Electrically insulating material

Abstract

The invention relates to an electrically insulating material (1) comprising a polymer matrix (11) containing a polymer or a polymer blend in which inorganic particles (12) are dispersed. The inorganic particles are hollow.

Description

[0001] Electrically Insulating Material

[0002] The present invention relates to the field of electrically insulating materials.

[0003] BACKGROUND OF THE INVENTION

[0004] A partial discharge is a rapid and localized electrical discharge.

[0005] A partial discharge can for example occur: - In the gas separating two electrical conductors (an electrical conductor can be an electrically powered conductor or a ground conductor);

[0006] - in a gas bubble (air vacuole) present in the insulating material placed between electrical conductors.

[0007] A partial discharge does not immediately short-circuit the electrical insulation but contributes to its degradation and premature failure through its mechanical, thermal and chemical action, when the electrical stress exceeds a critical value called the partial discharge onset voltage.

[0008] These phenomena lead to premature aging of the insulators, gradually reduce their breakdown voltage and can, in the medium / long term, lead to a dielectric breakdown inducing a short circuit in the winding.

[0009] When designing an electrical component, it is therefore important to ensure that the voltages involved in operation are lower than the partial discharge inception voltage (PDIV for Partial Discharge Inception Voltage, i.e. the voltage from which partial discharge appears).

[0010] In order to increase the Partial Discharge Onset Voltage (PDIV) of an electrical component, increasing the thickness of the insulation and controlling the distances of the air gaps in the component are the conventional solutions used to size insulation systems.

[0011] Unlike an electrical breakdown which generates an electrical discharge going from one conductor to another through the entire thickness of the insulating material (which is then destructive to the entire thickness of the insulating material), a partial discharge is localized on only a portion of the thickness of the insulating material containing a defect (air bubble, delamination...), without passing through it completely and without going from one electrical conductor to another.

[0012] Such a partial discharge can induce irreversible degradation of the electrically insulating material, whose insulating characteristics are progressively diminished. This results in accelerated aging of the insulating material (dielectric material).

[0013] Partial discharges typically occur in areas of the insulating material with defects such as air bubbles or delaminations, and in areas with air between two insulated conductors.

[0014] To reduce the risk of partial discharges or electrical breakdowns, increasing the thickness of the insulating material is known to raise the partial discharge onset voltage. Furthermore, increasing the thickness also raises the breakdown voltage, which is the potential difference that must be applied across the insulating material to induce electrical breakdown throughout its entire thickness.

[0015] However, this solution of increasing the thickness of the insulating material layer is costly and above all involves an increase in the consumption of polymers needed to manufacture the polymer matrix and an increase in the weight of the parts whose thickness is thus increased.

[0016] These solutions lead to an increase in the mass and volume of electrical systems and run counter to the current trend towards increasing mass and volume power density, particularly in the aeronautical sector where the mass power density of equipment must be as high as possible.

[0017] Another solution is to improve the manufacturing process of the insulating material in order to eliminate defects present in the matrix and delaminations (elimination of areas with air).

[0018] However, improving the process remains difficult and costly to implement and does not allow, for example, addressing the problems of discharges in the gaseous space separating insulated conductors.

[0019] SUBJECT OF THE INVENTION

[0020] One object of the present invention is to provide an insulating material that resolves all or part of the disadvantages of the aforementioned prior art.

[0021] SUMMARY OF THE INVENTION

[0022] To this end, according to a first aspect of the invention, an electrically insulating material is proposed comprising a polymer matrix containing a polymer or a mixture of polymers in which non-metallic inorganic particles are dispersed.

[0023] The insulating material according to the invention is essentially characterized in that the non-metallic inorganic particles are hollow.

[0024] Thus, the polymer matrix forms a dielectric material, i.e. an electrical insulator in which hollow inorganic particles are arranged. The insulating material according to the invention has a lower permittivity than the permittivity of the material constituting the polymer matrix due to the dispersion of the hollow inorganic particles.

[0025] The dispersion of hollow particles changes the voltage distribution in the insulating material and allows the partial discharge inception voltage (PDIV) to be increased.

[0026] The presence of hollow organic particles dispersed in the polymer matrix tends to reduce the permittivity of the insulating material by changing the distribution of electrical voltage in the insulating material (the dispersion of hollow inorganic particles tends to lengthen the path that electrons must travel through the insulating material for a partial discharge to occur).

[0027] Furthermore, in the event of the presence of an air bubble in the insulating material according to the invention, the presence of non-metallic and hollow organic particles tends to modify the distribution of the voltage between the air bubble (also called "air gap") and the polymer matrix, which has the effect of increasing the voltage at which partial discharges occur.

