ELECTRICAL FIELD GRADING MATERIAL AND USE THEREOF IN ELECTRICAL CABLE ACCESSORIES

Abstract

Electrical field grading material which comprises a non-polar elastomeric polymer, a phyllosilicate filler and a carbon black filler, wherein any carbon black filler present in the electric field grading material has a dibutyl phthalate (DBP) absorption number from 30 to 80 ml/100 g. The above material may be used in electrical cable accessories, particularly electrical cable joints or terminations for medium or high voltage cable. The electrical field grading material according to the present invention has varioresistive properties, particularly a significant variation of electrical conductivity as a function of the applied voltage within a reduced voltage range, so as to guarantee a high value of conductivity above a critical value of the electrical field, and therefore to ensure an even distribution of the electrical field lines within the material.

Claims

1. Electrical field grading material which comprises a non-polar elastomeric polymer, a phyllosilicate filler and a carbon black filler, wherein any carbon black filler present in the electric field grading material has a dibutyl phthalate (DBP) absorption number from 30 to 80 ml/100 g.

2. Electrical field grading material according to claim 1, comprising: (i) from 30% to 80% by weight of the non-polar elastomeric polymer; (ii) from 10% to 35% by weight of the phyllosilicate filler; and (iii) from 10% to 30% by weight of the carbon black filler; the percentages being calculated on the basis of the total weight of the material.

3. Electrical field grading material according to claim 2, comprising from 40% to 70% by weight of the non-polar elastomeric polymer.

4. Electrical field grading material according to claim 2, comprising from 15% to 30% by weight of the phyllosilicate filler.

5. Electrical field grading material according to claim 2, comprising from 15% to 25% by weight of the carbon black filler.

6. Electrical field grading material according to claim 1, wherein the phyllosilicate filler and the carbon black filler are in a weight ratio of from 0.9 to 1.2.

7. Electrical field grading material according to claim 1, wherein the phyllosilicate filler is selected from: kaolin, illite, montmorillonite, vermiculite, talc.

8. Electrical field grading material according to claim 7, wherein the phyllosilicate filler is kaolin.

9. Electrical field grading material according to claim 1, wherein the phyllosilicate filler has an average particle size of at least 1 m.

10. Electrical field grading material according to claim 9, wherein the phyllosilicate filler has an average particle size of less than 10 m.

11. Electrical field grading material according to claim 1, wherein the carbon black filler has a dibutyl phthalate (DBP) absorption number of from 60 to 80 ml/100 g.

12. Electrical field grading material according to claim 1 wherein the carbon black filler has a iodine adsorption of from 20 to 50 mg/g.

13. Electrical cable accessory which includes an element made from an electrical field grading material according to claim 1.

14. Electrical cable joint, particularly suitable for high voltage direct current cables, comprising: a central semiconducting electrode; two semiconducting deflectors; a field grading layer longitudinally extending between each one of the deflectors and the central electrode and in electric contact therewith; a joint insulating layer surrounding the central electrode, the two deflectors and the field grading layer; and a joint outer semiconductive layer surrounding and in direct contact with the insulating layer; wherein the field grading layer is made from an electrical field grading material containing a phyllosilicate filler.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0049] Further characteristics will be apparent from the detailed description given hereinafter with reference to the accompanying drawings, in which:

[0050] FIG. 1 is a partial longitudinal cross-section of a first joint, particularly suitable for HV cables, including an element made from an electrical field grading material according to the present invention;

[0051] FIG. 2 is a partial longitudinal cross-section of a second joint, particularly suitable for HV cables, including an element made from an electrical field grading material according to the present invention;

[0052] FIG. 3 is a partial longitudinal cross-section of a termination, particularly suitable for HV cables, including an element made from an electrical field grading material according to the present invention;

[0053] FIG. 4 is a graph reporting conductivity curves of samples of compositions according to the present invention and comparative ones.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0054] In FIG. 1, a longitudinal section of an embodiment of a joint (100) mounted on a pair of joined electrical cables (110) according to the present invention is schematically represented.

