Electric sector cables

Abstract

A high voltage electric cable having a longitudinal axis may include: a conductive core having a first cross sectional area; wherein the conductive core includes a solid, central conductor, and at least three solid, sector conductors stranded around the central conductor. The central conductor may have a second cross sectional area. A ratio of the second cross sectional area to the first cross sectional area may be of from 1/130 to 1/20.

Claims

1. A high voltage electric cable having a longitudinal axis, the cable comprising: a solid conductive core having a first cross sectional area; wherein the solid conductive core comprises: a solid, central conductor; and at least three solid, sector conductors stranded around the central conductor; wherein every sector conductor of the cable directly rests against the central conductor, wherein the central conductor has a second cross sectional area, wherein a ratio of the second cross sectional area to the first cross sectional area is from 1/130 to 1/20, wherein the first cross sectional area is greater than 1,000 square millimeters (mm.sup.2) and less than 4,000 mm.sup.2, and wherein a stranding step of the sector conductors is of from 1,200 millimeters (mm) to 1,800 mm.

2. The cable of claim 1, wherein the ratio of the second cross sectional area to the first cross sectional areas is from 1/65 to 1/20.

3. The cable of claim 1, wherein the ratio of the second cross sectional area to the first cross sectional areas is from 1/65 to 1/25.

4. The cable of claim 1, wherein the central conductor and the sector conductors are made of aluminum.

5. The cable of claim 1, wherein each one of the sector conductors rests against adjacent sector conductors.

6. The cable of claim 1, wherein all of the sector conductors have a same size and shape.

7. The cable of claim 1, wherein the conductive core comprises up to nine sector conductors.

8. The cable of claim 1, wherein the central conductor has a circular cross section.

9. The cable of claim 1, wherein each one of the sector conductors has a cross section with a substantially trapezoidal shape, and wherein the cross section has a major basis, a minor basis, a first side, and a second side.

10. The cable of claim 9, wherein the major basis is an arc of a circle.

11. The cable of claim 9, wherein the first side has a blocking recess, and wherein the second side has a blocking tooth whose shape and size matches a shape and size of the blocking recess.

12. The cable of claim 1, further comprising: water blocking material.

13. The cable of claim 1, wherein the cable is a submarine cable.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the invention will be better understood by reading the following detailed description, given by way of example and not of limitation, to be read with reference to the accompanying drawings, wherein:

(2) FIG. 1 is a cross section view of a high voltage electric sector cable according to an embodiment of the present invention;

(3) FIG. 2 is a cross section view of a high voltage electric sector cable according to another embodiment of the present invention; and

(4) FIG. 3 schematically shows an apparatus for manufacturing the cable of FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

(5) FIG. 1 shows a high voltage electric sector cable 1 (hereinafter simply referred to as cable) according to an embodiment of the present invention.

(6) The cable 1 comprises a conductive core 1a in turn comprising a central conductor 2 and a number M of sector conductors 3, all of the conductors 2, 3 being solid. The number M is preferably equal to or higher than three and equal to or lower than nine. By way of example, the cable 1 of FIG. 1 comprises six sector conductors.

(7) The central conductor 2 is a solid, elongated conductor having, in the present case, a substantially circular cross section. As mentioned above, the term solid means that the central conductor 2 is formed by a single conductive piece, whose cross section has no voids within its contour. The central conductor 2 has a longitudinal axis that substantially coincides with the longitudinal axis X of the cable 1 (i.e. the central conductor 2 is coaxial with the longitudinal axis X of the cable 1). The central conductor 2 is made of a conductive metal, preferably of aluminium, more preferably annealed aluminium.

(8) The sector conductors 3 are solid, elongated sector conductors. As mentioned above, the term solid means that each sector conductor 3 is formed by a single conductive piece, whose cross section has no voids within its contour. The cross section of each sector conductor 3 is substantially trapezoidal, and has a major basis 31, a minor basis 32, a first side 33a and a second side 33b. In the present case, the major basis 31 is curved, i.e. its profile is an arc of a circle. Such a configuration gives place to a cable 1 of substantially circular cross section. In the present case, the minor basis 32 is curved, i.e. is an arc of a cycle concentric with the major basis 31 and having substantially the same diameter as the central conductor 2. This latter minor basis 32 configuration provides an amount of voids in the conductive core 1a lower than the configuration wherein the minor basis is straight.