[0028] In the event of a partial discharge, within the insulating material according to the invention, outside of the hollow particles, the impacted matrix area remains limited since it is contained between neighboring inorganic and non-metallic particles that form thermal and mechanical shields, limiting the volume of polymer matrix impacted / degraded by the energy associated with the highly localized partial discharge. Thus, even in the event of a partial discharge, the electrical insulation characteristics conferred by the insulating material according to the invention are essentially preserved for longer periods.

[0029] The presence of hollow inorganic particles helps to limit the premature aging of the insulating material induced in the event of partial discharges, which is favorable to maintaining, in the long term, the resistance to a given breakdown voltage.

[0030] Finally, the use of hollow inorganic particles contributes to reducing the overall density of the insulating material.

[0031] The insulating material according to the invention thus makes it possible to reduce the density of the insulating material without compromising its electrically insulating character.

[0032] According to a second aspect, the invention relates to an electricity transmission cable comprising a central electrical conductor and an electrically insulating sheath surrounding said central electrical conductor.

[0033] This cable according to the invention is essentially characterized in that the sheath of electrically insulating material is composed of the insulating material according to any one of the embodiments of this insulating material described below.

[0034] The cable according to the invention has the inherent advantages of the insulating material according to the invention (reduced density of the insulator and reduced permittivity of the insulator), which allows use at higher voltages before the onset of partial discharges, improved resistance of the cable to partial discharges and a lower density.

[0035] For this reason, this cable is particularly suitable for use in an aircraft.

[0036] Thus, according to a third aspect, the invention relates to an aircraft electrical system, comprising at least one cable according to any one of the embodiments of the cable according to the invention described in this document.

[0037] Said at least one cable electrically connecting them via said at least one central electrical conductor of the cable, a first terminal of an electric voltage generator and a first terminal of an electrical component belonging to the list of electrical components including an electrical resistor, an electric induction coil, a capacitor, an electrical machine arranged to transform electrical energy into mechanical energy, a transformer.

[0038] The aircraft electrical system according to the invention has all the advantages inherent in the insulating material according to the invention and in the cable according to the invention.

[0039] BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Other features and advantages of the invention will become clear from the description given below, by way of example and not limitation, with reference to the accompanying drawings, in which:

[0041] [Fig. 1] Figure 1 shows a cable 100 according to the invention comprising an annular insulating sheath 102 of an electrical conductor 101, the insulating sheath 102 being made of the insulating material 1 according to the invention;

[0042] [Fig. 2] Figure 2 illustrates two Cl, C2 curves demonstrating that by dispersing hollow inorganic particles (called "filler") in the insulating polymer matrix (called "matrix"), we decrease the permittivity of the insulating material and its density (the term "hollow silica filler ratio" designates the volumetric rate of hollow silica particles in a given volume of the insulating material called mix);

[0043] [Fig. 3] Figure 3 illustrates an aircraft 20 according to the invention equipped with the aircraft electrical system 105 according to the invention which comprises the insulating material according to the invention (for example in the form of an insulating sheath of a cable 100 and / or other electrically insulating parts 107).

[0044] DETAILED DESCRIPTION OF THE INVENTION

[0045] With reference to Figure 1, the electrically insulating material 1 according to the invention comprises a polymer matrix 11 containing a polymer or a mixture of polymers in which are dispersed hollow, non-metallic inorganic particles 12.

[0046] The hollow inorganic particles dispersed in the matrix are selected so that the electrically insulating material has an electrical permittivity lower than the electrical permittivity of the polymer matrix alone 11 (By electrical permittivity of the polymer matrix alone 11 is meant the permittivity of the constituent material of the polymer matrix free from the hollow inorganic particles 12).

[0047] Thus, hollow inorganic particles make it possible to reduce the electrical permittivity of the insulating material and therefore the risk of the appearance of partial discharges (by increasing the voltage at which partial discharges appear, called PDIV).

[0048] To this end, hollow particles exhibit, in the temperature range -30°C to +100°C, an intrinsic electrical permittivity lower than the intrinsic electrical permittivity of the polymer matrix.

[0049] In the temperature range of -30°C to +100°C, the intrinsic electrical resistivity of the particles is preferentially greater than the intrinsic electrical resistivity of the polymer matrix.

[0050] The inorganic particles 12 are hollow to oppose the generation of partial discharges in the insulator while minimizing the density of the insulating material (the hollow areas 12b of the particles which contain a gas or vacuum have a density much lower than the density of the polymer matrix).

[0051] The density of the hollow particles contained in the polymer matrix is ​​strictly less than the density of the polymer constituting the polymer matrix.

[0052] Thus, the insulating material makes it possible to improve electrical insulation while having a minimized density and requiring less matrix material, i.e. less polymer.

