GB2196783A - Electric cable - Google Patents

Abstract

In an electric cable having stratified insulation (8) impregnated with a viscous insulating compound and a gas conduit for supplying pressurized dielectric gas to the insulation, the conduit is formed by an elastomeric hose (1) having gas outlets formed by slits (2) extending axially of the hose. In the embodiment the hose (1) is of neoprene, nitrile rubber or fluorinated polymer, the segments 6 of Cu or Al, the insulation 8 of paper tape impregnated with mineral oil and polyisobutylene and the sheath 10 of metal. Semiconductive layers 7 and 9 of paper filled with carbon black may be provided. <IMAGE>

Description

SPECIFICATION Electric cables This invention relates to electric cables of the type comprising a sheath housing at least one conductor surrounded by a stratified insulation impregnated with an insulating compound and a pressurized dielectric gas within the sheath and in contact with the said insulation.

In cables of this type, the stratified insulation inevitably contains micro-cavities, void of compound. These micro-cavities are formed during manufacture of the cable because of the difficulty of effecting complete impregnation by the high viscosity compound.

In use the cables are subjected to cyclic heating and cooling, which results in the micro-cavities continually tending to change their dimensions and positions. In particular, during periods of heating, the compound tends to expand more than the other materials of which the cable is composed. The prevalent tendency is for the compound to move radially, with a corresponding influence on the number and sizes of the micro-cavities. In the subsequent cooling cycles the compound contracts, again with resultant changes in the micro-cavities and their positions.

Under these conditions, in DC cables, particularly when there is a low pressure gas present, the facility with which the low pressure gas ionizes under the action of electrical stresses gives rise to the danger of electrical discharges, seated in the micro-cavities, which can perforate the insulation.

To counteract this danger, it is known to introduce under pressure into the cable sheath a dielectric gas, such as nitrogen or sulphur hexafluoride so that the gas diffuses through the compound and fills up the micro-cavities.

The diffusion of dielectric gas into the compound is very slow and in order to ensure filling of micro-cavities with gas in manufacture and the rapid filling of micro-cavities with gas during use of the cables it has been proposed to dispose inside the cable sheath a gas conduit in permanent communication with a pressurized gas source, the conduit having permanently open apertures distributed along its length within the cable. These conduits may, for example be hoses of rigid material with apertures in their side walls.

In practice, these known cables do not effectively eliminate the risk of perforation of the insulation for the full life of the cable, which will usually be some tens of years, in spite of the permanent connection to a source of pressurized gas.

This failure is probably due to the fact that the conduit becomes throttled over a period of time by compound passing through the apertures to reach the interior of the conduit and eventually plugging it, so as to restrict the flow of gas along the conduit and thereby prevent the rapid filling of micro-cavities. The presence of plugs of compound in gas conduits has been verified.

As a solution to this problem, it has been proposed (in Italian Patent Specification 476034) to form the gas conduit as an elastically deformable hose having a plurality of needled perforations through its wall. These perforations act as valves which, due to the elasticity of the wall of the hose can expand to permit the outward passage of pressurized gas, but which tend to close shut to resist the inward passage of compound. However, this solution has also proved ineffective, and plugs of compound have been found -to form in the bore of the hose.

In accordance with the present invention however there is provided an electric cable comprising a sheath housing at least one conductor surrounded by a stratified insulation impregnated with an insulating compound, and a gas conduit extending longitudinally of the cable for conveying a dielectric gas under pressure, into contact with the insulation, the said conduit being formed by a hose of elastomeric material having a plurality of slits through the thickness of its wall, the slits being disposed to extend axially of the hose and constituting one way valves for the release of gas into contact with the stratified insulation.

Because the openings for the escape of pressurized gas are formed as slits, extending axially of the hose, they are readily formed, e.g. by thin slitting blades, without removing any wall material and without any accompanying localized deformation such as accompanies the formation of perforations by needles.

A presently preferred form of electric cable in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a perspective view of- a piece of cable progressively sectioned for.clarity; and Fig. 2 is a perspective view of a detail from Fig. 1.

The cable illustrated is a unipolar cable comprising a hose 1 of an elastomeric material such as neoprene, fluorinated polymers or nitrile rubber. The material selected should not react in any way to contact by the impregnating compound employed in the cable insulation.

The wall of the hose 1 is formed with a plurality of slits 2, formed without removal of wall material, all extending completely through the wall and essentially axially of the hose, preferably, and as shown in Fig. 2, disposed in a radial plane or planes. The slits are preferably so dimensioned that the ratio of the axial length of each slit to the thickness of the hose wall is in the range 0.25 to 1. For example, if the wall thickness of the hose is 3mm, the axial length of each slit may be from 1 to 3mm.

