SE2051130A1 - Torsion-balanced electrical power cables - Google Patents

Abstract

There is provided an electric power cable comprising a conductor (201) having a central longitudinal axis (200), wherein the conductor (201) comprises stranded individual wires (203, 204) arranged in concentric wire layers (205, 206) around the central longitudinal axis (200), and wherein the lay direction of the stranded individual wires (203, 204) of one wire layer (205) is opposite to the lay direction of the stranded individual wires of another wire layer (206).

Description

TORSION-BALANCED ELECTRICAL POWER CABLES TECHNICAL FIELD

[0001] The present disclosure relates to the field of electric power cables and in particular to medium and high voltage electric power cables.

BACKGROUND

[0002] Electric power cables are regularly comprising, from the centre, at leastone conductor, at least one inner semiconducting layer, insulation, at least one outersemiconducting layer, a screen or sheath, and, externally, a jacket. This type of cableis normally manufactured by what is known as "triple extrusion", wherein all threeinner layers are extruded onto the conductor in a single process. The screen or sheathand the jacket are subsequently applied in a subsequent step. There are also mass- impregnated (MI) cables, i.e. cables with MI paper insulation.

[0003] The conductor of the electric power cable may be manufactured bystranding a number of individual wires. In stranded conductors, the individual wiresare arranged in concentric layers around the central longitudinal axis of the conductor, in a helical arrangement.

[0004] Electric power cables are used to transport electricity over long distances(several kilometres), both on land and on the bottom of the sea, i.e. submarine cables.When the cables are laid out, i.e. installed on the bottom of the sea, the cable issubjected to large tensions. However, if the conductor of the cable is not torsionallybalanced, the tension generates a torsion in the power cable which can be detrimental to the other layers of the electric power cable.

[0005] One problem is thus to design the electric power cable so that theconductor counteracts torsion when large forces are applied so that the cable does notbreak or kink, or form loops that can be pulled tight causing the minimum bending radius to be violated.

SUMMARY

[0006] One objective is to make available an electric power cable that is torsionally balanced.

[0007] There is thus provided an electric power cable comprising a conductorhaving a central longitudinal axis, wherein the conductor comprises strandedindividual wires arranged in concentric wire layers around the central longitudinalaxis, and wherein the lay direction of the stranded individual wires of one wire layer is opposite to the lay direction of the stranded individual wires of another wire layer.

[0008] That the stranded individual wires of one wire layer is opposite to the layerof the stranded individual wires of another wire layer allows for torsional balance. If,on the other hand, all individual wires of all wire layers are laid in the same direction,as exemplified in Fig 1, the conductor will be unevenly balanced leading to thattensional forces generate torsion. Imbalance can also occur if the majority of the wirelayers are unidirectional or if the lay length of the individual wire layers is not balanced.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Aspects and embodiments are now described, by way of example, with reference to the accompanying drawings, in which:

[0010] Fig 1 shows a schematic image of a concentric stranded conductor having acentral conductor wire, a first layer of a left-hand direction wire layer and a second layer of a left-hand direction wire layer.

[0011] Fig 2 shows a schematic image of a concentric stranded conductor having acentral conductor wire, a first layer of a right-hand direction wire layer and a second layer of a right-hand direction wire layer.

[0012] Fig 3 shows a radial cross section of an electric power cable.

DETAILED DESCRIPTION

[0013] The present disclosure will now be described more fully hereinafter with reference to the accompanying drawings.

[0014] Fig 1 illustrates a schematic image of a concentric stranded conductor 101having a central longitudinal axis 100, wherein the conductor 101 comprises a centralindividual wire 102 arranged along the central longitudinal axis 100 and strandedindividual wires 103, 104 arranged in concentric wire layers 105, 106 around thecentral longitudinal axis 100, wherein the lay directions of wire layers 105, 106 are left-hand directions. 3

[0015] With reference to Fig 2, the present disclosure provides an electric powercable comprising a conductor 201 having a central longitudinal axis 200, wherein theconductor 201 comprises stranded individual wires 203, 204 arranged in concentricwire layers 205, 206 around the central longitudinal axis 200, and wherein the laydirection of the stranded individual wires 203 of one wire layer 205 is opposite to the lay direction of the stranded individual wires 204 of another wire layer 206.

[0016] The conductor 201 may comprise a central individual wire 202 arranged along the central longitudinal axis 200.

[0017] When laying the wires, there are two different stranding directions: right- hand or left-hand, that also can be referred to as Z-lay and S-lay.

[0018] In a preferred embodiment, the stranded individual wires 203, 204 of anyadjacent pair of concentric wire layers 205, 206 are arranged with opposite laydirections. Typically, the individual wires of all wire layers are arranged in alternatinglay directions. Thereby, arranging the individual wires 203, 204 of the wire layers205, 206 of the conductor 201 so that a right-hand direction (Z-lay) is followed by aleft-hand direction (S-lay), and then another right-hand direction and so forth.

[0019] Typically, the electric power cable is a submarine power cable. The electric power cable can be a medium or a high voltage cable.