[0053] Preferably, hollow particles are selected to each have a hollow zone with a maximum external dimension DI of the particle, at atmospheric pressure (atmospheric pressure is 1013.25 hPa), preferably less than 3000 nm, preferably less than 1500 nm, preferably less than 1000 nm.

[0054] This maximum dimension limit DI of the particle which is less than 3000 nm is chosen using the Paschen curve which measures the breakdown voltage between two electrodes placed in air at a pressure of 1 atm, as a function of the distance between these electrodes.

[0055] We observe that below a limiting value of distance between the electrodes (i.e. the thickness of the air gap between these electrodes), which is on the order of 3000 nm, the breakdown voltage tends to increase towards infinity.

[0056] In other words, the maximum size of the hollow particles is chosen to have very small air gaps, allowing them to be located to the left of the Paschen curve at atmospheric pressure (and therefore at high breakdown voltage levels). Preferably, each inorganic particle 12 has a solid, hollow shell 12a, the material constituting the shell 12a having an electrical resistivity greater than an electrical resistivity value of said polymer or said mixture of polymers constituting the polymer matrix 11.

[0057] For the purpose of understanding the invention, the resistivity values ​​indicated in this application are, unless otherwise stated, given at ambient temperature and pressure (20°C and 1 atm).

[0058] By having inorganic particles dispersed in the polymer matrix with an electrical resistivity of these particles greater than the resistivity of the matrix alone, we statistically lengthen the path of electrical charges extending through the thickness of the insulating material.

[0059] Indeed, an electric charge will tend to pass through a path with the least electrical resistance, that is to say, through the mixed polymer(s) to bypass the inorganic particles with higher electrical resistivity.

[0060] Typically, polymers used to form insulating material have an electrical resistivity greater than 10 13 Q. cm, preferably greater than 10 15 Q. cm, in this case the resistivities of polyimide, polyamide-imide, silicone or their mixtures are greater than or equal to 1.10 15 Q. cm.

[0061] The material constituting the shell of a given particle is therefore preferentially chosen to have a resistivity higher than that of the polymer or polymer mixture of the matrix. This makes it possible to increase the breakdown voltage since the path of electrical charges through the insulating material is increased by the presence of the hollow particles.

[0062] The fact that these particles are hollow allows them to form a kind of thermal shield where the thermal energy associated with a partial discharge can dissipate, thus limiting the risk of degradation of the polymer matrix.

[0063] As previously stated, this limits the risk of damage to the polymer or polymer mixture of insulating material according to the invention.

[0064] A fortiori, this limits the risk of having a partial discharge destructive of the object containing the insulating material according to the invention (for example, an object containing the insulating material according to the invention may be an electrical cable whose insulating sheath is made of the insulating material according to the invention or a connector or a transformer or an electrical machine, for example a rotary electric motor).

[0065] Ideally, the constituent material of the hollow particles has a melting point above 300°C, preferably above 800°C.

[0066] With this characteristic, the thermal protection conferred to the polymer matrix by the hollow particles is generally retained even after a partial discharge.

[0067] The material constituting the particle shells can be made to have porosity. Such porosity is advantageous for reducing particle density.

[0068] The pore size is preferably less than 5 nm in order to prevent the constituent material of the polymer matrix from migrating into the hollow areas of the particles when it is in liquid or paste form.

[0069] Thus, the matrix polymer can, in a transient manner, be in a liquid or pasty phase (for example during the manufacture of the polymer matrix) without risk of migrating, towards the hollow areas of the particles, via the porosities.

[0070] Preferably, the shells 12a of the particles 12 are substantially spherical or ovoid, with an external diameter DI between 110 nm and 3000 nm, preferably less than 1500 nm.

[0071] Preferably, each solid and hollow particle shell has an internal hollow zone of substantially spherical or ovoid shape with diameter D0 between 100 nm and 1000 nm.

[0072] Preferably, in order to increase the particle volume ratio in the matrix, particles of different size distributions can be used. The diameter D0 of the maximum hollow part is typically between 50 nm and 1000 nm.

[0073] With this range of sphere dimensions, even during the aircraft's ascent to altitude, which is accompanied by a decrease in surrounding pressure, the breakdown stress in the space formed in the hollow particles does not tend to decrease, even if the hollow particles are not completely sealed.

[0074] Therefore, cracks in the wall (shell of the particles), porous walls or breaks of certain particles do not pose any particular problems since for the breakdown voltage to decrease there would have to be a defect in the insulating material involving a large number of hollow particles close to each other which would be both porous and whose hollow areas 12b would be communicating and / or a large number of neighboring particles broken and forming a large hollow area in the insulating material.

[0075] The risk of such a defect appearing simultaneously affecting a large number of neighboring hollow particles with additional gaseous communication between the hollow areas 12b is very unlikely.

