FR2977705A1 - Electric cable i.e. high voltage transmission cable such as overhead line, has external layer including assembly of two metal strands, where entire periphery of two metal strands includes hydrated alumina layer - Google Patents

Abstract

The cable (1) has an elongated central conducting element (4) surrounded by an external layer (2) including an assembly of two metal strands (2a, 3a), where the assembly of the metal strands is intended to impart an even surface to the external layer. The entire periphery of the two metal strands includes a hydrated alumina layer, where the hydrated alumina layer is one of a monohydrate alumina layer, a polyhydrate alumina layer, and a boehmite layer. The metal strands have a cross-section of Z-shape or trapezoidal shape, where the alumina layer breaks at a connecting region. An independent claim is also included for a method for manufacturing an electric cable.

Description

The present invention relates to the field of electrical conductors. It typically, but not exclusively, applies to high voltage electrical transmission cables or overhead power transmission cables, well known under the Anglicism "OverHead Lines" (OHL). In particular, the present invention relates to an electrical conductor of aluminum or aluminum alloy having a high corrosion resistance, so as to withstand harsh atmospheric conditions such as the salt atmosphere near the coast or the atmosphere sulfur environments industrialized cities. Overhead lines are usually made from aluminum. This material has a relatively low weight compared to other conductive materials. However, the latter has a fairly low resistance to corrosion. It has indeed been found that, after 2-3 years in a highly corrosive atmosphere (salty or sulfurous atmosphere), a conductor made of aluminum or aluminum alloy has cracks that can lead in the long run, the fall of the overhead line (breakage of the strands forming the cable). Therefore, it is known to protect aluminum or aluminum alloy cables by applying a layer of grease on their outer surface. However, this solution is not satisfactory since the grease layer has a limited action over time. In addition, the layer of fat generates a crown effect causing itself a noise nuisance which is unpleasant for the population installed in the vicinity of the line.

FR 676 889 discloses a high voltage electrical conductor comprising a central conductive element formed of aluminum round metal wires and covered with an outer layer formed of Z-shaped metal wires also made of aluminum. However, such a type of conductor does not sufficiently withstand time in atmospheres loaded with salt or sulfur.

The present invention aims to propose a new electrical conductor that avoids all or part of the aforementioned drawbacks. In particular, the electrical conductor according to the invention aims to withstand severe atmospheric conditions and thus avoid corrosion of overhead lines.

For this purpose, the subject of the invention is an electrical conductor comprising an elongated central electrically conductive element, surrounded by a first layer, referred to as an outer layer, said outer layer comprising an assembly of metal strands capable of conferring on said outer layer a surface substantially regular, each of the strands constituting the outer layer having a cross-section of complementary shape to the (x) strand (s) which is / are adjacent thereto, characterized in that at least a portion of the periphery of the strands, and preferably all the periphery of the strands, comprises (s) a layer of alumina (Al203), so that the outer surface of the conductor comprises said alumina layer along its longitudinal axis. The Applicant has surprisingly discovered that an outer layer formed of cross-section strands of complementary shape capable of conferring in addition to said outer layer a substantially regular surface, and whose edge or periphery of the strands is made of alumina, present extremely high corrosion resistance. According to the invention, by "assembly of metal strands capable of conferring on said outer layer a substantially regular surface, each of the strands constituting the outer layer having a cross-section of complementary shape to the (x) strand (s) which is / are adjacent (s) ", it is understood that: the juxtaposition or the nesting of all the constituent strands of the outer layer forms a continuous outer envelope (without irregularities), for example of circular or oval or square section. Thus, the Z-shaped or trapezoid-shaped cross-section strands are suitable for the present invention, whereas strands of circular section (whose assembly does not make it possible to obtain a regular external envelope), do not fit into the definition above. In particular, strands of Z-shaped cross-section are preferred. Even more preferably, the outer layer has a ring-shaped cross section. According to the invention, the term "outer layer" of the electrical conductor, the last layer of the cable, in particular, that which is intended to be in contact with the environment outside the cable, that is to say generally with the 'atmosphere. Preferably, the alumina layer is alumina monohydrate and even more preferably boehmite of formula AI203r nH20 with n between 1.5 and 2.5. In a first variant embodiment, said alumina layer 10 is not present on one or more portions of the conductor intended for the electrical connection and this, to facilitate its installation. In a second variant embodiment, the alumina layer is capable of breaking at a connection zone (eg electrical junction or electrical anchorage), so as to avoid, in the operational configuration of the cable, any overheating of the at the level of said connection. Conventionally, the connections at an electrical junction (cable-cable connection) or at an electrical anchorage (post-cable) are made via a sleeve of conductive material, such as steel . For example, at a junction, the end of two cables (about 80 cm long) is inserted into the sleeve which is then compressed by a clamping means. In the connection area, the ends of the conductor are thus protected from corrosion by the sleeve. The electrical conductors of the prior art do not comprise a layer of alumina on their outer surface, the current flowing in the conductor is removed from the material of the outer layer to the steel of the sleeve. In an electrical conductor according to the invention, the alumina layer, which covers the outer periphery of the conductor, is an electrical insulator (1 .mu.m alumina is used to electrically isolate a voltage of 40V). It could therefore be thought that it causes overheating at the outer layer by not allowing the current flowing in the conductor to be discharged to the sleeve. This would be all the more prejudicial since the IEC 61284 standard specifies that the temperature of a conductor must not exceed 105 ° C at the risk of causing a creep of the conductor (beyond this temperature is indeed observed a treatment of income which modifies the mechanical characteristics of the cable, in particular when this one is based on alloy of aluminum) and to cause the deflection of the overhead lines which could then be in contact with roofs of house or in contact trees. However, the Applicant has discovered that the presence of the alumina layer, especially at said connection area, was not binding and did not cause overheating since it breaks during installation. of the driver. Indeed, the compression exerted (according to the standards in force) on the sleeve through the clamping means is sufficient to break the alumina layer and thus pass the electric current between the outer layer and the steel sleeve. Preferably, the thickness of the alumina layer is at most 20 μm, and preferably at least 5 μm. In a particularly preferred manner, the thickness of the alumina layer can range from 6 to 15 μm, and even more preferably from 8 to 12 μm (limits included). According to a feature of the invention, between the conductive element and the outer layer is disposed a second layer, said inner layer.

