INSULATED ELECTRIC CABLE

Abstract: An electric cable of the mass-impregnated non-draining type impregnated with a dielectric impregnating material. The dielectric impregnating material is liquid and the viscosity of the liquid impregnating material exhibits a low temperature dependency that gives the cable pressure dynamic properties, whereby it acquires a capability to accommodate and compensate for thermal and pressure gradients building up in the cable under operation. The acquired pressure dynamic properties minimize the risks of unfilled voids and the like discontinuities developing. At the lower range of operating temperatures, the dielectric liquid has a viscosity suitably high to ensure the non-draining features while it allows the liquid to flow under the influence of shear stresses typically developing in the radial direction in the cable insulation of a DC cable during operation. Further, the impregnating material, within the first lower range of temperatures, exhibits a minimized weak temperature dependency of the viscosity. At a higher range of impregnating temperature, it has a significantly lower viscosity to ensure the impregnation result. It exhibits a significant change in the viscosity over the limited transition range of temperatures.

Description: Insulated electric cable TECHNICAL FIELD The present invention relates to an insulated electric cable. More specifically it relates to a cable having an insulation system comprising a solid part and a dielectric impregnating material. The solid part or body is porous and/or laminated. Further, the solid body is impregnated with the dielectric impregnating material in such a way that essentially all voids in the solid body are filled with the impregnating material. The dielectric impregnating material exhibits a thermo-reversible transition from a high viscous state at lower temperatures to a significantly less viscous state at higher temperatures. The insulated cable according to the present invention is favorable for use in direct current, DC, systems and installations for transmission and distribution of electric power. A dielectric impregnating material suitable for use in a cable according to the present invention exhibits a temperature-dependent viscosity such that it at typical impregnation temperatures and pressures is a free flowing and low viscous liquid, while it at operating temperatures is significantly more viscous. The transition occurs over a limited range of temperatures, the transition range. BACKGROUND ART Many of the first systems for transmission and distribution of electric power used DC technology. However, these DC systems were rapidly superseded by systems using alternating current, AC. The development of electrical supply systems in the first half of this century was exclusively based on AC transmission systems. By the 1950s there was a growing demand for long-distance transmission schemes. It was found economical to adopt DC-based transmission technology for these schemes. An important benefit of DC operation is the virtual elimination of dielectric losses, thereby offering a considerable gain in efficiency and savings in equipment. A typical insulation system for a DC transmission cable comprises an inner serni- conductive shield arranged in contact with and surrounding the conductor, an insulation body and an outer semi-conductive shield. The conductor and insulation system is supplemented with further functional layers and features such as a casing, mechanical reinforcement, means to withstand water penetration etc. Cables having a mass-impregnated solid, paper-based, insulation system are often found favorable. Cables of this type comprise a dielectric impregnating material selected to ensure that the cable is non-draining. Non-draining means in this art and in this application that the dielectric impregnating material is retained in the porous insulation also in case of damage to the cable. To date an essentially all paper insulation body impregnated with an electrically insulating oil has been used but laminated material such as a polypropylene paper laminate is being pursued. The most onerous condition for an insulation system of the mass- impregnated non-draining type in a DC cable occurs in the cooling phase when the operating load is lowered from full load. A commercially available insulated electric DC cable of the mass-impregnated non-draining type is, to be suitable for transmission or distribution of electric power, designed for operation at a high voltage, i.e. a voltage typically above 100 N. The insulation is applied by spinning or winding paper- or cellulose-based tapes or sheets around the conductor to form a solid but porous and laminated insulation. Thereafter, the wound insulation is impregnated with a dielectric fluid, e.g. an oil. The active part of the insulation system is the solid part. The dielectric impregnating material protects the insulation against moisture pick-up and fills all pores, voids or other interstices, whereby any dielectrically weak air in the insulation is replaced by the dielectric impregnating material. The dielectric impregnating material also acts as capsules to prevent water penetration. The impregnation process is typically time-consuming and delicate. It is often carried out in batches. It needs to be carefully monitored and controlled according to a carefully developed and strictly controlled process cycle with specified ramping of both temperature and pressure used during heating, holding and cooling to ensure a complete and even impregnation of the cable insulation. For a good impregnation result, a fluid exhibiting a low viscosity under impregnation conditions is desired. However, the fluid shall also be suitably viscous under operation conditions for the cable to avoid any axial migration of the fluid in the insulation and in particular to ensure that the cable is non-draining also in case of damage to the cable. As temperature fluctuations and temperature gradients will be expressed in a high- voltage DC cable migration and/or thermal expansion of the dielectric fluid must be carefully considered. Unfilled voids or other unfilled interstices or pores formed must be avoided. Such voids might constitute sites for space charge accumulation. An unfavorable space charge accumulation in the cable insulation increases the risk of a dielectric breakdown through an increased risk of discharges. Any discharges will degrade the insulation and ultimately might lead to its breakdown. The dielectric impregnating material shall, over the whole