Method of building insulation system of power cable

Abstract

To provide an insulation system of a power cable in which damage to the insulation system is suppressed.SOLUTION: There is provided a method of building an insulation system around an axial section of a conductor 5 of a power cable 1, the insulation system including an inner semiconducting layer 7a, 7b arranged around the conductor 5, an insulation layer 9a, 9b, and an outer semiconducting layer 11a, 11b. Further, an axial section is provided between a first insulation system section and a second insulation system section of the insulation system. A tape is wound around the conductor 5 along the axial section in a plurality of layers to form a plurality of layers of tape connecting with the first insulation system section and the second insulation system section, and the plurality of layers of tape is heated to melt and fuse the plurality of layers of tape to form an insulation system layer 17, 19 between the first insulation system section and the second insulation system section, and in a mid-section between lateral edges of the tape, the tape has a largest thickness, and wherein the thickness decreases from the mid-section towards both lateral edges.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates generally to power cables. [Background technology]

[0002] An insulation system for a power cable in a power grid infrastructure includes an inner semiconducting layer, an insulating layer disposed outside the inner semiconducting layer, and an outer semiconducting layer disposed outside the insulating layer.

[0003] Cable lengths of power cables having extruded insulation systems may be joined with so-called factory splices, which are often not easily detectable from the outside, for example because the splice of the insulation system is made before the outermost sheath or serving is provided around the cable core.

[0004] Typically, when performing factory splicing, the insulation systems of the two cable lengths are pencil-shaped, i.e. tapered and pointed towards each other after the conductors are spliced. Tape is then wrapped around the conductors to form the three layers mentioned above. The tape is cured by a heating process, for example as disclosed in JP 61-243680.

[0005] A drawback of this process is that edges between the inner or outer semiconducting layer and the insulating layer may form during the heating process, which may locally increase the electric field and result in discharges within the insulator that may damage or even lead to the breakdown of the insulation system. Summary of the Invention

[0006] It has been found that the tape forming the insulating layer softens more slowly during the heating process than the inner and outer semiconductor layers, so that the edges of the tape forming the insulating layer, which are usually arranged overlapping, are pressed into the inner semiconductor layer during the heating process, with the result that the edges between the insulating layer and the inner semiconductor layer locally increase the electric field when the cable is in use.

[0007] A general object of the present disclosure is to provide a method for constructing an insulation system for a power cable that solves or at least mitigates problems in the prior art.

[0008] Thus, according to a first aspect of the disclosure, there is provided a method of constructing an insulation system around an axial portion of a conductor of a power cable, comprising: a) installing a power cable, the power cable including the conductor and an insulation system disposed around the conductor, the insulation system including an insulation system layer including an inner semiconducting layer disposed around the conductor, an insulating layer disposed around the inner semiconducting layer, and an outer semiconducting layer disposed around the insulating layer, the power cable including at least an axial portion between a first insulation system portion and a second insulation system portion of the insulation system that is free of the outer semiconducting layer; a) placing a tape around the conductor along the axial portion in a plurality of layers to form a plurality of layers of tape connecting the first insulation system portion and the second insulation system portion; and c) heating the plurality of layers of tape to melt and fuse the plurality of layers of tape to form an insulation system layer between the first insulation system portion and the second insulation system portion, wherein the tape has a width defined by a distance between its side edges, the tape has a middle portion between the side edges, where the tape has a maximum thickness, and the thickness of the tape decreases from the middle portion toward both side edges.

[0009] Due to the reduction in tape thickness towards the side edges, the risk of edge formation during heating is significantly reduced. At the same time, the tapered shape provides a high fill factor.

[0010] According to one embodiment, the thickness decreases smoothly towards both side edges.

[0011] According to one embodiment, the thickness decreases symmetrically towards the side edges, the symmetry here being relative to the central longitudinal axis of the tape, i.e. along the axial extension of the tape.

[0012] According to one embodiment, in the cross section of the tape, the tape has a convex shape.

[0013] According to one embodiment, the convex shape is biconvex or plano-convex.

[0014] According to one embodiment, in step b), in each layer of the plurality of layers, adjacent turns are arranged overlapping each other.

[0015] According to one embodiment, in each layer of the plurality of layers, each turn of tape is axially shifted along the axial direction of the conductor relative to every turn of tape in any adjacent layer of tape.

[0016] According to one embodiment, each turn is axially shifted by at least 20% of the width of the tape, such as at least 30%, such as at least 40%.

[0017] According to one embodiment, in step c) the tape is heated under pressure above atmospheric pressure.

[0018] One embodiment includes providing an inner semiconductor layer around the conductor prior to step b), and in step b) the wrapping of the tape is around the inner semiconductor layer.

