TECHNICAL FIELD

The present disclosure generally relates to power cables. In particular it relates to power cables having a conductive core having sections made of different metal material.

BACKGROUND

A consideration for high voltage cable design is that there are often hot spots along part of the high voltage cable route. Therefore, the entire high voltage cable is designed according to the worst conditions that occur along the high voltage cable route. This would normally mean that the diameter of the entire cable had to be dimensioned based on the worst conditions, resulting in over- dimensioning of the cable, and high costs associated therewith.

A solution to the above problem is to adapt the conductive core of a high voltage cable. The same power transfer capacity may thereby be achieved along the entire length of the cable. For this purpose, a conductive core may comprise sections being made of different metal material, for example an aluminium section and a copper section. Copper for example has a higher conductivity than aluminium, and a copper section may thus be utilised along a portion of the high voltage cable which would otherwise require a larger diameter aluminium section.

Attachment of the different sections have traditionally been made by means of for example a sleeve or collar enclosing that part of the cable where the cable transitions from one metal material to another. These joints are however stiff or rigid.

SUMMARY

In view of the above, an object of the present disclosure is to provide a power cable which solves or at least mitigates the problems of the prior art. Hence, according to a first aspect of the present disclosure there is provided a power cable comprising: a conductive core, and a sheath enclosing the conductive core, wherein the conductive core comprises a first section made of a first metal material and a second section made of a second metal material, and wherein the conductive core has a joint member having a first end made of the first material and a second end made of the second material, the first end connecting with the first section and the second end connecting with the second section, wherein the first end and the second end of the joint member have been joined thermally. Installing a stiff joint of the prior art is usually combined with very significant costs since for example a laying ship and crew have to utilise several days for installation. By means of a joint member that has a first end and a second end which are thermally connected, a joint which is non-rigid compared to sleeve or collar solutions may be obtained, and which enables faster installation time.

According to one embodiment the first end of the joint member is welded to the first section of the conductive core.

According to one embodiment the second end of the joint member is welded to the second section of the conductive core. According to one embodiment the first metal material is aluminium and the second metal material is copper.

According to one embodiment the first end and the second end of the joint member are friction welded.

According to one embodiment the power cable is a high voltage power cable. According to one embodiment the power cable is a subsea cable.

According to a second aspect of the present disclosure there is provided a method of manufacturing a power cable, comprising: a) providing a joint member having a first end made of a first metal material and a second end made of a second metal material, the first end and the second end being joined thermally, b) providing a first conductive core part made of a first metal material, c) providing a second conductive core part made of a second metal material, d) joining the first conductive core part with the first end of the joint member, e) joining the second conductive core part with the second end of the joint member, thereby obtaining a conductive core having a first section defined by the first conductive core part and a second section defined by the second conductive core part, and f) providing a sheath to enclose the conductive core.

According to one embodiment the first end and the second end of the joint member are joined by means of friction welding.

According to one embodiment the first conductive core part is joined with the first end of the joint member by means of welding.

According to one embodiment the second conductive core part is joined with the second end of the joint member by means of welding. According to one embodiment the first metal material is aluminium and the second metal material is copper.

According to one embodiment the power cable is a high voltage power cable. According to one embodiment the power cable is a subsea cable.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, etc. are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, etc., unless explicitly stated otherwise. Moreover, the steps of the method need not necessarily have to be carried out in the indicated order unless explicitly stated. BRIEF DESCRIPTION OF THE DRAWINGS

The specific embodiments of the inventive concept will now be described, by way of example, with reference to the accompanying drawings, in which:

Fig. l schematically depicts an example of power cable prior to assembly of different sections; Fig. 2a depicts the power cable in Fig. ι in an assembled state;

Fig. 2b shows another example of a power cable in an assembled state; and

Fig. 3 is a flowchart of a method of manufacturing a power cable such as the power cable depicted in Figs 2a or 2b.

DETAILED DESCRIPTION

The inventive concept will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplifying

embodiments are shown. The inventive concept may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the inventive concept to those skilled in the art. Like numbers refer to like elements throughout the description.

