GB2341718A

GB2341718A - Mineral insulated electric cable with gas filled spaces

Info

Publication number: GB2341718A
Application number: GB9913384A
Authority: GB
Inventors: Oliver Sanders Johnson
Original assignee: Individual
Current assignee: Individual
Priority date: 1998-06-17
Filing date: 1999-06-10
Publication date: 2000-03-22
Also published as: GB9913384D0

Abstract

A mineral insulated cable with increased electrical strength has a gas phrase in the interstitial spaces between particles of a powdered mineral insulation. Preferably the gas phase is compressed air, nitrogen or sulphur hexafluoride and may be at an increased pressure, between 1.15 and 4 atmospheres. The powdered mineral may be a metallic oxalate, such as manganous, barium, strontium, calcium, magnesium or titanium oxalate, titanic nitrate or ammonium nitrate. The cable may have longitudinal passageways so that gas can diffuse into and out of the insulation.

Description

2341718 MINERAL INSULATED ELECTRIC CABLES

This invention relates to improvements in the materials and methods used for the manufacture of mineral insulated electric cables in order to increase the electrical strength of the insulation. Such cables, in common use, comprise a thin walled copper tube containing one or more elongate solid copper conductors, insulated from one another and from the copper sheath, by compressed mineral powder. Some 15-25% of the volume of this compressed powdered matrix comprises interstitial spaces between particles of the powder and these spaces contain gas at a pressure of the same order of magnitude as that of the atmosphere. These cables are the subject of British Standard 6207:Part 1: 1995 and related international specifications, which apply to cable designs having a highest rated voltage of 750 volts. The mineral powder generally consists predominantly of dry magnesium oxide, of the calcined or the fused variety. Sometimes a mixture of both is used and other inorganic minerals, such as fine silica sand, have also been employed. It is evident that not all inorganic minerals would be suitable as a component of such an insulation.

In all established methods of making mineral insulated cables, an initial preform is fabricated in which the conductors are placed within a copper tube in a disposition similar to that required in the finished cable. The tube of copper may either be in a prefabricated discrete length, or be made continuously from strip by shaping the strip into a tubular form and welding the edges together, while the conductor rod or rods and the insulation are being introduced into the tube.

The conductors are located, throughout the length of the cable, by the introduction of mineral insulation, either in the form of pre-shaped blocks of compacted mineral powder (GB 502344), or by packing the mineral powder around and between the conductor rods by ramming and/or by vibration (GB 669779), while the required spatial location of the conductor rod or rods is being established by passing them through guides.

1 The filled preform is then reduced in cross-section, with a corresponding increase in length, by passing it through pairs of shaped rolls, or through a sequence of dies of successively reducing bore, until the required dimensions of the finished cable have been achieved. At stages during the reduction process and at the end, it is necessary to anneal the preform, by heating, in order to restore the ductility of the work- hardened copper.

There are no less than five different methods, in current commercial use, for making mineral insulated power cables. All of them rely on a compact matrix of mineral powder to locate the conductors within the copper tube and to provide the requisite electrical insulation between the conductors and between the conductors and the sheath.

In the case of preforms which are being fabricated using blocks of compressed mineral insulation, the first one or two steps of diameter reduction break the blocks into smaller pieces and the fragments then become consolidated into a uniform compressed matrix as the diameter of the preform is further reduced. Some of the gas contained in (i) the interstital spaces between the mineral particles within the original blocks and (ii) in spaces between the blocks and (iii) between the blocks and the conductor rods and (iv) between the blocks and the inside of the copper tube, thereby becomes entrained in the matrix of the compressed mineral insulation in the finished cable.

Similarly, when a preform is fabricated by powder filling, the action of introducing the powdered insulation into the copper tube entrains some of the air, initially present in the tube, within the interstitial spaces of the compacted matrix of powder in the finished cable. The process of compacting the powder comminutes the initial grains of powder to create a coherent body of insulation which does not fall out of the cable when it is cut into lengths during installation.

Relative to the organic insulants in common use for cable manufacture, the electrical strength of the compact mineral 2 matrix is poor. Nevertheless, mineral insulated cables are widely used because of their excellent performance in hazardous conditions, particularly when there is a fire risk.

