INSULATED ELECTRIC CABLE

Abstract: An insulated electric cable having a polymer based insulation system comprising an insulation layer (12; 22) and one or more semi-conductive layers (11; 13; 21; 23). The insulation layer is based on an electrically insulating polymeric composition and comprises a polar enriched surface zone having a suitable raised content of a suitable polar substance. The cable is in particular suitable for use in a system or installation for transmission or distribution of electric power or as a conductor in a high voltage winding in an electric machine.

Description: Insulated Electric Cable TECHNICAL FIELD The present invention relates to an electπc cable having an insulation system compπsing a polymeric composition The polymeπc composition is typically extruded and cross-linked. In particular the invention relates to a cable having an extruded and cross-linked polyethylene, PE, composition, an XLPE composition. The present invention relates in particular to an m- sulated electπc cable with an insulation system compπsing a plurality of layers, such as an inner semi-conductive shield, an extruded insulation and an outer semi-conductive shield. At least the extruded insulation compπses a cross-linked polymeπc electπcally insulating composition with a system of additives typically compπsing cross-linking agent, scorch re- tarding agent and anti-oxidant. A cable according to the present invention is favorable used in systems for transmission and distπbution of electπc power and also as an insulated conductor in a high voltage winding in an electπc machine or other application wherein the electπcally insulation layer in the cable is subjected to a high voltage stress. BACKGROUND ART Many of the first electπcal supply systems for transmission and distπbution of electπcal power were based on DC technology However, these DC systems were rapidly superseded by systems using alternating current, AC. The AC systems had the desirable feature of easy transformation between generation, transmission and distπbution voltages. The development of modem electπcal 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 and it became clear that under certain circumstances there could in these schemes be benefits by adopting a DC based system. An important benefit of DC operation is the virtual elimination of dielectπc losses, thereby offeπng a considerable gam in efficiency and savings in equipment. The DC leakage current is of such small magnitude that it can be i Ogn' ored in current rating calculations, whereas in AC cables dielectπc losses cause a sianifi- cant reduction in current rating. This is of considerable importance for higher system voltages. Similarly, high capacitance is not a penalty in DC cables. A typical DC-transmission cable includes a conductor and an insulation system compπsing a plurality of layers, such as an inner semi-conductive shield, an insulation base body and an outer semi -conductive shield. The cable is also complemented with casing, reinforcement etc to withstand water penetration and any mechanical wear or forces duπng production, installation and use. Almost all the DC cable systems supplied so far have been for submaπne crossings or the land cable associated with them. For long crossings the mass-impregnated solid paper msu- lated type cable is chosen because there are no restπctions on length due to pressuπzing requirements. To date insulation systems with a solid essentially all paper insulation impregnated with an electπcally insulating oil are used. Application of laminated mateπal such as a polypropylene paper laminate is being pursued to gain advantage of the increased impulse strength and reduced diameter. As in the case of AC transmission cables, transient voltages is a factor that has to be taken into account when determining the insulation thickness of DC cables. It has been found that the most onerous condition occurs when a transient voltage of opposite polaπty to the operating voltage is imposed on the system when the cable is carrying full load. If the cable is connected to an overhead line system, such a condition usually occurs as a result of lightning transients. Extruded solid insulation based on a polyethylene, PE, or a cross linked polyethylene, XLPE, has for almost 40 years been used for AC transmission and distπbution cable insulation. Therefore the possibility of the use of XLPE and PE for DC cable insulation has been under investigation for many years. Cables with such insulations would have the same advantage as the mass impregnated cable. For DC transmission there are no restπctions on circuit length. Cable insulations based on XLPE also have a potential for being operated at higher temperatures than conventional mass impregnated paper based cable insulations. In the case of - XLPE, 90°C instead of 50°C for conventional DC-cables. Thus offeπng a possibility to increase the transmission load. However, it has not been possible to obtain the full potential of these polymeπc mateπals for full size cables. It is believed that one of the mam reasons is the development and accumulation of space charges in the dielectπc when subjected to a DC- field. Such space charges distort the electπcal stress distπbution and persist for long peπods because of the high resistivity of the polymers. Space charges in an insulation body do when subjected to the forces of an electπc DC-field accumulate in a way such that a polaπzed pat- tern similar to a capacitor is formed. Space charge accumulation is often divided into two basic types of patterns, diffeπng in the polaπty of the space charge accumulation. The space charge accumulation results in a local increase at certain points of the actual electπc field m relation to the field. This shall be contemplated when consideπng the geometπcal dimensions and dielectπc characteπstics of an insulation. The increase noted in the actual field might be 5 or even 10 times the contemplated field. Thus the design field for a cable insulation must include a safety factor taking into account this considerably higher field. Therefore, typically thick insulation layers and/or expensive mateπals are used in the cable insulation. The space charge accumulation is, at least in some aspects, a slow process, therefore this problem is accentuated when the polaπty of the cable after being operated for a long