EP2544190B1 - Electrical cable with reduced corrosion and improved fire resistance - Google Patents

Description

The present invention relates to the field of electric cables. 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).

OHL cables traditionally consist of bare electrically conductive elements, stretched over a suitable set of towers. These lines are conventionally intended for the transport of electrical energy under an alternating high voltage (225 to 800 kV).

The present invention relates to an electrical cable having high corrosion resistance, so as to withstand harsh atmospheric conditions such as the salt atmosphere near the coast or the sulfur atmosphere of industrialized urban areas.

OHL cables 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.

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

The present invention also relates to an electrical cable capable of withstanding heat, generated for example by a fire.

Aluminum or aluminum alloy cables, because of their low heat resistance (aluminum melting point being 658 ° C), are not used in electrical applications where the temperature can be high, for example where fire resistance is required (eg emergency exit lamp).

When such a requirement is required, it is known to use copper-based electrical cables in the prior art. The melting point of copper is indeed higher than that of aluminum and is of the order of 1083 ° C.

The present invention aims to propose a new electrical cable that avoids all or part of the aforementioned drawbacks. In particular, the electric cable according to the invention aims to withstand severe atmospheric conditions and thus avoid corrosion of overhead lines. It also aims to withstand high temperatures, such as fire temperatures that can be of the order of 600 to 1200 ° C, while allowing continuity of the electrical signal.

To this end, the subject of the invention is an electrical cable comprising an elongate element, surrounded by a first layer comprising an assembly of at least two metal strands made of aluminum or aluminum alloy (or aluminum or aluminum alloy metal wires). aluminum), characterized in that at least a portion of the periphery of said at least two metal strands, and preferably all around said at least two metal strands comprises a layer of hydrated alumina. In other words, said at least two metal strands are each surrounded at least in part, or even totally, by a layer of hydrated alumina.

The Applicant has surprisingly discovered that the first layer of the invention, formed with metal strands whose edge or periphery of said metal strands is made of hydrated alumina, presents a extremely high corrosion resistance.

In addition, said first layer of the invention has improved temperature resistance, while allowing continuity of the electrical signal. The electrical cable of the invention is thus capable of withstanding fires, and in particular despite the low melting point of the aluminum or aluminum alloys capable of forming the cable. Indeed, considering that the constituent metal strands of the first layer are aluminum or aluminum alloy, the hydrated alumina layer allows to liner aluminum or aluminum alloy, even when it is in fusion. In addition, the hydrated alumina layer will directly follow the expansion of the aluminum or molten aluminum alloy thus increasing the malleability and deformability of the strands forming the cable during thermal shocks. This is why, due to this expansion, the continuity of the electrical signal always takes place (the metal strands constituting the cable do not break under the effect of heat).

In a particular embodiment, each of the constituent metal strands of the assembly of the first layer comprises a layer of alumina around their entire periphery.

As a result, the entire outer surface of the first layer is covered with a layer of alumina. In other words, the outer surface of the first layer comprises said layer of alumina, this layer extending in particular along the longitudinal axis of the electric cable.

By "outer surface" is meant the surface that is farthest from the elongated element.

Preferably, the constituent metal strands of the first layer are capable of conferring on the said first layer a substantially regular surface, each of the strands constituting the first layer possibly having a cross section of shape complementary to the strand (s) which it is / are adjacent (s).

According to the invention, metal strands capable of conferring on said first layer a substantially regular surface, each of the constituent strands of the first layer possibly having a section cross-section of complementary shape to the (x) strand (s) which is / are adjacent thereto ", it is understood that: the juxtaposition or the nesting of all the constituent strands of the first layer forms a continuous envelope ( without irregularities), for example of circular or oval section or square.

Thus, the Z-shaped or trapezoid-shaped cross-section strands are suitable for the present invention, while strands of circular section (whose assembly does not make it possible to obtain a regular envelope), do not fit into the definition above. In particular, strands of Z-shaped cross-section are preferred.

Even more preferably, the first layer has a ring-shaped cross section.

According to a first variant embodiment, the first layer is an outer layer. According to the invention, the term "outer layer" of the electric cable, the last layer of the electric cable (ie the outermost layer of the electric 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. As a result, the electrical cable of the invention does not include other layers surrounding the first layer. Thus, when all the constituent metal strands of the first layer are surrounded by said layer of alumina, and the first layer is the outer layer, the outer surface of the electric cable of the invention comprises said layer of alumina along its longitudinal axis.

