FR2591792A1 - Aerial electrical cable with light wave guide core - Google Patents

Abstract

An aerial electrical cable comprises a core contg. a light wave guide, a sheath surrounding the core made of an elastomeric or thermoplastic material (I), and a pref. non-metallic reinforcement inside or outside the sheath. (I) is crosslinked after grafting of silane cpds. under the effect of moisture and contains additives such as metal oxides, carbides, etc., either alone or in combination, so as to improve the stray current resistance of the material. Prepn. of the cable is also claimed. Pref. the additive is a combination of hydrated Al oxide (30-80 pts.) and iron oxide (3-7 pts.) (w.r.t. 100 pts. polymer). Alternatively the additive comprises TiO2 or ZnO. (I) is LLDPE of density 0.88-0.95 g/cm2 or one of its copolymers and it may be mixed with other polymers.

Description

ELECTRIC AERIAL CABLE WITH PROBE GUIDES
LIGHT RESISTANT TO LEAK CURRENTS AND
PROCESS FOR ITS MANUFACTURE
The present invention relates to an overhead electrical cable, comprising an EMS containing light waveguides, a sheath surrounding the blade, made of a thermoplastic or elastomer material, as well as a plain reinforcement below or above this sheath, and in particular a metal-free sheath.

 In the field of energy supply by high voltage overhead networks, (ETZ Nd 106 (1985) Heft 4, S.154 ff) overhead cables are already provided with light waveguides for the transmission of 'information; there, in the case of what lton designates as long-range aerial targets, ranges of up to 435 m could be bridged. The problems which arise with such aerial targets can be of mechanical type, in particular difficulties due to vibrations or to loads, which, as the case may be, could be at least lessened by means of appropriate armatures of metallic or non-metallic type, that is to say without metal.

 However, electrical problems are also already known, which result from the insulating sheath. In the electric field of the phase conductors of a high voltage overhead line, the sheath, in particular, is charged on its surface; due to the high surface resistance of up to 1012 / cm, the induced charge cannot then discharge. Different soiling and humidity on the surface of a sheath means that even within the range of a high voltage overhead line, potential differences occur on the metal-free overhead cable, which cause leakage current discharges.

 To this is added that the surface of the sheath is necessarily set to a zero potential at each pylon due to the metal suspension, which is connected to the pylon according to an electrically conductive connection. This causes, in the area of the sheath / suspension transition, as is known, large differences in tension, which are always filled again by discharges harmful to the overhead cable.

 Starting from this state of the art, the invention aims to increase the operational safety of cables of the type indicated first and installed in the area of high voltage overhead lines.

 The invention relates for this purpose to an overhead electrical cable which is characterized in that the material, thermoplastic or elastomer, of the sheath, after grafting of silane compounds, is crosslinked under the action of moisture, and contains additives ensuring resistance to the leakage currents of this material, such as metal oxides, carbides and the like having an analogous action, individually or in combination.

 The particular advantage of an overhead cable thus formed is that the additives are integrated, by being distributed uniformly over the entire length of the cable and the cross section of the sheath, in a polymer matrix also uniformly crosslinked. This leads to a cable having great operational reliability for the intended purposes, since even in the case where the influences are exerted, occurring simultaneously in the electric field, of soiling and humidity, there is no need to fear damage or even destruction, under the effect of leakage currents, of the cable also free of metal.

 The resistance to leakage currents of the sheath (envelope) of the cable over the entire length also has the advantage of simpler mounting, compared to embodiments in which it is sought to protect against leakage currents of known cables. only at the places of suspension from the pylons, but also of a greater operational reliability due to the very advanced independence, obtained thanks to the invention, from the quality of the assembly works.

 It has been found advantageous, according to one embodiment of the invention, to constitute the additives by a combination of alumina hydrate and iron oxide, preferably in a proportion of 30 to 80 parts of alumina hydrate and 3 to 7 parts of iron oxide on 100 parts of polymer. The ready-to-use ducts have the required resistance to leakage currents; the manufacture of the sheaths, their extrusion and the subsequent crosslinking are carried out without problem.

 Instead of the additives mentioned, other materials which improve resistance to leakage currents can also be used. As materials have been found to be advantageous in this respect, for example, titanium oxide or zinc oxide.