As shown, the slits are all arranged in one radial plane (i.e. or a generatrix of the hose) and at regular intervals along the length of the hose. This arrangement is preferred because of its constructional simplicity but other arrangements will of course be possible, including random distributions of the slits. They should, however always be individually parallel with the hose axis 3, thus avoiding any tendency for the slits to open up when the hose is bent.

The hose 1 is loosely housed in a conduit 4 of an annular conductor 5 formed by laying up a plurality of segmented wires 6 of a highly conductive metal such as copper or aluminium. The diameter of the conduit 4 is some 0. 1 to 0.5mm greater than the outside diameter of the hose 1, so as to allow for elastic expansion of the hose and concomitant opening up of the slits 2. The conductor 5 is enclosed by a semiconductive layer 7 typically formed by windings of a tape of semiconductive material such as paper filled with carbonblack.

Over the layer 7 is formed the stratified insulation 8 formed by helical winding of a plurality of layers of tapes of insulating material such as cellulose paper, synthetic paper or plastics laminated paper.

The insulation 8 is impregnated with a viscous insulating compound, composed for example of 98% mineral oil and 2% poly-isobutylene. Another suitable compound comprises 93.5% mineral oil, 2.5% poly-isobutylene and 4% micro-crystalline wax.

The impregnated insulation 8 is covered by a semiconductive layer 9 of the same form as the layer 7, and the layer 9 is enclosed in a sheath 10, which may be metallic, adhesively bonded to the layer 9.

Further layers may be provided over the sheath 10 in known manner, such as layers for protecting the sheath against corrosion, or armouring, according to the intended operating conditions and environment of the cable.

Although the invention is described above with specific reference to a uni-polar cable, it is also applicable to other cables, such as tripolar and multi-polar cables.

With cables of the above described construction, there is very little tendency for the impregnating compound to pass inwardly through the hose wall and thereby plug the bore of the hose, compared with the known cable in which the hose is provided with perforations formed by needles.

A possible explanation for the shortcomings of the above-mentioned known cable is as follows.

When forming perforations without removal of material, a needle must be forcibly inserted, giving rise to the formations of very small fissures and tears around each perforation.

These fissures will extend in various directions, some of them essentially circumferentially. In practice cables are rarely laid strictly rectilinearly, and some local curvature of the cable and of course the hose is inevitable.

When the hose is held in a curved state, some of the fissures and perforations at the outside of the curve will be opened, and held open permanently, thus tending to enlarge the perforations.

Under the cyclical heating and cooling of the cable in operation, the compound, when hot, normally exerts a pressure on the outside of the hose greater than the internal gas pressure and there is accordingly a strong tendency for the compound to be forced into the perforations which are effectively enlarged by the contiguous fissures as explained above.

By contrast, the slits in the hose of the above described cable are formed by sliding a very thin blade into the wall material and there is very little tendency for fissures to form, even at the ends of the slits and particularly not in a transverse direction. Thus even in localised regions where the hose is slightly curved, there is no tendency for the mouths of the slits to be opened up and an increase in pressure of the compound will actually tend to close the slits more firmly.

With this construction, gas pressure can be maintained in the hose very efficiently and is only released to the extent that gas is required to fill small voids. The pressure that can be sustained within the hose may be increased by giving the hose wall a greater thickness, assuming of course that the number and size of the slits is not correspondingly increased. These facts, together with the fact that the entry of compound into the hose interior is virtually eliminated, give rise to the possibility of dispensing with any pressurized gas reservoirs permanently connected to the interior of the hose which may instead be pressurized and closed off, at least for limited periods of time.

Claims (5)

1. An electric cable comprising a sheath housing at least one conductor surrounded by a stratified insulation impregnated with an insulating compound, and a gas conduit extending longitudinally of the cable for conveying a dielectric gas under pressure into contact with the insulation, the said conduit being formed by a hose of elastomeric material having a plurality of slits through the thickness of its wall, the slits being disposed to extend axially of the hose and constituting one way valves for the release of gas into contact with the stratified insulation.

2. A cable according to claim 1, wherein the slits are so dimensioned that the ratio of the axial length of each slit to the wall thickness of the hose is in the range 0.25 to 1.

3. A cable according to claim 1 or 2, wherein at least some of the slits are disposed in a single plane extending longitudinally of the hose.

4. A cable according to claim 3, wherein the said single plane is a generatrix of the hose.

5. An electric cable substantially as herein described with reference to the accompanying drawing.