EP3358575B1 - Electric cable resistant to partial discharges - Google Patents

Description

The present invention relates to an electrical cable comprising an elongated electrically conductive member, an electrically insulating layer comprising polyimide (PI) surrounding said elongated electrically conductive member, and a fluorinated electrically insulating layer comprising a fluorinated polymer surrounding said electrically insulating layer comprising polyimide (PI), said layers having specific thicknesses according to the section of the elongated electrically conductive element.

The invention typically but not exclusively applies to electrical cables used in the field of aeronautics, for example on board aircraft.

In the prior art, it is known to equip aircraft with wiring son (eg more than 500 kilometers of cable in the A380), these son comprising a conductive element surrounded by a first polyimide layer of 0.017 to 0.065 mm thick, itself surrounded by a layer of PTFE polytetrafluoroethylene with a thickness of 0.1 to 0.22 mm for conductor nominal sections ranging from 0.15 to 95 mm ² . However, for such wiring son, the applied voltage is of the order of 115 V (between the phase and the neutral of the three-phase system). In order to reduce the cross-section of the cables to pass the same amount of current, the operating voltage of the aircraft cables has been increased to 230 V (between phase and neutral of the three-phase system). By doubling the voltage, the cable mass is divided by about two. However, the relatively high voltage, combined with aeronautical constraints, such as humidity, high temperature and low pressure, can generate partial discharges (DP) on electrical equipment, particularly at the surface and / or in the defects of electrical cable insulators. However, the partial discharges, which are tiny electric arcs, cause over time, a degradation of the insulating material until the breakdown of the dielectric causing the possible establishment of an electric arc.

In order to reduce partial discharges, EP 2 557 572 A1 has described an electrical cable comprising a conductive element, a layer comprising polyimide (PI) surrounding said conductive element, and a fluorinated layer comprising a fluorinated compound surrounding said layer comprising polyimide (PI), the total thickness of all fluorinated layers being at least 0.4 mm. However, the dimensions of the cable are not optimized to minimize its size and / or weight while ensuring optimal partial discharge resistance.

The present invention aims to provide a cable that avoids all or part of the aforementioned drawbacks. In particular, the present invention aims to provide a cable having a footprint and / or reduced weight, while ensuring good resistance to partial discharges, especially when the cable is intended for the field of aeronautics and suffered during a flight, high temperatures (around 150 ° C), low pressures (about 145 mbar) and high voltages, such as 230 V (between phase and neutral of the three-phase system) or 400 V between phases.

• un élément électriquement conducteur allongé ayant une section transversale s (en mm²),
• une couche électriquement isolante comprenant du polyimide (PI) entourant ledit élément électriquement conducteur allongé et ayant une épaisseur e₁ (en mm), et
• une couche électriquement isolante fluorée comprenant un polymère fluoré, entourant ladite couche électriquement isolante comprenant du polyimide (PI),

caractérisé en ce que l'épaisseur e₂ (en mm) de la couche électriquement isolante fluorée est inférieure à 0,4000 mm, et l'épaisseur totale e₁ + e₂ (en mm) est telle que e₁ + e₂ ≥ -0,02140 x In (s) + 0,41613.For this purpose, the subject of the present invention is an electrical cable comprising:

• an elongated electrically conductive element having a cross section s (in mm ² ),
• an electrically insulating layer comprising polyimide (PI) surrounding said elongated electrically conductive member and having a thickness e ₁ (in mm), and
• a fluorinated electrically insulating layer comprising a fluoropolymer, surrounding said electrically insulating layer comprising polyimide (PI),

characterized in that the thickness e ₂ (in mm) of the fluorinated electrically insulating layer is less than 0.4000 mm, and the total thickness e ₁ + e ₂ (in mm) is such that e ₁ + e ₂ ≥ -0.02140 x In (s) + 0.41613.

Thus, the thicknesses of the different layers of the cable of the invention are reduced, inducing a minimum weight and / or bulk in function of the diameter (ie of the section) of the elongated electrically conductive element, while guaranteeing the absence of conditions conducive to the appearance of partial discharges.

In a preferred embodiment, s (in mm ² ) is such that 0.25 ≤ s ≤ 85, and preferably 1 ≤ s ≤ 68.

According to one embodiment of the invention, the total thickness e ₁ + e ₂ (in mm) is such that e ₁ + e ₂ ≥ s 'x 0.00482 + 0.33012, with s' being the cross section of the electrically conductive elongated AWG element.