If a sufficiently large voltage is applied between two conductors or between one or more conductors and the copper sheath, an electrical current will flow between the two conductors, or between one of them and the sheath, and the cable will be said to have 'broken down'. The magnitude of the voltage causing this event is described as the 'breakdown voltage'. In the case of mineral insulated cables, the breakdown path occurs predominantly in the gas phase and the breakdown voltage is determined, to a large extent, by the electrical strength of the gas in the matrix of the mineral insulation.

Air is generally the gas which is entrained in the insulation when powder or compressed blocks of powder are introduced into the copper tube. However, since oxygen in the air (21% by volume) readily reacts with copper at high temperatures, after the preform has been annealed most of the oxygen will have been removed from the gas phase. In consequence, the gas pressure in the interstitial spaces between the particles of mineral powder, or in any fissures in the matrix, maybe up to 21% less than it would have been had such a reaction not taken place.

The electrical strength of a gas containing matrix of mineral powder depends on a number of factors, amongst which the most relevant, are:- 1 The mineral powder and its purity; the presence of moisture or of conducting particles reduces its strength. Also the lower the permittivity of the mineral the less electrical stress, for a given applied voltage, will fall on the gas phase.

2 The degree of uniformity of the matrix; fissures in the matrix, for example as a result of bending, reduce its strength; 3 3 The intrinsic electrical strength of the gas component of the matrix; 4 The pressure of the gas in the matrix. At pressures of the same order of magnitude as atmospheric pressure, the electrical strength of a gas is approximately proportional to the pressure of the gas.

It is the object of the present invention to provide improved materials and designs for, and improved manufacturing methods of, making mineral insulated cables which increase the electrical strength of a particular design of cable relative to that obtained by using the materials and methods of manufacture hitherto proposed or used.

In the improved methods according to the invention, the electrical breakdown strength of the insulation is enhanced by increasing the electrical strength of the gas phase of the matrix relative to that in mineral insulated cables in current production, or in such cables made by other methods which have been proposed or used, by one or more of the following procedures, which may be used individually, or some of them may be used in appropriate combinations:- 1 Manufacturing the cable in such a manner that the gas phase of the matrix, when it is first formed, consists of a relatively inert gas which does not react with other components of the cable.

2 Incorporating a gas into the matrix which has an intrinsic electrical strength greater than that of the air, or of the oxygen depleted air which normally comprises the gas phase of the matrix.

4 3 Manufacturing the cable in such a manner that the pressure of the gas in the interstices of the matrix, is at a higher pressure than if such a procedure is not used.

4 Including additive materials in the mineral insulation which generate a suitable gas when heated, for example during the annealing process, thereby increasing the pressure of the gas phase relative to that which would otherwise pertain.

Manufacturing a cable in such a manner that the electrical strength of the gas phase can be increased after the production of the cable has otherwise been completed.

Means for applying these procedures to the various possible methods of making mineral insulated power cables in current commercial use, or by other methods of making such cables which have hitherto been proposed or used, can be devised without great difficulty by someone skilled in the art. Theref ore, the invention is further illustrated by a description, by way of example but not exclusively, to its application only to one improved method of manufacture for each of the five procedures outlined above. However, although it may be impracticable to apply every one of these procedures to every possible method of making mineral insulated electrical cables, at least one of the procedures can be adapted to improve the insulation strength of cables made by all the processes in current use.

In the application of these procedures it is essential to take into account possible chemical reactions, which depend on the particular insulant and method of production in use. For example, if a manufacturer includes a silicone water repelant as a component of the mineral insulation it would not be possible to improve the electrical strength of the gas phase with sulphur hexafluoride because this gas reacts with, and is destroyed by, silicones at the temperatures used to anneal the cable.

As regards procedures 2,3 and 4 above, the extent to which the breakdown strength of a particular cable design can be increased depends on the degree of cohesiveness of the insulation and on the ability of the sheath and the cable terminations to withstand the gas pressure within the cable.