[0055] Each cable of the joined cable pair (110) comprises a conductor (not shown in the figure) and an insulating layer (11a, 11b) made, for example, from crosslinked polyethylene (XLPE).

[0056] The joint (100) comprises: an insulating element (13), made of, e.g., cross-linked EPDM, encircling and being in contact with the cable insulating layer (11a, 11b) of the electrical cable (110). A semiconductive electrode (12) which is embedded in said electrically insulating element (13) encircles the cable (200) where the cable insulating layers (11a, 11b) are removed to bare the cable conductors then joined, and can encircle a limited portion of the insulating layers (11a, 11b) in the vicinity of the removal zone.

[0057] The joint (100) further comprises an outer semiconducting layer (14) encircling the electrically insulating element (13) and two stress-relief cones (15a, 15b), each provided at a side of the electrically insulating element (13). The two stress-relief cones (15a, 15b) are made from an electrical field grading material according to the present invention.

[0058] In FIG. 2, a longitudinal section of another embodiment of a joint (200) according to the present invention is schematically represented. Differently from FIG. 1, the joint (200) is shown as such, before being mounted on a pair of joined electrical cables. The joint (200) extends along a longitudinal direction X between two opposite end portions (210, 220) and is suitable to be fit over the conducting core connection in the assembled configuration.

[0059] The joint (200) comprises a central electrode (240), made of semiconductive material, and two deflectors (250, 260), made of semiconductive material. The central electrode (240) is positioned in an intermediate position with respect to the end portion (210, 220) of the joint (200) and is arranged to surround the metal connector connecting the conducting cores of the two cables to be joined (not shown in the figure). The two deflectors (250, 260) are positioned at the end portions (210, 220) of the joint (200).

[0060] The joint (200) also comprises a field grading layer (270) which, in the embodiment of FIG. 2, longitudinally extends to partially cover and partially embed the two deflectors (250, 260) and to totally cover and partially embed the central electrode (240). The field grading layer (270) is made from an electrical field grading material according to the present invention.

[0061] As shown in FIG. 2, the field grading layer (270) overlaps the radially external surface and the longitudinal ends of the central electrode (240), and partially overlaps the radially external surface of the two deflectors (250, 260) and embeds their longitudinal end (252, 262) facing the central electrode (240). The field grading layer (270), transversally extends so as to be interposed between each one of the deflectors (250, 260) and the central electrode (240).

[0062] The joint (200) further comprises a joint insulating layer (280) that overlaps the field grading layer (270) so as to be positioned radially external thereto, and a joint outer semiconductive layer (290) overlapping the insulating layer (280) so as to be positioned radially external to such an insulating layer (280). In the embodiment of FIG. 2, the field grading layer (270) longitudinally extends so as to be interposed between the insulating layer (280) and the electrode (240) and the two deflectors (250, 260).

[0063] In FIG. 3, a longitudinal section of a preferred embodiment of a termination (300) mounted on an electrical cable (310) according to the present invention is schematically represented.

[0064] The electrical cable (310) comprises a conductor (not shown in the figure) and an insulating layer (31) made, for example, from crosslinked polyethylene (XLPE).

[0065] The termination (300) comprises: an insulating element (33), made for example from cross-linked EPDM, surrounding and in contact with the insulating layer (31) of the electrical cable (310) and a stress-relief cone (35) also surrounding and in contact with the insulating layer (31). The termination (300) further comprises an outer semiconducting layer (36) covering the insulating element (33) and the stress-relief cone (35). The stress-relief cone (35) is made from an electrical field grading material according to the present invention.

[0066] FIGS. 1, 2 and 3 show three embodiments of the present invention. Suitable modifications can be made to these embodiments according to specific technical needs and application requirements without departing from the scope of the invention.

[0067] The following examples are provided to further illustrate the invention.

