Electric DC-cable with an insulation system

Abstract

An insulated DC-cable and method for production of an insulated DC-cable with an insulating system comprising an extruded cross-linked polyethylene based insulation disposed around the conductor. The extruded polyethylene based compound comprises additives such as cross-linking agent, scorch retarding agent and antioxidant. The scorch retarding agent comprises a compound (D), 2,4 -diphenyl- 4 -methyl-pentene- 1 and the antioxidant comprises a compound (C), a diester of 3 -( 3,5 -di-tert-butyl- 4 -hydroxyphenyl)propionicacid and thiodiglycol. The compounded polyethylene based resin composition is extruded and cross-linked at a temperature and for a period of time sufficient enough to cross link the insulation. The temperature and the period of time upon extrusion and cross-linking are limited so as to substantially suppress or essentially avoid undesired polar by-products being formed in the cross-linked composition.

1 . An insulated direct current electric cable, a DC-cable, comprising a conductor and an extruded, cross-linked conductor insulation disposed around the conductor, wherein the extruded insulation comprises a polyethylene based compound with additives including a cross-linking agent, a scorch retarding agent comprises a compound (D), 2,4-diphenyl-4-methyl-pentene-1, and that the antioxidant comprises a compound (C), a diester of 3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionicacid and thiodiglycol.  
     
     
         2 . A DC-cable according to  claim 1 , characterized in that the extruded insulation has a thickness of less than 10 mm and that the antioxidant system is a combined two component antioxidant system consisting of compound (B), a di-alkyl-thio-dipropionate, and compound (C), a diester of 3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionicacid and thiodiglycol.  
     
     
         3 . A DC-cable according to  claim 2 , characterized in that the combined antioxidant system is added at a level of from 0.1 to 0.8% by weight.  
     
     
         4 . A DC-cable according to  claim 3 , characterized in that the combined antioxidant system is added at a level, preferably of from 0.2 to 0.5% by weight.  
     
     
         5 . A DC-cable according to  claim 1 , characterized in that the antioxidant system consists of compound (C), a diester of 3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionicacid and thiodiglycol, only  
     
     
         6 . A DC-cable according to  claim 5 , characterized in that compound (C), a diester of 3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionicacid and thiodiglycol, is added at a level of from 0.1 to 0.5% weight.  
     
     
         7 . A DC-cable according to  claim 6 , characterized in that compound (C), a diester of 3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionicacid and thiodiglycol, is added at a level of from 0.2 to 0.4% by weight.  
     
     
         8 . A DC-cable according to any of the preceding claims, characterized in that the scorch retarding agent, compound (D), 2,4-diphenyl-4-methyl-pentene-1, is added at a level of from 0.1 to 1.0% by weight.  
     
     
         9 . A DC-cable according to  claim 8 , characterized in that the scorch retarding agent, compound (D), 2,4-diphenyl-4-methyl-pentene-1, is added at a level of from 0.2 to 0.5% by weight.  
     
     
         10 . A DC-cable according to any of the preceding claims, characterized in that the peroxide cross-linking agent is added at a level of from 1.0 to 2.4% by weight.  
     
     
         11 . A DC-cable according to  claim 11 , characterized in that the peroxide cross-linking agent is added at a level of from 1.2 to 1.8% by weight.  
     
     
         12 . A DC-cable according to any of the preceding claims, characterized in that the peroxide cross-linking agent is dicumyl peroxide.  
     
     
         13 . A DC-cable according to any of the preceding claims, characterized in that the polyethylene resin is a low density polyethylene with a melt flow rate, MFR, within a range from 0.5 to 10 g/10 min.  
     
     
         14 . A method for production of an insulated DC-cable comprising a conductor and an extruded cross-linked polyethylene based conductor insulation according to any of the preceding claims, comprising the following steps; 
 providing a conductor;    compounding a polyethylene based resin composition, wherein a peroxide cross-linking agent, a scorch retarding agent and an antioxidant system is added;    extruding the compounded polyethylene based resin composition to form a conductor insulation disposed around the conductor in the DC-cable;    cross-linking the extruded insulation, characterized in that a scorch retarding agent comprising compound (D), 2,4-diphenyl-4-methyl-pentene-1, and an antioxidant system comprising compound (C), a diester of 3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionicacid and thiodiglycol is added to the polyethylene resin upon compounding; and that process variables such as the composition of polyethylene based resin, the extrusion and cross-linking temperatures and the time periods for extrusion and cross-linking are controlled such that any formation of water in the extruded, cross-linked insulation is suppressed.    
     
     
         15 . The method according to  claim 14 , characterized in that the additions of the peroxide cross-linking agent and the scorch retarding agent (D), 2,4-diphenyl-4-methyl-pentene-1, is balanced and that the composition is extruded and cross-linked at a temperature at which the cross-linking is completed to a desired level, that the amount of excessive peroxide cross-linking agent present in the cross-linked insulation is minimized.  
     
     
         16 . The method according to  claim 14  or  15 , characterized in that upon compounding a two component antioxidant system comprising compound (C), a diester of 3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionicacid and thiodiglycol, and compound (B), a di-alkyl-thio-dipropionate, is added to the polyethylene resin.  
     
     
         17 . The method according to  claim 16 , characterized in that the combined antioxidant is added at a level of from 0.1 to 0.8% by weight.  
     
     
         18 . The method according to  claim 16  or  17 , characterized in that the compounded resin composition is extruded and cross-linked at a product temperature below 230° C.  
     
     
         19 . The method according to  claim 18 , characterized in that the compounded resin composition is extruded and cross-linked at a product temperature within a range of from 200° C. to 225° C.  
     
     
         20 . The method according to  claim 14  or  15 , characterized in that upon compounding an antioxidant consisting of only of compound (C), a diester of 3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionicacid and thiodiglycol, is added to the polyethylene resin.  
     
     
         21 . The method according to  claim 20 , characterized in that compound (C), a diester of 3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionicacid and thiodiglycol, is added at a level of from 0.1 to 0.5% by weight.  
     
     
         22 . The method according to  claim 20  or  21 , characterized in that the compounded resin composition is extruded and cross-linked at a product temperature above 230° C.  
     
     
         23 . The method according to  claim 22 , characterized in that the compounded resin composition is extruded and cross-linked at a product temperature with a range of from 240° C. to 350° C.  
     
     
         24 . The method according to any of the preceding claims, characterized in that the scorch retarding agent, compound (D), 2,4-diphenyl-4-methyl-pentene-1, is added at a level of from 0.1 to 1.0% by weight.  
     
     
         25  The method according to any of the preceding claims, characterized in that the peroxide cross-linking agent is dicumyl oxide and added at a level of from 1.0 to 2.4% by weight.  
     
     
         26 . The method according to any of the preceding claims, characterized in that the polyethylene resin is a low density polyethylene with a melt flow rate, MFR, within a range from 0.5 to 10 g/10 min.