fresh or salt water, surrounds submarine cables. Therefore, a layer of insulation must be wrapped around the conductor to confine the current flow through the core conductor and not leak to the surrounding seawater. A protective layer is used to protect against water damage. A metallic (lead, aluminum, or copper) or polymer sheath is introduced around the insulation to keep water and water vapour from reaching the insulation. Lead is the ideal metal to use for the sheath because it can completely keep moisture away from the insulation [Worzyk, 2009; Pieroni and Fellows, 1979; Thue, 1998]. An armour is a layer or multiple layers of steel or copper round wire that make up the outermost layers of the cable required to protect the cable from random external causes of possible damage such as the tensional forces produced when laying the cable, or external sources. There are many considerations in designing the different layers of the cable. The differences in construction impact the performance of submarine power cables. The cable’s electrical Copyright Journal of Ocean Technology 2013 Figure 1: Assumed geometrical cable model. performance characteristics are the focus of this paper. Figure 1 shows a simplified geometric model for submarine cables with single and three conductors [Sadiku, 2007; Pieroni and Fellows, 1979; Hauge et al., 1988; El-Hawary, 1995; Thue, 1998; Worzyk, 2009]. The electrical resistance of the core conductor affects the cable’s efficiency and depends on factors such as the conductor’s material; construction type such as a solid core, stranded core, profiled core, or segmental core conductor; and electromagnetic phenomena such as proximity effect and skin effect [Worzyk, 2009; Sadiku, 2007; Thue, 1998]. The sheath/armour and armour/seawater insulation layers are used for protection between the sheath and armour and to keep water out of the armour, respectively [Worzyk, 2009; Pieroni and Fellows, 1979]. While the actual construction of submarine cables is not that simple, valid approximations are available. In three-core submarine cables there is a conductor screen between each of the core conductors and the insulation; outside the insulation layer there is an insulation screen, water swellable tapes, a metallic shield, and a polyethylene jacket. These layers are used to Copyright Journal of Ocean Technology 2013 prevent water and humidity from attacking the cable’s insulation as well as electrically shield each of the core conductors from themselves. Around the bundle of three conductors there is a binder tape and polypropylene string bedding. This keeps the three cables and filler bound together. Encompassing this are layers of galvanized steel armour with a polypropylene separator between armour layers and polypropylene serving outside the final layer of armour [ABB, 2010]. Noting that a metallic shield surrounds each of the conductors and knowing that this shield is electrically grounded means that each of the core conductors is electrically shielded from the others and the outside environment. This allows for the disregard or combination of materials that have relative permeability of one. The separation layers between the armour can be safely neglected since the layer is very thin and exists a significant distance away from the cable’s centre. The model developed to analyze the submarine cables does not account for the presence of extra conductors that are not core conductors. The assumptions are summarized as: • The conductor screen is part of the insulation. • Water swellable tape is part of the insulation. • Binder tape and polypropylene string bedding are part of the insulation layer between the bundle and armour layers. • Water swellable tape and polyethylene jacket that surround the individual core sheaths are part of the filler. • The armour layers are stacked directly on top of each other; the polypropylene insulation serves as the outer insulation layer of the cable. The Journal of Ocean Technology, Vol. 8, No. 1, 2013 107 • Potential pilot and fibre optic wires are neglected. performed to determine the surface impedances of the various cable layers. Applying the method of composition of impedances • The cable is totally surrounded by seawater. [MacNeill, 2012; Bianchi and Luoni, 1976; [Bianchi and Luoni, 1976] Schelkunoff, 1934] will allow the determination of the net total impedance – based on the LINE PARAMETERS resistance as the real part, and the inductive reactance 2πfL, with L= inductance – of the The line parameters – resistance, capacitance, cable layers in terms of geometry and material and inductance of the cables – are developed properties. The surface impedances of a hollow for single-core and three-core submarine cables. cylinder with an inner radius a, and outer These line parameters will be used in the tworadius b are given by [MacNeill, 2012; port network model of transmission links such Schelkunoff, 1934; Carson and Gilbert, 1921]: that the cable’s steady state performance can be evaluated. (1) Single-Core Cable Line Parameters The line parameters for a single-core submarine cable are determined based on the geometrical model shown in Figure 1. Figure 2 shows the cross sectional view of this cable, denoting the various layers of conductors and insulators and their associated thicknesses. (2) (3) Where , σ is the conductivity of the cylindrical shell, ω is the radial frequency of the applied electromagnetic wave, ε is the permittivity of the cylindrical shell, and μ is the permeability of the cylindrical shell. Applying the principle of composition of impedances to the cross section shown in Figure 2, the following equation relating to the surface impedances of the various cable layers can be developed [MacNeill, 2012]: (4) Figure 2: Cross sectional view of cable model. Electromagnetic field analysis of the cables (cross section is shown in Figure 2) can be 108 The Journal of Ocean Technology, Vol. 8, No. 1, 2013 Where L0, L01, and L02 are the inductances of the insulator layers given by [Schelkunoff, 1934]: Copyright Journal of Ocean Technology 2013 (5) where The value obtained, Z, represents the resistance and inductance of the cable through the following relationship [El-Hawary, 1995; Schelkunoff, 1934]: (6) and To find the capacitance of the cable while noting that the sheath is grounded [Westerwell and Price, 2006; Doyen et al., 1989; Wolff and Elberling, 2000; Thue, 1998] helps in reducing the capacitance model to that of the non-ideal coaxial cable given by [Sadiku, 2007; Schelkunoff, 1934]: (7) Where a is the inner radius of the sheath, and b is the radius of the core conductor of the cable. Three-Core Cable Line Parameters The line parameters of a three-core submarine cable are determined using the assumed geometrical configuration shown in Figure 1. Analyzing the magnetic field produced by the three-core conductors can be used to approximate the inductance of the three-core submarine cable. Figure 3 shows the geometry for the three-core cable. From this figure the inductance of all the cable layers just outside the bundle can be found using the following expression [MacNeill, 2012]: (8) Copyright Journal of Ocean Technology 2013 Figure 3: Three-core cable geometry. This can be applied to find the inductance contribution of the various cable layers that surround the bundle of core conductors. The inductance of the cable’s outer layers can be combined with the self-inductance of each core conductor and the mutual inductance of each core conductor with its adjacent conductors. It is assumed that the core conductors are transposed, such that the mutual inductance between core conductors is equalized. The mutual and self-inductance for each phase of the cable can then be evaluated as [El-Hawary, 1995]: The Journal of Ocean Technology, Vol. 8, No. 1, 2013 109 (9) The total equivalent inductance can be evaluated combining components of the inductance. (10) The capacitance of a three-conductor submarine cable can be determined effectively for two different sheathing configurations: sheaths surrounding each of the core conductors or a sheath surrounding the bundle. It is possible to have sheaths around each core conductor and another sheath that surrounds the total bundle, where one of the sheaths is required to be metallic and the other sheath may be metallic or polymer. Any of these combinations can be reduced to the same approach as that for the two sheathing techniques discussed above. In the first case where the metallic sheath surrounds each of the core conductors, the capacitance can be found from the non-ideal coaxial cable case [Sadiku, 2007; Schelkunoff, 1934]. (12) To support this assumption, we use the following observation concerning the resistance of submarine cables: the resistance is proportional to the energy lost in the core conductor and the energy lost in each of the cable’s layers due to the magnetic field’s induced electromotive force (EMF). This applies to both cable types; however, for the three-conductor submarine cable, there is a cancellation effect for balanced phase voltages, which are separated by 120 electrical degrees. This results in a sum of currents that is zero at each time instant. Due to the offsetting of each conductor from the centre of the cable, the magnetic field outside of the bundle will not be zero. The out of phase voltages reduce the magnetic field (11) This is true based on the same reasoning used for the single-core submarine cable. The capacitance can then be found from Equation 7. In the case where the metallic sheath surrounds the bundle of conductors the method of images can be applied for a coaxial cable with several off centre cores. The resistance of a three-conductor submarine cable can be found from the AC resistance of each core conductor given by: 110 The Journal of Ocean Technology, Vol. 8, No. 1, 2013 Figure 4: Variation of magnetic field outside three-core cable with distance from centre. Figure 5: Variation of magnetic field outside single-core cable with distance from centre. Copyright Journal of Ocean Technology 2013 appreciably outside the bundle of conductors. This allows for the approximations that there will be minimal magnetic field induced EMF in the outer layers of the submarine cable. Figures 4 and 5 show the magnetic field surrounding a triple-core and single-core cable where the line current is assumed to be 1 amp. (13A) (13B) Where the values A, B, C, and D are defined as follows [El-Hawary, 1995]: In Figures 4 and 5 each curve represents the magnetic field around the perimeter of the three-conductor cable, where the horizontal axis corresponds to the