GB2165689A

GB2165689A - High voltage cables

Info

Publication number: GB2165689A
Application number: GB08521986A
Authority: GB
Inventors: Timothy George Marsh; Anthony Edward Williamson
Original assignee: Associated Electrical Industries Ltd
Current assignee: Associated Electrical Industries Ltd
Priority date: 1984-10-08
Filing date: 1985-09-04
Publication date: 1986-04-16
Also published as: GB8521986D0; GB8425377D0

Abstract

A high voltage cable has a central conductor surrounded by two extruded layers of insulation with the innermost layer 2 being of higher permittivity than the outer layer 5 so that the average stress is reduced close to the conductor and increased away from it. Semiconductive layers 3, 4, 6, 7 may be provided. Layers 3, 4 may be made of ethylene copolymer with C. The insulation layers may be made of ethylene, propylene rubber with mica, cross-linked polyethylene with carbon black or an ethylene copolymer with mica. Each insulation layer 2, 5 is extruded between two semiconductive layers so that no dust particles can contaminate the insulation. <IMAGE>

Description

SPECIFICATION High voltage cables This invention relates to high voltage cables and more particularly to such cables rated at about 130 kV.

In conventional high voltage AC cables the stress gradient is exponential radially outwards from the conductor so that the outer parts of the insulation are under considerably lower average stress than the inner parts.

It is thus an object of the present invention to provide a high voltage cable in which the average stress is reduced close to the conductor and increased away from the conductor.

Accordingly, the invention provides a high voltage cable having a conductor surrounded by two or more extruded layers of insulation wherein the layer nearest the conductor is of highest permittivity and the outer layers are of successively lower permittivity.

In a preferred embodiment, the innermost layer of insulation has a semiconductor screen on its inside to form an interface with the conductor and there may also be a semiconductor screen on the outside of the outermost layer.

Preferably there are only two layers of insulation.

The invention also provides a method of manufacturing a high voltage cable wherein a first layer of insulation of high permittivity is first extruded around a conductor, and then at least one further layer of insulation of lower permittivity than the previous layer is extruded around the previous layer.

If each layer of insulation is extruded separately, then, preferably each layer of insulation is sandwiched between two layers of semiconductive material as it is extruded so that no dust particles can contaminate the insulation.

Thus a preferred embodiment of a high voltage cable comprises a conductor, which may be made up of several stranded conducting wires, a layer of semiconductive material, which may be a polyethylene copolymer containing conducting carbon black, a layer of high permittivity insulation, which may be either Ethylene Propylene Rubber (EPR) mixed with mica, a copolymer base mixed with mica or cross-linked polythene containing carbon particles, or any other high permittivity dielectric material, a second layer of semiconductive material, a layer of lower permittivity insulation, which may be either normal or crosslinked polyethylene and finally a third layer of semiconductive material, in order from the innermost conductor to the outermost layer of semiconductive material.

The invention will now be more fully described by way of example with reference to the drawing which shows a high voltage cable according to the invention.

A high voltage AC (130 kV) cable has a conductor made up of several copper wires 1 stranded together. This stranded conductor usually has a rough surface in places and may also have dust particles embedded between the strands. This can cause electrical discharges from the tips of the prominences so formed due to the large potential difference and thus damage the insulation. So the conductor is surrounded by an extruded layer of semi conducting material followed by a layer of insulation 2 of high permittivity made of EPR mixed with mica which is sandwiched by two layers 3, 4 of semiconductive material, made of polyethylene copolymer containing carbon black, forming a semi-conductor screen. The sandwiched layers are applied by extrusion in one pass so that no dust or other matter can get between the three layers.By having a semiconductor screen 3 next to the conductor 1 there is a smooth interface between the conductor and the insulation. In this way, any prominences on the particles on the conductor do not have a potential difference across them since the screen 3 equalises the potential across it.

A second layer of insulation 5 also sandwiched between two layers 6, 7 of semiconductive material is then applied by extrusion around the first-applied layers. Again, due to the sandwiching, no particles which may provide prominences for electrical discharges can get between the semiconductor screens 6, 7 and the insulation 5. Further, any dust which has settled on the outside of the semiconductor screen 4 is now between two layers of semiconductive material which have a large contact area and thus equalize the potential stresses between them. This second layer of insulation is made of polyethylene which may be cross-linked if desired.

A cable made as described above will have a lower maximum electrical stress than an equivalent size 'conventional' cable, or, if the same maximum stress is used as a conventional cable, a smaller thickness of insulation is required.

1. A high AC voltage cable having a conductor surrounded by two or more extruded layers of insulation wherein the layer nearest the conductor is of highest permittivity and the outer layers are of successively lower permittivity.

