GB2058489A

GB2058489A - Prevention of breakdown in cable joints and terminations

Info

Publication number: GB2058489A
Application number: GB8028192A
Authority: GB
Original assignee: BICC PLC
Current assignee: Balfour Beatty PLC
Priority date: 1979-09-04
Filing date: 1980-09-01
Publication date: 1981-04-08
Also published as: GB2058489B

Abstract

A fluid-filled cable joint or termination includes a path 1, for flow of oil between a cable conductor and the space 3, in which a sheet 6 of fibrous insulating material, e.g. paper, is situated in order to substantially reduce risk that particles of metal might settle along the path and produce electric breakdown of the conductor insulation. Preferably, the sheet 6 is preformed to the cross- sectional shape and/or profile of the path 1 and has an uncalendered rough surface texture and a low density and high porosity. <IMAGE>

Description

SPECIFICATION Improvements in electric cable joints and terminations This invention relates to joints and terminations for fluid-filled electric cables, that is to say cables in which the dielectric of the or each cable conductor connected or terminated at the joint or termination includes a dielectric fluid, usually oil, which is free to flow along the cable conductor or conductors. The insulation of the or each cable conductor is a laminated body built up by lapping tapes of paper or other insulating material onto the cable conductor.

The invention is particularly concerned with joints and terminations in which a path or paths is or are provided for free flow of the dielectric fluid from the or each cable conductor into the space bounded by the joint or termination enclosure or vice versa, for example straight feed joints, stop joints and terminations for oil-filled cables in which the oil serves as part of the dielectric of the cable. In some applications in joints and terminations of this kind the oil also serves as a cooling medium, the oil circulating through the cable conductor or conductors and through external cooling means. A stop joint is a joint which presents a barrier to the flow of the dielectric fluid from one to the other of the two cable lengths electrically connected at the joint.

The invention includes a joint and termination structures of the aforesaid kind and cable installations incorporating joints and terminations made therefrom.

The invention is mainly concerned with joints between and terminations of single core fluidfilled cables and will be described as applied to such joints and terminations.

Especially, but not exclusively, the invention relates to stop joints in which the barrier to the flow of the dielectric fluid comprises an elongate metal member, by which the conductors of the two cables are both electrically and mechanically connected, and a preformed body of insulating synthetic resin which surrounds at least part of and is directly or indirectly sealed to the metal member and to a fluid-tight enclosure for the joint.

The elongate metal member may be a solid or hollow rod extending between and connected to two ferrules into each of which one of the conductors fits or it may itself be a ferrule, into each end of which one of the conductors fits, the metal member being provided with a central barrier to prevent dielectric fluid flowing from one conductor to the other. The preformed body of insulating synthetic resin surrounding the metal member may for example be vacuum cast from an epoxy resin and it may be sealed directly to the metal member or to another body surrounding and sealed to the metal member.In practice, the or each path for free flow of dielectric fluid between a cable conductor and the space bounded by the joint enclosure usually comprises either a port extending through the preformed body of insulating sythetic resin or ports formed between the preformed body and two preformed end bodies of insulating synthetic resin positioned at opposite ends of the preformed body.

There is a risk that particles of metal or other materials inadvertantly produced during cable manufacture and jointing and carried by the flowing dielectric fluid to a joint or termination might settle along a path between a cable conductor, which is at the operating voltage of the cable installation, and the space bounded by the joint or termination enclosure, which enclosure is at earth or a low potential, and that the metal particles will initiate electric distress by contacting each other and producing an electric discharge or will become aligned and produce electric breakdown of the adjacent conductor insulation due to excessive electric stress at the tips of the aligned particles.

The present invention provides a joint or termination for fluid-filled electric cables which is of such a construction that the aforesaid risks are substantially reduced.

According to the present invention in a joint or termination in which the or each path for flow of dielectric fluid between a cable conductor and the space bounded by the joint or termination enclosure has a boundary wall which defines the path and which is a surface of at least one preformed body of an insulating sythetic resin, at least one sheet of fibrous insulating material is positioned in, and extends along at least a major part of the length of, the or each path.

Preferably, the or each sheet of fibrous insulating material is preformed to or approximately to the cross-sectional shape andior profile of the path in which it is positioned.

The or each sheet of fibrous insulating material is preferably of paper having an uncalendered rough surface texture and a low density and high porosity.

Since fibres of the or each sheet of fibrous insulating material protrude into the path in which it is positioned, these fibres restrict the mobility of any metal particles that may be carried by the flowing dielectric fluid, thereby reducing the risk that some metal particles will contact each other and initiate electric distress and the risk that some metal particles will become aligned and produce electric breakdown of the adjacent conductor insulation due to excessive electric stress at the tips of the aligned particles. Furthermore some metal particles will penetrate the fibrous insulating material, thereby reducing the risk of a concentration of particles building up on the surface of the sheet of fibrous insulating material.

At one or each end of the or each path, means may be provided for filtering from the flowing dielectric fluid particles of metal and other undesirable materials that may be carried by the fluid and that may not be entrapped by the or a sheet of fibrous insulating material.

Where the dielectric fluid is an insulating oil the materials of which the filtering means may be made include metal, paper, glass fibre, woven filter fabric such as PTFE and sintered products.

Where the filtering means is of metal it may be a fine metallic mesh serving as an electrical screen and it may be used in conjunction with other filtering means.

The invention is further illustrated, by way of example, in the accompanying drawing which shows a fragmental sectional side elevation of approximately one half of an oil-filled cable stop joint.

Referring to the drawing, an oil path 1 which extends between a port (not shown) in one end of a ferrule 2t which interconnects the conductors at the joint and has at an intermediate position along its length a barrier to flow of oil from one conductor to the other, and the space 3 bounded by the joint enclosure (not shown). The oil path 1 which is bounded by an internal surface of a preformed body 4 of insulating synthetic resin and an external surface of a preformed end body 5 of insulating synthetic resin and a frusto-conical preform 6 of paper having an uncalendered rough surface texture and a low density and high porosity is positioned in and extends along a major part of the length of the oil path. A filter 7 is provided at the end of the oil path 1 opening into the space 3; a filter (not shown) may also be provided at the end of the oil path opening into the port in the ferrule 2.

Claims

1. A fluid-filled electric cable joint or termination in which the or each path for flow of dielectric fluid between a cable conductor and the space bounded by the joint or termination enclosure has a boundary wall which defines the path and which is a surface of at least one preformed body of an insulating synthetic resin, wherein at least one sheet of fibrous insulating material is positioned in, and extends along at least a major part of the length of, the or each path.

2. A fluid-filled electric cable joint or termination as claimed in Claim 1, wherein the or each sheet of fibrous insulating material is preformed to or approximately to the crosssectional shape and/or profile of the path in which it is positioned.

3. A fluid-filled electric cable joint or termination as claimed in Claim 1 or 2, wherein the or each sheet of fibrous insulating material is of paper having an uncalendered rough surface texture and a low density and high porosity.

4. A fluid-filled electric cable joint or termination as claimed in any one of the preceding Claims, wherein, at one or each end of the or each path, means is provided for filtering from the flowing dielectric fluid particles of metal and other undesirable materials that may be carried by the fluid and that may not be entrapped by the or a sheet of fibrous insulating material.

5. A fluid-filled electric cable joint or termination as claimed in any one of the preceding Claims, wherein the filtering means is or includes a fine metallic mesh serving as an electrical screen.

6. An oil-filled electric cable stop joint substantially as hereinbefore described with reference to and as shown in the accompanying drawing.