Electrical conductor provided with a surrounding electrical insulation

Abstract: An electrical conductor is provided with an electrical insu­ lation (11) surrounding the conductor (10) and a surrounding protective layer (12) which protects the insulation against degradation caused by corona. The protective layer consists of an organic polymer containing at least 10 per cent by volume of a powdered filler material in the form of chromium oxide, iron oxide, or a mixture of chromium oxide and iron oxide. The filler preferably has an intrinsic resistivity of 10⁴ - 10⁸ ohm m. The thickness of the protective layer is preferably smaller than the thickness of the underlying insulation (11) and its resistivity preferably exceeds 10¹⁰ ohm m. The conduc­ tor is used, among other things, in windings and coils for electrical machines.

Description: The invention relates to an electrical conductor provided with a surrounding electrical insulation according to the precharacterising part of Claim 1. The US-A-4,493,873 and US-A-4,537,804 describe an electrical conductor which is provided with an electrical insulation consisting of two layers arranged on the conductor, one consisting of a filler-free organic polymer and one of an organic polymer containing finely-divided aluminium oxide. The incorporation of the aluminium oxide in the insulation will increase the resistance of the insulation against partial silent discharges, called corona. The US 32 28 883 describes electrical conductors which are insulated with one single layer of insulating material. To increase the corona resistivity of the polymeric insulating material it is suggested to add mineral filler material to the insulating polymer material. A large variety of such minerals are mentioned such as oxides, carbonates, sulphates and silicates. Among these filler materials is also mentioned anhydrous iron oxide without disclosing the specific kind of the iron oxide. Nothing is mentioned about the intrinsic resistivity of the filler material, but it has to be assumed as high in order not to degrade the insulating property of the insulating material of the conductor. The DE 20 50 581 discloses an electrical insulating material, particularly destined for electrical apparatus, of polymeric compounds with the track resistance being increased by the addition of aluminium oxide hydrate, particularly trihydrate, Al 2 O 3 x3H 2 O. In order to be able to reduce the necessary amount of aluminium oxide hydrate it is suggested to add a second component which may be selected from a large variety of metal oxides among them Fe 2 O 3 and Cr 2 O 3 . In any case this second component brings about a beneficial effect on the track resistance only when applied along with the mentioned main component of Al 2 O 3 x3H 2 O. These measures are in no way related to the protection of the insulating material against corona discharge attacks. The GB 21 08 130 describes an electric cable consisting of a conductor surrounded by an insulation layer the latter being surrounded by the cable sheath. Both, the insulating material and the sheath consist of polymer material. In order to reduce the occurrence of fume emission from the insulating material and the sheath in the event of fire and to render these fumes more transparent, it is suggested to add to both, the insulating material and the sheath, chromic anhydride. The invention aims at developing an electrical conductor of the above-mentioned kind whose insulation proves markedly more corona-resistant than previously known conductor insulations without increasing the total thickness of the overall insulation surrounding the conductor. To achieve this aim the invention suggests an electrical conductor provided with a surrounding electrical insulation according to the introductory part of Claim 1, which is characterized by the features of the characterizing part of Claim 1. Further developments of the invention are characterized by the features of the additional claims. According to the present invention, it has been found to be possible to provide an insulated conductor which withstands corona for a considerably longer period of time than the known conductor. According to the invention, a layer of an organic polymer is arranged outside the insulation containing a sufficient quantity of at least one of the substances chromium oxide and iron oxide in powder form. The powdered filler material has an intrinsic resistivity of 10 4 - 10 8 ohm m. The protective layer containing the filler preferably has a thickness which is smaller than the thickness of the underlying insulation layer, i.e. of the insulation located between the conductor and the protective layer. The protective layer preferably has a resistivity of above 10 10 ohm m. The aluminium oxide used in the described, known case has a considerably higher intrinsic resistivity than the filler material according to the invention. It is therefore surprising and has been