Power Cable

Abstract: An extra-high-voltage power cable is provided with extruded insulation 3 over a conductor 1 thereof. The insulation comprises an inner layer 5 of an unfilled high density polyethylene or polypropylene material having a higher electric strength than the material of the insulation adjacent thereto in an outer layer 6 thereof.

Description: This invention relates to power cables for voltages of 132 kV and above, which are provided with extruded insulation over their conductors. Currently cables up to and including 275 kV are being provided with extruded insulation comprising crosslinked low density polyethylene. However the use of such material for cables of higher voltages, for example 400 kV, requires the insulation to have a thickness which would result in unacceptable increases in the cable diametral dimensions both as regards to production and installation and, of course, material costs for the components of the cable radially outwardly of the insulation. In order to reduce the thickness of extruded insulation of cables it is known to form the insulation in layers which are graded according to their dielectric constant (also referred to as permittivity or specific inductive capacitance (sic)), with the inner layer of the insulation (wherein the electric stress will be higher) having a higher dielectric constant than the rest of the insulation. Examples of cables having such dielectric constant graded insulation layers are disclosed in US2717917, GB 2165689, GB 1194750 and US 4132858. US 3711631 discloses that cables with several layers of insulating materials arranged such that the inner insulations had a higher breakdown strength than the outer insulation had been proposed but that this structure had no particular technical advantages except possibly a cost advantage over a cable with a single insulating layer of highest breakdown voltage material. Instead, US 3711631 proposes the use of extruded insulation formed in layers which are graded according to a so-called 'strength constant' which is defined as the product of the dielectric constant and the maximum allowable dielectric stress. We have found that for cables for voltages of 132kV and above it is more important to grade the layers of the insulation according to their electric strength rather than their dielectric constant or so-called 'strength constant'. In this connection it will be appreciated that in general increasing the dielectric constant of the material by adding appropriate fillers will give rise to a decrease in its electric strength and may result in a change in the 'strength constant' in either direction. The present invention provides a power cable for voltages of 132kV and above provided with extruded insulation over a conductor thereof, said insulation comprising two layers, the material of the inner layer of which having a higher electric strength than the material of the outer layer characterised in that (i) said material of the inner layer is an un-crosslinked unfilled high density polyethylene or an un-crosslinked polypropylene material, (ii) said material of said outer layer comprises a crosslinked low density polyethylene, and (iii) the thickness of the inner layer is no more than a third of the thickness of the extruded insulation. The electric strength of the material of said inner layer may be at least 50 percent greater than that of the outer 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 with reference to the accompanying drawing in which the single figure is a schematic cross-sectional view of a core of a 400 kV cable. The core illustrated in the drawing comprises a central stranded conductor 1 an extruded, semiconducting screen layer 2 over the conductor, extruded insulation 3 over the screen layer 2 and an extruded semiconducting screen layer 4 over the extruded insulation 3. As thus far described the construction of the core is the same as that for a conventional 275 kV cable having extruded insulation. However, in the illustrated embodiment the extruded insulation 3 comprises an inner layer 5 and an outer layer 6. The inner layer is of a material selected for having a higher electric strength than the material of the outer layer 6. The material of the outer layer comprises a crosslinked low density polyethylene such as that presently conventionally used for the whole of the extruded insulation of conductor cores in 275 kV cables. The material of the inner layer in the embodiment is a high density polyethylene or a polypropylene and has an electric strength which is at least 30, and preferably at least 50%, greater than that of the crosslinked low density polyethylene of the outer layer. By utilising material with higher electric strength in the inner layer of the extruded insulation the overall thickness of the extruded insulation can be significantly reduced as compared with the thickness required if the insulation comprised crosslinked low density polyethylene throughout. The thickness of the inner layer 5 is not as great as the thickness of the outer layer 6 and is preferably no more than about 1/3 of the thickness of the extruded insulation. The inner layer 5 is not crosslinked as the form stability of the insulation is maintained by the greater thickness of the crosslinked outer layer. Furthermore, the bending stiffness of the extruded insulation is largely dependent upon the lower density polyethylene outer layer rather than the high density polyethylene or polypropylene inner layer and accordingly the flexibility of the core may be greater than that of a corresponding core where the extruded insulation comprises low density polyethylene throughout and accordingly has a greater thickness. The material of the inner layer is unfilled and accordingly translucent when being extruded. This is of particular advantage in that if the inner layer 5 is extruded upstream of the outer layer 6 it is possible to optically inspect through the inner layer the interface between the inner layer and the inner screen layer 2 prior to the outer layer 6 being extruded over the inner layer 5. In this way the interface can be checked for imperfections which may give rise to electrical breakdown. Thus in a preferred method of producing the illustrated core, the inner layer 5 is extruded onto or with the screen layer 2, the interface between the layers 5 and 2 are optically inspected and subsequently the layer 6 is extruded, possibly together with the screen layer 4, over the inner layer 5. It will of course be appreciated that subsequent to the manufacture of the core illustrated, that core would be provided with conventional outer layers. It will also be appreciated that although particularly applicable to 400 kV cables, the present invention is advantageous in connection with cables for voltages of 132 kV and above in that it enables the thickness of the extruded insulation to be reduced.

Claims: A power cable for voltages of 132kV and above provided with extruded insulation (3) over a conductor (1) thereof, said insulation (3) comprising two layers (5, 6), the material of the inner layer (5) of which having a higher electric strength than the material of the outer layer (6) characterised in that (i) said material of the inner layer (5) is an un-crosslinked unfilled high density polyethylene or an un-crosslinked polypropylene material, (ii) said material of said outer layer (6) comprises a crosslinked low density polyethylene, and (iii) the thickness of the inner layer (5) is no more than a third of the thickness of the extruded insulation (3). A cable as claimed in claim 1, wherein the electric strength of the material of said inner layer (5) is at least 50 percent greater than that of the outer layer (6).

Inventors: HEAD JULIAN GORDON, SIMMONS MALCOM ANTHONY HIGH VI, HEAD, JULIAN GORDON, SIMMONS, MALCOM ANTHONY HIGH VIEW
Applicants: PIRELLI GENERAL PLC
Application year: 1989
Main classification: H01B9
IPC codes: H01B7, H01B9