GB2269488A

GB2269488A - Belted cable termination with screen.

Info

Publication number: GB2269488A
Application number: GB9216534A
Authority: GB
Inventors: Colin David Hillyard
Original assignee: Prysmian Cables and Systems Ltd
Current assignee: Prysmian Cables and Systems Ltd
Priority date: 1992-08-04
Filing date: 1992-08-04
Publication date: 1994-02-09
Anticipated expiration: 2012-08-04
Also published as: GB9216534D0; GB2269488B

Abstract

In a belted cable termination where the cores 10, 12, 14 extending from the sheath 22 cross-over in order to be connected to respective conductor terminations, each core 10, 12, 14 are provided with a screen 24 at the cross-over regions. The screen may comprise a silicone rubber tube loaded with carbon or coated with a carbon-loaded varnish. Further stress reduction may be provided between the screen and the termination of each core. <IMAGE>

Description

Belted Cable Terminations This invention relates to belted cable terminations.

As is well known, a belted cable comprises a plurality of cores, each of which comprises a paper insulated conductor. The cores are twisted helically together, the resulting assembly is made substantially circular in cross section with paper or jute fillers and then lapped with a belt of paper insulation.

Thereafter other layers may be applied including a metal sheath. The cable inwardly of the metal sheath is impregnated with an insulating compound. Such cables are typically used for system voltages up to and including 22kV, and are cheaper size for size than screened cables (in which each core is screened) which are used for system voltages of llkV and above.

Whilst belted cables operate with acceptable electrical stress levels since there are no voids in the cable construction inwardly of the metal sheath (due to the use of fillers and insulating compound) and there is a regular electric field configuration along the length of the cable (due to the regular geometric relationship of the cores), an electrical stress problem arises in terminations of such cables.

When terminating a belted cable, the cores are exposed at the end of the cable to be terminated by stripping back all the cable layers radially outwardly thereof, the exposed cores are untwisted and the conductors thereof connected to respective conductor terminations. In making such connections it is necessary to connect each conductor with the appropriate conductor connection and in some cases the only convenient way of achieving this requires the, or some of the, exposed cores to cross over. Such crossing over of the cores can result in unacceptable electric stresses in the air gap between the cores at the cross over region which could give rise to discharging.

An object of this invention is to overcome the above problem and allow cores to cross over in belted cable terminations.

The invention includes a method-of terminating a belted cable including exposing the cores of the cable at the end thereof to be terminated, untwisting the exposed cores, and providing respective screening over the exposed cores at least at regions thereof where the cores cross over when connected to their respective conductor terminations.

The invention also includes a belted cable termination, wherein at least two cores extending from the sheath of the cable and connected to respective conductor terminations cross-over, and wherein said cores are provided with respective screening at least at the cross-over regions thereof.

In an embodiment of the invention, described hereinafter, the cable has three cores and each core is provided with said respective screening over a predetermined length thereof between the end of the sheath and the conductor termination for the core.

The screening may comprise a tube over the insulation of the conductor of the core. This tube may comprise a tubular member provided on the exterior thereof with a semiconducting layer, for example a coating of carbon black loaded varnish.

Alternatively the tube may comprise a tubular member loaded with carbon.

In each of the above-mentioned alternatives the tubular member may be formed of a silicone rubber.

Each core provided with said screening may also be provided with stress reducing means extending over the end portion of said screening closer to the conductor termination of said core and towards said conductor termination away from said screening end portion.

The stress reducing means may comprise a linear stress reducing tube.

In order that the invention may be well understood, the above-mentioned embodiment thereof, which is given by way of example only, will now be described with reference to the single figure in the accompanying drawings which shows an end of belted cable prepared for termination.

The cable illustrated is an 11 kV paper insulated, lead covered (PILC) belted cable for a 3 phase system.

Briefly, the cable comprises three cores 10, 12, 14, each of which comprises a stranded conductor 16 having a layer of insulation 18 formed by lapped paper tapes. The cores are twisted helically together and the resulting assembly is made substantially circular in cross-section with paper fillers and then lapped with a belt of paper insulation 20. A lead sheath 22 is provided over the belt insulation 20.

The cable inwardly of the lead sheath is impregnated with insulating compound. The above construction is well known.

In order to terminate the cable, the cores 10, 12, 14 are exposed at the end of the cable to be terminated by stripping back all of the cable layers radially outwardly thereof over a predetermined length and the exposed cores are untwisted. In order to connect the conductors of the cores to the appropriate conductor connection in the termination so that the phases are connected correctly the exposed cores may cross-over. In the illustration all three cores cross-over but it will be appreciated that in certain circumstances only two might do so.

