GB2043327A - Longitudinal sealing of electric cables - Google Patents

Abstract

An electric cable which is longitudinally sealed against water has an inner core (2) of conductors (1), a semi-conducting screen (3), cross- linked polyethylene (4) with grooves (6) extending in the longitudinal direction of the cable. The grooves 6 are filled with a mixture (8) comprising a sealing substance e.g. cellulose powder or an alumina silicate, semiconducting elements e.g. silicon or boron carbide and elements whose electrical resistance varies non-linearly with the magnitude of the capacitative currents e.g. C black to provide a semiconducting mixture having a non-linear variable resistance. An extruded PVC sheath (10) surrounds the aluminium screen (9). In another embodiment (Figure 2 not shown) the sheath 4 is covered with a semi conducting sheath (11) and the grooves are formed the sheath 11. <IMAGE>

Description

SPECIFICATION Improvements in the longitudinal sealing of electric cables The present invention relates to a method of longitudinally sealing a synthetic-insulation electric cable against water, and also to a cable constructed in accordance with this method. It applies more parti cularlyto a medium-voltage signle core cable having synthetic insulation.

A cable of the above referred to type consists of the following components: (a) a copper or aluminium conducting core, either stranded or solid; (b) a semi-conducting screen on the core; (c) an insulant consisting of an extruded synthetic material such as vinyl polychloride, chemically cross-linked polyethylene, or polyethylene; (d) a semiconducting screen such as varnish, tape or extruded material, optionally provided on the insulant; (e) a metal screen, either tape, extruded or longitudinal and (f) an outer sheath, usually extruded.

In the case of a three-core cable or, more generally, of multi-core cables, insulated elements are assembled under a packing sheath before adding an overall metal screen and an outer protective sheath.

During operation, the cables are subjected to heating and cooling cycles.

During the heating cycles, the metal core expands and the increase in diameter thereof with respect to the insulant may reach several millimetres. After the cable cools, a considerable gap is left between the insulant (or the semiconductor on the insulant) and the non-resilient metal screen. One method of sealing the cable is to insert a sealing substance which expands in the presence of water (e.g. cellulose powder, kaolin or a uniform mixture of aluminium sulphate, industrial vaseline and industrial oil) between the metal screen and the insulate (or semi-conductor on the insulant). This method is difficult to achieve in practice, because it is difficult to place a considerable quantity of sealing material on a smooth insulant (or semiconductor on the insulant).The method, therefore, has the disadvantage of failing to longitudinally seal the cable before and after the heating and cooling cycles. (Vaseline is a Registered Trade Mark).

If the outer sheath deteriorates, water may penetrate into the cable and consequently corrode and damage the metal screen, travel along the cable, penetrate into accessories such as cable boxes and connecting boxes, and cause a break-down.

It is known to obviate the above disadvantages by modifying the external shape of the insulant (or semi-conductor on the insulant) so asto limit the expansion of the metal screen and enable a sufficient quantity of sealing material to be introduced.

Although this procedure gives very satisfactory results, it has the disadvantage of being impractical and expensive for joining together such electric cables, for the outer screen of the able is removed in cable accessories such as cable ends and junctions and action is necessary as regards the distribution of the electric field to obviate bypassing and discharge problems likely to damage the insulant.

Accordingiy, the semiconducting screen above the insulant must be removed in the area where the screen has been removed and be replaced either by a deflector or by a tape or varnish having a non-linear resistance.

It is an object of the present invention to obviate the latter step, which takes time even though peelable and very expensive substances are used.

According to the present invention there is provided a method of longitudinally sealing a synthetic insulation electric cable against water including the steps of: (a) making a material which surrounds the core of the cable from a resilient substance; (b) forming grooves on the outer surface of said resilient material, said grooves extending in the longitudinal direction ofthe cable; (c) introducing a mixture into the grooves, said mixture comprising a sealing substance expanding in the presence of water, semi-conducting elements and elements whose resistance varies non-linearly with the strength of the capacitative currents; (d) surrounding the resilient material with a metal screen; and (e) surrounding the metal screen with an outer protective sheath.