(9) Each sector conductor 3 is made of a conductive metal. Preferably, each sector conductor 3 is made of aluminium, more preferably of annealed aluminium. Advantageously, the use of aluminium to manufacture both the central conductor 2 and the sector conductors 3 allows increasing the flexibility of the cable 1. Advantageously, the use of annealed aluminium increases the elongation at break of cable 1.

(10) Preferably, all the sector conductors 3 have cross sections with the same shape and area. Each sector conductor 3 has a cross section with an angular width substantially equal to 360/M. For the cable 1 shown in FIG. 1, comprising six sector conductors 3, the angular width of the cross section of each sector conductor 3 is substantially equal to 60.

(11) The sector conductors 3 are arranged around the central conductor 2. In particular, the sector conductors 3 are stranded around the central conductor 2. The stranding step of the sector conductors 3 may be, for instance, of from 1200 mm to 1800 mm.

(12) The sector conductors 3 are preferably tightly stranded around the central conductor 2. In particular, each sector conductor 3 has its minor basis 32 that substantially rests against the central conductor 2. Moreover, each sector conductor 3 has its first side 33a and second side 33b that substantially rest against the adjacent sector conductors 3 situated on its opposite sides.

(13) This way, the central conductor 2 and the sector conductors 3 form a substantially solid void-free conductive core having a circular cross section. The overall cross section area of the conductive core 1a is substantially equal to the sum of the cross section area of the central conductor 2 and the cross section areas of all the sector conductors 3.

(14) Preferably, the cross section area of each sector conductor 3 is such that the overall cross section area of the conductive core 1a is larger than 1000 mm.sup.2, more preferably larger than 1600 mm.sup.2.

(15) The cross section area of the central conductor 2 is equal to a fraction of the overall cross section area of the conductive core 1a, this fraction being of from 1/130 to 1/20 of the overall conductive cross section area. For instance, if the cross section area of the conductive core is equal to 2500 mm.sup.2, the cross section area of the central conductor 2 is at least 1/65 of said cross section area.

(16) Advantageously, a central conductor 2 whose cross section area is comprised within the range defined above guarantees a suitable support for the stranding of the sector conductors 3 and, in the meanwhile, provides the cable 1 with suitable flexibility, even when the overall cross section area of the conductive core is larger than 1000 mm.sup.2.

(17) The cable 1 of the present invention can comprise water blocking material 40 (depicted as a dashed line partially contouring one sector only in FIG. 1 for sake of simplicity).

(18) The water-blocking material 40 can be, for example, water swellable gel or powder tapes. The water blocking material 40 may be based, for example, on cellulose derivative such as carboxymethylcellulose or hydroxypropyl cellulose, optionally admixed with silicon grease. Alternatively, the water-blocking material may be made of a thermo-stable loaded silicone polymer (and added with silicone oil in a percentage equal to 7%). Advantageously, such polymer may also act as a lubricant and it has a substantially constant viscosity over temperature.

(19) The water-blocking material 40 advantageously fill the voids that may be accidentally formed between the central conductor 2 and the sector conductors 3 and between adjacent sector conductors 3 during the stranding operation. This way, accidental penetration of moisture and/or water into the cable 1 is advantageously prevented.

(20) The cable 1 comprises a number of tubular layers surrounding the cable conductive core 1a. Such tubular layers may comprise e.g. insulating layers, semiconductive layers and metal shields. In FIG. 1, for sake of simplicity, a single tubular layer 4 surrounding the sector conductors 3 is shown. The tubular layer 4 is made of a suitable semi-conductive polymeric material (for example polyethylene-based mixture containing carbon black).

(21) The cable 1 according to the present invention has a number of advantages.

(22) Firstly, the solid central conductor and the solid sector conductors substantially abutting allow minimizing the presence of voids within the conductive core of the cable, and therefore allow increasing the conductive area and the electric current density that the cable may transport.

(23) Further, the use of a central conductor having a relatively small cross section area allows, together with the use of aluminium, increasing the flexibility of the cable even when the cross section area of the cable conductive core is larger than 1000 mm.sup.2.

(24) The jointing of the cable according to the present invention is advantageously very simple. When cable lengths must be joined during installation, the junction between conductors, made for example by welding, is very simple, because it requires welding a very small number of conductors (M+1) in comparison to, for example, the number of conductive strands of the cable of U.S. Pat. No. 4,550,559. Advantageously, when sector and central conductors of the cable of the invention are made of aluminium, cold welding technique may be used, which is particularly convenient when submarine cables are considered.

