WO2002059910A1

WO2002059910A1 - An electrical cable

Info

Publication number: WO2002059910A1
Application number: PCT/GB2002/000009
Authority: WO
Inventors: Alan John Clark
Original assignee: Cortland Fibron Bx Ltd.
Priority date: 2001-01-24
Filing date: 2002-01-04
Publication date: 2002-08-01
Also published as: GB0101893D0

Abstract

An electrical cable (18) comprising an electrical conductor (20), an inner insulation layer (26) around the electrical conductor (230), and an outer insulation layer (28) around the inner insulation layer (26), the inner and outer insulation layers (26, 28) being such that they are made of materials which are different from each other and which have different permitivities, whereby the voltage drop across the inner and outer insulation layers (26, 28) is such as to reduce stress level build-up at the interface of the electrical conductor (20) with the inner insulation layer (26).

Description

AN ELECTRICAL CABLE

This invention relates to an electrical cable and, more especially, this invention relates to a high alternating voltage electrical cable.

Known high voltage electrical cables comprise an electrical conductor, an inner semi-conducting screen, an outer semi-conducting screen, and insulation between the inner and outer semi-conducting screens. The electrical conductor is usually a stranded conductor made of a plurality of electrically conducting strands. The insulation may be a high voltage cross linked polyethylene (XLPE) or ethylene propylene rubber (EPR) material. The inner semi-conducting screen is basically applied over the electrical conductor, whilst the outer semi-conducting screen is applied over the insulation. The inner semiconducting screen over the electrical conductor is very important. The purpose of this inner semi-conducting screen is to provide a smooth interface between the insulation and the electrical conductor. It is at this interface that the electrical stress is highest with an exponential decay over the remainder of the insulation. The electrical stress is increased if there are any sharp points or non-conformities along the electrical conductor surface. The inner semi-conducting screen is used to smooth out the undulations of the electrical conductor that consequently increase the electrical stress. Also, as the inner semi-conducting screen is usually made of a plastics material, it is able to be bonded to the insulation. It is essential that there are no voids between the inner semiconducting screen and the insulation as such voids may give rise to high stress points and Corona discharge.

Conventional high voltage electrical cable 6.6Kv and above manufacturing processes require triple layer extruders utilising catenary cooling and cross-linking tubes . These tubes are extensive and require a two storey building. Equipment is required to take the stranded electrical conductor from an input drum to a second storey of the building. Barrier tape is applied around the electrical conductor, followed by the inner semi-conducting screen, the insulation, and then the outer semi-conducting screen. The inner and outer semi-conducting screens and the intermediate insulation are applied by three separate extruders feeding into a triple layer head. Careful design and considerable engineering work are required, together with extrusion tooling for the head. From the head, the completed core is fed along a catenary line, for example, some 80 metres long. The first part of the catenary line is at a high temperature and pressure, and provides the medium for cross linking. Later sections of the catenary line provide graded cooling. This known manufacturing process is complex, and it involves the high capital cost together with a high operator competence level . It is inherent with electrical cable insulated with one type of material, that the majority of the applied voltage is dropped across the first small part of the insulation. The electrical stress (which is voltage divided by distance) is at its highest near the electrical conductor, and decays exponentially over the remainder of the insulation. The outer section of the insulation is not being used to its maximum effect .

It is an aim of the present invention to provide an electrical cable in which the voltage drop across the complete insulating material is better distributed, in order to reduce the stress levels building up at the inner semi-conductor interface.

Accordingly, in one non-limiting embodiment of the present invention, there is provided an electrical cable comprising an electrical conductor, an inner insulation layer around the electrical conductor, and an outer insulation layer around the inner insulation layer, the inner and outer insulation layers being such that they are made of materials which are different from each other and which have different permitivities, whereby the voltage drop across the inner and outer insulation layers is such as to reduce stress level build-up at the interface of the electrical conductor with the inner insulation layer.

Thus the electrical cable of the present invention uses two discrete materials of different permitivity for insulating the electrical conductor. It is possible by the careful selection of materials and permitivity to control the way in which the voltage is distributed across the entire insulation. The method makes effective use of the outer insulation layer, with more of the applied voltage being dropped across this outer insulation layer.

The reduction of alternating voltage stress levels within the electrical cable insulation operating in the voltage range of 5Kv to 15Kv may provide a significant improvement in the life expectancy, safety aspects and power transmission levels of dynamic electrical cables.

The invention may utilise the different electrical properties of polymeric material used in the inner insulation layer and the outer insulation layer, in order to distribute the voltage drop linearly across the full diameter of the insulating material, thereby reducing the likelihood of high electrical stress points within the structure of the electrical cable.

The electrical cable may be one in which the inner insulation layer is made of thermoplastic elastomer filled with a material that enables adjustment of the permitivity, and in which the outer insulation layer is made of a medium density polyethylene. Other materials may be employed.

