Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
  • H01B3/00—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties
  • H01B3/18—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties mainly consisting of organic substances
  • H01B3/30—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties mainly consisting of organic substances plastics; resins; waxes
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
  • H01B9/00—Power cables
  • H01B9/02—Power cables with screens or conductive layers, e.g. for avoiding large potential gradients

Definitions

• 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
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• polyethylene resins may be composed of ethylene and one or more monomers which are co-polymerisable with ethylene.
• LDPE low density polyethylene
• 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.;
• 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
• a typical polyethylene based resin composition to be used as an extruded, cross-linked insulation in an AC -cable comprises:
• antioxidants 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
• DICUP R® Hercules Chem
• 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.
• impregnation or degassing i.e. vacuum treatment of the cable.
• 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.
• 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.
• 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.
• 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 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.
• 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.
• the process for production of an insulated DC-cable comprising a conductor an extruded cross-linked polyethylene based conductor insulation includes the following steps:
• a polyethylene based resin composition comprising additions of a cross-linking agent, a scorch retarding agent, antioxidant and a spare charge reducing additive
• a space charge reducing additive comprising a glycerol fatty ester of the general formula ( I ), is added to the polyethylene resin upon compounding;
• 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.
• 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.
• the compound of structure ( I ) is a monoester, i.e.
• 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.
• 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.
• 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.
• 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.
• R ⁇ R ⁇ and R- may designate the same carboxylic acid residue, such as stearoyl, or different carboxylic residues, such as stearoyl and oleyl.
• 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.
• 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®.
• 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.
• 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.
• 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
• 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.
• 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.
• 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.
• 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.
• 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).
• EVA ethylene-vinylacetate
• EMA ethylene-methylacrylate
• EEA ethylene-ethylacrylate
• 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.
• 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.
• the DC-cable according to the embodiment of the present invention shown in figure 1 comprises from the center and outwards;
• 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.
• FIG 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.
• 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).
• PPA Pulsed Electro Acoustic
• 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).
• 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.
• 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.
• the space charge profile was recorded after 3 hours of voltage application.
• the space charge profile was recorded immediately after grounding of the high voltage electrode (i.e. after 3 hours at +40 kV).
• Example 1 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).
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.

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• Physics & Mathematics (AREA)
• Spectroscopy & Molecular Physics (AREA)
• Organic Insulating Materials (AREA)
• Compositions Of Macromolecular Compounds (AREA)
• Processes Specially Adapted For Manufacturing Cables (AREA)
• Extrusion Moulding Of Plastics Or The Like (AREA)

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• 1998
  • 1998-08-06 SE SE9802681A patent/SE512745C2/sv unknown
• 1999
  • 1999-08-03 AR ARP990103854A patent/AR019993A1/es not_active Application Discontinuation
  • 1999-08-04 AU AU55415/99A patent/AU760355B2/en not_active Ceased
  • 1999-08-04 MX MXPA01001363A patent/MXPA01001363A/es unknown
  • 1999-08-04 CN CN99811805A patent/CN1322362A/zh active Pending
  • 1999-08-04 KR KR1020017001528A patent/KR20010072260A/ko not_active Application Discontinuation
  • 1999-08-04 WO PCT/SE1999/001335 patent/WO2000008655A1/en not_active Application Discontinuation
  • 1999-08-04 EP EP99941942A patent/EP1103052A1/en not_active Withdrawn
  • 1999-08-04 JP JP2000564209A patent/JP2002522875A/ja active Pending
  • 1999-08-04 CA CA002339541A patent/CA2339541A1/en not_active Abandoned
• 2001
  • 2001-02-05 NO NO20010592A patent/NO20010592L/no not_active Application Discontinuation

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