[0076] For such a defect to be problematic and induce a drop in the voltage at the onset of a partial discharge, the size of the defect would have to be large (greater than or equal to 10,000 nm), which would make it easily detectable at the time of manufacturing the insulating material.

[0077] The electrical breakdown voltage between two electrodes separated only by an insulating material is greater with the insulating material according to the invention than it is with a simple insulating polymer matrix of the same thickness which does not contain said hollow particles.

[0078] Indeed, for there to be an electric arc, a breakdown, electric charges must be able to move between the electrodes which are separated from each other by the electrically insulating material.

[0079] The hollow inorganic and non-metallic particles dispersed in the polymer matrix have an intrinsic breakdown voltage much higher than the intrinsic breakdown voltage of the polymer or polymers of the matrix.

[0080] Thus, with this range of diameters of the maximum hollow part of the particles, we consider that the path of these charges through the insulating material is lengthened because it has to go around the hollow particles 12 to pass through the polymer of the matrix 11.

[0081] The lengthening of the charge path thus induces an increase in the breakdown voltage value. Preferably, each hollow particle shell has a maximum thickness between 5% and 45%, preferably between 5 and 25% of the external diameter of the spherical shell of the hollow particle (12).

[0082] In other words, for a particle with an external diameter DI of 100 nm, the maximum thickness of its shell, i.e. (Dl-D0) / 2, is between 5 nm and 45 nm, which leaves a minimum space of the hollow zone 12b between 90 nm and 100 nm.

[0083] Thanks to this, the particle has a reduced density and is easy to manufacture and has sufficient mechanical strength to be dispersible in the polymer matrix 11 while retaining its hollow spherical or ovoid shape which is necessary for homogeneity of electrical insulation in the insulating material 1.

[0084] Preferably, the volumetric rate of hollow particles (12) in the insulating material (1) represents between 1% and 80%, between 5% and 80%, preferably between 10% and 80%, preferably between 40% and 70%.

[0085] In other words, for a given volume of the insulating material according to the invention 1, the hollow particles 12 represent between 5% and 80% of this volume, preferably between 10% and 80%, preferably between 40% and 70% of this given volume.

[0086] The polymer matrix 11 can be obtained by molding the polymer or the mixture of different polymers in the form of solid grains (e.g. pellets or powder) or in liquid form.

[0087] Before molding the insulating material, the hollow inorganic particles can be pre-integrated: - into the polymer(s) constituting the solid grains intended for the formation of the polymer matrix; or between these solid grains; or - mixed into the polymer(s) in liquid / paste form which are intended to be injected to form the polymer matrix.

[0088] Thus, in the case where the matrix is ​​obtained by molding solid grains, the particles can be pre-mixed in the solid polymer grains, but they can also be mixed with these polymer grain(s).

[0089] We prefer that the particles be directly present in the solid grains because this promotes a homogeneous distribution of hollow particles in the polymer matrix.

[0090] Conversely, in the case where the matrix is ​​obtained by molding polymer(s) in liquid or paste form, an injection will be carried out into a mold of the mixture containing the solid particles and the polymer(s) in liquid or paste form.

[0091] Hollow particles each have an internal hollow zone that can contain a vacuum, a gas, or a mixture of several gases, for example dry air, with an electrical resistivity greater than 10 13 Q. cm.

[0092] The hollow zone 12b of a given particle 12 is delimited externally by an internal surface of the shell 12a of the particle 12 (here of diameter DO).

[0093] Any porosities internal to a shell 12b of a given hollow particle 12 are not part of the internal hollow zone of that given particle.

[0094] At least some of these internal hollow areas are filled with a gas or a mixture of gases, for example a mixture of gases containing at least 70%, preferably at least 80%, nitrogen. This mixture of gases is dry.

[0095] Gases containing fluorine, such as hydrofluorinated gases, hydrofluorocarbons CFO and sulfur hexafluoride SF6, can be used and exhibit higher breakdown stresses than dry air.

[0096] It should be noted that the lowering of permittivity is essentially linked to the inherent permittivity of the material constituting the hull rather than to the intrinsic permittivity of the gas contained in the hollow area of ​​the hull.

[0097] However, with regard to the permittivity of the polymer matrix alone, such a gas mixture is preferentially chosen to exhibit a lower permittivity at ambient temperature and pressure (i.e., at an ambient temperature of 20°C and an ambient pressure of 1 atm).

[0098] Preferably, the polymer matrix 11 consists of at least one of the polymers included in the group of polymers comprising a thermosetting polymer, a polyester, an epoxy polymer, polyimide, polyamide-imide, polyester-imide, thermoplastic polymer, polyethylene, polyurethane, elastomeric polymer, silicone-type elastomeric polymer and a mixture of at least some of these polymers.