Examples

[0068] The compositions as reported in Table 1 were prepared by using an internal Banbury mixer where all of the ingredients were added at the beginning, with the exception of the peroxide, which was added after discharging the composition in an open mill mixer. At the end of the mixing process, curing was effected by an electric press (15 min at 180 C. and 200 bar) to provide sample plates of 1.0 mm thickness. In the compositions reported in Table 1 the amounts of the various components are expressed as % by weight with respect to the total weight of the composition.

TABLE-US-00001 TABLE 1 EXAMPLE 1 2(*) 3(*) 4(*) 5(*) 6(*) EPDM rubber 51.2 24.2 65.1 51.2 51.2 51.2 HNBR 24.2 Kaolin 21.9 21.9 Carbon black (DBP = 65) 19.4 9.7 10.4 19.4 19.4 17.6 Carbon black (DBP = 118) 1.8 Polyisobutylene 2.2 3.1 2.2 2.2 2.2 Al-doped silicon carbide 20.8 Barium titanate 10.9 Titanium oxide 36.2 21.9 10.9 bis(t-butylperoxy-isopropyl) 1.0 1.5 1.0 1.0 1.0 1.0 benzene Additives 5.3 5.8 0.5 5.3 5.3 5.3 (*)comparative HNBR = hydrogenated nitrile butadiene rubber.

[0069] The additives reported in Table 1 are a mixture of conventional products selected from: plasticizers, antioxidant agents, thermal stabilizers, crosslinking coagents.

[0070] The compositions of Table 1 were tested for dielectric strength (DC), according to IEC 60243-2 (2001). The results are reported in Table 2.

TABLE-US-00002 TABLE 2 EXAMPLE 1 2(*) 3(*) 4(*) 5(*) 6(*) Dielectric strength (DC) 15.0 30.0 15.0 5.0 4.0 <1 (kV/mm) (*)comparative

[0071] The sample of comparative Example 6 had a very low dielectric strength for the application in an accessory for power cable. Samples of comparative Examples 4 and 5 had a poor dielectric strength.

[0072] Samples of Examples 1-5 were tested to evaluate their conductivity, according to IEC 60093 (1980). The results are reported in FIG. 4, where the curves are plotted with the applied voltage (kV/mm) in abscissae versus conductivity (S.Math.m) in ordinates.

[0073] The compositions of comparative Examples 2 and 5, containing respectively titanium oxide and barium titanate/titanium oxide, are not vario-resistive as their conductivity increases very negligibly at voltage increase, therefore they cannot be used as field grading materials in cable accessories. In the case of the composition of Example 2, the polar polymeric component (HNBR) is believed to have played a negative role in the vario-resistive behaviour.

[0074] The composition of comparative Example 4 is too much conductive for insulating applications (like the compositions of comparative Examples 5) and its conductivity kept on increasing at voltage increase.

[0075] Samples of the composition of Example 1, according to the invention, and of comparative Example 3 (containing doped silicon carbide) showed satisfactory vario-resistive properties making them suitable as field grading materials.

[0076] Samples of the composition of Example 1 and of comparative Example 3 were tested to evaluate their mechanical properties, in particular elastic recovery after ageing, according to the following procedure.

[0077] Samples were kept under tensile stress at 200% elongation for 7 days in an oven at 70 C. Then, the elastic recovery of each sample was measured after 5 minutes from the sample release. The samples of Example 1 recovered about 60% of their initial length, thus the composition was suitable for electric cable accessories. The samples of comparative Example 3 were not able to complete the test because of rupture of the same. Such poor elastic properties made the composition of comparative Example 3 unsuitable for electric cable accessories.

[0078] The Applicant also observed that the composition of comparative Example 3 caused manufacturing problem. In particular, during the preparation of the samples, abrasion in the mixing apparatus occurred, probably due to the presence of silicon carbide. Such a drawback is to be carefully evaluated while considering a large scale production.

[0079] The present description shows only some embodiments of the present invention. Suitable modifications can be made to these embodiments according to specific technical needs and application requirements without departing from the scope of the invention.