MXPA00007989A - An electric direct current cable - Google Patents

Abstract

An insulated electric DC-cable having a polymer based insulation system comprising an extruded and cross-linked polyethylene, XLPE, based composition, disposed around a conductor and a method for production of such a cable. The XLPE based composition comprises a polar modification in the form of a polar segment comprising a polar co-monomer having the general formula:CH2=CR-CO-X-(CH2)n-N(CH3)2 or CH2=CR-CO-O-(CH2-CH2O)m-H where n is equal to 2 or 3;m is equal to a number from 1 to 20;R is H or CH3;and X is O or NH. The polar comonomer is introduced into the XLPE composition.

Description

A DIRECT ELECTRIC POWER CORD FIELD OF THE INVENTION The present invention relates to a direct electric power cable, a CD cable, having an insulating system comprising a composition of polyethylene, PE. The polyethylene composition is a cross-linked and extruded PE composition, an XLPE composition. The present invention relates in particular to an isolated electric DC cable for the transmission and distribution of electrical energy. The insulating system comprises a plurality of layers, such as an internal semiconductor metal sheath, an extruded insulation and an outer semiconductive metal sheath.

At least the extruded insulation comprises a cross-linked polyethylene electrically based on the insulating composition with a system of additives, typically comprising cross-linking agent, combustion retarding agent and an antioxidant.

ART BACKGROUND Although many of the first electrical supply systems for the transmission and distribution of energy REF .: 122205 electric were based on CD technology, these CD systems were quickly replaced by systems using alternating current, C. The AC systems had the desirable feature of easy transformation between the generation, transmission and distribution voltages. The development of modern electricity supply systems in the first half of this century was based exclusively on AC transmission systems. However, before 1950 there was a demand for growth for long-run schemes and it became clear that in certain circumstances it could be beneficial to adopt a CD-based system. The expected benefits include a reduction of problems typically found in the association with the stability of the AC systems, a more effective use of the equipment as a power factor of the system is always the unit and a capacity to use a given insulation thickness or height to a higher operating voltage. Against these very significant advantages, the high cost of thermal equipment for converting CA to CD back to CA has been valued. However, for a given transmission power, terminal costs are constant, and therefore, CD transmission systems became economical for schemes involving long distances. In this way CD technology becomes economical for systems intended for transmission over long distances as for when the transmission distance typically exceeds the length for which the savings in transmission equipment exceed the cost of the terminal plant.

An important benefit of the CD operation is the virtual elimination of dielectric losses, which offers a considerable gain in efficiency and savings in the equipment. The DC leakage current is of such a small magnitude that it can be ignored in the current specification calculations, while the dielectric losses of AC cables cause a significant reduction in the current specification. This is of considerable importance for voltages of higher systems. Similarly, high capacitance is not a disadvantage in CD cables. A typical CD transmission cable includes a conductor and an insulating system comprises a plurality of layers, such as an internal semiconductor metal cover, an insulating base body and an outer semi-conductive metal cover. The cable is also complemented by wrapping, re-feeding, etc. to overcome the penetration of water and any wear or mechanical forces during the production of installation and use.

Almost all CD cable systems supplied so far, have been for underwater crossings or the ground wire associated with these. For large traverses, the isolated paper cable impregnated with solid mass is chosen because there are no restrictions in length due to pressurization requirements. It has been supplied for operating voltages of 450 kV. To date, an essentially paper-insulated body impregnated with an electrical insulating oil has been used, but the application of the laminated material such as a sheet of polypropylene paper is continued for the use of voltages of up to 500 kV to gain advantage of Increased impulse resistance and reduced diameter.

As in the case of AC transmission cables, transient voltages are a factor that must be taken into account when determining the insulation thickness of the CD cables. It has been found that the most onerous condition occurs when a transient voltage of polarity opposite to the operating voltage is imposed on the system when the cable is carrying a total load. If the cable is connected to an airline system, such a condition usually occurs as a result of lightning transitions.

