AN ELECTRIC DC-CABLE WITH AN INSULATION SYSTEM COMPRISING AN EXTRUDED POLYETHYLENE COMPOSITION AND A METHOD FOR MANUFACTURING SUCH CABLE

Abstract: An electric DC-cable with an insulation system comprising an extruded polyethylene composition and a method for manufacturing such a cable. The insulating system comprises an extruded cross-linked polyethylene based insulation disposed around the conductor. The extruded insulation system in addition to the polyethylene based compound includes an additive, which is a glycerol fatty acid ester of the general formula (I): R<1>O(C3H5(OR<2>)O)nR<3> where n>/=1, R<1>, R<2>, and R<3>, which are the same or different, designate hydrogen or the residue of a carboxylic acid with 8-24 carbon atoms, with the proviso that there are at least two free OH groups and at least one residue of a carboxylic acid with 8-24 carbon atoms in the molecule. In the method for producing the DC-cable the compounded polyethylene based composition is extruded and cross-linked at a temperature and for a period of time sufficient enough to cross-link the insulation.

Description: AN ELECTRIC DC-CABLE WITH AN INSULATION SYSTEM COMPRISING AN EXTRUDED POLYETHYLENE COMPOSITION AND A METHOD FOR MANUFACTURING SUCH CABLE TECHNICAL FIELD ' The present invention relates to an insulated electric direct current cable, a DC- cable, with a current- or voltage-carrying body, i.e. a conductor and an insulation system disposed around the conductor, wherein the insulation system comprises an extruded and cross-linked polyethylene composition. The present invention relates in particular to an insulated electric DC-cable for transmission and distribution of electric power. The extruded insulation system comprises a plurality of layers, such as an inner semi-conductive shield, an insulation and an outer semi- conductive shield. At least the extruded insulation comprises a cross-linked polyethylene based electrically insulating composition with a system of additives such as cross-linking agent, scorch retarding agent and anti-oxidant BACKGROUND ART Although many of the first electrical supply systems for transmission and distribution of electrical power were based on DC-technology, these DC-systems were rapidly superseded by systems using alternating current, AC.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 distribution voltages. The development of modern electrical supply systems in the first half of this century was exclusively based on AC-transmission systems. However, by the 1950s there was a growing demand for long transmission schemes and it became clear that in certain circumstances there could be benefits by adopting a DC based system. The foreseen advantages include a reduction of problems typically encountered in association with the stability of the AC-systems, a more effective use of equipment as the power factor of the system is always unity and an ability to use a given insulation thickness or clearance at a higher operating voltage. Against these very significant advantages has to be weighed the high cost of the terminal equipment for conversion of the AC to DC and for inversion of the DC back again to AC. However, for a given transmission power, the terminal costs are constant and therefore, DC-transmission systems were rendered economical for the schemes involving long distances. Thus DC-technology becomes economical for systems intended for transmission over long distances as for when the transmission distance typically exceed the length for which the savings in the transmission equipment exceeds the cost of the terminal plant. An important benefit of DC operation is the virtual elimination of dielectric losses, thereby offering a considerable gain in efficiency and savings in equipment. The DC leakage current is of such small magnitude that it can be ignored in current rating calculations, whereas in AC-cables dielectric losses cause a significant 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 include a conductor and an insulation system comprises 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 during, production installation and use. Almost all the DC-cable systems supplied so far have been for submarine crossings or the land cable associated with them. For long crossings the mass-impregnated solid paper insulated type cable is chosen because there are no restrictions on length due to pressurizing requirements. It has been supplied for operating voltages of 450 kV. To date an essentially all paper insulation body impregnated with a electric insulation oil has been used but application of laminated material such as a polypropylene paper laminate is being persuaded for use at voltages up to 500 kV 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 polarity 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 distribution 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 insulation have the same advantage as the mass impregnated cable in that for DC transmission there are no restrictions on circuit length and they also have a potential