Production

This section describes the way in which cables are manufactured, from raw materials to finished lengths of cable.

Click here to see cross section of a typical unarmoured high voltage power cable

This picture shows, from the centre, the copper conductive core, the white Cross-Linked Polyethylene insulator (XLPE), the black semiconductor layer (for high voltage electrical performance, an aluminium moisture-resistant layer and the outer high density polyethylene protective sheath

An unarmoured 400kV underground cable

How are XLPE cables made?

Cables are made in extrusion lines which take copper wire, plastic granules and other materials and in a continuous process convert them into the finished cable.

The process is:

• Copper wire is bunched and twisted into the correct size of conductor.
• The wire is fed into the head of an extrusion machine, which melts the plastic granules, shapes and solidifies them around the copper in a thick insulation layer of carefully controlled thickness.
• A sheathing operation welds a continuous layer of aluminium sheet around the insulator to act as an electrical shield and to prevent water from diffusing into the plastic.
• A final extrusion step covers the metallic screening sheath with a layer of tough protective plastic.
• Optional extra layers of steel wire armour and protective plastic may be added on top.

When these steps have taken place, the whole cable is passed through a heat treatment at over 200C. This high temperature activates the additives incorporated into the plastic granules at the plastic manufacturing plant, causing them to create strong bonds between the polymer molecules. These bonds, or cross-links prevent the molecules from sliding over each other and melting. This cross-linked polyethylene (XLPE) is stable under high service temperatures and allows the cable to carry high current loads without the conductor shifting inside the hot plastic insulation.

Why are cables made in short lengths?

In theory the cheapest way to give a length of installed underground cable would be to produce it in one long length. This would avoid the requirement for any joints in the system, other than at the very ends. However there is a limit to the length of cable which can be transported around the road network and handled at the other end. This limit is imposed by the size of cable reel which road tunnels, bridges and other obstructions can let past. An additional factor is that within this size restriction, the thicker cables can only be coiled up a certain amount so as to prevent damage from over bending. This means that large diameter cable can only be supplied in lengths of 600 meters or so.

What are the raw materials used in the cable?

The main raw materials for the cable are the copper conductors, the Cross-Linked Polyethylene Insulator (XLPE), the metallic screen and the outer protective cable sheath.

The most important and complex of these materials is the XLPE insulation layer. While polyethylene has excellent insulating characteristics, ordinary grades are too soft at high temperatures to be used in high voltage power cables. Over time, the copper conductor might drift in the cable, reducing the insulation thickness and compromising the ability of the cable to resist shorting out. If the polyethylene molecules are cross-linked, these linkages prevent the molecules from moving, so melting or softening is prevented, keeping the copper conductor in the same place as it started out. In contrast to normal polyethylene granules, XLPE granules contain special additives, usually peroxides, for generating the molecular cross-links. During initial moulding processes, these additives are not activated, but during later heat treatments at high temperatures they activate and set the plastic into a flexible but non-meltable form.

A similar process is used in tire manufacture, where the rubber is moulded under heat and only after a certain period at temperature does the cross-linking process set the rubber in its final shape.

Click here to see schematic of the process for producing XLPE granules.

Click here to see a plant for producing polyethylene for blending with additives to produce a curable flexible polymer - Cross-Linked Polyethylene (XLPE).

By building a fibre optic temperature sensor into the cable structure during production, the fire sensor is very close to the heated conductors and has excellent sensitivity. However the fibre optic must also be spliced at every cable joint, adding to the cost and complexity of the joining process.

Click here to see pictures of cables with fibre optic temperature sensors built in during the production process.

By installing a fibre optic temperature sensor alongside a cable, a continuous run of fibre optic can be installed along the project. This saves installation time and costs and improves optical performance. However the fact that the sensor is outside of the cable means that the sensitivity to short-term temperature events is less good and the external mount means that the cable cannot be dragged through ducts during installation.

Click here to see a fibre optic temperature sensor in a tubular duct running alongside a cable.

(back to top)