Electrical cable for aeronautics with limited shrinkage of the insulating layer

Abstract

The present invention relates to an electrical cable suitable in particular for aeronautical applications, in which insulating layer shrinkage is limited. This cable comprises an insulation system including at least one electrically insulating layer (typically based on an extruded fluoropolymer) surrounding an elongated electrically conductive element consisting of a plurality of wires forming a cylindrical strand, with a stranding pitch of less than 8 times the diameter of the strand.

Description

Description Title of the invention: Electrical cable for aeronautics with limited shrinkage of the insulation layer

[0001] The present invention relates to the field of electrical cables comprising an elongated electrically conductive element and at least one layer of an insulating material surrounding this elongated electrically conductive element.

[0002] The invention applies particularly, but not exclusively, to electrical cables intended for use in the aeronautical field, typically on board aircraft. The invention is especially suited to the conditions imposed in this type of application, involving the transmission of high-voltage electrical currents, combined with demanding conditions of humidity, temperature, and low pressure. In the aeronautical or aerospace sector, electrical cables must withstand extreme temperatures ranging from -65°C to +260°C (and more typically between -55°C and 200°C) and low pressures on the order of tens of hPa, particularly around 90 hPa, while remaining compact and / or lightweight.

[0003] The problem addressed by the present invention is that of the shrinkage of the insulating layer which is observed in cables subjected to strong temperature variations and which is particularly sensitive in the case of cables used in aeronautics or aerospace, where variations in the aforementioned ranges are classically observed.

[0004] The phenomenon known as "insulation shrinkage" (also referred to simply as "insulation shrinkage") manifests as a reduction in the length of the insulating layer deposited around an elongated electrically conductive element, along the longitudinal axis of the conductive element. In other words, particularly when subjected to temperature variations, the insulating layer can shrink longitudinally, hence the use of the term "shrinkage" to describe this effect. Extruded insulating layers are particularly affected by this phenomenon, to a greater or lesser degree depending on their exact composition and the conditions of their application and use. Extruded insulating layers used in the aeronautics and aerospace industries are especially susceptible to this phenomenon, particularly those based on fluoropolymers such as perfluoroalkoxy (PFA) copolymers.

[0005] The overall shortening of the insulating layer along its length results in partial exposure of the conductor at its ends. This leaves the conductor exposed to the external environment, which can lead to partial discharges. In particular, when the current carried by the cable has a voltage exceeding the minimum of the Paschen curve in the environment where the cable is used, electric arcs can occur, potentially compromising the safety and / or integrity of the electrical circuit where the cable is located.

[0006] Shrinkage can be quantified by the length of insulation exposed at each end of the cable. In the case of a cable connected to connectors at both ends, the connectors can compensate for shrinkage, but only to a very small extent, namely for shrinkage of 1 to 2 mm at most. However, shrinkage becomes problematic when the length of exposed cable is greater, which is the case for cables commonly used in the aerospace industry.

[0007] In particular, the work carried out by the inventors in the context of the present invention has now made it possible to establish that cables intended for use in aeronautics or aerospace for which the conductors are made according to the standard of the aerospace series, EN2083 or EN4434, lead, on each side of the cable, to a bare length of the order of 10mm with an insulator based on a PFA type polymer.

[0008] The aforementioned shrinkage effect and partial exposure of the conductor is a well-known problem, for which few solutions have yet been found. In this regard, it has been suggested that the extrusion temperature profile or the cooling of the insulation could have an impact, but the work carried out by the inventors within the framework of the present invention has now established that these parameters are insufficient to significantly reduce the shrinkage phenomenon.

[0009] There is therefore currently a need to reduce the phenomenon of insulation shrinkage, particularly for cables subjected to large temperature variations, especially between -65°C and +260°C, such as cables used in the aeronautical and aerospace fields.

[0010] One aim of the present invention is to provide a cable suitable in particular for applications in the fields of aeronautics and aerospace, and which exhibits the lowest possible insulation shrinkage.

[0011] To this end, the present invention proposes to use a particular conductive element, namely consisting of several elongated conductors (of the metallic wire type) stranded together to form a strand with a shorter strand lay than those imposed by current standards (in particular, a shorter lay than the strand lays recommended by standards EN2083 or EN4434, which specifies a strand lay between 8 and 16 times the diameter of a conductor for a conductor with a cross-section ranging from 14 to 107 mm²). 2 ). The strand pitch referred to in this description is that measured on the outermost layer of the conductor strand.