In a first improved method in accordance with the invention, the pressure of the gas in the interstitial spaces between the particles of mineral powder is increased, relative to that of the gas in the same design of cable which is made by a similar method, by replacing air as the initial gas phase by a suitable gas which does not react with any other component of the preform of the cable during the course of its manufacture or subsequently. Nitrogen is a gas suitable for use in this procedure. Gases which have a lower intrinsic electrical strength than nitrogen would not be suitable.

The invention is further illustrated, in regard to the first improved method, by a description, by way of example but not exclusively, to an improvement whereby the gas initially contained in the preform consists predominantly of nitrogen. The process of making the cable starts with forming continuously a strip of copper, which will become the sheath of the finished cable, into tubular form and welding the opposing edges of the strip together to make a tube. This operation is carried out in the vertical plane with the tube moving downwards. As it proceeds, the copper rods, which will subsequently become the conductors in the finished cable, and powdered mineral insulation are introduced into the tube through a conductor guide which places the conductor rods in the required spatial distribution within the copper tube. The conductor guide also contains a longitudinal tube through which nitrogen is introduced into the copper tube below the point of welding. In consequence, the grains of powder fall into a zone of the copper tube in which the gas is predominantly nitrogen, rather than air. Thus, as the usual sequence of reductions in diameter and intermediate annealings, which are necessary to reduce the diameter of the preform to that required for the finished cable,

6 is performed, the loss of pressure (which is normally circa 20%), due to the reaction between copper and oxygen when the workpiece is annealed, is considerably reduced and the electrical strength of the finished cable is correspondingly higher.

In a second improved method according to the invention, the gas phase of the matrix comprises, in part or in total, a gas which has a higher intrinsic electrical strength than that of air or of oxygen depleted air. A suitable gas for this purpose is sulphur hexafluoride. Sulphur hexafluoride has an intrinsic electrical strength some 2.3 times greater than that of air at the same pressure, and in an admixture with air the electrical strength of the mixture increases as the proportion of sulphur hexafluoride increases.

The invention is further illustrated, in regard to the second improved method, by a description, by way of example but not exclusively, of a method in which one or more straight copper rods, which will form the conductor or conductors in the finished cable, is or are placed within a length of copper tube which will form the sheath. The rod(s) and the tube are each of approximately the same length. One end of the tube is sealed in a manner which holds the rods in the required spatial distribution. A tubular ram, which is a little longer than the copper tube, is introduced into the assembly. This ram has one or more bores, throughout its length, to accommodate and to locate the conductor or conductors and the ram has other longitudinal bores through which the powdered insulation and the high strength gas may be introduced. The assembly is then placed in the vertical plane with the lower end held in a friction grip encompassing the copper tube.

Sulphur hexafluoride is then caused to flow, through one of the bores in the ram, into the space between the closed end of the copper tube and the bottom end of the ram. Powdered mineral insulation, consisting mainly of calcined magnesium oxide, is introduced into the lower part of the assembly, through the bores in the ram, and the ram is moved up and down in a reciprocating motion to compact the powder. As the powder becomes compacted by 7 this movement the assembly is forced downwards through the friction grip and thus it becomes uniformly filled with compacted powder. Under these conditions the gas in the interstices between the grains of powder consists predominantly of sulphur hexafluoride.

An alternative means of achieving this result is to fill the preform, as described above, but with air in the preform as is usual when mineral insulated cables are made by this process. The degree of compaction of the powder in the completed preform is much less than it is in a finished cable and it is therefore possible to replace most of the air in the preform with sulphur hexafluoride. This can be done by applying a vacuum, to one or both ends of the preform, for a sufficient period to extract the majority of the air and then introducing sulphur hexafluoride into the preform to take its place.

Once the pref orm has been f illed with mineral insulation, one end is pointed and it is then transferred to a draw bench where its diameter is reduced by pulling it through a die. After two or three such reductions in diameter the copper components have become hardened and it is necessary to restore their ductility by annealing. Further reductions in diameter, with such intermediate annealing as is required, are carried out until the required diameter of the cable has been reached.