The extruded solid insulation based on polyethylene, PE, or a cross-linked polyethylene, XLPE, has been used for almost 40 years for the insulation of the AC transmission and distribution cable. Therefore, the possibility of using XLPE and PE for the isolation of the CD cable has been under investigation for many years. Cables with such insulators have the same advantage as mass-impregnated cable in that for CD transmission there are no restrictions on the length of the circuit and may also have a potential to operate at higher temperatures. In the case of XLPE, 90 ° C instead of 50 ° C for conventional CD cables. In this way a possibility is offered to increase the transmission load. However, it has not been possible to obtain the full potential of these materials for full-size cables. It is believed that one of the main reasons is the development and accumulation of space charges in the dielectric when subjected to a CD field. Such spatial charges distort the distribution of electrical stress and persist for long periods due to the high resistivity of the polymers. Space charges in an insulating body when subjected to the forces of an electric DC field accumulate, in a way that a polarized pattern similar to a capacitor is formed. There are two basic types of spatial charge accumulation patterns, which differ in the polarity of space charge accumulation. The accumulation of space charge results in a local increase in certain points of the current electric field in relation to the field, which will be considered when considering the geometric conditions and the dielectric characteristics of an insulator. The increase observed in the current field could be 5 or even 10 times the field contemplated. In this way, the design field for a cable insulator must include a safety factor that takes this upper field into account considerably, which results in the use of thicker and / or more expensive materials in the cable insulation. The construction of the accumulation of the space charge is a slow process, therefore, this problem is accentuated when the polarity of the cable after it is operated for a long period of time at the same polarity is reversed. As a result of the inversion, a capacity field is superimposed on the field that results from the accumulation of the space charge and the maximum field strength point moves from the interface and into the insulator. Attempts have been made to improve the situation by using additives that reduce the strength of the insulator without seriously affecting other properties. To date it has not been possible to match the electrical development achieved with insulated paper impregnated cables and commercial polymer insulated CD cables have not been installed. However, successful laboratory tests have been reported on a 250 kV cable with a maximum effort of 20 kV / mm using XLPE insulator with mineral filler (Y. Maekawa et al., Research and Development of DC XLPE Cables, JiCable '91, pp. 562-569). This stress value is compared to 32 kV / mm used as a tip value for the impregnated paper mass cables.

An extruded resin composition for the insulation of the AC cable typically comprises a polyethylene resin as the base polymer supplemented with various additives, such as a peroxide cross-linking agent., a combustion retardant agent and an antioxidant or a system of antioxidants. In the case of an extruded insulator, the semiconducting metal covers are typically also extruded and comprise a resin composition, which in addition to the base polymer and an electrically semi-conductive or conductive filler comprises essentially the same type of additives. The various layers extruded in an insulated wire generally are often based on a polyethylene resin. The polyethylene resin generally means and in this application, a polyethylene-based resin or an ethylene copolymer, wherein the ethylene monomer constitutes a major part of the mass. In this way, the polyethylene resins could be composed of ethylene and one or more monomers that are co-polymerizable with ethylene. LDPE, low density polyethylene, is currently the predominant insulating base material for AC cables. To improve the physical properties of the extruded insulation and its ability to overcome degradation and decomposition under the influence of prevailing conditions under production, loading, laying of layers and the use of such a cable, the polyethylene-based composition typically comprises additives such as : stabilization additives, e.g. antioxidants, electronic precipitators to counteract the decomposition due to oxidation, radiation etc .; - lubricant additives, e.g. stearic acid, to increase processability; - additives of increased capacity to overcome electrical stresses, e.g. an increased water tree resistance, e.g. polyethylene glycol, silicones etc .; and - cross-linking agents such as peroxides, which decompose due to heating in free radicals and initiate cross-linking of the polyethylene resin, sometimes used in combination with - unsaturated compounds that have the ability to improve the cross-linking density; combustion retarders to avoid premature cross-linking.