[0012] The inventors have now demonstrated that the strand pitch is a key parameter for controlling insulation shrinkage phenomena and that a sufficiently short strand pitch drastically reduces the shrinkage phenomenon, with a length of exposed conductor on each side of the conductor that is small enough to reduce, or even inhibit, the risk of partial discharges at the ends of the cable.

[0013] More specifically, the present invention relates to an electrical cable comprising at least: - an elongated electrically conductive element; and - an insulation system including at least one electrically insulating layer, completely surrounding said elongated electrically conductive element, wherein said electrically insulating layer preferably comprises at least one fluorinated polymer and in which: The elongated electrically conductive element consists of several twisted metal wires forming a cylindrical strand, the twist pitch of which is less than 8 times the diameter of the strand.

[0014] In an electrical cable according to the invention, the strand pitch can, for example, be less than or equal to 7 times the strand diameter, preferably less than or equal to 6 times the strand diameter. In particular, to limit the cable diameter and its linear mass, the strand pitch can be greater than or equal to 3 times the strand diameter, preferably greater than or equal to 4 times the strand diameter, and more preferably greater than or equal to 5 times the strand diameter. For example, the strand pitch can be between 3 and 8 times, typically between 4 and 8 times the strand diameter, and especially between 5 and 8 times the strand diameter.

[0015] It is particularly advantageous to use a strand pitch less than 8 times the strand diameter insofar as the phenomenon of shrinkage of the insulating layer (and more generally of the insulating system when the cable includes other layers than the insulating layer, for example semiconducting layers of the type described below) can be substantially reduced and thus lead to very small portions of exposed conductor at the end of the cable, which generally remain less than or equal to 5 mm, typically less than or equal to 4 mm, for example on the order of 3 mm or even 2 mm or 1 mm.

[0016] It is also particularly advantageous to use a strand pitch less than 8 times the strand diameter as this can allow a smaller bending radius of the cable, avoiding ovalization of the conductor and therefore of the cable when the cable is bent, which is in high demand due to installation constraints.

[0017] The strand pitch of a strand is a well-known characteristic that corresponds to the spatial repetition period of the outermost pattern of the strand. Typically, the twisted metal wires of the strand each form a helix, and the strand pitch is equal to the pitch of each helix, namely the distance between the turns of one of the helices. The strand pitch can thus be easily measured in practice by drawing a line parallel to the longitudinal axis of the strand. Each of the helical wires of the strand crosses this line at regular intervals: the strand pitch is the distance between two consecutive points of intersection between the wire and the drawn line on a given wire.

[0018] The total thickness of the insulation system may advantageously be less than or equal to 5 mm, preferably less than or equal to 4 mm, more preferably less than or equal to 3 mm, and even more preferably less than or equal to 2 mm.

[0019] The total thickness of the insulation system can also be greater than or equal to 0.5 mm, for example between 0.5 mm and 5 mm, or even between 0.5 mm and 4 mm, between 0.5 mm and 3 mm or between 0.5 mm and 2 mm.

[0020] Typically, the cable insulation system according to the invention can be applied by extrusion around the strand of several twisted metallic wires constituting the elongated electrically conductive element, in particular by coextrusion, or by successive extrusions when the insulation system comprises layers other than the insulating layer.

[0021] Various features and variants of the invention are described in more detail below.

[0022] THE ELECTRICALLY CONDUCTIVE EXTENDED ELEMENT

[0023] The elongated electrically conductive element of the cable of the invention is a multi-core conductor comprising a plurality of twisted metallic wires forming a strand. This strand may optionally include some non-metallic wires in addition to the metallic wires.

[0024] The constituent wires of the strand may be made of aluminium, aluminium alloy, copper, copper alloy, or a mixture or combination thereof (for example, a combination of several wires of distinct composition or wires comprising aluminium and copper-based composites).

[0025] In one possible embodiment, the constituent wires of the strand may be, in whole or in part, coated with a metal or alloy different from the core metal or alloy, such as nickel, a nickel alloy, tin, a tin alloy, silver, a silver alloy, or mixtures thereof. Such a coating (plating) may, for example, protect the conductor from corrosion and / or improve its contact resistance.

