GB2269712A

GB2269712A - Stress-reduction in power cable joint.

Info

Publication number: GB2269712A
Application number: GB9217088A
Authority: GB
Inventors: Peter Ronald Gilbert
Original assignee: Associated Electrical Industries Ltd
Current assignee: Associated Electrical Industries Ltd
Priority date: 1992-08-12
Filing date: 1992-08-12
Publication date: 1994-02-16
Anticipated expiration: 2012-08-12
Also published as: GB9217088D0; AU4718393A; GB2269712B; WO1994005064A1

Abstract

A joint between insulated conductors (3) of two electric power cables (1) incorporating insulation screens (7) is made by a method wherein after electrically connecting the ends of the conductors of the cables, the ends of the connected conductors are surrounded by a sleeve (11) of solid electrically insulating material having along its internal surface an insert (19) of a material which is substantially more electrically conductive than the sleeve and electrically contacts the connected conductors. The sleeve is positioned so that its ends project over the cable insulation (5) to provide an annular gap (17) between sleeve (11) and insulation, and the cable insulation screens are joined by a tubular electrically conducting surface (13, 23) surrounding the sleeve. The joint is completed by introducing into the joint a material in a sufficiently fluid state to penetrate and fill the annular gaps between the sleeve and cable insulation. Material between the sleeve and the cables provides respective tubular paths (15) each extending between a respective end of the insert and the corresponding end of the tubular surface which joins the cable insulation screens and having an impedance which reduces with increasing electrical stress. The method is tolerant of cable size and shape. <IMAGE>

Description

Electric Power Cable Jointing This invention relates to methods of forming joints between electric power cables and to joints between such cables, more especially such cables designed for use at medium and high operating voltages, that is to say operating voltages above 600 volts.

A known method of forming a joint between insulated conductors of two electric power cables comprises the steps of: electrically connecting the ends of the conductors of the cables; surrounding the ends of the connected conductors with a sleeve of solid electrically insulating material having along its internal surface an insert of a material which is substantially more electrically conductive than the material of the sleeve, so that the insert is in electrical contact with the connected conductors and the ends of the sleeve project over the cable conductor insulation to provide an annular gap therebetween; and introducing to the joint a material in a sufficiently fluid state to penetrate and fill the annular gap between the sleeve and cable insulation. Such a method of jointing forms the subject of United Kingdom Patent No.

2211386B.

This method of jointing has the advantage that it can be used with cables of different cross-sectional sizes and shapes, and different types of cable insulation. It is also extremely tolerant of localised depressions, flats, scores or other imperfections in the cable insulation. However, in the particular form described by way of example in GB 2211386B this method of jointing is not convenient for use with higher voltage cables of the kind incorporating insulation screens, that is cables having a layer of electrically conducting material in intimate contact with the cable insulation, all along the length of the cable.

It is an object of the present invention to provide a method of jointing which allows the method of GB 2211386B to be used with cables incorporating insulation screens.

According to the present invention a method of forming a joint between insulated conductors of two electric power cables incorporating insulation screens comprises the steps of: electrically connecting the ends of the conductors of the cables; surrounding the ends of the connected conductors with a sleeve of solid electrically insulating material having along its internal surface an insert of a material which is substantially more electrically conductive than the material of the sleeve, so that the insert is in electrical contact with the connected conductors and the ends of the sleeve project over the cable insulation to provide an annular gap therebetween; providing a tubular electrically conducting surface between the insulation screens of the cables and surrounding the sleeve; and introducing to the joint a material in a sufficiently fluid state to penetrate and fill the annular gap between the sleeve and cable insulation; the material between the sleeve and the cables providing respective tubular paths each extending between a respective end of said insert and the corresponding end of said tubular conducting surface, and having an impedance which reduces with increasing electrical stress.

The invention also provides a joint between insulated conductors of two electric cables incorporating insulation screens comprising: means electrically connecting the ends of the conductors of the two cables; a sleeve surrounding the ends of the connected cables and having ends projecting over the cable insulation to define at each end, with the cable insulation, an annular gap; an internal insert in the sleeve in electric contact with the connected conductors; material filling the annular gaps between the sleeve and the cable insulation, said insert being substantially more electrically conductive than the material of said sleeve and said material filling the annular gaps; and a tubular electrically conducting surface extending around the outside of said sleeve between the insulation screens of the cables, the material between said sleeve and the cables providing respective tubular paths each extending between a respective end of said insert and the corresponding end of said tubular conductive surface and having an impedance which reduces with increasing electrical stress.

In a method or joint according to the invention the sleeve preferably projects at each end over the end of the insulation screen of the adjacent cable.

In one particular embodiment of the method or joint according to the invention the paths of electrical stress dependent impedance are provided by a layer of material on the inner surface of said sleeve.

In another particular embodiment of the invention the paths of electrical stress dependent impedance are provided by said material filling said annular gaps.

One method of forming a joint in accordance with the invention and the joint so formed will now be described, by way of example, with reference to the accompanying drawing which is a schematic axial cross-sectional view of the joint.