The electrical cable may be one in which the permitivity of the inner insulation layer is high, for example in the range of 3.5 - 6.0, and in which the permitivity of the outer insulation layer is low, for example in the range of 2.3 - 2.7. The outer insulation layer may be a conventional high- voltage material suitable for use up to 30kV. The inner insulation layer may be an extrudable material with good insulating properties and suitable for operation at voltages up to 4kV (phase to phase) . The inner insulation layer may be a single material or several materials having different permitivity values. The permitivity value for the inner insulation layer needs to be varied in dependence on operating voltage and conductor size. The above mentioned thermoplastic material may be blended with other materials, for example with low levels of a very high permitivity material.

The electrical cable may include barrier tape provided around the electrical conductor and positioned between the electrical conductor and the inner insulating layer.

The electrical cable may include a metallic outer layer.

An embodiment of the invention will now be described solely by way of example and with reference to the accompanying drawing in which:

Figure 1 is a section through a known electrical cable; and

Figure 2 is a section through an electrical cable of the present invention.

Referring to Figure 1, there is shown an electrical cable 2 comprising an electrical conductor 4 which is made of individual electrically conducting strands 6. The electrical cable 2 also comprises barrier tape 8 which surrounds the electrical conductor 4. An inner semiconducting screen 10 surrounds the barrier tape 8. A layer of insulation 12 surrounds the inner semi-conducting screen 10. An outer semi -conducting screen 14 surrounds the insulation 12. A metallic layer 16 surrounds the outer semi-conducting screen 14.

The electrical cable 2 is such that the inner semiconducting screen 10 is of prime importance. The purpose of this inner semi-conducting screen 10 is to provide a smooth interface between the insulation 12 and the electrical conductor 4. It is at this interface that the electrical stress is highest. The electrical stress is increased if there are any sharp points or non-conformities along the surface of the electrical conductor 4. The inner semi-conducting screen 10 is used to smooth out the undulations caused by the individual strands 6 of the electrical conductor 4, which undulations can increase the electrical stress. Also, the inner semi-conducting screen 10 is made of a plastics material so that it can be bonded to the insulation 12. It is essential that there are no voids between the inner semi-conducting screen and the insulation 12, or again there is a potential for a high stress point and Corona discharge .

The manufacture of the electrical cable 2 is complex, and it involves very high capital cost together with high operator competence level . Referring now to Figure 2, there is shown an electrical cable 18 of the present invention, the electrical cable 18 comprising an electrical conductor 20 which is made of individual electrically conducting strands 22. An optional barrier tape 24 is provided around the electrical conductor 20. Thus far, the electrical cable 18 is of the same type of construction as the electrical cable 2.

The electrical cable 18 is provided with an inner insulation layer 26 which surrounds the electrical conductor 20. An outer insulation layer 28 surrounds the inner insulation layer 26. The inner and outer insulation layers 26, 28 are such that they are made of materials which are different from each other and which have different permitivities. The material for the inner insulation layer has high permitivity, and the material for the outer insulation layer has low permitivity. The electrical cable 18 then becomes one in which the voltage drop across the inner and outer insulation layers 26, 28 is linearly distributed such as to reduce stress level buildup at the interface of the electrical conductor with the inner insulation layer 26.

The , inner insulation layer 26 is made of a high permitivity insulation material in the form of a thermoplastic elastomer having a permitivity of 3.5 - 6.0. The outer insulation layer 28 is made of a low permitivity insulation material in the form of a medium density polyethylene having a permitivity of 2.1 - 2.3.

The electrical cable 18 may have reduced high alternating voltage stress levels within the cable insulation operating in the voltage range of 5Kv to 15Kv. Significant improvement in the life expectancy, the safety aspect and the power transmission levels of the electrical cable 18 may be obtained.

It is to be appreciated that the embodiment of the invention described above with reference to the accompanying drawings has been given by way of example only and that modifications may be effected. Thus, for example, the electrical cable 18 may include a metallic outer layer 30 and barrier tape.

Claims

1. An electrical cable comprising an electrical conductor, an inner insulation layer around the electrical conductor, and an outer insulation layer around the inner insulation layer, the inner and outer insulation layers being such that they are made of materials which are different from each other and which have different permitivities, whereby the voltage drop across the inner and outer insulation layers is such as to reduce stress level build-up at the interface of the electrical conductor with the inner insulation layer.

2. An electrical cable according to claim 1 in which the inner insulation layer is made of thermoplastic elastomer filled with a material that enables adjustment of the permitivity, and in which the outer insulation layer is made of medium density polyethylene.

3. An electrical cable^' according to claim 1 or claim 2 in which the permitivity of the inner insulation layer is in the range of 3.5 - 6.0, and in which the permitivity of the outer insulation layer is in the range of 2.3 - 2.7.

4. An electrical cable according to any one of the preceding claims and including barrier tape provided around the electrical conductor and positioned between the electrical conductor and the inner insulation layer.

5. An electrical cable according to any one of the preceding claims and including a metallic outer sheath.