[0026] The strand constituting the electrically conductive element can typically have a cross-section ranging from 3 mm 2 (A WG 12) at 107 mm 2 (AWG 0000), preferably ranging from 14 mm 2 (AWG 6) at 107 mm 2 (AWG 0000), preferably ranging from 34 mm 2 (AWG 2) at 107 mm 2 (AWG 0000), and even more preferably ranging from 68 mm 2 (AWG00) at 107 mm 2 (AWG0000).

[0027] The strand constituting the electrically conductive element can typically have an outside diameter ranging from 2.0 mm to 20 mm, preferably ranging from 4.5 mm to 18 mm, preferably ranging from 7.0 mm to 16 mm, and even more preferably ranging from 10 mm to 15.2 mm.

[0028] THE ELECTRICALLY INSULATING LAYER

[0029] In a cable according to the invention, the thickness of the electrically insulating layer, referred to as the "insulating layer" for brevity, present in the insulation system (as the sole constituent layer of the insulation system; or in association with other layers, in particular semiconducting layers described below) is preferably less than or equal to 3 mm, in particular less than or equal to 2 mm.

[0030] The thickness of the insulating layer is preferably greater than or equal to 0.5 mm, for example between 0.5 mm and 3 mm, or even between 0.5 mm and 2 mm.

[0031] This layer is typically deposited around the strand constituting the conductive element by extrusion.

[0032] Preferably, the electrically insulating layer comprises at least one fluorinated polymer. Advantageously, the electrically insulating layer comprises no polymers other than fluorinated polymers.

[0033] The fluorinated polymer(s) present in the electrically insulating layer may, for example, be chosen from polytetrafluoroethylene (PTFE); fluorinated ethylene and propylene copolymers (FEP) such as poly(tetrafluoroethylene-co-hexafluoropropylene); perfluoroalkoxy (PFA) copolymers such as perfluoro(alkylvinyl ether) / tetrafluoroethylene copolymers; perfluoro methoxy (MFA) copolymers; and poly(ethylene-co-tetrafluoroethylene) (ETFE); or a mixture thereof.

[0034] Preferably, the fluorinated polymer(s) in the electrically insulating layer are chosen from perfluoroalkoxy (PFA) copolymers, such as perfluoro(alkylvinyl ether) / tetrafluoroethylene copolymers. Perfluoroalkoxy (PFA) copolymers are particularly suitable for their electrical properties and high temperature resistance. Advantageously, the insulating layer may consist solely of perfluoroalkoxy (PFA) copolymers, excluding any other polymer. In this case, the PFA-based insulating layer is preferably applied by extrusion.

[0035] INTERNAL SEMICONDUCTIVE LAYER (OPTIONAL)

[0036] According to an advantageous embodiment, in the electrical cable of the invention, the insulation system surrounding the elongated electrically conductive element comprises, in addition to the insulating layer, at least one internal semiconducting layer surrounding the elongated electrically conductive element, said internal semiconducting layer being itself surrounded by the insulating layer. The internal semiconducting layer is typically in direct contact with the insulating layer without an intermediate layer between the two.

[0037] In this description, "semiconductor layer" means a layer whose volume resistivity is less than 10000 Qxm (Ohm meter) (at room temperature), preferably less than 1000 Qxm, and particularly preferably less than 500 Qxm.

[0038] The internal semiconductor layer, when present, advantageously comprises a fluorinated polymer, in particular selected from polytetrafluoroethylene (PTFE); fluorinated ethylene and propylene copolymers (FEP) such as poly(tetrafluoroethylene-co-hexafluoropropylene); perfluoroalkoxy (PFA) copolymers such as perfluoro(alkylvinyl ether) / tetrafluoroethylene copolymers; perfluoro methoxy (MFA) copolymers; and poly(ethylene-co-tetrafluoroethylene) (ETFE); or a mixture thereof.

[0039] In a particularly preferred manner, the internal semiconductor layer, when present, may comprise one or more perfluoroalkoxy (PFA) copolymers.

[0040] The internal semiconductor layer, when present, advantageously comprises at least 70% by weight of polymer(s), even more preferably at least 80% by weight of polymer(s), and even more preferably at least 90% by weight of polymer(s).

[0041] The inner semiconductor layer also contains electrically conductive materials in sufficient quantity to make it semiconductive. For example, it may contain from 0.1% to 40% by weight of electrically conductive materials, such as carbon black or carbon nanotubes.

[0042] The internal semiconductor layer, when present, can typically be a layer extruded around the elongated electrically conductive element.