According to a first variant embodiment, the inner layer comprises an assembly of metal strands, each of the constituent strands of the inner layer having a cross section of shape complementary to the strand (s) which is / are adjacent thereto. Preferably, the strands of the inner layer, once assembled, thus form an outer envelope having a regular section, for example circular, oval or square. Even more preferably, the strands of the inner layer, once assembled have a ring-shaped cross section. For example, the strands of the inner layer may have a Z-shaped or trapezoidal cross-section, the Z-shape being preferred. In an alternative embodiment, the strands of the inner layer may have a cross section of round shape. According to one embodiment, at least a portion of the periphery of the strands, and preferably all around the strands of the inner layer is also formed of a layer of alumina or a layer of alumina monohydrate. The thickness of this layer also varies from 5 to 20 μm, preferably from 6 to 15 μm and even more preferably from 8 to 12 μm (inclusive). In particular, the central conductive element, the outer layer and / or the inner layer are made of aluminum or aluminum alloy. Aluminum alloy means the aluminum alloys defined in the Washington DC 2086 Aluminum Association Directive or alloys meeting the European standard EN573. These standards define several classes of aluminum alloy with references ranging from 1000 to 8000. Preferably, the conductor is an overhead power cable.

The present invention also relates to a method of manufacturing an electrical conductor as described above, characterized in that it comprises the following steps: a) perform anodizing on the surface of several aluminum strands or in aluminum alloy, so as to form a layer of alumina on at least a portion of the periphery of said strand, and preferably all around said strands, b) assemble several strands obtained according to step a) to form the outer layer around the central conductive member, each strand having a cross-section of complementary shape to the adjacent strand (s), and being adapted to impart to said outer layer a substantially regular surface. By definition, anodization is a controlled and electrochemical oxidation of the surface of a material, such as an aluminum or aluminum alloy material.

Preferably, the strand obtained in step a) undergoes hot hydration so as to transform the alumina layer into alumina monohydrate. Advantageously, the strand obtained in step a) or the strand obtained after hot hydration is rinsed with osmosis water. The invention will be better understood, and other objects, details, features and advantages thereof will appear more clearly in the following description of a particular embodiment of the invention, given solely for illustrative purposes and not limiting, with reference to the accompanying drawings. In these drawings: - Figure 1 is a schematic sectional view of a conductor according to one embodiment of the present invention; Fig. 2 is an enlarged view of the outer layer of the conductor according to Fig. 1; FIG. 3 is a block diagram of an accelerated corrosion test conducted by the present applicant; Fig. 4 is a photograph showing the surface of a conductor according to the prior art ("standard OHL with internal grease") after it has undergone the corrosion test of Fig. 3; Fig. 5 is a photograph showing the conductor surface according to the invention after said conductor has undergone the corrosion test of Fig. 3; and FIG. 6 is a graph showing the evolution of corrosion (average depth of corrosion-time-based breaches) for three cables: a first conductor according to the prior art comprising an outer layer having section strands Z-shaped cross-section ("standard OHL without inner grease"), a second according to the prior art comprising an outer layer having Z-shaped cross-section strands with an internal grease filler ("standard OHL"), and another conductor according to the invention ("OHL Solution").