[0099] The insulating material according to the invention is a solid which, depending on the application, can be deformable or not.

[0100] When integrated into an electrical power transmission cable 100, the insulating material according to invention 1 will be formed with a deformable and flexible polymer matrix, without risk of breakage of the insulating material, within the limit of a radius of curvature of the cable which must be greater than 2 times, preferably greater than 5 times, the width of the cable, i.e. its diameter. Preferably, at least some and preferably all of said hollow inorganic particles 12 are made of technical ceramic.

[0101] The term "technical ceramic" refers to an item having a vitrified or non-vitrified body, of crystalline or partially crystalline structure, or made of glass, whose body is formed of essentially mineral and non-metallic substances, and which is formed by a molten mass which solidifies upon cooling, or which is formed and brought to maturity, simultaneously or subsequently, by the action of heat.

[0102] Ceramic is an inorganic, non-metallic material that exhibits high electrical resistivity and is resistant to thermal shock with strong temperature gradients, which is the case when a partial discharge occurs.

[0103] One advantage of ceramic particles is that they can be obtained industrially with high dimensional accuracy both in terms of particle size and dimensions and in terms of the size and dimensions of the hollow area of ​​the particles.

[0104] A hollow ceramic particle also makes it possible to form an effective shield between a possible partial discharge zone and the surrounding polymer matrix zone, which is thus protected from the energy induced by the discharge.

[0105] The hollow particles used in the insulating material according to the invention can be, for example, hollow sphere-type particles made of silica in the form of ceramics marketed under names such as Nanoshel ®, alphananotech ® or Sigma Aldrich ®.

[0106] The 12b hollows of the 12 particles can be obtained by various processes such as laser ablation, template-based synthesis, pyrolysis for example by heat flow or by a rapid drying process of the inorganic material.

[0107] Preferably, at least some of the particles 12 and preferably all of the hollow inorganic particles 12 having a low relative permittivity typically less than 10 dispersed in the matrix 11 are composed of at least one of the elements of the group comprising Silica - SiO2 (silica is silicon dioxide), Magnesium Pyrophosphate - Mg2P2O7, Zinc Phosphate - Zn3 (PO4 ) 2, Aluminium Oxide A12O3, Aluminium Nitride A1N, Silicon Nitride S13N4, Beryllium Oxide BeO, or a mixture of these elements of the group.

[0108] These elements of the group have the advantage of being inexpensive to produce and particularly stable over time in terms of low permittivity and coherence of bonding with the polymer matrix.

[0109] Furthermore, the dimensions of parts made with these inorganic ceramic materials can be obtained with an accuracy of plus or minus 10 nm, which makes it possible to precisely define the electrical properties of hollow particles.

[0110] In this respect, these elements of the group promote the preservation over time of the structural homogeneity of the insulating material according to the invention and the preservation of its electromagnetic characteristics.

[0111] With such particles, the insulating material according to the invention can undergo partial discharges while retaining its electrically insulating character over a prolonged life.

[0112] Preferably, at least some of said inorganic particles have an external surface treated with an organo-silicon polymer-based coupling agent, which is preferably a silane-based coupling agent.

[0113] The organo-silicon polymer coupling agent, in particular the silane-based coupling agent, is used to promote mechanical bonding between inorganic particles treated with this coupling agent and the polymer matrix.

[0114] The bond between the polymer matrix and the external surface of the inorganic particles treated with the coupling agent is greatly improved.

[0115] The continuity of mechanical and chemical bonding between the particles and the matrix reduces the risk of having partial discharges between the polymer matrix and the external surface of the inorganic hollow particles.

[0116] In particular, the silane molecule has two different chemical structures at both ends of the molecule, which allows the silane-based coupling agent to act as a chemical and mechanical bond between the particles (which here form a ceramic charge) and the polymer matrix.

[0117] One end of the coupling agent is chemically reactive with the polymer while the other end is chemically reactive with the external surface of the inorganic particle which is here of ceramic type.

[0118] In general, the insulating material according to invention 1 can be used to make electrically insulating parts intended for various applications.

[0119] Such parts may be: - one or more electrically insulating sheaths of an electrical cable with an electrical conductor; - one or more connectors equipped with a layer of insulating material surrounding a part of the electrical conductor of the conductor;

[0120] one or more electrically insulating parts of a transformer or electrical machine.

[0121] We will now describe, with reference to figure 1, an electricity transmission cable 100 according to the invention having a sheath 102 of insulating material 1 according to any one of the embodiments of this insulating material according to the invention.

[0122] As previously stated, this cable is an electrical transmission cable 100 comprising at least one central electrical conductor 101 and at least one electrically insulating sheath 102 surrounding said central electrical conductor 101 (the sheath is an annular sheath surrounding the entire conductor, except for its ends which are each intended to be connected to a connection terminal associated with one of the ends of the conductor).