operating temperature range, exhibit a viscosity sufficiently high to ensure that the cable is non-draining. The viscosity shall also be high enough to minimize any axial flow of dielectric impregnating material in the cable during operation. But the viscosity shall be low enough to allow a sufficient radial flow of dielectric impregnating material during operation to avoid formation of unfilled voids and similar discontinuities, which might form sites for space charge accumulation. A conventional DC cable of the non-draining type having a porous or laminated insulation system is impregnated with dielectric oil. A conventional oil dielectric impregnating material, which in this application is referred to as a type I dielectric impregnating material, exhibits a strongly temperature-dependent viscosity that typically decreases essentially exponential as the temperature increases. In particular, the type I dielectric impregnating material also has a strongly temperature-dependent viscosity in the typical operation temperature interval of a HNDC cable. This might contribute to unsatisfactory pressure dynamics in the cable. The expression pressure dynamics or pressure dynamic properties means in this application that the flow characteristics of the impregnating material will allow the impregnating material to flow or not flow as a function of the pressure and temperature gradients building up in the cable. Pressure dynamics considered to be favorable will allow a suitable radial flow, outwards and inwards, within the cable to compensate for said gradients and if no gradients, which need compensation, are present in the cable there will be no flow. However, any axial flow in the cable will be suppressed and minimized. A type I impregnating material requires the use of high impregnation temperatures. That is an impregnation temperature that is substantially higher than the operation temperature. This high impregnation temperature is required to ensure that a type I dielectric impregnating material in a non-draining insulation system is sufficiently fluid under impregnation conditions. Oils used as type I dielectric impregnating materials are typically selected to be suitably viscous at expected operation temperatures to ensure the non-draining criteria. However, a high impregnation temperature might be disadvantageous for several reasons. It might damage the solid insulation material. Further, it can promote chemical reactions within and between any material present in the cable. A high impregnation temperature will negatively affect energy consumption, during production, and overall production costs. The thermal expansion and shrinkage of the insulation must also be considered. This means that the cooling rate during cooling must be controlled and slow, which adds time and complexity to an already time-consuming and complex process. Another type of impregnated insulation system for a cable employs oil having a low viscosity. These cables need tanks or reservoir along the cable to ensure that the cable insulation remains fully impregnated upon thermal cycling experienced during operation. With these cables there is a risk of oil spillage from a damaged cable. International Patent Application with publication No. WO98/01869 discloses a DC cable of the non-draining type having an insulation system with a dielectric impregnating material. The dielectric impregnating material has been modified by the addition of a thickening agent or gelator. Such a gelling dielectric impregnating material is, for the remainder of this application, referred to as a type II dielectric impregnating material. Further examples of gelling dielectric impregnating materials and method for producing cables with gelling dielectric impregnating materials can be found in the International patent applications with the following publication Nos. WO99/33066, WO99/33067, WO99/33068 and WO99/33071. A gelled type II dielectric impregnating material has a positive effect on the pressure dyna ics in the insulation system of a cable during operation as the gelled dielectric impregnating material will have a viscosity so high that it will not flow at operation temperatures. The gelled dielectric impregnating material has a viscosity of 500Pas or more at typical operating temperatures. Operating temperatures are typically from -50 °C to 80 °C. Shear yield stress of such a type II gelled impregnating material is typically in the order of 100 Pa within this range of temperatures. That is, a type II impregnating material will typically not flow under these conditions as long as it is not subjected to shear stresses of 100 Pa or more. The shear stresses due to pressure dynamics in a high voltage DC cable are in the range 0.01-10 Pa during operation. Most often the shear stresses due to pressure dynamics in a high voltage DC cable are in the range 0.1-1 Pa. A type II dielectric impregnating material typically comprises a gelling polymer additive. The gelling polymer additive imparts a thermo-reversible transition between a gelled state at low temperatures and liquid state at high temperatures to the type II dielectric impregnating material. At high temperatures the liquid dielectric impregnating material is essentially Newtonian and has a low viscosity. Typically, a viscosity lower than a type I impregnating material under similar conditions. The transition occurs over a limited temperature range at temperatures higher than the range of typical operation temperatures. The production of a cable with an insulation system comprising a type II dielectric impregnating material exhibits a substantial potential for reduction of the time period needed for impregnation. It also exhibits potentials for improving many other desirable features during production and use. But it still requires a strictly controlled temperature cycle during impregnation. The gelling polymer additive and the dielectric fluid to be used in a type II dielectric impregnating material must be carefully matched to meet the typically conflicting demands of high fluidity during irnpregna- tion and non-draining when the cable is used. There is a strong desire to reduce impregnation temperatures and at the same time there is a desire to increase the current densities in the DC cables, which with today's conductors and cable design will increase the operation temperatures in the DC cable. This means that there is a trend and desire to further reduce the temperature span between the im- pregnation temperature and operation temperature. Consequently, it will be even more