for being operated at higher temperatures. In the case of XLPE, 90 °C instead of 50 °C for conventional mass-impregnated DC-cables. Thus offering a possibility 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 being the development of space charge in the dielectric when subjected to a DC-field. Such space charges distort the stress distribution and persist for long periods because of the high resistivity of the polymers. Space charges in an insulation body do 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 space charge accumulation patterns, differing in the polarity of the space charge accumulation in relation to the polarity. The space charge accumulation results in a local increase at certain points of the actual electric field in relation to the field, which would be contemplated when considering the geometrical dimensions and dielectric characteristics 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 account for this considerably higher field resulting in the use of thicker and/or more expensive materials in the cable insulation. The build up of the space charge accumulation is a slow process, therefore this problem is accentuated when the polarity of the cable after being operated for a long period of time at same polarity is reversed. As a result of the reversal a capacity field is superimposed on the field resulting from the space charge accumulation 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 seriously affecting the other properties. To date it has not been possible to match the electrical performance achieved with the impregnated paper insulated cables and no commercial polymeric 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, JiCable'91, pp. 562- 569). This stress value compares with 32 kV/mm used as a typical value for mass-impregnated paper cables. An extruded resin composition for AC-cable insulation typically comprises a polyethylene resin as the base polymer complemented with various additives such as a peroxide cross-linking agent, a scorch retarding agent and an anti-oxidant or a system of antioxidants. In the case of an extruded insulation the semi-conductive shields are also typically extruded and comprise a resin composition that in addition to the base polymer and an electrically conductive or semi-conductive filler comprises essentially the same type of additives. The various extruded layers in an insulated cable in general are often based on a polyethylene resin. Polyethylene resin means generally and in this application a resin based on polyethylene or a copolymer of ethylene, wherein the ethylene monomer constitutes a major part of the mass. Thus polyethylene resins may be composed of ethylene and one or more monomers which are co-polymerisable with ethylene. LDPE, low density polyethylene, is today the predominant insulating base material 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 comprises additives such as; - stabilizing additives, e.g. antioxidants, electron scavengers to counteract decomposition due to oxidation; radiation etc.; - lubricating additives, e.g. stearic acid, to increase processability; - additives for increased capability to withstand electrical stress, e.g. an increased water tree resistance , e.g. polyethylene glycol, silicones etc.; and - cross-linking agents such as peroxides, which decompose upon heating 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-linking density; - scorch retarders to avoid premature cross-linking. The number of various 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 effects of a change in the system of additives. In other cases the improvements sought for are of such dignity that some minor negative 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 insulation in an AC -cable comprises: 100 parts by weight of low density polyethylene (922 kg/m.3) with melt flow rate (MFR 2 ) of 0,4 - 2,5 g/lO min. 0,1 - 0,5 phr (parts per hundred resin) of an antioxidant, e.g. SANTONOX R® (Flexsys Co) with the chemical designation 4,4'-thio-bis(6-tert-butyl-m-cresol), or other antioxidants or combination of antioxidants 1,0 - 2,5 phr of a cross linking agent, DICUP R® (Hercules Chem) with the chemical designation dicumyl peroxide. However, it is well known that all cross linked polyethylene compositions used as extruded insulation in AC-cable systems exhibit strong tendency to accumulate space charge under DC-electric stress, thus making them unsuitable for use in insulation systems for DC-cables. It is also known that extended degassing, i.e. exposing the cross linked cable at high temperatures to a high vacuum for long periods 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 accumulation 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. OBJECTS OF THE INVENTION It is an object of the present invention to provide an insulated DC-cable with an electrical insulation system suitable for use as a transmission and distribution cable in networks and installations for DC-transmission and distribution of electric power. The cable shall comprise a solid