GB2108329A

GB2108329A - Joining electric cables

Info

Publication number: GB2108329A
Application number: GB08132697A
Authority: GB
Inventors: Julian Gordon Head; Stephen Richard Guy
Original assignee: Pirelli General Cable Works Ltd; Pirelli General Cable Works
Current assignee: Pirelli General Cable Works Ltd; Pirelli General Cable Works
Priority date: 1981-10-29
Filing date: 1981-10-29
Publication date: 1983-05-11
Also published as: GB2108329B

Abstract

In forming a joint between two electric cables having elastomeric or thermoplastic insulations 1, 2, a screen 5 is formed by applying semi-conducting tapes and then a thin insulation layer 6 is formed over the screen before a 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 might cause electrical breakdown). <IMAGE>

Description

SPECIFICATION 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 to apply layers of thermoplastics or cross-linkable thermoplastics insulating tapes. Before applying these insulating tapes, the layers of semi-conducting tapes are moulded and if applicable cross-linked by the application of heat and pressure in order 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 preparation of the cable ends, 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 reformed over the jointing zone is moulded and if applicable cross-linked by the application of heat and pressure, to eliminate the possibility of voids within itself or between itself and the underlying screen.

We have discovered during our development tests on such joints that electrical breakdown may occur at a ridge formed on the screen, which ridge we have found follows a helical path corresponding to the overiap or space between the overlying insulating tapes (depending whether these are applied with adjacent turns overlapping or butt-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 that 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 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.

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 layer also being of XLPE. It is then found that the electric stress at the screen insulation interface is reduced by 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 itforthe complete joint insulation since the thermal resistance of such a joint would be much greater than that of 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 way of example only, 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 ofthetwo cables are prepared by cutting back the elastomeric (e.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 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.

Athin 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 or 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 of these cable insulations. Heat and pressure are then applied to the insulation layers to mould them (and if applicable cross-link them to form eiastomeric layers). During this process the soft and deformable material of the thin insulation layer 6 (preferably of similar softness or softer than the screen 5) deforms to apply a uniform pressure to the screen, so cushioning the latter from deformation by the over-lying tapes of the main insulation layer.

Also, the thin cushioning layer 6 deforms into the spaces between 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.

1. A method of forming a joint between two electric cables having elastomeric or thermoplastics insulation, comprising forming a joint screen 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

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION 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 to apply layers of thermoplastics or cross-linkable thermoplastics insulating tapes. Before applying these insulating tapes, the layers of semi-conducting tapes are moulded and if applicable cross-linked by the application of heat and pressure in order 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 preparation of the cable ends, 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 reformed over the jointing zone is moulded and if applicable cross-linked by the application of heat and pressure, to eliminate the possibility of voids within itself or between itself and the underlying screen. We have discovered during our development tests on such joints that electrical breakdown may occur at a ridge formed on the screen, which ridge we have found follows a helical path corresponding to the overiap or space between the overlying insulating tapes (depending whether these are applied with adjacent turns overlapping or butt-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 that 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 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. 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 layer also being of XLPE. It is then found that the electric stress at the screen insulation interface is reduced by 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 itforthe complete joint insulation since the thermal resistance of such a joint would be much greater than that of 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 way of example only, 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 ofthetwo cables are prepared by cutting back the elastomeric (e.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 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. Athin 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 or 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 of these cable insulations. Heat and pressure are then applied to the insulation layers to mould them (and if applicable cross-link them to form eiastomeric layers). During this process the soft and deformable material of the thin insulation layer 6 (preferably of similar softness or softer than the screen 5) deforms to apply a uniform pressure to the screen, so cushioning the latter from deformation by the over-lying tapes of the main insulation layer. Also, the thin cushioning layer 6 deforms into the spaces between 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 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.

3. A method of forming a joint between two electric cables having elastomeric orthermoplastics insulation, said method being substantiaily as herein described with reference to the accompanying drawing.