GB1558657A - Jointing electric cables - Google Patents

Description

(54) JOINTING ELECTRIC CABLES (71) We, PIRELLI GENERAL CABLE WORKS LIMITED, a British company, of Thavies Inn House, 3-4, Holborn Circus, london EC1N 2QA, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be perfonned, to be particu marly described in and by the following statement: This invention relates to jointing electric cables.

A method used in the jointing of low and high voltage electric cables is to enclose the jointed conductors in a casing or shell and then pour a setting compound into the casing so as to embed the jointed conductors. This compound, when set, serves as the primary electrical insulation, but in addition it serves as a rigid support for the cable cores, as protection against mechanical damage and as a barrier against the entry of water into the joint. Research has shown that a rigid compound which will fully satisfy the important mechanical requirements is unlikely to have an electrical rperformance adequate for the jointing of high voltage cables rated at 7.2 kV and above. In particular it has been found that the inability of rigid thermosetting compounds (for example rigid silica-filled acrylic resins) to deform in sympathy with the expansion and contraction of polymeric core 4insulation during conductor load cycling can lead to a path of electrical weakness along the core surface between the conductor connector and the terminated core screen at earth potential.

This invention provides a cable joint comprising an inner casing enclosing the jointed conductors of the cables being jointed, this inner casing being filled with a compound serving as electrical insulation, and an outer casing enclosing the inner casing and filled with a set compound serving as a mechanical support for the jointed conduc tors, the compound filling the inner casing having better electrically insulating properties than the compound filling the outer casing.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a longitudinal section through a straight joint between two high voltage cables; and Figure 2 is a cross-section through the straight joint shown in Figure 1.

Referring to the drawings, there is shown a straight joint between two three-core high voltage cables. The individual conductors are insulated with extruded polymeric insulation, e.g. ethylene-propylene rubber (EPR)or cross-linked polyethylene, shown at 1. Respective core conductors of the two cables are jointed by three connectors, two of which are shown in Figure 1 at 2. Individual screening layers for the conductors are shown at 3.

The joint comprises an inner casing 10 which encloses the jointed conductors and which is filled, by pouring through the inlet 11, with a compound whose function is solely to provide adequate electrical insulation over the conductor connectors and the lengths of extruded conductor insulation from which the cable screens have been stripped. The inlet 11 is closed, as shown, after the inner casing has been filled.

The inner casing includes a metal (e.g.

aluminium) sleeve 12 which is maintained at earth potential by connections 13 to the metal sheaths or screens of the cables themselves. Each end of the inner casing is closed by a flexible moulded disc 14 provided with three circular apertures for the core conductors, holding the three cores at their appropriate spacings. The disc 14 is moulded in a semi-conducting material such as carbon-loaded EPR and is made suffi ciently flexible to fit over non-circular (e.g.

sectoral) cores and in any event form a seal against loss of compound from the inner casing. This sealing can be assisted by a clip or binding around the periphery of the disc.

Each such disc provides electric stress relief at the ends of the core screens by being positioned to embrace these screens at their ends and by being contoured as shown around the core apertures on its inwardly facing side.

The inner casing is completed at each end by a semi-conducting rubber shroud 15 which embraces and seals onto both the sleeve 12 and disc 14.

The joint further comprises an outer casing 20, which may comprise a vacuum-formed plastics sheet. This outer casing is filled, by pouring through a subsequently- closed inlet 21, with a compound which sets rigid and which is chosen for its ability to provide mechanical support and protection to the jointed conductors. There is no electrical requirement for this compound since the electrical field is confined within the inner casing and within the core screens 3.

A core spreader element 22 is placed between the cores to maintain the correct spacings between the three phases and also to support the metal sleeve 12. The joint in accordance with the invention, whether for 3 core or single core cables, allows substantial cost savings over conventional joints using solidified resin as filling compound.

This is because the volume of electrical grade compound is kept to a minimum (approximately 30% of the total volume of the 3 core joint) and because neither compound has to satisfy both electrical and mechanical requirements. In particulair, the compound filling the inner casing has better electrical properties than the compound filling the outer casing By way of example, the compound filling the inner casing may comprise a flexible polyurethane resin, and the compound filling the outer casing may comprise a rigid silica-filled polyester or polyurethane resin.

WHAT WE CLAIM IS: 1. A cable joint comprising an inner casing enclosing the jointed conductors of the cables being jointed, this inner casing being filled with a compound serving as electrical insulation, and an outer casing enclosing the inner casing and filled with a set compound serving as a mechanical support for the jointed conductors, the compound filling the inner casing having better electrically insulating properties than the compound filling the outer casing.

2. A cable joint as claimed in claim 1, in which the compound filling the inner casing comprises a flexible polyurethane resin.

