EP0129485B1 - Electric-cable structure and its application - Google Patents

Description

The present invention relates to a new electric cable structure, in which the conductor is coated with several successive layers of materials, comprising a hydrophobic and semiconductor sealing gel, placed between a polymer layer, also semiconductor, and a screen. metallic.

The invention also relates to the application of this structure to the continuous earthing of electrical conductors and to the radialization of the field in the power cables.

• Les fig. 1a et 1b sont des coupes transversales de deux types de câble de la technique antérieure;
• les fig. 2a et 2b sont des coupes analogues des mêmes câbles présentant un perfectionnement conforme à l'invention;
• la fig. 2c représente une coupe d'un câble coaxial conforme à l'invention;
• la fig. 3 est une vue en perspective avec arrachés illustrant une structure de câble conforme à la présente invention.

The appearance of semiconductor polymeric materials has, as we know, brought about a great improvement in the manufacture of electric cables, both for telecommunication cables and for energy transport cables. Such known cable structures will be described below, with reference to FIGS. 1a and 1b of the accompanying drawings, in which:

• Figs. 1a and 1b are cross sections of two types of cable of the prior art;
• fig. 2a and 2b are similar sections of the same cables having an improvement in accordance with the invention;
• fig. 2c shows a section of a coaxial cable according to the invention;
• fig. 3 is a perspective view with parts broken away illustrating a cable structure according to the present invention.

The cable structure shown in fig. 1a is that of a telecommunication cable of a conventional type. This cable comprises, for example, a plurality of conductive wires 1 made of a conductive material such as copper or aluminum, surrounded by an insulating layer 2. All of the conductive wires thus sheathed are surrounded by a conductive metallic shield 3 , forming a screen, which is itself surrounded by a protective layer constituted by a semiconductor polymer 4, ensuring good physical contact with the metal surface 3. The space 5 left free between the insulating sheaths 2 and the metal surface 3 can be filled in the conventional way with a sealant.

The energy transport cable shown in fig. 1b, which is also of a known type, comprises, for its part, a strand of conducting wires 6, which is surrounded by a sheath or semiconductive polymer layer 7. Around this sheath 7 is arranged an insulating material 8, itself surrounded by a second semiconductor polymer layer 9, surrounded by a layer of conductive metal 10 forming a screen, for example consisting of copper, steel or aluminum. The outer belt 11 can itself be constituted by an insulating or semi-conductive polymer sheath.

The usual cables of the type of those illustrated in figs. 1a and 1b or made up of strand assemblies such as multipolar cables, however have the drawback of not being perfectly sealed against humidity and of not ensuring perfect contact between the semiconductor sheath and metal surface. Indeed, the area between the semiconductor polymer (referenced 4 in fig. 1 a and 9 in fig. 1 b) and the metal screen (referenced 3 in fig. 1a and 10 in fig. 1b ) is always susceptible, following a shock, a cable twist, a cracking, a condensation occurring at the level of the free spaces or a longitudinal propagation starting from the junctions or the splices of the cable, to allow traces of moisture to come into contact with the metal, thus causing the deterioration of the latter by a phenomenon of branching, oxidation and / or corrosion. This drawback can be partially limited by incorporating between the metallic layer and the semiconductor polymer a layer of a hydrophilic material such as carboxymethylcellulose or of a hydroscopic material such as a semiconductor clay, the swelling of which in the presence of moisture prevents water from spreading along the conductive metal. However, these products do not prevent the phenomena of local corrosion of the screens.

US-A-4095039 describes a cable structure in which a sealing layer having semiconductive properties and ensuring perfect sealing is interposed between a metallic screen and a semiconductive polymer layer surrounding a plurality of conductive cables. The sealing layer comprises a polyisobutylene rubber, a large amount of carbon black and, if necessary, to lower its viscosity, an oil, in as small an amount as possible.

The object of the present invention is therefore to produce such structures of electric cables in which a perfect seal is ensured between a metal screen and a semiconductor polymer layer and which ensure a continuous earthing of the cables.