delicate to match the specific demands. The dielectric impregnating material shall also be able to accommodate the thermal fluctuations, thermal and pressure gradients and other phenomena occurring upon thermal cycling or under the influence of thermal gradients. This to ensure that no unfilled voids or other similar discontinuities likely to enhance space charge accumulation are formed. Further, the dielectric impregnating material shall exhibit such thermal and electric properties and stability such that it opens for an increase in load, i.e. an increase in both operation voltages and current densities used in the device. It is desirable to provide an insulated DC cable of the non-draining type having an electrical insulation system that ensures stable dielectric properties irrespective of the op- eration temperatures. The dielectric impregnating material employed shall, under conditions prevailing during: - impregnation, have a sufficiently low viscosity, i.e. a viscosity deemed suitable and technically and economically favorable for impregnation; and - operation of the cable, have a viscosity and elasticity deemed suitable, i.e. a vis- cosity that ensures that it will be essentially retained and evenly distributed in the insulation body at all temperatures within the range of temperatures for which the DC cable is designed to operate. At the same time, the cable shall have pressure dynamic properties whereby the internal flow of the dielectric impregnating material, both inwards and outwards, can accommodate thermal cycling and the pressure gradients associated therewith. The flow shall ensure that no unfilled voids or pores will be formed in the insulation system as thermal gradients and electrical forces act on the cable and its integral parts, the conductor, the solid part of the insulation and the dielectric impregnating material. A DC cable according to the present invention shall provide an opportunity for a substantial reduction in the lengthy time-consuming batch-treatment for impregnation of the insulation system. It shall, in particular, exhibit pressure dynamic properties that, by allowing a suitable radial flow, both outwards and inwards, ensure stable dielectric properties also when operated under varying temperatures or temperature gradients, thereby mamtaining or improving the high reliability, low maintenance requirements and long working life of conventional DC cables of the non-draining type while offering potentials for lower production costs and an increased range of operating temperatures and current loads. A DC cable according to the present invention shall have: - stable and consistent dielectric properties; - a high and consistent electric strength over wide ranges of operating temperatures and current loads; and - as an extra advantage, be open for an increase in the electrical strength and thus allow an increase in operation voltages, improve manageability and robustness of the cable. SUMMARY OF THE INVENTION It is an object of the present invention to provide an insulated electric cable hav- ing a mass-impregnated insulation of the non-draining type. The cable shall have pressure dynamic properties that accommodate for the stresses and other forces building up in the cable in operation in such a way that no unfilled voids or similar discontinuities are formed. The stresses and forces are building up under the thermal and pressure gradients formed in the cable during operation, also electric and magnetic fields and forces will act within the cable. A cable according to the present invention as defined in the preamble of claim 1 comprises at least one conductor and an insulation system with a mass-impregnated solid part. The mass-impregnated solid part is porous and/or laminated and impregnated with a dielectric impregnating material. Typically, the solid part is comprised of wound tapes or the like. The tapes comprise paper, other fibrous materials or polymer films or any combination thereof, giving the tapes a structure comprising pores and/or being laminated. Pores or other voids are accessible for impregnation. The dielectric impregnating material in a cable according to the present invention is essentially liquid and exhibits a viscosity with a weak temperature dependency. At temperatures in a first lower temperature range the dielectric impregnation material is a viscous liquid. At those temperatures it has a viscosity suitably high to ensure the non- draining features. At temperatures in a second higher range of temperatures the dielectric im- pregnating liquid has a significantly lower viscosity than in the first lower temperature range. This low viscosity ensures good properties for a satisfactory impregnation. The significant change of viscosity takes place over a limited range of temperatures, the transition range. As this range typically is very narrow it is often, and at times also in this application, referred to as the transition temperature. A dielectric material with a low temperature viscosity as defined in the foregoing provides a cable according to the present invention with the non-draining features such that the insulating liquid will be retained in the cable also if the cable is damaged. Further, it minimizes any axial flow of dielectric impregnating material in the cable. The first lower range of temperatures typically includes at least the essential part of the operation range of temperatures. The low viscosity at the high temperature range ensures that the laminated and/porous insulation is fully impregnated and essentially free of unfilled voids or similar discontinuities after impregnation. Such discontinuities would provide sites for space charge accumulation under an electric DC field. The second higher range of temperatures typically includes at least the impregnation temperatures. To meet the object of the present invention, the viscosity of the dielectric impregnating material for a cable according to the preamble of claim 1 is chosen to impart pressure dynamic properties to allow a sufficient radial flow of dielectric impregnating material during operation to avoid formation of unfilled voids. This is accomplished with the feature according to the characterizing part of claim 1. Within the first lower range of temperatures, the dielectric impregnating material will be allowed to flow in radial direction, both outwards and inwards, under the influence of shear stresses typically developing in the radial direction in the cable insulation under operation. Further, the impregnating will, within this first lower range of