angular position around the cable. Each line in each figure corresponds to a distance away from the cable’s centre; the distance from the cable’s centre is given in the legend of each figure. These assumptions yield close approximations for the line parameters for a three-core submarine cable. Two-Port Network Analysis Having obtained the line parameters from the submarine cable geometry and material properties, a two-port network analysis can be used to find the relationship between the cables’ input (sending end) and output (receiving end) variables. The two-port network is an equivalent circuit that can be used to represent a transmission line as shown in Figure 6. (14) (15) (16) (17) where and , is the length of the transmission line. With the sending-end voltage, apparent power, and power factor specified, the receiving end voltage, voltage angle, power, reactive power, power factor, efficiency, voltage regulation, and Thevenin equivalent impedance can be determined. These indices define the steady state performance of the line. The receiving end voltage and current are found directly from knowledge of the sending end voltage and current and solving Equations 13A and 13B [El-Hawary, 1995]. The receiving end power is given by: Figure 6: Two-port network model. Analyzing this equivalent circuit, the following relationships relate the input (sending end) to the output (receiving end) of the cable [El-Hawary, 1995]: Copyright Journal of Ocean Technology 2013 (18) The receiving end reactive power is given by: (19) The Journal of Ocean Technology, Vol. 8, No. 1, 2013 111 The transmission efficiency is given by: (20) The receiving end power factor is then: Figure 7: Thevenin’s equivalent model. (21) The voltage regulation at the receiving end of the transmission line is defined as the difference between rated and no load voltages normalized by the rated voltage. The no load voltage can be found from Equation 13A by setting Ir=0. The voltage regulation is then expressed as follows: (22) The Thevenin equivalent impedance of the cable is defined as follows: (23) The Thevenin equivalent voltage is the receiving end voltage at no load (zero receiving end current). (24) This and the transmission line’s Thevenin equivalent impedance can be used to model the transmission line interconnection with the grid. Note also that the grid can be represented as a Thevenin equivalent circuit. Figure 7 shows the interconnection between the Thevenin equivalent models for the submarine cable and the grid. 112 The Journal of Ocean Technology, Vol. 8, No. 1, 2013 In Figure 7 it is important to note that under normal operating conditions, power is meant to flow from the receiving end of the submarine cable to the grid. The current flow through this equivalent circuit model is defined as the difference between the grid voltage and receiving end voltage divided by the sum of the equivalent Thevenin impedances. If the receiving end voltage and grid voltage are of the same magnitude then to have a current flow there must be a difference in phase between the two sources. If the receiving end phase angle is assumed to be zero degrees, then the grid phase angle must be a small negative angle in order to have the correct current flow. RESULTS Based on manufacturer supplied cable specification sheets, the line parameters for both a single-core and three-core submarine cable can be calculated. Simulation results establish three curves for each of the performance indices. One curve (red) represents the simulation using line parameters provided by the manufacturer of the specific cable type; a second curve (blue) uses the line parameters that were calculated from the manufacturer’s cable geometry; and finally a curve (green) represents the equivalent set of overhead transmission lines. The equivalent set of overhead transmission lines uses the same core conductor radius as used for the submarine cable. The Copyright Journal of Ocean Technology 2013 Table 1: Cable geometry case study. phase separation and height for the equivalent set of overhead conductors are the standard values used for the voltage class considered. The overhead lines study was included as a reference to compare with the submarine cables electrical characteristic. The cable geometry used is shown in Table 1 [ABB, 2010]. Single-core submarine cable spacing of 10 m is assumed. The equivalent set of overhead transmission lines is assumed to have a height of 10 m and phase separation of 0.813 m [El-Hawary, 1995]. This is based on the assumption of a 32.5 kV sending end voltage. A line frequency of 60 Hz has been assumed. It is also assumed that there are no harmonics introduced on the transmission line. The sending end power is assumed to be 3 MVA at a 0.95 lagging power factor. A lagging power factor is chosen because this would simulate the sending end generator being an induction machine operating as a generator. The performance indices evaluated are the receiving end voltage, voltage angle, real power, reactive power, line efficiency, voltage regulation, and the Thevenin equivalent impedance at the receiving end of the transmission line. Copyright Journal of Ocean Technology 2013 Single-Core Cable Steady State Performance Figures 8 through 16 show the steady state performance of the single-core submarine cable for