completely unpredictable that the filler material according to the invention, which is a more low-resistive insulating materials than aluminium oxide, gives a better insulation than the corresponding insulation with aluminium oxide. One possible explanation of the favourable results obtained according to the invention may be that the corona sputters away insulating polymer material from the surface layer to which chromium oxide or iron oxide has been added, and that the chromium oxide or iron oxide thus exposed, because of its lower resistivity, increases the electrical conductivity locally on the surface sufficiently for the extremely concentrated effect of a corona discharge to spread over a larger surface area thus being reduced in its effect. In order for the dielectric losses in the insulation to be maintained at a low level, the layer of chromium oxide or iron oxide is arranged preferably in a thickness which is smaller than the thickness of the rest of the insulation. The electrical conductor according to the invention has the shape of a wire, rod, tape or bar, i.e. is elongated with a constant cross-section of circular or otherwise curved or multi-corner shape. It can be used to advantage in windings for electrical machines, electrical transformers and other electrical high voltage equipment of varying types in which the insulation of the conductor is often subjected to corona. Chromium is a preferred filler material because of its property to give the protective layer a corona resistance which is not - or only insignificantly - reduced by the influence of external factors, such as moisture. The particle size of the powdered filler is suitably 0.005-30 µm (micrometer) and preferably 0.005-5 µm. The content of the powdered filler suitably amounts to 10-40% of the volume of the protective layer. Particularly preferred is a content of powdered filler of 10-30% of the volume of the protective layer. The organic polymer in the protective layer may, among other things, consist of a wire enamel of such types as are normally used when enamelling winding wire such as terephtalic acid alkyds, polyester imides, polyamide imides, polyimides, polyurethanes, epoxy resins, polysulphons, silicones, polyamides, and polymers based on polyhydantoin. The insulation surrounded by the protective layer may consist of filler-free resins, as specified in the preceding paragraph. However, it may also consist of a prefabricated polymer film, for example a film of polyamide imide, polyimide, polypropylene, polymethyl pentene, polyethylene glycol terephthalate, polycarbonate, or polysulphon wound helically around the conductor or wrapped around the conductor with a longitudinal joint, or of a wrapping of a glass fibre wire which is impregnated with a resin, for example an epoxy resin, and applied around the conductor, or a wrapping of a tape built up of a felt of fibres of an organic polymer such as aromatic polyamide (e.g. NOMEX R 410 from Du Pont, USA). The protective layer may be arranged as the outermost layer on the conductor. However, it may be provided with a coating if special demands are placed on the surface of the insulated conductor, such as diffusion density or surface fineness, and therefore a relatively thin filler-free organic polymer is applied as surface layer. The organic polymer may be of a kind as exemplified for the organic polymer in the protective layer. In certain cases, such as when there is a risk that the insulation may become detached from the surface of the conductor, for example because of the effect of oxygen diffused therein, it may be suitable to apply a layer of the same kind as the protective layer nearest the conductor, i.e. inside the insulation surrounded by the protective layer. The thickness of the protective layer suitably amounts to 5-100 µm and preferably to 10-50 µm, the thickness of the insulation located between the conductor and the protective layer suitably to 10-500 µm and preferably to 10-200 µm. The invention will now be described in greater detail with reference to the accompanying drawings showing - by way of example - in Figures 1 to 4 different embodiments of an insulated electrical conductor according to the invention, Figure 5 a side view of a coil for an electrical machine, built up with a conductor according to the invention, Figure 6 a cross-section of the coil in Figure 5. Example 1 According Figure 1, a wire 10 with a cross section of 10 mm x 2 mm is coated with polyester-imide wire enamel, for example Terebec R 533 L-33 from BASF (Fed. Rep. of Germany) in a conventional manner using multipass coating and wiping dies or felts, the wire passing twelve times through the wiping devices and a heating device providing a temperature of about 400° C for curing of the enamel. The increase in thickness upon each passage is approximately 5 µm. The finished layer 11 thus has a thickness of about 60 µm. On top of the layer 11 there is applied, using