 According to an embodiment of the invention, it is also advantageous to use as the sheath material a linear polyethylene (LLDPE) having a density of 0.88 to 0.95 g / cm 3 or one of its copolymers, alone or in blending with other polymers. This linear polyethylene is a polyethylene polymerization product which combines the characteristic properties of linear low pressure polyethylene (HDPE) with those of highly branched high pressure polyethylene (LDPE).

 The structure of this material, which is produced according to various processes with comparatively low pressures, similarly only contains HDPE as branches in very short chains. The main chain of the polymer is thus, as in the case of HDPE, determining for some essential properties of the macromolecule. Therefore, the melting zones of the LLDPE are located, around 120-1250C, in the vicinity of the fusion zone of the HDPE.

 Different from HDPE and thus approaching LDPE, the number of ramifications is significantly higher. As a result, the density and crystallinity are significantly reduced. The designation LLPDE brings together the properties that appear traditionally contradictory, namely linear and low density molecular structure ("low density").

 Sheaths made of such a material are distinguished, for example, by an increase in abrasion resistance, thermoforming stability, as well as cold stability.

To this is added that during the manufacture of a cable according to the invention, its can count on smaller quantities of silane and of peroxide, for the purpose of crosslinking, than is usually the case; after grafting the silane compounds onto the basic molecules of the linear polyethylene, the crosslinking operation in the presence of moisture takes place by being accelerated.

 In the case of a sufficiently high humidity, one can even do without the use of saturated steam or water insertion. The manufacture of a cable according to the invention can therefore also be carried out more economically.

 To make an aerial cable according to the invention, the procedure is advantageously as follows: first mixing the base material with the constituents of the silane and then grafting the silane onto the base polymer, into the base material as well prepared, add the additives and distribute them evenly; and finally the sheath is formed and exposed to moisture for crosslinking.

 The invention is based on the knowledge that the base polymer must first be made capable of crosslinking, in order to ensure subsequent uniform crosslinking even of different cross sections, before in this material already pre-programmed for the crosslinking process, essential additives are mixed to improve resistance to leakage currents.

Further improvements in the quality of the final product can be obtained during the manufacturing process, by mixing the additives, in the base material having received the grafting.
silanes, at temperatures between 1000
and 1400C.

There is also benefit
that the crosslinking catalyst, used for the
crosslinking under the action of humidity, either
incorporated by mixing only just before forming
of the crosslinkable sheath. We avoid this way
pre-crosslinking, after addition of the oxides
of metal, hydrates and other similar additives.

The invention will be better understood
using the example of mixing indicated below
as well as the description of the example of realization
tion shown in the accompanying drawings, in which,
- Figure 1 shows schematically,
in cross-section, the constitution of a cable
aerial according to the invention;
- Figure 2 schematically shows a
mounting such an air target.

Example of mixing
- 100 parts polyethylene copolymer
(proportion of copolymer approximately 20 leaks)
- 1.7 parts vinyl trimetoxysilane
- Peroxide
(e.g. dicumyl peroxide) 0.006 parts
- Polyethylene copolymer
(proportion of copolymer approximately 3X) 20 parts
- Alumina hydrate
("martinal" OL104C)
(from the Martinswerke Factory) 42 games
- Iron oxide (fie304) 5 parts
- Stabilizer (AnosHB) 0.5 parts
- Polyethylene copolymer
(proportion of copolymer approximately 15%) 100 parts - Catalyst (naftovine S) 0.85 parts
The polyethylene constituents in the example mixture above can also be replaced for example by an ethylene-propylene rubber or the LLDPE mentioned above.

 To manufacture a sheath resistant to leakage currents for overhead cable, the procedure is for example as follows: firstly, corresponding to A, the polymer-based material is introduced into what is called a "grafting extruder" "- - in its inlet hopper - together with the silane and the peroxide which is dissolved therein, and the silane is grafted onto the polyethylene copolymer, thus making it crosslinkable. The polyethylene material thus treated is extruded and granulated.

 At the same time, a very concentrated ("batch") mixture is produced according to B, containing additives to improve the resistance to leakage currents of the alumina hydrate and iron oxide. To mix the various constituents, an appropriate kneader is advantageously used.