In the present invention, the term "AWG" means "American Wire Gauge" and refers to a unit of measurement for measuring the diameter of an electrical cable. A table of correspondence available in the literature makes it possible to convert the section s in mm ² in section AWG (http://www.astm.org/Standards/B258.htm, http://www.astm.org/ Standards / B286.htm ).

Correspondence tables that take into account the multi-strand nature of a conductor will be preferred.

In a preferred embodiment, s' (in AWG) is such that -2 (ie AWG000) ≤ s' ≤ 24 (ie AWG24), and preferably s' (in AWG) is such that -1 (ie AWG00) ≤ s' ≤ 10 (ie AWG10).

The cable of the invention makes it possible to avoid partial discharges under the conventional conditions of use. It therefore preferably has a partial peak discharge threshold voltage, peak value (also well known as the PDIV for " partial discharge inception voltage ") greater than or equal to 800 V at a pressure of 145 mbar and a temperature of 150 ° C.

It preferably has a threshold voltage of appearance of the partial discharges, value in volts RMS (also well known under the anglicism PDIV in V RMS for " partial discharge inception voltage in voltage root mean square ") greater than or equal to 566 V for a sinusoidal voltage, at a pressure of 145 mbar and a temperature of 150 ° C.

The elongated electrically conductive member is preferably central.

The fluoropolymer is preferably a polymer obtained by polymerization of monomers among which at least one of said monomers is tetrafluoroethylene or vinyl fluoride.

By way of example, the fluoropolymer may be a fluorinated homopolymer or copolymer, and preferably it is chosen from a polytetrafluoroethylene (PTFE), a poly (tetrafluoroethylene-co-hexafluoropropylene) (FEP), a perfluoro (alkylvinyl ether) copolymer / tetrafluoroethylene (PFA), a poly (ethylene-co-tetrafluoroethylene) (ETFE) and a combination thereof.

Preferably, a layer is called "comprising a fluorinated polymer" when it comprises, in mass with respect to the mass of said layer, at least about 50% of fluorinated polymer (s), preferably at least 70% about fluorinated polymer (s), and even more preferably at least about 80% fluorinated polymer (s), and even more preferably about 90% fluorinated polymer (s) , such as in particular PTFE, PFA, ETFE, FEP or a combination thereof.

Advantageously, the fluoropolymer is PTFE.

Preferably, the thickness e ₂ (in mm) of the fluorinated electrically insulating layer comprising a fluorinated polymer, such as, for example, PTFE, PFA, ETFE, FEP or a combination thereof, is such that 0.2000 mm ≤ e ₂ <0.4000 mm, preferably such as 0.2000 mm ≤ e ₂ ≤ 0.3950 mm, and more preferably such that 0.2500 mm ≤ e ₂ ≤ 0.3850 mm.

In the invention, the thickness e ₂ of the fluorinated electrically insulating layer is measured after sintering of said layer. Indeed, during sintering, the fluoropolymer may lose in volume. In particular, PTFE can lose about 25% by volume.

In the invention, the thickness e ₁ + e ₂ is measured after sintering of said layers.

The fluorinated electrically insulating layer is preferably sintered.

According to the invention, the fluorinated electrically insulating layer may be banded and / or extruded, and preferably banded.

When banded, the fluorinated electrically insulating layer may correspond to the winding of one or more ribbons of fluorinated polymer (s). It is then sintered to give it its mechanical properties.

Preferably, the fluorinated electrically insulating layer comprises one or more ribbons of fluorinated polymer (s), preferably one or more PTFE ribbons.

The cable may further comprise at least one fluorinated adhesive layer comprising a fluoropolymer, the fluoropolymer included in said adhesive layer being in particular identical to or different from that included in the fluorinated electrically insulating layer.

According to a characteristic of the invention, the fluorinated adhesive layer or layers are composed of one or more fluorinated polymers. This is called fluorinated adhesive layer.

In particular, the fluoropolymer or polymers of the fluorinated adhesive layer are chosen from poly (tetrafluoroethylene-cohexafluoropropylene) (FEP), a perfluoro (alkyl vinyl ether) / tetrafluoroethylene (PFA) copolymer, a polytetrafluoroethylene (PTFE), a poly ( ethylene-co-tetrafluoroethylene) (ETFE) and a combination thereof, said aforementioned fluorinated compounds having adhesion properties.