The number of various additives is large and the possible combinations thereof are essentially unlimited. When selecting an additive or a combination or group of additives the objective is that one or more properties will be improved, while the others will be maintained or, if possible, also improved. However, it is almost always impossible to predict all possible side effects of a change in the additive system. In other cases the improvements sought are of such dignity that some minor inconveniences have to be accepted, although there is always a purpose to minimize such negative effects.

A typical polyethylene-based resin composition for use as an extruded cross-link insulator in an AC cable comprises: 97. 1-98.9% by weight of the low density polyethylene (922 kg / m3) melt flow rate 0.4-2.5 g / 10 min with an additive system as described above.

These additives may comprise: 0. 1-0.5% by weight of an antioxidant, such as, but not limited to SANTONOX R® (Flexsys C o) with the chemical designation 4, 4'-t io-bis (6-tert-but-il-m-cresol ) , Y 1. 0-2.4% by weight of a crosslinking agent such as, but not limited to, DICUP R® (Hercules Chem) with the chemical designation dicumyl peroxide.

Although some disadvantages with the use of such an XLPE composition, its advantages have been known for a long time, e.g. its ability to avoid combustion i.e. premature cross-linking, they have imported more than these drawbacks. It is also well known that this type of XLPE composition exhibits a strong tendency to form spatial charges under the electric DC fields, thus rendering it unsuitable in CD cable insulation systems. However, it is also known that extended degassing i.e. Exposure of the cross-linked cable insulation at high temperatures to a high vacuum for long periods of time will result in a rather diminished tendency for the accumulation of space charge under DC voltage stresses. In general, it is believed that the vacuum treatment removes the peroxide decomposition products, such as "acetophenone" and "cumyl alcohol", from the insulator, whereby the accumulation of space charge is reduced. Degassing is a batch process that consumes time compared to the impregnation of paper insulators, and thus costly. Therefore, it is advantageous if the need for degassing is removed. Most known cross-link polyethylene compositions used as an extruded insulator in the AC cable exhibit a tendency for spatial charge accumulation which makes them unsuitable for use in insulating systems for CD cables.

It is known to add low amounts of an additive comprising carbonyl groups to an LDPE for the dual purpose of increasing the resistivity and decreasing the accumulation of space charge. Such carbonyl addition is carried out by oxidizing the polyethylene or by a copolymerization of carbon monoxide with ethylene. Carbonyl groups are thought to act as trapped sites for space charges, whereby the mobility of any space charge is restricted and the development of a polarized pattern within the cross-linked insulator as a result of the accumulation of space charge when the insulator is subjected to a CD field. However, a tendency to untangle and therefore, an increased space charge at elevated temperatures has been observed, e.g. temperatures above 40 ° C. Also the additives in the form of organic acids and anhydrides have been shown to give similar effects. In addition to the molar modifications of the polyethylene, it has been suggested to introduce polar units into the polymer to obtain a superior CD breaking strength. For example, Japanese Patent Publication JP-A-210610 reports that an anhydride such as maleic acid anhydride, MAH, has been introduced into polyethylene for this purpose. The resulting crosslink insulator material exhibited a decrease in the spatial charge accumulation attributed to the increased polarity of the cross-linked polymer chain structure and, it was concluded that the introduced MAH groups, which are fixed within the structure of Cross link, acts as trapped sites for many space charges. In JP-A-210610 it was reported that cross-linked polyethylene with additions of MAH at levels that correspond from about 0.02 to about 0.5% by weight, resulted in the cross-link composition for use as an insulator in the CD cable with an accumulation of diminished space charge. Other additions used for such polar modification of the cross-linking structure and the associated reduction in the accumulation of space charge in the crosslinked insulation are ionomers, salts of acrylic metals, carboxylic acid and acetates.