According to a second variant embodiment, the first layer is covered with an electrically insulating layer or an insulating sheath.

In the invention, the hydrated alumina layer is an aluminum oxide hydroxide layer or in other words an alumina hydroxide layer.

According to a first variant, the hydrated alumina layer is a monohydrate layer.

By way of example, mention may be made, as alumina monohydrate, of boehmite, which is the gamma polymorph of AlO (OH) or Al ₂ O ₃ .H ₂ O; or diaspore, which is the alpha polymorph of AIO (OH) or Al ₂ O ₃ .H ₂ O

According to a second variant, the hydrated alumina layer is a polyhydrated layer, and preferably a trihydrate layer.

By way of example, mention may be made, as alumina trihydrate, of gibbsite or hydrargillite, which is the gamma polymorph of Al (OH) ₃ ; bayerite, which is the alpha polymorph of Al (OH) ₃ ; or nordstrandite, which is the beta polymorph of Al (OH) ₃ .

The alumina layer of the invention (i.e. hydrated alumina layer) is a layer whose thickness is controlled. In other words, it is obtained by a manufacturing method making it possible to obtain a substantially constant and homogeneous thickness all around the metal strand (s). By way of example, this layer of hydrated alumina can be obtained by anodization (see controlled oxidation).

In a first variant embodiment, said hydrated alumina layer is not present on one or more portions of the electrical cable intended for the electrical connection and this, to facilitate its installation.

In a second variant embodiment, the hydrated 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 or aluminum. For example, at a junction, the end of two cables (about 80 cm long) is inserted inside the sleeve which is then compressed by a clamping means. In the connection area, the ends of the cable are thus protected from corrosion by the sleeve.

The electrical cables of the prior art do not include a layer of hydrated alumina on their outer surface, the current flowing in the cable is removed from the material of the outer layer to the conductive material of the sleeve.

In the electrical cable according to the invention, the alumina layer hydrated, which preferably covers the outer periphery of the first layer of the electric cable, is an electrical insulator (1 micron of alumina can electrically isolate a voltage of 40V). It could therefore be thought that it causes overheating at the first layer by not allowing the evacuation of the current flowing in the electric cable 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 hydrated alumina layer, especially at the level of said connection zone, was not restrictive and did not cause overheating since it breaks when the installation of the electric cable. Indeed, the compression exerted (according to the standards in force) on the sleeve by means of the clamping means is sufficient to break the alumina layer and thus pass the electric current between the first layer and the sleeve, especially when the first layer is an outer layer.

Preferably, the thickness of this alumina layer (see strands of the first layer) is at most 20 microns, and preferably at least 5 microns. In a particularly preferred manner, the thickness of the alumina layer may range from 6 to 15 μm, and even more preferably from 8 to 12 μm (inclusive).

The elongated element of the electrical cable of the invention may preferably be positioned at the center of the cable (i.e. central position). It can be an electrically conductive element, and / or a mechanical reinforcing element.

According to one characteristic of the invention, between the elongated element and the outer layer is disposed a second layer. We can talk more particularly a second layer called the 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 metal strands, and preferably all around the metal strands of the inner layer is also formed of a layer of alumina, and preferably a layer of alumina monohydrate.

The thickness of this layer of alumina (see strands of the second layer) also varies from 5 to 20 μm, preferably from 6 to 15 μm, and even more preferably from 8 to 12 μm (limits included). .

In particular, the elongated element, the first layer (or more particularly the constituent metal strands of the first layer) and / or the second layer (or more particularly the constituent metal strands of the second layer) are preferably 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 electrical cable of the invention is a high voltage electrical transmission cable (OHL).

Another subject of the invention relates to an electric cable comprising at least one metal strand (or wire), in particular of aluminum or of aluminum alloy, characterized in that said metal strand comprises over its entire periphery a layer of alumina hydrated, said metal strand and the hydrated alumina layer being as defined in the present description. This metal strand surrounded by its hydrated alumina layer may in particular be obtained by step a of the manufacturing method described below, and more particularly by controlled oxidation.

Thus, the metal or strands whose edge or periphery is completely surrounded by hydrated alumina, on the one hand has an extremely high resistance to corrosion, and on the other hand an improved temperature resistance, while allowing continuity electrical signal.

This metal strand may be conventionally surrounded by an electrically insulating layer or an insulating sheath.