Advantageously, at least one fluorinated adhesive layer is disposed on at least one of the two faces of the electrically insulating layer comprising polyimide. An adhesive layer has the function of allowing adhesion between the layers that it connects or between the elongate electrically conductive element and the layer that it connects.

The fluororesic adhesive layer is generally capable of adhering the elongated electrically conductive element to the electrically conductive layer PI insulating material or the electrically insulating layer of PI to the fluorinated electrically insulating layer (eg layer comprising PTFE, PFA, FEP, ETFE or a combination thereof). In fact, the fluoropolymer or polymers of the adhesive layer undergo prior treatment that gives them their adhesive property, as is the case for the Kapton FN® product marketed by the company Dupont.

According to one embodiment, the electrically insulating layer comprising a polyimide and the fluorinated electrically insulating layer (e.g. comprising PTFE) are separated by a fluorinated adhesive layer.

The electrically insulating layer comprising a polyimide may be covered on each of its faces with a fluorinated adhesive layer, and in particular a fluorinated ethylene propylene copolymer (FEP) coating.

The thickness of the fluorinated adhesive layer (e.g. FEP) can range from about 2 to 2.5 μm before sintering.

The thickness of the electrically insulating layer comprising a polyimide may range from about 20 to 30 microns, and preferably from about 23 to 27 microns.

The electrical cable of the invention may further comprise one or more electrically insulating layers comprising additional polyimide (PI), each of the electrically insulating layers comprising a polyimide which may be covered on each of its faces with a fluorinated adhesive layer, and in particular a fluorinated ethylene propylene copolymer (FEP) coating.

In this case, the thickness e ₁ (in mm) as defined in the invention refers to the total thickness of the electrically insulating layers comprising polyimide (PI),
The Kapton FN® product is suitable for the present invention. It is in the form of a ribbon comprising an electrically insulating layer of polyimide (PI) covered on each of its faces with a layer of FEP (FEP / PI / FEP).

Two electrically insulating layers comprising a polyimide can thus be obtained by winding at least two thicknesses of said tape to overlap, and a total thickness of polyimide electrically insulating layers before sintering (or after sintering) is obtained. order of about 50 to 51 microns, and a total thickness of FEP adhesive layers before sintering (or after sintering) of the order of about 9 to 10 microns. Thus, the thickness of the assembly: FEP / PI / EFF / FEP / PI / FEP before sintering (or after sintering) is of the order of 60 microns.

The elongate electrically conductive element that is suitable according to the invention is, for example, of the solid or multi- stranded type .

The elongate electrically conductive member may be copper (Cu), tin-plated Cu alloy, silver-plated Cu alloy, nickel-plated Cu alloy, aluminum (Al), nickel-plated aluminum or aluminum alloy. copper-plated and nickel-plated aluminum (well known under the Anglicism " nickel plated copper clad aluminum ").

The elongated electrically conductive element according to the invention is preferably multi-stranded.

When the cable comprises other fluoride layer (s) (ie other layer (s) comprising a fluoropolymer as defined in the invention), the thickness e ₂ (in mm) designates the thickness of the fluorinated electrically insulating layer cumulated with the respective thicknesses of the other optional fluorinated layers, in particular comprising at least one fluorinated homo- or copolymer such as in particular PTFE, PFA, ETFE, FEP or a combination thereof.

In particular, when one or more fluorinated adhesive layers are present, the thickness e ₂ (in mm) denotes the thickness of the fluorinated electrically insulating layer cumulated with the thicknesses of the fluorinated adhesive layers.

According to a preferred embodiment of the invention, the fluorinated electrically insulating layer is the outermost fluorinated layer of the cable.

According to another preferred embodiment of the invention, the cable further comprises one or more other fluorinated layers (i.e. additional fluorinated layers).

In this embodiment, the cable may comprise at least one additional fluorinated layer, in particular chosen from a fluorinated semiconductor layer, another fluorinated electrically insulating layer and an outer fluoride (superficial) layer capable of being labeled (ie a marking layer ), and preferably a fluorinated semiconductor layer.

The fluorinated semiconductor layer may comprise at least one fluorinated polymer, the fluorinated compound included in said semiconductor layer being in particular identical to or different from that included in the fluorinated electrically insulating layer.

The fluorinated semiconductor layer may be in the form of a ribbon, an extrudate, a varnish, or a combination thereof.

According to the invention, it is more particularly considered that a layer is semiconductive when its electrical conductivity is at least 0.001 Sm ^-1 (siemens per meter).

