XLPE Insulated Power Cable
XLPE is the recognized abbreviation for cross-linked polyethylene. This and other cross-linked synthetic materials, of which EPR (ethylene propylene rubber) is a notable example, are being increasingly used as cable insulants for a wide range of voltages.
Polyethylene has good electrical properties and in particular a low dielectric loss factor, which gives it potential for use at much higher voltages than PVC. Polyethylene has been and still is used as a cable insulant, but, as a thermoplastic material, its applications are limited by thermal constraints.
Cross-linking is the effect produced in the vulcanization of rubber and for materials like XLPE the cross-linking process is often described as ‘vulcan- ization’ or ‘curing’. Small amounts of chemical additives to the polymer enable the molecular chains to be cross-linked into a lattice formation by appropriate treatment after extrusion.
The effect of the cross-linking is to inhibit the movement of molecules with respect to each other under the stimulation of heat and this gives the improved stability at elevated temperatures compared with the thermoplastic materials. This permits higher operating temperatures, both for normal loading and under short-circuit conditions, so that an XLPE cable has a higher current rating than its equivalent PVC counterpart.
The effects of ageing, accelerated by increased temperature, also have to be taken into account, but in this respect also XLPE has favourable characteristics.
BS 5467 specifies construction and requirements for XLPE and EPR-insulated wire-armoured cables for voltages up to 3.3kV. The construction is basically similar to that of PVC cables to BS 6346, except for the difference in insulant. Because of the increased toughness of XLPE the thicknesses of insulation are slightly reduced compared with PVC.
33kV XLPE cable
The standard also covers cables with HEPR (hard ethylene propylene rubber) insulation, but XLPE is the material most commonly used. From 3.8kV up to 33kV, XLPE and EPR insulated cables are covered by BS 6622 which specifies construction, dimensions and requirements.
The polymeric forms of cable insulation are more susceptible to electrical discharge than impregnated paper and at the higher voltages, where the electrical stresses are high enough to promote discharge, it is important to minimize gaseous spaces within the insulation or at its inner and outer surfaces.
To this end XLPE cables for 6.6 kV and above have semiconducting screens over the conductor and over each insulated core. The conductor screen is a thin layer extruded in the same operation as the insulation and cross-linked with it so that the two components are closely bonded. The screen over the core may be a similar extruded layer or a layer of semiconducting paint with a semiconducting tape applied over it.