Referring to the drawing, the joint is between two lengths of cable 1 each having a central core of electrical conductors 3 provided with a sheath of insulating material 5 which, in turn, is provided with an external electrically conducting screen 7.

In forming the joint appropriate lengths of the insulating sheath 5 and screen 7 are first stripped from the ends of the two cables 1, and an electrical connection made between the exposed ends of the conductors 3 by means of a metal sleeve connector 9 fitted around the ends of the conductors 3 and clamped to them in known manner.

After so joining the ends of the conductors 3, the ends of the cables 1 are enclosed in a housing. The housing includes a rigid sleeve 11 of a high grade insulating polymer, such as polyethylene or ethylene propylene rubber, co-extruded with an external electrically conducting sheath 13 consisting, for example, of a polymer, such as carbon-loaded ethylene propylene rubber, and with an internal coating 15 of a material with electrical stress dependent impedance properties, such as polychloroprene loaded with silicon carbide particles and carbon. The sleeve 11 is threaded onto one of the cables 1 prior to joining the ends of the cable conductors 3, and then positioned centrally around the connector 9 with an annular gap 17 between each end of the sleeve 11 and the insulating sheath 5 of the adjacent cable 1.The sleeve 11 is sufficiently long for its ends to extend over the ends of the screens 7 of the cables 1.

Centrally within the sleeve 11 with its co-extruded sheath 13 and internal coating 15, there is provided a tubular, resilient electrically conducting insert 19, suitably moulded from carbon-loaded nitryl rubber. The insert 19 is appreciably shorter than the sleeve 11, typically about one third of its length, and is fixed to the coating 15, prior to the sleeve 11 being threaded onto one of the cables 1, using an electrically conductive adhesive such as carbon-loaded epoxy resin. The insert 19 is provided internally with integral radially extending projections 21 which establish electrical contact with the connector 9 when the sleeve 11 is moved axially into the desired position.

The joint housing is completed by conical end caps 23 of a resilient electrically conducting polymer material such as carbon-loaded nitryl rubber. Prior to joining the ends of the cable conductors 3 with connector 9 the end caps 23 are each threaded on to a respective one of the two cables 1. After the cable conductors 3 are joined the end caps 23 are push-fitted onto respective ends of the sleeve 11. Each of the end caps 23 has at its wider inner end two circular flanges 25 and 27 of different radius which by reason of the resilience of the material of the end caps 23 respectively make electrical contact with the external sheath 13 and internal coating 15 on the sleeve 11. At its outer narrower end each of the end caps 23 has a flange 29 which makes electrical contact with the screen 7 of the adjacent one of the cables 1.

A hardenabe electrically insulating material (not shown) is then introduced in a fluid form to the interior of the housing, to fill the space therein not occupied by the cables 1 and the connector 9, and is allowed to harden. A low viscosity resin such as silicone rubber is suitably used. The material is suitably introduced by low pressure injection through a nozzle 31 in one of the end caps 23 until the space within the housing is completely filled, as indicated by the passage of material through the nozzle 33 in the other end cap 23.

In use of the completed joint the cable screens 7, end caps 23, and the external sheath 13 of sleeve 11 are all in electrical contact with one another, and maintained at ground potential, whereas the insert 19 is at or about the potential of the cable conductors 3. In consequence, there exists a longitudinal electric stress between the ends of the two tubular interfaces 35, 37 on either side of insert 19 between the hardenable insulating material in the housing and the part of the housing incorporating the sleeve 11.However, the presence of the internal coating 15 on the sleeve 11, more particular the electrical stress dependent electrical impedance property of the coating 15, serves to distribute these electrical stresses uniformly along the lengths of the interfaces 35, 37 to avoid excessive electrical stress at the extremities of these interfaces 35, 37 which are liable to occur in the absence of the coating 15. It will be appreciated that such excessive electrical stress could give rise to electrical activity at the interfaces 35, 37, causing creation of voids, conductive paths and other progressive deterioration lending eventually to electrical failure of the joint.

A further advantage of the presence of the coating 15 and its connection, at its outer ends, to the end caps 23 is that the cable sheaths 7 can extend through the end caps and terminate within the sleeves where the coating 15 provides stress control. Without the coating 15 it would be necessary to terminate the sheaths 7 at the other ends of the end caps 23, where the caps 23 contact the sheaths 7, and to geometrically taper the end caps 23 to achieve stress control at the terminations of the sheaths 7. Because it is not necessary to geometrically taper the ends of the housing, the housing can be manufactured by cost effective methods e.g. extrusion instead of moulding.

The presence of the insert 19 and its overlap at its ends of the insulating sheaths 5 of the cables 1 avoids extreme electric stress adjacent the terminations of the insulating sheaths 5, and allows the use of a relatively low grade insulating material for the hardenable material injected into the housing. Thus the advantages of the arrangement forming the subject of GB-22113688 are retained.

In an alternative embodiment of the invention to that described above by way of example, the required tubular paths of electric stress dependent impedance material, instead of being provided by an internal coating on the sleeve 11, are provided by the material injected into the housing. To this end the injected material may be compounded with an additive of silicon carbide particles and carbon particles.