In order to overcome problems relating to electrical stress at the cross-over regions, the cores are provided with respective screening at least at these regions.

In more detail, and in particular referring to core 12 which is shown in axial cross section, each core is provided with a respective screening over a predetermined length thereof between the end of the sheath 22 and the end of the core to be connected to the conductor termination (not shown) for the core.

The position of the screening along the length of the exposed core and the length of the screening is such that the screening is provided at least over the cross-over region of that core with an adjacent core.

As illustrated, the screening in the embodiment comprises a tube 24 fitted over the paper insulation 18 of the conductor of the core. In the embodiment, the tube 24 comprises a tubular member formed of a silicone rubber and provided on the exterior thereof with a semi-conducting layer comprising a coating of carbon black loaded varnish. As one alternative, the screening tube 24 may comprise instead a tubular member formed of a silicone rubber loaded with carbon.

Electrical stress reducing means in the form of a linear stress reducing tube 26 of rubber loaded with silicon-carbide extends over the end portion 28 of the screening tube 24 which is closer to the conductor termination of the core and towards the conductor termination away from that end portion.

A rubber sleeve 30 having better surface tracking properties than the stress relief tube 26 extends over the entire length of the stress relief tube 26 and adjacent portions of the screening tube 24 and insulation 18 at each end of the stress relief tube.

Stress relief mastic 32 is provided at the junction of the stress relief tube 26 and the screening tube 24.

The termination also comprises a three core rubber glove 34 which fits tightly over the lead sheath 22 and the exposed cores extending therefrom.

As clearly illustrated the portions of the glove which extend over the exposed cores fit over the end portion of the screening tube 24 closer to the lead sheath 22.

Barrier tape 36 is applied over the belted insulation 20 and the adjacent end portion of the lead sheath and a butyl sealing mastic 38 fills the space left internally of the glove 34.

The free end of each conductor 16 has a terminating lug 40 fitted thereto. A rubber lug cover 42 extends over a collar portion of the lug and the adjacent end portion of the rubber sleeve 30. The space within the lug cover is filled with a butyl sealing mastic 44.

To complete the termination the lug terminations are fitted to conductor terminations (not shown) of the electrical apparatus with which the cable is to be connected.

As a practical matter, it is envisaged that when a termination is made in which only two of the cores cross over, all three cores will be provided with screening described above. However, it will be appreciated that it may only be necessary to provide the two cores which cross over with such screening.

It is also envisaged that the stress relief tube may be dispensed with if the screening is made appreciably longer.

Claims

CLAIMS:

1. A method of terminating a belted cable including exposing the cores of the cable at the end thereof to be terminated, untwisting the exposed cores, and providing respective screening over the exposed cores at least at regions thereof where the cores cross over when connected to their respective conductor terminations.

2. A belted cable termination, wherein at least two cores extending from the sheath of the cable and connected to respective conductor terminations crossover, and wherein said cores are provided with respective screening at least at the cross-over regions thereof.

3. A cable termination as claimed in claim 2, wherein said cable has three cores.

4. A cable termination as claimed in claim 3, wherein each core is provided with said respective screening over a predetermined length thereof between the end of the sheath and the conductor termination for the core.

5. A method as claimed in claim 1 or a cable termination as claimed in any one of the claims 2 to 4, wherein said screening comprises a tube over the insulation of the conductor of the core.

6. A method or termination as claimed in claim 5, wherein said tube comprises a tubular member provided on the exterior thereof with a semiconducting layer.

7. A method or termination as claimed in claim 6 wherein said semiconducting layer comprises a coating of carbon black loaded varnish.

8. A method or termination as claimed in claim 5, wherein said tube comprises a tubular member loaded with carbon.

9. A method or termination as claimed in claim 6, 7 or 8 wherein said tubular member is formed of a silicone rubber.

10. A method as claimed in any one of claims 1 and 5 to 9 or a cable termination as claimed in any one of claims 2 to 9, wherein each core provided with said screening is also provided with stress reducing means extending over the end portion of said screening closer to the conductor termination of said core and towards said conductor termination away from said screening end portion.

11. A method or cable termination as claimed in claim 10, wherein said stress reducing means comprises a linear stress reducing tube.

12. A method of terminating a belted cable or a belted cable termination substantially as herein described with reference to the accompanying drawing.