In this way, it is desired to provide an isolated CD cable with a polymer based on the electrical insulation system comprising a composition of Extruded XLPE, suitable for use as a transmission and distribution cable in networks and installations for the transmission and distribution of electric power CD. The cable will typically be produced with a process for the application and processing of the extruded XLPE based on the insulation that can be carried out in a manner, so that there is no need for any long-term consumer batch treatment, such as impregnation or degassing. , ie vacuum treatment of the cable to ensure the stable and consistent dielectric properties and a high and consistent electrical resistance of the cable insulation. The cable insulation will also exhibit a low tendency to build up space charge, a high DC shear resistance, high pulse resistance and high insulation resistance. This will offer technical and economic advances on prior art methods as the time of production costs can be reduced and the possibility is provided for an essentially continuous or at least semi-continuous process for the application and processing of the cable insulation system. . In addition to reliability, the low maintenance requirements and long working life of a conventional CD cable, comprising an impregnated paper base insulation, will be maintained or improved. The replacement of an insulation based on cellulose or impregnated paper with an extruded polymeric insulation will allow an additional advantage for an increase in the electrical resistance and in this way, will allow an increase in the operating voltages, will improve the way of handling and the robust form of the cable.

In particular, it is desired to provide an isolated electric CD cable, wherein the cross-linked and extruded PE composition contained in the insulation system comprises a three-dimensional cross-linking structure exhibiting trap sites, whereby the mobility of any load spatial restriction and the development of a polarized space charge profile within the extruded insulation. Such a reduction in the tendency for the accumulation of space charge in the insulation provides as an additional economic advantage a capability to reduce the safety factors in the design values used to size the insulation of the cable. In particular, such a cable is desired for operation under the specific conditions prevailing in a network or installation for the transmission and distribution of electrical energy.

BRIEF DESCRIPTION OF THE INVENTION It is an object of the present invention to provide an insulated electrical CD cable that satisfies the wishes as specified above. This is in accordance with the present invention achieved by a CD cable as defined in the preamble of claim 1, having an insulator system based on polymer comprising an extruded cross-linked polyethylene composition disposed around a conductor, characterized by the additional measures according to the characterizing part of claim 1. Further developments of the invented CD cable are characterized by the features of the additional claims 2 to 12.

It is an object of the present invention to provide a method for the production of an isolated electric CD cable as specified above. This is in accordance with the present invention achieved by a method, as defined in the preamble of claim 13 for the fabrication of an insulated CD cable, having one having an insulator system based on polymer comprising a polyethylene composition. of extruded cross-link arranged around a conductor, characterized by additional measures according to the characterizing part of claim 13. Further developments of the method invented by the features of the additional claims 14 to 20.

DESCRIPTION OF THE INVENTION To use extruded polyethylene or crosslinked polyethylene (XLPE) as an insulator for CD cables, several factors have been taken into account. The most important result is the accumulation of space charge under the DC voltage stress. The present invention. The present invention accomplishes such a decrease in the spatial charge accumulation that typically occurs in an operational CD cable by implementing a low amount of a polar comonomer in the polyethylene chain, wherein the polar comonomer is of the general formula: CH2 = CR-CO-X- (CH2) n-N (CH3) 2 or CH2 = CR-CO-0- (CH2.CH20) m-H where 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.

This has been done by introducing such a comonomer to form a segment in the chain structure during the polymerization or as hanging side groups in an insertion operation. The amount of the polar monomer in the insulating compound is in the range of 0.1% by weight of the total polymer, preferably 0.1 to 5% by weight, and more preferably 0.5 to 1.5% by weight.

According to one embodiment of the invention, the polar comonomer is based on methacrylamide and is of the general formula: CH2 = C (CH3) -CO-NH- (CH2) n-N (CH3) 2 where n is equal to 2 or 3.

In the case of n = 3 the monomer is designated dime t i lamino-propylme tacr i 1-amide (DMAPMA).

According to another second embodiment of the invention, the polar comonomer is based on acrylic amide and is of the general formula: CH2 = CH-CO-NH- (CH2) n-N (CH3) 2 where n is equal to 2 or 3.

According to a third additional embodiment of the invention, the polar comonomer is based on methacrylic ester and is of the general formula: CH2 = C (CH3) -CO-0- (CH2) n-N (CH3) 2 where n is equal to 2 or 3 According to a fifth additional alternative embodiment of the invention, the polar comonomer is based on acrylic ester and is of the general formula: CH2 = CH-C0-0- (CH2) n-N (CH3) 2 where n is equal to 2 or 3.