temperatures, exhibit a viscosity with a weak temperature dependency. More specifically the impregnating material will exhibit a viscosity suitably low to allow it to flow under the shear stresses typically developing in a DC cable under operation and a viscosity which within the typical range of operating temperatures for a DC cable exhibits a weak temperature dependency. In a DC cable under operation shear stresses in the order of 0.01-10 Pa will develop and in a high- voltage DC cable used for transmission or distribution of electric power shear stresses of 0.1 - 1 Pa will develop. A cable according to the present invention ensures that the non-draining charac- teristics are combined with the pressure dynamic properties required to ensure that the insulation will remain essentially free of unfilled voids. This will be ensured not only after impregnation, but it will also remain so under and after operation. This is beneficial as a guarantee for a long service life under high voltage and high current densities as unfilled voids and similar discontinuities might form sites for space charge accumulation. Further, an unfavorable space charge accumulation in the cable insulation increases the risk of a dielectric breakdown through an increased risk of discharges. Any discharges will degrade the insulation and ultimately might lead to its breakdown. Such a dielectric impregnating material as used in a cable according to the present invention will, in the remainder of this application, at times, be referred to as a type III dielectric impregnating material. A dielectric insulating liquid suitable for use in a cable according to the present invention has a viscosity at operation temperatures that is: - high enough to ensure that the impregnating liquid is retained in the insulation; but - low enough to ensure pressure dynamic properties, which accommodate and compensate for the thermal and pressure gradients associated with operation of a DC cable and thereby mini- mize any risks of unfilled voids being formed under operation. At impregnation temperatures the liquid is an essentially free-flowing liquid with a low viscosity allowing good and easy impregnation. Further, it exhibits the desired features discussed in the foregoing to ensure good impregnation. Further developments of the present invention are characterized by the features of additional claims 2 to 14. A type III dielectric impregnating material exhibits a rheology modification, which during operation gives the cable according to the present invention its advantageous pressure dynamic properties. A type III dielectric impregnating material is modified to remain liquid but viscous at the temperatures of operation. The viscosity of such liquid type III di- electric impregnating materials will generally be 250 Pas or less in the whole operating range of temperatures. Typically the viscosity in a temperature range of - 50 °C to 80 °C is from 1 Pas to 200 Pas and preferably in the range of 1 Pas to 100 Pas. The liquid will flow at the typical shear stresses applied to it in a DC cable insulation system during operation. This will make the capability of the liquid to flow outwards essentially equal to the capability to flow inwards and the radial flow will be controlled by the shear stresses in the cable insulation system as they develop due to pressure gradient developing under thermal cycling and the like. Shear stresses, acting on the insulating system in a HVDC cable under operation, will typically be in the order of 0.1-1 Pa. The cable is in general operated at temperatures up to 80 °C. Typically, it is op- erated at temperatures from -50 up to 80 °C. Preferred operating temperatures includes temperatures from -5 to 80 °C. It is often preferred to limit the range of operating temperatures further and to strive for a range of - 5 °C to 70 °C. Type III dielectric impregnating materials will at impregnation temperature in general exhibit a viscosity lower than 1 Pas, typically a viscosity in the range lxlO "3 Pas (1 mPas) to 1 Pas and preferably a viscosity in the range of lxlO "3 Pas (1 mPas) to 100 mPas, i.e. a viscosity similar to that of most suggested type II dielectric impregnating materials at impregnation temperature. Typically, the impregnation is carried out at temperatures over 80 °C, preferably at temperatures from 80 °C to 120 °C. Typically, the ratio between the viscosity at operating temperatures and the vis- cosity at impregnation temperatures for a dielectric impregnating liquid to be used in a cable according to the present invention is 5:1 or more. Suitably, the ratio is within a range from 10:1 to 1000:1. Preferably, the ratio is within a range from 40:1 to 1000:1. A cable according to a typical embodiment of the present invention comprises a dielectric impregnating material having a viscosity which: - a operating temperatures including temperatures from - 50 °C to 80 °C is within a range from 1 Pas to 200 Pas; - at impregnating temperatures including temperatures above 80 °C is within a range from 1 Pas to 1 Pas; and which has - a ratio between the viscosity at operating temperatures and the viscosity at impregnation temperatures for the dielectric impregnating liquid of 5:1 or more. The impregnating material has a viscosity that, within the lower temperature range of from - 50 °C to 80 °C, allows it to flow at shear stresses developing in the radial direction of the cable under operation, the shear stresses being in the order of from 0.01 Pa to 10 Pa. Suitably, a cable according to the present invention has an insulation system impregnated with an impregnating material comprising oil and a styrene-olefin block copolymer. Typically, it comprises a mineral oil with additions of styrene-olefin block copolymers. Suitable block polymers have been found to be styrene-ethylene-butylene-styrene block polymers, various styrene-butadiene, styrene-butadiene-styrene, styrene-iosprene-styrene and styrene- ethylene-propylene block polymers. A cable according to one preferred embodiment of the present invention com- prises a dielectric impregnating material having a viscosity which: - at operating temperatures from - 5 °C to 80 °C is within a range from 1 Pas to 200 Pas; - at impregnating temperatures from 80 °C to 120 °C is within a range from 1 mPas to 1 Pas; and which has - a ratio between the viscosity at operating temperatures and the viscosity at impregnation temperatures for the dielectric impregnating liquid from 10:1 to 1000:1. - The impregnating material has a viscosity that, within the lower temperature range