ELECTRIC CABLE JOINTING OR TERMINATING METHOD

Abstract: An electric cable having a current carrying conductor pro­ vided with an extruded polymeric insulating layer, has a layer of a high permittivity material extruded over the whole length of the insulating layer, which layer is covered with semi-conducting tape in contact therewith.

Description: This invention relates to a method of making an electric cable termination or joint. In known high voltage cables in which the current carrying conductors are provided with extruded polymeric insulation, a layer of polymeric insulation is disposed over the conductor with a first semi-conducting layer between the conductor and insulation and a second semi-conducting layer over the insulation, and a conducting sheath is provided over the second semi-conducting layer to provide an earth screen. It is conventional to co-extrude the semi-conducting layers with the insulating layer whereby the semi-conducting layers are bonded to the insulating layer. The purpose of the semi- conducting layer is to prevent electrical discharges occurring due to voids being present between the inner surface of the insulating layer and the conductor and between the outer surface of the insulating layer and the conducting sheath. The semi-conducting layer over the insulating layer has to be removed during jointing or termination of the cable in the region of the joint or termination as it normally has a resistance such that it would carry unacceptable high currents if it were allowed to form a path between the current carrying conductor and earth in the joint or termination. Removal of the semi-conducting layer over the insulating layer therefrom is a difficult operation which involves the risk of damage being caused to the insulation layer. To avoid this problem, it has been proposed to provide over the whole length of the insulation layer a layer of highly resistive material whose resistance is voltage dependent instead of a semi-conducting layer since such a voltage dependent resistive layer does not have to be removed during jointing or termination. However, this solution suffers from the disadvantage that material with the appropriate voltage dependent resistivity is not suitable for application by conventional extrusion equipment. The present invention seeks an alternative solution in which the semi-conducting layer over the insulating layer is replaced by an alternative layer which can be readily extruded over the insulating layer but which does not have to be removed during jointing or terminating. In GB-A-2095024 there is disclosed an electric cable in which respective layers of material having a relative permittivity which is higher than the relative permittivity of the insulating layer are provided under and over the insulating layer in order to reduce dielectric losses. EP-A-0076579 discloses an electric cable having at least one current carrying conductor provided with an extruded insulating layer, over the whole length of which is applied a layer of material having a relative permittivity which is higher than the relative permittivity of the material of the first-mentioned layer, the second mentioned layer being covered with a semi-conducting tape in contact therewith. Also disclosed in EP-A-0076579 is a method of making a cable termination with such a cable which method includes stripping back the semi-conducting tape along an end portion of the cable. When the semi-conducting tape is removed prior to cable termination most of the second-mentioned layer is removed with it and to this end the second-mentioned layer adheres strongly to the tape and modestly to the first-mentioned layer and preferably comprises a paint-type material which is applied by spraying, dipping, washing or wiping although it may comprise a material which is extruded over the first-mentioned layer. The present invention provides an electric cable termination or joint including: (a) providing an electric cable having at least one current carrying conductor provided with an extruded polymeric insulating layer, over the whole length of which is an extruded layer of a material having a relative permittivity which is higher than the relative permittivity of the material of the first-mentioned layer, the second-mentioned layer being covered with a semi-conducting tape in contact therewith; and (b) stripping back the semi-conducting tape along an end portion of the cable; characterised in that the relative permittivity of the material of said second layer is at least 15 and in that none of said second-mentioned layer exposed by the stripping back of said tape is stripped from said first-mentioned layer. In order that the invention may be well understood, an embodiment thereof, which is given by way of example only, will now be described The embodiment comprises a method of making an electrical termination or joint with a high voltage electric cable having: a single current carrying conductor; a layer of semi-conducting material extruded over the conductor; a layer of polymeric insulating material extruded over the layer of semi-conducting material; and a layer of high permittivity material, that is a material with a relative permittivity of at least 15 and preferably for example 20 to 30, or even more, extruded over the layer of insulating material. All three extruded layers extend the whole length of the conductor and the insulating layer is bonded to the layers on each side of it. The three layers may be co-extruded or extruded in separate operations on the cable length. In order to fulfil its functions of preventing electrical discharges the high permittivity layer is covered with semi-conducting tape which is lapped or longitudinally wrapped around the high permittivity layer in contact with the outer surface thereof. The semi-conducting tape may include a dessicant. Whilst it is preferable that the entire surface of the high permittivity layer be contacted by the semi-conducting tape small breaks in such contact will not give rise to electrical discharges because of the high permittivity of the layer. A conventional metallic earth screen is provided over the semi-conducting tape. When making a joint or termination the semi-conducting tape over the high permittivity layer is stripped back by simple unwinding or unwrapping thereof to the termination of the earth screen, but since the high permittivity layer will carry negligible current if left in a position at a joint or termination, it does not have to be stripped from the insulation layer. This not only simplifies jointing and terminating operations and eliminates the possibility of damage to the insulation layer's outer surface but also assists stress distribution within the joint or termination as the high permittivity layer tends to reduce the surface gradient at the earth screen termination. An example of a suitable high permittivity material is nitrile rubber containing anatase.

Claims: A method of making an electric cable termination or joint including: (a) providing an electric cable having at least one current carrying conductor provided with an extruded polymeric insulating layer, over the whole length of which is an extruded layer of a material having a relative permittivity which is higher than the relative permittivity of the material of the first-mentioned layer, the second-mentioned layer being covered with a semi-conducting tape in contact therewith; and (b) stripping back the semi-conducting tape along an end portion of the cable; characterised in that the relative permittivity of the material of said second layer is at least 15 and in that none of said second-mentioned layer exposed by the stripping back of said tape is stripped from said first-mentioned layer.

Inventors: BALL, EDMUND HUGH
Applicants: PIRELLI GENERAL PLC
Application year: 1986
Main classification: H01B9
IPC codes: H01B9