peπod of time at same polaπty is reversed. As a result of such a reversal a capacitive field is supeπmposed on the applied field. As a consequence the space charge accumulation peak in the profile and the point of maximal field stress is moved from the interface and into the insulation. Attempts have been made to improve the situation by the use of additives to reduce the insulation resistance without seπously affecting other properties. To date it has not been possible to match the electπcal performance achieved with the impregnated paper insulated cables and no commercial polymeπc insulated DC cables have been installed. However, successful laboratory tests have been reported on a 250 kV cable with a maximum stress of 20 kV/mm using XLPE insulation with mineral filler (Y.Maekawa et al, Research and Development of DC XLPE Cables, JiCablePl, pp. 562- 569). This stress value compares with 32 kV/mm used as a typical value for mass-impregnated paper cables. Another aspect to consider for extruded polymeπc insulation is charge injection at any interface between the insulating layer in an insulation system and conducting or semi-conducting layer or body arranged in contact with the insulating layer. The πsk for charge injection is drastically increased if the electπcal properties of the conducting body or semi-conducting shield in contact with the insulating layer is inhomogenous. Earlier it has been considered that charge injection is likely to occur in LDPE insulating composition under AC conditions when the load is higher than 15-20 kV/mm. See e.g. J. Jonsson, B Ranby, D. Mary, C. Laurent and C. Mayoux, LEEE Trans. Diel. Electr. Insul.,2. 107(1995). or T. Mizuno. Y.S. Liu, W. Shionoya, K. Yasuoka, S. Ishn, H. Miyata and A Yokoyama. LEEE Trans. Diel. Electr. Insul., 4, 433(1997). At higher loads, e.g. the loads anticipated to occur in the mter- face between the insulating layer and a semi-conductive shield within a cable for high voltage direct current transmission of electπc power, approx. 100 kV/mm, the πsk of charge injection is substantial. As charge injection πsk to enhance aging of the polymeπc mateπal in the insulating layer it also put the expected life-time of the cable at πsk. An extruded res composition for AC cable insulation typically compπses a polyethylene resin as the base polymer complemented with vaπous additives such as a peroxide as cross- linking agent, a scorch retarding agent and an anti-oxidant or a system of anti-oxidants. In the case of an extruded insulation the semi -conductive shields are typically also extruded and compπse a resin composition that in addition to the base polymer and an electπcally conduc- tive or semi-conductive filler compπses essentially the same type of additives. The vaπous extruded layers in an insulated cable in general are often based on a polyethylene resm. Polyethylene resin means generally and m this application a resm based on polyethylene or a co- polymer of ethylene, wherein the ethylene monomer constitutes a major part of the mass. Thus polyethylene resms may be composed of ethylene and one or more monomers which are co-polymeπsed with ethylene. LDPE, low density polyethylene, is today the predominant insulating base mateπal for AC-cables. To improve the physical properties of the extruded insulation and its capability to withstand degradation and decomposition under the influence of the conditions prevailing under production, shipment, laying, and use of such a cable the polyethylene based composition typically compπses additives such as; - stabilizing additives, e.g. antioxidants, electron scavengers to counteract decomposition due to oxidation; radiation etc.; - lubπcating additives, e.g. steaπc acid, to increase processabihty; - additives for increased capability to withstand electπcal stress, such as being more water tree resistant, examples of such additives are polyethylene glycol, si cones etc.; and - cross-linking agents such as peroxides and silanes, which, upon a following treatment such as heating or exposing for radiation, e.g. X-ray, Infrared, Ultraviolett. decompose into free radicals and initiate cross-linking of the polyethylene resin, sometimes used in combination with unsaturated compounds having the ability to enhance the cross-hnkmg density, - scorch retarders to avoid premature cross-hnkmg. The number of vaπous additives is large and the possible combinations thereof is essentially unlimited. When selecting an additive or a combination or group of additives the aim is that one or more properties shall be improved while others shall be maintained or if possible also improved. However, in reality it is always next to impossible to forecast all possible side ef- fects of a change in the system of additives. In other cases the improvements sought for are of such dignity that some minor negative effects have to be accepted, although there is always an aim to minimize such negative effects. A typical polyethylene based resin composition to be used as an extruded, cross-linked msu- lation in an AC-cable compπses: 97.1 - 98.9 % by weight of low density polyethylene (922 kg m 3 ) of melt flow rate 0.4 - 2.5 g/10 mm with a system of additives as descπbed m the forgoing. These additives can compπse: 0.1 - 0 5 % by weight of an antioxidant, such as but not limited to SANTONOX R® (Flexsys Co) with the chemical designation 4,4'-thιo-bιs(6-tert-butyl-m-cresol), and 1 0 - 2 4 % by weight of a cross-lmking agent such as but not limited to , DICUP R® (Hercules Chem) with the chemical designation dicumyl peroxide. It is well known that this type of XLPE composition exhibits a strong tendency to form space charges under DC electπc fields, thus making it unusable in insulation systems for DC cables. However, it is also known that extended degassing, i.e. exposing the cross-lmked cable insulation at elevated temperatures to a vacuum for long peπods of time, will result in a somewhat decreased tendency to space charge accumulation under DC voltage stress. It is generally believed that the vacuum treatment removes the peroxide decomposition products, such as acetophenone and cumyl alcohol, from the insulation whereby the space charge accumula- tion is reduced. Degassing is a time-consuming batch-process comparable with impregnation of paper insulations and thus as costly Therefore it is advantageous if the need for