Electrical cable that limits partial discharges

Abstract

An aerospace cable that limits the occurrence of partial discharges includes an elongate electrically conductive element surrounded by a semiconductor layer and an insulation system around the semiconductor layer. This insulation system has an electrically insulating layer surrounding the semiconductor layer. Each layer of the insulation system has a radial thickness (ei . . . ), and in that an equivalent insulation thickness (teq) of this insulation system is greater than or equal to the result of a third-degree polynomial function equation of a minimum pressure and of a maximum temperature at which the cable is intended to be used.

Claims

1. An aerospace cable that limits the occurrence of partial discharges comprising: an elongate electrically conductive element; a semiconductor layer surrounding the elongate electrically conductive element; and an insulation system having at least one electrically insulating layer surrounding said semiconductor layer, wherein each layer of the insulation system has a radial thickness (ei . . . ), and in that an equivalent insulation thickness (teq) of this insulation system is greater than the result of a polynomial function equation of a minimum pressure and of a maximum temperature at which the cable is intended to be used.

2. The cable according to claim 1, wherein the polynomial equation is a third-degree polynomial equation and is a function of a partial discharge inception voltage (PDIV) in the cable under the minimum pressure and maximum temperature conditions.

3. The cable according to claim 2, wherein the partial discharge inception voltage (PDIV) under the minimum pressure and maximum temperature conditions is higher than or equal to a breakdown voltage in air between two planar electrodes, one of which is covered with an insulation system whose material and thickness are identical, respectively, to those of the insulation system of the cable.

4. The cable according to claim 1, wherein the equivalent insulation thickness t.sub.eq, in mm, is such that: $\begin{array}{cc}{t}_{\mathrm{eq}}\ge \underset{n=1}{\overset{4}{.Math.}}{a}_n.Math.{\mathrm{PDIV}}^{n-1}& \left[\mathrm{Math}.1\right]\end{array}$ with $\begin{array}{cc}{a}_n=\underset{m=1}{\overset{4}{.Math.}}{b}_{n,m}.Math.p_T^{n-1}& \left[\mathrm{Math}.2\right]\end{array}$ and $\begin{array}{cc}{p}_T=\frac{273,15}{273,15+T}& \left[\mathrm{Math}.3\right]\end{array}$ where p.sub.T is the minimum pressure in mbar or hPa T is the maximum temperature in degrees Celsius and PDIV represents an amplitude of the partial discharge inception voltage, in kV (kVpeak), under the minimum pressure and maximum temperature conditions.

5. The cable according to claim 4, wherein the coefficients are the following: TABLE-US-00005 b.sub.n, m n = 1 n = 2 n = 3 n = 4 m = 1 −0.58133 0.81629 0.02282 0.00864 m = 2 2.11632 −2.91153 −0.02497 −0.03217 m = 3 −2.88833 3.94018 0.01584 0.04238 m = 4 1.31095 −1.78528 −0.01122 −0.01814

6. The cable according to claim 1, wherein the equivalent insulation thickness t.sub.eq is determined as a function of the respective radial thicknesses of the layers forming the insulation and such that: $\begin{array}{cc}{t}_{\mathrm{eq}}=\underset{x=1}{\stackrel{\mathrm{Nombre}\mathrm{de}\mathrm{couches}\mathrm{de}l\text{'}\mathrm{isolation}}{.Math.}}\frac{e_x}{{\epsilon }_{\mathrm{rx}}}& \left[\mathrm{Math}.4\right]\end{array}$ where e.sub.x is the thickness of a layer x and ε.sub.rx is the permittivity of the layer x.

7. The cable according to claim 1, wherein the maximum temperature is at most 260° C., preferably higher than −65° C.

8. The cable according to claim 1, wherein the minimum pressure is at least 90 hPa and at most 1100 hPa.

9. The cable according to claim 1, wherein the insulation system fully or partly defines the outer periphery of the cable exposed to an exterior atmosphere which may correspond to the minimum pressure and/or to the maximum temperature.