To this end, the subject of the invention is an electrical cable structure of the type comprising at least one screen and at least one semi-conductive polymer layer surrounding at least one conductive cable, characterized in that between said metallic screen and said layer a semiconductive polymer is interposed with a sealing layer with a dynamic viscosity of less than 100,000 centipoises at 20 ° C. and between 50,000 and 100,000 centipoises at 100 ° C., this sealing layer comprising a semiconductor jelly containing between 50 and 95% by weight of at least one compound of a paraffinic or naphthenic hydrocarbon oil of petroleum, vegetable or synthetic origin, and between 5 and 50% by weight of carbon black and / or graphite, or a powder of at least one metal chosen from the group comprising zinc, copper, and aluminum, or of at least one oxide of one of these metals; in that the resistivity of said sealing layer is less than 40,000 ohms x cm; and in that the resistivity of said semiconductor polymer layer is less than 20,000 ohms x cm.

Within the meaning of the present application, the term “metallic screen” designates not only a conductive shielding of the type illustrated in figs. 1a and 1b, but also any ply of metallic threads, woven, braided or "guippés", to use the term used in the art.

The semiconductor and hydrophobic jelly used in accordance with the invention is designated by the references 12 and 13, respectively, in FIGS. 2a or 2b, on which the elements already described with reference to FIGS. 1a and 2a keep the same reference figures. This jelly is interposed between the metal screens 3, respectively 10, and the semiconductive polymer sheaths 4, respectively 9. By virtue of its hydrophobic properties, it insulates the electric cables from humidity, while effectively ensuring a the earth continuously, thanks to its special dielectric properties.

Of course, such a continuous earthing is also applicable, according to the same principle, to other types of cables, in particular energy transport cables.

Fig. 2c shows a particular application of the cable structure according to the invention, in a low noise coaxial cable. In common coaxial cables, the friction of the metal braid against the dielectric is generally the source of triboelectric noise. In fig. 2c, the semiconductor jelly constitutes the sealing layer represented by the reference 13 which is interposed between the semiconductor polymer layer 9 which covers the insulating material 8, and the metal braid represented by the reference 10. This arrangement makes it possible to remove much of the triboelectric noise.

The introduction of the semiconductor and hydrophobic sealing jelly between the metal screen and the semiconductor polymer layer also makes it possible, thanks to the dielectric properties of this layer, to effectively ensure the radialization of the field in the transport cables. of energy.

A first advantage of the present invention is linked to the fact that the semiconductor jelly is perfectly compatible with both the metal strip, to which it adheres completely and that it protects from possible traces of moisture or other forms of corrosion of the metal. , than with the semiconductor polymer layer, due to the very nature of its constituents, insofar as these cannot diffuse into the polymer layer and where additives and conductive fillers of the same kind are preferably added than those used in the composition of the jelly.

A second advantage of the present invention lies in the fact that, given the presence of the semiconductor jelly, the semiconductor polymer layer no longer has to simultaneously provide effective protection of the metal strip and maximum adhesion to the metal : the semiconductor polymer layer can therefore be chosen according to the only mechanical properties required for cable protection, in addition to the desired electrical properties.

A third advantage of this cable belt structure resides in the fact that the semiconductor jelly ensures, by its fluidity and by its plasticity, in addition to a perfect seal and, therefore, an excellent electrical contact between the semiconductor polymer layer and the metallic screen which surrounds it, whatever the mechanical deformations imposed on the cable, while maintaining effective protection of these elements.

An additional advantage of the cable belt structure according to the invention finally results from the fact that the fluidity and plasticity properties of the sealing layer are not very susceptible to the effect of temperature since the dynamic viscosity is at 20 °. C, less than 100,000 centipoises and, at 100 ° C, remains between 50,000 and 100,000 centipoises.

This cable belt structure finally considerably facilitates the operations of connecting the cables during their installation.

This new type of cable belt structure therefore protects, with increased reliability, the metal screen against corrosion and ensures excellent grounding or excellent radialization of the electric field, while better protecting the cable itself by strengthening its outer sheath.

