Characteristics of XLPE Insulated Cables With Reference to the UK Standards _ EEP

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Characteristics of XLPE insulated cables withreference to the UK standards

Characteristics of XLPE insulated cables with reference to the UK standards (on photo: 10kV Aluminium Conductor XLPE 

Insulated Aerial Cable)

Cross-linking effect

XLPE is the recognized abbreviation for cross-linked polyethylene. This and other cross-linked

synthetic materials, of which EPR (ethylene propylene rubber) is a not able example, are being

incr easingly 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

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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 ratingthan 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 BS6346, 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 (photo credit: openelectrical.org)

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 dischargethan 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.

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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.

Single-core and three-core designs  are employed, and there is scope for 

constructional variation depending on the conditions of use, subject to the cores

being surrounded individually or as a three-core assembly by a metallic layer,

which may be an armour, sheath or copper wires or tapes.

 A typical arm oured construction which has been supplied in substantial quantities is shown

in Figure 1  below.

Figure 1 – XLPE cable construction

Where:

1. Circular stranded conductor 

2. Semiconductor XLPE screen

3. XLPE insulation

4. Semiconducting tape screen

5. Copper tape screen

6. PVC filler 7. Binder 

8. Extruded PVC sheath

9. Galvanized steel wire armour 10. Extruded PVC oversheath

In the UK this type of cable, mainly in single-core form, is favoured for power station cabling,

where lightness and convenience of terminating are major considerations. Three-core designs are

also used for site supplies.

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Underground direct buried power cables

For underground distribution at 11kV, the XLPE cable does not compete economically with the

paper-insulated aluminium-sheathed cable, but work is in progress on standardizing and assessing

XLPE cable design, including trial installations, in preparation for any change in the situation.

Overseas, where circumstances are different, XLPE cable is the type in major demand.

With manufacturing facilities increasingly orientated to this market, XLPE insulated cables

constitute a large proportion of UK production.

Cold Shrink Cable Joint – XLPE Single Core High Voltage Cable

Cant see this video? Click here  to watch it on Youtube.

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Reference:  Newnes Electrical Pocket Book – E.A. Reeves DFH(Hons), CEng, MIEE Martin J.

Heathcote BEng, CEng, FIEE