In particular, when the fluorinated semiconductor layer is in the form of a ribbon or extrudate, it may be composed of at least one fluorinated polymer or copolymer and from 0.1% to 40% by weight approximately charging (electrically) conductive, with respect to the total mass of said fluorinated semiconductor layer.

When the fluorinated semiconductor layer is in the form of a varnish, it may be composed of at least one fluorinated polymer or copolymer, of the FEP, PFA or PTFE dispersions type, and from 0.1% to About 40% by weight of (electrically) conductive filler, based on the total mass of said fluorinated semiconductor layer.

Preferably, the fluorinated semiconductor layer comprises at least about 10% by mass of electrically conductive filler, and even more preferably at least about 25% by mass of electrically charged filler. conductive, relative to the total mass of said fluorinated semiconductor layer.

The electrically conductive filler may advantageously be chosen from carbon blacks, carbon nanotubes and a mixture thereof.

According to one characteristic of the invention, the fluorinated semiconductor layer has a longitudinal resistivity of 0.04 to 100 Ohm.m, and preferably 0.06 to 0.6 Ohm.m.

The outer layer (surface) capable of being marked may be in the form of a ribbon, an extrudate or a varnish. It may in particular comprise at least one fluorinated polymer or copolymer, such as, for example, PTFE, FEP, PFA, ETFE, and at least one metal complex-type pigment.

As indicated above, when one or more other fluorinated layers are present, the thickness e ₂ (in mm) denotes the thickness of the fluorinated electrically insulating layer cumulated with the thicknesses of the other fluorinated layers such as those mentioned above (layer of fluorinated marking, fluorinated semiconductor layer, other fluorinated electrically insulating layer, etc ...).

The other fluorinated layers of the cable of the invention, if they exist, are preferably sintered (e.g. fluoridated adhesive layer (s)).

The electrically insulating layer comprising polyimide may be made by taping (winding a polyimide tape), by coating varnish (mixture of components polymerizing in situ ) or by extrusion, according to techniques known to those skilled in the art.

The cable comprising the above-mentioned characteristics is intended to be used in the field of aeronautics, in particular at 230 V (between the phase and the neutral of the three-phase system) and is in particular intended to equip the aircraft.

For a better understanding of the invention, the description will reference to the accompanying drawing and which is only illustrative and not restrictive.

The figure 1 illustrates a cross-sectional view of an electrical cable at the insulation stage (without sheath) according to a preferred embodiment of the invention.

Example of realization:

For the sake of clarity, only the essential elements for understanding the invention have been shown schematically, and this without respect of the scale on the figure 1 .

According to a first embodiment, the hook up wire or the power cable 1, shown in FIG. figure 1 , comprises: a central elongate electrically conductive element 2, in particular of copper or aluminum, of multi-stranded type, and, successively and coaxially around this central elongated electrically conductive element 2, a first FEP adhesive layer 5a, an electrically insulating layer of polyimide (PI) 3, a second FEP adhesive layer 5b and an electrically insulating PTFE layer 4, here representing the outer layer of the cable 1. The various layers are obtained by taping. The cable is then heat-treated to sinter the outer layer of PTFE. For this, a temperature above 340 ° C is applied.

The FEP / PI / FEP assembly preferably corresponds to Kapton FN® tape from Dupont comprising a 25.4 μm thick layer of PI coated on each of its faces with a 2.5 μm FEP layer. thick before sintering.

After laying (or taping) the electrically insulating layer of PTFE, the electrical cable thus isolated is heat-treated in an oven at a temperature above the melting temperature of the PTFE, ie at a temperature above 340 ° C, for obtain sintering of PTFE and layers of FEP. By this single heat treatment step which includes the heat-sealing step of the polyimide and the sintering step of PTFE and FEP layers, it ensures the adhesion of all the thicknesses of ribbons. In Indeed, the heat treatment leads to the cohesion of the electrically insulating layer of PTFE on the electrically insulating layer PI and the bonding of the electrically insulating layer PI on itself and on the elongated electrically conductive element.

In Table 1 below, two cables according to the invention are illustrated and for comparison two cables as described in FIG. EP 2 557 572 A1 , with for each of the cables the total thickness of the electrically insulating layers of PI after sintering, the thickness of the fluorinated layers after sintering (adhesive layers and electrically insulative fluorinated layer), their section in mm ² and in AWG (which corresponds to the most close to the section in mm ² for multi-strand conductors) and their emergence voltage of PDIV discharges in V RMS. In this example, two layers of Kapton FN® ribbon from Dupont were used for overlap. There is thus a fluorinated electrically insulating layer of PTFE, two electrically insulating layers of polyimide (PI) and four adhesive layers of FEP according to the following arrangement of the outermost layer to the innermost layer: PTFE / FEP / PI / FEP / FEP / PI / FEP / elongated electrically conductive element.

