The subject of the invention is a capacitive insulating core of a high- voltage bushing applicable in electric power equipment, and especially in power transformers, comprising a resin-impregnated insulating core situated around a cylindrical mandrel. There are widely known bushings with insulating cores of type RIP (Resin Impregnated Paper), which are manufactured by spirally winding a number, ranging between a few dozen and a few hundred, of layers of a special insulating paper onto a metallic mandrel, where the wound paper layers overlap each other partially or completely and, every few or so layers of paper, between the paper layers there is inserted a conducting layer in the form of a sheet of aluminum foil of suitable dimensions adjusted to the size of the bushing and to its performance parameters. The size of the aluminum foil sheets and their number is selected to ensure the proper shaping of the electric field in the bushing. The mandrel, together with the wound-on insulating paper sandwiched with inserted aluminum foil sheets is placed in a cylindrical mould which is filled with epoxy resin and the insulation paper is impregnated with resin, and then the resin hardening process is carried out. Hardened resin together with the paper and the foil form a material with high mechanical resistance and good insulating properties, which are required for the insulating bushing cores. After removal from the mould, the core is machined in order to give it its final shape. Due to the size of high-voltage bushings and large quantities of resin, the resin hardening process releases large amounts of heat as a result of the exothermic reaction, which causes the emergence of considerable temperature gradients, which in turn cause local, large internal stresses. Internal stresses can cause cracks and delamination on the resin and foil contact surfaces. Such faults are caused mainly by large differences between the thermal expansion coefficients of both materials. Another inconvenience of manufacturing RIP bushings are difficulties connected with an even placement of aluminum foil on insulating paper as foil tends to fold and crease when the layers are wound onto the bushing mandrel, and the even placement of the aluminum foil on the insulating paper is of great importance for the even shaping of the electric field and the dielectric strength of the bushing. The use of electrically conducting metal sheets as intermediate layers between the layers of insulating paper, used to shape the electric field, is known from many solutions applicable to bushings with capacitive insulating cores. Examples of the solutions are shown in patent descriptions of US, Nos. 3875327, 4-362 897, 4 338 487, 4 387 266, 4 500 745, and GB 991 546, GB 1 125 964. An insulating bushing that contains a capacitive core formed by winding an insulating material onto a tube enclosing a conductor is known from European patent description no. EP 1 103 988. The insulating material of which the core is formed is paper impregnated with resin. Inside the capacitive core, between the layers of the insulating material, there are placed conducting or semiconducting foils serving as the elements shaping the electric field.

The essence of the capacitive insulating core of a high-voltage bushing according to the invention, formed by winding layers of electrically insulating material around a cylindrical mandrel, between which layers there are placed sheets of conducting material whose function is to shape the electric field in the bushing is that at least one sheet of the conducting material that is placed in the core between the layers of the electrically insulating material is a structure made on the basis of paper or woven or unwoven fabric and it contains conducting particles suspended in it, forming a percolative network electrically conducting in the sheet plane, the conducting particles having a basically elongated shape and such dimensions that the ratio of their length to the largest cross section dimension exceeds 10. Preferably in the first embodiment variant of the invention, the sheet of the conducting material placed in the core between the layers of the electrically insulating material contains conducting particles in the form of carbon nanotubes. Preferably in the first embodiment variant of the invention, the structure of a single sheet of conducting material is dry paper material made on the basis of cellulose fibers, and the conducting particles have the form of single- wall or multiwall carbon nanotubes less than 50 nm in diameter. Preferably in the first embodiment variant of the invention, multiwall carbon nanotubes between 10 nm and 30 nm in diameter and between 1 μm and 10μm in length are used. Preferably in the second embodiment variant of the invention, the sheet of the conducting material placed in the core between the layers of the electrically insulating material contains conducting particles in the form of carbon nanofibers. Preferably in the second embodiment variant of the invention, the structure of a single sheet of conducting material is dry paper material made on the basis of cellulose fibers, and the conducting particles have the form of carbon nanofibers of diameters between 50 nm and 300 nm. Preferably in the second embodiment variant of the invention, graphite carbon nanofibers 100 nm to 200 nm in diameter and 30 μm to 100μm long are used. Alternatively in the second embodiment variant of the invention, graphite carbon nanofibers 60 nm to 150 nm in diameter and 30 μm to 100μm in length are used. Preferably in the third embodiment variant of the invention, the sheet of the conducting material placed in the core between the layers of the electrically insulating material contains conducting particles in the form of metallic microfibers. Preferably in the third embodiment variant of the invention, the structure of a single sheet of conducting material is dry paper material made on the basis of cellulose fibers, and the conducting particles have the form of metallic microfibers of diameters below 30 μm. Preferably in the third embodiment variant of the invention, steel microfibers 5 to 10 μm in diameter and 1 to 6 mm long are used. In the second version of the invention embodiment, the capacitive insulating core of the high-voltage bushing is characterized by that at least one sheet of conducting material that is placed in the core between the layers of the electrically insulating material is a structure made on the basis of paper or woven or unwoven fabric and it contains conducting particles suspended in it, forming a percolative network electrically conducting in the sheet plane, the conducting particles having a form of grains of dimensions less than 1 μm. Preferably in the second version of the invention embodiment, the sheet of conducting material placed in the core between the layers of the electrically insulating material contains conducting particles in the form of metallic nanograins. Preferably in the second version of the invention embodiment, the structure of a single sheet of conducting material is dry paper material made on the basis of cellulose fibers, and the conducting particles have the form of metallic nanograins 10 nm to 1000 nm in diameter. Preferably in second version of the invention embodiment, aluminum nanograins 50 nm to 150 nm in diameter are used.

