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.

Description

[0001] 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 thc conductor, wherein the insulation system comprises an extruded and cross-linked polyethylene composition.[0002] 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.[0003] 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for transmission and distribution of electrical power were based on DC technology, these DC systems were rapidly superseded by sy...

AI Technical Summary

Benefits of technology

[0017] 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, improve handleability and robustness of the cable.

[0018] 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

Problems solved by technology

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.

However, it has not been possible to obtain the full potential of these materials for full size cables.

Such space charges distort the stress distribution and persist for long periods because of the high resistivity of the polymers.

Thus the design 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.

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, in reality it is always next to impossible to forecast all possible side effects of a change in the system of additives.

Furthermore it is well known that this cross linked composition exhibits a strong tendency to form space charges under DC electric fields, thus making it unusable in insulation systems for DC cables.

However, 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.

Degassing is a time-consuming batch-process comparable with impregnation of paper insulations and thus as costly.

Most known cross-linked polyethylene compositions used as extruded insulation in AC-cable exhibit a tendency for space charge accumulation which renders them unsuitable for use in insulation systems for DC-cables.

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