The degree of compactness of the insulation increases during the initial stages of the reduction in the diameter of the preform. In consequence, there is considerable impedance to the longitudinal movement of the gas trapped in the matrix of mineral powder. As a result, there is no significant loss of sulphur hexafluoride from the workpiece as its diameter is being reduced, or f rom the cable when it is being cut into lengths in preparation for installation. However, if it is found, with a particular cable design and choice of insulating material, that there is an unacceptable loss of gas during the reduction and annealing processes, both ends of the preform may be sealed to prevent this loss. Because of the high intrinsic electrical strength of sulphur hexafluoride, the breakdown strength of a 8 cable made in this fashion is higher than that of a mineral insulated cable otherwise made with the same materials, in the same manner and to the same dimensions.

In a third improved method in accordance with the invention, the pressure of the gas in the interstitial spaces between the particles of mineral powder is increased by increasing the pressure of the gas in the copper tube, while the powder is being introduced, and maintaining this increased pressure during the process of reducing the diameter of the preform to that of the required cable.

The invention is further illustrated, in regard to the third improved method, by a description, by way of example but not exclusively, to an improvement in the method of making mineral insulated cables in which the initial workpiece for the manufacture of a single core mineral insulated cable is made by threading hollow cylindrical blocks of compressed calcined magnesium oxide onto a length of copper rod, which rod will subsequently become the conductor in the finished cable. The blocks are placed touching one another along the length of the rod, except for short spaces at the two ends of the rod. At each end these spaces are filled by short (50mm) hollow annealed copper cylinders, both of which are also placed on the rod. One of the copper cylinders has a longitudinal bore through its body through which compressed gas can be introduced into the preform at a later stage. These cylinders are 'sliding fits' both on the rod and within the copper tube into which the assembly, consisting of the copper rod, blocks of mineral insulation and the two copper cylinders, is then pushed. The copper tube, which will become the sheath of the cable, has a length similar to that of the rod so that, after the assembly has been pushed into the copper tube, the two copper cylindrical blocks are inside and located at the ends of the tube.

The ends of the copper tube are then radially compressed onto the copper cylinders with sufficient force that the cylinders are compressed onto the copper rods, so as to form gas-tight seals at both ends of the preform. Air is then introduced into the 9 assembly through the longitudinal bore in one of the copper cylinders, until the required pressure has been attained. Further radial compression is then applied to collapse and seal the bore through which the additional air has been introduced into the preform. The finished preform is then reduced in diameter by succesive reductions, with intermediate annealings, until the required diameter of the cable has been reached.

The extent to which the pressure of the gas within the cable can be increased in this manner, is limited by two factors. If the pressure is too high some of the the mineral insulation will be ejected by the gas pressure when the cable is cut. The degree to which this occurs will depend on the cohesiveness of the compacted mineral insulation and on the design of the cable being made. Also, the gas pressure will increase as the temperature of the cable is increased, both during the annealing treatment and in fire conditions. If the gas pressure becomes too high it will cause the sheath to burst.

It is therefore necessary to carry out simple experiments in order to determine the extent to which the gas pressure within the matrix, and therefore, the degree of the improvement in electrical performance, can be achieved with different cable designs and mineral insulation materials.

In a fourth improved method in accordance with the invention, the pressure of the gas within the finished cable is increased by including in the mineral insulation a small proportion of a material, or materials, which will evolve a suitable gas when the cable is heated during the course of its manufacture. The compression of the inert mineral insulation, caused by the succesive stages in the reduction in the diameter of the workpiece, comminutes the larger particles of the powder and reduces the total volume of the interstitial spaces to some 10% to 15% of the total volume of the cable insulation. Only a small proportion of a gas evolving solid material is therefore required to increase the gas pressure in the interstitial spaces, but the material (s) and the gas evolved must not have any deleterious effects on the cable and the gas, or the products of any reaction of it with other components of the preform, must not be absorbed within the preform to an extent which negates the object of including the additive(s). In some instances, for example if an additive which releases carbon monoxide when heated is used to increase the pressure of the gas phase, some of the carbon monoxide will be lost by reaction with the comparatively small amount of copper oxide normally present within the cable. This can be compensated for by increasing the proportion of the additive. It is also important to ensure that there is no significant loss of gas during the annealing and diameter reduction stages of manufacture. If necessitated by the particular method of manufacture used, this loss can be prevented by a suitable sealing of the ends of the preform. This can be done, f or example, by the procedure set out above in the description of the third method for closing the ends of the preform. In the case of cables being made by forming the sheath continuously, while the conductor rods and the additive containing mineral insulation are being introduced, such an end sealing procedure is not usually necessary.