of from - 5 °C to 80 °C, allows it to flow at shear stresses developing in the radial direction of the cable under operation, the shear stresses being in the order of from 0.1 Pa to 1 Pa. According to one more preferred embodiment of the present invention the vis- cosity of the dielectric impregnating liquid is within the range of 10 Pas to 100 Pas at operating temperatures from - 5 °C to 65 °C. Suitably, the dielectric impregnating liquid exhibits a low temperature dependency in the range of temperatures from - 50 °C to 80 °C. This means that the viscosity of the dielectric impregnating material is essentially constant within this the first lower range of tem- peratures. The temperature dependency is typically less than 4 [%/°C] for temperatures be- tween 0 °C to 80 °C. According to a preferred embodiment the temperature dependency is less than 3 [%/°C] for temperatures between 0 °C to 80 °C. By the temperature dependency expressed as x [ /°C] is meant in this application that the viscosity is increased with x% for a temperature decrease of °C. Of course, a temperature increase of 1 °C would result in a viscosity decrease of x %. A DC cable according to the present invention typically comprises from the center and outwards: - a conductor of any desired shape and constitution, such as a stranded multi-wire conductor, a solid conductor or a sectional conductor; - a first semi-conducting shield disposed around and outside the conductor and inside the conductor insulation; - a woimd and impregnated insulation according to the present invention with a dielectric electrically insulating solid part exhibiting a porous and/or laminated structure as described in the foregoing impregnated with a type III dielectric impregnating material; - a second semi-conducting shield disposed outside the conductor insulation; and - an outer protective sheath. The two semi-conducting shields are typically wound and impregnated. Typically, also the semi-conducting shields comprise a dielectric electrically insulating solid part exhibiting a porous and/or laminated structure as described in the foregoing and impregnated with a dielectric impregnating liquid. The liquid is favorably a type III dielectric impregnating material. The cable can, when deemed appropriate, be supplemented with reinforcing and sealing compounds or a water-swelling powder for filling any interstices in and around the conductor, other metal/polymer interfaces may be sealed in order to prevent water from spreading along such interfaces. A DC cable according to the present invention is ensured long-term stable and consistent dielectric properties and a high and consistent electric strength as good as or better than any conventional DC-cable. To a substantial extent this can be attributed to the flow properties of the impregnating material. This stability is especially important due to the long life such installations typically are designed for, and the limited access for maintenance to such installations of being installed in remote locations or even in sub-sea locations. The special selection dielectric impregnating materials ensure the long-term stable properties of the insula- tion system also when used at elevated temperatures, at excessive thermal fluctuations and/or under thermal gradients. This opens for an increase in the operation load, both in regard to increased voltages and current densities. Further, there is an object of the present invention to provide a cable suitable for use in a system or installation for transmission or distribution of electric power, in particular in a system or installation for high- voltage direct current (HVDC) transmission or distribution of electric power. Further, it is an object to indicate a favorable use of a type III dielectric impregnating material. The type III material is essentially liquid and exhibits a weakly temperature dependent viscosity, i.e. a viscosity with a weak temperature dependency of 4 [%/°C] or less. The viscosity of the type III material is featured such that: - at a first lower range of temperatures it is in a high viscosity state with a viscosity suitably high to ensure the non-draining features; - at a second higher range of temperature it has a significantly lower viscosity than in the high- temperature high- viscosity state to ensure good impregnation result, wherein the insulation is essentially free of unfilled voids; - over a third limited range of temperatures, the transition range, it exhibits a significant change in the viscosity; and - that the dielectric impregnating material, within the first lower range of temperatures, has a viscosity low enough to allow it to flow under the influence of shear stresses in the order typically developing in the radial direction in the cable insulation of a DC cable under operation. In the insulation system of a cable according to the present invention, under operation a shear stress in the order of 0.01-10 Pa will typically develop. These shear stresses act essentially in the radial direction. More typically, shear stresses in the order of 0.1 -1 Pa will develop under operation. The cable has least one conductor and an electric insulation system. The insulation system comprises a solid part and a dielectric impregnating material. The solid part has a structure that is porous and/or laminated. The solid part is impregnated with the dielectric impregnating material. The use of a type III material for impregnation of such a cable imparts the unique pressure dynamic properties exhibited by a cable according to the present invention. These pressure dynamic properties allow the cable to accommodate and compensate for the thermal and pressure gradients building up in the cable under operation. The pressure dynamic properties essentially eliminate or at least substantially reduce or minimize the risks of unfilled voids and the like discontinuities developing under operation. This is important for a DC cable, and in particular for a high voltage DC cable for transmission or distribution of electric power, as this provides a tool to limit space charge accumulation and also to control the pattern of any developing space charge accumulation in the insulation system. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be described in more detail with reference to the drawings and examples. Figure 1 shows a cross-section of an embodiment of a DC cable according to the present invention, favorable for use as a cable for transmission of electric power; Figure 2 shows schematically the temperature dependency of the viscosity for type I, type II and type III dielectric impregnating materials; Figure 3 shows the temperature dependency of the viscosity for the type III dielectric impregnating materials used in example A; and Figure 4 shows the temperature dependency of the viscosity for the type III dielectric impregnating materials used in example B. DESCRIPTION OF PREFERRED EMBODIMENTS, EXAMPLES. The DC cable according to the embodiment of the present invention shown in figure 1 comprises from the center and outwards; - a stranded multi-wire conductor 10; - a first semi-conducting shield 11 disposed around and outside the conductor 10 and inside a conductor insulation 12; - a wound and impregnated conductor insulation 12 comprising a gelling additive as described in the foregoing; - a second semi-conducting shield 13 disposed outside the conductor insulation 12; - a metallic screen 14; and - a protective sheath 15 arranged outside the metallic screen 14. The cable is supplemented with a reinforcement in form of metallic, preferably steel, wires outside the outer extruded shield 13, and a sealing compound or a water-swelling powder is introduced in interstices in and around the conductor 10. A DC cable according to the present invention can have any arbitrary insulation system comprising a solid part, which is porous and/or laminated. The solid part is impregnated with a dielectric fluid or mass such that essentially all voids are filled with the dielectric impregnating material. The cable according to the present invention can have any arbitrary conductor configuration. Further, it can comprise any arbitrary functional layer and any arbitrary configuration of these layers. ADC cable according to the present invention can also be used in any system for DC transmission of electric power independent of the configuration of the system. The DC cable according to the present invention can be a single conductor DC cable having a multi-wire core as shown in Figure 1 or a DC cable with two or more conductors. A DC cable comprising two or more conductors can be of any known type with the conductors placed side-by-side in a flat cable arrangement, or in a two-conductor arrangement with one first central conductor surrounded by a concentrically arranged second outer conductor, i.e. a coaxial two-conductor cable. The outer conductor is typically arranged in the form of an electrically conductive sheath, screen or shield. A DC cable according to the present invention is suitable for use in bipolar or mono-polar DC systems or installations for transmission of electric power. A bipolar system typically comprises two or more associated single conductor cables or at least one multi-conductor cable, while a mono- polar installation has at least one cable and a suitable current return path arrangement. The graphs shown in figure 2 are schematic representations of the temperature dependency of dielectric impregnating materials of type I, type II and type III. The graph labeled type I is a representation of the temperature dependency of a type I impregnating material typically used in a conventional DC cable of the mass- impregnated non-draining type. Such a conventional dielectric impregnating exhibits a strongly temperature dependent viscosity that typically decreases essentially exponentially as the temperature increases. In particular, the type I dielectric impregnating material also has a strongly temperature-dependent viscosity in the typical operating temperature interval of a HNDC cable. This might contribute to unsatisfactory pressure dynamics in the cable. Oils used as type I dielectric impregnating materials are typically selected to exhibit suitably viscosity to be non- draining at expected operation temperatures. An impregnation temperature substantially higher than the typical operating temperatures, must be used to ensure that the type I oil is sufficiently fluid at impregnation conditions. However, a high impregnation temperature has several disadvantages: - It might damage the solid insulation material; - It can promote chemical reactions within and between any material present in the cable; - It will increase energy consumption during production and thus overall production costs. The graph labeled type II is a representation of the temperature dependency of a type II impregnating material typically used in a DC cable of the mass-impregnated non- draining type as described in e.g.WO98/01869. Such a type II dielectric impregnating material has been modified by the addition of a thickening agent or gelator. A gelled type II dielectric impregnating material has a positive effect on the pressure dynamics in the insulation system of a cable since during operation it will not flow at operating temperatures. The gelled dielectric impregnating material will have a viscosity of 500 Pas or more at typical operating temperatures, i.e. temperatures from -50 °C to 80 °C. Shear yield stress of such a type II gelled im- pregnating material is typically in the order of 100 Pa. The impregnating material will not flow as long as it is not subjected to shear stresses of 100 Pa or more. The shear stresses due to pressure dynamics in a high voltage DC-cable are in the order of 0.01 - 10 Pa, most often the shear stresses are typically in the range 0.1-1 Pa during operation. A type II dielectric impregnating material typically comprises a gelling polymer additive. The gelling additive imparts a thermo -reversible transition between a gelled state at low temperatures and a liquid state at high temperatures to the type II dielectric impregnating material. At high temperatures used for impregnation, the liquid dielectric impregnating material is essentially Newtonian and exhibits a low viscosity, typically less than 0.1 Pas. The transition occurs over a limited temperature range. The production of a cable with an insulation system comprising a type II di- electric impregnating material exhibits a substantial potential for reduction of the time period needed for impregnation. It also exhibits potentials for improving many other desirable features during production and use. However, it is not self-healing as it does not offer the pressure dynamic properties required to accommodate for thermal and pressure gradients building up during operation. Thus it will not allow a sufficient radial flow of impregnating material to compensate for thermal expansion and the gradients mentioned to eliminate or at least mini- mize the formation of unfilled voids under operation. Also one must be cautious and observant so the gel is stable and no phase separation