The resulting cable thus has grooves formed on the cable insulant or on a semiconducting screen located on the cable insulant. Moreover, it therefore becomes unnecessary to remove the sealing substance, and furthermore the size of the terminal and the junction boxes can be reduced and the production cost of the cables will faill below that of conventional cables.

The present invention will now be described in greater detail by way of examples with reference to the accompanying drawings, wherein: Figure lisa section through a single core cable made in accordance with one preferred method, without a semiconducting screen on the insulant; and Figure2 is a section through a single core cable made in accordance with an alternative method, with a semiconducting screen on the insulant.

Referring first to Figure 1, aluminium conductors 1 constitute the conducting core 2 of a cable. A semi-conducting screen 3 surrounds the conducting core 2. Outside the screen, a chemically cross linked polyethylene insulant 4 is extruded so as to form ridges 5 and grooves 6 arranged in alternate sequence around the circumference of the insulant 4. The groove 6 extend in the longitudinal direction of the cable, i.e. along an axis perpendicular to the plane of the drawing. The outer surfaces 7 of the ridges 5 are flattened against the inner surface of an aluminium screen 9. A mixture 8 the composition of which will be described later is placed in each grove 6. An extruded outer sheath 10 made of vinyl polychloride surrounds the screen 9.

Referring to Figure 2, identical elements are represented by the same reference numbers. The only difference from Figure 1 is that the cable comprises a semiconducting screen 11 extruded over the insulant 4. The grooves 6 are formed on the semiconducting screen 11 instead of on the insulant 4.

Before being placed inside a metal screen, the hollow parts are filled with the mixture 8. During the heating cycles, the outer surfaces 7 of the ridges 5 flatten and act as a shock-absorbing mattress, limiting the expansion of the metal screen 9. In addition, this feature ensures permanent contact between the metal screen 9 and the semiconductor 11 on the insulant, in spite of the presence of the mixture 8.

The mixture 8 comprises a sealing substance, semiconducting elements and elements whose electrical resistance varies non-linearly with the strength of the capacitative currents, thus providing a semiconducting mixture having a non-linearly variable resistance. Accordingly, one of the elements of the mixture 8 is in the form of carbon black which serves to protect the insulant in partial electrical discharges, while the other element of the mixture 8 is a silicon or boron carbide which imparts the non-linear characteristic to the resistance of the mixture.

It has been found that an increase in the capacitative current, for instance, near a screen interruption, produces a voltage drop in the mixture and therefore reduces the concentration of the electric field.

Consequently, the lines of force are better distributed and it becomes unnecessary to use deflecting cones or linear voltage distributors at cable end junctions.

The sealing substance is in the form of a waterexpandable agent such as a cellulose powder or an alumina silicate avoiding the propagation of any corrosion.

Claims (7)

1. A method of longitudinally sealing a synthetic insulation electric cable against water including the steps of: (a) making a material which surrounds the core of the cable from a resilient substance; (b) forming grooves on the outer surface of said resilient material, said grooves extending in the longitudinal direction of the cable; (c) introducing a mixture into the grooves, said mixture comprising a sealing substance expanding in the presence of water, semi-conducting elements and elements whose resistance varies non-linearly with the strength of capacitative currents; (d) surrounding the resilient material with a metal screen; and (e) surrounding the metal screen with an outer protective sheath.

2. A method according to claim 1, wherein said mixture includes carbon black and silicon carbide.

3. A method according to claim 1, wherein said mixture includes carbon black and boron carbide.

4. A cable manufactured by the method accord- ing to claim 1, wherein the resilient material is the actual cable insulant.

5. Acable manufactured by the method accord- ing to claim 1,wherein the resilient material is an extruded semiconducting screen located on the cable insulant.

6. Acable manufactured by the method accord- ing to claim 1, wherein the substance under the metal screen is the actual cable insulant.

7. The method of longitudinally sealing a synthetic insulation electric cable against water substantial ly as herein described with reference to Figure 1 or Figure 2 of the accompanying drawing.