GB2259400A

GB2259400A - Extra-high-voltage power cable

Info

Publication number: GB2259400A
Application number: GB9219285A
Authority: GB
Inventors: Malcom Anthony Simmons; Julian Gordon Head
Original assignee: Prysmian Cables and Systems Ltd
Current assignee: Prysmian Cables and Systems Ltd
Priority date: 1988-10-17
Filing date: 1992-09-11
Publication date: 1993-03-10
Anticipated expiration: 2012-09-11
Also published as: GB2259400B; GB9219285D0

Abstract

An extra-high-voltage power cable is provided with extruded insulation 3 over a conductor 1 thereof. The insulation comprises a layer 5 of unfilled high density polyethylene or polypropylene having a higher electric strength than the material of the insulation in layer 6. In the embodiment layer 5 may or may not be cross-linked, has an electric strength fifty times greater than layer 6 and a thickness a third of the extruded insulation. The outer insulation layer 6 may be cross-linked low density polyethylene. Semiconductive screening layers 2, 4 are provided. Since the layer 5 is unfilled it is translucent thereby assisting inspection of the integrity of layer 2 prior to extrusion of the outer layer 6. <IMAGE>

Description

EXTRA-HIGH-VOLTAGE POWER CABLE This invention relates to extra-high-voltage power cables, that is 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.

An object of the present invention is to enable the thickness of extruded insulation over conductors of extra-high-voltage power cables to be reduced and whilst this is particularly applicable to cables of 400 kV for the reasons discussed above, it is also applicable to such cables of lower voltages.

This object is achieved in accordance with the invention by providing the insulation with an inner layer, where electric stress will be higher, formed of an unfilled high density polyethylene or polypropylene material having a higher electric strength than the material of the insulation adjacent thereto.

Thus, in accordance with the invention there is provided an extra-high-voltage power cable provided with extruded insulation over a conductor thereof, said insulation comprising an inner layer of an unfilled high density polyethylene or polypropylene material having a higher electric strength than the material of the insulation adjacent thereto.

The electric strength of the material of said inner layer may be at least 50 percent greater than that of the material of the insulation adjacent thereto.

Whilst the material of said inner layer may be cross-linked it may also be un-crosslinked.

The material of the insulation adjacent the inner layer may comprise a cross linked low density polyethylene, i.e. a material currently commonly used for the whole of the extruded insulation.

The thickness of the inner layer is preferably no more than a third of the thickness of the extruded insulation.

In a preferred embodiment, the insulation comprises two layers.

The invention also includes a method of manufacturing an extra-high-voltage cable including the step of extruding insulation over a conductor of the cable such that the insulation has an inner layer of an unfilled high density polyethylene or polypropylene material having a higher electric strength than the material of the insulation adjacent thereto.

Preferably the inner layer is extruded over the conductor upstream of the material of the insulation adjacent to the inner layer being extruded over the inner layer, such that the interface between the inner layer and a screen over the conductor may be optically inspected through the inner layer prior to the material of the insulation adjacent to the inner layer being extruded over the inner 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.

In the embodiment the material of the outer layer comprises a cross linked 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 cross linked 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 need not be 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 also advantageous in connection with other extra-highvoltage cables in that it enables the thickness of the extruded insulation to be reduced.

Attention is directed to the claims of our copending application No. 8824285.4 (Publication No.

2223877A) from which the present application is directed.

Claims

CLAIMS:

1. An extra-high-voltage power cable provided with extruded insulation over a conductor thereof, said insulation comprising an inner layer of an unfilled high density polyethylene or polypropylene material having a higher electric strength than the material of the insulation adjacent thereto.

2. A cable as claimed in claim 1, wherein the electric strength of the material of said inner layer is at least 50 percent greater than that of the material of the insulation adjacent thereto.

3. A cable as claimed in claim 1 or 2, wherein said material of the inner layer is cross-linked.

4. A cable as claimed in claim 1 or 2, wherein said material of the inner layer is un-crosslinked.

5. A cable as claimed in any one of the preceding claims wherein the material of the insulation adjacent said inner layer comprises a cross linked low density polyethylene.

6. A cable as claimed in any one of the preceding claims wherein the thickness of the inner layer is no more than a third of the thickness of the extruded insulation.

7. A cable as claimed in any one of the preceding claims wherein the insulation comprises two layers.

8. A method of manufacturing an extra-high-voltage cable including the step of extruding insulation over a conductor of the cable such that the insulation has an inner layer of an unfilled high density polyethylene or polypropylene material having a higher electric strength than the material of the insulation adjacent thereto.

9. A method as claimed in claim 8 wherein the inner layer is extruded over the conductor upstream of the material of the insulation adjacent to the inner layer being extruded over the inner layer, such that the interface between the inner layer and a screen over the conductor may be optically inspected through the inner layer prior to the material of the insulation adjacent to the inner layer being extruded over the inner layer.

10. A method as claimed in claim 9, and including the step of optically inspecting said interface through the inner layer.

11. An extra-high-voltage power cable substantially as hereinbefore described with reference to the accompanying drawing.

12. A method of manufacturing an extra-high-voltage cable substantially as hereinbefore described with reference to the accompanying drawing.