10. The cable according to claim 1, wherein the insulation system is covered with a braid.

11. The cable according to claim 1, wherein the insulation system is covered with a braid, which is itself covered with a sheath.

12. The cable according to claim 1, wherein said cable comprises a single semiconductor layer.

13. The cable according to claim 1, wherein the insulation system comprises a layer comprising colored pigments.

14. The cable according to claim 1, wherein said cable is an aircraft cable whose maximum flight altitudes are such that the minimum pressure at which the cable is used is a function of this maximum flight altitude.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0077] The attached drawings illustrate the invention:

[0078] FIG. 1 shows a cross-sectional view of a cable according to the invention; and

[0079] FIG. 2 shows a graph of the changes in an equivalent insulation thickness (teq) of a cable according to the invention as a function of various minimum pressures and for a maximum temperature of 150° C. at which the cable is intended to be used.

DESCRIPTION OF ONE OR MORE EMBODIMENTS

[0080] For reasons of clarity, only those elements that are essential to the understanding of the embodiments described below have been presented diagrammatically, without regard to scale.

[0081] As illustrated in FIG. 1, a cable forming a single insulated electrically conductive element 1 according to one embodiment of the invention comprises an elongate electrically conductive element 2, a semiconductor layer 3 surrounding the elongate electrically conductive element 2, and an electrically insulating layer 4 surrounding the semiconductor layer 3.

[0082] The elongate electrically conductive element 2 is formed by 37 strands made of copper covered with a layer of nickel and thus has a diameter of 12 AWG (American Wire Gauge).

[0083] The material of the semiconductor layer 3 and of the insulating layer 4 comprises PFA. The semiconductor layer is formed from a polymeric mixture A comprising at least 60% by weight of perfluoroalkoxy alkane (PFA) copolymer in relation to the total weight of the polymeric mixture. The polymeric mixture A is, for example, sold under the reference S185.1 B by PolyOne.

[0084] The electrically insulating layer is formed from a second polymeric mixture B comprising at least 95% by weight of perfluoroalkoxy alkane (PFA) copolymer in relation to the total weight of the polymeric mixture. The polymeric mixture B is, for example, sold under the reference AP-210 by DAIKIN. The permittivity of this material obtained by polymerizing the polymeric mixture B is 2.1.

[0085] The polymeric mixtures A and B were each introduced into one of the two extruders for the two-layer co-extrusion and extruded around the elongate electrically conductive element 2.

[0086] The cable according to the invention for withstanding a “pressure at flight temperature” PT is determined such that

[00005]$\begin{array}{cc}{p}_T=\frac{273,15}{273,15+T}& \left[\mathrm{Math}.2\right]\end{array}$

[0087] where p.sub.T is the minimum pressure

[0088] T is the maximum temperature. And such that the coefficients b.sub.n,m are the following

TABLE-US-00001 b.sub.n, m n = 1 n = 2 n = 3 n = 4 m = 1 −0.58133 0.81629 0.02282 0.00864 m = 2 2.11632 −2.91153 −0.02497 −0.03217 m = 3 −2.88833 3.94018 0.01584 0.04238 m = 4 1.31095 −1.78528 −0.01122 −0.01814

[0089] and allow the coefficients of the third-degree polynomial equation to be calculated for a given minimum pressure and a given maximum temperature, where

[00006]$\begin{array}{cc}{a}_n=\underset{m=1}{\overset{4}{.Math.}}{b}_{n,m}.Math.p_T^{n-1}& \left[\mathrm{Math}.2\right]\end{array}$

[0090] And when a user sets a PDIV resistance threshold expressed in amplitude (kVpeak) representing the amplitude of the partial discharge inception voltage (PDIV) under these minimum pressure and maximum temperature conditions at a given value, it is then possible, by virtue of the invention, to determine the equivalent insulation thickness t.sub.eq required for this cable by applying the equation below.