In the present invention, whatever the subject of the invention considered, the metal strand (s) preferably do not comprise a layer of ceramic alumina, and more generally do not comprise a ceramic layer, surrounding the layer of hydrated alumina. Thus, the fire resistance can be optimized by the non-presence of a layer of ceramic alumina, or the non-presence of a ceramic layer, around the hydrated alumina layer.

Indeed, during a fire, a layer of ceramic alumina surrounding the hydrated alumina layer could significantly damage the metal strand. The ceramic alumina layer thus limit, during a fire, the continuity of the electrical signal of the electric cable in question, that is to say when the metal strands or are melt.

The electric cable thus defined in this other object of the invention can be used in particular in the field of aeronautics, in the railway field or in buildings, for example to supply a lamp of an emergency exit panel.

1. a) réaliser une oxydation contrôlée sur la surface d'au moins un brin métallique en aluminium ou en alliage d'aluminium, de manière à former une couche d'alumine hydratée sur au moins une partie du pourtour dudit brin métallique, et de préférence sur tout le pourtour dudit brin métallique, et
2. b) assembler plusieurs brins obtenus selon l'étape a) afin de former la première couche, et optionnellement la deuxième couche, autour de l'élément allongé.

The present invention also relates to a method of manufacturing an electric cable as described above, characterized in that includes the following steps:

1. a) performing a controlled oxidation on the surface of at least one metal strand of aluminum or aluminum alloy, so as to form a layer of hydrated alumina on at least a portion of the periphery of said metal strand, and preferably on all around said metal strand, and
2. b) assembling several strands obtained according to step a) to form the first layer, and optionally the second layer, around the elongate member.

Controlled oxidation makes it possible to obtain a layer of hydrated alumina whose thickness is substantially constant and homogeneous around the periphery of the metal strand, contrary to what could be obtained with an oxidation called "in the open air" .

By way of example, the controlled oxidation can be carried out by anodization. Anodizing is more particularly a controlled and electrochemical oxidation of the surface of a material, such as an aluminum or aluminum alloy material.

Preferably, the metal strand obtained in step a) can undergo clogging of the hydrated alumina layer, in order to improve its compactness.

This clogging may for example be carried out by performing a hot hydration of the metal strand obtained in step a), by dipping said strand in boiling water. This clogging step is performed prior to step b).

Advantageously, the strand obtained in step a) or the strand obtained after clogging, is rinsed with osmosis water.

In a preferred embodiment, in the first layer, and optionally in the second layer, each strand has a cross-section of complementary shape to the strand (s) adjacent thereto, and being capable of conferring on the layer question a substantially regular surface.

The invention will be better understood, and other purposes, details, Features and advantages thereof will become more apparent in the following description of particular embodiments of the invention, given solely by way of illustration and not limitation, with reference to the accompanying drawings.

• La figure 1 est une vue schématique de section d'un câble électrique selon un mode de réalisation de la présente invention ;
• La figure 2 est une vue agrandie de la couche externe du câble électrique selon la figue 1 ;
• La figure 3 est une vue schématique de section d'un câble électrique selon un autre mode de réalisation de la présente invention ;
• La figure 4 est une vue agrandie de la couche externe du câble électrique selon la figue 3 ;
• La figure 5 est une photographie montrant la couche d'alumine hydratée, formée selon le procédé de linvention ;
• La figure 6 est un schéma de principe d'un test de corrosion accélérée mené par le présent demandeur ;
• La figure 7 est une photographie montrant la surface d'un câble électrique selon l'art antérieur (« OHL standard avec graisse intérieur ») après que celui-ci ait subi le test de corrosion de la figure 6 ;
• La figure 8 est une photographie montrant la surface d'un câble électrique selon l'invention après que ledit câble électrique ait subi le test de corrosion de la figure 6 ;
• La figure 9 est un graphique montrant l'évolution de la corrosion (profondeur moyenne de brèches formées par la corrosion en fonction du temps) pour trois câbles électriques : un premier câble électrique selon l'art antérieur comprenant une couche externe comportant des brins de section transversale en Z (« OHL standard sans graisse intérieure »), un second câble électrique selon l'art antérieur comprenant une couche externe comportant des brins de section transversale en Z avec un bourrage intérieur en graisse (« OHL standard»), et un autre câble électrique selon l'invention (« OHL Solution ») ;
• La figure 10 est une photo macroscopique d'un fil d'alliage d'aluminium brut ayant subit un test thermique (puissance thermique de 440 watt); et
• La figure 11 est une photo macroscopique d'un fil d'alliage d'aluminium anodisé selon l'invention ayant subit le même test thermique que le fil de la figure 10 (puissance thermique de 440 watt).