[0019] According to one embodiment, the insulation system layer is an insulating layer or an outer semiconducting layer.

[0020] Each of the three layers of the insulation system, i.e. the inner semiconducting layer, the insulating layer and the outer semiconducting layer, is preferably subjected to an individual heating step. Thus, after the tape is wrapped to form the inner semiconducting layer, the tape forming the insulating layer is heated and cured before the tape forming the outer semiconducting layer is wrapped around the insulating layer. Furthermore, the tape forming the insulating layer is heated and cured before the tape forming the outer semiconducting layer is wrapped around the insulating layer. Thus, the risk of edge formation is usually with respect to the underlying insulation system layer.

[0021] According to one embodiment, the tape comprises a polymeric material.

[0022] According to one embodiment, the polymeric material includes one of polyethylene, polypropylene, ethylene propylene rubber, EPR, and ethylene propylene diene monomer rubber.

[0023] According to one embodiment, the construction of the insulation forms part of a cable joining or repair operation of the insulation system of a power cable.

[0024] According to one embodiment, the power cable is a high voltage power cable.

[0025] The power cable may be an AC or a DC power cable.

[0026] The power cable may be a submarine power cable or an underground power cable.

[0027] According to a second aspect of the present disclosure, a power cable obtainable by the method of the first aspect is provided.

[0028] In general, all terms used in the claims should be interpreted according to their ordinary meaning in the art unless otherwise expressly defined herein: "All references to a / an / the element, apparatus, component, means, etc. should be interpreted without limitation as referring to at least one instance of the element, apparatus, component, means, etc., unless otherwise specified."

[0029] Specific embodiments of the inventive concept will now be described, by way of example, with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0030] [Figure 1] 1 shows a schematic longitudinal cross section of a power cable with a splice; [Diagram 2] 1 is a flow chart of a method of constructing an insulation system around an axial portion of a conductor of a power cable. [Figure 3a] An example of a tape is shown diagrammatically. [Figure 3b] 1 shows a cross section of an example tape. [Figure 3c] 1 shows a cross section of an example tape. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] The inventive concept will now be more fully described below with reference to the accompanying drawings, in which exemplary embodiments are shown. However, the inventive concept may be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided as examples 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 reference numerals refer to like elements throughout the description.

[0032] FIG. 1 shows diagrammatically an example of a longitudinal section of a power cable 1 with a factory splice 3 .

[0033] The power cable 1 comprises a first portion 1 a and a second portion 1 b joined by a factory joint 3 .

[0034] The first part 1a includes a first conductor portion 5a, an inner semiconducting layer 7a, also called a conductor screen, arranged around the first conductor portion 5a, an insulating layer 9a, arranged around the inner semiconducting layer 7a, and an outer semiconducting layer 11a, also called an insulating screen. The inner semiconducting layer 7a, the insulating layer 9a, and the outer semiconducting layer 11a arranged around the first conductor portion 5a form a first insulation system part of the power cable 1.

[0035] The first insulation system portion is an extruded insulation system. Each of the inner semiconducting layer 7a, the insulating layer 9a, and the outer semiconducting layer 11a may include a polymeric material such as polyethylene, polypropylene, ethylene propylene rubber, EPR, or ethylene propylene diene monomer rubber (EPDM).

[0036] Similarly, the second portion 1b comprises a second conductor portion 5b, an inner semiconductor layer 7b arranged around the second conductor portion 5b, an insulating layer 9b arranged around the inner semiconductor layer 7b, and an outer semiconductor layer 11b. The inner semiconductor layer 7b, the insulating layer 9b, and the outer semiconductor layer 11b arranged around the second conductor portion 5b form a second insulation system portion of the power cable 1.

[0037] The second insulation system portion of the second portion 1b is an extruded insulation system. Each of the inner semiconducting layer 7b, the insulation layer 9b, and the outer semiconducting layer 11b can include a polymeric material such as polyethylene, polypropylene, ethylene propylene rubber, EPR, or ethylene propylene diene monomer rubber (EPDM).

[0038] The first conductor portion 5a and the second conductor portion 5b are bonded by way of example such that the conductor portions 5a, 5b are connected via a conductor joint 13 to form a single conductor 5. The conductor portions 5a and 5b may be thermally bonded, for example by welding.

[0039] The first insulation system portion and the second insulation system portion are joined to form the insulation system of the power cable 1 .

[0040] The first insulation system part and the second insulation system part are joined using tape to form an inner semiconductor layer 15 connecting the inner semiconductor layers 7a, 7b of the first insulation system part and the second insulation system part, to form an insulating layer 17 connecting the insulating layers 9a, 9b of the first insulation system part and the second insulation system part, and to form an outer semiconductor layer 19 connecting the outer semiconductor layers 11a, 11b of the first insulation system part and the second insulation system part.

