Coating processes – Electrical product produced – Wire conductor
Reexamination Certificate
2001-12-27
2004-11-30
Dye, Rena (Department: 1774)
Coating processes
Electrical product produced
Wire conductor
C427S117000, C174S1100PM, C428S379000
Reexamination Certificate
active
06824815
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for producing an electrical cable, particularly for high voltage direct current transmission or distribution.
More particularly, the present invention relates to a process for preparing an electrical cable, particularly for high voltage direct current transmission or distribution, which is suitable for either terrestrial or submarine installation, comprising the stage of producing at least one insulating coating for said cable by hot cross-linking of a polymeric composition comprising a polyethylene, a radical initiator and a small amount of an unsaturated carboxylic acid.
The present invention moreover relates to a cable for high voltage direct current transmission or distribution in which the insulating coating consists of the abovementioned polymeric composition.
For the purposes of the present description and the claims, the term “high tension” means a tension of greater than 35 kV.
2. Description of the Related Art
The cables generally used for high voltage direct current transmission, either along terrestrial lines or, particularly, along submarine lines, are cables commonly known in the art, such as mass-impregnated cables in which the conductor, coated with a first semiconducting layer, is electrically insulated by being wound with an insulating material, generally paper or paper/polypropylene/paper multilayer laminates, which is then totally impregnated with a mixture with high electrical resistivity and high viscosity, generally a hydrocarbon oil containing a viscosity-increasing agent. The cable then comprises a further semiconducting layer and a metal screen, generally made of lead, which is itself surrounded by at least one metal armouring structure and by one or more plastic protective sheaths.
Although mass-impregnated cables are characterized by high reliability in operation even at very high voltages (greater than 150 kV), they have a number of drawbacks mainly associated with migration of the insulating fluid inside the cable. Particularly, during use, the cable is subjected, owing to variations in the intensity of the current transmitted, to thermal cycles which cause migrations of the fluid in the radial direction. As a matter of fact, when the current carried increases and the cable heats up, the viscosity of the insulating fluid decreases and the fluid is subjected to a thermal expansion greater than all the other components of which the cable is made. This leads to migration of the fluid from the insulating layer towards the exterior and, consequently, to an increase in the pressure exerted on the metal screen, which is deformed in the radial direction. When the current carried decreases and the cable cools down, the impregnating fluid contracts, whereas the metal screen, which is made of a plastic material (usually lead), remains permanently deformed. This therefore results in a decrease in the internal pressure of the cable, leading to the formation of microcavities in the insulating layer with a consequent risk of electric discharges and, hence, of perforation of the insulation. The risk of perforation increases as the thickness of the insulating layer increases and, hence, as the maximum voltage for which the cable was intended increases.
Another solution for high voltage direct current transmission consists of cables with fluid oil, in which the insulation is provided by a pressurized oil of low viscosity and high electrical resistivity (under a hydrostatic head). Although this solution is highly effective in terms of avoiding the formation of microcavities in the cable insulation, it has a number of drawbacks mainly associated with the complexity of construction and, particularly, results in a limitation of the maximum permissible length of the cable. This limitation of the maximum length is a major drawback, especially as regards submarine use, in which the lengths required are usually very great.
For many years, research has been directed towards the possibility of using cross-linked polyolefins, and particularly cross-linked polyethylene (XLPE), to produce insulating materials for cables for direct current transmission. Insulating materials of this type are already widely used in the case of cables for alternating current transmission. The use of said insulating materials also in the case of cables for direct current transmission would allow said cables to be used at higher temperatures, for example at 90° C. instead of 50° C., compared with the mass-impregnated cables described above (higher working temperatures, making it possible to increase the amount of current transported) and would eliminate limitations in the maximum permissible length of the cable, in contrast with the cables containing fluid oil described above.
However, it has not hitherto been possible to adequately and fully exploit said insulating materials, particularly for direct current transmission. It is commonly believed that one of the main reasons for this limitation is the development and accumulation of so-called space charges in the dielectric insulating material when said material is subjected to a direct current. It is thought that space charges alter the distribution of the electrical field and persist for long periods on account of the high resistivity of the polymers used. The accumulation of space charges leads to a local increase in the electrical field, which is consequently greater than that which would be expected considering the geometrical dimensions and the dielectric properties of the insulating material.
The accumulation of space charges is a slow process: however, the problem is accentuated when the direct current transported by the cable is reversed (in other words, if there is a reversal of polarity). As a result of this reversal, a capacitive field is superimposed on the whole electrical field and the value of the maximum gradient can be localized within the insulating material.
It is known that a prolonged degassing treatment, which may be carried out, for example, by subjecting the insulating material based on a cross-linked polymer to high temperatures and/or to a high vacuum for a long period, makes it possible to obtain an insulating material which is capable of limiting the accumulation of space charges when the cable is subjected to polarity reversal. In general, it is thought that, by virtue of the removal of the decomposition products of the cross-linking agent (for example dicumyl peroxide which forms acetophenone and cumyl alcohol on decomposition) from the insulating material, said degassing treatment reduces the formation of space charges. However, a prolonged degassing treatment obviously leads to an increase in the production times and costs.
In efforts to reduce the accumulation of space charges, it is known practice to modify cross-linked polyethylene (XLPE) by introducing small amounts of polar groups.
Patent application EP-A-0 463 402 discloses an ethylene (co)polymer containing polar groups chosen from ketone, nitrile and nitro groups in an amount of between 20 ppm and 8000 ppm, said polar groups having a dipole moment of greater than 0.8 debye. Said (co)polymer is said to be usable as an insulating material for high voltage cables with improved dielectric rigidity. Said polar groups may be introduced into the polyethylene by various processes such as, for example:
by copolymerization of a comonomer containing said polar groups with ethylene;
by blending an ethylene polymer or copolymer containing said polar groups with a conventional polyethylene;
by oxidation of a conventional polyethylene;
by grafting comonomers containing said polar groups onto a conventional polyethylene.
Japanese patent application JP 10/283 851 discloses a cable for direct current transmission which has improved dielectric rigidity, in the presence of polarity reversals or following applications of electrical pulses, in which the insulating coating consists of a polymeric composition comprising a cross-linked polyolefin containing (i) a dicarboxylic acid anhydrid
Albizzati Enrico
Perego Gabriele
Dye Rena
Finnegan Henderson Farabow Garret & Dunner, L.L.P.
Gray J.
Pirelli Cavi E Sistemi S.p.A.
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