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(11) | EP 0 001 494 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Electric cables |
| (57) The dielectric of a high-voltage cable is of lapped tapes that comprise alkene polymer
film (13) preferably laminated between two paper layers (14). The dielectric is impregnated
with an insulating liquid that is a aryl alkane having two or more separate benzene
rings and 1-6 aliphatic non-cyclic carbon atoms per benzene ring. The benzene rings
(or two of them if there are more than two in the molecule) are separated by no more
than two aliphatic carbon atoms. Some of the aliphatic carbon atoms may be in hydrocarbon
substituents in the benzene rings. |
[0001] This invention relates to electric cables for service at very high voltages and incorporating alkene polymer film (especially polypropylene or high- density polyethylene) as part of the insulation. Such film may, and preferably does, form part of a laminate having surface layers of cellulosic paper.
[0002] At the highest voltages for which insulated cables are currently made (around 400 - 500 kV) oil- imprenated paper insulation maintained under hydraulic pressure is used without exception, but dielectric losses increase rapidly with voltage, and it is clear that different materials will be required if further significant voltage increases are to be achieved. Alkene polymers, and especially polypropylene and polyethylene, are attractive as partial (or if possible total-) replacements for paper as they have very low loss tangents and good impulse behaviour and are not unduly expensive. They are, however, susceptible to swelling on contact with all the conventional hydrocarbon dnsulating oils, with a consequent risk that the film will become so tightly wound on the cable core that the cable cannot be wound on a drum for delivery to site (or if impregnated in a wound condition cannot be unwound), or cannot be moved for rejointing or repair after service, without risk of damage to the film and consequent failure; in fact, in some cases tensions induced by swelling in experimental cables have been so great as to cause fracture of the laminate.
[0003] We have previously shown that this tendency can be controlled by using a carefully chosen paper/polymer/ paper laminate in combination with a carefully chosen oil, and our British patent specifications numbers 1311867 and 1458422 disclose cables in accordance with the prior-art portion of Claim 1.
[0004] Although satisfactory and useful high-voltage cables can be made in this way, we have subsequently found that the characteristics of the oil and the paper, and the conditions of lamination, need to be very closely regulated if a cable with good and consistent properties is to be obtained.
[0005] We have now found a group of hydrocarbon oils that have electrical properties that compare favourably with the insulating mineral oils and alkylbenzene oils conventionally used in the cable industry and that swell alkene polymers to a significantly smaller extent, thus allowing a higher operating temperature and/or providing a useful margin of safety that allows reasonable variations in paper quality and laminating conditions to be accomodated.
[0006] The invention is characterised by the use of an insulating liquid comprising an aryl alkane having at least two separate (uncondensed) benzene rings and from one to six aliphatic non-cyclic'carbon atoms per benzene ring, one or more of which carbon atoms may be present in one or more hydrocarbon substituents in one or both of the benzene rings, the benzene rings or two of them being separated by not more than two aliphatic carbon atoms.
[0007] Preferably the aryl alkane has a viscosity less than 57 centistokes at 20°C and less than 11 centistokes at 60°C.
[0008] It is very desirable, and may be essential if a sufficient control of swelling is to be obtained to allow winding and unwinding of the cable, for the alkene polymer film to be present solely as a laminate, preferably prestressed by extrusion bonding, having two layers of cellulosic paper forming the exposed surfaces of the laminate. This also assists in resisting degradation, if, under overvoltage conditions, electrical discharges should occur in the gaps between the edges of lapped tapes.
[0009] The aryl alkane (or a mixture of aryl alkanes) may be used as the sole impregnant, or may be mixed with other suitable impregnants. In particular, they may be mixed with suitable mineral oils, preferably having a viscosity less than 57 centistokes at 20°C and less than 11 centistokes at 60°C, to reduce cost, or with diorganosiloxamer (silicone) oils, in accordance with our British patent no. 1515847, to improve swelling performance still further.
[0010] Aryl alkanes that are already commercially available include:
1-phenyl 1-(3,4 dimethylphenyl) ethane (or
1-phenyl 1-xylyl ethane), sold'by Mitsui & Co.,. under the designation Nisseki Condenser Oil S; diphenyl methane;
1,2 diphenylethane; and mono-isopropyl biphenyl (mixed isomers).
The first of these,hereinafter for convenience called PXE, is particularly attractive because it has excellent electrical properties, in some respects better than conventional cable oils. The following table is illustrative:
[0011] When the alkene polymer film forms part of a laminate, it preferably constitutes at least half the thickness of the laminate, and the paper layers correspondingly not more than one quarter each. The total thickness will usually be in the range 50-250 micrometres.
[0012] The paper layers are preferably of low density, to secure a low overall power factor and permittivity, but on the other hand, they should be of a fine and uniform structure with a high impermeability, preferably at least 10,000 Gurley seconds.