[0041] 2 to 3c, a method of constructing an insulation system around the exposed first and second conductor portions 5a and 5b to form a power cable, such as the power cable 1, will now be described.

[0042] The construction of the insulation system may form part of a cable joining operation of the power cable 1 or of a repair operation of the insulation system of the power cable 1 .

[0043] When the method concerns a cable joining operation, the first and second parts 1a and 1b are not joined at first. In this case, the first and second conductor parts 5a and 5b are not initially connected. Before joining the first and second conductor parts 5a and 5b, the ends of the first and second conductor parts 5a and 5b are exposed. Furthermore, for each of the first and second parts 1a and 1b, the inner semiconductor layers 7a, 7b, the insulating layers 9a, 9b and the outer semiconductor layers 11a, 11b are exposed in turn. Furthermore, each insulating layer 9a, 9b may be formed as a pencil shape, i.e. a cone, tapered towards each other, as shown in FIG. 1.

[0044] The first conductor portion 5a and the second conductor portion 5b are joined together to form a conductor joint 13.

[0045] Alternatively, if the method concerns repair work of an insulation system, the conductor 5 may not require bonding in the area around which the insulation system is constructed. In this case, the first conductor portion 5a and the second conductor portion 5b are exposed, allowing the reconstruction of the insulation system in the damaged area. Furthermore, for each of the first and second portions 1a and 1b, the insulating layers 9a, 9b and the outer semiconductor layers 11a, 11b are exposed in turn. In this case, the insulating layers 9a, 9b may be formed as pencils, i.e. cones, tapered towards each other.

[0046] In both cases, in this respect the first and second insulation system parts are axially spaced apart from each other, in particular the power cable 1 has at least an axial section between the first and second insulation system parts that is free of the outer semiconducting layer.

[0047] In step a) of the method, a power cable 1 is provided having an axial section between the first and second insulation system parts which is free of at least the outer semiconducting layer.

[0048] Typically, the inner semiconducting layer 15 is provided around the conductor portions 5a, 5b that were exposed and joined before step a). Alternatively, the inner semiconducting layer 15 may be provided around the conductors 5 in the exposed, unjointed areas, for example for repair of the insulation system of the power cable 1.

[0049] Inner semiconductor layer 15 may be formed, for example, by wrapping tape around exposed first and second conductor portions 5a, 5b and heating the tape to harden the tape.

[0050] In step b), a tape is wrapped around the conductor 5 along an axial portion between the first insulation system part and the second insulation system part.

[0051] The tape is wound in multiple layers to form multiple layers of tape that connect with the first insulation system portion and the second insulation system portion of the power cable 1.

[0052] According to one example, in step b), in each layer of the plurality of layers, adjacent turns are arranged in an overlapping manner.

[0053] According to one example, in each layer of the plurality of layers, each turn of tape is disposed axially shifted along the axial direction of the conductor 5 relative to all turns of tape of any adjacent layer of tape. Each turn may, for example, be axially shifted by at least 20% of the width of the tape, such as at least 30%, for example at least 40%.

[0054] In step c) the tape layers are heated to melt and fuse the tape layers to form an insulation system layer between the first insulation system part and the second insulation system part. Furthermore, the insulation system layer thus obtained is also fused to the first insulation system part and the second insulation system part. In particular, the insulation system layer is fused to the corresponding insulation system layers of the first insulation system part and the second insulation system part.

[0055] The insulation system layer formed in step c) may be the insulating layer 17 or the outer semiconducting layer 19 connecting the first insulation system part and the second insulation system part.

[0056] In step c) the tape may be heated under a pressure higher than atmospheric pressure, for example the pressure may be at least 2 bar, such as at least 3 bar, or at least 4 bar.

[0057] Figure 2 shows an example of a tape 21-1 used in step b). The tape 21-1 has a width w defined by the distance between its side edges 21a, 21b. The tape 21-1 has an intermediate portion 21c between the side edges 21a, 21b. In the intermediate portion 21c, the tape 21-1 has its maximum thickness t. The thickness of the tape 21-1 decreases in the direction from the intermediate portion 21c towards the side edge 21a and in the direction towards the side edge 21b. Thus, the thickness of the tape 21 at the side edges 21a, 21b is smaller than the maximum thickness t at the intermediate portion 21c.

[0058] The thickness of the tape 21 may preferably decrease smoothly towards both side edges 21a, 21b.

[0059] The thickness of the tape 21 may taper symmetrically towards the side edges 21a, 21b. With respect to a central axis A passing through the intermediate portion 21c, the taper is symmetric on either side of the central axis A.