equipment of the same kind with four passages of wiping devices and associated heating devices, a protective layer 12 consisting of an epoxy resin and chromium oxide (Cr 2 O 3 ) with a particle size of less than 5 µm. The protective layer has a thickness of 20 µm and the chromium oxide constitutes 25% of its volume. The epoxy resin used for the application of the protective layer may consist of 100 parts by weight of an epoxy resin of bisphenol A-type containing 5.15-5.5 moles epoxy groups per kg (e.g. Epikote 828 from Shell, Holland), and 50 parts by weight curing agent of amino type with an amino value of 400-460 mg KOH/g, dissolved in equal parts xylene and butanol. The epoxy resin contains 25 per cent by volume chromium oxide, calculated on the solid constituents of the resin without solvent. The thickness of the protective layer 12 constitutes 33% of the thickness of the insulation 11. Example 2 On a rectangular wire 10 with the cross-section of 8 mm x 2.5 mm, to which a wire enamel layer 11 of the same kind as described in Example 1 has been added, there is applied a protective layer 12 consisting of a polyesterimide containing chromium oxide of the same kind as described in Example 1. The protective layer 12 has a thickness of 20 µm and 30% of its volume consists of chromium oxide. The polyesterimide enamel used for the application of the protective layer may be of the same kind as that in the wire enamel layer 11, however chromium oxide and further cresol being added to the polyesterimide enamel so that the viscosity becomes suitable for the application of the protective layer using the same method as described in Example 1. The enamel then contains 30 per cent by volume chromium oxide, calculated on the contents of solid substance of the enamel without solvent. The thickness of the protective layer 12 constitutes 33% of the thickness of the insulation 11. Example 3 On a rectangular cross-sectional wire with the cross-section 8 mm x 2.5 mm, to which a wire enamel layer 11 of the same kind as described in Example 1 has been added, there is applied a protective layer 12 consisting of a polyamideimidecontaining chromium oxide of the same kind as described in Example 1. The protective layer 12 has a thickness of 20 µm and 25% of its volume consists of chromium oxide. The polyamideimide enamel used for the application of the protective layer may consist of, for example Lack E 3561/27 from Herberts GmbH, Fed. Rep. of Germany, to which chromium oxide and N-methylpyrrolidone have been added so that the viscosity becomes suitable for the application of the protective layer using the same method as described in Example 1. The enamel then contains 25 per cent by volume chromium oxide, calculated on the solid constituents of the enamel without solvent. The thickness of the protective layer 12 constitutes 33% of the thickness of the insulation 11. Example 4 A round wire 13 according to Figure 2 with a diameter of 1 mm is enamelled with a polyamideimide wire enamel, for example Lack E 3560/32 form Herberts GmbH, Fed. Rep. of Germany. It is applied in the manner described in Example 1 using dies to form an enamel layer 14 having a thickness of 40 µm. On the layer there is applied a protective layer 15 of the same kind as the protective layer 12 in Example 3 but having a thickness of 10 µm and having a percentage by volume of chromium oxide of 15%. The application of the protective layer is also carried out using dies. The thickness of the protective layer 15 constitutes 25% of the thickness of the insulation 14. Example 5 Around a rectangular cross-sectional wire 16 according to Figure 3 with a cross-section of 10 mm x 4 mm there is wound, with half an overlap, a tape of a polyimide film (e.g. Kapton R Type F from Du Pont, USA) having a thickness of 25 µm, which is provided on the side facing the conductor with a 12 µm thick layer of fluoridated ethyleneporpylene polymer (e.g. Teflon FEP R from Du Pont, USA). This results in providing a 12 µm thick layer 17 of the fluoridated polymer nearest the conductor and outside of this a layer 18 of polyimide-fluorinated polymer-polyamide with a thickness of 62 µm. Overlapping parts of the tape are fixed to one another, and parts of the tape making contact with the conductor 16 are fixed to the conductor by the fluorinated ethylene propylene by the film being heated to a temperature of 270° C in a heating device suited for the purpose. Around the conductor thus insulated there is applied a protective layer 19 of the same kind and thickness as the protective layer 12 in Example 1 and in the manner specified there, with the difference that the percentage by volume of chromium oxide amounts to 15% instead of 25%. The thickness of the protective layer 12 constitutes 20% of the thickness of the insulation (layers 17 and 18). Example 6 A rectangular cross-sectional wire 10 according to Figure 1 with the cross-section 10 mm x 2 mm is provided with an insulating