In the sealing compositions for semiconductor jellies which can be introduced into the structure of the electrical cable belt which is the subject of the present invention, a proportion of the order of 50 to 95% by weight of paraffinic hydrocarbon compounds is used or Naphthenics selected to not diffuse at temperatures of the order of 50 ° C or more in polyethylene, polypropylene, polybutylene, polyvinyl chloride or any other cellular insulation material used in the composition of the belt sheath.

These hydrocarbon compounds can be of petroleum, vegetable or synthetic origin, or be composed of mixtures of several of these oils. Advantageously, distillation cups or oils and / or petrolatum obtained from the latter are used. Generally, less than 5% of these oils have a boiling point below 350 ° C.

When they are of synthetic origin, these hydrocarbon compounds are advantageously constituted by polymers obtained from olefins having three or four carbon atoms, or by mixtures of these. Advantageously, then, synthetic oil cuts having a molecular weight by weight of between 200 and 4000 and, more particularly, between 400 and 1500 are used.

To these oils, a conductive filler such as a metal powder or metal oxide, the metal of which may advantageously be zinc, copper or aluminum, or carbon black, is added in a manner known per se. , a mixture of carbon black of varying particle size, or graphite or, finally, a mixture of the latter. The proportion of the conductive charge, relative to that of the oil, is determined above all by considerations of resistance. electrical and viscosity of the desired semiconductor and hydrophobic jelly, depending on the conditions of manufacture and use of the electrical cable in the belt from which it will be introduced. This proportion can therefore vary between 5 and 50% by weight of the sealing jelly, depending on the case, and, more particularly, between 5 and 40%.

A particularly advantageous composition according to the invention is obtained by the use of very conductive carbon blacks of the Ketjen EC or Phillips XE2 type; these blacks, which can be used in a lower concentration than conventional blacks, for the same resistivity, make it possible to obtain compositions which are all the more hydrophobic; the concentration of these blacks is between 5 and 15% by weight, depending on whether they are used alone or not and according to the desired resistivity.

In the composition of the jelly, one can finally add, without however this addition being necessary for all oils, stabilizing agents, adhesiveness agents such as resins of petroleum origin, thickening agents such as unsaturated polyolefins in proportion which may be between 0 and 20%, and finally metal passivators such as benzotriazoles, substituted or not, or any other substance known per se capable of performing a similar function, in proportion which may be between 0 and 2%, depending on the nature of the oil, of the conductive filler or metal used in the composition of the strip (or armor) of the cable.

• - une résistivité électrique inférieure à 40.000 et de préférence inférieure à 10.000 ohms x cm, lorsque le câble est destiné à être mis à la terre, ou une résistivité inférieure à 20.000 ohms x cm pour les câbles dits homopolaires;
• - une viscosité à 100 °C comprise entre 10000 et 100000 centipoises;
• - une bonne adhérence au métal à basse température (-10 °C, conformément à la norme CNET CM 35), et
• - une température bille-anneau, mesurée selon la norme NFT 66008, supérieure à 50 °C et, de préférence, entre 100 et 200 °C.

The semiconductor and hydrophobic jellies entering the cable belt structure object of the present invention will have the following physical properties:

• - an electrical resistivity less than 40,000 and preferably less than 10,000 ohms x cm, when the cable is intended to be earthed, or a resistivity less than 20,000 ohms x cm for so-called homopolar cables;
• - a viscosity at 100 ° C of between 10,000 and 100,000 centipoises;
• - good adhesion to the metal at low temperature (-10 ° C, in accordance with standard CNET CM 35), and
• - a ball-ring temperature, measured according to standard NFT 66008, greater than 50 ° C and, preferably, between 100 and 200 ° C.

Tests have been carried out for many years to make the cladding materials thermoplastic semiconductor, by incorporating therein metals, metal oxides or carbon blacks of current quality. However, to obtain sufficient electrical conductivity, it was necessary to introduce significant quantities of conductive charge, which had the consequence of deteriorating the mechanical properties of the thermoplastics and of adversely affecting their properties of adhesion to the metal strip which they had to protect. The introduction of a semiconductor jelly which ensures complete sealing between the sheath and the metal therefore allows the use of sheath materials with improved properties.

