Electrical cable that is resistant to partial discharges

Abstract

An electrical cable (1) is provided having (1) a conductive element (2), a first layer (3) having polyimide (PI) surrounding said conductive element (2), a second fluorinated layer (4) having at least one fluorinated compound, surrounding the first layer, and optionally at least one fluorinated semiconductor layer having at least one fluorinated compound, where the total thickness of the assembly of fluorinated layers is at least 0.4 mm.

Claims

1. Electrical cable comprising: a conductive element having a nominal cross section in the range of 0.95 mm.sup.2 to 70 mm.sup.2; a first layer comprising polyimide (PI) surrounding said conductive element; a second fluorinated layer of polytetrafluoroethylene (PTFE) surrounding said first layer; and optionally at least one fluorinated semiconductor layer comprising at least one fluorinated compound, wherein the total thickness of the assembly of PTFE layers is at least 0.4 mm, and that the thickness ratio of the fluorinated layer(s) to the PI layer ranges from 4 to 22, and wherein the combination of the materials selected for said first layer of PI, said second fluorinated layer, and optionally at least one fluorinated semiconductor layer and the nominal cross section in the range of 0.95 mm.sup.2 to 70 mm.sup.2 of said conductive element are such that the combined insulations layers results provide said cable with a partial discharge inception voltage above 800 volts.

2. Electrical cable according to claim 1, wherein the total thickness of the assembly of fluorinated layers is at least 0.5 mm.

3. Electrical cable according to claim 1, in which the fluorinated compound is PTFE.

4. Electrical cable according to claim 1, in which the second layer is in the form of one or more tapes and of extrudate, or a combination thereof.

5. Electrical cable according to claim 4, in which the second layer comprises one or more PTFE tapes, covered with an extruded layer of one of said fluorinated compounds.

6. Cable according to claim 1, in which the second layer is totally sintered.

7. Electrical cable according to claim 1, in which said semiconductor layer is positioned at the surface around the second layer, or between the first layer and the second layer, or between the conductive element and the first layer, or a combination thereof.

8. Electrical cable according to claim 1, in which the semiconductor layer is in the form of a tape, an extrudate or a varnish, or a combination thereof.

9. Electrical cable according to claim 8, wherein, when the semiconductor layer is in the form of a tape or an extrudate, said semiconductor layer is composed of fluorinated polymer or copolymers comprising, on a weight basis relative to the total weight of said semiconductor layer, from 0.1% to 40% of (electrically) conductive filler.

10. Electrical cable according to claim 8, wherein, when the semiconductor layer is in the form of a varnish, said semiconductor layer is composed of fluorinated components comprising, on a weight basis relative to the total weight of said semiconductor layer, from 0.1% to 40% of (electrically) conductive filler.

11. Electrical cable according to claim 1, wherein the semiconductor layer has a longitudinal resistivity of from 0.04 to 100 ohm.Math.m.

12. Electrical cable according to claim 1, wherein the thickness of the first layer ranges from 0.028 mm to 0.1 mm.

13. Electrical cable according to claim 1, wherein at least one adhesive layer is arranged on: at least one of the two faces of the first layer, or between the conductive element and the semiconductor layer when said adhesive layer is between the conductive element and the first layer, or a combination thereof.

14. Electrical cable according to claim 13, wherein the adhesive layer is composed of one or more fluorinated polymers.

15. Electrical cable according to claim 14, wherein the fluorinated polymer(s) of the adhesive layer are selected from the group consisting of: poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP), perfluoro(alkyl vinyl ether)/tetrafluoroethylene (PFA), polytetrafluoroethylene (PTFE), and poly(ethylene-co-tetrafluoroethylene) (ETFE), or a combination thereof, the abovementioned fluorinated compounds having adhesive properties.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) For better understanding of the invention, the description will make reference to the attached drawings, which are given merely as non-limiting illustrations.

(2) In these drawings:

(3) FIG. 1 is a view in cross section of an electrical cable at the insulation stage (without sheath) according to one preferred embodiment of the invention; and

(4) FIG. 2 is a graph showing the discharge inception voltage (PDIV) at 150 C. for a pressure of 150 mbar as a function of the thickness in millimeters of the second PTFE layer and for various types of cable, and of nominal conductive cross sections from 0.95 to 70 mm.sup.2.

DETAILED DESCRIPTION

Implementation Examples

Example 1: Example of Composition of a Cable According to the Invention (FIG. 1)

(5) For reasons of clarity, only the elements that are essential for understanding the invention have been represented schematically, without being to scale, in FIG. 1.

(6) According to a first embodiment, the hook-up wire or the power cable 1, represented in FIG. 1, comprises: a central conductive element 2, especially made of copper or aluminum, of multistrand type, and, successively and coaxially around this element, a first adhesive layer made of FEP 5a, a layer of polyimide PI 3 known as the first layer, a second adhesive layer 5b and a PTFE layer 4, known as the second layer, representing here the outer layer of the cable 1. The various layers are obtained by taping. The cable is then heat-treated in order to sinter the PTFE outer layer. To do this, a temperature above 340 C. is applied.

(7) In this example, the thickness of PI is 0.058 mm and the thickness of the PTFE second layer after sintering is about 0.56 mm, such that the ratio PTFE/PI=0.56/0.058=9.65.

Example 2: Process and Another Composition of a Cable According to the Invention

(8) According to another embodiment, an electrical cable comprises an electrical, conductor, for example made of copper or copper alloy coated with a layer of nickel, generally of multistrand type.

(9) Said electrical conductor is covered with an adhesive FEP layer, which is itself covered with a PI layer, which is itself covered with another adhesive FEP layer. This FEP/PI/FEP assembly preferably corresponds to the tape Kapton FN from DuPont comprising a layer of PI 25.1 m thick coated on each of its faces with a layer of FEP 2.5 m thick.