layer 11 in the form of a wrapping 11 of glass yarn which has been impregnated with a binder in the form of an epoxy resin, which has thereafter been cured. The epoxy resin may consist of a polyamide-modified epoxy resin (such as AF-42 from Minnesota Mining and Manufacturing Company, USA), or of an epoxy resin of the kind described in Example 1. The insulating layer 11 has a thickness of 100 µm. Around the layer 11 there is applied a protective layer 12 of the same kind as the protective layer 12 in Example 1 and in the manner specified there, but with the difference that the percentage by volume of chromium oxide amounts to 10% instead of 25%, and the thickness of the protective layer 12 amounts to 40 µm instead of 20 µm. The thickness of the protective layer 12 constitutes 40% of the thickness of the insulation 11. Example 7 A rectangular cross-sectional wire according to Figure 4 with the cross-section 10 mm x 2 mm is enamelled with a layer 11 of a polyesterimide enamel in the manner described in Example 1 into a thickness of 60 µm and is provided with a protective layer 12 of epoxy resin with addition of chromium oxide also in the manner described in Example 1. On the protective layer 12 there is applied a coating 20 in the form of a polyamideimide enamel, for example Lack E 3561/27 form Herberts GmbH, Fed. Rep. of Germany, in a thickness of 20 µm. The coating can be applied in accordance with the method using multipass coating and wiping dies or felts, the wire passing through wiping equipment and associated heating equipment with a temperature of about 400° C four times with an increase in thickness of the coating of about 5 µm upon each passage, so that the finished coating attains a thickness of about 20 µm. Instead of chromium oxide (Cr 2 O 3 ) in the cases exemplified above there can be used an equally great percentage by volume of iron oxide (Fe 2 O 3 ) with the same size of the grains, or an equally great percentage by volume of a mixture of iron oxide and chromium oxide in arbitrary proportions, for example in equal parts by weight. A conductor according to the present invention is excellently well suited for use in coils for electrical machines for operating voltages in excess of 3 kV. In such coils there are arranged around the conductor insulation layers of mica to render the conductor insulation corona-resistant. When using conductors according to the present invention in such coils it is possible to make the insulation and the corona protection around the conductors considerably thinner than with the described previously known corona-resistant conductor insulation. This results in an increased percentage of conductor material in a coil of given dimensions, and consequently in an increase in power of a machine of given dimensions or in reduced dimensions for a machine of a certain power. Such a coil comprises a bundle of several adjacently arranged lengths of a conductor according to the invention with rectangular cross-section and a main insulation surrounding the conductor bundle for insulating the bundle against the walls of the machine slot. The conductor bundle may consist of several turns of one single insulated conductor which has been bent while forming the final shape of the bundle. It may also consist of several separate insulated conductors, which at their ends are electrically connected to each other, usually in parallel. Particularly great advantages are obtained according to the invention for the former type of conductor bundle, since the stresses between adjacent conductors are in that case greater and a short-circuit caused by a damaged conductor insulation is more serious. The main insulation may be formed in different, previously known ways. Thus, it may consist of a wrapping of mica tapes or mica sheets. These may be made of large mica flakes of conventional kind, which are attached to a carrier material of paper, woven glass fibre or the like, for example, with a resinous binder such as an alkyd resin or a thin thermoplastic film. The mica materials may also consist of self-supporting tape or sheet of small mica flakes, overlapping each other, manufactured by splitting of ordinary mica, these mica materials then usually also being attached to a carrier material. On the mentioned mica tapes or mica sheets there may be applied a resinous binder which subsequently, when the materials are applied around the conductor bundle, binds the various layers in the conductor insulation to one another. Such a binder, however, may be supplied after the insulating material has been applied around the conductor bundle by an impregnation. As examples of suitable binders for the turns in the wrapping may be mentioned solvent-free resins such as epoxy resins and unsaturated polyester resins. A conductor of a kind described in any of Examples 1 to 3 and 5 to 6 is bent into a coil containing several turns of the conductor positioned adjacent to each other, as