(25) In FIG. 2, the cross section of a cable 1 according to another embodiment of the present invention is shown.

(26) Cable 1 is substantially similar to the cable 1 shown in FIG. 1. However, differently from the cable 1, in the cable 1, the first side 33a of each sector conductor 3 has a blocking recess 34, while the second side 33b of each sector conductor 3 has a blocking tooth 35. Both the blocking recess 34 and the blocking tooth 35 of each sector conductor 3 extend longitudinally along the entire longitudinal length of the sector conductor 3.

(27) Shape and size of the blocking recess 34 match shape and size of the blocking tooth 35.

(28) Blocking teeth 35 and blocking recesses 34 are suitable for hooking each sector conductor 3 to the adjacent ones. In particular, the blocking recess 34 of a sector conductor 3 engages with the blocking tooth of the sector conductor facing the first side 33a of the sector conductor 3. Similarly, the blocking tooth 35 of the same sector conductor 3 engages with the blocking recess of the sector conductor facing the second side 33b of the sector conductor 3.

(29) Advantageously, blocking recesses 34 and blocking teeth 35 can provide the cable 1 with an increased stability, especially when the cable is bent.

(30) FIG. 3 schematically shows an apparatus for manufacturing the cable of FIG. 1.

(31) The apparatus of FIG. 3 comprises a conductor supply tool 100, an optional water-blocking material supply tool 200, a stranding tool 300, an extruder 400, a cooling tool 500, and a collecting spool 600, all connected in cascade. The various parts (tools and extruder) of the apparatus are concatenated each other so as to form a continuous manufacturing line (or plant). The apparatus may comprise further devices not shown in FIG. 3 since they are not relevant to the present description.

(32) The conductor supply tool 100 preferably supplies M+1 conductors, namely the central conductor 2 and the M sector conductors. The conductor supply tool 100 may comprise spools from which the conductors are unwound. The central conductor 2 and the sector conductors 3 are then supplied to the stranding tool 300.

(33) The stranding tool 300 comprises rotating devices, not shown in FIG. 3, that strand the sector conductors 3 around the central conductor 2. As an example, the stranding tool 300 may strand the sector conductors 3 according to a helix pattern thereby forming the conductive core of the cable.

(34) Optionally, during the stranding operation, the stranding tool 300 receives water-blocking material from the water-blocking material supply tool 200, e.g. gel or powder of water-swellable material. For instance, the water-blocking material may be interleaved with the sector conductors 3 just before they are stranded around the central conductor 2, thereby forming layers of water-blocking material 40 as shown in FIG. 1.

(35) The conductive core then passes through the extruder 400. The extruder 400 preferably comprises a supply unit 400b and an extrusion head 400a. The supply unit 400b preferably supplies the extrusion head 400a with the polymeric material suitable for providing the core with insulating and semiconductive layers, said material being well-known to the skilled person.

(36) After extrusion, the layer/s of polymeric material are cooled down through the cooling tool 500. After cooling, the cable 1 is wound on the collecting spool 600.

(37) Tests were carried out on a so produced cable. In particular, some mechanical tests were carried on the cable in order to analyze its performance when subject to bending.

(38) A conductive core comprising six sector conductors and a central conductor was manufactured. The conductive core was surrounded by a 50% Boston tape and a thermoshrinking layer. The overall cross section of the resulting cable was about 2,500 mm.sup.2 and the central conductor cross section was of about 78 mm.sup.2. The ratio between the area of the central conductor cross section and the area of the overall cross section is then equal to about 1/30. The length of the manufactured cable was about 6,000 mm with a stranding step of 1,600 mm.

(39) The cable was bent on a spool having a diameter equal to about 4.2 m. One end of the cable was fixed to the spool so that it could not rotate about its axis during bending. A dynamometer suitable for measuring the bending stress was fixed to the other end of the cable.

(40) The spool was made to rotate so that the cable was made to bend about the spool. During bending of the cable, the dynamometer measured a bending stress ranging between about 30 Kg and 50 Kg. During bending, the structure of the cable did not split, i.e. the sector conductors remained in reciprocal tight contact and in tight contact also with the central conductor along the whole bent portion of the cable. After bending, the cable was straightened up. The cable exhibited sufficient flexibility to be substantially straightened up with little tensile stress.