[0123] Typically each conductor has a solid circular cross-section.

[0124] The sheath made of electrically insulating material 102 is composed of the insulating material 1 according to any one of the embodiments of the insulating material according to the invention.

[0125] In general, the sheath 102 made of insulating material 1 of the cable according to the invention 100 has a reduced permittivity and a reduced density compared to an insulating sheath of the same dimensions made of the same polymer(s) but which would not include the said hollow inorganic particles 12 dispersed in the polymer matrix 11. Thus, the sheath 102 according to the invention has an improved electrical insulation capacity while having a reduced density.

[0126] The cable 100 according to the invention, compared to a cable having the same characteristics but whose insulating material sheath would be exclusively made of the same polymer or mixture of polymers, without the said hollow inorganic particles, makes it possible, when subjected to a given current at high voltage, to limit the risk of the appearance of partial discharges (PD) in the cable 100.

[0127] In this sense, the 100 cable according to the invention is particularly suitable for transmitting currents with voltage levels likely to generate partial discharges (PD), i.e. a maximum voltage greater than or equal to 300V and / or having AC (Alternating Current) or PWM (Pulse Width Modulation) type waves, or pulsed waves (The problem related to the presence of partial discharges is generally observed in these wave forms).

[0128] Depending on the level of electrical insulation required, and the voltages to be applied through the electrical conductor of the cable, the sheath has a thickness greater than 0.2 mm and between 5 and 1000% of a conductor diameter.

[0129] The central electrical conductor 101 is metallic, for example copper or a copper alloy containing, by mass, mostly copper or aluminium or an aluminium alloy containing, by mass, mostly aluminium.

[0130] The properties of the insulating material according to the invention (composite material including the polymer matrix 11 and the hollow inorganic particles 12) can be estimated from the main known properties of the matrix 11 and the hollow particles 12 and the volume ratio of hollow particles dispersed in the matrix.

[0131] The dispersion of the particles is such that for a given unit volume of the matrix containing at least three of the hollow particles, the volumetric rate at any point of the insulating material per unit volume is within + or - 10% around an average volumetric rate value.

[0132] Thus, the volumetric ratio of hollow particles in the matrix is ​​relatively homogeneous over the entire length of the cable.

[0133] The formula below uses the effective media theory (EMT) and allows us to estimate the permittivity value of the insulating material, denoted Σmix, as a function of the permittivity value of the polymer matrix (denoted Σm), the permittivity value of the material constituting the hollow particles (denoted Σi), the volume of the matrix Vm, and the volume Vi of particles (here, spherical or ovoid particles) dispersed in the matrix. To account for the morphology of the ceramic particles, a shape factor n is introduced to generalize the equation. The value n = 0.165 is considered for particles in the form of spheres.

[0134] [Form 1]

[0135]

[0136] As an example, this formula is verified by the experimental curves in Figure 2, which illustrate two curves Cl, C2 of the evolution of the permittivity Σmix of the insulating material 1 as a function of the volumetric charge rate it contains (filling rate of the matrix in spherical particles expressed as a % of the total volume of the insulating material).

[0137] These Cl, C2 curves are obtained by measurements of different materials according to the invention which differ from each other only in the volumetric loading rate of hollow inorganic particles (each of these insulating materials has the same polymer matrix composition and the same type of hollow inorganic particles).

[0138] These Cl, C2 curves in Figure 2 are obtained by measurements at ambient temperature and pressure (20°C and 1 atm), with a polyimide polymer matrix (density 1.45 g / cm3 and s' = 4, s' denoting the permittivity) and hollow spherical silica particles (density 1.05 g / cm3 and permittivity s' = 1.5).

[0139] These Cl, C2 curves allow us to determine: - on the one hand the variation of the relative permittivity Σmix of the insulating material including the matrix and the dispersed particles (using the theory of effective media (EMT)); and - on the other hand the variation of density of this insulating material as a function of the volumetric loading rate (that is to say the total percentage of the volume of the insulating material layer which is occupied by spherical hollow silica particles with an outside diameter between 100nm and 600nm.

[0140] The volumetric loading rate of hollow silica particles is noted as "hollow silica filler ratio voi".

[0141] From the curves in Figure 2, we understand that by increasing the volumetric content (vo) of hollow particles in the matrix (vo increasing from 0%, when the matrix contains no hollow particles, up to 80% by volume of hollow particles), we observe: - A decrease in the permittivity value Σmix of the insulating material, which goes from 4 to 1 (in units of relative permittivity) for the series of measurements of curve C2 and from 4 to 1.2 for the series of measurements of curve C1; and

[0142] - A decrease in the density of said insulating material which goes from 1.4 g / cm3 (for curves Cl and C2) to approximately 1.1 to 1.25 g / cm3 depending on the measurement curve Cl, or C2 chosen.