The conductor 1, illustrated in FIGS. 1 and 2, corresponds to a high voltage electrical transmission line conductor.

For the sake of clarity, only the essential elements for understanding the invention have been schematically represented in these figures, and this without respect for the scale. This conductor comprises: a central electrically conductive element 4 and, successively and coaxially around this central conductive element 4, an inner layer 3, and an outer layer 2. The inner 3 and outer 2 layers are also electrically conductive. In particular, the central element 4 is in contact with the inner layer 3, which is itself in contact with the outer layer 2. The conductive element 4 is formed of round cylindrical strands 4a of aluminum or aluminum alloy. aluminum, seven in number, each strand 4a being covered with grease 5. This grease 5 thus fills both the interstices present between the cylindrical strands 4a and between the strands 4a and the inner layer 3. The inner layer 3 and the layer external 2 consist of an assembly of strands (3a and 2a) also aluminum or aluminum alloy whose cross section is Z-shaped (or shaped "S" according to the orientation of Z). The geometry of the strands in the shape of "Z" thus makes it possible to obtain a surface almost provided with no gaps that can generate accumulations of moisture and therefore poles of corrosion.

As shown in Figure 1, the inner layer 3 comprises 13 strands 3a and outer layer 18 strands 2a. The inner layer 3 differs from the outer layer 2 in that the outer layer is composed of strands 2a whose periphery (of each strand) is formed of a layer of alumina 9, preferably monohydrate. This layer of alumina 9 is generally formed by anodization. The particular geometry of the strands 2a (Z cross section) and their protection by the alumina layer 9 thus form a barrier against corrosion, even if the electrical conductor 1 is in severe conditions of marine and industrial exposure (presence in the air of elements: sodium, chloride, sulfur ...). This will be demonstrated in Test 1 below. In variants of this particular embodiment, it is possible to modify the number of strands 3a, 2a of the inner and outer layer, the number of internal layers or the number of round wires, and the nature of the aluminum. . A method of manufacturing the conductor according to the invention will now be described. This process comprises several steps: a degreasing step-stripping strands, a first rinsing step, a neutralization step, a second rinsing step, an anodizing step under current in a sulfuric acid-based electrolyte, a third step rinsing, a step of clogging the pores with hot water and a fourth rinsing step.

The starting material is, for example, a ZS aluminum alloy strand or cross-section wire type AGS (aluminum, magnesium, silica, bearing reference 6201 of the European standard EN573), the Z height is 2.9 mm is an equivalent diameter of 3.2 mm. The wire is packaged on a reel. These yarns are marketed with a grease film related to the drawing process. Therefore, for the manufacturing process, it is generally necessary to proceed to a degreasing step. The degreasing and stripping of the yarns are mostly done chemically or electrolytically. The purpose of the degreasing operations is to eliminate the various bodies and particles contained in the greases while the stripping operation serves to remove the oxides present on the metal. There are several methods of stripping: chemical, electrolytic or mechanical. These methods are known to those skilled in the art. Chemical etching consists in eliminating the oxides by dissolution, or even bursting of the layer, without attacking the underlying metal. For degreasing / stripping, it is possible for example to use an industrial solution to 45m1 / L GARDOCLEAN ° (Company CHEMETALL). The solution consists essentially of soda (about 30g / L to 45m1 / L) and surfactants.