Among the semiconductor polymers which can be used in the electric cable structure which is the subject of the present invention, there are compositions comprising mainly an ethylene polymer, or a mixture of a homopolymer and an ethylene copolymer. , or a mixture of ethylene copolymer with a propylene monomer, vinyl acetate, ethyl acrylate or any other monomer, in a manner known per se. In particular, compositions containing more than 70% of high or medium density ethylene or polyethylene compolymer will be used, in order to give this sheath the required rigidity and solidity. The polyethylene used may advantageously have a density between 0.90 and 0.95 and a melt index between 0.1 and 2. It is also possible to use any plastic material capable of incorporating the conductive fillers and, in particular, polychloride plasticized vinyl.

The polymer composition also contains a conductive filler, which will advantageously be of the same nature as that contained by the semiconductor jelly entering the cable belt structure. The proportion of this load can also vary between 5% and 45%, depending on the resistivity and robustness that can be expected from this type of sheath and the expected conditions of use of the electric cable. For the purposes of continuous earthing, this proportion will advantageously vary between 8 and 15% by weight.

• - poyéthylène ou copolymère éthylène-acrylate d'éthyle ou copolymère éthylène-acétate de vinyle ou d'un copolymère éthylène-polypropylène ou d'une combinaison quelconque de ces, quatre polymères 10 à 100%
• - noir de carbone 5 à 20%
• - mélange d'antioxydant 0,1 à 2%

The semiconductor polymer layers can advantageously have the following composition (% by weight):

• - polyethylene or ethylene-ethyl acrylate copolymer or ethylene-vinyl acetate copolymer or of an ethylene-polypropylene copolymer or of any combination of these, four polymers 10 to 100%
• - carbon black 5 to 20%
• - mixture of antioxidant 0.1 to 2%

• - résistivité inférieure à 10.000 et de préférence à 1.000 ohms x cm, lorsque l'écran est destiné à la mise à la terre, ou de 10 à 10.000 ohms x cm, lorsqu'il s'agit de radialiser le champ au sein d'un isolant;
• - allongement à la rupture supérieure à 100% et, de préférence, à 300% (Norme NFT 51034);
• - dureté Shore D comprise entre 35 et 70 et, de préférence, entre 50 et 70.

The polymer layers entering the cable belt structure which is the subject of the present invention preferably have the following physical properties:

• - resistivity less than 10,000 and preferably 1,000 ohms x cm, when the screen is intended for earthing, or from 10 to 10,000 ohms x cm, when it is a question of radializing the field within an insulator;
• - elongation at break greater than 100% and preferably 300% (Standard NFT 51034);
• - Shore D hardness between 35 and 70 and preferably between 50 and 70.

The sheaths must finally have good resistance to stress cracking.

In order to check the robustness, longevity and quality of the earthing of the cable structures in accordance with the present invention, the Applicant has carried out comparative tests between them and cable structures of a conventional type.

Three cables A, B and C, 50 meters long, with a structure like those shown schematically in fig. 1a, for cable A, and in fig. 2a, for cables B and C, were thus buried in various types of land.

The compositions of these cables are listed in Table 1 below:

Although the resistances of the screens with respect to earth are comparable for the three types of cables, when they are earthed (of the order of 10 to 25 ohms per 50 meters), only the resistance of the screens of cables B and C with respect to the earth remains substantially constant over time and is already between 40 and 60% below the resistance of cable A, after two years, under the same conditions of use.

Thus, in waterproof cables having the structure according to the present invention, the presence of a hydrophobic semiconductor jelly between the metal screen and the semiconductor polymer layer allows this screen and this layer to remain constantly in contact. electric, without the use of any auxiliary earthing of the screen, and without risk of accidental corrosion of the latter as a result of ramification phenomena following imperfect contacts between the screen and the semiconductor layer.

Additional comparative tests were carried out with two other types of cables, D and E, buried under the same conditions, in order to show the best electrical continuity of the cable structures according to the invention.