There are a number of materials, in a dry and anhydrous form, which may be used as additives for this purpose and some examples, but not exclusive, of suitable materials are set out in this and the next paragraph. The oxalates of calcium, magnesium, strontium, barium, manganese and a number of other metals decompose, at annealing temperatures, to produce carbon monoxide and, in some cases, carbon dioxide as well. An oxide and/or the carbonate of the metal remain(s) as an innocuous component of the solid phase. The evolution of carbon dioxide, in addition to the carbon monoxide, makes it difficult to regulate the required increase in gas pressure in those cables in which magnesium oxide is a component of the mineral insulation. Reversible and temperature dependant reactions take place between magnesium oxide and carbon dioxide. It is therefore preferable, if an oxalate is to be used, to use strontium or barium oxalate as the additive material because virtually no carbon dioxide is generated at the temperatures used to anneal the cable. If, for some reason, it is preferred to use an 11 alternative oxalate, the problem of pressure regulation, caused by the evolution of carbon dioxide, can be overcome by including an appropriate quantity of strontium or barium oxide in the mineral insulation in order to absorb the unwanted carbon dioxide.

Titanic nitride yields nitrogen to leave a harmless residue of titanous nitride. Ammonium nitrate evolves nitrous oxide and water at the annealing temperature. The unwanted water can be removed by including an appropriate quantity of barium oxide as an additional additive to the mineral insulation. This will absorb the water to form innocuous barium hydroxide.

The invention is further illustrated, in regard to the fourth method, by a description, by way of example but not exclusively, to an improvement in the materials used in the manufacture of mineral insulated cables whereby anhydrous powdered barium oxalate is uniformly mixed with the powdered magnesium oxide, in the proportion of one part by weight of barium oxalate to 3,000 parts of magnesium oxide. This mixture is then incorporated into the cable by any one of the manufacturing processes described above or which are in current use or which have hitherto been proposed or used. When the workpiece is heated to anneal the copper components, the barium oxalate decomposes with the evolution of carbon monoxide and the formation of barium carbonate. The addition of the small proportion of barium oxalate to the mineral insulation increases the pressure of the gas in the interstitial spaces by more than f if teen percent and thereby causes a corresponding increase in the comparative electrical strength of the cable. The proportion of the barium oxalate can be increased to achieve a higher level of increase in strength, subject to the same constraints as set out above in the illustrations of the third method. The proportion of barium oxalate, or other suitable additive material, to the mineral powder to achieve the optimum improvement in electrical strength, within the limits of the constraints for any particular method of production and composition of the mineral insulation, can best be established by simple trial procedures.

12 In the fifth improved method, in accordance with the invention, the cable is so constructed that gas may be caused to flow into and through one or more longitudinal passage ways throughout the length of the cable so that it can then difuse from these passage ways into, and out from, the compacted insulation. By this means the pressure of the gas in the interstitial spaces between particles of mineral insulation can be increased substantially after the ends of the cable system have been terminated. The end seals are of such a design as to permit the introduction of the gas into the passage ways and also to provide for its evacuation from the cable, should this be necessary. In this context, 'substantially' means pressures above one and a half times atmospheric pressure and up to pressures which can not safely be further increased without incurring a risk of the sheath bursting due to the internal gas pressure. The higher the pressure and the larger the thickness of insulation, the greater is the voltage rating that can be assigned to the cable.

Cables made in this way are intended for the transmission of electrical power and operate at potentials, between conductors, in excess of 10,000 volts. For such use, the cables are generally required to be supplied in lengths exceeding 200 metres. They are also appreciably larger in diameter than the majority of the cables described in BS 6207:Part 1:1995. It would therefore be impracticable to make mineral insulated versions of such cables starting from discrete lengths of copper tube, for instance in the manner described in GB 502344, GB 669779 or by similar methods. It is, however, possible to make them by a continuous sheath forming and welding process or by some of the processes described in UK Patent GB 2 292 634 B. By these methods, it would generally be unnecessary to make multiple reductions in the diameter after the sheath has been fabricated and filled with mineral insulation. Instead, only sufficient reduction in the sheath diameter would be required to compact the insulation into a cohesive mass.

