Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Rod – strand – filament or fiber
Reexamination Certificate
2002-01-25
2004-02-24
Kelly, Cynthia H. (Department: 1774)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Rod, strand, filament or fiber
C428S375000, C428S383000, C174S1100PM, C524S100000, C524S495000
Reexamination Certificate
active
06696154
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention concerns a cable, in particular for transport or distribution of electrical energy and insulating composition used therein.
In particular, the present invention concerns a cable, in particular for transport or distribution of electrical energy, preferably at medium or high voltage, comprising an insulating covering consisting of a polymeric composition comprising at least one polyolefin having improved electrical properties.
Within the scope of the present invention, “medium voltage” means a voltage ranging from 5 to 35 kV, while “high tension” means a voltage greater than 35 kV.
DESCRIPTION OF THE RELATED ART
At present, for the production of insulating layers of cables for transport of energy, cross-linked polyolefins are preferred. Typically, this polyolefin is a cross-linked polyethylene (XLPE).
Generally, the covering structure of such cables comprises three different layers of extruded material:
internal semiconducting layer;
insulating layer;
external semiconducting layer.
This covering structure is generally made by passing a metallic conductor through an extrusion head into which together flow three extruders (triple-head extrusion), which deposit the aforesaid layers onto the said metallic conductor in the order indicated above. In the case where it is desired to subject the said external layer to cross-linking, immediately after the extrusion the cable passes into a suitable device, also referred to as a vulcanising tube, where the said cross-linking is effected.
Generally the cross-linking is achieved by via radicals by thermal decomposition of organic peroxides, for example dicumyl peroxide, tert-butyl cumyl peroxide and the like, which are added to the polyolefin before the extrusion or injected directly into the extruder.
The extrusion temperature of the material which constitutes the insulating layer must not exceed the limit imposed by the decomposition temperature of the peroxide utilised. For example, when dicumyl peroxide is used, the temperature of the extruder is maintained below 130° C. to avoid premature cross-linking of the insulating material.
Advantageously, the cross-linking process is performed at a temperature ranging from 200 to 400° C. and the time necessary to achieve complete cross-linking of the insulating material varies from case to case depending on parameters well known to the technician of the field. Preferably, upon completion of the cross-linking, the cable is subjected to a degassing treatment, generally at a temperature of about 70-90° C., to eliminate decomposition products of the peroxide such as, for example, methanol and water since their presence within the insulating layer can prejudice its performance in time. Then, the cable is cooled and collected on reels.
Finally, the cable is completed by the addition of a metallic screen, an external sheath and, in some cases, other protective coverings (armouring).
It is well known that, in general, dielectric strength (DS) values measured on the insulating layer of a real cable are markedly lower than the values obtained when the same insulating material is in the form of flat samples (plates). The reasons for these differences are not fully known but it is believed this limitation of the DS values on the cable may be due to the presence of defects (for example: voids, protrusions, metallic particles and contaminants), formed in the insulating layer during the extrusion process. The quantity of such defects would increase on increasing thickness of the insulating layer.
The presence of such defects would also be responsible for a considerable diminution in the lifetime of the cable.
In the art, various attempts have been described intended to limit the adverse consequences of such effects by adding small quantities of additives commonly referred to as “voltage stabilizers” to the material which forms the insulating layer.
For example, EP-A-0 089 490 and EP-A-0 111 043 teach the use, as voltage stabilizer, of a mixture consisting of one or more divalent aliphatic alcohols having from 4 to 15 carbon atoms and of aliphatic or aliphatic/aromatic compounds, which are liquids or liquefy below 80° C. and which contain alcoholic and/or ketonic functional groups. The said stabilizing mixture is added to the electrical insulating material in quantities of between 0.3 and 5% by weight. The insulating material is based on a polyolefin such as, for example, a low-density polyethylene cross-linked via peroxide. The aforesaid insulating material is said to show improved dielectric resistance over time even in the presence of moisture, offering protection against growth of the so-called “water trees” and against occurrence of the so-called “electrical trees”. Both acetophenone and benzophenone are mentioned among the materials constituting the aforesaid stabilizing mixture.
DE 2 709 139 describes the use of a diaryl-ketonic carboxylic acid or of an ester thereof, in quantities that range from 0.1 to 5% by weight, as electrical voltage stabilizer in a polyolefin-based insulating material. The said voltage stabilizer is said not to interfere with the cross-linking process and would not be inactivated by peroxides only because it is generated “in situ” by decomposition of the cross-linking agent itself. As an example of a voltage stabilizer, benzophenone-2-carboxylic acid, deriving from the decomposition of the cross-linking agent 3-t-butylperoxy-3-phenylphthalide, is in fact mentioned.
JP 47-28042 describes the use of benzophenone or benzophenone substituted with alkyl groups, aryl groups, halogen, or OH groups with poly alpha-olefins to improve dielectric breakdown strength in insulation of high-voltage cables or electrical machine. As an example of benzophenone derivatives 2-hydroxy-4-n-octyloxy benzophenone, 2,2′-dihydroxy-4-n-oxyloxybenzophenone, 2,2′-dihydroxy-4-n-dodecylbenzophenone, 2,2′-dihydroxy-4-n-dodecyloxybenzophenone, 2,2′-dichloro-4-methylbenzophenone, 2,2′-dihydroxy-4-n-butyloxybenzophenone, 2-bromo-4-methylbenzophenone are mentioned.
GB 1304112 describes a method for the polymerization of monomers and crosslinking of polymers with radiation. Insulation on electrical conductors can be crosslinked with the predominantly continuum visible light radiation from the radiation source by exposing the insulated conductor to the predominantly continuum visible light radiation. The rate and extent of crosslinking can be enhanced by blending the crosslinkable polymer with photosensitizers among which benzophenone, 3- or 4-methylbenzophenone or 3- or 4-methoxybenzophenone are mentioned.
Therefore, the need to produce electrical cables equipped with a polyolefin insulating covering having improved electrical properties, in particular high values of dielectric strength and stability over time, but using conventional cross-linking systems, is still keenly felt.
SUMMARY OF THE INVENTION
The Applicant proposed to satisfy this need by adding a voltage stabilizer having the following group of properties to the material that constitutes the insulating layer of the cable:
ability to increase the lifetime and the dielectric strength of the insulating layer without substantially altering the other electrical properties required for an insulating material, in particular low dielectric loss (tandelta) values;
high stability in the insulating material over time thanks to a reduced ability to migrate towards the external surface of the insulating layer itself. In fact, the migration of the additive involves a loss of the stabilizing effects over time, above all in the interface zone between the internal semiconducting layer and the insulating layer where probability of partial discharges is the greatest;
substantial inertness towards commonly used cross-linking agents, in particular organic peroxides, thus avoiding phenomena of inhibition of the cross-linking reaction and/or alteration or destruction of the additive itself during the cross-linking process;
chemical and physical properties which make the use of t
Del Brenna Marcello
Martinotto Luca
Peruzzotti Franco
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Gray J.
Kelly Cynthia H.
Pirelli Cavi E Sistemi S.p.A.
LandOfFree
Cable, in particular for transport or distribution of... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Cable, in particular for transport or distribution of..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Cable, in particular for transport or distribution of... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3317506