[0043] Preferably, the inner semiconductor layer, when present, and the insulating layer are extruded simultaneously (co-extrusion) to allow good cohesion between the layers and to avoid the formation of air pockets between the layers in which partial discharges could appear.

[0044] The semiconductor layer can typically have a thickness ranging from 0.05 mm (millimeter) to 1.0 mm, preferably ranging from 0.07 mm to 0.8 mm, and particularly preferably a thickness ranging from 0.09 mm to 0.5 mm.

[0045] EXTERNAL SEMICONDUCTIVE LAYER (OPTIONAL)

[0046] According to another possible embodiment (compatible if necessary with the internal semiconducting layer described above, but also conceivable in the absence of it) the insulation system surrounding the elongated electrically conductive element of the electrical cable of the invention may include, in addition to the insulating layer, at least one external semiconducting layer surrounding the insulating layer (and typically in direct contact with the insulating layer without an intermediate layer between the two layers).

[0047] The external semiconductor layer, when present, advantageously comprises a fluorinated polymer, which may in particular be selected from polytetrafluoroethylene (PTFE); fluorinated ethylene and propylene (FEP) copolymers such as poly(tetrafluoroethylene-co-hexafluoropropylene); perfluoroalkoxy (PFA) copolymers such as perfluoro(alkylvinyl ether) / tetrafluoroethylene copolymers; perfluoro methoxy (MFA) copolymers; and poly(ethylene-co-tetrafluoroethylene) (ETFE); or a mixture thereof.

[0048] In a particularly preferred manner, the outer semiconductor layer, when present, may comprise one or more perfluoroalkoxy (PFA) copolymers.

[0049] The external semiconductor layer, when present, advantageously comprises at least 50% by weight of polymer(s), preferably at least 70% by weight of polymer(s), even more preferably at least 80% by weight of polymer(s), and even more preferably at least 90% by weight of polymer(s).

[0050] The outer semiconductor layer also contains electrically conductive materials in sufficient quantity to make it semiconductive. For example, it may contain from 0.1% to 40% by weight of electrically conductive materials, such as carbon black or carbon nanotubes.

[0051] The outer semiconductor layer, when present, can typically be a layer extruded around the elongated electrically conductive element.

[0052] Preferably, the inner semiconductor layer, when present, the insulating layer, and the outer semiconductor layer, when present, are extruded simultaneously (co-extrusion) to allow good cohesion between the layers and to avoid the formation of air pockets between the layers in which partial discharges could occur.

[0053] The outer semiconductor layer can typically have a thickness ranging from 0.05 mm (millimeter) to 1.0 mm, preferably ranging from 0.07 mm to 0.8 mm, and particularly preferably a thickness ranging from 0.09 mm to 0.5 mm.

[0054] THE CABLE ACCORDING TO THE INVENTION

[0055] The electrical cable according to the invention may further comprise a metallic screen forming an electromagnetic shield and placed around the insulating system.

[0056] The metallic screen can be a "wire" screen, composed of a set of copper or aluminum conductors arranged around the insulating system; a "tape" screen, composed of one or more conductive metallic tapes laid in a helix around the insulating system; a "waterproof" screen, a type of metallic tube around the insulating system; or a "braided" screen, forming a braid around the insulating system. The metallic screen is preferably braided, particularly to give the electrically conductive cable flexibility.

[0057] All types of metallic screens can act as grounding for the electrical cable and can thus carry fault currents, for example in the event of a short circuit in the network concerned.

[0058] In addition, the cable of the invention may include a protective sheath. When the cable includes a metallic shield, the protective sheath may surround the metallic shield. If the cable does not include a metallic shield, the protective sheath surrounds the insulation.

[0059] The protective sheath can be a polymer-based layer such as those described for the electrically insulating layer. For aeronautical applications, the protective sheath is preferably based on one or more fluoropolymers (such as PTFE, FEP, PFA and / or ETFE) and / or polyimide.

[0060] Preferably, the protective sheath is the outermost layer of the cable. The protective sheath can be in the form of a tape, an extrudate, or a varnish.

[0061] BRIEF DESCRIPTION OF THE DRAWINGS

[0062] The attached drawings are provided for schematic and illustrative purposes only, without regard to scale.

[0063] [Fig. 1] Figure 1 is intended to illustrate the concept of "no stranding" referred to in the description. The figure schematically represents a strand 1 made up, for example, of two conductor wires 2 and 3, stranded together.