In the case of single core cables, the passage way(s) for the longitudinal transmission of the gas may be formed as part of the conductor. There are several ways by which this may be done, for 13 example by using a design for the conductor which has previously been employed in single core oil-filled cables. Such conductors are made up of a small number of relatively large wires, each of which has a cross section similar in shape to that of the stones used to form an arch.

Such wires can be used to form a conductor with, typically, a 4MM central bore throughout its length and, for example, a cross section of 1000 sq mm. Some of these shaped wires may be knurled, on one or both radial faces, to facilitate the flow of gas from the bore of the conductor into the mineral insulation.

Alternatively, smaller conductors, for example 185 sq mm conductors, which are commonly used both in single core and three-core cables, may be made from a number of round wires, for example 37 wires of equal diameter, which are stranded together in the established manner. This construction provides adequate spaces between the wires of the conductor for the required longitudinal and radial flow of the gas.

The invention is further illustrated, in regard to the fifth improved method, by a description, by way of example but not exclusively, of a means of manufacturing a mineral insulated cable for the transmission of electrical power at a high voltage.

At the first stage of the manufacturing operation, a long length, for example 1000 metres, of 185 sq mm conductor is made by stranding together 10 segmental shaped copper wires to form a circular conductor of 16.6 mmdiameter, which has a central bore of approximately 4 mm. The mating radial faces of some of the segmental wires are knurled to allow easy passage of gas to and from the central bore of the conductor and into, or out from, the compressed insulation surrounding it.

This conductor is then fed vertically downwards through a welding mill which is forming 1 mm thick copper strip into the copper tube which will be the sheath of the finished cable. The external diameter of the welded tube is 30 mm. While the 14 conductor and tube are then moving vertically downwards, with the conductor being held by guides in the centre of the tube,a dry mixture of fused and calcined magnesium oxide powder is introduced through a guide tube so as to fill the annular space between the conductor and the inside of the copper tube, at a position below the point at which the tube is being welded.

The assembly is tapped at intervals by a light hammer in order to aid consolidation and then the diameter of the tube is reduced by passing it between two pairs of rolls until the diameter has been reduced to 28.6 mm. This reduction in diameter is suf f icient to consolidate the insulation into a coherent mass and thereby provides a radial thickness of about 5 mm of compressed insulation between the conductor and the sheath.

The cable is then annealed, provided with an anticorrosion sheath and tested. Once the cable has been installed, with successive lengths jointed together and terminated with appropriate designs of end fittings, dry nitrogen is introduced, under pressure, through an appropriate valve in one of the end fittings. The pressure is then increased to a level sufficient to raise the electrical performance to correspond with the designed voltage range. If, after manufacture and/or after installation, the cable is evacuated and sulphur hexafluoride introduced in place of nitrogen, then the same improvement in electrical strength performance is achieved with less than half the rise in pressure.

An alternative mineral insulation which may be used consists of dry calcined magnesium oxide mixed with fine, dry, silica sand. If the mixture contains a large proportion of silica sand the permittivity, of the mineral component, is usefully reduced by comparison with one consisting of magnesium oxide alone. As a consequence the electrical stress on the gas phase is reduced and the electrical strength performance of such a cable is higher than is the case of an otherwise identical cable but with the insulation consisting predominantly of magnesium oxide.

Higher performance results can also be achieved if the sheath is reinforced by a layer, or layers, of high tensile metal tapes over the copper sheath, or by using an appropriate stronger other metal, for example 18:8 stainless steel, in place of copper for the sheath. Such measures enable higher pressures to be used and further raise the rated voltage of the cable system. If 18:8 stainless steel, or other suitable metal, with a greater strength at high temperatures than copper, is used, the fire performance temperature of the cable is enhanced.