extruded conductor insulation that can be applied and processed without the need for any lengthy time consuming batch- treatment such as impregnation or degassing, i.e. vacuum treatment of the cable. Thereby reducing the production time and thus the production costs for the cable and thereby offering the possibility for an essentially continuous or at least semi-continuous production of the cable insulation system. Further, the reliability, low maintenance requirements and long working life of conventional DC-cables comprising a mass impregnated paper-based insulation shall be maintained or improved. That is, the cable according to the present invention shall have stable and consistent dielectric properties and a high and consistent electric strength. The cable insulation shall exhibit a low tendency to space charge accumulation, a high DC breakdown strength, a high impulse strength and high insulation resistance. The replacement of the impregnated paper or cellulose based tapes with an extruded polymeric insulation shall as an extra advantage open for an increase in the electrical strength and thus allow an increase in operation voltages, make the cable handy and improve robustness. It is also an object to provide a cable comprising an extruded, cross linked insulation based on polyethylene which has low or no space charge accumulation in the insulation during DC-electric stresses, thereby eliminating or at least substantially reducing any problem associated with space charge accumulation. It shall also provide a capacity to reduce safety factors in design values used for dimensioning the cable insulation It is further the object to provide a method for manufacturing the insulation of such an insulated DC-cable according to the present invention. The process according to this aspect of the present invention for application and processing of the conductor insulation shall be essentially free from operating steps requiring a lengthy batch treatment of complete cable lengths or long lengths of cable core. The process shall also exhibit a potential for being used in a continuous or semi-continuous way for production of long lengths of DC- cable. SUMMARY OF THE INVENTION It has now surprisingly been found that excellent results with regard to accumulation of space charge under the influence of a DC-electric field can be achieved by incorporating in XLPE compositions for electric cables a specific glycerol fatty acid ester additive, optionally in combination with further additives. The present invention thus provides a DC-electric power cable comprising a conductor and an extruded, cross linked solid insulation system comprising at least three layers disposed around the conductor, characterized in that the extruded insulation system comprises a polyethylene based compound to which additives including a cross linking agent, a scorch retarding agent, an antioxidant and an additive comprising a glycerol fatty acid ester of the general formula ( I ) R'O(C 3 H 5 (OR 2 )O) n R 3 ( I ) where n > 1, preferably, because of commercial availability, n = 1-20, and more preferably n = 3-8, R 1 , R 2 , and R 3 , which are the same or different, designate hydrogen or the residue of a carboxylic acid with 8-24 carbon atoms, with the proviso that there are at least two free OH groups and at least one residue of a carboxylic acid with 8-24 carbon atoms in the molecule. In case R 2 and R 3 both represent hydrogen (H) atoms and R 1 = R the carboxylic residues the formula will take the simple form of ( II ) RO(CH 2 CH(OH)CH 2 O) n H ( II ) The compounded polyethylene based insulation is typically extruded and heated to an elevated temperature and for a period of time long enough to cross link the insulation. The temperature and the period of time is controlled so as to optimize the cross linking process. The cable insulation system can be applied on the conductor with an essentially continuous process without the need for lengthy batch treatments as e.g. vacuum treatment. The low tendency for space charge accumulation and increased DC breakdown strength of conventional DC-cables comprising an impregnated paper insulation is maintained or improved. The insulating properties of the DC-cable according to the present invention exhibit a general long term stability such that the working life of the cable is maintained or increased. The present invention also provides a method for the production of a DC-cable as described in the foregoing. In its most general form the process for production of an insulated DC-cable comprising a conductor an extruded cross-linked polyethylene based conductor insulation includes the following steps: laying or otherwise forming a conductor of any desired shape and constitution; compounding a polyethylene based resin composition comprising additions of a cross-linking agent, a scorch retarding agent, antioxidant and a spare charge reducing additive extruding the compounded polyethylene based resin composition to form a conductor insulation disposed around the conductor in