[0143] The aforementioned advantages of the invention on the decrease in permittivity of the material (and consequently the decrease in the voltage at which partial discharges occur) and on the decrease in density induced by this insulating material are clearly evident from each of the series of measurements Cl, C2 in Figure 2.

[0144] As mentioned above, by choosing the maximum particle size and the volume ratio, we ensure that the permittivity decreases. Consequently, the overall voltage ratio seen by the insulator is higher than that of a material with a higher permittivity, and therefore the overall voltage ratio seen by the gas will be reduced. The gas present in or surrounding insulator defects can break down at a lower voltage when the pressure decreases (this occurs when the aircraft's altitude increases, for example, from 0 to 50,000 feet).

[0145] Thus, the invention is perfectly suited to aeronautical applications.

[0146] With reference to Figures 1 and 3, the invention finally relates to an aircraft electrical system 105 20, comprising at least one cable 100 according to any one of the embodiments of the cable according to the invention.

[0147] This at least one cable 100 electrically connects them, via said at least one central electrical conductor 101 of the cable 100, a first terminal of an electrical voltage generator 106 and a first terminal of an electrical component 107.

[0148] Typically, electrical component 107 belongs to the list of electrical components including an electrical resistor, an electrical induction coil, a capacitor, an electrical machine arranged to transform electrical energy into mechanical energy 107, a transformer.

[0149] The cable according to the invention is particularly suitable for use on an aircraft electrical system because it limits the risk of partial discharges occurring in the insulating material 1 or on its surface while allowing the thickness of the sheath 102 of insulation 1 to be reduced (typically pressures below 1 atm).

[0150] Thus, the electrical insulation capacity is improved while minimizing the density of the insulating material.

[0151] This mass gain is favorable for the use of the insulation according to the invention in an aircraft 20.

[0152] Furthermore, it has been observed with Paschen's law, which allows the breakdown voltage of air to be expressed for a given distance between two conductors as a function of the variation in air pressure, that as long as this distance in air is greater than a predefined minimum distance (usually more than 1 mm thick), the breakdown voltage in air always decreases with the decrease in local air pressure around the insulator (the decrease in local air pressure is linked to the increase in the flight altitude of the aircraft).

[0153] In other words, the probability of an air electrical breakdown occurring tends to increase with the aircraft's current cruising altitude, which is potentially dangerous for aircraft safety.

[0154] This risk is minimized thanks to the insulating material according to the invention which reduces the percentage of the voltage that is seen by the air by changing the voltage distribution and the material retains its properties / its interest for all flight altitudes of the aircraft (typically flight altitudes are between level 0 and 50,000 feet - ft).

[0155] Preferably, the aircraft electrical system 105 20 according to the invention has the particularity that the central electrical conductor 101 of the cable 100 electrically connects them:

[0156] a first terminal of an electrical voltage generator 106; and

[0157] - a first terminal of an electrical component 107. This electrical component 107 belongs to the list of electrical components including an electrical resistor, an electrical induction coil, a capacitor, an electrical machine arranged to transform electrical energy into mechanical energy, an electrical transformer.

[0158] For example, the electrical machine 107 arranged to transform electrical energy into mechanical energy can be a rotary electric motor, and more specifically:

[0159] - a drive motor for a wheel 107 of an aircraft landing gear (this type of motor 107 requires high electrical voltages to move the aircraft); or

[0160] a drive motor 108 of an aircraft propulsion system (for example a fully electric propulsion system or a hybrid propulsion system, i.e. driven both with energy in electrical form and in another form such as a hydrocarbon). Preferably, said cable 100 of the aircraft electrical system 105 20 is a first cable which electrically connects said first electrical terminals together.

[0161] The aircraft electrical system 105 comprising at least one second cable, the first and second cables, when observed in cross-section, having identical structures to each other, the second cable having a central electrical conductor of the second cable which electrically connects them:

[0162] a second terminal of the electrical voltage generator 106; and

[0163] - a second terminal of said electrical component 107 or of another electrical component belonging to said list of electrical components.

[0164] In other words, the first and second cables are structurally identical to each other when these cables are observed in cross-section (the cross-section of the first cable is structurally identical with a cross-section of the second cable).

[0165] Apart from their respective lengths, these first and second cables are identical to each other in terms of dimensions, structure and constituent materials.

