Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Rod – strand – filament or fiber
Reexamination Certificate
2003-08-19
2004-11-30
Dye, Rena (Department: 1774)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Rod, strand, filament or fiber
C428S383000, C174S1100PM, C174S1200SR, C525S333700
Reexamination Certificate
active
06824870
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a cable with recyclable covering. In particular, the invention relates to a cable for transporting or distributing medium or high voltage electrical energy, wherein an extruded covering layer based on a thermoplastic polymer material in admixture with a dielectric liquid with high mechanical and electrical properties is present, enabling, in particular, the use of high operating temperatures and the transportation of high power energy.
2. Description of the Related Art
The requirement for products of high environmental compatibility, composed of materials which, in addition to not being harmful to the environment both during production and utilization, can be easily recycled at the end of their life, is now fully accepted in the field of electrical and telecommunications cables.
However the use of materials compatible with the environment is certainly conditioned by the need to limit costs and, for the more common uses, guaranteeing a performance equivalent to or even better than that of conventional materials anyway.
In the case of cables for transporting medium and high voltage energy, the various coverings surrounding the conductor commonly consist of polyolefin-based crosslinked polymer material, in particular crosslinked polyethylene (XLPE), or elastomeric ethylene/propylene (EPR) or ethylene/propylene/diene (EPDM) copolymers, also crosslinked. The crosslinking, effected after the step of extrusion of the polymer material on the conductor, gives the material satisfactory mechanical performance even under hot conditions during continuous use and with current overload.
It is well known however that crosslinked materials cannot be recycled, so that manufacturing wastes and the covering material of cables which have reached the end of their life can be disposed of only by incineration.
Electric cables are also known having their insulation consisting of a multi-layer wrapping of a paper or paper/polypropylene laminate impregnated with a large quantity of a dielectric liquid (commonly known as mass impregnated cables or also oil-filled cables). By completely filling the spaces present in the multi-layer wrapping, the dielectric liquid prevents partial discharges arising with consequent perforation of the electrical insulation. As dielectric liquids, products are commonly used such as mineral oils, polybutenes, alkylbenzenes and the like (see for example U.S. Pat. No. 4,543,207, U.S. Pat. No. 4,621,302, EP-A-0987718, WO 98/32137).
It is however well known that mass impregnated cables have numerous drawbacks compared with extruded insulation cables, so that their use is currently restricted to specific fields of application, in particular to the construction of high and very high voltage direct current transmission lines, both for terrestrial and in particular for underwater installations. In this respect, the production of mass impregnated cables is particularly complex and costly, both for the high cost of the laminates and for the difficulties encountered during the steps of wrapping the laminate and then of impregnating it with the dielectric liquid. In particular, the dielectric liquid used must have low viscosity under cold conditions to allow rapid and uniform impregnation, while at the same time it must have a low tendency to migrate during installation and operation of the cable to prevent liquid loss from the cable ends or following breakage. In addition, mass impregnated cables cannot be recycled and their use is limited to an operating temperature less than 90° C.
Within non-crosslinked polymer materials, it is known to use high density polyethylene (HDPE) for covering high voltage cables. HDPE has however the drawback of a lower temperature resistance than XLPE, both to current overload and during operation.
Thermoplastic low density polyethylene (LDPE) insulating coverings are also used in medium and high voltage cables; again in this case, these coverings are limited by too low an operating temperature (about 70° C.).
WO 99/13477 describes an insulating material consisting of a thermoplastic polymer forming a continuous phase which incorporates a liquid or easily meltable dielectric forming a mobile interpenetrating phase within the solid polymer structure. The weight ratio of thermoplastic polymer to dielectric is between 95:5 and 25:75. The insulating material can be produced by mixing the two components while hot either batchwise or continuously (for example by means of an extruder). The resultant mixture is then granulated and used as insulating material for producing a high voltage electric cable by extrusion onto a conductor The material can be used either in thermoplastic or crosslinked form. As thermoplastic polymers are indicated: polyolefins, polyacetates, cellulose polymers, polyesters, polyketones, polyacrylates, polyamides and polyamines. The use of polymers of low crystallinity is particularly suggested. The dielectric is preferably a synthetic or mineral oil of low or high viscosity, in particular a polyisobutene, naphthene, polyaromatic, &agr;-olefin or silicone oil.
U.S. Pat. No. 4,410,869 describes dielectric compositions comprising a mixture of ditoluyl ether isomers, optionally in the presence of hydroquinone or a derivative thereof, used for impregnating electrical devices, including capacitors and transformers.
U.S. Pat. No. 4,543,207 describes dielectric compositions comprising dielectric oils and aromatic mono-olefins and/or diolefins having condensed or non-condensed aromatic nuclei. Said compositions comprise, in particular, mixtures of organic acid esters, vegetable or animal oils and aromatic ethers with 0.01-50% aromatic mono- and/or diolefins having two condensed or non-condensed aromatic rings. The compositions are used to impregnate capacitors, transformers and electric cables.
The Applicant considers as still unsolved the technical problem of producing an electric cable with a covering made from a thermoplastic polymer material having mechanical and electrical properties comparable to those of cables with an insulating covering of crosslinked material. In particular, the Applicant has considered the problem of producing a cable with a non-crosslinked insulating covering having good flexibility and high mechanical strength under both hot and cold conditions, while at the same time possessing high dielectric strength.
In view of said problem, the Applicant considers that the addition of dielectric liquids to polymer materials as proposed in the cited WO 99/13477 gives totally unsatisfactory results. In this respect, the Applicant maintains that adding a dielectric liquid to an insulating material should on the one hand determine a significant increase in its electrical properties (in particular its dielectric strength), while on the other hand maintaining the material characteristics (thermomechanical properties, manageability) unchanged, even at high operating temperature (at least 90° C. and beyond).
SUMMARY OF THE INVENTION
The Applicant has now found it possible to solve said technical problem by using, as recyclable polymer base material, a thermoplastic propylene homopolymer or copolymer mixed with a dielectric liquid as hereinafter defined. The resultant composition possesses good flexibility even when cold, excellent thermomechanical strength and high electrical performance, such as to make it particularly suitable for forming at least one covering layer, and in particular an electrical insulating layer, of a medium or high voltage cable of high operating temperature, of at least 90° C. and beyond. The dielectric liquid suitable for implementing the invention has high compatibility with the base polymer and high efficiency in the sense of improving electrical performance, consequently allowing the use of small quantities of additive such as not to impair the thermomechanical characteristics of the insulating layer.
High compatibility between the dielectric liquid and the base polymer ensures homogeneous dispersion of the liquid in the poly
Albizzati Enrico
Castellani Luca
Dell'Anna Gaia
Scelza Cristiana
Dye Rena
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Gray Jill M.
Pirelli S.p.A.
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