[0060] The tapes may have a convex cross-section, as shown in Figures 3b and 3c. According to the example of Figure 3b, tape 21-2 is plano-convex. In the example shown in Figure 3c, tape 21-3 is biconvex.

[0061] The tapes 21-1, 21-2, and 21-3 include a polymer material, which may include, for example, one of polyethylene, polypropylene, ethylene propylene rubber, and ethylene propylene diene monomer rubber.

[0062] Tapes with the geometrical structure as shown in Figures 3a-c can be used for the insulating layer or the outer semiconducting layer or for both of these layers. However, the materials of the tapes 21-1, 21-2, 21-3 are different for the two layers: for the insulating layer they are made of insulating material and for the outer semiconducting layer they are made of semiconducting material.

[0063] The inventive concept has been described above primarily with reference to certain examples, however, as will be readily appreciated by those skilled in the art, other embodiments than those disclosed above are equally possible within the scope of the inventive concept, as defined by the appended claims.

Claims

1. A method for constructing an insulation system around the axial portion of a conductor (5) of a power cable (1), a) Providing a power cable (1), wherein the power cable (1) includes a conductor (5) and an insulating system (7a, 7b, 9a, 9b, 11a, 11b) arranged around the conductor (5), and the insulating system (7a, 7b, 9a, 9b, 11a, 11b) comprises an insulating system layer including an inner semiconductor layer (7a, 7b) arranged around the conductor (5), an insulating layer (9a, 9b) arranged around the inner semiconductor layer (7a, 7b), and an outer semiconductor layer (11a, 11b) arranged around the insulating layer (9a, 9b), and the power cable (1) includes an axial portion between the first insulating system portion and the second insulating system portion of the insulating system (7a, 7b, 9a, 9b, 11a, 11b) that lacks at least an outer semiconductor layer (19), b) In order to form layers of tapes (21-1, 21-2, 21-3) that connect to the first insulation system portion and the second insulation system portion, the tapes (21-1, 21-2, 21-3) are wrapped around the conductor (5) along the axial portion in the multiple layers, c) Heating the layers of the plurality of tapes (21-1, 21-2, 21-3) to melt and fuse the layers of the plurality of tapes, thereby forming an insulating system layer (17, 19) between the first insulating system portion and the second insulating system portion, Includes, The tape (21-1, 21-2, 21-3) has a width (w) defined by the distance between its side edges (21a, 21b), the tape (21-1, 21-2, 21-3) has an intermediate portion (21c) between its side edges (21a, 21b), the tape (21-1, 21-2, 21-3) has its maximum thickness (t) in the intermediate portion (21c), and the thickness (t) of the tape (21-1, 21-2, 21-3) decreases from the intermediate portion (21c) toward both side edges (21a, 21b). method.

2. The method according to claim 1, wherein the thickness smoothly decreases toward both side edges (21a, 21b).

3. The method according to claim 1 or 2, wherein the thickness decreases symmetrically toward the side edges (21a, 21b).

4. The method according to claim 1 or 2, wherein the cross-section of the tape (21-2, 21-3) has a convex shape.

5. The method according to claim 4, wherein the convex shape is biconvex or planoconvex.

6. The method according to claim 1 or 2, wherein in step b), adjacent windings are arranged to overlap in each of the plurality of layers.

7. The method according to claim 1 or 2, wherein in each of the plurality of layers, each winding of the tape (21-1, 21-2, 21-3) is positioned to be axially shifted along the axial direction of the conductor (5) with respect to all windings of the tape (21-2, 21-3) in any adjacent layer of tape (21-1, 21-2, 21-3).

8. The method according to claim 7, wherein each winding is shifted axially by at least 20%, for example, at least 30%, for example, at least 40%, of the width of the tape (21-1, 21-2, 21-3).

9. The method according to claim 1 or 2, wherein in step c), the tapes (21-1, 21-2, 21-3) are heated under a pressure higher than atmospheric pressure.

10. The method according to claim 1 or 2, wherein, before step b), an inner semiconductor layer (15) is provided around the conductor (5), and in step b), the winding of the tapes (21-1, 21-2, 21-3) is around the inner semiconductor layer (15).

11. The method according to claim 1 or 2, wherein the insulating system layer is an insulating layer (17) or an outer semiconductor layer (19).

12. The method according to claim 1 or 2, wherein the tapes (21-1, 21-2, 21-3) include a polymer material.

13. The method according to claim 12, wherein the polymer material comprises one of polyethylene, polypropylene, ethylene propylene rubber, EPR, and ethylene propylene diene monomer rubber.

14. The method according to claim 1 or 2, wherein the construction of the insulation system forms part of the cable joining work or repair work of the insulation system of the power cable (1).

15. The method according to claim 1 or 2, wherein the power cable (1) is a high-voltage power cable.

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