Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
  • H01B1/00—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors
  • H01B1/20—Conductive material dispersed in non-conductive organic material
  • H01B1/22—Conductive material dispersed in non-conductive organic material the conductive material comprising metals or alloys
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
  • H01B9/00—Power cables
  • H01B9/02—Power cables with screens or conductive layers, e.g. for avoiding large potential gradients
  • H01B9/027—Power cables with screens or conductive layers, e.g. for avoiding large potential gradients composed of semi-conducting layers
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
  • H01F27/00—Details of transformers or inductances, in general
  • H01F27/28—Coils; Windings; Conductive connections
  • H01F27/32—Insulating of coils, windings, or parts thereof
  • H01F2027/329—Insulation with semiconducting layer, e.g. to reduce corona effect

Definitions

• This invention relates to electric cables with polymeric insulation for service at moderately high voltages, say from about lOkV to about 150kV. At such voltages there is a risk of breakdown by local discharges occurring at the inner or outer surface of the insulation, and it is the practice to minimise this risk by applying to one or both those surfaces a thin layer of an electrically conductive material that will adhere more securely than would a metallic conductor.
• This layer generally has a resistivity much higher than that of a metal, and is consequently known as a "semiconductive" screen (though it has no connection with the "semiconductive” materials used in electronic devices).
• the (or each) semiconductive screen is made of particles of a "conductive" grade of carbon black dispersed in a suitably chosen polymer. Serious problems sometimes arise, however, owing to the high adsorptive power of carbon black, which tends on the one hand to introduce moisture into the screen composition and on the other hand tends to withdraw additives from the polymer phase, which may be harmful owing to the presence of the additive at the carbon/polymer interface or to its loss from the polymer phase or both.
• Semiconductive compositions can also be made by incorporating fine metallic particles in a polymeric matrix, and compositions of this kind have been proposed for certain screening purposes, as for example in US-A-3576387; but such compositions have such poor flow properties and are so highly abrasive that they cannot be extruded or applied by any other technique applicable to the screening of long lengths of cable.
• the cable in accordance with the invention comprises at least one metallic conductor enclosed in insulation of a polymeric material of a sufficient radial thickness to withstand at least a D.C. potential of l5kV and, completely covering at least one of the inner and outer surfaces of the insulation, an extruded semiconducting screening layer comprising conductive particles dispersed in a matrix of a polymeric material which adheres both to the particles and to the insulation, the polymeric material in the matrix at least being cross-linked by the technique involving grafting silane side-chains to the polymer and then cross-linking them by hydrolytic condensation and the proportion of dispersed conductive particles being sufficient to give a resistivity (measured in the direction of the length of the cable) of no more than 5000 ohm-cm, and is distinguished by the fact that the conductive particles are of a metal with a resistivity not exceeding 15 times that of annealed copper and a hardness not greater than 100 Brinell and have an average length of at least 1 mm and an aspect ratio of
• the aspect ratio of a particle is meant the ratio of its length to its shortest dimension perpendicular to its length, for example in the case of a cylindrical fibre (a preferred shape) the ratio of length to diameter and in the case of a flake the ratio of length to thickness.
• Both fibres and flakes are commercially available (for instance from Transmet Corporation of Columbus, Ohio, USA); they are mostly made by the "melt-extraction” and “melt drag” techniques developed by Battelle Laboratories Inc., (also of Columbus, Ohio), but fibres can also be made by chopping fine wire.
• the length of the particles there is no clearly defined upper limit to the length of the particles but our present preference is for particles with a length up to about 20 mm and more especially up to about 6 mm. It will be noted that-the lengths of the particles are many times greater than those of conductive carbon particles (typically of the order of micrometres) and will almost always exceed the thickness of the screening layer. The extrusion process aligns the particles to such an extent that there is no significant risk that they will extend through the whole thickness of the layer.
• the particles are of aluminium or a dilute aluminium alloy having resistivities relative to copper in the range 1.6 to 2.0 times and hardness in the range 15 to 40 Brinell.
• Other metals that may be used include lead (12.5 and 5), zinc (3.5 and 50), copper (1 and 50) and subject to cost considerations tin (6.6 and 10), nickel (4.1 and 100) and silver (0.95 and 60).
• Composite particles, such as tinned or nickel-plated copper and silvered aluminium can be used, as may mixtures of particles of different metals (and/or composites).
• the proportion of particles to matrix required will vary with the material shapes and sizes of the particles, with the degree of orientation brought about by extrusion (which can be increased by using a long-land extrusion die, preferably with a passage that reduces in cross-section towards the outlet) and to some extent with the nature of the matrix polymer. In most cases less than 10% by volume will be needed.
• the compatibility of the metal particles with the matrix is improved by the presence of the silane reagents, but if desired they may be treated with a specific adhesion promoter, such as a functional organosilane or organotitanate.
• the matrix polymer may of course be compounded with other appropriate ingredients, such as cross-linking additives, stabilisers, antioxidants and copper inhibitors; carbon black (conductive or reinforcing) may be used in small amounts and may in some cases be used to increase radial conductivity of the screening layer; large quantities of carbon black are, of course, undesirable.
• a series of 33kV power cables is made using the general procedure and the insulation composition of Example 1 of Specification No. 1526398. Both surfaces of the insulation are coated with a thin semiconductive screening layer
• the screening layers are of the following formulation (in parts by weight):
• This formulation has a volume resistivity of about 100 ohm-cm, and in the hot set test specified in IEC '502 at 200°C has an elongation of about 100%.
• Example 2 This is similar to Example 1 except that the aluminium alloy fibres are pre-dispersed in 10 parts of the polyethylene using a Banbury mixer. This produces a more uniform dispersion but some fibres are deformed in the mixing process and the average conductivity is a little lower.
• 100 aluminium wires nominally 0.025 mm in diamater (produced by the technique of our UK Patent No. 1394058) are formed into a bunch and thinly sheathed with polyethylene.
• the sheathed bunch is then chopped into 8 mm lengths to give fibres with an aspect ratio of about 320 in compact granules with minimal trapped air that readily mix with granulated polyethylene. They are tumble-mixed with such polyethylene granules in proportions to give 15 parts by weight of aluminium to 100 parts total polyethylene; the same grafting agents, antioxidant and catalyst are metered in at the extruder as before, and the subsequent procedure is the same.
• Examples 3 and 4 provide an outlet for very fine drawn wires that are unsuitable for their intended purpose (e.g. because, through breakages, they do not meet the customer's specification for supply length).
• a formulation comprising, in parts by weight: was used in place of the formulation set out in Example 1 to make a readily strippable screen.

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• Physics & Mathematics (AREA)
• Chemical & Material Sciences (AREA)
• Dispersion Chemistry (AREA)
• Spectroscopy & Molecular Physics (AREA)
• Conductive Materials (AREA)
• Manufacturing Of Electric Cables (AREA)
• Insulated Conductors (AREA)
• Extrusion Moulding Of Plastics Or The Like (AREA)

Applications Claiming Priority (2)

Publications (1)

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Family Applications (1)

Country Status (5)

Cited By (3)

Citations (8)

• 1981
  • 1981-09-15 ZA ZA816401A patent/ZA816401B/xx unknown
  • 1981-09-15 DK DK410281A patent/DK410281A/da not_active Application Discontinuation
  • 1981-09-16 NO NO813151A patent/NO813151L/no unknown
  • 1981-09-23 EP EP81304385A patent/EP0049104A1/fr not_active Withdrawn
  • 1981-09-23 FI FI812955A patent/FI812955L/fi not_active Application Discontinuation

Patent Citations (8)

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