These effects are illustrated in the following table, which relates to a cable constructed to the dimensions and in the manner described above, i. e. to a cable with a 185 sq.mm. conductor having a diameter over the metal sheath of 28.6 mm. The suitability of the cable construction and the approximate gas pressure requirements for a particular system are determined in relation to the impulse strength test requirements for such systems in the United Kingdom.

System Voltage Nitrogen Pressure SF6 Pressure kV atmospheres atmospheres 11 2.3 1 22 3.8 1.6 33 5 2.2 132 16 7 275 15 A three core cable can be constructed by laying up three single core cables made in the manner described above.

An alternative means for making a three-core cable, by way of example, but not exclusively, is by introducing three conductors into the cable sheath while it is being formed, either by welding or by extrusion.

The conductors can be made from segmental wires, as described above, or by the conventional stranding together of a required number of smaller wires. The larger diameter three-core cables are not easy to handle unless the conductors are made from a number of small wires, for example, using 37 or 61 wires, and the conductors need to be disposed in a spiral within the insulation in order to the achieve maximum flexibility of the completed cable.

One way of achieving this disposition, is to lay-up together the circular bare multi-wire conductors in the conventional manner used for the assembly of a three-core cable. Immediately before entry into the sheathmaking and filling section of the manufacturing process, the conductor assembly is passed through a caterpillar traction device which causes the conductors to travel, at this point, at a speed slightly greater than the speed of the cable as it emerges from the sheathing unit. This causes the conductors to 'bird-cage', that is to be forced apart, as they enter a 'bell-mouthed' guide tube. This guide tube establishes the required radial separation of the conductors within the cable sheath. The cable sheath can be made by forming a strip into a tube and by welding together the opposing edges. The most simple means of filling the cable with mineral insulation, is for this operation to be performed in the vertical plane. Alternatively, the sheath can be welded in the horizontal plane or be made f rom aluminium by extrusion. In either case, the mineral insulation is introduced into the sheath and consolidated by reducing the diameter in ways the same as, or similar to, those described in UK Patent GB 2 292 634 B. The cable sheath may then be provided with a reinforcement of metal tapes, if these are necessary to enable the sheath to withstand higher pressures. It may then be provided with an anti-corrosion serving. After testing and installation, appropriate terminations are fitted to the ends of the cable and the system pressurised with Nitrogen and re-tested as described above. If a higher electrical performance is required, after the installation has been completed, the gas contained in the cable, joints and terminations can be largely removed by vacuum pumps 17 and replaced by a gas, such as sulphur hexafluoride, having an intrinsic electrical strength greater than that of nitrogen. In either case it is necessary to construct the terminations and the joints so that gas can flow from one end of the cable system, or a section of the system, to the other.

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Claims

Claims:

1. A mineral insulated cable, as hereinbefore described, in which the electrical strength of the insulation of the cable is increased, by comparison with the insulation of mineral insulated cables which have been manufactured, or proposed to have been manufactured, by increasing the electrical strength of the gas phase in the interstitial spaces between the particles of the powdered mineral insulation.

2. A mineral insulated cable as claimed in Claim 1 in which the electrical strength of the gas phase is increased by ensuring that the gas which is present within the powdered insulation of the preform of the cable, contains no component gas which will react with other components of the cable and by so doing, reduces the electrical strength of the cable.

3. A mineral insulated cable as claimed in Claim 2 in which the gas which is present in the powdered insulation, when the powder is introduced into the preform of the cable, is nitrogen.

4. A mineral insulated cable as claimed in Claim 1 in which the electrical strength of the gas phase is increased by incorporating a gas into the cable which has a higher intrinsic electrical strength than nitrogen.

5. A mineral insulated cable as claimed in Claim 4 in which the gas is sulphur hexafluoride.

6. A mineral insulated cable as claimed in Claim 1 in which the pressure of the gas in the interstitial spaces of the powder matrix is increased by at least 15% above atmospheric pressure and up to 4 atmospheres pressure, or up to such lower level of pressure increase as may be determined by limitations imposed by other features of the cable design and structure. Such increase in pressure is obtained by introducing compressed gas into the cable during its manufacture.

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7. A mineral insulated cable as claimed in Claim 6 in which the introduced compressed gas is air.