The step of neutralizing the wires makes it possible not to pollute the bath allowing the anodization. In addition, this step makes it possible to eliminate certain traces of oxides that may be detrimental to anodization. This step is done in a bath identical to the anodizing bath. A solution of sulfuric acid H2SO4 at 2008 / L at room temperature will eliminate any soda residues related to degreasing. Neutralization makes it possible to put the surface of the aluminum at the same pH as the anode bath. Then, the strands are anodized. Anodizing is based on the principle of electrolysis of water. In a tank filled with process treatment, i.e., in an acid medium such as sulfuric acid, the part is placed at the anode of a DC generator. The cathode of the system is usually lead (inert in the middle). It can also be aluminum or stainless steel, in some installations. During electrolysis, the oxide layer is produced from the surface towards the core of the metal, unlike an electrolytic deposit. For aluminum, a layer of alumina is formed which has an electrical insulating power. Thus the current no longer reaches the substrate, and is then protected. The reactions are as follows: - at the cathode: 2H + + 2e- - + H2 at the anode: Al -> 3e- + AI3 +, then: 2 AI3 + + 3 H2O -> AI2O3 + 6 H + - Balance equation: 2 Al + 3 H2O - * Al2O3 + 3 H2 These reactions therefore cause the formation of an aluminum oxide layer 9, the alumina which is an insulator. The current no longer reaches the layer. For this reason it is necessary to use an electrolyte which dissolves the layer such as sulfuric acid, phosphoric acid, chromic acid or oxalic acid. Equipotential spheres are then obtained which progress by producing porous hexagonal structures. The anodizing process depends on the dissolution rate. Indeed: - If Vdissolution> Voxydation, one has a stripping 30 Si Vdissolution = Voxydadon, one has an electrolytic polishing - Si Vdissolution <Voxydadon, one has anodization.

The alumina layer 9 in sulfuric anodization is formed towards the outside towards the inside. The coloration is carried out by impregnation of the dye by absorption in the pores. The electrolytic parameters are imposed by a current density and a conductivity of the bath. For the desired thickness on the prototype wire is 8-10pm, the current density will be set at 55-65A / dm2 and the voltage will be set at 20-21V and an intensity of 280-350A. This gives the strand or son 2a. Clogging is the technique for closing or closing existing porosities in each cell of the oxide layer. This obturation is obtained by transformation of the alumina constituting the anodic layer, resulting in expansion and thus progressive closure of the pores. This operation is performed by immersing the anodized parts in boiling water (osmosis water having a temperature greater than 80 ° C) to promote the kinetics of reaction. The anhydrous alumina absorbs water molecules and becomes an alumina monohydrate and becomes boehmite. Clogging thus promotes a good resistance to corrosion. The different rinses are defined by 3 steps: rough rinsing, clean rinsing, drying with compressed air. Rinsing is done by reverse osmosis water. Finally, the strands 2a of Z cross section are assembled in a standard manner so as to obtain a conductor with a section of 455 mm 2. The latter consists of a central conductor element formed of 19 round wires AGS 6201, on which is disposed an inner layer composed of 18 strands / cross-section Z son of aluminum alloy AGS 6201 and on which is disposed an outer layer with 24 son also Z section obtained by the method described above. The electrical conductor according to the invention makes it possible to obtain anti-corrosion characteristics superior to the standard conductor as will be demonstrated below.

Examples: Test 1: corrosion test An anti-corrosion test was carried out in order to compare the mechanical strength of the electrical conductor according to the invention with standard cables of the prior art. For this, the conductor according to the invention "OHL solution" tested is the conductor obtained according to the process above and having as a reminder the characteristics below: a central element made of AGS 6201 composed of 19 round wires, on which is disposed an inner layer formed of 18 sections of Z section AGS 6201 and on which is disposed an outer layer comprising 24 ZS section strands of AGS 6201 whose edge is formed of a layer of alumina monohydrate of 8 at 10 μm thick (hereinafter referred to as AEROZ conductor 1). The "standard OHL without inner grease" conductor is a conductor comprising a conductive element formed from 19 round wires of AGS 6201, surrounded by a first layer formed of 18 Z-section strands of AGS 6201 on which is disposed a second layer of 24 ZS section of AGS 6201. The conductor has a section of 455mm2. For this cable, the grease has been removed. The "standard OHL" conductor is the same conductor as previously described except that the inner grease has been left behind. The accelerated corrosion test combines two standard tests: the salt spray test and the kersternich test. The salt spray shows wet corrosion with the presence of sodium chloride (NaCl) allowing an increase in the conductivity of the moisture consequently a larger ion exchange accelerating the corrosion phenomenon. The Kersternich test makes it possible to demonstrate an observable corrosion in an industrial or urban environment by injecting sulfur products into a humid atmosphere. The test set up combines the two tests as shown in FIG. 2. A solution of 50/0 NaCl is placed at the bottom of a closed chamber and is heated to 50-60 ° C. in order to reproduce the salt fog while 11 intake of gaseous products is created from a dissolution of copper in sulfuric acid and is sprayed into the enclosure. The samples 6 are placed in the enclosure in an orderly manner, allowing a homogeneous circulation of the polluted environment.

The follow-up parameters to obtain a reproducible test are: the temperature of the NaCl solution, the concentration of the NaCl solution, the air flow rate injected into the sulfuric acid for a return to the enclosure, the quantity of dissolved copper and the concentration of sulfuric acid allowing the dissolution of copper.