shown in Figure 5 and Figure 6, respectively. The mentioned turns positioned adjacent to each other form the conductor bundle 21 of the coil. As will be clear from Figure 5, the bending is carried out such that the coil has straight parts 22 and 23 intended to be positioned in the slots 24 of the machine, as well as bent end parts 25 and 26 which extend outside the slots. The terminals of the coil are designated 27 and 28. The conductor bundle 21 is then wound turn-by-turn with, for example, half an overlap with a 25 mm wide mica tape consisting of a 0.09 mm thick self-supporting layer of small mica flakes which overlap each other and which are fixed to an 0.04 mm thick woven glass fibre with an 0.006 mm thick polyethyleneglycol terephthalate film. Such an insulating tape is described in SE-A-200 820. After the bundle has been provided with a wrapping 29 of, for example, twenty layers of the mica tape placed on top of each other, the winding is first dried at a pressure of 0.1 mm Hg and at a temperature of 40° C, whereupon an impregnating resin is supplied at this pressure. When all impregnating resin has been supplied, the pressure is raised to, for example, 10 kg/cm 2 . The resin may consist of 100 parts by weight of an epoxy resin, which in a known manner has been produced from epichlorhydrin and 4.4'-dioxydiphenyl-dimenthylmethane and which has an epoxy equivalent of 192, and 100 parts by weight of a curing agent consisting of a mixture of 75 parts by weight hexahydrophthalic acid anhydride and 25 parts by weight tetrahydrophthalic acid anhydride. In order for the resin not to penetrate out of the insulation during a subsequent curing operation, the impregnated conductor bundle with the mica tape wrapping can be surrounded by a sealing tape of polytetrafluoroethylene or the like. The coil is then placed in a forming tool for curing of the resinous material. The curing takes place at a temperature of about 160° C for a period of about 10 hours. In an alternative embodiment a coil is manufactured using an insulating tape which is impregnated with impregnating resin prior to its wrapping around the bundle 21 (Figure 6), which may be of the same kind as in the example above. The resin contains an unsaturated polyester resin manufactured from adipic acid (11 mole per cent), phthalic acid anhydride (11 mole per cent), maleic acid anhydride (23 mole per cent) and ethylene glycol (55 mole per cent) to which have been added, on the one hand, diallyl phthalate in such an amount that the diallyl phthalate constitutes 40% of the total weight of polyester resin and diallyl phthalate and, on the other hand, bensoyl peroxide in an amount corresponding to 1% of the entire weight of the resin. The polyester resin itself can be manufactured by reaction of a mixture of the mentioned acids and alcohol in an inert atmosphere by increasing the temperature to 220° C and maintaining this temperature until the acid value of the reaction product is about 30. Prior to the impregnation of the insulating tape, 100 parts by weight of the resin is diluted with 100 parts by weight acetone. The insulating mica tape, which may be of the same kind as that described in Example 1, can be impregnated with the resinous material at room temperature and at atmospheric pressure. The impregnated mica tape is then wound, after driving off the solvent, around the conductor bundle 21 with half an overlap into thirty layers placed one above the other to form the main insulation 29. The coil is then placed in a forming tool for curing the resinous material at a temperature of 160° C for a period of three hours. Although many unsaturated polyester resins and epoxy resins suited for impregnation of electrical insulations are known, a few additional examples of such resins will be mentioned. Thus, there may be used, for example, a polyester resin consisting of 60 parts by weight of a reaction product of 3 moles maleic acid anhydride, 1 mole adipic acid, and 4.4 moles ethylene glycol with an acid value of 30, and of 40 parts by weight diallyl phthalate and containing 0.75% bensoyl peroxide, and a polyester resin consisting of 70 part by weight of a reaction product of 1 mole fumaric acid, 1 mole phthalic acid and 2.2 moles propylene glycol with an acid value of 25, and of 30 parts by weight monostyrene and containing 0.5% bensoyl peroxide, as well as an epoxy resin consisting of 100 parts by weight "Epon 828" (Shell Chemical Co.) and 65 parts hexahydrophthalic acid anhydride, an epoxy resin consisting of 85 parts by weight "Araldit F", 100 parts by weight "Curing agent 905" (both form Ciba AG, Switzerland) and 15 parts by weight phenylglycidylether, an epoxy resin consisting of 100 parts by weight "DER 331" (Dow Chemical Co.) and 65 parts by weight tetrahydrophthalic acid anhydride or 100 parts by weight epoxynovolak "DEN 438" (Dow Chemical Co.) and 3 parts by weight of the boron trifluoride complex "Hz 935 J 50" (Ciba Geigy AG).