An advantage of the invention is that it provides sufficiently good conductivity of the conducting material used to shape the electric field in the bushing, while at the same time it allows to produce the insulating core material not susceptible to cracking and delamination during the production process. In particular, the elongated shape of the conducting particles located in the paper or woven or unwoven fabric structure and their large length-to- transverse dimension ratio results in good conductivity of the conducting material with only a small content of these particles in the structure, because the conductivity is of the percolation type. Especially advantageous is the use of carbon nanofibers, carbon nanotubes or metallic microfibers showing, besides the large length-to-diameter ratio, a very high conductivity along their length. Good electric conductivity can also be obtained using particles which do not have an elongated shape, but which are very small, for example metallic grains of submicron dimensions. Another advantage of the invention is that it eliminates local thermal stresses in the bushing due to similar thermal properties of the insulating material and the structure made on the basis of paper or woven or unwoven fabric containing conducting particles. Moreover, the use of such a conducting structure allows to avoid difficulties connected with its placement on the layers of insulating material which occur if metal foil is used, and therefore the quality of the bushing increases. If paper is used as the insulating material which has to be dried before impregnation with resin, then an additional advantage of using conducting layers made on the basis of paper, woven or unwoven fabric characterized by much better gas permeability than aluminum foil, is that it allows to shorten the process of drying the wound insulating core prior to its impregnation.

The subject of the invention is presented as an embodiment example in the drawing where fig. 1 shows schematically the capacitive insulating core of the high-voltage bushing in longitudinal section A-A, and fig. 2 shows the insulating core of the high-voltage bushing in transverse section B-B.

A capacitive insulating core 1 is situated around a cylindrical mandrel 2 and placed inside a standard insulating casing intended for high-voltage bushings, which is not shown in the drawing. The capacitive core 1 consists of many layers of electrically insulating crepe paper 3 which are wound onto each other and around the mandrel. The insulating paper layers are spirally wound onto the mandrel 2, and their dimensions, i.e. the thickness of an individual sheet and its width and length, depend on the size of the insulating bushing and its technical parameters. Between the insulating paper sheets there are inserted single sheets of conducting material 4 which serve to shape the electric field in the bushing. A single sheet of the conducting material 4 is a structure made on the basis of paper and containing conducting particles forming a percolative conducting network in the sheet plane. The sizes of the conducting particles are such that at least one external dimension of a single particle is less than 30 μm. Instead of paper, woven or unwoven fabric of adequate properties can be used as the basis for the structure containing conducting particles. In the first example of the invention embodiment, the structure of an individual sheet of conducting material is dry paper material made on the basis of cellulose fibers, and the conducting particles have the form of single- wall or multiwall carbon nanotubes of diameters smaller than 50 nm. For example, multiwall carbon nanotubes 10nm to 30nm in diameter and 1 μm to 10μm long made by Sun Nanotech Co. Ltd., China can be used. In the second example of the invention embodiment, the structure of an individual sheet of conducting material is dry paper material made on the basis of cellulose fibers, and the conducting particles have the form of carbon nanofibers 50 nm to 300 nm in diameter. For example, graphite carbon nanofibers made by Applied Science Inc., USA, type Pyrograph® type PR-19 that are 100 nm to 200 nm in diameter and 30 μm to 100 μm in length, or type PR-24 that are 60 nm to 150nm in diameter and 30 μm to 100 μm in length can be used. In the third example of the invention embodiment, the structure of an individual sheet of conducting material is dry paper material made on the basis of cellulose fibers, and the conducting particles have the form of microfibers less then 30 μm in diameter. By way of example, steel microfibers 6 μm in diameter and 5 mm in length produced by Nippon Seisen Co., Ltd., Japan can be used. In the second variant of the invention embodiment, in an example of its embodiment, the structure of an individual sheet of conducting material is dry paper material made on the basis of cellulose fibers, and the conducting particles have the form of metallic nanograins 10 nm to 1000 nm in diameter. As an example, aluminum nanograins 50 nm to 150nm in diameter, produced by QinetiQ Nanometerials ltd., UK can be used.

In another example of the invention embodiment, the structure of an individual sheet of conducting material contains a mixture of particles containing carbon nanotubes and nanofibers, or a mixture of carbon particles and metallic particles.

In all examples of the invention embodiments, the layers of electrically insulating material 3 together with the sheets of conducting material 4 which form the insulating core 1 are impregnated with impregnating resin. Impregnation is carried out in suitably shaped moulds. After the mould has been filled, the insulating core 1_ takes the shape of the mould. Then this core undergoes a hardening process and when this is finished, it is machined to obtain the required shape. The ready insulating core 1 is placed in the insulating casing of the high-voltage bushing.