degassing is removed Most known cross-linked polyethylene compositions used as extruded insulation in AC-cable exhibit a tendency for space charge accumulation which renders them unsuitable for use in insulation systems for DC-cables It is known to add low amounts of an additive compπsing carbonyl groups to a LDPE for the dual purpose of increasing resistivity and decreasing space charge accumulation. Such addition of carbonyl is accomplished either by oxidizing polyethylene or by a co-polymeπzation of carbon monoxide with ethylene. Further it has been noted, J. Jonsson and C. Tornkvist, Proc. Nord-IS, 217(1996) that an addition of carbonyl groups to a content of 0.4 -1.0 % by weight to polyethylene might reduce the charge injection. One plausible cause offered for this is that the carbonyl groups act as trap sites for space charges, whereby the mobility of any space charges is restπcted and also any development of a polaπzed pattern within the insulation as a result of space charge accumulation when the insulation is subjected to a DC- field. However, a tendency for detrapping and thereby an increased space charge accumulation has also been noted at elevated temperatures, e.g. temperatures above about 40°C. Also additives in the form of organic acids and anhydπdes have shown to give similar effects. Further molar modifications of the polyethylene has been suggested by introduction of polar units into the polymer to obtain a higher DC-breakdown strength. For example Japanese Patent Publication JP-A-210610 reports that an anhydπde such as Maleic Acid Anydπde, MAH, have been grafted onto the polyethylene for this purpose. The resulting cross-linked msula- tion mateπal exhibited a decrease m space charge accumulation attπbuted to the increased polaπty of the cross-lmked polymer chain structure and it was concluded that the grafted MAH groups, which are fixed withm the cross-linked structure, act as trap sites for any space charges. In JP-A-210610 it was reported that cross-linked polyethylene with additions of MAH at levels corresponding to from about 0.02 to about 0.5 % by weight resulted in cross- linked composition suitable for use as insulation in a DC-cable with a decreased space charge accumulation Other additions used for such polar modification of the cross-linked structure and associated with reduction in space charge accumulation in the cross-linked insulation is lonomers, acrylic metal salts, carboxyhc acid and acetates. Thus it is desirous to provide an insulated cable with a polymer based electπcal insulation system compπsing an extruded XLPE composition suitable for use as a transmission and distπbution cable in networks and installations for transmission and distπbution of electπc power. The cable shall typically be produced with a process for application and processing of the extruded XLPE based insulation which can be earned out in a manner such that there is no need for any lengthy time consuming batch-treatment such as impregnation or degassing, i.e. vacuum treatment of the cable to ensure stable and consistent dielectπc properties and a high and consistent electπc strength of the cable insulation The cable insulation shall further exhibit a low tendency to space charge accumulation, a high DC breakdown strength, a high impulse strength, high insulation resistance and a high capability to withstand charge injection. This would offer both technical and economical advances over pπor art methods as production time and production costs can be reduced and the possibility for an essentially con- tinuous or at least semi-continuous process for the application and processing of the cable insulation system is provided. Further the reliability, the low maintenance requirements and the long working life of a conventional cable, e.g. a transmission cable compπsing a mass impregnated paper-based insulation, shall be maintained or improved. The cable according to the present invention shall as an extra advantage open for an increase in the electπcal strength and thus allow an increase in operation voltages, improve handleability and robustness of the cable. In particular it is desirous to provide an insulated electπc cable wherein the extruded and cross-linked PE composition contained in the insulation system compπses a three dimen- sional cross-linked structure with a capability to restπct the mobility of charges within the insulation layer of the cable thereby reducing the tendency to space charge accumulation, raising the capability to withstand charge injection and providing means for controlling any developing space charge profile or pattern within the insulation layer. Restπcting the mobility of charges would thus be of great value in most insulated conductors operating at high voltages. The reduction in mobility of any charges which would be achieved with a cable according to the present invention provides as an extra economical advantage a capability to reduce safety factors in design values used for dimensioning the cable insulation In particular such a cable is desirous for operation under the specific conditions prevailing in a network or installation for transmission or distπbution of electπcal power. Such a cable would be attractive to use as transmission cable and as a conductor winding in a high voltage electπc device, such as a generator, a transformer, a reactor or a motor or any other cable application wherein the insulation will be subjected to a high voltage stress SUMMARY OF THE INVENTION It is an object of the present invention to provide an insulated electπc cable that meets the desires as specified in the foregoing. This is according to the present invention achieved by a cable as defined in the preamble of claim 1 having a polymer based insulation system compπsing an extruded cross-linked polyethylene composition disposed around a conductor char- acteπzed by the further measures according to the characteπzmg part of claim 1. Further developments of the invented cable are characteπzed by the features of the additional claims 2 to 17. It is also an object of the present invention to present the particullary advantageous uses of the cable according to the present invention as defined in claims 18 and 19 DESCRIPTION OF THE INVENTION In order to use polymer based cable insulation systems compπsing at least one polymer based insulation layer and one