An extruded high voltage cable generally comprises a conductor, a first conducting layer arranged around the conductor, an insulation layer comprising a polymer arranged concentrically around the first conducting layer and a second conducting layer arranged around the insulation layer. Usually there are also protective layers arranged concentrically around the second conducting layer. The polymer in the insulating layer generally is a cross-linked polymer, for example, polyethylene, ethylene-propylene rubber (EPM, EPDM) or silicone rubber. The conducting layers are usually made of one of the above mentioned polymers and carbon black. Sometimes a longitudinal semiconducting tape is arranged between the conductor and the first conducting layer to prevent material from the first conducting layer to be pushed into gaps between adjacent wires in the conductor. The longitudinal tape is, for example, made of polyester and carbon black and has a width that is a greater than the circumference of the conductor.
A conductor for an extruded high voltage cable is usually made either by arranging a plurality of metal wires in segments, a so-called segmented conductor, or by stranding together a plurality of metal wires in concentric layers, a so-called concentric lay conductor.
The geometry of a concentric lay conductor may, for example, be arranged according to the following: Six wires are firmly arranged around a single central wire in a first layer. A second layer comprising 12 wires is concentrically arranged around the first layer. A third layer comprising 18 wires is concentrically arranged around the second layer, etc. Each layer has six wires more than the underlying layer. The number of layers in a concentric lay conductor is decided with regard to the required current of the cable. There exist several standards regarding the number of wires in the different layers. Usually the wires of the second, third and each consecutive layer are helically wound around the preceding layer. Instead of a central wire with six surrounding wires in a first layer, a solid conductor or a hollow conductor may, for example, be used.
Arranging the wires in concentric layers creates interstices in the conductor, and the conductor is therefore compacted to increase the fraction of metal in the conductor cross section and to reduce the diameter of the conductor. This compacting is usually made for each layer of wires by a wire drawing type die or by rollers. The compacting could also be done for the complete conductor after the outermost layer has been laid.
For the manufacturing of an extruded high voltage cable the next step after the conductor has been made is to extrude the conducting layers and the insulation layer concentrically around the conductor. The compacted conductor is usually wound on a cable drum and transported to the extrusion line. In a step before the extrusion a longitudinal semiconducting tape may be folded around the conductor to prevent material from the inner conducting layer to be pushed into gaps between adjacent wires in the outer layers of the conductor. The extrusion is made in an extrusion line, where the conductor is fed into an extrusion head where usually the inner conducting layer, the insulation layer, and the outer conducting layer are extruded around the conductor in the same operation step.
During extrusion of the inner conducting layer it is important for the outer layer of the conductor to be tight, i.e. that there are no gaps between adjacent wires in the outer layer. This is especially the case for conductors with a large cross section, as for example between 800-3000 mm2. If a loose conductor, i.e. where the outer layer is not tight, is fed to the crosshead of the extrusion line, the outer layer of the conductor may be pushed backwards by the crosshead and when the diameter becomes too large for the crosshead, the outer layer will get stuck and a so-called “bird-cage” structure will be formed in a short time. If this is the case the extrusion line must be stopped immediately. The conductor is exposed to bending when it is transported from the wire drawing machine and after extrusion when the cable is wound on a cable drum.
Occasionally a loose conductor can be run through the extrusion line without an immediate problem, and without being discovered. The inner interface of the inner conducting layer may become irregular due to gaps between adjacent wires in the outer layer of the conductor. This may cause an increase of the electric field at the interface and may result in electrical breakdown at high voltage testing of the cable.
To minimize the risk of a loose conductor getting stuck in the extrusion die, the outer surface of the conductor is usually helically wound with a semiconducting tape before the extrusion, or larger tolerances is allowed for the crosshead in the extrusion line, than what would have been necessary if the risk of having a loose conductor would be very low. Large tolerances for the cross head might give a cable where the centering of the conductor in the cable is not as good as if the tolerances of the crosshead would not need to be increased due to the risk of a loose conductor.