WO2001028055A1

WO2001028055A1 - Prefabricated cable joint for high electric al stresses

Info

Publication number: WO2001028055A1
Application number: PCT/SE2000/001965
Authority: WO
Inventors: Lars Palmqvist; Jörgen Svahn; Ingemar Sundberg
Original assignee: Abb Ab
Priority date: 1999-10-11
Filing date: 2000-10-10
Publication date: 2001-04-19
Also published as: SE9903652L; SE9903652D0; AU1067601A; SE514822C2

Abstract

The present invention relates to a jointing body (1) of an essentially elastic material, preferably for high-voltage cables with an extruded insulation, comprising an inner elongated cylindrical electrodes (2), of an electrically conductive material, forming a space (3), which is essentially free of electric field, for jointing together the conductors of the cables, an insulating casing (4) extending around and past the electrode (2) and an outer conductive layer (5), wherein the jointing body (1) exhibits a cylindrical hole (6) for the cables, the diameter of said hole being smaller than the diameter across the insulation of the cables which are to be jointed together, such that an active pressure arises at the boundaries towards the respective cable. The essential feature of the invention is that at least one part (7, 9) of the insulating casing (4) comprises an insulating material with increased permittivity, to control the electric field.

Description

Prefabricated cable joint for hiσh electrical stresses

The present invention relates to a jointing body of an essentially elastic material, preferably for high-voltage cables with extruded insulation, comprising an inner elongated cylindrical electrode, of an electrically conductive material, forming a space which is essentially free from electric field, for jointing together the conductors of the cables, an insulating casing extending around and past the electrode and an outer conductive layer, the jointing body exhibiting a cylindrical hole for the cables, the diameter of the hole being smaller than the diameter across the insulation of the cables which are to be jointed together, such that an active pressure arises at the boundaries towards the respective cable.

It is previously known to manufacture prefabricated joints for cables with extruded insulation, made of an elastic material. The cylindrical opening of the joint shall be smaller than the diameter across the insulation of the cable on which the joint is intended to be used. The joint has an outer conductive layer, which by its shape creates a geometrical guide, at the end of the outer conductive layer of the cable. Further, it is known to arrange, in the centre part of the joint, an electrode of conductive layers, which is designed such that the electrical stresses are kept down because of amply dimensioned radii.

The most critical region is the so-called triple point where the conductive casing and insulation of the joint meet the insulation of the cable. Above all, high fields along the boundary of the cable insulation must be avoided here.

The object of the present invention is to reduce the electrical stresses in the cable joint. These objects are achieved according to the invention by means of a jointing body of the kind described in the introduction, which is characterized in that at least part of the insulating casing comprises an insulating material with increased permittivity, to control the electric field.

Thus, the invention comprises introducing, in the region where the conductive casing and insulation of the joint meet the insulation of the cable, a material with a higher permittivity than the insulating material in the joint. This measure results in the electrical stress decreasing along the boundary.

It is also known that material with a high permittivity has high electric losses . This is avoided by limiting the length of the field-controlling layer.

To reduce the stresses at the edges of the central elec- trode, a material with a high permittivity can be introduced also here. To reduce the losses, it is substantially applied at the ends of the electrode where the field is largest .

A further possibility is to start from a normal prefabricated joint without additions of material with a high permittivity and, in addition thereto, to manufacture an adapter comprising the field-controlling material, which adapter is fitted onto the prepared cable before the normal prefabricated joint is installed. The adapter may, of course, be viewed as a special embodiment of a joint according to the previous description, made in two parts of which the adapter makes up one part .

The invention will now be described in greater detail with reference to the accompanying drawings, wherein Figure 1 shows a schematic cross-section view of a jointing body according to the present invention, and

Figure 2 shows a schematic cross-section view of an adapter .

In the drawings, Figure 1 shows a jointing body 1 of an elastic material, for jointing together high-voltage cables with extruded insulation. The jointing body 1 comprises an inner elongated cylindrical electrode 2, of conductive material, to create a space 3 which is essentially free of electric field. In this space 3, a jointing of the conductors of the cables is carried out (not shown) . An electrically insulating casing 4 extends around and past the electrode 2 and is surrounded by an outer conductive layer 5.

The jointing body 1 exhibits a cylindrical through-hole 6, into which the cables are inserted, the diameter of the hole 6 being smaller than the diameter across the insulation of the cables to be jointed together, such that, by stretching the jointing body 1, before or during mounting thereof, an active pressure arises at the boundaries towards the cable during operation. The insulating casing 4 comprises one part 7 consisting of an insulating material with increased permittivity. This part 7 is arranged from a region 8 , where the boundary layer between the insulating casing 4 and the outer conductive layer 5 meets the cable, and extends along the cable for a dis- tance which is smaller than half the distance between the mentioned region 8 and the inner electrode 2. Another part 9 of said insulating material with increased permittivity is arranged substantially at the ends 10 of the inner electrode, where the electric field is otherwise largest. Figure 2 shows an alternative embodiment, where the field- controlling material is accommodated in an adapter 11, the outer diameter of which is such that a standard jointing body 12 can be mounted over the adapter 11. Thus, it is possible, when using an adapter 11, to utilize a standard jointing body 12 intended for cable with a large insulating diameter on a cable with a relatively small diameter across the insulation. The jointing body 12 comprises an inner elongated cylindrical electrode 13, of conductive material, to create a space 14 which is essentially free of electric field. In this space 14, a jointing of the conductors of the cables is carried out (not shown) . An electrically insulating casing 15 extends around and past the electrode 13 and is surrounded by an outer conductive layer 16. The jointing body 12 exhibits a cylindrical through-hole 17, into which the adapter 11 is inserted.

The adapter 11 comprises a conductive material 18 and an electrically insulating material 19. The insulating material 19 comprises one part 20 consisting of insulating material with increased permittivity. This part 20 is arranged from a region 21, where the boundary layer between the insulating material 19 and the conductive material 18 meets the cable, and extends along the cable for a distance smaller than half the distance between the mentioned region 21 and that end of the adapter 11 which is located at the inner electrode 13 of the jointing body 12.

Claims

1. A jointing body (1) of an essentially elastic material, preferably for high-voltage cables with extruded insula- tion, comprising an inner elongated cylindrical electrode (2), of an electrically conductive material, forming a space (3), which is essentially free of electric field, for jointing together the conductors of the cables, an insulating casing (4) extending around and past the electrode (2) and an outer conductive layer (5), whereby the jointing body (1) exhibits a cylindrical hole (6) for the cables, the diameter of said hole being smaller than the diameter across the insulation of the cables which are to be jointed together, such that an active pressure arises at the boundaries towards the respective cable, characterized in that at least one part (7, 9) of the insulating casing (4) comprises an insulating material with increased permittivity to control the electric field.

2. A jointing body (1) according to claim 1, characterized in that said part (7) with increased permittivity is arranged from a region (8) where the boundary layer between the insulating casing (4) and the outer conductive layer (5) meets the cable, and a distance along the cable, which is smaller than half the distance between said region (8) and the inner electrode (2) .

3. A jointing body (1) according to claim 1 or 2 , characterized in that said part (9) with increased per- mittivity is arranged substantially at the ends (10) of the inner electrode, where the electric field is otherwise largest .

4. A jointing body (1) according to claim 1, characterized in that the insulating material with increased permittivity is accommodated in an adapter (11) , the outer diame- ter of which is such that a standard jointing body can be mounted over the adapter (11) .