GB2295501A - Cable joint - Google Patents

Abstract

A high voltage cable joint for three-core cables is formed by jointing and individually insulating the cable cores (1). Insulating spacers (7) separate the cores (1), which are enclosed within a thick rigid insulating housing (11) that is sealed by end fittings (9) on to the cable sheaths (2). An electrical continuity/screening layer (14) extends along the outer surface of the joint casing (11, 9). The interior of the casing (11, 9) is filled with a cold-pourable insulating filler. <IMAGE>

Description

CABLE JOINT This invention relates to a high voltage cable joint between 3-core cables.

Generally, the invention is concerned with the jointing of electric power cables for a 17.5kV system voltage, but it is also suitable for lower voltages such as 15kV or 10kV, and also for higher voltages, for example 24kV. An advantageous application is in the jointing of paper insulated, lead-sheathed cables, but the invention is also suitable for forming a transition joint from a paper cable to a polymeric cable.

Known joints for such cables employ heat-shrinkable polymeric technology, and suitable kits are available from Raychem, in which one or more recoverable sleeves having appropriate electrical characteristics are heat shrunk into position around the cables. An alternative technology is to use oil as an insulating medium, and the oil is poured at an elevated temperature into a rigid cylindrical metal canister which, with end fittings, encloses the jointed conductors and seals on to the cables. The canister may be provided with inner insulation that is thinner than the canister wall. The end fittings may be made of metal, or alternatively polymeric heat-shrinkable moulded parts may be used. The cable conductors are exposed, connecting elements such as crimps applied, and each core is then individually re-insulated.An insulating core spreader may be used physically to separate the cores in the region of the insulated connectors.

In some applications, however, it is not possible, or not desirable, to use heat shrink technology. The oil/metal canister type of joint, however, has the following disadvantages: the relatively high weight of the metal canister; the fact that the metal can suffer corrosion when the joint is buried in the ground; the fact that the oil needs to be heated before becoming pourable; and the inherent danger of having to work with hot oil.

GB-A-2269712, GB-A-358188, US-A-3845235 and US-A-3996081 disclose electric cable joints, but each is only suitable for enclosing within a rigid outer insulating member a joint between cables that have a single core. Although each core of a three-core cable could in theory be separately enclosed in the manner disclosed in these publications, the resulting joint would be complex, bulky, and very time-consuming to produce. Such a three-core cable joint is disclosed in GB-A-881866. GB-A-1387366 discloses a single core cable joint in which the jointed conductors are disposed within a tubular insulating member which itself is disposed within an insulating compound that is contained within an outer metal casing radially spaced therefrom.

It is one object of the present invention to provide a simplified high voltage joint that overcomes, or at least alleviates at least some of the disadvantages as set out above of known joints.

Thus, in accordance with one aspect of the present invention, there is provided a high voltage cable joint between two cables each of which has three cores, wherein successive layers of each cable are stripped back, the conductors of one cable are connected to respective conductors of the other cable, each conductor connection is individually insulated, the cores are separated from each other by an insulating spacer arrangement, the connected cores are surrounded by and laterally spaced apart from a rigid relatively thick insulating tubular member, the insulating tubular member is sealed to each cable, the region contained within the insulating tubular member and surrounding the connected conductors is substantially filled with a pourable insulating medium, and wherein the joint comprises an electrically conductive, relatively thin layer that is disposed completely around the rigid insulating tubular member in abutment therewith along substantially its entire length and that is electrically connected to the conductive earthing sheath or screen of each cable, thereby to provide screening of the joint.

Thus, the insulating tubular member is of a greater wall thickness than the thickness of the conductive layer, and preferably the insulating member provides a structurally rigid housing around the joint region.

A joint in accordance with the present invention accordingly allows a single rigid insulating tubular member to enclose all the cores of the joint, reducing its cost, complexity and bulk. The cores are not only individually insulated, but are also separated by the spacer arrangement, which may comprise one or more generally spoke-shape spacers, for enhancing the electrical isolation therebetween. Furthermore, the tubular member can itself provide a rigid casing for mechanical protection of the joint, thus dispensing with the need for any additional structure. Electrical continuity across the joint, for earth current or electromagnetic screening, may then conveniently be provided by a conductive component mounted directly on the outer surface of the tubular member, thus also reducing bulk. A simple and inexpensive joint construction is thus obtained.

The rigid tubular insulating member may be formed, for example extruded, as a tube, or it may be formed into a tubular shape during construction of the joint, for example as by the closure of two half-shells around the joint. Preferably, the wall thickness of the member along at least part and preferably all its length is at least 5 mm, and is within the range 3 mm to 10 mm.

The insulating member is advantageously formed from polymeric material, preferably polyethylene but PVC, acrylic or another suitable polymer may be used.

The advantage of the rigid tubular member being electrically insulating, and preferably with a wall thickness of greater than 5 mm, is that functionally it can form part of the primary insulation of the joint, that is to say it can provide the insulation between the phases (of a cable joint in a three-phase electrical supply system) and the earth screen of the joint. The joint has an electrically conductive layer disposed completely around the outside of the insulating member and electrically connected to the conductive earthing sheath or screen of each cable, thereby to provide continuity across the joint.This layer, the joint earth screen, may be provided as a metallic mesh, for example of copper, that is wrapped around the tubular member, or it may alternatively be applied, by metal spraying or extrusion for example, as a conductive coating to the tubular member during or subsequent to its manufacture.

A further advantage of the rigid tubular member being electrically insulating is that if the cores of the cables within the joint are not centrally located, either initially or subsequently during operation of the joint, a reduction in their spacing from the tubular member, or even contact therewith, is less likely to result in electrical failure than would be so were a metal canister, or other metal layer to be provided directly around the connections.