both the calculated values of the line parameters and the measured values of line parameters provided by the cable manufacturer Figure 8: Receiving end voltage magnitude, single-core. Figure 9: Receiving end voltage angle, single-core. The Journal of Ocean Technology, Vol. 8, No. 1, 2013 113 in Table 1 [ABB, 2010]. In addition the equivalent set of overhead transmission lines will be analyzed for comparison purposes. reactive power zero crossing for a certain line length. This zero crossing is possible since the sending end power factor is lagging. Figure 8 shows that a submarine cable’s receiving end voltage reaches a maximum value when the receiving end reactive power is zero. For the overhead transmission lines the voltage increases over the whole range of line lengths. Figure 9 shows that the receiving end voltage phase angle decreases at a faster rate than for the overhead transmission line. Figures 12 and 13 display the transmission line efficiency and voltage regulation, respectively. These figures provide the following information: the overhead transmission lines are more efficient than the submarine cables. This is due to the extra losses incurred in the submarine cable’s layers. The voltage regulation is better for the overhead transmission lines than for submarine cable. This means that the submarine cable’s receiving end voltage will be more sensitive to load changes. Figure 10 shows that the real power delivered to the shoreline station decreases with line length, and decreases at a faster rate for submarine cables. Figure 11 shows that the reactive power for overhead lines is nearly constant, while for submarine cables it is increasing with line length. This results in a Figure 12: Transmission line efficiency, single-core. Figure 10: Receiving end power, single-core. Figure 13: Receiving end voltage regulation, single-core. Figure 11: Receiving end reactive power, single-core. 114 The Journal of Ocean Technology, Vol. 8, No. 1, 2013 Figure 14 shows the power factor at the receiving end of the transmission line. The power factor for the overhead transmission line is approximately constant, while for the Copyright Journal of Ocean Technology 2013 submarine cable it achieves a maximum at a certain line length. This means that for this specific cable, there is a line length where the generated reactive power and added reactive power from the transmission line sum to zero. This implies that a cable could potentially be tailored to a specific marine application so as to achieve improved power factors at the receiving end. Figure 14: Receiving end power factor, single-core. Figure 15: Thevenin equivalent impedance, single-core. Figure 16: Thevenin equivalent impedance angle, single-core. Copyright Journal of Ocean Technology 2013 Figures 15 and 16 show that the Thevenin equivalent impedance increases linearly with transmission line length, and that the impedance phase angle is approximately constant with line length. From Figures 8 to 16 it can be seen that the calculated values for the line parameters provide very good approximations of the performance indices for line lengths less than 20 km. Three-Core Cable Steady State Performance Figures 17 through 25 show the steady state performance of the three-core submarine cable for both the calculated values of the line parameters and the measured values of line parameters provided by the cable manufacturer in Table 1 [ABB, 2010]. In addition the equivalent set of overhead transmission lines will be analyzed for comparison purposes. Figure 17: Receiving end voltage magnitude, three-core. Figure 18: Receiving end voltage angle, three-core. The Journal of Ocean Technology, Vol. 8, No. 1, 2013 115 It can be seen from Figures 17 and 18 that for three-core submarine power cables, the receiving end voltage decays as the line length increases. For the overhead lines the voltage increases over the whole range of line lengths. The receiving end voltage phase angle decreases at a slower rate than for the overhead transmission line. Figure 19 shows that the real power decreases with line length, and decreases at a faster rate for submarine cables. Figure 20 shows that the reactive power for overhead lines is nearly constant, while for submarine cables it is increasing with line length; this results in a receiving end reactive power zero crossing for the submarine cable. Figures 21 and 22 display the transmission line efficiency and voltage regulation, which provides the following information: the overhead transmission lines are marginally more efficient than the submarine cable. This is due to the minimal losses incurred in the cable’s extra layers. The voltage regulation for the overhead transmission line is much better than for the submarine cable. This means that the submarine cable receiving end voltage will be more sensitive to load changes. Figure 23 shows the power factor at the receiving end of the transmission line. The results are similar to the single-core cable case. Again, a cable could potentially be tailored to achieve a desired power factor. Figure 19: Receiving end power, three-core. Figure 20: Receiving end reactive power, three-core. Figure 21: Transmission line efficiency, three-core. Figures 24 and 25 show that the Thevenin’s equivalent impedance increases linearly with