[原创]英语原文1
Cables
GENERAL DESIGN CRITERIA
(1) The cross-sectional area of the conductors chosen should be of the optimum size to carry the specified load current or short circuit short term current without overheating and should be within the required limits for voltage drop.
(2) The insulation applied to the cable must be adequate for continuous operation at the specified working voltage with a high degree of thermal stability, safety and reliability.
(3) All materials used in the construction must be carefully selected in order to ensure a high level of chemical and physical stability throughout the life of the cable in the selected environment.
(4) The cable must be mechanically strong, and sufficiently flexible to withstand the re-drumming operations in the manufacturer′s works, handing during transport or when the cable is installed by direct burial, in trenches, pulled into ducts or laid on cable racks.
(5) Adequate external mechanical and or chemical protection must be applied to the insulation and metal or outer sheathing to enable it to withstand the required environmental service conditions.
CABLE CONSTRUCTION
(1) conductor materials
Copper is still the predominant conductor material in stranded, shaped and segmental formats. Solid or stranded, shaped or segmental aluminum is also often specified on the basis of cost in the manufacturer′s country at the time of cost in the manufacturer′s country at the time of tender. Aluminum is also lighter and assists with ease of handling large cables. Additional care has to be taken when jointing aluminum cables. It is necessary to ensure that the contact surfaces are free from oxide and that when connecting to copper or brass terminals no corrosion cell is formed.
(2) Insulation
Oil-impregnated, paper-insulation cables have a history of satisfactory use at all voltage levels. However , paper insulation deteriorates rapidly because of its hygroscopic nature if exposed to moisture. In order to prevent this, the paper layers are protected against ingress of water, usually by a lead alloy or corrugated aluminum alloy metal sheath.
PVC insulation is specified and is suitable for cable rated up to 7.2KV. PVC has the advantage over paper insulation in that it is non-hygroscopic and does not therefore required a metallic sheath. Moreover, it is both lighter and tougher and inherently more flexible than paper.
XLPE insulation is a thermo sate achieved by a process akin to the vulcanization of rubber. The resulting material combines the advantages of PVC insulation with thermal stability over a wide temperature range. XLPE has no true melting point and remain elastic at high temperatures therefore permitting greater current carrying capacity, overload and short circuit performance in comparison with PVC-and paper-insulated cables.
EPR insulation has a cross-linked molecular structure like XLPE and are produced by a similar process. Both EPR and XLPE have the same durable and thermal characteristics but EPR has a higher degree of elasticity that is maintained over a wide temperature range.
Mineral insulated copper conductor cables are manufactured for 600V and 1000V installations that could involve high temperature, rough mechanical handing, surface knocks or contact with oil.
(3) Sheaths
Very little lead sheathing is now specified except for special HV cables. Lead and lead alloy sheaths have been traditionally used to prevent the ingress of moisture into paper-insulated cables or other cabbies installed in particularly marshy conditions. Lead corrosion and improvements are obtained by the addition of other elements. Alloy sheaths are used with unarm cables where vibration problems might be encountered.
(4) Insulation levels and screening
Generally if the network is solidly earthed the voltage will not rise above the maximum system phase-to-neutral voltage under fault conditions. However, if under fault conditions the earth arrangement is such as to allow the voltage to neutral to rise to the line voltage then the cable insulation must be specified accordingly.
To minimize the possibility discharges at the inner surfaces of cable core dielectric a grading screen is introduced. This screen comprises of one or two layers of semiconducting tapes or compounds over the core insulation.
(5) Armouring
In order to protect cables from mechanical such as pick or spade blows, ground subsidence or excessive vibrations they are lapped by one or two layers of galvanized steel tapes, galvanized steel wire braid or galvanized steel wires helically wound over the cable.
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2007-6-10 20:25:34