or more semi-conductive layers several factors have to be taken into account. One important issue is the restπction of the mobility of charges as a successful re- stπction of charges will: - reduce the tendency for space charge accumulation withm the polymeπc insulation layer when subject to a high voltage stress and in particular a high DC-voltage stress; - increase the capability of the insulation layer to withstand charge injection; - and provide means for controlling any developing space charge profile or pattern within the insulation system. The present invention accomplish a significant decrease in the mobility of charges and thereby the sought for reduction in space charge accumulation, increased capa- bility to withstand charge injection and the also sought for tool to control any developing space charge profile or pattern occurπng within the insulation layer by implementing a polar enπched surface zone within the insulation layer. The polar enπched surface zone compπses a suitable raised content of a suitable polar substance. The polar groups act as trap sites for charges, whereby the mobility of any charges is restπcted. Thereby substantially reducing the accumulation of space charges with the cross-lmked insulation layer when the insulation is subjected to an electπc DC field. Further the polar enπched surface zone(s) also increases the capability of the insulation layer to withstand charge injection. Polar groups within the polar enπched surface zone(s), acting as traps sites for any space charges, also provides means for controlling any developing space charge profile or pattern to avoid harmful polaπzed patterns or profiles developing within the insulation layer. The present invention is in particular favorable in cables having an insulation system wherein the insulation system compπses an ex- truded, cross-linked polyethylene based composition, an XLPE composition. Typically such an insulation system also compπses one or more semi-conductive layers arranged adjacent to the surfaces of the insulating layer. Preferably a cable according to the present invention compπses. - a first inner semi-conductive shield disposed on and around the conductor; and - an insulation layer disposed on and around the first inner semi-conductive shield; wherein the insulation layer compπses a first, inner, polar enπched surface zone disposed withm the insulation layer at the inner interface between the inner semi-conductive shield and the insulation layer. According to one favorable embodiment the insulation system further compnse a second outer semi-conductive shield disposed on and around the insulation layer, wherein a second, outer, polar enπched surface zone disposed withm the insulation layer at the outer interface between the outer semi-conductive shield and the insulation layer. Polar substances which have been found suitable for use in the polar enπched zone in a cable according to the present invention are carbonyl groups. The carbonyl groups are introduced into the polar enπched zone either through oxidizing polyethylene or by co-polymeπzation of carbon monoxide with ethylene withm the polar enπched zone. Favorable contents of carbonyl groups have been found to be from 0.1 to 1.5 % by weight. Most preferred is contents of carbonyl groups ranging from 0.4 to 1.0 % by weight. Other embodiments of the present invention compπses insulating layers to which additives in the form of organic acids or anhy- dπdes have been made. These additives were found to have similar effects on the accumulation of space charges, the capability to withstand charge injection and possibility to control any developing space charge pattern to avoid the development of any polaπzed space charge profile as discussed in the foregoing According to another embodiment of the present invention the insulating layer compπses 10 polar units introduced into the polymer chain. Also these polar units were found to act as trap sites for space charges, whereby also in this case the mobility of any space charges is re- stπcted and the accumulation space charges within the cross-linked insulation layer is substantially reduced. The polar units act as traps sites for any space charges thereby reducing the space charge accumulation, increasing the capability to withstand charge injection and also providing means for controlling the developing pattern of space charges to avoid the development of any polarized pattern withm the insulation layer. A cable according to one embodiment of the present invention compπses an XLPE based insulation layer wherein Maleic Acid Anydπde, MAH, has been grafted onto the polyethylene withm the polar en- πched surface zone for this purpose. The resulting cross-linked insulation layer exhibits a decreased tendency for space charge accumulation and an increased capability to withstand charge injection. Both these effects are attπbuted to the increased polaπty of the cross-linked polymer chain structure within the polar enπched surface zone. The polar units withm the polymer chain structure also offers a tool for controlling any developing charge pattern within the insulation layer which is valuable to avoid any polaπzed pattern or profile. Cross- linked polyethylene with additions of MAH at levels corresponding to from about 0.01 to about 1.0 % by weight has been found to result in cross-lmked composition suitable for use m the polar enπched zone of the insulation layer m a DC-cable to achieve the desired decrease in space charge accumulation, increased capability to withstand charge injection and possi- bility to control any developing space charge accumulation profile. Other additions deemed suitable for use to achieve this type of polar modification of the cross-lmked structure and thus to achieve the desired reduction in space charge accumulation, increased capability to withstand charge injection and possibility to control any developing space charge accumulation profile in the cross-linked insulation is lonomers, acrylic metal salts, carboxyhc acid and acetates. According to one further embodiment of the present invention an XLPE insulation layer compπsed a polar enπched surface zone wherein a polar co-monomer having the general formula CH2*=CR-CO-X-(CH 2 )n-N(CH 3 ) 2 or CH 2 =CR-CO-O-(CH 2 -CH 2 O) m -H was mtro- duced into the polyethylene chain. In this general formula n is equal to 2 or 3, m is equal to a number between 1 and 20, R is H or CH 3 and X is O or NH. Preferably m is equal to 1, 5, 6. or 9. The polar co-monomers was in this case introduced in to the polyethylene chain and 11 either formed a segment in the chain backbone duπng the polymeπzation or a pending side group in a grafting operation. Typically an amount of polar monomer in excess of 0.1 % by weight of the total polymer in the polar enπched surface zone of the insulation layer has been found suitable. Preferably an amount of 0.1 to 5 % by weight of the polar co-monomer shall be used. Most preferred is a content of the polar co-monomer ranging from 0.5 to 1. 