According to another sixth alternative embodiment of the invention, the polar comonomer is based on methacrylic acid and oligomeric ethylene glycol and is of the general formula: CH2 = C (CH3) -CO-0- (CH2-CH20-) mH where m is equal to a number between 1 and 20, preferably m is equal to 1, 5, 6 or 9.

According to still another seventh alternative embodiment of the invention, the polar comonomer is based on acrylic acid and oligomeric ethylene glycol and is of the general formula: CH2 = CH-CO-0- (CH2-CH20-) mH where m is equal to a number between 1 and 20, preferably m is equal to 1, 5, 6 or 9.

A method for the production of an isolated electric CD cable comprising the steps of; - combining a PE composition, - extruding the combined polyethylene composition, as a part of a polymer-based insulation system arranged around a conductor; Y subsequently cross-linking the PE composition in an XLPE composition in the general manner according to the present invention carried out such that a polar comonomer of the type described above and having the general formula CH2 = CR-C0-X- (CH2) n-N (CH3) 2 or CH2 = CR-C0-0- (CH2-CH20) m-H where 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 is introduced into the composition of XLPE. Preferably m is equal to 1, 5, 6 or 9.

According to a first embodiment of the method according to the present invention the polar monomer is added to the ethylene before or during the polymerization reaction, in this way the comonomer will be built into the polymer structure and integrated with the polyethylene chain. The added amount of the comonomer is in the order of 1% by weight of the finished polymer, typically in an amount of 0.1 to 5% by weight of the finished polymer and more preferred in an amount of 0.5 to 1.5% by weight of the finished polymer.

According to another second embodiment of the method according to the present invention, ethylene and polar monomer are copolymerized in the same manner as in the method of the first embodiment, except that the amount of comonomer is now greater than 5-40% by weight, and preferably 25-35% by weight of the finished polymer. This copolymer with high amount of polar comonomer is subsequently diluted by combining the copolymer with narrow polyethylene until the average polar comonomer content is about 1% by weight, typically the content is from 0.1 to 5% by weight of the finished polymer and more preferred 0.5 to 1.5% by weight of the finished polymer.

According to still another third embodiment of the method according to the present invention, the polar monomer is introduced into an ethylene homopolymer. The insertion process can be performed in a separate step after the polymerization process or it could be performed during the extrusion and / or cross-linking of the polyethylene based on the insulation of the cable.

The number of polar groups corresponds to approximately 1 polar group per 1000 carbon atoms in the polyethylene structure A CD cable according to the present invention with an extruded, cross-linked insulator system comprising a cross-linked polyethylene composition, XLPE, with a polar monomer introduced into the XLPE exhibits considerable advantages such as: A substantially reduced tendency for the accumulation of space charge resulting in the low tendency for the development of a polarized space charge profile, - An increased CD breaking strength.

The cable according to the present invention in this way offers good performance and stability of the extruded cable insulation system also when high temperatures have been employed during extrusion, cross-linking or other high-temperature conditioning.

It is, as always, favorable whether the content of any unreacted peroxide cross-linking agent or any by-products or degradation products in the extrudate, the XLPE composition can be minimized to further reduce any tendency for space loading. Thus, the peroxide content of the PE composition to be extruded and crosslinked is less than 5% and preferably less than 2%. In this way, a CD cable according to the present invention is appropriately adapted to meet the specific requirements for use as a CD cable without resorting to time-consuming batch treatments. The essential elimination or substantial reduction of the excess peroxide moieties in the insulation of the DC cable is advantageous considering the cost of the cross-linking agent of peroxide and more important considering the fact that the cross-linking agent of peroxide due to degradation, it is likely to form undesirable byproducts such as methane to cumyl alcohol, which is a source of spatial charges.