2. A high AC voltage cable according to Claim 1, wherein the innermost layer of insulation has a semiconductor screen on its inside to form an interface with the conductor.

3. A high AC voltage cable according to Claim 2, wherein there is a semiconductor screen on the outside of the outermost insulating layer.

4. A high AC voltage cable according to

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION High voltage cables This invention relates to high voltage cables and more particularly to such cables rated at about 130 kV. In conventional high voltage AC cables the stress gradient is exponential radially outwards from the conductor so that the outer parts of the insulation are under considerably lower average stress than the inner parts. It is thus an object of the present invention to provide a high voltage cable in which the average stress is reduced close to the conductor and increased away from the conductor. Accordingly, the invention provides a high voltage cable having a conductor surrounded by two or more extruded layers of insulation wherein the layer nearest the conductor is of highest permittivity and the outer layers are of successively lower permittivity. In a preferred embodiment, the innermost layer of insulation has a semiconductor screen on its inside to form an interface with the conductor and there may also be a semiconductor screen on the outside of the outermost layer. Preferably there are only two layers of insulation. The invention also provides a method of manufacturing a high voltage cable wherein a first layer of insulation of high permittivity is first extruded around a conductor, and then at least one further layer of insulation of lower permittivity than the previous layer is extruded around the previous layer. If each layer of insulation is extruded separately, then, preferably each layer of insulation is sandwiched between two layers of semiconductive material as it is extruded so that no dust particles can contaminate the insulation. Thus a preferred embodiment of a high voltage cable comprises a conductor, which may be made up of several stranded conducting wires, a layer of semiconductive material, which may be a polyethylene copolymer containing conducting carbon black, a layer of high permittivity insulation, which may be either Ethylene Propylene Rubber (EPR) mixed with mica, a copolymer base mixed with mica or cross-linked polythene containing carbon particles, or any other high permittivity dielectric material, a second layer of semiconductive material, a layer of lower permittivity insulation, which may be either normal or crosslinked polyethylene and finally a third layer of semiconductive material, in order from the innermost conductor to the outermost layer of semiconductive material. The invention will now be more fully described by way of example with reference to the drawing which shows a high voltage cable according to the invention. A high voltage AC (130 kV) cable has a conductor made up of several copper wires 1 stranded together. This stranded conductor usually has a rough surface in places and may also have dust particles embedded between the strands. This can cause electrical discharges from the tips of the prominences so formed due to the large potential difference and thus damage the insulation. So the conductor is surrounded by an extruded layer of semi conducting material followed by a layer of insulation 2 of high permittivity made of EPR mixed with mica which is sandwiched by two layers 3, 4 of semiconductive material, made of polyethylene copolymer containing carbon black, forming a semi-conductor screen. The sandwiched layers are applied by extrusion in one pass so that no dust or other matter can get between the three layers.By having a semiconductor screen 3 next to the conductor 1 there is a smooth interface between the conductor and the insulation. In this way, any prominences on the particles on the conductor do not have a potential difference across them since the screen 3 equalises the potential across it. A second layer of insulation 5 also sandwiched between two layers 6, 7 of semiconductive material is then applied by extrusion around the first-applied layers. Again, due to the sandwiching, no particles which may provide prominences for electrical discharges can get between the semiconductor screens 6, 7 and the insulation 5. Further, any dust which has settled on the outside of the semiconductor screen 4 is now between two layers of semiconductive material which have a large contact area and thus equalize the potential stresses between them. This second layer of insulation is made of polyethylene which may be cross-linked if desired. A cable made as described above will have a lower maximum electrical stress than an equivalent size 'conventional' cable, or, if the same maximum stress is used as a conventional cable, a smaller thickness of insulation is required. CLAIMS

4. A high AC voltage cable according to Claim 3, wherein each layer of insulation is sandwiched between two layers of semiconductive material.

5. A high AC voltage cable according to any preceding Claim, wherein at least one semiconducting layer is a polyethylene copolymer containing conducting carbon black.

6. A high AC voltage cable according to any preceding Claim, wherein at least one layer of insulation is either, Ethylene Propylene Rubber mixed with mica, or, a copolymer base mixed with mica or cross-linked polythere containing carbon particles.

7. A high AC voltage cable according to any preceding Claim, wherein there are only two layers of insulation.

8. A method of manufacturing a high AC voltage cable wherein a first layer of insulation of high permittivity is first extruded around a conductor, and then at least one further layer of insulation of lower permittivity than the previous layer is extruded around the previous layer.

9. A method of manufacturing a high AC voltage cable according to Claim 8, wherein each layer of insulation is sandwiched between two layers of semiconductive material as it is extruded.

10. A high AC voltage cable or a method of manufacturing a high AC voltage cable substantially as described herein with reference to the accompanying drawing.