5 % by weight. A preferred polar co-monomers according to this embodiment is based on metacrylic amide and has the general formula CH 2 =C(CH 3 )-CO-NH-(CH 2 ) n -N(CH 3 ) 2 , where n is equal to 2 or 3. The co-monomer having n=3 and designated dimethylammo-propylmetacryl-amide (DMAPMA) has been found most suitable. Other polar co-monomers found suitable for introduction into the polymeπc chain in the polar enπched zone(s) of the insulating layer of a cable according to the present invention are: - co-monomers based on acrylic amide, having the general formula CH 2 =CH-CO-NH-(CH 2 ) n -N(CH 3 ) 2 , where n is equal to 2 or 3; - co-monomers based on metacrylic ester, having the general formula CH 2 =C(CH 3 )-CO-O-(CH 2 ) n -N(CH 3 ) 2 , where n is equal to 2 or 3 ; - co-monomers based on acrylic ester, having the general formula CH 2 =CH-CO-O-(CH 2 ) n -N(CH 3 ) 2 , where n is equal to 2 or 3; - co-monomers based on metacrylic acid and ohgomeπc ethyleneglycol, having the general formula CH 2 =C(CH 3 )-CO-O-(CH2-CH 2 O-) m H, where m is equal to a number between 1 and 20, preferably m is equal to 1, 5, 6, or 9; and - co-monomers based on acrylic acid and ohgomeπc ethyleneglycol, having the general formula CH 2 =CH-CO-O-(CH2-CH 2 O-) m H, where m is equal to a number between 1 and 20, preferably m is equal to 1, 5, 6, or 9. An insulated electπc cable according to the present invention can be produced with a process which typically compπses the following steps, not necessaπly in the order given below; - compounding or otherwise producing composition in a way such that it is given a suitable content of polar groups or polar units for the desired polar enπched composition. - extruding the compounded polyethylene composition having a raised content ol polar units or polar groups, as a surface zone contained within an insulation layer of a polymer based insulation system disposed around a conductor and compπsing an insulation layer and at least one semi-conductive layer; and 12 - subsequently cross-linking the polar enπched PE composition into a polar enπched XLPE composition contained as a surface of the insulation layer In the following will suitable processing for a cable according to the present invention be examp fied. To achieve the polar groups within the polar enπched surface zones can a polymeπc compound, e.g. polyethylene be oxidized to form a polar enπched polymeπc composition compπsing carbonyl groups suitable for use in surface zones of the insulating layer in a cable according to the present invention. Alternatively can the carbonyl groups be introduced through a co-polymeπzation of carbon monoxide and ethylene. A composition to be used in the polar enπched surface zone(s) of the insulation layer can also be produced in a compounding step wherein a polar monomer is added to the ethylene pπor to or duπng the po- lymeπzation reaction. In this way the co-monomer will be built into the backbone of the polymer and be integrated with the polyethylene chain. The polar co-monomer is typically added to the polymer in an amount of from 0.05 to 5 % by weight of the finished polymer and preferably in an amount of from 0.3 to 1.5 % by weight of the finished polymer. Alternatively the ethylene and the polar monomer is copolymeπzed in a similar manner but the amount of comonomer is now considerably higher, e.g. 5 - 40 % by weight, and preferably 25 - 35 % by weight of the finished polymer. Such a copolymer with high amount of polar comonomer is subsequently diluted by compounding the copolymer with straight polyethylene until the av- erage polar comonomer content is the same as descπbed in foregoing Still another suitable way to produce a composition to be used m the polar enπched zone(s) of a cable according to the present invention is to graft a the polar monomer on a homopolymer of ethylene. The grafting process can be performed either in a separate step after the polymeπzation process or it might be executed duπng the extrusion and/or cross-hnkmg of the polyethylene based cable insulation A cable according to the present invention with an extruded, cross linked insulation system, such as an insulating system compπsing an extruded, cross-linked polyethylene based composition, an XLPE composition, with a polar enπched surface zone introduced into the msu- lating layer exhibit considerable advantages such as, - A substantially reduced tendency for space charge accumulation, - A substantially increased capability to withstand charge injection 13 - A tool for control of any developing space charge pattern or profile; and - An increased DC breakdown strength. A cable according to the present invention thus offers good performance and stability of the extruded cable insulation system also when high temperatures have been employed duπng extrusion, cross-linking or other high temperature conditioning. It is, as always, favorable if the content of any unreacted peroxide cross-hnkmg agent or any by products or degradation products in an extruded XLPE composition can be minimized to further reduce any tendency for space charge. Thus the peroxide content of a PE composition to be extruded and cross-linked into an insulation system for a cable according to the present invention shall be less than 5 % and preferably less than 2 %. The essential elimination or substantial reduction of excess peroxide remnants in the insulation of the cable is advantageous consideπng the cost of the peroxide cross-hnkmg agent and more important considering the fact that the peroxide cross-linking agent upon degradation is likely to form undesired by-products such as methane, cumyl alcohol or acetophenone, which is a source of space charges. A cable according