All of these advantageous properties and improvements over prior art cables having an insulating system comprising an extruded XLPE composition, are for a CD cable produced according to the present invention, it is performed without many disadvantages associated with some cables produced from the cable. previous art The substantially reduced tendency for the accumulation of space charge resulting in the low tendency for the development of a polarized space charge profile to be maintained or improved ensures that the high CD shear strength of conventional CD cables comprising an insulator of impregnated paper. Furthermore, the insulation properties of a CD cable according to the present invention exhibit a general long-term stability, so that the working life of the cable is maintained or increased. This is achieved in particular, by the combined implementation of a polar segment in the XLPE and the controlled processing of the PE composition before and during the extrusion and cross-linking and the conditioning carried out in association with extrusion and cross-linking, where the process variables such as temperatures, pressures, processing times, atmospheric composition are controlled.

The CD cable according to the present invention offers the ability to be produced by an essentially continuous process without any time-consuming batch stage, such as impregnation or degassing, whereby substantial reduction in production time is started and so the production costs without risking the technical development of the cable.

A CD cable as defined above, is especially advantageous for operation under the specific conditions prevailing in high voltage transmission or distribution cables used in a network or installation for transmission or distribution of electric power, due to the improved thermal properties combined with electrical properties maintained or improved. This is especially important due to the long life for which such facilities are designed, and the limited access for the maintenance of such facilities that are going to be installed in distant places or even submarines. An additional advantage for a high voltage direct current cable that is produced according to the present invention, is that the production time can be substantially reduced by adopting an essentially continuous process free of the operating stages that require treatments by Batches of complete cable lengths or lengths in part offer cost advantages compared to conventional cables.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be described in more detail, while reference is made to the drawings and examples. Figure 1 shows a sectional view of a cable for high voltage direct current transmission of electrical energy according to an embodiment of the present invention. Figures 2a to 2d show the records of the spatial loading for comparative tests on plates with XLPE compositions as used in the above isolated AC cables and for the compositions according to the present invention.

DESCRIPTION OF THE MODALITIES, PREFERRED EXAMPLES The CD cable according to the embodiment of the present invention shown in Figure 1 comprises the center and the exteriors; - a braided stranded conductor 10; - a first extruded semiconductor sheath 11 arranged around and outside the conductor 10 and inside a conductive insulator 12; an extruded conductive insulation 12 with a crosslinking composition, extruded as described above; - a second extruded semiconductor sheath 13 disposed outside the conductive insulation 12; - a metallic screen 14; Y - an outer covering or cladding 15 arranged outside the metallic screen 14.

The CD cable when it can be considered appropriate is further complemented in several ways with several functional layers or other features. For example, it may be complemented by a reinforcement in the form of metal wires outside the outer extruded liner 13, a seal compound or a powder that swells with water introduced into the metal / polymer interfaces or a radial system made by e.g. a laminate of polyethylene metal resistant to corrosion and of longitudinal water seal achieved by the material that swells in water, e.g. tape or powder below the coating 15. The conductor does not need to be braided, but can be of any desired shape and constitution, such as a stranded stranded conductor, a solid conductor or a segmental conductor.

EXAMPLE 1 Comparative tests The test plates with XLPE compositions as used in the CA cables isolated from the prior art and in accordance with the present invention for use in isolated CD cables, were produced, processed and subjected to a trend evaluation for the accumulation of space charge by recording the space charge profiles using the Pulse Elect RoAccoust ic (PEA) technique. The PEA technique is well known within the art and is described by Takada et al. in IEEE Trans. Electr. Insul. Vol. EI-22 (No. 4). pp 497-501 (1987). to. A polyethylene composition was prepared by adding about 1% by weight of dimethylamino-propylmethacrylamide, DMAPMA to about 99% by weight of the low density polyethylene composition, wherein this PE composition comprised about 98% by weight of low density polyethylene (922 kg / m3) with a melt flow rate of 0.8 g / 10 min and about 2% by weight of a conventional antioxidant system and peroxide cross-linking agent.