The results shown in FIGS. 4 to 6 are obtained. As is shown in the graph of FIG. 6, the conductor according to the invention has no corrosion mark / breccia, which is not the case with cables according to the art. prior. After 120 days in a saline and sulfurous atmosphere, the driver of the prior art without grease has more than 350 microns of depth of observed corrosion bites, more than 150 for the driver with grease and no or almost none for the driver according to the invention. This graph also shows the important role of the grease which by its drip point will flow to the outside of the driver to protect it from corrosion.

The photographs of FIGS. 4 and 5 show the external surfaces of the conductor according to the invention (FIG. 5) and the conductor according to the prior art with grease (FIG. 4) after these have been exposed for more than 200 days to the hostile atmosphere of the experimental enclosure. It can be seen that the surface of the conductor according to the invention is not damaged, unlike that of the conductor according to the prior art. Numerous erosions and gaps are indeed visible in FIG. 4. Consequently, the conductor according to the invention effectively resists a very corrosive atmosphere.

Test 2: Validity test according to the IEC 61284 standard Tests were carried out by an independent laboratory, DERVAUX company, to measure the conductor temperature according to the invention. For these tests, the driver AEROZ 1 was tested, as well as a conductor AEROZ 2 (conductor identical to AEROZ 1 except that the Z-section strands of the inner layer also has an alumina thickness of 8 to 10 μm) . In order to measure the temperature, the company DERVAUX followed the protocol stated in the standard IEC61284.

This independent company found that for both types of conductor according to the invention, the temperature did not exceed 105 ° C and therefore complied with IEC61284. Although the invention has been described in connection with a particular embodiment, it is obvious that it is not limited thereto and that it comprises all the technical equivalents of the means described and their combinations if they are within the scope of the invention.

Claims (12)

REVENDICATIONS1. An electrical conductor (1) comprising an elongated central electrically conductive element (4) surrounded by a first layer, said outer layer (2), said outer layer (2) comprising an assembly of metal strands (2a) capable of conferring on said layer external (2) a substantially regular surface, each of the strands (2a) constituting the outer layer having a cross-section of complementary shape to (x) strand (s) which is / are adjacent thereto, characterized in that at least a portion of the periphery of the strands (2a), and preferably all around the strands, comprises (s) a layer of alumina (AI203) (9), so that the outer surface of the conductor comprises said layer of alumina (9) along its longitudinal axis. 2. Electrical conductor (1) according to claim 1, wherein the alumina layer (9) is alumina monohydrate. 3. Electrical conductor (1) according to one of claims 1 or 2, wherein the cross section of the strands (2a) has a shape of Z or trapezium. 4. Electrical conductor (1) according to one of the preceding claims, wherein the alumina layer is capable of breaking at a connection area. 5. Electrical conductor (1) according to one of the preceding claims, wherein the alumina layer (9) has a thickness ranging from 5 to 20 pm. 25 6. Electrical conductor (1) according to one of the preceding claims, wherein between the central electrically conductive element (4) and the outer layer (2) is disposed a second layer, said inner layer (3). 7. Electrical conductor (1) according to claim 6, wherein the inner layer (3) comprises an assembly of metal strands (3a), each of the strands (3a) constituting the inner layer having a cross section of shape complementary to ( x) strand (s) which is / are adjacent to it (s). 8. Electrical conductor (1) according to claim 7, wherein at least a portion of the periphery of the strands (3a), and preferably all around the strands of the inner layer (3) is formed of a layer of alumina or a layer of alumina monohydrate. Electric conductor (1) according to one of the preceding claims, in which the central electrically conductive element (4), the outer layer (2) and / or the inner layer (3) are made of aluminum or alloy aluminum. 10. Electrical conductor (1) according to one of the preceding claims, characterized in that it is an electrical conductor of overhead lines. 11. A method of manufacturing an electrical conductor (1) according to one of claims 1 to 10, characterized in that it comprises the following steps: a) perform anodizing on the surface of several strands (2a) d aluminum or aluminum alloy, so as to form a layer of alumina (9) on at least a portion of the periphery of said strand (2a), and preferably all around said strands, b) assemble several strands ( 2a) obtained according to step a) to form the outer layer around the central conductive element (4), each strand having a complementary cross section complementary to the adjacent strand (s), and being able to impart to said outer layer a substantially regular surface. 12. A method of manufacturing a conductor according to claim 11, wherein the strand (2a) obtained in step a) undergoes hot hydration so as to convert the alumina layer to alumina monohydrate.