to the present invention is suitably adapted to meet the specific requirements for use as a cable for transmission or distπbution of electπc power, such as a High Voltage Direct Current transmission cable, or other cable wherein the insulation layer is subjected to a high voltage stress without resorting to time consuming batch treatments. All the advantageous properties and improvements over pπor art cables having an insulation system compπsing an extruded XLPE composition, is for a cable according to the present invention achieved without the many disadvantages associated with some pπor art cables. The substan- tially reduced tendency for space charge accumulation, and the increased capability to withstand charge injection ensures that the high DC breakdown strength of conventional DC- cables compπsing an impregnated paper insulation is maintained or improved. The possibility to control any developing space charge profile provides means for avoiding any unfavorable space charge pattern within the insulating layer of the cable insulation. Further the insulating properties of a cable according to the present invention exhibit a general long term stability such that the working life of the cable is maintained or increased. This is achieved in particular by the implementation ol a polar enπched zone(s) in the insulation layer. It is further 14 enhanced by the controlled processing of the polymeπc compositions to be used in the insulation system pπor to and duπng extrusion and cross-linking, wherein process vaπables such as temperatures, pressures, processing times, atmosphere composition are closely controlled A cable according to the present invention offers as an extra advantage the capability of being produced by an essentially continuous process without any time consuming batch step such as impregnation or degassing, thereby opening for substantial reduction in production time and thus the production costs without πsking the technical performance of the cable A cable as defined in the foregoing is especially advantageous for operation under the specific conditions prevailing in high-voltage transmission or distπbution cable used in a network or installation for transmission or distπbution of electπcal power, due to the improved thermal properties combined with maintained or improved electπcal properties This is especially important due to the long life such installations are designed for, and the limited access for maintenance to such installations of being installed m remote locations or even sub-sea. One further advantage for a high-voltage cable produced according to the present invention is that the production time can be substantially reduced by the adoption of an essentially continuous process free from operating steps requiπng batch treatment of complete cable lengths or part lengths offer cost advantages compared to conventional cables A cable according to the present invention is also beneficial for use as an insulated conductor in a high voltage winding for an electπc machine such as a generator, a transformer, a reactor or a motor as the reduced tendency space charge accumulation and in particular the increased capability to withstand charge injection offers a possibility to increase the field stress, i.e. the voltage stress, without πsking any detπmental effects at the interfaces between the insulation layer and the semi-conductive layers due to uncontrolled space charge accumulation or charge injection BRIEF DESCRIPTION OF THE DRAWINGS The present invention shall be descπbed more in detail while referπng to the drawings and examples 15 Figure 1 shows a cable suitable for high-voltage transmission of electπc power according to one embodiment of the present invention. Figure 2 shows a cable to be used as an insulated conductor for a high-voltage winding in an electπc machine according to another embodiment of the present invention. Figures 3a, 3b, 3c and 3d show space charge recordings for comparative tests on plates with XLPE compositions as used in pπor art insulated AC -cables and for compositions suitable as polar enπched compositions to be used in insulated cables according to the present invention. DESCRIPTION OF PREFERRED EMBODIMENTS, EXAMPLES. The high voltage transmission cable according to the embodiment of the present invention shown in figure 1 compπses from the center and outwards; - a stranded multi-wire conductor 10; - a first extruded inner semi-conductmg shield 11 disposed around and outside the conductor 10 and inside a conductor insulation 12; - an extruded conductor insulation 12 with an extruded, cross-linked composition as de- scnbed in the foregoing compπsing one first polar enπched surface zone 120 adjacent to the interface between the insulation layer 12 and the inner semi-conductive shield 11 and one second polar enπched surface zone 125 adjacent to the interface between the insulation layer 12 and the outer semi -conductive shield 13; - a second extruded semi-conducting shield 13 disposed outside the conductor insulation 12; - a metallic screen 14; and - an outer coveπng or sheath 15 arranged outside the metallic screen 14. Both semiconducting shields 11, 13 and the insulation layer 12 compnsmg the two polar enπched zones 120, 125 are typically extruded and crosslmked and are also typically polyethylene based. A cable according to the present invention can when deemed appropπate be further complemented in vaπous ways with vaπous functional layers or other features. It can for example be complemented with a reinforcement in form of metallic wires outside the outer extruded shield 13. a sealing compound or a water swelling powder introduced in metal/polymer mter- faces or a system of radial achieved by e.g. a corrosion resistant metal polyethylene laminate and longitudinal water sealing achieved by water swelling mateπal, e.g. tape or powder beneath the sheath 15 The conductor can be of any desired shape and constitution, such as a 16 stranded multi-wire conductor or a segmental conductor. The insulated cable according to the embodiment of the present invention shown in figure 2 is intended to be used in a high voltage winding in an electric machine and comprises from the center and outwards; - a stranded multi-wire conductor 20; - a first inner semi -conducting shield 21 disposed around and outside the conductor 20; - a conductor