8. A mineral insulated cable as claimed in Claim 6 in which the introduced compressed gas is nitrogen.

9. A mineral insulated cable as claimed in Claim 6 in which the introduced compressed gas is sulphur hexafluoride.

10. A mineral insulated cable as claimed in Claim 1 in which the gas initially in the preform is extracted and replaced by a gas having a higher intrinsic electrical strength than nitrogen.

11 A mineral insulated cable as claimed in Claim 10 in which the gas replacing that which was initially present in the preform is sulphur hexafluoride or a mixture of sulphur hexafluoride and nitrogen.

12. A mineral insulated cable as claimed in Claim 1 in which the pressure of the gas in the interstitial spaces of the powder matrix is increased by at least 15% above atmospheric pressure and by up to 4 atmospheres pressure or to such lower level of pressure increase as may be determined by limitations imposed by the design factors of the particular cable. Such increase in the pressure of the gas phase is obtained by the generation of a gas within the sheath of the cable, during its manufacture or subsequently, by the effect of heat on a material or materials which evolves or evolve gas under these conditions.

13. A mineral insulated cable as claimed in Claim 12 in which the material is a metalic oxalate.

14. A mineral insulated cable as claimed in Claim 13 in which the material is manganous oxalate.

15. A mineral insulated cable as claimed in Claim 13 in which the material is barium oxalate.

16. A mineral insulated cable as claimed in Claim 13 in which the material is strontium oxalate.

17. A mineral insulated cable as claimed in Claim 13 in which the material is calcium oxalate.

18. A mineral insulated cable as claimed in Claim 13 in which the material is magnesium oxalate.

19. A mineral insulated cable as claimed in Claim 12 in which the material is titanic nitride.

20. A mineral insulated cable as claimed in Claim 12 in which the material is ammonium nitrate.

21. A mineral insulated cable as claimed in Claim 12 in which the material is any suitable material or combination of materials which evolve a gas under the influence of heat and thereby raise the gas pressure within the cable and which materials, the gas which they evolve and any residual solid materials have no deleterious effect on the cable.

22. - A mineral insulated cable as claimed in Claim 1 in which longitudinal passage ways are provided throughout the length of the cable so that gas can diffuse from these passage ways into and out from the compressed mineral insulation.

23. A mineral insulated cable as claimed in Claim 22 in which these passage ways are incorporated as part of the conductor or conductors.

24. A mineral insulated cable as claimed in Claim 23 in which the passage ways are constructed as hereinbefore described.

25. A mineral insulated cable as claimed in Claims 22-24 in which the pressure of the gas phase within the cable is increased by introducing additional gas into the cable to increase the gas pressure by a minimum of 15% above atmospheric pressure and by up to such a pressure as can safely be contained 21 by the sheath of the cable, by any joints between lengths of cable and by the end terminations.

26. A mineral insulated cable as claimed in Claim 25 in which the additional gas is air or nitrogen.

27. A mineral insulated cable as claimed in Claim 25 in which the gas has an intrinsic electrical strength greater than nitrogen.

28. A mineral insulated cable as claimed in Claim 27 in which the gas is sulphur hexafluoride.

29. A mineral insulated cable as claimed in Claim 27 in which the gas is a mixture of sulphur hexafluoride and nitrogen.

30. A mineral insulated cable as claimed in Claims 28 - 29 in which the majority of the gas, originally present in the cable, is extracted and replaced by sulphur hexafluoride or by a mixture of sulphur hexafluoride and nitrogen.

31. A mineral insulated cable system, comprising more than one length of cable as claimed in Claims 22 - 30, in which the passage ways are continued through joints connecting together successive lengths of cable.

32, A mineral insulated cable system as claimed in Claim 31 in which the cable system is terminated by sealing ends, which incorporate valves through which gas may be introduced into or extracted from the cable.

33. The manufacture of mineral insulated cables as claimed in any one or more Claims 12-21 wherein the material or materials is, or are, dispersed within the mineral insulation.

34. A mineral insulated cable as in herebefore described.

35. Methods of manufacturing mineral insulated cables and mineral insulated cable systems as hereinbefore described.

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