Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
  • H01B13/00—Apparatus or processes specially adapted for manufacturing conductors or cables
  • H01B13/004—Apparatus or processes specially adapted for manufacturing conductors or cables for manufacturing rigid-tube cables

Definitions

• This invention relates to the manufacture of mineral insulated electric cable, that is to say, cables which comprise at least one elongate electrical conductor and a surrounding metal sheath, the or each elongate conductor being insulated from the sheath, and from any other conductor, by means of compacted mineral insulating powder.
• Such cables have been manufactured for many years, and are widely employed for example where performance may be needed at high temperatures for indefinite periods, such as in systems intended to operate during fires.
• the cables were originally manufactured by a so-called 'vertical-fill' process in which the conductors are inserted into a vertically oriented metal tube, and mineral insulant is poured into the tube while compacting it, to form a cable preform.
• the cable preform is then subjected to a number of die drawing and annealing operations to reduce its cross-sectional area by upto about 99%, thereby to form the finished cable.
• manufacturing economics have required a move to continuous processes, at least for the more commonly sold sizes of cable.
• EP-A-0 384 778 (the disclosure of which is incorporated herein by reference), in which a strip of metal and one or more elongate conductors are transported along their length and the strip is continuously formed into a tube that encloses the or each conductor, opposed longitudinally extending edges of the strip are welded together, mineral insulant is inserted into the tube to form a cable preform, and the cable preform is subjected to one or more reduction operations in which its diameter is reduced to form the cable.
• Reduction operations have traditionally been performed in the 'vertical-fill' method by pulling the cable preform through a number of dies, the steps being separated by annealing stages.
• the die reduction stage is replaced by banks of shaped rollers arranged in pairs about the cable preform, alternate roller pairs in each bank being arranged at 90° to one another, so that the cross-sectional area of the preform is reduced by about 80 to 99% percent as it passes through each bank of rollers.
• An annealing stage is located between the banks of rollers and after the last bank of rollers.
• the drawing stage will comprise three banks of rollers, each with 14 pairs of rollers, and three annealing stages.
• the annealing temperature usually lies in the range of 350 to 650°C, depending on the speed of the cable preform through the annealing stage.
• magnesium oxide has a relative permitivity ( ⁇ r ) of about 4.58 which leads to a capacitance in the order of about 230 pFm "1 for a typical MI cable as compared with capacitance values of about 100 to 120 pFm "1 for a soft insulated cable.
• the annealing temperature is raised to a value in the range of from 450 to 550°C, the preform will split at the third bank of rollers rather than at the second. It is, in general, not possible to increase the annealing temperature yet further because the cable becomes too soft and oxidises.
• the mineral insulant contains from 0.01 to 20% by weight of a binder that causes the silica to resist deformation of the tube when the tube is subjected to the reduction operations.
• the present invention is based at least in part on our observation that, in a continuous process, during the diameter reduction operations when the cable preform is subject to forces applied sequentially at different points on the preform, a silica mineral insulant will allow the cable preform to deform under the applied forces whereas a magnesia mineral insulant will not, or will only allow deformation to a much lesser extent.
• a silica mineral insulant will allow the cable preform to deform under the applied forces whereas a magnesia mineral insulant will not, or will only allow deformation to a much lesser extent.
• the cross-section of the preform is deformed into an ellipse by rollers bearing on opposite sides of the preform.
• a number of pairs of rollers are employed, alternate pairs bearing on the the preform at an angle of 90° to the adjacent roller pairs.
• the addition of a small quantity of a binder material to the silica will reduce the tendency of the silica to flow, and will thereby enable the silica to resist the oscillating elliptical deformation of the cable preform without preventing reduction of the diameter thereof as it passes through the roller pairs.
• the binder material will preferably exhibit the similar temperature resistance to the silica, and so is preferably a refractory, for example a metal oxide, especially an oxide of calcium, aluminium, or magnesium, or a material such as boron nitride.
• the relative permitivity of the binder is not of critical importance since it will usually be employed at relatively low concentrations in the mineral insulant.
• the binder may comprise a mixed metal oxide, and especially a mixed oxide of aluminium, silicon and magnesium. Such mixed oxides include periclase, spinel, corundum, forsterite, cordierite, mullite, clinoenstatite, and cristobalite.
• the mineral insulant contains at least 0.01% binder, preferably at least 0.1% binder, and especially at least 1% binder (all percentages expressed herein being by weight). However, the insulant should not contain more than about 30% binder since such levels will cause the overall relative permitivity of the mineral insulant to be unacceptably increased. Preferably the mineral insulant contains not more than 25% binder, and especially not more than 20% binder.
• a polymerised silicone resin may be employed (although this may require a reduction in the annealing temperature).
• a polymerised silicone resin coating in a mineral insulant is described in our copending UK patent application No. 9702827.8, the disclosure of which is incorporated herein by reference. Indeed it could be possible to employ incompletely polymerised silicone, and to complete polymerisation during the annealing stage.