On these drawings:

• The figure 1 is a schematic sectional view of an electric cable according to an embodiment of the present invention;
• The figure 2 is an enlarged view of the outer layer of the electric cable according to Fig 1;
• The figure 3 is a schematic sectional view of an electric cable according to another embodiment of the present invention;
• The figure 4 is an enlarged view of the outer layer of the electric cable according to Fig. 3;
• The figure 5 is a photograph showing the hydrated alumina layer, formed according to the method of the invention;
• The figure 6 is a schematic diagram of an accelerated corrosion test conducted by the present applicant;
• The figure 7 is a photograph showing the surface of an electrical cable according to the prior art ("standard OHL with internal grease") after it has undergone the corrosion test of the figure 6 ;
• The figure 8 is a photograph showing the surface of an electric cable according to the invention after said electric cable has undergone the corrosion test of the figure 6 ;
• The figure 9 is a graph showing the evolution of corrosion (mean depth of corrosion-formed gaps as a function of time) for three electrical cables: a first electrical cable according to the prior art comprising an outer layer having Z-shaped cross-section strands ("Standard OHL without inner grease"), a second prior art electrical cable comprising an outer layer having Z-shaped cross-section strands with an internal grease filler ("Standard OHL"), and another electrical cable according to the invention ("OHL Solution");
• The figure 10 is a macroscopic photograph of a raw aluminum alloy wire that has undergone a thermal test (thermal power of 440 watt); and
• The figure 11 is a macroscopic photo of an anodized aluminum alloy wire according to the invention having undergone the same thermal test as the wire of the figure 10 (thermal power of 440 watt).

For the sake of clarity, only the essential elements for understanding the invention have been shown schematically in these figures, and this without respect of the scale.

The electric cable 1, illustrated on Figures 1 and 2 , corresponds to a high voltage electrical transmission cable of the OHL type.

This electric cable 1 comprises: a central electrically conductive element 4 elongated 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 seven in number, each strand 4a being covered with greases 5. This grease 5 thus fills both the interstices present between the strands. cylindrical 4a and between the strands 4a and the inner layer 3.

The inner layer 3 and the outer layer 2 consist of an assembly of strands (3a and 2a) also made of aluminum or aluminum alloy, the cross section of which is Z-shaped (or of "S" shape according to FIG. 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 represented on the figure 1 , the inner layer 3 comprises 13 strands 3a and the 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.

The electric cable 1, illustrated on figures 3 and 4 , corresponds to a high-voltage electrical transmission cable of the OHL type, but with a structure slightly different from that of the electric cable described in Figures 1 and 2 .

This electric cable 1 comprises: a central electrically conductive element 4 elongated 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 elongate element 4 is formed of round cylindrical strands 4a of aluminum or aluminum alloy 19 in number, each strand 4a being covered with grease.

The inner layer 3 and the outer layer 2 consist of an assembly of strands (3a and 2a) also made of aluminum or aluminum alloy whose cross section is trapezoidal. The geometry of the trapezium-shaped strands has the advantage of obtaining a surface virtually provided with no interstices that can generate moisture accumulations and therefore corrosion poles. As represented on the figure 3 the inner layer 3 comprises 18 strands 3a and the outer layer 24 strands 2a. The inner layer 3 is composed of strands 2a whose periphery (of each strand) is formed of a layer of hydrated alumina 9, preferably of one bohemite (see Figures 4 or 5 ). This layer of alumina 9 is generally formed by anodization. The alumina layer 9 thus forms an envelope capable of containing the aluminum or the aluminum alloy when the latter is melted because of high temperature. This effect will be demonstrated in test 3 below.

In alternative embodiments shown on the Figures 1 to 4 it is possible to modify the number of strands 3a, 2a of the inner and outer layer, their shape, the number of internal layers or the number of round wires, as well as the nature of the aluminum.

A method of manufacturing the electric cable 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 of removing the oxides by dissolution, see bursting layer, without attacking the underlying metal. For degreasing / pickling, it is possible for example to use a 45ml / L industrial solution of GARDOCLEAN ^® (Company CHEMETALL). The solution consists essentially of soda (about 30g / L to 45ml / 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 H ₂ SO ₄ at 200 g / 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.