the DC-cable, (preferably the three layered insulation system comprising the insulation layer complemented with the two semi-conducting shields is applied using a true triple extrusion process) cross-linking the extruded insulation wherein according to the present invention a space charge reducing additive comprising a glycerol fatty ester of the general formula ( I ), is added to the polyethylene resin upon compounding; R'O(C 3 H 5 (OR2)O) n R 3 ( I ) where n > 1, R 1 , R 2 , and R 3 , which are the same or different, designate hydrogen or the residue of a carboxylic acid with 8-24 carbon atoms, with the proviso that there are at least two free OH groups and at least one residue of a carboxylic acid with 8-24 carbon atoms in the molecule. and wherein the compounded polyethylene based resin composition is extruded and cross-linked at an elevated temperature and applied pressure and for a period of time long enough to cross link the insulation. Other distinguishing features and advantages of the present invention will appear from the following specification and appended claims. DETAILED DESCRIPTION OF THE . INVENTION In order to use extruded polyethylene or cross linked polyethylene (XLPE) as an insulation for DC-cables several factors have to be taken into account. The most important issue is the space charge accumulation under DC-voltage stress. The present invention accomplish such significant decrease in the space charge accumulation typically occurring in an operating DC-cable by incorporating a low amount of an additive of the general structure ( I ) into the polyethylene or the cross linkable polyethylene compound. The compound of the general structure ( I ) is a mono- or polyglycerol ether where at least one OH group forms an ester with a carboxylic acid with 8-24 carbon atoms. Preferably, the compound of structure ( I ) is a monoester, i.e. it contains one carboxylic acid residue with 8-24 carbon atoms per molecule. Further, the ester forming carboxylic acid preferably forms ester with a primary hydroxylic group of the glycerol compound. The compound of formula ( I ) may include 1-20, preferably 1-15, most preferably 3-8 glycerol units, i.e. n in the formula ( I ) is 1-20, preferably 1-15, and most preferably 3-8. When R 1 , R 2 , and R 3 in formula ( I ) do not designate hydrogen they designate the residue of a carboxylic acid with 8-24 carbon atoms. These carboxylic acids may be saturated or unsaturated and branched or unbranched. Illustrative, non-limiting examples of such carboxylic acids are lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, linoleic acid, linolenic acid, and behenic acid. When the carboxylic residue is unsaturated the unsaturation may be utilized for binding the compound of structure ( I ) to the ethylene polymer of the composition and thus effectively prevent migration of the compound of structure ( I ) from the composition. In formula ( I ) R\ R^ and R- may designate the same carboxylic acid residue, such as stearoyl, or different carboxylic residues, such as stearoyl and oleyl. To prevent migration and exudation, the compound of structure ( I ) should be compatible with the composition in which it is incorporated, and more particularly with the ethylene base resin of the composition. The compounds of structure ( I ) are known chemical compounds or may be produced by known methods. Thus, a compound of formula ( I ) where n = 3 is commercialized as Atmer®184 (or 185) by ICI, Great Britain, and one where n in average is 8, having one fatty acid residue per molecule, can be obtained from ICI under the denomination SCS 2064®. Other known commercial compounds which can be described by formula ( II ) are TST 221® (n = 6 and R = linoleic acid residue (unsaturated C18 acid)) TST 215® (n = 6 and R = stearic acid (saturated C18 acid)), and TST 216® (n = 6 and R = behenic acid (unsaturated C22 acid)) all supplied by Danisco, Denmark. The compound of formula ( I ) is incorporated in the composition of the invention in an amount effective for inhibiting space charge accumulation under DC-stress. Generally this means that the compound of formula ( I ) is incorporated in an amount of about 0,05-2 % by weight, preferably 0,1-1 % by weight of the composition. In addition to the compound of formula ( I ) the composition of the compounds for the DC-cables of the present invention may include conventional additives, such as antioxidants to counteract decomposition due to oxidation, radiation, etc.; lubricating additives, such as stearic acid; cross linking additives, such as peroxides which decompose upon heating and initiate cross linking; and other additives such as scorch retardant agents and compatibilizers. The overall amount of additives, including the compound of formula ( I ) in the composition of the present invention should not exceed about 10 % by weight of the composition. Besides the compound of formula ( I ) and other conventional and optional additives mentioned above the composition of the invention predominantly comprises an ethylene