It will also be appreciated that an insulating tubular member is not liable to corrosive effects due to acids or salts in the ground, and can generally be expected to offer significant weight savings with respect to a metal member that is otherwise equivalent, for example in terms of structural rigidity.

The individual insulation of the conductors may be provided by sleeves having a revolving action, as disclosed in European Patent No. 0210807 and as sold under the Raychem trademark RAYVOLVE, or by other cold-applied sleeves, or by heat-shrinkable sleeves.

Preferably, the insulating filler is a cold filler, that is to say one that is pourable at ambient temperature. A suitable insulating filler medium comprises (a) a vegetable oil and a compatible resin, comprising from about 40 to 100 parts by weight of the oil and 60 to 0 parts by weight of the resin; (b) a latex, selected so as to be capable of forming a gel with the oil/resin mixture; (c) a desiccant, present in an amount sufficient to absorb water from the latex on mixing therewith, and (d) an inert filler present in an amount from 0 to 75 % by weight of the total composition. "Resin" is herein understood to comprise rosin, modified rosin, rosin derivatives, hydrocarbon resins, terpene resins, and bitumen.Such compositions form the subject of British Patent Application No. 9400079.1, the subject of which is now contained in International Patent Application No. PCT/GB94/02763, the entire contents of which are included herein by this reference.

When the cable is a screened cable, it is necessary to maintain the screening across the joint, and this can conveniently be achieved by a conductive layer disposed completely around the insulating member.

The joint is preferably covered by an outermost sleeve of insulating material, that encloses the rigid insulating member and the conductive layer, and that seals on to each cable. This outermost sleeve may be a heat recoverable polymeric sleeve, or it may be a cold-applied sleeve.

The joint of the invention may be between more than two cables. The cables may have more than three cores.

A high voltage joint, in accordance with the present invention, will now be described, by way of example, with reference to the accompanying drawing, showing a 17.5kV three core paper cable screened joint in cross-sectional elevation.

Referring to the drawing, each cable comprises three insulated cable cores 1 (only two being seen) that extend beyond a metal cable sheath 2 that is contained within a cable jacket, or serving, 3. The conductors of the cores 1 of the two cables are electrically and mechanically connected together by screw connectors 4 that are then enclosed within a void filling mastic 5 and covered by an insulating RAYVOLVE sleeve 6. The jointed and insulated cores 1 are separated on each side of the connectors 4 by a 3-way insulating core spreader 7.

The joint region is enclosed within a 7 mm thick rigid polyethylene cylindrical tube 11 that has a closure fitting 9 at each end that reduces in diameter and is sealed at 12 to the cable sheath 2 on each side, to form a joint casing. The closures 9 have a port 10 therethrough, and the tube 11 is filled therethrough with a cold pourable insulating material 8 that occupies all the interstices between and around the jointed cores 1 and the spreaders 7. The insulating and sealing of the joint region is thus completed.

An earth screen in the form of a tinned copper mesh 14 is provided around the tube 1, closures 9, and extends across the joint. An earth continuity conductor 15 extends across the joint in electrical connection with the metal cable sheath 2 on each side. The braid 14 and conductor 15 are connected to the cable sheaths 2 by roll springs 13 so as to ensure a good earthing connection. Finally, a polymeric heat recoverable outer protective sleeve 16 is shrunk in to position to extend from the cable jacket 3 on one side of the joint to the jacket 3 on the other side.

Claims (9)

1. A high voltage cable joint between two cables each of which has three cores, wherein successive layers of each cable are stripped back, the conductors of one cable are connected to respective conductors of the other cable, each conductor connection is individually insulated, the cores are separated from each other by an insulating spacer arrangement, the connected cores are surrounded by and laterally spaced apart from a rigid relatively thick insulating tubular member, the insulating tubular member is sealed to each cable, the region contained within the insulating tubular member and surrounding the connected conductors is substantially filled with a pourable insulating medium, and wherein the joint comprises an electrically conductive, relatively thin layer that is disposed completely around the rigid insulating tubular member in abutment therewith along substantially its entire length and that is electrically connected to the conductive earthing sheath or screen of each cable, thereby to provide screening of the joint.

2. A cable joint according to claim 1, wherein the insulating filler medium was poured into the connection region substantially at ambient temperature.

3. A cable joint according to any one of the preceding claims, wherein the rigid insulating tubular member has a wall thickness of at least 3 mm along at least part and preferably all, of its length.

4. A cable joint according to any one of the preceding claims, wherein the rigid insulating member comprises a cylinder of substantially uniform diameter, and the joint comprises two end fittings that seal on to respective ends of the cylinder and reduce in diameter so as to seal on to respective ones of the cables.

5. A cable joint according to claim 4, wherein each end fitting has an aperture therethrough for receiving the filling material and/or allowing expulsion of air from around the connection.

6. A cable joint according to claim 4 or claim 5, wherein the end fittings are made of electrically insulating material.

7. A cable joint according to any one of claims 4 to 6, wherein the end fittings are rigid, or alternatively are recovered into position by the application of heat thereto.

8. A cable joint according to any one of the preceding claims, comprising an outer insulating layer protective sleeve that encloses the rigid insulating member and seals on to each cable.

9. A cable joint according to any one of the preceding claims, wherein the insulating filler medium comprises (a) a vegetable oil and a compatible resin, comprising from about 40 to 100 parts by weight of the oil and 60 to 0 parts by weight of the resin; (b) a latex, selected so as to be capable of forming a gel with the oil/resin mixture; (c) a desiccant, present in an amount sufficient to absorb water from the latex on mixing therewith, and (d) an inert filler present in an amount from 0 to 75% by weight of the total composition.