[0020] Typically, the stranded individual wires 203, 204 of the concentric wirelayers 205, 206 have a pitch. The pitch is also known as conductor lay length. Thepitch is the length measured along the longitudinal axis 200 of the conductor 201 required for a single strand of wire 203, 204 to make one complete turn (precisely 360 degrees) around the axis 200 of the conductor 201.

[0021] Typically, the pitch of individual wires 203, 204 of the wire layers 205, 206is determined by the number of individual wires 203, 204 in combination with thediameter of the individual wire layers 205, 206 so that torsional forces that arises ineach wire layer due to tensional forces are cancelled out. If the pitch is properlyadjusted, the torsional response from the different layers can cancel each other out toa certain extent. The torsional forces that arises in each wire layer due to tensionalforces are typically cancelled out to at least 25 %, such as at least 50 %, such as at least 75 %. 4

[0022] Typically, the stranded individual wires 203, 204 of the concentric wire layers 205, 206 have a pitch in the range of 0.1-1 m, such as 0.2-0.8 m.

[0023] A non-limiting example of the pitch of a stranded conductor according to the present disclosure is presented in Table 1.

Table 1. Example of the pitch of a stranded conductor.

Wire layer Lay length Right/ Left hand layCentral wire Straight (no lay length) Straight 1st layer 0.31 m Right-hand lay 2Hd layer 0.40 m Left-hand lay gfd layer 0.50 m Right-hand lay 4fl1 layer 0.59 m Left-hand lay 5fh layer 0.68 m Right-hand lay

[0024] The pitch of the stranded individual wires 203, 204 of a wire layer 205,206 may be shorter than the pitch of the stranded individual wires of a different wire layer.

[0025] The current-carrying conductor is either made of copper or aluminium orpartly of copper and partly of aluminium, such as copper where the temperature ofthe surroundings is less cold and aluminium where the temperature of thesurroundings is colder. One non-limiting example of such a scenario is a submarinecable having a copper conductor near land and aluminium conductor in the deepwaters. The centre wire conductor may also be made of aluminium and the outer wire layers may be copper. Typically, if the conductor is a hybrid conductor.

[0026] The conductor may be stranded from round wires. In such a case, the laidwires are compressed to reduce the interstices between the strands. Alternatively andpreferably, the conductor is composed from profiled wires, also known as shapedwires, that forms a keystone conductor also known as profiled wire conductor. In the stranding machine, the wires combine to a circular conductor, wherein filling grades of up to 96 % can be achieved. It is beneficial to have as high filling grade as possible since a high current density is achieved.

[0027] In one embodiment, the conductor 201 has a cross sectional area of at least600 mm2, such as at least 640 mm2, which is beneficial for higher voltages, typically above 150 kV.

[0028] Fig 3 shows a radial cross section of a non-limiting example of an electricpower cable having one power core. The cable 300 comprises one insulated strandedconductor 301 surrounded by an inner conducting layer 302, insulation 303, e.g.cross-linked polyethylene (XLPE) and an outer conducting layer 304. In order for thecable to be longitudinally watertight, one alternative is that the cable is provided witha semiconducting water swellable tape 305 with a surrounding metallic moisturebarrier sheath 306 made from e.g. lead or copper, and outside of the metallic sheath306 it is arranged an outer sheath 307 consisting of an insulating polymer layer, e.g.PE.

[0029] The electric power cable in Fig 3 is a non-limiting example of an electricpower cable where the stranded conductor according to the present disclosure can bearranged. The electric power cable can also have several power cores, such as threepower cores, in a case where the electric power cable is a three-phase electric power cable.

Claims (1)

1. An electric power cable comprising a conductor (201) having a centrallongitudinal axis (200), wherein the conductor (201) comprises strandedindividual wires (203, 204) arranged in concentric wire layers (205, 206)around the central longitudinal axis (200), and wherein the lay direction of thestranded individual wires (203, 204) of one wire layer (205) is opposite to the lay direction of the stranded individual wires of another wire layer (206). The electric power cable according to claim 1, wherein the individual wires (203,204) of the wire layers (205, 206) have a pitch that is determined by the numberof individual wires (203, 204) in combination with the diameter of theindividual wire layers (205, 206) so that torsional forces that arises in each wire layer due to tensional forces are cancelled out. The electric power cable according to claim 2, wherein the torsional forces thatarises in each wire layer due to tensional forces are cancelled out to at least 25 %, such as at least 50 %, such as at least 75 %. The electric power cable according to any one of the preceding claims, whereinthe conductor (201) comprises a central individual wire (202) arranged along the central longitudinal axis (200). The electric power cable according to any one of the preceding claims, whereinthe conductor (201) has a cross sectional area of at least 600 mm2, such as at least 640 mm2. The electric power cable according to any one of claims 1 or 2, wherein thestranded individual wires (203, 204) of the concentric wire layers (205, 206) have a pitch in the range of 0.1-1 m, such as 0.2-0.8 m. The electric power cable according to any one of the preceding claims, whereinthe pitch of the stranded individual wires (203, 204) of a wire layer (205, 206) is shorter than the pitch of the stranded individual wires of a different wire layer. The electric power cable according to any one of the preceding claims, whereinthe stranded individual wires of any adjacent pair of concentric wire layers are arranged with opposite lay directions. 7 9. The electric power cable according to any of the preceding claims, wherein the conductor is a keystone conductor.