transmission line length, and that the impedance phase angle is constant with line length. The trends of the performance indices for both 116 The Journal of Ocean Technology, Vol. 8, No. 1, 2013 Figure 22: Receiving end voltage regulation, three-core. Copyright Journal of Ocean Technology 2013 maximum receiving end voltage; and the receiving end power factor approaches unity for a certain line length. CONCLUSIONS Figure 23: Receiving end power factor, three-core. This study has shown that submarine cables have unique electrical characteristics that need to be considered when specifying underwater transmission of power. • Submarine cables provide considerable reactive power to the system. • Figure 24: Thevenin equivalent impedance, three-core. Figure 25: Thevenin equivalent impedance angle, three-core. single-core submarine cables and three-core submarine cables are very similar. The difference between the two cables can be found by looking at the rate of change of the performance indices with respect to line length. The design of three-core cable lines is more complex than single-core. The important characteristics of submarine cables are as follows: submarine cables are less efficient; the cables incur a Copyright Journal of Ocean Technology 2013 In the case where sending end power factor is lagging there is a line length where the receiving end power factor could be close to unity. • The receiving end voltage magnitude fluctuates (rises, maximizes, and then decreases). One of the advantages is that a submarine cable is able to feed the reactive power demand of the electric power generator (induction and permanent magnet generators) that are commonly used in offshore wind and marine energy converters. This balancing of reactive power demand from the generator and production from the cable means that there is an enhanced quality of power reaching shore. The consequence of this is the possibility of introducing resonances in the transmission line. The findings from this evaluation would assist in tailoring the cables’ geometry to improve electrical characteristics of the submarine cables and hence improve the quality of electric power transmission to shore, contributing to overall reliability of marine energy electric generation. In general, determining submarine cable performance characteristics has important implications on The Journal of Ocean Technology, Vol. 8, No. 1, 2013 117 how to analyze underwater power systems and offshore generation interconnections with specific focus on marine energy. ACKNOWLEDGEMENTS The authors would like to thank CanmetENERGY/Natural Resources Canada (NRCan) for providing funding support for this study; it was funded under the Program for Integrated Renewable Electricity Systems allocated by the Program of Energy Research and Development POL 5.1 Marine Renewable Energy. Funding support from Natural Sciences and Engineering Research Council of Canada Discovery Grant and support of Dalhousie University (Electrical and Computer Engineering Department and Faculty of Engineering) are hereby acknowledged. Finally, the advice and support of NRCan’s Ghanashyam Ranjitkar are hereby acknowledged. REFERENCES ABB [2010]. XLPE submarine cable systems attachment to XLPE land cable systems – user’s guide. ABB, Sweden. Bianchi, G. and Luoni, G. [1976]. Induced currents and losses in single-core submarine cables. IEEE Transactions on Power Apparatus and Systems, Vol. PAS- 95, No. 1. Carson, John R. and Gilbert, J.J. [1921]. Transmission characteristics of the submarine cable. Bell System Technical Journal, Journal of the Franklin Institute. Doyen, H., Eberling, H., Fiss, H.J., and Labrenz, M. [1989]. Experiences with different cable laying designs and laying methods in conjunction with the power 118 The Journal of Ocean Technology, Vol. 8, No. 1, 2013 supply of the islands in the North and Baltic Sea. IEEE Electrical Distribution. El-Hawary, M.E. [1995]. Electrical power systems design and analysis. IEEE Press: New York, NY. Hauge, O., Johnsen, J.N., Holte, T.A., and Bjorlolw-Larsen, K. [1988]. Performance of the +/-250 kV HVDC Skagerrak submarine cables: further development of the HVDC paper-insulated, mass impregnated (solid type) submarine cable. IEEE Transactions on Power Delivery. MacNeill, A.M. [2012]. Submarine power cable transmission line parameters and performance. Dalhousie University. Pieroni, C.A. and Fellows, B.W. [1979]. Life evaluation of a 35-kV submarine power cable in a continuous flexing environment. IEEE Journal of Oceanic Engineering, Vol. OE-4, No. 1. Sadiku, M.N.O. [2007]. Elements of electromagnetics. Fourth Edition. Oxford University Press: New York, NY. Schelkunoff, S.A. [1934]. The electromagnetic theory of coaxial transmission lines and cylindrical shields. Bell System Technical Journal, Vol. XIII, pp. 532-578. Thue, W.A. [1998]. Electrical power cable engineering. CRC Press: New York, NY. Westerwell, Th. and Price, J.J. [2006]. Basslink HVDC interconnector – system design considerations. IEEE AC and DC Power Transmission. Wolff, C. and Elberling, T. [2000]. The Kontek HVDC link between Denmark and Germany. IEEE Power Engineering Society. Worzyk, T. [2009]. Submarine power cables design, installation, repair, environmental aspects. Springer: Heidelberg, Berlin. Copyright Journal of Ocean Technology 2013