CA1201880A

CA1201880A - Jointing electric cables

Info

Publication number: CA1201880A
Application number: CA000418254A
Authority: CA
Inventors: Julian G. Head; Stephen R. Guy
Original assignee: Prysmian Cables and Systems Ltd
Current assignee: Prysmian Cables and Systems Ltd
Priority date: 1982-12-21
Filing date: 1982-12-21
Publication date: 1986-03-18

Abstract

ABSTRACT

JOINING ELECTRIC CABLES

In forming a joint between two electric cables having elastomeric or thermoplastic insulations 1,2, the usual joint screen 5 is formed by applying semi-conducting tapes and then a thin insulation layer 6 is formed over this screen before the main insulation 7 of thermoplastics or cross-linkable thermoplastics tapes is applied. The material of the thin insulation layer 6 is sufficiently soft and deformable so as substantially to cushion the screen 5 against deformation by the tapes of the main insulation layer (which deformation we have now found might cause electrical breakdown).

Description

~2()~0 JOINTING ELECTRIC CABLES

This invention relates to jointing high voltage electric cables having elastomeric or thermoplastics insulations.
It is known in jointing such cables, and once the ends of the two cables have been prepared by cutting back the insulation to expose the conductors and the conductors have been connected by welding or by use of a ferrule, to apply a number of layers of semi-conducting tape over the exposed conductors and ferrule and subsequently *o applylayer~ of thermoplastics or cross-linkable thermoplastics insulating tapes. Before applying the~e insulating tapes 7 the layers of semi-conducting tapes are moulded and if applicable cross-linked by the application of heat and pressure in order 15f to provide a continuous electrical screen. If necessary the screen is then treated (e.g. abraded) to provide a smooth outer surface. In the initial prepara*ion of the cable ends1 the cut back ends of the insulation are formed into a taper and, when subsequently applying the thermoplastics or cross-linkable thermoplastics insulating tapes, these are applied so as to fill in the space between the tapered insulation ends to a diameter equal to or greater than that of the cable insulation. Then the insulation thus re~ormed over the jointing zone is moulded and if applicable cross-lin~ed ~' ~L21!~

by -the application of hea-t and pressure, to elimin~te the possibility of voids wi-thin itself or be-tween itse:Lf ~nd the unclerlying screen.
We have discovered d~lring our d~velopment tests on such joints that electrical breakdown m~y occur at a ridge formed on the screen, which ridge we have found follows a helical path corresponding to the overlap or space between the overlying insulating *apes (depending whether these are applied with adjacen-t turns overlapping or but-t-spaced). Our laboratory investigations have indicated that such ridges result from deformation of material of the screen into the spaces which exist between adjacent turns of the insulating tapes (whether they are overlapped or butt-spaced), and have further indicated tha-t such deformation was due to the application of pressure at the interface between the screen and insulation of the joint.
In accordance with this invention, there is provided a method of forming a joint be-tween two electric cables having elastomericorthermoplastics insulation, comprising forming a join-t screen by applying semi-conducting tapes, forming a thin insulation layer over the screen and forming a main insulation layer over the thin insulation layer by applying thermoplastics or cross-linkable thermoplastics insulating tapes, the material of the thin insulation layer being sufficiently sof* and deformable so as substantially to cushion the screen against deformation by the tapes of the main insulation layer.
The thin, cushioning insulating layer may have a thickness of 2mm or less. By way of example, in the case of jointing cables having cross-linked polyethylene (XLPE) insulation, we have found that cross-linkable ethylene/propylene rubber (EPR) is suitable for the cushioning layer~ the tapes of the main joint insulation ~2~

layer also being of XLPE. It is then found -that the electric stress at -the screen/insulatiorl in-terface is reduced ~y approximately 20% owing to the difference in permittivity between XLPE and EPR. Although EPR is a material with good electrical performance it would not be desirable to utilise it for the complete joint insulation since the thermal resistance of such a joint would be much greater than that o~ an equivalent XLPE
joint or of the cable. The thin layer of EPR does not however significantly affect the thermal resistance.
An embodiment of this invention will now be described, by ~ay of example only7 with reference to the accompanying drawing, the single figure of which is a schematic side view of a joint constructed in accordance with the invention.
Referring to the drawing, in the jointing process the ends of the two cables are prepared by cutting back the elastomeric le.g. XLPE) insulations 1, 2 and connecting the conductors 3, 4 by welding as shown or by using a ferrule. A conductor screen 5 -for the joint is then formed by applying tapes of semi-conducting-plastics (e.g. EPR or polyethylene loaded with carbon black and optionally in cross-linkable form) to a number of layers, and moulded ~and if applicable Z5 cross-linked to form an elastomeric layer) by the application of heat and pressure. If necessary the screen may be treated to ensure a smooth outer surface.
A thin insulation layer 6 (of thickness 2mm or less, say) is formed directly over the screen 5 by applying elastomeric tapes (e.g. EPR). Then the main insulation layer 7 is formed by applying tapes of thermoplastics Dr cross-linkable thermoplastics insulating material (e.g. cross-linkable polyethylene), filling in the space between the tapered ends of the cable insulations to a diameter equal to or greater than that ~/

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of these cable insulations. Heat and pressure are -then applied to the insulation layers to mould them (and if applicablc cross~link them -to form elastomeric layers).
During this process the soft and deformable material of the thin insulation layer 6 (preferably of similar softness or softer than *he screen 5) deforms to apply a uniform pressure to the screen, so cushioning the latter from deformation by the o~er-lying tapes of the main insulation layer. Also, the thin cushionins layer 6 deforms into the spaces bet~een the adjacent turns of the overlying tapes of the main insulation layer but this has no significant electrical effect as the material of the cushioning layer is itself insulating.
After -this application of heat and pressure to the insulation layers, an external screen 8 (and any joint sleeve or other protective layers) are applied.

Claims

1. A method of forming a joint between two electric cables having elastomeric or thermoplastics insulation, comprising forming a joint screen by applying semi-conducting tapes, forming a thin insulation layer over the screen and forming a main insulation layer over the thin insulation layer by applying thermoplastics or cross-linkable thermoplastics insulating tapes, the material of the thin insulation layer being sufficiently soft and deformable so as substantially to cushion the screen against deformation by the tapes of the main insulation layer.

2. A method as claimed in claim 1, in which the insulation of the electric cables comprises cross-linked polyethylene, and in which method the material of the thin insulation layer comprises cross-linkable ethylene/
propylene rubber and the material of the main joint insulation comprises cross-linkable polyethylene, the method including the step of cross-linking these materials.