Field grading layer

Abstract

A method is provided for applying a field grading layer to a power cable includes preparing a polymer composition having a polymer matrix, at least one conductive filler different from a non-linear conductive organic filler, and at least one crosslinking agent. The method includes extruding the polymer composition into a crosslinkable field grading tape and winding the crosslinkable field grading tape onto a section of power cable in need of field grading. The crosslinkable tape is crosslinked to obtain a field grading layer.

Claims

1. Method of applying a field grading layer to a power cable, said method comprising the steps of: i) preparing a polymer composition comprising a polymer matrix, at least one conductive filler different from a non-linear conductive organic filler, and at least one crosslinking agent, ii) extruding the polymer composition into a crosslinkable field grading tape, iii) winding the crosslinkable field grading tape onto a section of power cable in need of field grading, and iv) crosslinking the crosslinkable tape to obtain a field grading layer.

2. Method according to claim 1, wherein the polymer matrix is selected from polyolefins, copolymers thereof, polymethyl(methacrylate), and mixtures thereof.

3. Method according to claim 2, wherein the polyolefins are selected from the group consisting of polypropylene (PP), polyethylene (PE), low density polyethylene (LDPE), cross-linked polyethylene (XLPE), and mixtures thereof.

4. Method according to claim 1, wherein the at least one conductive filler is selected from metal oxides, ceramics, carbon nanotubes, SiC, graphene oxide, and mixtures thereof.

5. Method according to claim 1, wherein the polymer composition further comprises at least one passive conductive filler.

6. Method according to claim 5, wherein the at least one passive conductive filler is selected from carbon black and antimony doped SnO.sub.2.

7. Method according to claim 1, wherein the at least one crosslinking agent is an organic peroxide.

8. Method according to claim 1, wherein a first section of the power cable has an insulation layer, and the crosslinking of the crosslinkable field grading tape results in crosslinkage to the insulation layer.

9. A sub-sea cable, wherein said subsea cable includes a field grading layer applied according to the method of claim 1.

10. Method according to claim 1, wherein the polymer matrix is different from an elastomer matrix.

11. Method according to claim 1, wherein the power cable comprises a central conductor, an insulation layer surrounding the central conductor, an inner semiconducting layer arranged in between the central conductor and the insulation layer, and on a first section of the power cable a field grading layer on the outer surface of said insulation layer, wherein the field grading layer comprises said polymer matrix and said at least one conductive filler different from a non-linear conductive organic filler, and wherein said field grading layer is crosslinked internally and crosslinked to the insulation layer.

12. Method according to claim 11, wherein the power cable on a second section comprises an outer semiconducting layer, and wherein the field grading layer is in electrical contact with the outer semiconducting layer and in electrical contact with the central conductor.

13. Crosslinkable field grading tape formed of an extruded polymer composition comprising: a polymer matrix; at least one conductive filler different from a non-linear conductive organic filler; and at least one crosslinking agent.

14. Crosslinkable field grading tape according to claim 13, wherein said crosslinkable field grading tape further comprises at least one passive conductive filler.

15. Crosslinkable field grading tape according to claim 13, wherein the polymer matrix is selected from polyolefins, copolymers thereof, polymethyl(methacrylate), and mixtures thereof.

16. Crosslinkable field grading tape according to claim 15, wherein the polyolefins are selected from the group consisting of polypropylene (PP), polyethylene (PE), low density polyethylene (LDPE), cross-linked polyethylene (XLPE), and mixtures thereof.

17. Crosslinkable field grading tape according to claim 13, wherein the at least one conductive filler is selected from metal oxides, ceramics, carbon nanotubes, SiC, graphene oxide, and mixtures thereof.

18. A sub-sea cable, wherein, said subsea cable has a crosslinkable field grading tape according to claim 13.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The enclosed figures illustrate one embodiment of the present invention.

(2) FIG. 1a illustrates a cable termination prior to the application of a crosslinkable field grading tape according to one embodiment of the present invention.

(3) FIG. 1b illustrates the same cable termination as FIG. 1a but after a crosslinkable field grading tape according to the present invention has been applied to the cable termination.

(4) FIG. 2 represents a longitudinal cross sectional view of a power cable comprising a field grading layer according to the invention, and a termination.

(5) FIG. 3 represents a longitudinal cross sectional view of a power cable comprising a field grading layer according to the invention, and a junction.

(6) For reasons of clarity, the same elements are designed by identical references. Similarly, only those elements that are essential for understanding the invention are shown in diagrammatic manner, and scale is not complied with.

