PVC and PE cables were used for LV circuits in the 1950’s and they started to gain wider acceptance in the 1960’s. Because they were cleaner, lighter, smaller and easier to install than paper-insulated type cables. During the 1970’s the particular benefits of XLPE and HEPR insulations were being recognized for LV circuits. Today it is these cross-linked insulations, mainly XLPE, which dominate the LV market with PVC usage in decline for power circuits, although still used widely for low voltage wiring circuits.
The LV XLPE cables are more standardized than MV polymeric types. But even so there is a choice of copper or aluminium conductor, single-core or multi-core, SWA or un-armoured, and PVC or Low Smoke and Fume (LSF) sheathed. A further option is available for LV in which the neutral or earth conductor is a layer of wires applied concentrically around the laid-up cores rather than as an insulated core within the cable. For this reason, the concentric earth conductor can replace the armour layer as the protective metal layer for the cable.
Design and Construction of Polymeric Cables:
This concentric wire design is mainly used in the LV electricity distribution network. Where the armoured design were primarily used in the industrial applications. Polyethylene and PVC were shown to be unacceptable for use as general MV cable insulation in the years following the 1960’s. Because, their thermoplastic nature resulted in significant temperature limitations. The XLPE and EPR were required in order to give the required properties. They allowed higher operating and short circuit temperatures within the cable. It had an advantage of easier jointing and terminating than for paper-insulated cables. This meant that in some applications a smaller conductor size could be considered than had previously been possible in the paper insulated case.
The MV polymeric cables comprise copper or aluminium conductors insulated with XLPE or EPR. It covered with a thermoplastic sheath of MDPE, PVC or LSF material. Within this general construction there are options of single core or three core types, individual or collective screens of different sizes and armoured or un-armoured construction. Single-core polymeric cables are more widely used than single-core paper-insulated cables. It was particularly for electricity supply industry circuits. Unlike paper-insulated cables, MV polymeric three core cables normally have circular section cores. This is mainly because the increases in price and cable diameter are usually outweighed in the polymeric case by simplicity and flexibility of jointing and termination methods using circular cores. Screening of the cores in MV polymeric cables is necessary for a number of reasons, which combine to result in a two-level screening arrangement.
This comprises extruded semiconducting layers immediately under and outside the individual XLPE or EPR insulation layer. A metallic layer in contact with the outer semiconducting layer. The semiconducting layers are polymeric materials containing a high proportion of carbon. This carbon gives an electrical conductivity well below that of a metallic conductor. But well above that required for an insulating material.
These charge transfers result in erosion of the insulation surface and premature breakdown. In order to achieve intimate contact, the insulation and screens are extruded during manufacture as an integral triple layer and this is applied to the individual conductor in the same operation. The inner layer is known as the conductor screen and the outer layer is known as the core screen or dielectric screen.
When the cable is energized, the insulation acts as a capacitor and the core screen has to transfer the associated charging current to the insulation on every half-cycle of the voltage. It is therefore necessary to provide a metallic element in contact with the core screen so that this charging current can be delivered from the supply. Without this metallic element, the core screen at the supply end of the cable would have to carry a substantial longitudinal current to charge the capacitance which is distributed along the complete cable length, and the screen at the supply end would rapidly overheat as a result of excessive current density. However the core screen is able to carry the current densities relating to the charging of a cable length of stay 200 mm, and this allows the use of a metallic element having an intermittent contact with the core screen, or applied as a collective element over three laid-up cores. A 0.08-mm thick copper tape is adequate for this purpose.
The normal form of armouring is a single layer of wire laid over an inner sheath of PVC or LSF material. The wire is of galvanized steel for three core cables and aluminium for single-core cables. Aluminium wire is necessary for single-core cables to avoid magnetizing or eddy-current losses within the armour layer. In un-armoured cable, the screen is required to carry the earth fault current resulting from the failure of any equipment being supplied by the cable or from failure of the cable itself. In this case, the copper tape referred to previously is replaced by a screen of copper wires of cross section between say 6 mm2 and 95 mm2, depending on the earth fault capacity of the system.
Ref: Electrical Power Engineers