3. A cable joint as claimed in claim 1 or 2, in which the compound filling the outer casing comprises a rigid silica-filled polyester resin.

4. A cable joint as claimed in claim 1 or 2, in which the compound filling the outer casing comprises a rigid silica-filled polyurethane resign.

5. A cable joint as claimed in any preceding claim, in which the inner casing is electrically conductive and is electrically connected to metal sheaths or screens of the respective cables.

6. A cable joint as claimed in claim 5, in which the inner casing includes a pair of discs, closing the opposite ends of the inner casing, which discs are provided with appertures through which the respective cable conductors extend, the discs being flexible so as to seal around the conductors against loss of compound from the inner casing.

7. A cable joint as claimed in claim 6, in which said discs are form'end of semi-conducting material.

8. A cable joint as claimed in claim 7, in which said discs are formed of carbonloaded EPR.

9. A cable joint as claimed in any one of claims 6 to 9, in which said inner casing further includes a metal sleeve and a pair of semi-conducting rubber shrouds, each shroud embracing, at its opposite ends, the metal sleeve and a respective said disc.

10. A cable joint as claimed in any one of claims 6 to 9, in which said discs embrace individual screening layers of the cable conductors.

11. A cable joint as claimed in any preceding claim, in which the outer casing comprises a vacuum-formed plastics sheet.

12. A cable joint substantially as herein described with reference to and as shown in the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (12)

**WARNING** start of CLMS field may overlap end of DESC **. ciently flexible to fit over non-circular (e.g. sectoral) cores and in any event form a seal against loss of compound from the inner casing. This sealing can be assisted by a clip or binding around the periphery of the disc. Each such disc provides electric stress relief at the ends of the core screens by being positioned to embrace these screens at their ends and by being contoured as shown around the core apertures on its inwardly facing side. The inner casing is completed at each end by a semi-conducting rubber shroud 15 which embraces and seals onto both the sleeve 12 and disc 14. The joint further comprises an outer casing 20, which may comprise a vacuum-formed plastics sheet. This outer casing is filled, by pouring through a subsequently- closed inlet 21, with a compound which sets rigid and which is chosen for its ability to provide mechanical support and protection to the jointed conductors. There is no electrical requirement for this compound since the electrical field is confined within the inner casing and within the core screens 3. A core spreader element 22 is placed between the cores to maintain the correct spacings between the three phases and also to support the metal sleeve 12. The joint in accordance with the invention, whether for 3 core or single core cables, allows substantial cost savings over conventional joints using solidified resin as filling compound. This is because the volume of electrical grade compound is kept to a minimum (approximately 30% of the total volume of the 3 core joint) and because neither compound has to satisfy both electrical and mechanical requirements. In particulair, the compound filling the inner casing has better electrical properties than the compound filling the outer casing By way of example, the compound filling the inner casing may comprise a flexible polyurethane resin, and the compound filling the outer casing may comprise a rigid silica-filled polyester or polyurethane resin. WHAT WE CLAIM IS:

1. A cable joint comprising an inner casing enclosing the jointed conductors of the cables being jointed, this inner casing being filled with a compound serving as electrical insulation, and an outer casing enclosing the inner casing and filled with a set compound serving as a mechanical support for the jointed conductors, the compound filling the inner casing having better electrically insulating properties than the compound filling the outer casing.

2. A cable joint as claimed in claim 1, in which the compound filling the inner casing comprises a flexible polyurethane resin.

3. A cable joint as claimed in claim 1 or 2, in which the compound filling the outer casing comprises a rigid silica-filled polyester resin.

4. A cable joint as claimed in claim 1 or 2, in which the compound filling the outer casing comprises a rigid silica-filled polyurethane resign.

5. A cable joint as claimed in any preceding claim, in which the inner casing is electrically conductive and is electrically connected to metal sheaths or screens of the respective cables.

6. A cable joint as claimed in claim 5, in which the inner casing includes a pair of discs, closing the opposite ends of the inner casing, which discs are provided with appertures through which the respective cable conductors extend, the discs being flexible so as to seal around the conductors against loss of compound from the inner casing.

7. A cable joint as claimed in claim 6, in which said discs are form'end of semi-conducting material.

8. A cable joint as claimed in claim 7, in which said discs are formed of carbonloaded EPR.

9. A cable joint as claimed in any one of claims 6 to 9, in which said inner casing further includes a metal sleeve and a pair of semi-conducting rubber shrouds, each shroud embracing, at its opposite ends, the metal sleeve and a respective said disc.

10. A cable joint as claimed in any one of claims 6 to 9, in which said discs embrace individual screening layers of the cable conductors.

11. A cable joint as claimed in any preceding claim, in which the outer casing comprises a vacuum-formed plastics sheet.

12. A cable joint substantially as herein described with reference to and as shown in the accompanying drawings.