(10) The FEP/PI/FEP assembly is then surrounded by a layer, comprising a winding of three PTFE tapes: a first PTFE tape having a thickness before sintering of about 180 m, a second PTFE tape having a thickness before sintering of about 180 m and a third PTFE tape which advantageously comprises a pigmented PTFE layer (3%) and has a thickness before sintering of about 76 m. The pigment of the third PTFE tape is a metal complex. This allows marking by UV laser of the surface of the outer layer of said third tape. Generally, the pigments do not represent more than 5% by weight of said third tape. It is preferable not to exceed this value of 5%, or even to minimize it so as not to degrade the electrical properties of the cable.

(11) After laying (or taping) of the first PTFE tape, the electrical conductor thus insulated is heat-treated in an oven at a temperature above the melting point of the PTFE, namely at a temperature above 340 C., to obtain sintering of the PTFE. Via this single heat-treatment step, which comprises the step of heat-welding of the polyimide and the step of sintering of the PTFE, the adhesion of all the thicknesses of tapes is ensured. Specifically, the heat treatment leads to the cohesion of the PTFE tape on the PI tape and the bonding of the PI tape to itself and to the conductive element.

(12) After this baking step, the second and then the third tape are laid.

(13) Next, the step of marking of the third tape (UV laser marking) is performed according to techniques known to those skilled in the art.

(14) Finally, a second heat treatment at a temperature above 340 C. is performed, so as to sinter the second and third PTFE tapes (i.e. the tapes laid after the first heat treatment). The various heat-treatment steps are generally performed in an oven or an array of ovens.

(15) After baking, the total thickness of the PTFE layer is about 0.68 mm.

(16) Next, the cable is advantageously covered with a metal shield (braiding of metal wires) and a composite sheath. The latter two elements are known to those skilled in the art.

Example 3: Test of Resistance of Cables Obtained According to Example 2 for Various Thicknesses of PTFE and Various Nominal Cross Sections of Conductors (FIG. 2)

(17) The discharge inception voltage was measured for various cables. These cables were prepared according to the process of Example 2. The first heat treatment takes place after laying the first tape and the second and final heat treatment takes place after laying the last tape. The characteristics of the cables at the insulation stage (without metal braiding and sheath) according to the invention are represented in the table below:

(18) The mean thickness of Kapton FN represents the thickness measured on the cable once manufactured, whereas the thickness of the PTFE tapes corresponds to the thickness of the PTFE tapes used (before manufacture of the cable) and the cumulative thickness corresponds to the thickness measured once the cable has been manufactured.

(19) TABLE-US-00001 Nature of the Mean conductor/ thickness Thicknesses Cumulative nominal of Kapton of PTFE PTFE cross section FN tapes (m) thickness Ex. (mm.sup.2) (mm) 1 2 3 4 5 6 (mm) 1 NPC/0.95 0.060 64 64 0.19 2 NPC/0.95 0.060 76 76 76 76 0.50 3 NPC/0.95 0.060 76 76 76 76 76 76 0.72 4 NPC/1.45 0.062 100 100 0.32 5 NPC/1.45 0.062 100 0.16 6 NPC/1.45 0.069 100 76 76 76 76 0.653 7 NPC/4.1 0.062 100 76 76 76 76 0.61 8 NPC/4.1 0.062 100 0.16 9 NPC/6.6 0.069 180 180 76 0.683 10 NPA/27.1 0.076 76 76 0.224 11 NPA/27.1 0.074 180 100 0.466 12 NPA/27.1 0.058 180 180 76 0.557 13 NPA/42.9 0.060 180 180 76 0.497 14 NPA/70 0.056 130 180 76 0.659 Examples 1 to 9 are hook-up wires and Examples 10 to 14 are power cables. NPC: nickel-plated copper NPA: nickel-plated aluminum

(20) For each sample of 10005 mm, a cable loop is made. The diameter of the loop is between 8 and 12 times the outside diameter of the cable.

(21) The cable braiding is undone over about 5 mm to allow it to be connected to the earth.

(22) The cable loop is positioned in an oven configured so as to allow the application of a vacuum, a voltage and connection to the measuring system (oscilloscope).

(23) When the temperature and negative pressure conditions have been reached and stabilized, a 50 Hz alternating voltage is applied between the conductor and the braid.

(24) The voltage is increased by 50 volts/s up to the inception of partial discharges. The corresponding voltage (partial discharges inception voltage) is recorded.

(25) One of the functionalities of the oscilloscope makes it possible to count the number of discharges exceeding a predefined gauge (5 pC).

(26) The definition of the partial discharge inception voltage is that voltage that is reached when at least one discharge per second takes place over a period of 30 seconds.

(27) The voltage is then increased by 100 volts above the PDIV before redescending in order to determine the partial discharge extinction voltage, which is defined by the oscilloscope as the voltage at which the last discharge was detected.

(28) As shown by graph 2, the cables have a cumulative thickness of layers comprising PTFE of greater or equal to 0.4 mm, can withstand a higher discharge inception voltage (PDIV) than cables with a total thickness of layers comprising PTFE of less than 0.4 mm, irrespective of the cross section of the conductor or of its nature. A PDIV limit above 800 V, the peak value, which is the minimum voltage without discharges currently experienced for the application of 230 V of hook-up wires and power cables, was considered.

(29) These tests thus show that a thickness of layer(s) comprising PTFE of at least 0.4 mm makes it possible to obtain cables that show good resistance to partial discharges at a high temperature of 150 C. and at a low pressure of 150 mbar.

(30) Although the invention has been described in connection with a particular embodiment, it is obvious that it is not in any way limited thereto and that it includes all the technical equivalents of the means described and also combinations thereof if they fall within the context of the invention.