A 2 mm thick test plate of a prepared polyethylene was molded at 130 ° C, then, two semi-conductor electrodes were molded on the test plate and the assembly was cross-linked in an electric press at 180 ° C for 15 minutes. minutes The 2 mm thick cross-linked test plate was subsequently evaluated at 50 ° C in a device for PEA analysis, the plate was inserted between two flat electrodes and subjected to a direct voltage electric field of 40 kV. This electrode was connected to the ground and the other electrode was maintained at a voltage potential of +40 kV. The space charge profile as shown in Figure 2a was recorded for the test plate. The arbitrary units for the space / volume charge are presented as a function of the thicknesses of the test plate, i.e. 0 is the electrode connected to the ground and x indicates the distance of the electrode connected to the ground in the direction towards the +40 kV electrode. b. A 2 mm thick test plate of the same polyethylene composition comprising DMAPMA as prepared in comparative example a, was also molded at 130 ° C. Two semi-conductor electrodes were molded in this test plate and the assembly cross-linked in an electric press at 250 ° C for 30 minutes.

The 2 mm thick cross-linked test plate was subsequently evaluated at 50 ° C in a device for PEA analysis, the plate was inserted between two flat electrodes and subjected to a direct voltage electric field of 40 kV. This electrode was connected to the ground and the other electrode was maintained at a voltage potential of +40 kV. The space loading profile as shown in Figure 2b was recorded for the test plate. Where the arbitrary units for the space charge / volume are presented as a function of the thickness of the test plate, i.e. 0 is the electrode connected to the ground and x indicates the distance of the electrode connected to the ground in the direction towards the +40 kV electrode. c. A 2 mm thick test plate of a conventional polyethylene composition comprising as used in examples a and b, but without the DMAPMA was molded at 130 ° C.

Two semi-conductor electrodes were molded into the test plate and the assembly was cross-linked in an electric press at 180 ° C for 15 minutes.

The 2 mm thick cross-linked test plate was subsequently tested at 50 ° C on a device for PEA analysis, the plate was inserted between two flat electrodes and subjected to a direct voltage electric field of 40 kV. This electrode was grounded and the other electrode was maintained at a voltage potential of +40 kV. The space charge profile as shown in Figure 2c was recorded for the test plate. Where the arbitrary units for the space charge / volume are presented as a function of the thickness of the test plate, i.e. 0 is the electrode connected to the ground and x indicates the distance of the electrode connected to earth in the direction of 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-conductor electrodes were molded into the test plate and the assembly cross-linked in an electric press at 250 ° C for 30 minutes.

The 2 mm thick cross-linked test plate was subsequently tested at 50 ° C on a device for PEA analysis, the plate was inserted between two flat electrodes and subjected to a direct voltage electric field of 40 kV. This is one electrode connected to the ground and the other electrode was maintained at a voltage potential of +40 kV. The space loading profile as shown in Figure 2d was recorded for the test plate. Where the arbitrary units for the space charge / volume are presented as a function of the thickness of the test plate, i.e. 0 is the electrode connected to earth and x indicates the distance of the electrode connected to the earth in the direction of the +40 kV electrode.

CONCLUSIONS OF COMPARATIVE EVIDENCE The spatial loading profiles of the samples in example la, Ib, le and Id were recorded 3 hours after application of the DC voltage, the results are shown in figures 2a, 2b, 2c and 2d respectively. It can be clearly seen that the accumulation of space charge in the insulating material traditionally used in the AC XLPE cables (see Figures 2c and 2d) is high and that the tendency for the accumulation of space charge is substantially reduced for the two compositions according to the present invention shown with Figures 2a and 2b of the comparative examples.

It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Having described the invention as above, the content of the following is claimed as property.

Claims (20)

1. An isolated electric DC cable having a polymer-based isolation system comprising an XLPE-extruded, cross-linked polyethylene-based composition disposed around a conductor, characterized in that the XLPE-based composition comprises a polar modification in the shape of a polar segment comprising a polar comonomer having the general formula: CH2 = CR-CO-X- (CH2) nN (CH3) 2 or CH2 = CR-CO-0- (CH2.CH20) mH 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 0 or NH.