Claims: Electrical conductor (10,13,16) provided with a surrounding electrical insulation (11,14,17,18) and a protective layer (12,15,19) surrounding said insulation to protect it against degradation caused by corona, said protective layer being built up of an organic polymer and a powdered filler, characterized in that said protective layer contains at least 10 per cent by volume of a powdered filler consisting of iron oxide (Fe 2 O 3 ), or chromium oxide (Cr 2 O 3 ) or a mixture of iron oxide (Fe 2 O 3 ) and chromium oxide (Cr 2 O 3 ), and that said powdered filler material in the protective layer has an intrinsic resistivity of 10 4 - 10 8 ohm m. Electrical conductor according to Claim 1, characterized in that the protective layer (12,15,19) has a thickness which is smaller than the thickness of the insulation (11;14;17,18). Electrical conductor according to claim 1 or 2, characterized in that the protective layer (12,15,19) has a resistivity of more than 10 10 ohm m. Electrical conductor according to any of the preceding Claims, characterized in that the content of chromium oxide, iron oxide or a mixture of chromium oxide and iron oxide constitutes 10-40% of the volume of the protective layer (12). Electrical conductor according to any of Claims 1 to 3, characterized in that the content of chromium oxide, iron oxide or a mixture of chromium oxide and iron oxide constitutes 10-30% of the volume of the protective layer (12). Electrical conductor according to any of the preceding Claims, characterized in that on top of the protective layer (12) there is arranged a surface layer (20) of a filler-free organic polymer. Electrical conductor according to any of the preceding Claims, characterized in that a layer of the same kind as said protective outer layer is applied nearest the conductor, i.e. inside the insulation surrounded by the protective layer. Electrical conductor according to any of the preceding Claims, characterized in that the thickness of the protective layer (12,15,19) amounts to at most 30% of the thickness of the insulation (11;14;17,18). Electrical conductor according to any of the preceding Claims, characterized in that the organic polymer in the protective layer (12,15,19) consists of a cured resinous binder and the insulation (11;14;17,18) of one or more layers of an organic polymer. Electrical conductor according to Claim 8, characterized in that the organic polymer in the insulation (11;14;17,18) consists of an organic resinous binder. Use of a conductor (10,16) according to any of the preceding claims in a coil designed to be arranged in slots (24) in the stator or rotor of an electrical machine, which coil comprises a bundle (21) of adjacently located lengths of said conductor having rectangular cross-section and a main insulation (29) surrounding said bundle for insulation of the bundle against the walls of the machine slots.

Inventors: HJORTSBERG ARNE, HOLMSTROEM GOERAN, JOHANSSON LENNART, KARLSSON THOMMY, HJORTSBERG, ARNE, HOLMSTROEM, GOERAN, JOHANSSON, LENNART, KARLSSON, THOMMY, Holmström, Göran
Applicants: ASEA BROWN BOVERI, ASEA BROWN BOVERI AB
Application year: 1988
Main classification: H01B3
IPC codes: C08K3, H01B7, H02K3, H01B3