GB2029630A - Improvements in or relating to a conductor for a power cable - Google Patents

Abstract

A conductor for an insulated power cable comprises an inner core (1) of twisted metal wires and a tubular mantle 2 made of aluminium or aluminium alloy, having a smooth outer surface. The cross sectional area of the mantle 2 is 5 to 50 per cent of the total cross sectional area of the conductor. The mantle 2 is arranged around the inner core 1 with a clearance corresponding to a loose slip fit. Insulating layer 4, semiconductive layers 3, 5, metal layer 6 and plastics layer 7 are also provided. The mantle may be formed by extrusion or welding an aluminium strip. The core 1 may be of Al or alloys thereof and reinforced with steel wires. A sector shaped conductor is also disclosed (Figure 2 not shown). <IMAGE>

Description

SPECIFICATION Improvements in or relating to a conductor for a power cable This invention relates to a conductor for a power cable and more particularly to a conductor for an insulated low or high voltage power cable comprising an insulating layer surrounding said conductor.

It has been proposed to provide a power cable in which the conductor merely comprises twisted wires. Such a prior proposed cable suffers from the disadvantage of a very irregular and un-even surface, which, among other things, may cause a harmful corona phenomen in high-voltage cables.

It is a commonly recognised disadvantage of tubular and, in particular, solid conductors that the indictive resistance, when using alternating current, is somewhat higher than in a conventional conductor of twisted wires. This is especially true in the case of a conductor having a large cross-sectional area.

However, the relative significance of this indicative resistance can be reduced by utilising a tube which has a relatively thin wall.

According to one aspect of this invention there is provided a conductor for an insulated power cable which comprises an insulating layer and an electric conductor located inside said layer, said conductor comprising an inner core twisted of metal wires and a tubular aluminium oralluminium alloy mantle enclosing said inner part and having a substantially smooth outer surface and a cross sectional area which is 5 to 50 per cent of the total cross sectional area of the electric conductor, the mantle being mounted around the inner core with a clearance at least approximately corresponding to a loose slip fit.

Preferably the metal mantle is made of aluminium by means of extrusion.

Alternatively the metal mantle is made of aluminium strip by means of welding.

Conveniently said clearance is of the order of 0.1 mm.

Advantageously said metal wires are of substantiallythe same metal as the mantle.

According to another aspect of this invention there is provided an insulated power cable comprising a conductor located within an insulating layer, said conductor according to said one aspect.

A preferred embodiment of the invention comprises a conductor with a tubular outer layer which has a continuous smooth surface. Within the tube, there is provided any core such as a sealed core of twisted aluminium or copper wires. The tube and the inner core have a small tolerance in the manufacturing dimensions so that the core can slip within the tube.

By means of the invention, nearly all advantages of a solid conductor are achieved, but the flexibility of the conductor is better. The surface integrity of the conductor is of advantage when coating the conductor with plastics or a corresponding insulating material to produce a cable such as a high voltage cable with a conductor shield and insulation, or a low voltage cable with an insulation. When coating a conventional twisted conductor, plastics or a similar insulating material can penetrate between the wires which often results in the cable being damaged when the conductor is loaded. Another advantage resulting from a reliable continuous surface for the mantle is apparent when the cable is eventually damaged: as the water cannot reach the conductor at other points only the damaged point need by repaired.If the mantle of the conductor is damaged water can get inside the conductor but cannot escape elsewhere to damage the insulation. Also in this case, repairs effected at the point of damage will be sufficient. The smoothness of the conductor surface, on the other hand, offers the advantage that in a high voltage cable the conductor shield to be arranged on it can be thinner than when using a relatively uneven twisted conductor. Correspondingly, in a low voltage cable, the insulation to be arranged immediately on the conductor will have a smooth surface underneath it which improves the reliability of the cable and facilitated its insulation.

When bendng the conductor, the tubular mantle, moreover, makes it easier for the conductor to yield to the bending because the manufacturing method itself, usually extrusion involves the use soft aluminiium or alloy. In addition, the clearance between the tube and the inner part permits a relative sliding motion of the parts during bending. At the same time the tube eliminates the risk that adjacent wires in the surface layer of the twisted conductor will damage the plastics on the cable as they slide during bending.