[0064] To distinguish them, wire 2 is shown without hatching and wire 3 is hatched. However, in practice, they are most often two metallic wires of the same type.

[0065] Each of the wires 2 and 3 takes the form of a helix, which has the same repetition period L shown in the diagram, which corresponds to what is commonly called the helix pitch. This repetition distance L corresponds to the "strand pitch" of strand 1.

[0066] [Fig. 2] Figure 2 schematically represents a possible structure for a cable according to the invention.

[0067] This structure includes at its center a metallic conductor 1 consisting of a strand of metallic wires (of the type shown in Figure 1). The strand pitch of the strand constituting this conductor is a short pitch in the sense of the invention, namely less than 8 times the diameter of the strand.

[0068] Around this conductor 1, the structure includes an insulating system 10, which can be single-layer or multi-layer, and which comprises: - an optional internal semiconductor layer 12; - an insulating layer 14, systematically present, which is typically based on a fluorinated polymer such as a PFA; - an optional external semiconductor layer 16.

[0069] Around the insulating system 10, the cable may contain an optional metallic armor 20.

[0070] Around the insulating system 10 and the possible armor 20, the cable includes, according to a preferred method, an outer protective sheath 30.

[0071] Each of the layers 12, 16, 20 and 30 is optional: it may or may not be present, independently of the other layers.

Claims

Demands

1. Electrical cable comprising at least: - an elongated electrically conductive element (1); and - an insulation system (10) including at least one electrically insulating layer (14) completely surrounding said elongated electrically conductive element (1), where: - said electrically insulating layer (14) comprises at least one fluorinated polymer, and - the elongated electrically conductive element (1) consists of several twisted metal wires (2,3) forming a cylindrical strand, the twist pitch of which is less than 8 times the diameter of the strand.

2. Electric cable according to claim 1, wherein the strand pitch is greater than or equal to 3 times the strand diameter, in particular greater than or equal to 4 times the strand diameter, in particular greater than or equal to 5 times the strand diameter.

3. Electric cable according to claim 1 or 2, wherein each of the twisted metal wires (2,3) of the strand forms a helix, and wherein the twisting pitch is equal to the pitch of each of these helices.

4. Electric cable according to any one of claims 1 to 3, wherein the total thickness of the insulation system (10) is less than or equal to 5 mm, preferably less than or equal to 4 mm, more preferably less than or equal to 3 mm, and even more preferably less than or equal to 2 mm, for example between 0.5 mm and 5 mm.

5. Electrical cable according to any one of claims 1 to 4, wherein the insulation system is applied by extrusion around the strand of several twisted metallic wires (2,3) constituting the elongated electrically conductive element (1),

6. Electric cable according to any one of claims 1 to 5, wherein the thickness of the insulating layer (14) present in the insulation system is less than or equal to 3 mm, in particular less than or equal to 2 mm, for example between 0.5 mm and 3 mm, or even between 0.5 mm and 2 mm.

7. Electric cable according to any one of claims 1 to 6, wherein the electrically insulating layer (14), which comprises at least one fluorinated polymer, is applied by extrusion.

8. Electrical cable according to claim 7, wherein the fluorinated polymer is selected from polytetrafluoroethylene (PTFE); fluorinated ethylene and propylene (FEP) copolymers such as poly(tetrafluoroethylene-co-hexafluoropropylene); perfluoroalkoxy (PFA) copolymers such as perfluoro(alkylvinyl ether) / tetrafluoroethylene copolymers; perfluoro methoxy (MFA) copolymers; and poly(ethylene-co-tetrafluoroethylene) (ETFE); or a mixture thereof.

9. Electrical cable according to claim 8, wherein the fluorinated polymer is a perfluoroalkoxy (PFA) copolymer, preferably applied by extrusion.

10. Electric cable according to any one of claims 1 to 9, wherein the insulation system surrounding the elongated electrically conductive element comprises, in addition to the insulating layer, at least one internal semiconducting layer (12) surrounding the elongated electrically conductive element, said internal semiconducting layer being itself surrounded by the insulating layer (14), this internal semiconducting layer (14) preferably comprising a fluorinated polymer.

11. Electric cable according to any one of claims 1 to 10, wherein the insulation system surrounding the elongated electrically conductive element comprises, in addition to the insulating layer, at least one external semiconducting layer (16) surrounding the insulating layer, this external semiconducting layer (16) preferably comprising a fluorinated polymer.

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