2. A CD cable according to claim 1, characterized in that in the polar comonomer it is presented as radicals in the structure of the XLPE chain.

3. A CD cable according to claim 1, characterized in that in the polar comonomer it is presented as lateral groups introduced into the XLPE.

4. A CD cable according to any of claims 1, 2 or 3, characterized in that the polar monomer is present in the XLPE composition in an amount exceeding 0.1% by weight.

5. A CD cable according to any of claims 4, characterized in that the polar monomer is present in the XLPE composition in an amount of 0.5 to 1.5% by weight of the total polymer.

6. A CD cable according to any of the preceding claims, characterized in that the polar monomer is a polar comonomer based on methacrylamide and is of the general formula: CH2 = C (CH3) -C0-NH- (CH2) n-N (CH3) 2 where n is equal to 2 or 3.

7. A CD cable according to claim 6, characterized in that n = 3 and the polar monomer is dimethylamino-propylmetacrylamide (DMAPMA).

8. A CD cable according to any of claims 1 to 5, characterized in that the polar monomer is based on acrylic amide and is of the general formula: CH2 = CH-CO-NH- (CH2) n-N (CH3) 2 where n equals 2 or 3

9. A CD cable according to any of claims 1 to 5, characterized in that the polar monomer is based on methacrylic ester and is of the general formula: CH2 = C (CH3) -C0-0- (CH2) n-N (CH3) 2 where n is equal to 2 or 3

10. A CD cable according to any of claims 1 to 5, characterized in that the polar monomer is based on acrylic ester and is of the general formula: CH2 = CH-C0-0- (CH2) n-N (CH3) 2 where n is equal to 2 or 3.

11. A CD cable according to any of claims 1 to 5, characterized in that the polar monomer is based on methacrylic acid and oligomeric ethylene glycol and is of the general formula: CH2 = C (CH3) -CO-O- (CH2.CH20-) mH where m is equal to a number from 1 to 20, preferably n is equal to l, 5, 6 or 9.

12. A CD cable according to any of claims 1 to 5, characterized in that the polar monomer is based on acrylic acid and oligomeric ethylene glycol and is of the general formula: CH2 = CH-C0-0- (CH2. CH20- "H where m is equal to a number from 1 to 20, preferably n is equal to l, 5, 6 or 9.

13. A method for the production of an insulated electric CD cable comprising the steps of combining a PE composition, extruding the combined polyethylene composition, as a part of a polymer based insulation system disposed around a conductor and subsequently cross-linking the composition of PE in an XLPE composition, characterized in that a polar comonomer having the general formula: CH2 = CR-CO-X- (CH2) n-N (CH3) 2 or CH2 = CR-CO-0- (CH2.CH20) 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 0 or NH, is introduced into the PE composition.

14. A method according to claim 13, characterized in that the polar monomer copolymerizes or introduces into the polyethylene chain.

15. A method according to claim 13 or 14, characterized in that the polar monomer is added to the ethylene before or during the polymerization reaction and the polar comonomer is subsequently introduced as radicals into the structure of the PE chain during the polymerization reaction.

16. A method according to claim 15, characterized in that the polar comonomer is added in an amount of 0.1% to 5% by weight of the finished polymer.

17. A method according to claim 15, characterized in that the polar comonomer is added in an amount of 5 to 40% by weight of the finished polymer and the high amount of the polar comonomer is subsequently diluted by combining the copolymer with the linear polyethylene until the content of average polar comonomer is from 0.1% to 5% by weight

18. A method according to claim 13 or 14, characterized in that the polar monomer is introduced into a homopolymer of ethylene.

19. A method according to claim 18, characterized in that the polar monomer is introduced into a homopolymer of ethylene in a separate step after the polymerization process.

20. A method according to claim 18, characterized in that the polar monomer is introduced into a homopolymer of ethylene during the extrusion and / or cross-linking of the insulation of the cable based on polyethylene.