In order that the invention may be more readily understood and so that further features thereof may be appreciated the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a cross sectional view of an electric conductor of a cable according to the invention, having a circular cross section; Figure 2 is a cross sectional view of an electric conductor of a cable according to the invention, having a cross section of sector shape; and Figure 3 is a cross sectional view of a cable according to the invention incorporating the conductor of Figure 1.

The inner core 1 of a conductor is formed of aluminium wires which are twisted together, and can be unsealed or sealed in order to save space. On the inner core 1 there is extruded a tubular mantle 2 of aluminium such that a small gap is left between the mantle and the inner core or such that, at the most, the mantle is lightly pressed against the inner core 1. In any event the outer mantle must be a looase slip fit on the inner core. Instead of extrusion, any other appropriate method known may be em ployed to mount the tubular mantle in position so that the mantle will have a continuous (i.e. non perforated) smooth outer surface. For example, various tube welding methods may be used in which aluminium strip is used as starting material.

The tubular mantle 2 should be as thin as possible but not so thin as to become wrinkled on being mounted in position. On the other hand, the tubular mantle 2 must forum a substantial portion, for example, 5 to 50 per cent preferably 20 to 50 per cent, of the total cross sectional area of the con ductor.

Figure 2 illustrates a second embodiment of the invention in which the core 1 and the tubular mentle 2 have a cross section which is non-circular, in this case it is section shaped. Conductors in accordance with the invention can have any appropriate cross section.

Figure 3 illustrates a 110 kV cable in which the conductor core 1 is made of aluminium and has a total metal cross sectional area of 1400 mm2. The conductor core 1 diameter is 44 mm. The average thickness of the mantle 2 is about 3 mm (lower limit 2.75 mm). The aluminium mantle 2 is enclosed by a semi-conducting conductor shield 3 having a thickness of about 1 mm. The thickness of an insulation layer4surrounding this layer3 is about 15 mm. The insulation layer 4 itself is surrounded by a 0.5 to 2.0 mm thick semi-conducting insulation shield 5, a 3 mm thick contact protection or similar metal layer 6, and outermost a 3.5 thick plastics sheath 7.

The clearance between the mantle 2 and the inner core 1 is of the order of 0.1 mm, which corresponds to a very loose slip fit.

Because the outer surface of the mantle 2 is smooth, there will occur no harmful corona phenomen in this surface.

Naturally, the structure of the inner part of the conductor may differ from that described above.

Thus, it can be, for example, steel reinforced. Of course, appropriate aluminium alloys may be used instead of aluminium.

Claims (11)

1. A conductor for an insulated power cable which comprises an insulating layer and an electric conductor located inside said layer, said conductor comprising an inner core twisted of metal wires and a tubular aluminium or aluminium alloy mantle enclosing said inner part and having a substantially smooth outer surface and a cross sectional area which is 5 to 50 per cent of the total cross sectional area of the electric conductor, the mantle being mounted around the inner core with a clearance at least approximately corresponding to a loose slip fit.

2. A conductor as claimed in claim 1, wherein the metal mantle is made of aluminium by means of extrusion.

3. A conductor as claimed in claim 1, wherein the metal mantle is made of aluminium strip by means of welding.

4. A conductor as claimed in any one of the preceding claims wherein said clearance is of the order of 0.1 mm.

5. A conductor as claimed in any one of the preceding claims wherein the cross sectional area of the metal mantle is 20 to 50 per cent of the total cross sectional area of the electric conductor.

6. A conductor according to any one of the preceding claims wherein said metal wires are of substantially the same metal as the mantle.

7. An insulated power cable comprising a conductor located within an insulating layer, said conductor comprising a conductor according to any one of claims 1 to 6.

8. A conductor substantially as herein described with reference to and as shown in Figure 1 of the accompanying drawings.

9. A conductor substantially as herein described with reference to and as shown in Figure 2 of the accompanying drawings.

10. An insulated power cable substantially as herein described with reference to and as shown in Figure 3 of the accompanying drawings.

11. Any novel feature or combination of features disclosed herein.