• à la cathode : 2H⁺ + 2e^- → H₂
• à l'anode : Al→ 3e^- + Al³⁺, puis : 2 Al-³⁺ + 3 H₂O → Al₂O₃ + 6 H⁺
• Équation bilan : 2 Al + 3 H₂O → Al₂O₃ + 3 H₂

The reactions are as follows:

• at the cathode: 2H ⁺ + 2e ^- → H ₂
• at the anode: Al → 3e ^- + Al ³⁺ , then: 2 Al- ³⁺ + 3 H ₂ O → Al ₂ O ₃ + 6 H ⁺
• Balance equation: 2 Al + 3 H ₂ O → Al ₂ O ₃ + 3 H ₂

• Si V_dissolution > V_oxydation, on a un décapage
• si V_dissolution = V_oxydation, on a un polissage électrolytique
• si V_dissolution < V_oxydation, on a une anodisation.

These reactions thus 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 V _dissolution > V _oxidation , we have a stripping
• if V _dissolution = V _oxidation , we have an electrolytic polishing
• if the _{solution is oxidized} , it is anodized.

The hydrated alumina layer 9 in sulfuric anodization is formed from 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 8-10μm, the current density will be set at 55-65A / dm ² 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 hydrated 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. 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 an electric cable with a section of 455 mm ² . The latter consists of a central conductor element made up of 19 AGS 6201 round wires, on which is disposed an inner layer consisting of 18 strands / Z-shaped cross-section wires of aluminum alloy AGS 6201 and on which is arranged an outer layer comprising 24 son also of Z section obtained according to the method described above.

The electric cable according to the invention makes it possible to obtain anti-corrosion characteristics superior to the standard conductor as will be demonstrated below.

Test 1: anticorrosion test

An anti-corrosion test was conducted to compare the mechanical strength of the electric cable according to the invention with standard cables of the prior art.

For this, the electrical cable according to the invention "OHL solution" tested is the electrical cable obtained according to the above method and having as a reminder the characteristics below: a central electrically conductive element made of AGS 6201 composed of 19 round wires, on which is disposed an inner layer formed by 18 ZS section strands of 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 8 to 10 microns thick (hereinafter referred to as AEROZ conductor 1).

The "OHL standard without internal grease" electrical cable is an electrical cable consisting of a central electrically conductive element made up of 19 round wires of AGS 6201, surrounded by a first layer consisting of 18 ZS section strands of AGS 6201 on which a second layer of 24 ZS section strands of AGS 6201 is arranged. The conductor has a section of 455mm ² . For this cable, the grease has been removed.

The "OHL standard" electrical cable is the same electrical cable as previously described except that the inner grease has been left.

The accelerated corrosion test combines two standard tests: the salt spray test and the kersternich test. The salt spray shows a 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 brings together the two tests as shown on the figure 6 . A solution of 5% NaCl is placed at the bottom of a closed chamber and is heated to 50-60 ° C to reproduce the salt spray while the supply of sulfurous products in gaseous form 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.

We obtain the results appearing on the Figures 7 to 9 .

As shown in the graph of the figure 9 , the electric cable according to the invention has no mark / corrosion breach which is not the case of electric cables according to the prior art. At the end of 120 days in a saline and sulfurous atmosphere, the prior art electric cable without grease has more than 350 microns of depth of observed corrosion bites, more than 150 for the electric cable with grease and no or almost none for the electric cable according to the invention. This graph also shows the important role of the grease which by its point of drop will flow outward of the electric cable to protect it from corrosion.

The photographs of Figures 7 and 8 show the outer surfaces of the electric cable according to the invention ( figure 8 ) and the electric cable according to the prior art with grease ( figure 7 ) after these have been exposed for more than 200 days to the hostile atmosphere of the experimental chamber. It can be seen that the surface of the electric cable according to the invention is not damaged, unlike that of the electric cable according to the prior art. Many erosions, gaps are indeed visible on the figure 7 . Therefore, the electric cable according to the invention effectively resists a very corrosive atmosphere.

Test 2: Validity test according to IEC61284

Tests were conducted by an independent laboratory, DERVAUX company, to measure the temperature of the electrical cable 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.

The electric cable according to the invention also provides fire characteristics superior to the standard conductor.

Test 3: Heat resistance test

To carry out the heat resistance test, raw aluminum wires were compared to wires according to the invention, in particular to AGS 6201 aluminum alloy wires covered with a boehmite layer. The thickness of the hydrated alumina layer varied from 7 to 10 μm along the wire. The son tested all have a diameter of 8 mm.