Advantageously, a "batch" corresponding to C is also produced on a kneader, containing the crosslinking catalyst, (stentexld "batch" kneader: batch kneader).

 After granulation of the crosslinkable base mixture (A), it is mixed together with the highly concentrated mixture (B) for example in an appropriate kneading machine or a corresponding mixer, and the mixture is then brought to the inlet hopper of an extruder for the continuous production of the sheath according to the invention The mixture corresponding to C is also introduced at most t8t into the forming extruder; during the mixing and homogenization phase in the extruder, the crosslinking catalyst is distributed in a sufficiently homogeneous manner.

 Figure 1 shows an overhead cable free of metal, with light mowing guides (LwL), constituted according to the invention, while in Figure 2 is shown its assembly according to a high voltage overhead line over a long distance.

 All around the insulating core 1 are targeted the hollow conductors 2, through which the light mowing guides pass. This wired assembly is surrounded by the insulating sheath (envelope) 4, which, according to the invention, is made of a material based on polymer resistant to fluid currents, crosslinked under the action of humidity, after grafting of silanes. A frame 5, consisting, for example of high-rigidity synthetic material son, in particular based on homo or aromatic co-polyamides, serves to absorb the tensile forces.

 As a variant to this exemplary embodiment, such overhead cables may also include a metallic reinforcement, or else in the core of the cable, below the sheath, metallic conductive cords for transmitting information can be arranged.

 Figure 2 shows schematically the assembly as practiced today of an overhead cable with light wave guides in the field of energy supply.

Between the pylons 6 and 7 located at a distance of 400 m for example, the earth cable 8 and the three phase conductor cables 9 are stretched.

At a distance of 6 m, for example from the phase conductors 9, there is an overhead cable 11 constituted as shown in FIG. 1; this overhead cable is fixed to the pylons 6 and 7 by means of metallic and grounded suspensions 10.

 Particularly in the area of these suspensions, in the case of a metal-free overhead cable, an increased load must be expected by induced leakage currents on the surface of the sheath 4. Due to corresponding additional electrical charges - unavoidable during service - subjected to the effect of dirt and moisture, vibrations of the cabal, constructional peculiarities, etc. other sections of cable are however also threatened with leakage currents. By the same token, the invention remedies it thanks to the special constitution of the cable sheath,

Claims (8)

 1- Electric aerial cabal, comprising a core containing light wave guides, a sheath surrounding the core, made of a thermoplastic or elastomer material, as well as a frame below or above this sheath, and in particular a free frame of metal, overhead cable characterized in that the material, thermoplastic or elastomer, of the sheath (4), after grafting of silane compounds, is crosslinked under the action of moisture, and contains additives ensuring resistance to currents leakage of this material, such as metal oxides, carbides and the like having an analogous action, individually or in combination.  2- Aerial target according to claim 1, comprising a sheath containing metal oxides, characterized in that the additives are a combination of alumina hydrate and iron oxide.  3- Aerial cable according to claim 2, characterized in that the proportion of alumina hydrate is 30 to 80 parts and that of iron oxide from 3 to 7 parts per 100 parts of polymer.  4- aerial cabin according to claim 1, comprising a sheath containing metal oxides, characterized in that these consist of titanium dioxide and zinc oxide.  5- aerial cable according to any one of claims 1 to 4, characterized in that the material used for the sheath is constituted by a linear polyethylene (LLDPE) having a density of 0.88 to 0.95 g / cm3, or else l 'one of its copolymers, alone or in the form of a cut with other polymers.  6- A method for implementing an overhead cable comprising a sheath (4) resistant to leakage currents according to any one of claims 1 to 5, characterized in that the basic material is first mixed with the constituents of the silane and the silane is then grafted onto the base polymer, in the base material thus prepared, the additives are incorporated and distributed in a homogeneous manner, and finally the sheath is formed and exposes it to moisture for cross-linking.  7- A method according to lfi claim 5, characterized in that the incorporation by admixture of additives in the base material on which silanes have been grafted, is carried out at temperatures between 100o and 14000.  8- Method according to moon or 1poor of claims 5 and 6, characterized in that the crosslinking catalyst used for crosslinking under the action of humidity is incorporated by mixing just before the formation of the crosslinkable sheath.