insulation 22 with a composition as described in the foregoing comprising one first polar enriched surface zone 220 adjacent to the interface between the insulation layer 22 and the inner semi-conductive shield 21 and one second polar enriched surface zone 225 adjacent to the interface between the insulation layer 22 and the outer semi-conductive shield 23; - a second semi-conducting shield 23 disposed outside the conductor insulation 22. Both semi-conducting shields 21, 23 and the insulation layer 22 comprising the two polar enriched zones 220, 225 are typically extruded and crosslinked and are also typically polyethylene based. The cable typically comprises no further reinforcement. Nor does it comprise any outer covering or sheath as the outer semi-conductive shield is to be disposed in contact with the groove or similar object in which the winding is arranged. The conductor can be of any desired shape and constitution, such as a stranded multi-wire conductor or a segmental conductor. EXAMPLE 1 Comparative tests for XLPE compositions to be used in the polarenriched XLPE zones within the insulation layer of the insulation system in a cable according to the present invention. Test plates with XLPE compositions as used in prior art insulated AC-cables and for compositions to be used in the polar enriched zones in accordance with the present invention for use in insulated high voltage cables or insulated conductors for a high voltage winding in an electric machine were produced, processed and subjected to an evaluation of the tendency for space charge accumulation by recording space charge profiles using the Pulsed ElectroAc- 17 coustic (PEA) technique. The PEA technique is well known within the art and descπbed by Takada et al. in LEEE Trans. Electr. Insul. Vol. EI-22(No.4). pp 497-501(1987) a, A polyethylene composition was prepared by adding about 1 % by weight of dimeth- ylamino-propylmetacryl-amide, DMAPMA to about 99 % by weight of low density polyethylene composition, wherein this PE composition compπsed about 98 % by weight of low density polyethylene (922 kg/m 3 ) with a melt flow rate of 0,8 g/10 min and about 2 % by weight of a conventional system of antioxidant and peroxide cross-linking agent. A 2 mm thick test plate of a this prepared polyethylene was molded at 130°C, whereafter, two semi-conductive electrodes were molded on the test plate and the assembly was cross-lmked in an electπc press at 180°C for 15 minutes. The 2 mm thick cross-linked test plate was subsequently tested at 50°C in a device for PEA analysis where the plate was inserted between two flat electrodes and subjected to a 40 kV direct voltage electπc field. That is one electrode was grounded and the other electrode was held at a voltage potential of + 40kV. The space charge profile as shown m figure 3a was recorded for the test plate. Where arbitrary units for space charge/volume is presented as a function of the test plate thickness, i.e. 0 is at the grounded electrode and x indicates the dis- tance from the grounded electrode in the direction towards the + 40 kV electrode. b, A 2 mm thick test plate of a the same polyethylene composition compπsing DMAPMA as prepared in comparative example a was molded, also at 130°C. Two semi-conductive electrodes were molded on this test plate and the assembly was cross-linked in an electπc press at 250°C for 30 minutes. The 2 mm thick cross-l ked test plate was subsequently tested at 50°C in a device for PEA analysis where the plate was inserted between two flat electrodes and subjected to a 40 kV direct voltage electπc field. That is one electrode was grounded and the other electrode was held at a voltage potential of + 40kV The space charge profile as shown in figure 3b was recorded for the test plate. Where arbitrary units for space charge/volume is presented as a function of the test plate thickness, i.e. 0 is at the grounded electrode and x indicates the distance from the grounded electrode in the direction towards the + 40 kV electrode. c, A 2 mm thick test plate of a conventional polyethylene composition compπsing as used in examples a and b but without the DMAPMA was molded at 130°C. Two semi -conductive electrodes were molded on the test plate and the assembly was crosslmked in an electπc press at 180°C for 15 minutes. The 2 mm thick cross-linked test plate was subsequently tested at 50°C in a device for PEA analysis where the plate was inserted between two flat electrodes and subjected to a 40 kV direct voltage electπc field. That is one electrode was grounded and the other electrode was held at a voltage potential of + 40kV. The space charge profile as shown in figure 3c was recorded for the test plate. Where arbitrary units for space charge/volume is presented as a function of the test plate thickness, i.e. 0 is at the grounded electrode and x indicates the distance from the grounded electrode in the direction towards the + 40 kV electrode. d, A 2 mm thick test plate of a polyethylene composition as in example c was molded at 130°C. Two semi-conductive electrodes were molded on the test plate and the assembly was cross- linked in an electπc press at 250°C for 30 minutes. The 2 mm thick cross-lmked test plate was subsequently tested at 50°C in a device for PEA analysis where the plate was inserted between two flat electrodes and subjected to a 40 kV direct voltage electπc field. That is one electrode was grounded and the other electrode was held at a voltage potential of + 40kV. The space charge profile as shown in figure 3d was recorded for the test plate. Where arbitrary units for space charge/volume is presented as a function of the test plate thickness, i.e. 0 is at the grounded electrode and x indicates the dis- tance from the grounded electrode in the direction towards the + 40 kV electrode. 19 CONCLUSIONS OF COMPARATIVE TESTS The space charge profiles of the samples in example la, lb, lc, and Id recorded 3 hours after the application of the DC voltage are shown in figures 3a, 3b, 3c, and 3d, respectively. It is clearly seen that the space charge accumulation in the insulation mateπal traditionally used in AC XLPE cables (see figure 3c and 3d) is high and that the tendency for space charge accumulation is substantially reduced for the two compositions according to the present invention represented with comparative examples figures 3a and 3b.