A first cable D presents the structure illustrated in FIG. 3. A corrugated metallic screen 14, made of copper, surrounds the conductive wires 21, sheathed with an insulator 22. Around the screen 14 are successively arranged an intermediate semiconductive polymer layer 15, a steel screen 16 arranged in a helix and a semi-conductive external polymer sheath 17.

Between layers 14 and 15, 15 and 16, and 16 and 17 was injected a semiconductor jelly, respecti clothing 18, 19 and 20, ensuring the tightness of the cable.

The polymer layers and the semiconductor jelly used in the composition of the cable D are produced with formulations identical to those of the cable C previously described.

The electrical properties of this cable D were compared with those of a cable E built on the same model, but without the introduction of semiconductive sealing jelly in 18, 19 and 20.

Table II below gives the resistance values of the screens in ohms for 50 meters of buried cable of these cables D and D.

This table therefore shows that the best results are obtained with cable D; in fact, if the resistance values of the screen 16 with respect to the earth are comparable, the resistance value with respect to the earth of the screen 14, in the waterproof version D is lower by a factor of about 15 compared to that of the unsealed version E of said cable, while the resistance between screens is divided by a factor of the order of 10.

Thus, in the structure of the waterproof cable D, conforming to the metal surface of the screen or screens and the semiconductive polymer layer, a semiconductive and hydrophobic sealing gel, promotes the electrical conductivity between screens and sheaths, while ensuring longitudinal sealing. The three constituents of this cable belt are therefore placed in continuous parallel contact, which makes it possible to avoid frequent earthing of the external structure of the cables and to promote the reducing effect.

Claims (10)

1. An electric cable structure of the type comprising at least one metallic screen (3, 10) and at least one semi-conducting polymer layer (4, 9) surrounding at least one conductor cable (2, 6), characterised in that between said metallic screen and said semiconducting polymer layer there is disposed a sealing layer (12, 13) of a dynamic viscosity less than 100,000 centipoises at 20 °C, and comprising between 50,000 and 100,000 centipoises at 100 °C, this sealing layer comprising a semiconducting gel containing between 50 and 95% by weight of at least one compound of a hydrocarbon oil of paraffinic or naphthenic nature and of petroleum, vegetable or synthetic origin, and between 5 and 50% by weight of carbon black and/or graphite, or of a powder of at least one metal selected from the group comprising zinc, copper and aluminium or at least one oxide of one of these metals; in that the resistivity of said sealing layer is less than 40,000 ohms x cm; and in that the resistivity of said semiconducting polymer layer is less than 20,000 ohms x cm.

2. An electric cable structure according to claim 1, characterised in that the resistivity of the semi- conducting polymer layer is less than 10,000 ohms x cm and that of the sealing layer is less than 40,000 ohms x cm when said semiconducting polymer layer is used for sheathing, particularly as an outer sheath.

3. An electric cable structure according to claim 1, characterised in that the resistivity of each of the two polymer and sealing layers is less than 20,000 ohms x cm when said metallic screen is disposed externally of said layers in relation to the cable axis.

4. An electric cable structure according to any of claims 1 to 3, characterised in that the sealing layer contains stabilising agents, thickening agents and adhesive agents in a proportion of between 0 and 20%.

5. An electric cable structure according to any of claims 1 to 4, characterised in that the semiconducting polymer layer contains, in % by weight, between 10 and 100% of polyethylene, or of an ethylene-ethylacrylate copolymer, or of an ethylene-vinylacetate copolymer, or of a propyleneethylene copolymer or a combination of these four polymers, between 5 and 20% of carbon black and between 0.01 and 2% of at least one stabiliser.

6. An electric cable structure according to any of claims 1 to 5, characterised in that said semiconducting polymer layer and said sealing layer comprises semiconducting fillers and like protective additives.

7. An electric cable structure according to claim 1, characterised in that the metallic screen is made of steel, zinc, copper or aluminium.

8. Use of cables having a structure according to either of claims 1 and 2 for the continuous earthing of electric conductors.

9. Use of cables having an annular structure according to either of claims 1 and 3 for the radial distribution of electric fields within the insulation.

10. Use of cables having a structure according to any of claims 1 to 3 for the production of coaxial cables.

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