Fig. la depicts a portion of a power cable 1 prior to assembly thereof. The power cable 1 comprises a sheath (not shown), a conductive core la having a first section lb made of a first metal material, a second section IC made of a second metal material, and a joint member id. The sheath, which may be made of a polymer material for example, is arranged to enclose the

conductive core la for the purpose of protection thereof. Moreover, the power cable may comprise one or more layers of armour wires protecting the conductive core. Depending on the number of electrical phases, a number of such conductive cores may be arranged within the sheath, for example one conductive core in case the power cable is a DC cable, and three conductive cores in case the power cable is a three-phase AC cable. However for the purpose of illustration, an example having only one conductive core la is shown in Fig. l.

The joint member id is arranged to join the first section lb with the second section ic. To this end, the joint member id has a first end le made of the first metal material and a second end if made of the second metal material. The first end le and the second end if are joined thermally, for example by means of welding such as friction welding. Thus, initially, the joint member comprises two separate pieces, which are then thermally joined. The thermal joint is schematically shown by means of the dashed lines indicated by reference numeral lg. Friction welding is obtained by rotating one of the first end le and the second end if relative to the other end. It would typically not be possible to utilise friction welding directly for the first section lb and the second section ic of the conductive core la without the joint member id because of the length of these section. The first section lb is joined with the first end le. The first section lb and the first end le are both made of the first metal material, and can thus easily be joined for example by heating, e.g. by means of welding.

The second section ic is joined with the second end if. The second section ic and the second end if are both made of the second metal material, and can thus easily be joined for example by heating, e.g. by means of welding.

The first metal material may for example be aluminium, and the second metal material may for example be copper. It is however envisaged that other conductive metal materials could be used for the components of the conductive core la. Fig. 2a depicts the power cable ι in an assembled state, again without the sheath visible.

Fig. 2b schematically depicts a portion of a power cable 2, with its sheath removed to expose a conductive core 2a. The power cable 2 thus comprising a conductive core 2a having a first section 2b which has a first diameter, and a second section 2c which has a second diameter, larger than the first diameter. The conductive core 2a further comprises a joint member 2d which has a first end 2e joined with the first section 2b, having a diameter essentially corresponding to the first diameter, i.e. the diameter of the first section 2b. The joint member 2d has a second end 2f joined with the second section 2c, having a diameter essentially corresponding to the second diameter, i.e. the diameter of the second section 2c. The joint member 2 may thus enable the connection of the first section 2b and the section 2b in case they have different diameters. Similarly to the example described with reference to Fig. 1, the first end 2e and the second end 2f are joined thermally, for example by means of friction welding. Moreover, the first end 2e is joined with the first section 2b and the second end 2f is joined with the second section 2c in the same manner as previously described. According to one variation, the first end 2e and the second end 2f are first thermally joined, e.g. by means of friction welding. Both the first end 2e and the second end 2f may have the same diameter when they are thermally joined, i.e. corresponding essentially at least to the second diameter which is the larger of the two diameters of the first section 2b and the second section 2c of the conductor core 2a. After joining the first end 2e and the second end 2f thermally, the joint member 2d may be subjected to a turning process in a lathe such that the first end 2e obtains a diameter at its end face essentially corresponding to the first diameter. The joint member 2d may hence have a tapering portion, tapering in a direction from the second end 2f to the first end 2e. A method of manufacturing a power cable such as power cable 1 or 2 will now be described with reference to Fig. 3. In a step a) a joint member having a first end made of a first metal material and a second end made of a second metal material are provided. The first end and the second end are joined thermally, for example by means of friction welding as described above.

In a step b) a first conductive core part made of a first metal material is provided.

In a step c) a second conductive core part made of a second metal material is provided. In a step d) the first conductive core part is joined with the first end of the joint member.

In a step e) the second conductive core part is joined with the second end of the joint member. A conductive core having a first section defined by the first conductive core part and a second section defined by the second conductive core part is thereby obtained.

In a step f) a sheath is provided to enclose the conductive core.

It is envisaged that the power cable presented herein may be utilised in for example subsea applications e.g. power transmission or power distribution. The power cable may for example be utilised for landfalls for subsea cables. The inventive concept has mainly been described above with reference to a few examples. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended claims.