• binders may be classified into a number of categories depending on the degree to which they prevent the silica mineral insulant from flowing after compaction.
• the binder is one that causes the at least some of the silica to agglomerate into lumps after compaction at a defined pressure.
• Some binders may cause higher degrees of agglomeration, for example causing the silica to be formed into a pellet after compaction, and such binders may also be employed.
• other binders do not cause any significant change in the ability of the silica to flow, and such binders are not considered suitable.
• the mineral insulated cable in accordance with the present invention may be of any form that is currently employed, for example it may have any number of internal conductors, and may, if desired, include an intermediate screen (triaxial construction). Often the cable will employ copper conductors and a copper sheath, which would, for example, be suitable for a power and signal cable that is employed in conditions where it could be subjected to a fire. However, other materials could be employed, for example, if the cable is intended to be employed for applications in which it will experience prolonged use at high temperatures, materials such as stainless steel, or nickel based alloys may be used. Examples of nickel based alloys include alloys of nickel with copper, e.g.
• nickel having between 25% and 75% nickel, such as cupronickel, and Monel, and other alloys comprising nickel with chromium and/or with cobalt, e.g. Ni,Cr,Fe,Co alloys.
• alloys include those sold under the trademarks 'Inconel' and 'Incoloy'.
• pure nickel may be employed.
• Such cables may be employed for example as thermocouples (where two conductors of different composition are employed) or cables for capacitive transducers (for example for monitoring rotor blade tip clearances).
• Figure 1 is a schematic block diagram of a typical continuous process for the manufacture of mineral insulated cable
• Figure 2 is a schematic cross-sectional view of a mineral insulated cable undergoing reduction by means of rollers.
• Figure 3 is a graphical representation showing the agglomeration characteristics of a range of binders.
• metal (e.g. copper) strip 1 is unwound from supply spools 2, cleaned, degreased and edge-trimmed at station 3.
• the strip 1 is then continuously formed into a tube around a number of copper conductors 4 by means of forming rolls (not shown) at forming station 6 until the strip 1 encloses the conductors, and opposite longitudinally extending edges of the strip face each other.
• the opposite edges are then welded together at a TIG arc welding station 8.
• the welded tube so formed Downstream of the arc welding station the welded tube so formed is filled with mineral insulant supplied from hopper 9, the mineral insulant being introduced into the tube by means of a lance 10 that extends along the interior of the tube from the forming station 6 to a position beyond the welding station 8.
• the cable preform 12 so formed is passed through three banks of shaped rollers (only the first bank 14 being shown) whose profiles cause the diameter of the tube to be reduced until the the final desired diameter of the cable is achieved.
• An annealing furnace (not shown) is located between adjacent banks of rollers, and after the last bank of rollers.
• FIG. 2 shows schematically, and exaggerated for the sake of clarity, the mineral insulated cable preform as is passes between a pair of rollers in one of the banks.
• the rollers do not exert a radially uniform force on the preform, but instead apply the force on opposite sides of the preform in the direction shown by the arrows, thereby causing the preform sheath to be deformed into a generally elliptical shape.
• These rollers alternate with similar rollers oriented at 90° to them, with the result that, the metal sheath of the preform is caused to oscillate rapidly between ellipses having vertical and horizontal major axes, thereby causing rapid work hardening of the sheath.
• the degree to which the preform is distorted by the rollers is believed to depend on the ability of the mineral insulant within the sheath to flow and thereby to accommodate the pressure of the rollers, with the result that when amorphous silica is employed as the mineral insulant, the sheath exhibits in the region of 200 splits km "1 during manufacture.
• binders were examined for their ability to prevent flowing of the amorphous silica insulant.
• the binders were mixed with the silica and a 20 ml quantity of the resulting mixture was pressed in a pelletiser for 15 minutes at a pressure of 4.5 kN. After 15 minutes the pressure was released and the powder was removed from the press and examined.
• the binders were graded into six catagories depending on the properties of the powder as shown in the table.
• figure 3 shows the number of splits per kilometre of cable when the mineral insulant is employed in a cable manufactured by the process described in EP- A-0 384 778 (where tested). From the figure, it can be seen that AZ 44 and microspheres show no cohesion on compaction. Amorphous silica showed a tendency to compact, but did not coalesce to any great extent. Additions of other powders to silica increased its dendency to coalesce up to cagatories 2-3. From these tests, preferred binders are mullite in the range of about 13 to 20%, and lime in the range of from 1 to 20% by weight.

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• Engineering & Computer Science (AREA)
• Manufacturing & Machinery (AREA)
• Inorganic Insulating Materials (AREA)
• Insulated Conductors (AREA)

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• 1997
  • 1997-03-26 GB GBGB9706257.4A patent/GB9706257D0/en active Pending
• 1998
  • 1998-03-06 AU AU65062/98A patent/AU6506298A/en not_active Abandoned
  • 1998-03-06 EP EP98910830A patent/EP0970484B1/de not_active Expired - Lifetime
  • 1998-03-06 WO PCT/GB1998/000665 patent/WO1998043254A1/en active IP Right Grant

Non-Patent Citations (1)

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