DETAILED DESCRIPTION

(7) The present invention will now be described in further details. Compounding and especially molding and/or crosslinking a conductive filler having field grading properties in an elastomer matrix according to the prior art method for preparing premolded joints or stress cones, can induce stress to the conductive filler and reduce the field grading properties of the conductive filler. Indeed, when molding the elastomer matrix containing the conductive filler, a high temperature has to be maintained for a long time (often more than one hour) to achieve crosslinking. One reason for this is that premolded joints are quite large and the thermal inertia of the system causes the temperature to be maintained for an extended time. In addition, the adhesion between the obtained field grading layer and the insulation(s) is not optimized.

(8) According to one aspect of the present invention, a crosslinkable field grading tape is produced by preparing a polymer composition comprising at least one conductive filler different from a non-linear conductive organic filler, a polymer matrix such as low density polyethylene (LDPE) matrix, and at least one crosslinking agent, said polymer composition being then extruded into said tape.

(9) Thus, the crosslinkable field grading tape is formed of an extruded polymer composition comprising a polymer matrix, at least one conductive filler different from a non-linear conductive organic filler, and at least one crosslinking agent.

(10) This crosslinkable field grading tape is according to this aspect of the present invention applied to an element, such as a power cable, in need of a field grading layer and then, easily crosslinked. The thin crosslinkable field grading tape can be crosslinked with in a considerable shorter time period (5-20 minutes) which induces less thermal stress on the conductive filler than the molding and crosslinking of a conductive filler in an elastomer matrix or a polymer matrix according to the prior art. In addition, the adhesion between the field grading layer and the insulation(s) is improved.

(11) In one aspect of the present invention, the polymer matrix used for the crosslinkable field grading tape is of the same type or equivalent to the polymer employed in the insulation layer of the power cable or the joint, thereby improving the compatibility and the adhesion of the different parts. The insulation layer is preferably made of crosslinked polyethylene (XLPE) or low density polyethylene (LDPE).

(12) The polymer of the insulation layer may further comprise a peroxide to facilitate the crosslinking.

(13) Making a crosslinkable field grading tape instead of molding an elastomer matrix-based composition makes it possible to use application techniques well known from the manufacturing of factory joints using for example crosslinked polyethylene (XLPE) tape. This reduces the time during which the conductive filler is subjected to high temperature during application, allowing a higher degree of non-linearity or linearity to remain through crosslinking (step iv)) and compounding (i.e. step i)). Crosslinking also ensures good adhesion between the cable insulation layer and the crosslinkable field grading tape and some amount of crosslinking will also occur between the crosslinkable field grading tape and the previously extruded and eventually crosslinked insulation layer. Thus, the crosslinking of the crosslinkable field grading tape induces chemical bonding with great adhesion of the field grading layer to the insulation layer of the power cable.

(14) In one embodiment of the present invention, a crosslinkable field grading tape is obtained by preparing a polymer composition comprising LDPE, at least one peroxide, and one or more conductive fillers different from a non-linear conductive organic filler, and extruding said polymer composition.

(15) Applicable peroxides include but are not limited to organic peroxides, more specifically dialkyl peroxides.

(16) Examples of dialkyl peroxides include, but not limited to, dicumyl peroxide, tert-butyl cumyl peroxide, di(tert-butylperoxyisopropyl)benzene, di-tert-butyl peroxide, and mixtures thereof.

(17) In the manufacturing process of factory joints used in subsea high voltage cables, an extruded crosslinkable field grading tape in which the polymer matrix is polyethylene, is used. This polyethylene with peroxides for crosslinking is modified into a field grading layer by adding at least one conductive filler having field grading properties in the form of metal oxides, ceramics, carbon nanotubes, SiC, graphene oxide, and mixtures thereof, and more preferably in the form of SiC, ZnO, FeO.sub.2, TiO.sub.2, Al.sub.2O.sub.3, BaTiO.sub.3, SnO.sub.2, SbO.sub.2 and mixtures thereof. This crosslinkable field grading tape is then applied to the cable, heated to initiate crosslinking.

(18) The crosslinking step i) can be performed at a temperature ranging from about 150 C. to about 250 C., and preferably from about 170 C. to about 210 C., and optionally during from about 5 min to about 60 min, and preferably from about 10 to about 30 min.

(19) After application, conventional terminations, premolded joints or factory joints can be applied on top of the insulation layer of the cable.

(20) FIGS. 1a and 1b illustrates schematically a cable termination making use of the present invention. FIG. 1a illustrates a power cable PC.sub.1 comprising a central conductor (1a), radially surrounded by an inner semiconducting layer (2a), surrounded radially by and insulation layer (3a) which initially is surrounded radially with an outer semiconducting layer (4a). To allow for termination of the cable, the outer semiconducting layer (4a) has been removed from a part of the illustrated end section of the cable. At the termination end also, a section of the insulation layer (3a) and the inner semiconducting layer (2a) have been removed providing a section of the conductor (1a) for electrical connection.