It will be understood that whilst in the methods and joints described above, by way of example, silicon carbide and carbon particles are utilised to confer the required electric stress dependent impedance properties, other materials may be utilised to confer these properties in other methods and joints according to the present invention.

It will further be understood that the present invention is applicable to joints between more than two cables i.e. to branch connections, as well as to simple straight connections between two cables, as described above by way of example.

It is also pointed out that the present invention is applicable to joints and methods of forming joints between cables including outer protective armouring and/or corrosion protection.

Since in joints and methods of forming joints in accordance with the invention the housing provides an electrically conducting, normally grounded, surface completely surrounding the joint, the reinstatement of such armouring and/or corrosion protection around the housing can be effected without interfering with the correct electrostatic operation of the joint. For the same reason the invention is also applicable to multicore cables as well as single core cables.

It is pointed out that whilst in the particular embodiments of the invention described above by way of example the fluid material injected into the housing is a hardenable material, this is not necessarily the case. Hence a material which cures to a jelly like state rather than hardens might be used.

Claims

1. A method of forming a joint between insulated conductors of two electric power cables incorporating insulation screens comprising the steps of: electrically connecting the ends of the conductors of the cables; surrounding the ends of the connected conductors with a sleeve of solid electrically insulating material having along its internal surface an insert of a material which is substantially more electricity conductive than the material of the sleeve, so that the insert is in electrical contact with the connected conductors and the ends of the sleeve project over the cable insulation to provide an annular gap therebetween; providing a tubular electrically conducting surface between the insulation screens of the cables and surrounding the sleeve; and introducing to the joint a material in a sufficiently fluid state to penetrate and fill the annular gap between the sleeve and cable insulation; the material between the sleeve and the cables providing respective tubular paths each extending between a respective end of said insert and the corresponding end of said tubular conducting surface and having an impedance which reduces with increasing electrical stress.

2. A joint between insulated conductors of two electric cables incorporating insulation screens comprising: means electrically connecting the ends of the conductors of the two cables; a sleeve surrounding the ends of the connected cables and having ends projecting over the cable insulation to define at each end, with the cable insulation, an annular gap; an internal insert in the sleeve in electric contact with the connected conductors; material filling the annular gaps between the sleeve and the cable insulation, said insert being substantially more electrically conductive than the material of said sleeve and said electrically insulating material; and a tubular electrically conducting surface extending around the outside of said sleeve between the insulation screens of the cables; the material between said sleeve and the cables providing respective tubular paths each extending between a respective end of said insert and the corresponding end of said tubular conductive surface and having an impedance which reduces with increasing electrical stress.

3. A method according to Claim 1 or a joint according to Claim 2 wherein the sleeve projects, at each end, over the end of the insulation screen of the adjacent cable.

4. A method according to Claim 1 or Claim 3 or a joint according to Claim 2 or Claim 3 wherein the paths of electrical stress dependent impedance are provided by a layer of material on the inner surface of said sleeve.

5. A method according to Claim 1 or Claim 3 or a joint according to Claim 2 or Claim 3 wherein the paths of electrical stress dependent impedance are provided by said electrically insulating material filling said annular gaps.

6. A method or a joint according to any preceding claim wherein the insert projects, at each end, over the end of the insulation of the adjacent cable.

7. A method or a joint according to any preceding claim wherein said tubular conducting surface incorporates a sheath on the outer surface of said sleeve.

8. A method or a joint according to any preceding claim wherein said tubular conducting surface incorporates conical end caps.

9. A method or a joint according to Claim 8 when dependent on Claims 4 and 7 wherein each said cap has two circular flanges of different radius at its inner, wider end which respectively electrically contact said sheath on the outer surface of said sleeve and said layer of material on the inner surface of said sleeve.

10. A method or joint according to Claim 8 or Claim 9 wherein each said end cap has a flange at its outer, narrower end which electrically contacts said insulation screen of the adjacent cable.

11. A method or a joint according to any one of Claims 8 to 10 wherein said end caps are made of a polymer material.

12. A method or a joint according to Claim 11 wherein said end caps are made of carbon-loaded nitryl rubber.

13. A method or joint according to any one of the preceding claims wherein the stress dependents properties of said tubular paths are conferred by silicon carbide and carbon particles in said material between the sleeve and the cables.

14. A method or a joint according to any one of the preceding claims wherein said sleeve comprises an extrudable polymer material.

15. A method or a joint according to Claim 14 wherein said polymer material is ethylene propylene rubber.

16. A method or a joint according to Claim 14 wherein said polymer material is polyethylene.

17. A method or a joint according to Claim 14, 15 or 16 when dependent on Claim 4 and Claim 7 wherein said layer on the inner surface of the sleeve and said sheath on the outer surface of the sleeve are co-extruded with said sleeve.

18. A method or a joint according to Claim 17 wherein said layer is of polychloroprene material.

19. A method or a joint according to Claim 17 or Claim 18 wherein said sheath is made of a carbon-loaded ethylene propylene rubber material.

20. A method of forming a joint substantially as hereinbefore described with reference to the accompanying drawing.

21. A joint substantially as hereinbefore described with reference to the accompanying drawing.