polymer as indicated earlier. The choice and composition of the ethylene polymer varies depending on whether the composition is intended as an insulating layer of an electric cable or as an inner or outer semi conductive layer of an electric cable. A composition for an insulating layer of an electric cable according to the invention may for example comprise about 0,05 % to about 2 % by weight of the compound of formula ( I ) together with other conventional and optional additives; 0 to about 4 % by weight of a peroxide cross linking agent; the remainder of the composition substantially consisting of an ethylene polymer. Such ethylene polymer preferably is an LDPE, i.e. an ethylene homopolymer or a copolymer of ethylene and one or more alpha-olefins with 3-8 carbon atoms, such asl-butene, 4-methyl-l-pentene, 1-hexene, and 1-octene. The amount of alpha-olefm comonomer(s) may be in the range from about 1 % to about 40 % by weight of the ethylene monomer. A copolymer of ethylene together with minor amounts, i.e. up to 5 % by weight of one or more polar comonomer(s), eg. vinyl acetate, methylacrylate, ethylacrylate, butylacrylate or dimethylamino-propylmethacrylamide (DMAPMA) can also be used. Similarly, a composition for a semiconductive layer of an electric cable may comprise about 0,05 % to about 2 % by weight of the compound of formula ( I ) together with other conventional and optional additives; about 30-80 % by weight of an ethylene polymer; carbon black in an amount at. least sufficient to make the composition semiconductive, preferably about 15-45 % by weight of carbon black; 0 to about 30 % by weight of an acrylonitrile-butadiene copolymer; and 0 to about 4 % by weight of a peroxide cross linking agent. In this connection the ethylene polymer is an ethylene copolymer of the composition as described for the insulating layer or an ethylene copolymer, such as EVA (ethylene-vinylacetate), EMA (ethylene-methylacrylate), EEA (ethylene-ethylacrylate), or EBA (ethylene-butylacrylate). A DC-cable according to the present invention with an extruded, cross linked insulation system comprising a cross-linked polyethylene composition, XLPE, and an additive of structure ( I ) exhibit considerable advantages such as; - A substantially reduced tendency for space charge accumulation and accordingly an increased DC breakdown strength. The cable according to the following examples the present invention also offers good performance and stability of the extruded cable insulation system even when high temperatures have been employed during extrusion, cross linking or other high temperature conditioning.. The DC-cable according to the present invention offers 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 risking the technical performance of the cable. In order to further facilitate the understanding of the invention some illustrating, non-limiting examples will be given below. In the examples all compositions are given as part per hundred parts of resin by weight, unless otherwise stated. BRIEF DESCRIPTION OF THE DRAWINGS The present invention shall be described more in detail while referring to the drawings and examples. Figure 1 shows a section- view of a cable for high-voltage direct current transmission of electric power according to one embodiment of the present invention. Figure 2 shows the configuration of the test plates. Figures 3 to 14 show space charge recordings for measurements on plates with XLPE compositions as used in prior insulated AC-cables and for compositions according to the present invention. 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 extruded semi-conductive 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; - a second extruded semi-conductive shield 13 disposed outside the conductor insulation 12; - a metallic screen 14; and - an outer covering or sheath 15 arranged outside the metallic screen 14. The DC-cable can when deemed appropriate be further complemented in various ways with various 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 interfaces or a system of moisture barriers achieved by e.g. a corrosion resistant metal polyethylene laminate and longitudinal water sealing achieved by water swelling material, e.g. tape or powder beneath the sheath 15. The conductor need not be stranded but can be of any desired shape and constitution, such as a stranded multi-wire conductor, a solid conductor or a segmental conductor. The test plate 20 used for measurement of the space charge distribution shown in figure 2, comprises two semi-conductive electrodes 21 made of a carbon black filled ethylene copolymer and the insulation body 22 with the composition given in Table 1. Figure 3, 5, 7, 9, 11, and 13 show the distribution of space charge in arbitrary units in the "voltage-on" mode as a function of distance from the grounded electrode. Similarly figure 4, 6, 8, 10, 12, and 14 show the distribution of space charge in arbitrary units in the "voltage-off" mode as a function of distance from the grounded electrode (note the scales in "voltage-on" mode