[0013] The dielectric may consist of laminate tape, or it may include also a layer or layers of paper tape. In particular, it is desirable for the dielectric to include thin inner and outer layers of paper only, preferably each of these layers consisting of two paper tapes exactly, in accordance with our British patent no. 1442634.
[0014] The invention will be further described, by way of example, with reference to the accompanying drawings in which Figure 1 is a cross-section of a cable, and Figure 2 is an enlarged detail of part of the dielectric. In Figure 1, 1 indicates a steel tape helix defining a central oil duct within the metallic conductor 2. Over the conductor is applied a bronze binder tape 3 and a conductor screen 4 formed of tapes metallised and/or loaded with conductive material. The dielectric is made up of: an inner layer 5 consisting of two paper tapes each 75 micrometres thick; a main body 6 and a thin outer layer 7 made up of two paper tapes each 100 micro-metres thick. The cable is completed by a semiconductive dielectric screen 8, conductive bedding tape 9 (made of copper wires and textile threads or glass fibres woven together), lead or aluminium sheath 10, bronze tape 11 or other pressure resisting reinforcement and a plastics oversheath 12. The dielectric is impregnated with 100% commercial PXE maintained at a minimum pressure of 20 kN/m2.
[0015] The main dielectric body is made up of three parts, 6a, 6b and 6c, each of which is made up of a large number of extrusion-bonded laminate tapes each comprising a polypropylene layer sandwiched between and bonded to two paper tapes. In the inner part 6a, the polypropylene layer is 50 micrometres thick and each paper layer 25 micrometres; in the middle part 6b the polypropylene layer 60 micrometres and each paper layer 30 micrometres; and in the outer part 6c the polypropylene layer 80 micrometres and each paper layer 40 micrometres. The parts 6a and 6b each have a radial thickness of 5mm; the outer part may have a radial thickness of 5mm to give a voltage rating of 400kV, or of 14mm to give a voltage rating of 750V.
[0016] Figure 2 illustrates, for either of these cables, the layers of laminate tape, each tape comprising a centre layer 13 of polypropylene and outer layers 14 of paper. Butt spaces 15 between tape edges are filled with the PXE.
[0017] The benefits of the invention are illustrated by a test in which an extrusion-bonded (prestressed) laminate with a central layer (13) of propylene homopolymer with a melt flow index of'6 at 230°C and 2.16 Kg load (sold by Imperial Chemical Industries Ltd. under the designation PXC 3391) 50 micrometres thick and surface layers (14,14) of cellulosic kraft coil- winding paper with a density of 0.75 Mg/m3 and a minimum impermeability of 10,000 Gurley seconds, each layer 25 micrometres thick, were exposed to four different insulating oils at 850C and the swelling measured at a constant interfacial pressure of 0.5 MN/m2
in the way described by Drage Secrett and Reynolds in a paper entitled "Compatibility of Liquids and . Polymers in Mixed E.H.V. Dielectrics" appearing in the annual report of the NAS-NRC Conference on Electrical Insulation and Dielectric Phenomena 1970, pages 100-106, the relevant part being on pages 101-102. Each of these oils produced an initial temporary reduction in thickness followed by swelling to a final thickness greater than the original, and swelling was calculated separately on the basis of (1) original (dry) thickness and (2) minimum thickness. The results were as follows:
1. An electric cable for high voltage including at least one conductor surrounded
by a dielectric made of lapped tapes comprising an alkene polymer film, preferably
pre-stressed laminate tapes in which the said film is sandwiched between and bonded
to two cellulosic paper tapes forming the exposed surface of the film, impregnated
with an insulating liquid maintained under pressure in an enclosing sheath, CHARACTERISED
BY THE USE
of an insulating liquid comprising an aryl alkane having at least two separate (uncondensed) benzene rings and from one to six aliphatic non-cyclic carbon atoms per benzene ring, one or more of which carbon atoms may be present in one or more hydrocarbon substituents in one or both of the benzene rings, the benzene rings or two of them being separated by no more than two aliphatic carbon atoms.
of an insulating liquid comprising an aryl alkane having at least two separate (uncondensed) benzene rings and from one to six aliphatic non-cyclic carbon atoms per benzene ring, one or more of which carbon atoms may be present in one or more hydrocarbon substituents in one or both of the benzene rings, the benzene rings or two of them being separated by no more than two aliphatic carbon atoms.
2. An electric cable as claimed in claim 1 CHARACTERISED IN THAT the aryl alkane has
a viscosity less than 57 centristokes at 20°c and less than 11 centistokes at 600c.
3. An electric cable as claimed in claim 1 or claim 2 CHARACTERISED IN THAT the aryl
alkane is mixed with a mineral oil.
4. An electric cable as claimed in Claim 1 or Claim 2 CHARACTERISED IN THAT the aryl
alkane is mixed with a diorganosiloxamer oil.