occurs. The graph labeled type III shows a representation of the temperature dependency of a type III impregnating material as used in a DC cable of the mass-impregnated non- draining type according to the present invention. A type III dielectric impregnating material exhibits a rheology modification, which during operation gives the cable according to the present invention its advantageous pressure dynamic properties. A type III dielectric impregnating material is modified to remain Hquid but viscous at the temperatures of operation. The viscosity of such liquid type III dielectric impregnating materials will generally be 100 Pas or less in the whole operating range of temperatures. The viscosity in a temperature range of - 50 °C to 80 °C is generally from 1 Pas to 100 Pas. The liquid will flow at the typical shear stresses applied to it in a DC cable insulation system during operation, which for a HVDC cable typically are in the order of 0.1-1 Pa. This will give the liquid an essentially equal capability to flow outwards and inwards, whereby the flow is controlled by the shear stresses in the cable insulation system as they develop due to pressure gradient developing under thermal cycling and the like. Type III dielectric impregnating materials will at impregnation temperature typically exhibit a viscosity in the range lxl0 "3 Pas (1 mPas) to 1 Pas. Typically, the impregnation is carried out at temperatures from 80 °C to 120 °C. Typically, the ratio between the viscosity at operating temperatures and the viscosity at impregnation temperatures for a dielectric impregnating liquid to be used in a cable according to the present invention is 5:1 or more. The type III liquid dielectric impregnating material is typically produced by admixing a styrene-olefin block co-polymer with an oil or a mixture of oils to achieve a dielectric impregnating material having the desired properties. The desired properties are, for example, a viscosity in the range 1 Pas to 100 Pas in a temperature range of - 50 °C to 80 °C and a viscosity in the range of 1 mPas to 1 Pas at temperatures between 80 °C and 120°C. The oil or mixture of oils is preferably a mineral oil or a mixture of at least two mineral oils with different characteristics, but could also be a synthetic oil or a vegetable oil. Different types of oils may also be mixed to achieve an oil with desired characteristics. EXAMPLES Examples of type III liquid dielectric impregnating materials Example A. A type III impregnating material was produced by admixing 1.5 % by weight of KRATON G1654 (Shell), a styrene-ethylene-butylene-styrene block copolymer, with a mineral oil NSlOO (Nynas Petroleum) and thereafter heating the mixture to 120 °C under nitrogen for 3 h until all polymer was dissolved. KRATON is a trademark of Shell Chemicals Ltd. The temperature dependency of the viscosity for the type III dielectric impregnating material according to example A was found to be as described in figure 3. Figure 3 shows a low temperature viscosity approaching 100 Pas at temperatures around 5 °C. The viscosity decreased slowly for temperatures up to around 80 °C, where it rapidly dropped several orders of magnitude so that, at temperatures around 100 °C, it was as low as 0.1 Pas. The temperature dependency of the viscosity within the operation range of temperatures, i.e. temperatures from -5°C to 80°C, was on average over the range about 2.6 %/°C In the range 5 °C to 40 °C the temperature dependency was about 2.9 %/°C. At higher operating temperatures of 40 °C to 80 °C, it was slightly decreased to about 1.9 % /°C . Example B. The mixture of 0.8 w-% KRATON G 1651 (Shell Chemicals), a styrene-ethylene-butylene- styrene block copolymer, and an oil BT250 (Nynas Petroleum), was heated to 120°C under nitrogen for 1 h until all the polymer was dissolved. The resulting dielectric impregnating liquid had the properties described in figure 4. KRATON is a trademark of Shell Chemicals Ltd. The temperature dependency of the viscosity for the type III dielectric impregnating material according to example B was found to be as described in figure 4. Figure 4 shows a low temperature dependency of the viscosity just above 200 Pas at temperatures around 5 °C. The viscosity was found to be almost constant; only a slow stable decrease was detected for temperatures up to around 80 °C. At temperatures above 80 °C, the viscosity was found to rapidly drop several or- ders of magnitude so that, at temperatures around 100 °C, it was approaching 0.1 Pas. This low viscosity was reached at a temperature of 120 °C. The temperature dependency within the operation range of temperatures, i.e. temperatures from -5°C to 80°C, was steady and as low as 0.5 %/°C. Although the examples above are restricted to the use of a styrene-ethylene- butylene-styrene block copolymer, other tests have indicated that other styrene-olefin di-block or tri-block copolymers would give similar temperature dependency for the viscosity. Such feasibility tests have been performed with styrene-butadiene block copolymers (KRATON 1101), styrene-butadiene-styrene block copolymers (KRATON 1102) and styrene-ethylene- propylene (KRATON 1701). KRATON is a trademark of Shell Chemicals Ltd. The results of these examples show that it is probable that a DC cable according to the present invention, with a conductor insulation comprising a dielectric impregnating material of this type III, will allow faster impregnation rates and lower impregnation temperatures compared with cables comprising conventional type I dielectric impregnating materials. A cable comprising a type III dielectric impregnating material will be non-draining cable, i.e. the impregnating material will fully retained in the insulation. Further, a cable comprising a type III impregnating material will, due to the low temperature dependency of the viscosity of these type III materials, provide the desired pressure dynamic properties. These pressure dynamic properties allow a flow of dielectric liquid in the radial direction within a cable under the influence of shear forces developing in a cable under operation. This will accommodate for the forces and stresses building up due to thermal expansion, electric and magnetic field and the like. A sufficient radial flow, both inwardly and outwardly as required, will compensate for thermal expansion and stresses and gradients mentioned. Thereby any formation of unfilled voids during operation is avoided. At the same time, is any axial flow in the cable is minimized and the non-draining characteristics maintained.