Claims: 2 0 CLAIMS 1. An insulated electπc cable having a polymer based insulation system compπsing an insulation layer (12;22) and one or more semi-conductive layers (11;13;21;23) disposed around a conductor (19;20), wherein the insulation layer compπses an electπcally insulating polymeπc composition, characterized in that the polymeπc composition compπses a polar enπched surface zone (120; 125;220,225) having a suitable raised content of a suitable polar substance 2. A cable according to claim 1, characterized in that the insulation system compπses: - a first inner semi-conductive shield (11;21) disposed on and around the conductor (10;20); - an insulation layer (12;22) disposed on and around the first inner semi-conductive shield; wherein the insulation layer compπses a first, inner, polar enπched surface zone (120;220) disposed at the inner interface between the inner semi-conductive shield and the insulation layer 3. A cable according to claim 2, characterized in that the insulation system further compπses a second outer semi-conductive shield (13;23) disposed on and around the insulation layer (12;22) and that the insulation layer compπses a second, outer, polar enπched surface zone (125:22%) disposed at the outer interface between the outer semi-conductive shield and the insulation layer. 4. A cable according to claim 1, 2 or 3, characterized in that the insulation layer (12;22) compπses an extruded and cross-linked, polyethylene based, XLPE, composition. 5. A cable according to claim 1, 2, 3 or 4, characterized in that the polar enπched surface zone (120.125;220;225) compπses a polar group introduced into the polymeπc composition. 6. A cable according to claim 5, characterized in that the polar group is a carbonyl group. 7 A cable according to claim 1, 2, 3 or 4, characterized in that the polymer chain compπses a polar modification in the form of polar segments. 21 8. A cable according to claim 7, characterized in that the polar segments are present as polar side-groups grafted on to the polymer chain. 9. A cable according to claim 7, characterized in that the polar segments are present as moieties in the chain backbone of the polymer chain. 10. A cable according to claim 9, characterized in that comprising the polar segment comprises a polar co-monomer having the general formula: CH 2 =CR-CO-X-(CH 2 ) n -N(CH 3 ) 2 or CH 2 =CR-CO-O-(CH 2 -CH 2 O) m -H where n is equal to 2 or 3; m is equal to a number from 1 to 20; R is H or CH 3 ; and X is O or NH. 11. A cable according to claim 10, characterized in that the polar comonomer is based on metacrylic amide with the general formula: CH 2 =C(CH 3 )-CO-NH-(CH 2 ) n -N(CH 3 ) 2 where n is equal to 2 or 3. 12. A cable according to claim 11 characterized in that n=3 and the polar monomer is dimethylamino-propylmetacryl-amide (DMAPMA). 13. A cable according to claim 10, characterized in that the polar monomer is based on acrylic amide and of the general formula: CH 2 =CH-CO-NH-(CH 2 ) n -N(CH 3 ) 2 where n is equal to 2 or 3. 14. A cable according to claim 10, characterized in that the polar monomer is based on metacrylic ester and of the general formula: CH 2 =C(CH 3 )-CO-O-(CH 2 )n-N(CH 3 ) 2 where n is equal to 2 or 3. 15. A cable according to claim 10, characterized in that the polar monomer is based on acrylic ester and of the general formula: CH 2 =CH-CO-O-(CH 2 )n-N(CH 3 ) 2 where n is equal to 2 or 3. 22 16. A cable according to claim 10, characterized in that the polar monomer is based on metacrylic acid and o gomeπc ethyleneglycol and of the general formula: CH 2 =C(CH 3 )-CO-O-(CH 2 -CH 2 O-) m H where m is equal to a number from 1 to 20, preferably n is equal to 1, 5, 6, or 9. 17 A cable according to claim 10, characterized m that the polar monomer is based on acrylic acid and ohgomeπc ethyleneglycol and of the general formula CH 2 =CH-CO-O-(CH 2 -CH 2 O-) m H where m is equal to a number from 1 to 20, preferably n is equal to 1, 5, 6, or 9. 18. Use of a cable according to any of the preceding claims in a system or installation for transmission or distπbution of electπc power. 19. Use of a cable according to any of claims 1-17 as a conductor in a high voltage winding in an electπc machine.

Inventors: JONSSON JONAS, TOERNKVIST CHRISTER, JONSSON, JONAS, TOERNKVIST, CHRISTER
Applicants: ASEA BROWN BOVERI, JONSSON JONAS, TOERNKVIST CHRISTER, ASEA BROWN BOVERI AB, JONSSON, JONAS, TOERNKVIST, CHRISTER
Application year: 1999
Main classification: H01B3
IPC codes: C08F8, C08F255, C08J3, H01B3, H01B9, H01B13, H02K3