and "voltage-off mode are different). In order to facilitate the understanding of the invention some illustrating, non- limiting examples will be given below. In the following examples test plates with various compositions were manufactured and subjected to measurements of space charge accumulation by recording the space charge profiles. The profiles were recorded using the Pulsed Electro Acoustic (PEA) technique. The PEA technique is well known within the art and described by Takada et al. in IEEE Trans. Elec. Insul. Vol. EI-22 (No 4), pp. 497-501 (1987). The space charge profiles shown in the following examples are either "voltage-on" i.e. the recorded space charge profiles under electrical stress after 3 hours DC-voltage application, or "voltage-off, i.e. the recorded space charge profiles immediately after grounding of the electrodes (prior to grounding a DC-voltage was applied for 3 hours). The compositions shown in Table 1 were all made in a conventional manner by compounding the components in an extruder. The test plates were manufactured in a two- step process. In the first step the insulation was press molded from an extruded tape at 130 ° C for 10 minutes into circular plates with a diameter of 210 mm and a thickness of 2 mm. In the second operation two semiconductive electrodes were mounted in the center on each side of the circular insulation plates and the assembly was heated to 180 °C for 15 minutes in an electric press unless otherwise stated. The high temperature cycle was made in order to complete the cross linking. The test plates were hereafter cooled to ambient temperatures under pressure. Mylar® films were used as backing during the press molding. The semiconductive electrodes were made of a commercial product, LE 0500® from Borealis, Sweden. This compound comprises ethylene-butylacrylate copolymer and acetylene black. The dimensions of these electrodes were 1 mm in thickness and 50 mm in diameter. Figure 2 show the configuration and the dimensions of the test plates. The space charge profiles of the test plates were recorded by a device for PEA analysis at 50 °C. One electrode was grounded and the other was held at a voltage of +40 kV, i.e. the electric field strength in the plate was 20 kV/mm. In the space charge profiles figure 3-14 the electric charge per unit volume is presented as a function of the test plate thickness, i.e. zero is the position of the grounded electrode and x indicates the distance from the grounded electrode in the direction towards the high voltage (+40 kV) electrode. In the "voltage on" mode the space charge profile was recorded after 3 hours of voltage application. In the "voltage-off mode the space charge profile was recorded immediately after grounding of the high voltage electrode (i.e. after 3 hours at +40 kV). The space charge profiles are given in arbitrary units of charge per volume insulation. The amplification used during "voltage-off is higher than during "voltage-on". However, the scales used for all samples in either mode are comparable. Example 1, 2, and 3 are comparative examples. The composition of the insulation material in these examples correspond to the invention disclosed in the Swedish patent application No. 9704825-0 (1997-12-22). EXAMPLE 1 A 2 mm thick test plate of polyethylene of composition A (see Table 1) equipped with two semiconductive electrodes and cross linked at 180 °C for 15 minutes was tested at 50 °C in a device for PEA analysis. The plate was inserted between two flat electrodes and subjected to a 40 kV direct voltage electric 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 3 was recorded, in the so called "voltage-on" mode after 3 hours of exposure to the DC-voltage stress. The charge per unit volume is presented in arbitrary units 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. Figure 4 shows the space charge profile immediately after grounding of the high voltage electrode at the end of the 3 hours high voltage electrification in the so called "voltage-off mode. The charge per unit volume is presented in arbitrary units (different from that used in the "voltage-on" mode) 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 original high voltage electrode. EXAMPLE 2 In order to test the effect of removing all volatile from the insulation system a test plate of the same kind as in example 1 and cross linked at 180 °C for 15 minutes was treated in a high vacuum at 80 °C for 72 hours. After this treatment the space charge profiles were recorded. Figure 5 shows the "voltage-on" mode and figure 6 the "voltage-off mode. EXAMPLE 3 In order to test the effect of cross linking conditions a test plate of the same kind as in example 1 was cross linked at 250 °C for 30 minutes. The test plate was tested in a device for PEA analysis. Figure 7 shows the "voltage-on" mode and figure 8 the "voltage- on" mode. EXAMPLE 4 A 2 mm thick test plate of polyethylene of composition B (see Table 1) equipped with two semiconductive electrodes and cross linked at 180 °C for 15 minutes was