Claims: CLAIMS 1. An electric cable having least one conductor and an electric insulation system having a solid part and a dielectric impregnating material, wherein the solid part is porous and/or laminated, the solid part is impregnated with a dielectric impregnating material, and the dielectric impregnating material is essentially hquid and exhibits a temperature-dependent viscosity such that the dielectric impregnating material; - at a first lower range of operating temperatures, is in a high- viscosity state with a viscosity suitably high to ensure non-draining features, - at a second higher range of impregnating temperature, has a significantly lower viscosity than in the low-temperature high- viscosity state, - over a third intermediate transition range of temperatures, exhibits a significant change in the viscosity, characterized in that the dielectric impregnating material - within the first lower range of operating temperatures, comprising temperatures from - 50 °C to 80 °C, has a viscosity within a range from 1 Pas to 200 Pas, - within the first lower range of temperatures, has a viscosity low enough to allow it to flow under the influence of shear stresses in the order typically developing in the radial direction in the cable insulation of a DC cable under operation, the shear stresses for which the dielectric impregnating material starts to flow being in the order of from 0.01 Pa to 10 Pa, - at the second higher range of impregnating temperatures, comprising temperatures above 80 °C, has a viscosity within a range from 1 mPas to 1 Pas, - over the third intermediate transition range of temperatures, exhibits a change in the viscosity such that the ratio between the viscosity at the first lower range of temperatures and the viscos- ity at the second higher range of temperatures for the dielectric impregnating liquid is 5:1 or more, and - that within a range of temperatures from 0 °C to 80 °C, the impregnating material exhibits a weak temperature dependency of the viscosity of 4 %/°C or less. 2. A cable according to claim 1, characterized in that the first lower range of operating temperatures includes temperatures from -5 °C to 80 °C. 3. A cable according to any of the preceding claims, characterized in that the second higher range of impregnating temperatures includes temperatures from 80 °C to 120 °C. 4. A cable according to any of the preceding claims, characterized in that the vis- cosity of the dielectric impregnating material in the first lower range of operating temperatures ranges from 1 Pas to 100 Pas. 5. A cable according to any of the preceding claims, characterized in that the viscosity of the dielectric impregnating material in the second higher range of impregnating tem- peratures ranges from 10 mPas to 100 mPas. 6. A cable according to any of the preceding claims, characterized in that the ratio between the viscosity at the first lower range of operating temperatures and the viscosity at the second higher range of impregnating temperatures for the dielectric liquid is from 10:1 to 1000:1. 7. A cable according to any of the preceding claims, characterized in that within the first lower range of operating temperatures, the shear stresses for which the dielectric impregnating material starts to flow is in the order of from 0.1 Pa to 1 Pa and that within a range of temperatures from 0 °C to 80 °C the impregnating material exhibits a weak temperature dependency of the viscosity of 3 %/°C or less. 8. A cable according to any of the preceding claims, characterized in that the dielectric impregnating liquid comprises oil and a styrene-olefin block copolymer. 9. A cable according to claim 8, characterized in that the oil comprises a mineral oil. 10. A cable according to claim 8, characterized in that the oil comprises a synthetic oil. 11. A cable according to claim 8, characterized in that the oil comprises a vegetable oil. 12. A cable according to any of claims 1 to 7, characterized in that the dielectric impregnating Hquid comprises a mixture of at least two oils with different viscosity and a styrene- olefin block copolymer, and that the mixture of oils comprises at least one of the following; a mineral oil, a synthetic oil and a vegetable oil. 13. Use of an electric cable according to any of the preceding claims in a system or installation for transmission or distribution of electric power. 14. Use of an electric cable according to any of claims 1 to 12 in a system or installation for high- voltage direct current (HNDC) transmission or distribution of electric power. 15. Use of a dielectric impregnating material that is essentially Hquid and exhibits a temperature-dependent viscosity for impregnating the soHd part of a cable insulating system such that the impregnating material imparts to the cable pressure dynamic properties, whereby the cable insulation system acquires a capabiHty to accommodate and compensate for thermal and pressure gradients building up in the cable during operation to minimize the risks of unfilled voids and the like discontinuities developing, wherein the impregnating material - at a first lower range of temperatures, is in a high- viscosity state with a viscosity suitably high to ensure the non-draining features, - at a second higher range of temperature, has a significantly lower viscosity than in the high- temperature high- viscosity state, - over a third limited range of temperatures, the transition range, exhibits a significant change in the viscosity, - that within the first lower range of temperatures, the dielectric impregnating material has a viscosity low enough to aUow it to flow under the influence of shear stresses in the order typically developing in the radial direction in the cable insulation of a DC cable during operation, and - that within the first lower range of temperatures, the impregnating material exhibits a mini- mized weak temperature dependency of the viscosity.

Inventors: BERGKVIST MIKAEL, EKANDER HANS, FELIX JOHAN, GUSTAFSSON KARIN, HANSSON OLA, LILJENBERG ANNIKA, NORDBERG PER, BERGKVIST, MIKAEL, EKANDER, HANS, FELIX, JOHAN, GUSTAFSSON, KARIN, HANSSON, OLA, LILJENBERG, ANNIKA, NORDBERG, PER
Applicants: ABB AB, BERGKVIST MIKAEL, EKANDER HANS, FELIX JOHAN, GUSTAFSSON KARIN, HANSSON OLA, LILJENBERG ANNIKA, NORDBERG PER, BERGKVIST, MIKAEL, EKANDER, HANS, FELIX, JOHAN, GUSTAFSSON, KARIN, HANSSON, OLA, LILJENBERG, ANNIKA, NORDBERG, PER
Application year: 2001
Main classification: H01B3
IPC codes: H01B3, H01B9