Patent Application
20050217890
The present invention concerns an insulated electrical cable with a shield of metallic foil for making it watertight in the radial direction and a jacket arranged outside of the shield.
Insulated electrical cables for high voltages (>3 kV) are normally constructed in such a manner that that they consist of, from the centre, at least one conductor, at least one inner conducting layer, insulation, at least one outer conducting layer, a shield, and, externally, a jacket. The said type of cable is normally manufactured by what is known as “triple extrusion”, in which all three inner layers are extruded onto the conductor in a single process. The shield and the jacket are subsequently applied in a subsequent step. The most common insulation material is cross-linked polyethene (PEX).
The role of the shield is both to ensure that the outer conducting layer is maintained at electrical earth potential by conducting any capacitive eddy currents that may arise, and also to provide in the event of damage that gives rise to a short-circuit a return path of sufficiently low ohmic resistance for the current, in order to ensure adequate personal safety and in order to ensure sufficient short-circuit current such that existing protection will disconnect the supply voltage.
The role of the jacket is not only that of electrically insulating the shield from its surroundings, but also that of providing mechanical and chemical protection from the surroundings.
It has become apparent that a phenomenon known as “water treeing” can arise in the insulation, which degrades the insulation, possibly leading to flash-over. Water treeing principally occurs for cables with voltages exceeding 3 kV AC in combination with the insulation being exposed to a humidity exceeding 70%. For this reason, therefore, it is required that a moisture barrier against water is introduced for certain cables. This barrier should consist of metallic material.
Cable designs for voltages greater than 3 kV are also available, having insulation of XPLE. The shield in these cables consists of a thick tape of longitudinal aluminium folded over the outer conducting layer. This type of cable is often more rigid than a cable having wire shield and at the same time it may be difficult to make contact with a tape or a foil of aluminium at the end of the cable and at joins.
When a cable is to be radially sealed with a longitudinal foil, it is required that an underlying construction is fairly round. This is often solved for a multi-conductor cable by an underlying filler material being extruded onto the underlying construction before the foil is applied.
When using cable that requires shield for personal protection and for protection against short-circuits, the shield is normally constructed from copper wires, or a shield of copper wires is used, possibly also having aluminium foil applied to its outside. A galvanic element may arise when copper and aluminium come into contact with each other. Solutions are thus available for cables having copper shield and aluminium foil that minimise this effect. Despite this, major problems with corrosion often arise when the jacket is punctured, and these problems frequently lead to increased pressure and thus degradation of the outer conducting layer and the underlying insulation. The consequence of this is the risk of a complete break-down of the cable and subsequent interruption in electrical supply.
Another problem that may arise is that poor contact between different shield materials may give rise to differences in potential between these materials in the event of excess voltage transients, and this may degrade the outer conducting layer and the underlying insulation, or it may puncture the jacket, leading to the risk of subsequent cable break-down and interruption in electrical supply.
This galvanic corrosion is currently a problem for existing cable designs, in particular in the event of a hole arising in the foil and water penetration occurring. Even if the underlying construction is longitudinally sealed for water, the galvanic corrosion can give rise to a local break in the shield of the cable.
This can be solved by using similar metallic materials for shield wires and the external foil, or by preventing direct contact between the different metallic materials by, for example, baking the shield wires into a filler material that protects from corrosion when different materials for shield wires and foil are used.
In order to prevent the risk for damage to a cable as described above, therefore, the shield of aluminium wires in the present invention can be arranged in contact with an externally applied aluminium foil, whereby no problems arise when conducting away capacitive eddy currents, which currents can arise in the outer conducting layer of the cable when an alternating voltage or a pulsating direct voltage is applied to the cable. This means that differences in galvanic potential between different metallic materials can be avoided such that the problems described above do not arise.
Recycling of cables consisting of different metallic materials is another problem. It is considerably more advantageous with one preferred embodiment, in which conductors and shield are made from aluminium, to recycle material than it is with a construction consisting of different metallic materials. Furthermore, spreading of copper, a heavy metal, in the environment can be avoided by the use of aluminium.
A further advantage with the use of aluminium as material in the shield is that the weight of an aluminium shield is only half that of a shield made from copper if the same resistance is to be obtained in the shield construction.
One difficulty that arises for all cable designs in which a sealing layer of aluminium foil is required, is the presence of a pressure under the foil that resists when the warm jacket is pressed onto the cable and heats the foil layer against itself and against the externally applied jacket.