tested at 50 °C in a device for PEA analysis. The plate was inserted between two flat electrodes and subjected to a 40 kV direct voltage electric 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 9 was recorded, in the so called "voltage-on" mode after 3 hours of exposure to the DC-voltage stress. The charge per unit volume is presented in arbitrary units 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. Figure 10 shows the space charge profile immediately after grounding of the high voltage electrode at the end of the 3 hours high voltage electrification in the so called "voltage-off mode. The charge per unit volume is presented in arbitrary units (different from that used in the "voltage-on" mode) 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 original high voltage electrode. EXAMPLE 5 In order to test the effect of removing all volatile from the insulation system a test plate of the same kind as in example 4 and cross linked at 180 °C for 15 minutes was treated in a high vacuum at 80 °C for 72 hours. After this treatment the space charge profiles were recorded. Figure 11 shows the "voltage-on" mode and figure 12 the "voltage-off mode. EXAMPLE 6 In order to test the effect of cross linking conditions a test plate of the same kind as in example 4 was cross linked at 250 °C for 30 minutes. The test plate was tested in a device for PEA analysis. Figure 13 shows the "voltage-on" mode and figure 14 the "voltage- on" mode. When comparing the space charge profiles in example 1, 2, and 3 with the space charge profiles in example 4, 5, and 6 it is evident that the compound of formula ( I ) is an extremely effective space charge reducing agent. It is clearly seen from table 2 that the space charge accumulated under similar conditions is more than 50 % lower when a compound of formula ( I ) is added to the insulation composition. In order to show the robustness of the space charge accumulation suppressing effect of the compound of formula ( I ) the following experiments, presented in example 7, 8, and 9, were performed. EXAMPLE 7 In order to check eventual concentration dependence of compound of formula ( I ) a 2 mm thick test plate of polyethylene of composition C (see Table 1) equipped with two semiconductive electrodes and cross linked at 180 °C for 15 minutes was tested at 50 °C in a device for PEA analysis. The space charge profiles in "voltage-on" mode and "voltage-off mode were identical to figure 9 and 10, respectively. EXAMPLE 8 In order to check the influence of the antioxidant system on the space charge reducing power of the compound of formula ( I ) three test plates of composition D, E, and F (see Table 1), respectively, was manufactured and tested as described in example 1. All three test plates showed space charge profiles in both "voltage-on" mode and "voltage-off mode which were identical to figure 9 and figure 10, respectively. EXAMPLE 9 In order to investigate the influence of other additives on the space charge reducing power of a compound of formula ( I ), three different compositions G, H, and I (see Table 1), respectively, was manufactured and tested as described in example 1. All three test plates showed space charge profiles in both "voltage-on" mode and "voltage-off mode which were identical to figure 9 and figure 10, respectively. It is evident from the results of example 7, 8, and 9 that the addition of a compound of formula ( I ) is an effective space charge reducing agent in a very broad range of cross linked polyethylene compositions. TABLE 1 Composition of XLPE insulation compounds * LDPE, low density polyethylene, i.e. polyethylene prepared by radical polymerization at high pressure (density = 0,922 g/cm 3 ). ** Irganox 1035®, diester of 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionicacid and thiodiglycol, Ciba-Geigy. *** Irganox PS 802®, di-stearyl-thio-dipropionate, Ciba-Geigy. **** ICI, Great Britain ***** 2,4-diphenyl-4-methyl-pentene-l, Nofmer MSD®, Nippon Oil and Fats. TABLE 1 (cont.) Composition of XLPE insulation compounds * LDPE, low density polyethylene, i.e. polyethylene prepared by radical polymerization at high pressure (density = 0,922 g/cm 3 ). ** Irganox 1035®, diester of 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionicacid and thiodiglycol, Ciba-Geigy. *** Irganox PS 802®, di-stearyl-thio-dipropionate, Ciba-Geigy. **** ICI, Great Britain 2,4-diphenyl-4-methyl-pentene-l, Nofmer MSD®, Nippon Oil and Fats. TABLE 1 (cont.) Composition of XLPE insulation compounds * LDPE, low density polyethylene, i.e. polyethylene prepared by radical polymerization at high pressure (density = 0,922 g/cm 3 ). ** Irganox 1035®, diester of 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionicacid and thiodiglycol, Ciba-Geigy. *** Irganox PS 802®, di-stearyl-thio-dipropionate, Ciba-Geigy. **** ICI, Great Britain ***** 2,4-diphenyl-4-methyl-pentene-l, Nofmer MSD®, Nippon Oil and Fats. TABLE 2 Relative magnitudes of the accumulated space charge in "voltage-off mode . (after 3 hours of DC electrification at 20 kV/mm).

