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High voltage cable
Tuesday, 09 January 2007
Electric power cables are used to transfer large amounts of electrical power by conducting large electric currents at high voltages. A high voltage transmission system comprises, depending on whether it is single-phase, double-phase or three-phase, one or more high voltage cables. The principle structure of a high voltage cable with an envelope of plastic material is such that the conductor which usually is consisting of a plurality of wires is encircled by a field controlling or field smoothing layer. This layer is encircled by an insulation layer of plastic material. The insulation layer is encircled by an outer semiconductor layer. In order to deliver these high voltages, special high-voltage cables, high-voltage cables are required to have good insulating properties. Also, it is often required that the cables possess sufficient flexibility to sustain bends and turns in the pathway between the high-voltage source and the item of equipment, and also to permit flexing of the cable during operation. Cables for power transmission or distribution at medium or high voltage generally have a metallic conductor coated, respectively, with a first inner semiconductive layer, an insulating layer and an outer semiconductive layer. Cables for the transmission of high or very high voltages comprise a stranded conductor wrapped with a stratified insulation constituted by a paper/polypropylene/paper laminate impregnated with an insulating fluid. High voltage cables usually contain one or more electrically semiconductive layers for limiting the electromagnetic field and one or more electrically insulating layers. Both the electrically insulating and the electrically semiconductive layers may consist of polymeric mixtures based on ethylene homopolymers and/or copolymers. The electrically semiconductive layers usually contain large amounts of an electrically conductive pigment. The internal semiconductor layer is covered and surrounded by a polythene containing insulation sheath. The insulation sheath may consist entirely of polythene (PE), of crosslinked polythene (XLPE) or of a copolymer of ethene and, for example, another alkene or a diene, such as the copolymer of ethene and propene (EPM) or the terpolymer of ethene, propene and a diene (EPDM). Around the insulation sheath there is provided an external semiconductor screen of usually the same composition as the first semiconductor layer. Around the external semiconductor layer there is present a conductive layer of, for example, a metal foil, such as a copper foil, which in turn is covered with a layer of a synthetic resin, over which the usual reinforcement and finally an outer cover of a synthetic resin of for example PVC, is provided. The semiconductor layers are relatively thin if compared with the insulation layer and are also of plastic material. For conducting purposes these layers are added with graphite in order to achieve a weak conducting property. The purpose of these semiconductor layers is to influence the electric field strength within the insulation layer. Environmental moisture diffusing into the cable insulation can promote the development of harmful "electrochemical trees" which shorten the useful service life of the cable. In order to avoid the entry of water or water vapours through the outer coating to the cable conductor the outer coating preferably consists of metal or a metal sheet in combination with an outer layer of PE or PVC (layer coating). The avoidance of water intrusion is of particular significance, since intruded water accelerates the aging processes of the insulating materials of the cable made of cross-linked polyethylene and thus leads to an early damage or failure of the cable. Cables insulated with oil/paper insulation have been successfully used for high-voltage direct current (HVDC) applications. Cables insulated with crosslinked polyethylene can have several advantages over cables insulated with oil/paper for HVDC applications. The advantages of crosslinked polyethylene include lower manufacturing costs, lower operation costs, easier maintenance for utilities, higher temperature ratings to utilities, and environmental friendliness due to no oil leakage. It is well known to employ stress control means to control electrical stress in a region of high electrical field strength due to a shield discontinuity in high voltage cable or electrical equipment, for example, electrical bushings, and joints or terminations of high voltage cables. Such stress control means typically comprise stress cones and tapes or tubular articles of semi-conductive stress control material.