The principle of the test that has been performed on the son (samples) is based on induction. Via a coil, a magnetic field is created around the samples. By a physical principle, the electrons of the material (aluminum) will be excited. This excitation will generate heat until at a given point (in the middle of the coil), the fusion of the substrate. The melting temperature of the aluminum (658 ° C.) is then reached.

The heating parameters of the samples will depend on the power emitted by the inductor.

For this test, this power is varied and the time taken by the sample to reach melting and eventually to break.

During the test, a camera measures the exact time the samples will eventually break.

The different results obtained are emitted in Table I below. Aluminum wire Wire according to the invention Thermal power (watt) Time before wire break (min) Thermal power Time before wire break 467 1 min 26 sec 440 8 min 10 sec 436 1 min 55 sec 436 > 10 min 440 1 min 23sec 440 > 15 min 440 1 min 32sec 440 > 15 min 720 30 sec 720 > 2 min 720 3 5sec 720 1 min 34 sec 720 36 sec 720 1m 45 sec Table I: Comparative results with the same power of the resistance to fusion between an aluminum alloy wire and anodized aluminum alloy wire.

As shown by the test above, the strands according to the invention withstand high temperatures ( figure 11 ) and do not intersect, unlike pure aluminum strands ( figure 10 ).

Although the invention has been described in connection with particular embodiments, it is obvious that it is not limited thereto and that it includes all the technical equivalents of the means described and their combinations if they are within the scope of the invention.

Claims (19)

1. An electric cable (1) comprising an elongated element (4) surrounded by a first layer (2) comprising an assembly of at least two aluminium or aluminium alloy metal strands (2a), characterized in that the whole perimeter of said at least two metal strands includes a hydrated alumina layer (9).
2. The electric cable (1) according to claim 1, characterized in that the outer surface of the first layer comprises said alumina layer (9).
3. The electric cable (1) according to claim 1 or 2, characterized in that the assembly of the metal strands is able to give said first layer a substantially regular surface.
4. The electric cable (1) according to one of the preceding claims, characterized in that each of the constitutive metal strands (2a) of the first layer has a cross-section with a shape mating the strand(s) which is(are) adjacent to it.
5. The electric cable (1) according to one of the preceding claims, characterized in that the first layer is an external layer.
6. The electric cable (1) according to one of the preceding claims, wherein the alumina layer (9) is an alumina monohydrate layer.
7. The electric cable (1) according to one of the preceding claims, wherein the alumina layer (9) is a boehmite layer.
8. The electric cable (1) according to one of claims 1 to 5, wherein the alumina layer (9) is an alumina polyhydrate layer.
9. The electric cable (1) according to one of the preceding claims, wherein the cross-section of the metal strands (2a) has the shape of a Z or a trapezium.
10. The electric cable (1) according to one of the preceding claims, wherein the alumina layer is capable of breaking at a connecting area.
11. The electric cable (1) according to one of the preceding claims, wherein the alumina layer (9) has a thickness of at most 20 µm.
12. The electric cable (1) according to one of the preceding claims, wherein the alumina layer (9) has a thickness of at least 5 µm.
13. The electric cable (1) according to one of the preceding claims, wherein a second layer, a so-called internal layer (3), is positioned between the elongated element (4) and the first layer (2).
14. The electric cable (1) according to claim 13, wherein the internal layer (3) comprises an assembly of metal strands (3a), each of the constitutive strands (3a) of the internal layer having a cross-section with a shape mating the strand(s) which is(are) adjacent to it.
15. The electric cable (1) according to claim 14, wherein at least one portion of the perimeter of the metal strands (3a), and preferably the whole perimeter of the metal strands (3a), of the internal layer (3) is formed with a hydrated alumina layer.
16. The electric cable (1) according to any of the preceding claims, wherein the elongated element (4), the first layer (2) and/or the second layer (3) are in aluminium or in aluminium alloy.
17. The electric cable (1) according to any of the preceding claims, characterized in that it is a high voltage electric transmission cable (OHL).
18. A method for manufacturing an electric cable (1) according to one of claims 1 to 17, characterized in that it includes the following steps:

    a) achieving controlled oxidation over the surface of at least one metal strand (2a) in aluminium or in aluminium alloy, so as to form a hydrated alumina layer (9) over the whole perimeter of said metal strand; and

    b) assembling several strands (2a) obtained according to step a) in order to form the first layer, and optionally the second layer, around the elongated element (4).
19. The method for manufacturing an electric cable (1) according to claim 18, characterized in that the controlled oxidation step is an anodization step.

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