PDIV in V RMS was measured according to the latest edition (2015) of the standard EN 3475-307, method B. <b> TABLE 1 </ b> Section (in mm ² ) Section (AWG) Thickness of the fluorinated layers (in mm) Thickness of the PI layer (in mm) PDIV (in V RMS) Cable 1 3 12 0.35 0.0508 ≥566 Cable 2 42 1 0.30 0.0508 ≥566 Cable 3 ^(*) 3 12 0.50 0.0508 ≥700 Cable 4 ^(*) 42 1 0.50 0.0508 ≥1050 (*) Cable not forming part of the invention

It appears that a reduced weight of 4.9% is obtained respectively for the cable 1 and 4.4% for the cable 2, with respect to the cables 3 and 4 of the prior art, while ensuring a sufficient level of voltage of appearance of partial discharges.

Claims (14)

1. Electric cable (1) comprising:

   - an elongated electrically conductive element (2) having a cross-section s (in mm²),

   - an electrically insulating layer comprising polyimide (PI) (3) surrounding said elongated electrically conductive element (2) and having a thickness e₁ (in mm), and

   - a fluorinated electrically insulating layer comprising a fluoropolymer (4), surrounding said electrically insulating layer comprising polyimide (PI) (3),

   characterized in that the thickness e₂ (in mm) of the fluorinated electrically insulating layer is less than 0.4000 mm, while the total thickness e₁ + e₂ (in mm) is such that e₁ + e₂ ≥ - 0.02140 x In (s) + 0.41613.
2. Electric cable according to claim 1, characterized in that s is such that 0.25 ≤ s ≤ 85.
3. Electrical cable according to claim 1 or 2, characterized in that it has a threshold voltage of the appearance of partial discharges, with an RMS value in volts greater than or equal to 566 V at a pressure of 145 mbar and a temperature of 150°C.
4. Electrical cable according to any one of the preceding claims, characterized in that the fluoropolymer is chosen from a polytetrafluoroethylene (PTFE), a poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP), a perfluoro(alkyl vinyl ether) tetrafluoroethylene copolymer (PFA), a poly(ethylene-co-tetrafluoroethylene) (ETFE) or a combination thereof.
5. Electrical cable according to any one of the preceding claims, characterized in that the fluoropolymer is PTFE.
6. Electrical cable according to any one of the preceding claims, characterized in that the thickness e₂ of the fluorinated electrically insulating layer comprising a fluoropolymer (4) is such that 0.2000 mm ≤ e₂ ≤ 0.3950 mm.
7. Electrical cable according to any one of the preceding claims, characterized in that the thickness e₂ of the electrically insulating fluorinated layer (4) is measured after sintering of said layer.
8. Electrical cable according to any one of the preceding claims, characterized in that the fluorinated electrically insulating layer (4) is taped.
9. Electrical cable according to any one of the preceding claims, characterized in that it further comprises at least one fluorinated adhesive layer comprising a fluorinated polymer (5a, 5b), the fluoropolymer included in said adhesive layer (5a, 5b) being identical or different from that comprised in the fluorinated electrically insulating layer (4).
10. Electrical cable according to any one of the preceding claims, characterized in that the electrically insulating layer comprising a polyimide (3) is covered on each of its faces with a fluoride adhesive layer (5a, 5b).
11. Electrical cable according to any one of the preceding claims, characterized in that the elongated electrically conductive element (2) corresponds to copper (Cu), a tin-plated Cu alloy, a silver-plated Cu alloy, a nickel-plated copper alloy, aluminum (Al), nickel-plated aluminum or copper-plated and nickel-plated aluminum.
12. Electrical cable according to one of the preceding claims, characterized in that the fluorinated electrically insulating layer (4) is sintered.
13. Electrical cable according to one of the preceding claims, characterized in that it further comprises at least one fluorinated semiconductor layer.
14. Electrical cable according to claim 13, characterized in that the fluorinated semiconductor layer comprises at least one fluoropolymer, the fluorinated compound comprised in said semiconductor layer being identical to or different from that comprised in the fluorinated electrically insulating layer (4).

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