(21) As an example, a polymer composition comprising LDPE, an organic peroxide, and a conductive filler having field grading properties is prepared so as to obtain a crosslinkable field grading tape. In the termination, the crosslinkable field grading tape would be applied as indicated below in FIG. 1b, forming the field grading layer (5a). A conventional stress cone for AC terminations can then be mounted on top of the field grading tape layer (5a). As illustrated, the field grading layer (5a) is in electrical contact with the outer semiconducting layer (4a) at the end (6a) of the outer semiconducting layer (4a).

(22) The conductor (1a) can consist of a multitude of individual copper or aluminum conductors.

(23) The outer semiconducting layer (4a) and/or the inner semiconducting layer (2a) can be made of a polymer filled with carbon particles.

(24) Application of a field grading material as a cross-bonded tape as compared with elastomer matrix has the following advantages: excellent adhesion between the insulation layer (3a) and the field grading layer (5a), simpler application process as there is no need to mold a separate accessory, and less thermal stress on the conductive filler during preparation of the crosslinkable field grading tape and crosslinking as compared with preparing a premolded accessory

(25) The present invention is especially a feasible technique for joints and terminations for subsea cables, which benefit from the improved field grading properties.

(26) FIG. 2 represents a power cable PC.sub.2 positioned in a termination (7), the termination (7) surrounding the power cable PC.sub.2.

(27) The power cable comprises a central conductor (1b), surrounded successively by an inner semiconducting layer (2b), an insulation layer (3b), and an outer semiconducting layer (4b).

(28) At the end (8b) of the power cable, a field grading layer (5b) replaces a part of the outer semiconducting layer (4b) of the power cable, so that the layer (5b) longitudinally extends in the prolongation from the outer semiconducting layer (4b) to said end (8b).

(29) Hence, said layer (5b) surrounds and is physically in contact with the insulation layer (3b) of the power cable PC.sub.2.

(30) In addition, the field grading layer (5b) is electrically connected to the central conductor (1b) by a conducting connection (9).

(31) The termination (7) includes: a stress cone body (10), and a deflector (11).

(32) The stress cone body (10) surrounds a part of the field grading layer (5b), the stress cone body (10) being an electrically insulating body. The deflector (11) is a semiconducting cone surrounding a part of the outer semiconducting layer (4b) and a part of the field grading layer (5b). The deflector (11) is physically in contact with the outer semiconducting layer (4b) of the power cable, and in physically in contact with the field grading layer (5b). Hence, at least inside the termination (7), the field grading layer (5b) separates the insulation layer (3b) from the deflector (11).

(33) FIG. 3 represents a first power cable PC.sub.3 and a second power cable PC.sub.4, positioned in a junction (12), the junction (12) surrounding each power cable (PC.sub.3, PC.sub.4).

(34) Both power cables (PC.sub.3, PC.sub.4) respectively comprise a central conductor (1c, 1d), surrounded successively by an inner semiconducting layer (2c, 2d), an insulation layer (3c, 3d), and an outer semiconducting layer (4c, 4d).

(35) At the end (8c, 8b) of each power cable (PC.sub.3, PC.sub.4), a field grading layer (5c, 5d) replaces a part of the outer semiconducting layer (4c, 4d) of the power cable (PC.sub.3, PC.sub.4), so that the layer (5c) longitudinally extends in the prolongation from the outer semiconducting layer (4c) to the end (8c) of the first power cable, and the layer (5d) longitudinally extends in the prolongation from the outer semiconducting layer (4d) to the end (8d) of the second power cable.

(36) Hence, said layer (5c, 5d) surrounds and is physically in contact with the insulation layer (3c, 3d) of each power cable (PC.sub.3, PC.sub.4).

(37) The junction (12) includes: an insulation of premolded joint (13), an outer semiconducting screen (14), and a connector (15) surrounded by a conducting screen (16), to electrically connect the first power cable and the second power cable together in the junction (12).

(38) The insulation (13) surrounds all the field grading layers (5c) and (5d), the premoulded joint being an electrically insulating joint.

(39) The outer semiconducting screen (14) surrounds a part of the outer semiconducting layers (4c, 4d) of said power cables, and a part of the field grading layers (5c, 5d). The outer semiconducting screen (14) is physically in contact with the outer semiconducting layers (4c, 4d) of the power cables (PC.sub.3, PC.sub.4), and in physically in contact with the field grading layers (5c, 5d). Hence, at least inside the junction (12), the field grading layer (5c) separates the insulation layer (3c) from the outer semiconducting screen (14), and the field grading layer (5d) separates the insulation layer (3d) from the semiconducting screen (14).

(40) The connector (15) allows to electrically connect the central conductor (1c) of the first power cable PC.sub.3 with the central electrical conductor (1d) of the second power cable PC.sub.4.

(41) In addition, the field grading layers (5c, 5d) are electrically connected to the central conductors (1c, 1d) by the intermediate of the connector (15) surrounded by its conducting screen (16).