This has been solved in the present design by inserting profiles into the space that is formed between the included insulated cable conductors/the parts of the cable construction. These profiles/strips can thus also be constructed of filler material that protects against corrosion, where the shield wires are baked into the filler material in order to further ensure that the shield is not broken in the event of damage, such as a hole, to the foil that would cause corrosion to the underlying shield wires.
In order to make the construction watertight in the longitudinal direction, cavities are filled preferably with swelling powder/swelling tape during the cabling process. It is usually sufficient, if the profiles have the correct design, to apply the swelling powder in specially designed chambers in which the electrostatically charged powder is placed. A major advantage of electrostatic application of the powder is a significant reduction in the formation of dust. The second advantage is that all the components, if they conduct to a certain degree, attract powder to themselves, even if they are obscured relative to the location of powder application, in that they attract the electrostatically charged powder particles. This ensures that all component parts of the construction become covered with powder, and in this way the longitudinal watertight sealing in the event of water penetration of the construction is ensured.
Another problem that exists with longitudinally folded tapes is that the change in diameter that occurs on heating can readily give rise to distortion over the join in the foil. In order to minimise this distortion, elements such as soft tapes, milling or the equivalent are often inserted into the construction in order to take care of a part of this heat expansion. Alternatively, or in addition, the plastic jacket may also be of a plastic material that has high strength at high temperatures, such as cross-linked polyethene (PEX).
This has been solved in the present invention for multi-conductor cables in that the tape of metal foil is applied as a tape during the cabling process. This means that the join does not need to absorb all of the heat expansion, and the heat expansion can be distributed more evenly around the foil and on the externally applied jacket. Another difficulty that arises with compact designs such as this one is the ability to open it at the ends and at joins. This can be solved through the invention in that one or several tear-strips are arranged under the external tape of metal foil or, possibly, in at least one shield strip.
The invention will now be described in more detail with the aid of preferred embodiments and with reference to the attached drawings.
FIGS. 2A-E show various radial cross-sections of shield tapes for a multi-conductor cable arranged according to the invention.
In
In order for the construction to be longitudinally water-tight, cavities under the aluminium foil are filled, preferably with swelling powder/swelling strips, during the cabling process. Given correctly designed profiles of the strips, it is usually sufficient that the swelling powder is applied in specially designed chambers, into which electrostatically charged powder is applied. A major advantage with electrostatic application of powder is a considerable reduction in dust formation. The second advantage is that all the components, if they conduct to a certain degree, attract powder to themselves, even if they are obscured relative to the location of powder application, in that they attract the electrostatically charged powder particles. This ensures that all component parts of the construction become covered with powder, and in this way the longitudinal watertight sealing in the event of water penetration of the construction is ensured.
By dividing the shield into a number of sectors with conductors of conducting material incorporated into them, and by surrounding these shield sections with a metallic foil that is in contact with the conductors, an arc can be obtained, in the event of a fault on the cable, that creates a conducting plasma through all parts that are included and that are in electrical contact with each other. The light arc or the plasma at the location of the fault are not hindered or delayed given that the contacts partially consist of conducting plastic and rubber material or other conducting material such as carbon-baked paper or non-woven tape. This means that the construction of the shield provides satisfactory current transport to the shield wires, which can then release electrical protection and disconnect the cable from the electrical network.
It is preferable that the aluminium foil used as tape for taping around the cable is milled. A higher flexibility in the manufacturing process is obtained by milling an aluminium-coated plastic tape. The milling also reduces the risk for gaps arising at the tape when the cable is bent over, for example, a cable drum for transport to the next stage in a manufacturing process. The milling also gives a more secure and tighter sealing join at overlaps by reducing the risk for gaps. The milling also provides a greater tolerance for angular deviation, which makes it possible to use a somewhat broader tape for a taping operation of the cable. The tape that will preferably be used consists of an aluminium foil on a polyester foil with copolymer (melting glue), that can be easily glued to foil overlaps and to the surrounding jacket.
A jacket 7, preferably of a polymer material such as polyethene, lies outside of the shield construction 5. Items 2-4 can, when lower voltages, under 3000 volts, are used, be replaced by a homogeneous insulating material.
Naturally, the invention is not limited to the embodiments described above and shown in the drawings, and it can be modified within the framework of the attached claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0201589-9 | May 2002 | SE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/SE03/00864 | 5/27/2003 | WO | 5/17/2005 |