[0166] The electrical voltage generator 106 is preferably adapted to deliver an electrical voltage greater than 300 V between its first and second terminals, which are electrically connected to each other via the conductors of the first and second cables. In summary, the presence of hollow inorganic particles 12 in the insulating sheath 102 of the cables allows for an insulating sheath 102:

[0167] whose partial discharge onset voltage is increased (lower permittivity of the insulator with hollow particles) and

[0168] whose material's resistance to partial discharges (in the event of partial discharges) is improved (use of inorganic particles less subject to degradation under partial discharges); and

[0169] - whose density is reduced compared to the same insulating sheath not containing these hollow particles (the density of a hollow particle is less than the density of the polymer or polymer blend of the insulating sheath); and

[0170] - whose breakdown voltage is higher (at constant thickness of the insulating sheath) than an identical polymer matrix not containing hollow inorganic particles.

[0171] Thus, thanks to their electrically insulating sheaths which contain hollow inorganic particles, the first and second cables 100 according to the invention are particularly suited to an aeronautical application for which the mass of the cables must be minimized while maintaining or increasing the breakdown voltage threshold that the cable can withstand at ambient pressure and temperature.

[0172] Since the breakdown voltage of the insulating material according to the invention is improved, it is therefore possible, without compromising the breakdown resistance compared to a cable of the prior art, to reduce the thickness of the insulating sheath (to reduce the linear mass of the cable) and / or increase the voltage delivered between the ends of the cable (this allows a gain in delivered power without having to increase the cross-section of the cable).

[0173] This is perfectly consistent with the current trend of increasing the voltage delivered to aircraft electrical systems, whose current voltages are frequently above 300V.

Claims

DEMANDS 1. Aircraft electrical system (105) of an aircraft (20), comprising at least one electrically powered cable (100) having a central electrical conductor (101) and an electrically insulating sheath (102) surrounding said central electrical conductor (101), the electrically insulating sheath (102) having an electrically insulating material (1), said at least one cable (100) electrically connecting, via said at least one central electrical conductor (101) of the cable (100), a first terminal of an electrical voltage generator (106) and a first terminal of an electrical component (107) belonging to the list of electrical components including an electrical resistor, an electrical induction coil, a capacitor, an electrical machine arranged to transform electrical energy into mechanical energy, a transformer,characterized in that the electrically insulating material (1) comprises a polymer matrix (11) containing a polymer or a mixture of polymers in which non-metallic inorganic particles (12) are dispersed, the inorganic particles being hollow.

2. Aircraft electrical system (105) of (20) according to claim 1, wherein each inorganic particle (12) has a solid hollow shell (12a), the constitutive material of the shell (12a) having an electrical resistivity greater than an electrical resistivity value of said polymer or said mixture of polymers constituting the polymer matrix (11).

3. Aircraft electrical system (105) of (20) according to claim 2, wherein the particle shells are substantially spherical or ovoid with an external diameter (Dl) between 110 nm and 3000 nm, preferably less than 1500 nm.

4. Aircraft electrical system (105) of (20) according to at least one of claims 2 or 3, wherein each solid hollow particle shell has an internal hollow zone of substantially spherical or ovoid shape with diameter (DO) between 100 nm and 1000 nm.

5. Aircraft electrical system (105) of (20) according to any one of claims 3 or 4, wherein each hollow particle shell has a maximum thickness of between 5% and 45%, preferably between 5% and 25% of the external diameter of the spherical hollow particle shell (12).

6. Aircraft electrical system (105) of (20) according to any one of claims 1 to 5, wherein the volumetric rate of hollow particles (12) in the insulating material (1) is between 1% and 80%, preferably between 10% and 80%, preferably between 40% and 70%.

7. Aircraft electrical system (105) of (20) according to any one of claims 1 to 6, wherein the polymer matrix (11) is made up of at least one of the polymers included in the group of polymers comprising a thermosetting polymer, polyester, epoxy polymer, polyimide, polyamide-imide, polyester-imide, thermoplastic polymer, polyethylene, polyurethane, elastomeric polymer, silicone-type elastomeric polymer and a mixture of at least some of these polymers.

8. Aircraft electrical system (105) of aircraft (20) according to any one of claims 1 to 7, wherein at least some and preferably all of said hollow inorganic particles (12) are made of technical ceramic.

9. Aircraft electrical system (105) of aircraft (20) according to any one of claims 1 to 8, wherein at least some and preferably all of said hollow inorganic particles (12) are composed of at least one of the elements of the group comprising Silica - SiO2 (silica is silicon dioxide), Magnesium pyrophosphate - Mg2P2O7, Zinc phosphate - Zn3 (PO4)2, aluminum oxide A12O3, aluminum nitride A1N, silicon nitride S13N4, beryllium oxide BeO, or a mixture of these elements of the group.

10. Aircraft, characterized in that it comprises the aircraft electrical system (105) of aircraft (20) according to any one of claims 1 to 9.

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