[00007]$\begin{array}{cc}{t}_{\mathrm{eq}}\ge \underset{n=1}{\overset{4}{.Math.}}{a}_n.Math.{\mathrm{PDIV}}^{n-1}& \left[\mathrm{Math}.1\right]\end{array}$

[0091] It is thus determined for a minimum pressure of 1000 mbar and a given maximum temperature of 80° C., and a PDIV resistance threshold with an amplitude of 3.946 kV (Vpeak), the equivalent insulation thickness (teq) of this insulation system must be 0.4762 mm.

TABLE-US-00002 a1 −0.06576 a2 0.09544 a3 0.00779 a4 0.00072

[0092] This equivalent insulation thickness teq for this insulation system is then translated into a radial thickness, as a function of the permittivity of the one or more materials forming the insulation system.

[0093] One exemplary embodiment of a cable that addresses the constraints of a minimum pressure of 1000 mbar and a given maximum temperature of 80° C., and a PDIV resistance threshold of 3.946 kV then has the below dimensions

[0094] mean diameter of the conductor=2.15 mm (±10%);

[0095] mean thickness of the single layer of insulation of the insulation system when the permittivity is 2.10 is of the order of 1 mm.

[0096] Another subject of the invention is a multilayer insulation system such that the equivalent insulation thickness t.sub.eq is determined as a function of the respective radial thicknesses of each of the layers forming the insulation system and such that:

[00008]$\begin{array}{cc}{t}_{\mathrm{eq}}=\underset{x=1}{\stackrel{\mathrm{Nombre}\mathrm{de}\mathrm{couches}\mathrm{de}l\text{'}\mathrm{isolation}}{.Math.}}\frac{e_x}{{\epsilon }_{\mathrm{rx}}}& \left[\mathrm{Math}.4\right]\end{array}$

[0097] where e.sub.x is the radial thickness of a layer x

[0098] and ε.sub.rx is the permittivity of the layer x.

[0099] For a thickness of the electrically insulating layer ei of 0.50 mm and a permittivity of 2.10, good agreement between the measured results and the calculated results are obtained, as the results below show. The measured partial discharge inception voltages (PDIVs) are expressed as the mean of 10 measurements and the coefficient of variation (the ratio of the standard deviation to the mean). The measured PDIV is close to the calculated PDIV and is slightly higher. Thus, the invention ensures operation without partial discharges within the specified PDIV limit without an excessive margin with regard to determining the PDIV, which would result in an overly high thickness and therefore weight.

TABLE-US-00003 Difference between Maximum Minimum Mean of the Calculated calculated PDIV and means temperature pressure measurements Coefficient PDIV of the measurements [° C.] [mbar] [Vpeak] of variation [Vpeak] [%] 40 200 1216 4% 1213 0.3% 40 400 1761 4% 1655 6.4% 40 600 2227 3% 2218 0.4% 40 800 2616 4% 2536 3.2% 40 1000 2871 5% 2802 2.5%

[0100] For a thickness of the electrically insulating layer ei of 1 mm and a permittivity of 2.10, the below results are obtained. Here again, good agreement between the measured results and the calculated results are demonstrated for various minimum pressures and various maximum temperatures.

TABLE-US-00004 Difference between Maximum Minimum Mean of the Calculated calculated PDIV and means temperature pressure measurements Coefficient PDIV of the measurements [° C.] [mbar] [Vpeak] of variation [Vpeak] [%] 40 200 1716 6% 1655 3.7% 40 400 2665 2% 2433 9.5% 40 600 3336 3% 3326 0.3% 40 800 4048 3% 3820 6.0% 40 1000 4642 3% 4225 9.9% 80 200 1710 4% 1601 6.8% 80 400 2494 3% 2231 11.8% 80 600 3174 5% 3046 4.2% 80 800 3725 4% 3655 1.9% 80 1000 4149 3% 3946 5.2%

[0101] In another example, for a PDIV threshold of 1000 Vrms, i.e. 1414 Vpeak in amplitude, for a maximum flight altitude of 35 k ft, corresponding to exposure to a minimum pressure of 238 mbar and a maximum temperature of 150° C., a teq=0.367 mm corresponds to an electrically insulating layer formed with 95% by weight of perfluoroalkoxy alkane (PFA) copolymer with a radial thickness of 0.79 mm.