Claims: CLAIMS 1. An insulated electric DC-cable with a conductor and a polymer based insulation system comprising at least three layers of extruded and cross-linked polyethylene, XLPE, based compositions, disposed around the conductor, characterized in that the extruded insulation system in addition to the polyethylene based compounds includes an additive which is a glycerol fatty acid ester of the general formula R 1 O(C 3 H 5 (OR 2 )O) n R 3 (I) where π≥l, Rl, R2 and R3, which are the same or different, designate hydrogen or the residue of a carboxylic acid with 8-24 carbon atoms, with the proviso that there are at least two free OH groups and at least one residue of a carboxylic acid with 8-24 carbon atoms in the molecule. 2. A DC-cable according to claim 1, characterized in that in compound (I) both R2 and R3 represent hydrogen atoms and R1=R the residue of carboxylic acid with 8-24 carbon atoms i.e. the compound has the formula RO(CH 2 CH(OH)CH 2 O) n H (II) 3. A DC-cable according to claim 1, characterized in that n is 1- 20, preferably 1-15 and most preferably 3-8. 4. A DC-cable according to any of claims 1-3 characterized in that the compound of formula (I) is a monoester. 5. A DC-cable according to any of the preceding claims characterized in that the ester is formed between the carboxylic acid and a primary hydroxylic group of the glycerol compound. 6. A DC-cable according to any of the preceding clams characterized in that the compound of formula (I) is included in both the insulation and semiconductive layers. 7. A DC-cable according to any of clams 1-5 characterized in that the compound of formula (I) is included only in the insulation layer(s). 8. A DC-cable according to any of claims 1-5 characterized in that the compound of formula (I) is included only in the semiconductive layer(s). 9. A DC-cable according to any of the preceding claims characterized in that the compound of formula (I) is present in the polymer composition(s) in an amount at least 0.05 % by weight based on the actual composition. 10. A DC-cable according to claim 9, characterized in that the compound of formula (I) is present in the polymer composition(s) in an amount of from 0.05 to 2 % by weight, preferably from 0.1 to 1% by weight, of the actual composition. 11. A DC-cable according to any of the preceding claims, characterized in that the polymer composition(s) include(s) one or more conventional additives such as antioxidants, cross-linking agents, lubricating additives, scorch-retarding agents and compatibilisers. 12. A DC-cable according to claim 11 characterized in that the overall amount of conventional additives, including the compound of formula (I), in the actual composition, is not more than about 10% by weight of the actual composition. 13. A DC-cable according to any of the preceding claims characterized in that the polyethylene (PE) is selected from homopolymers of ethylene, copolymers of ethylene with one or more cc-olefins with 3-8 carbon atoms and copolymers of ethylene with vinyl acetate, methylacrylate, ethylacrylate, butylacrylate or dimethylamino- propylmethacrylamide (DMAPMA). 14. A method for production of an insulated electric DC-cable comprising the steps of compounding a polyethylene (PE) composition, extruding said compounded PE composition as a part of polymer-based insulation system disposed around a conductor and subsequently crosslinking the PE composition into an XLPE composition characterized in that a compound of the general formula R 1 O(C 3 H 5 (OR 2 )O) n R 3 (I) where n> 1, Rl, R2 and R3, which are the same or different, designate hydrogen or the residue of a carboxylic acid with 8-24 carbon atoms, with the proviso that there at least two free OH groups and at least one residue of a carboxylic acid with 8-24 carbon atoms in the molecule; is added to the PE composition. 15. A method according to claim 14, characterized in that the compound (I) is added in an amount of at least 0.05 % based on the weight of the actual composition. 16. A method according to claim 15, characterized in that the compound (I) is added in an amount of 0.05 - 2 %, preferably from 0.1 to 1% by weight of the actual composition. 17. A method according to any of the preceding claims characterized in that one or more other additives such as antioxidants, lubricating additives, cross-linking agents, scorch retarding agents and compatibilisers also are added to the composition. 18. A method according to any of claims 15- 17 characterized in that the total amount of additives, including the compound of formula (I) added to each composition is not more than 10 % by weight of the actual composition.

Inventors: BOSTROEM JAN-OVE, CARSTENSEN PETER, FARKAS ANDREAS, GUSTAFSSON ANDERS, GUSTAFSSON BILL, JOHANNESSON KENNETH, NILSSON ULF, NYLANDER PERRY, BOSTROEM, JAN-OVE, CARSTENSEN, PETER, FARKAS, ANDREAS, GUSTAFSSON, ANDERS, GUSTAFSSON, BILL, JOHANNESSON, KENNETH, NILSSON, ULF, NYLANDER, PERRY
Applicants: ABB AB, BOSTROEM JAN OVE, CARSTENSEN PETER, FARKAS ANDREAS, GUSTAFSSON ANDERS, GUSTAFSSON BILL, JOHANNESSON KENNETH, NILSSON ULF, NYLANDER PERRY, BOSTROEM, JAN-OVE, CARSTENSEN, PETER, FARKAS, ANDREAS, GUSTAFSSON, ANDERS, GUSTAFSSON, BILL, JOHANNESSON, KENNETH, NILSSON, ULF, NYLANDER, PERRY
Application year: 1999
Main classification: H01B9
IPC codes: H01B3, H01B9, H01B13