INSULATION ELEMENT WITH MANUFACTURED FIBERS FOR ELECTRICAL INSULATION IN HIGH-VOLTAGE RANGE

Description

TECHNICAL FIELD

[0001] The present invention relates to an insulating element with low electrical conductivity for electrically insulating an electrical component in the high-voltage range. The invention also relates to an electrical component comprising such an insulating element and a method for producing such an insulating element. STATE OF THE ART

[0002] Insulation elements are an important component of electrical components, especially transformers. They serve to electrically insulate two electrical conductors that are at different voltage potentials during operation. This can create an electromagnetic field within the insulation element itself. Insulation elements impregnated with transformer oil are often used to insulate transformer windings to achieve efficient cooling of the windings.

[0003] For various reasons, in many applications, and particularly in HVDC converter transformers, it is desirable for the insulation elements not to have a completely insulating effect, but rather to have a precisely adjustable, slight conductivity. The conductivity of the insulation element can, for example, be matched to that of the transformer oil in order to improve the dielectric strength of the transformer insulation overall. By making the insulation element even more conductive than the transformer oil, the electric field can be pushed more into the insulating oil and thus preventing excessively high local field strengths in the insulation element. This relieves the load on the solid insulation and allows it to be made smaller. Smaller dimensions, i.e. lower thickness, of the insulation element means less material consumption and therefore lower costs in manufacturing the insulation element.On the other hand, the insulation element also takes up less volume in the electrical component, which also allows it to be smaller and manufactured more cost-effectively. Depending on the type and application of the electrical component, certain conductivities are certainly desirable with regard to the insulation elements used.

[0004] To ensure the most constant operating conditions possible for the electrical component, for example, during the commissioning of a transformer, the electrical properties of the insulation element should be as temperature-independent as possible. However, the electrical conductivity of most common insulation elements is highly temperature-dependent.

[0005] Document WO 2008 / 119705 A1 discloses an insulating element with a matrix made of a polymer material. To achieve a certain electrical conductivity, electrically conductive particles are embedded in the polymer material.

[0006] Insulation elements containing natural fibers, such as cellulose, and / or chemical fibers, such as aramid fibers, are often used, particularly in transformers. Natural fibers made from cellulose have excellent dielectric properties and sufficient dielectric strength. Furthermore, insulation elements made from cellulose are characterized by their good impregnation with transformer oil. Chemical fibers, on the other hand, are generally characterized by better temperature resistance, especially over extended periods of use.

[0007] A cellulose-based material in which a certain conductivity is achieved by incorporating conductive particles is disclosed in EP 0 953 680 A1. However, the material produced according to this document has a conductivity of at least 2.0 mS / cm, which is far too high for the insulation of many electrical components, and in particular for transformer insulation.

[0008] DE 29 34 007 discloses a paper or cardboard product with a metallic filler incorporated therein. Therefore, the electrical conductivity of the paper or cardboard product specified in this document is too high for many electrical applications.

[0009] DE 10 2010 041 630 A1 discloses a transformer insulation based on cellulose material, the specific resistance of which is adapted to the resistance of the oil using semiconducting or non-conducting nanoparticles. For this purpose, the nanoparticles distributed in the cellulose material are coated with an electrically conductive polymer. However, the production of an insulation element according to the teachings of this document is difficult and associated with relatively high costs, since the nanoparticles are largely washed out during the necessary dewatering of the cellulose material, resulting in significant particle loss. Furthermore, the insulation element exhibits uneven electrical conductivity.

[0010] WO 2012 / 003166 discloses a multi-layer insulation element with two layers, each containing aramid fibers, and with a layer of cellulose arranged between them.

[0011] WO 2016 / 131698 A1 by the same applicant discloses an insulating element for the high-voltage range, in which electrically conductive particles are embedded in a natural and chemically unprocessed fiber material, such as cellulose, to achieve a precisely adjustable conductivity of the insulating element. For this purpose, the particles have an electrically non-conductive core and an electrically conductive or semiconductive sheath surrounding the core. By also incorporating a cationic polymer into the insulating element, a particularly homogeneous conductivity distribution can be achieved.

[0012] WO 2014 / 154478 A1 discloses a cellulose material with low electrical conductivity, which is achieved by means of an impregnation of polyethyleneimine. PRESENTATION OF THE INVENTION

[0013] It is therefore an object of the present invention to provide an inexpensively producible insulation element for the electrical insulation of an electrotechnical component in the high-voltage range, which has a precisely adjustable, low and homogeneously distributed electrical conductivity.

[0014] To achieve this object, an insulating element is proposed as defined in claim 1. Furthermore, claim 13 defines an electrical component comprising such an insulating element, and claim 15 defines a method for producing such an insulating element. Preferred embodiments are defined in the dependent claims.

[0015] The present invention thus provides an insulating element with low electrical conductivity for the electrical insulation of an electrical component in the high-voltage range, comprising chemical fibers comprising chemical fibers made from synthetic polymers or inorganic chemical fibers, as well as electrically conductive particles with an electrically non-conductive core and an electrically conductive or semiconductive sheath surrounding the core. The insulating element also comprises a cationic polymer.

[0016] The cationic polymer enables a homogeneously distributed low electrical conductivity within the insulation element, in particular a homogeneously distributed low electrical conductivity across the thickness of the insulation element. For this purpose, the electrically conductive particles and / or the cationic polymer are preferably homogeneously distributed within the insulation element. The homogeneous distribution is preferably achieved across the thickness of the insulation element and advantageously across the entire insulation element.

[0017] The insulation element is preferably used for electrical insulation in the high-voltage range. However, it can also be used for insulation at voltages below the high-voltage range. The high-voltage range typically includes alternating voltages with an effective value of at least 1000 volts and direct voltages of at least 1500 volts. The insulation element is preferably even designed such that it has an electrically insulating effect at any voltage of at least 100 kV, and particularly preferably at least 350 kV.

[0018] The insulation element is advantageously impregnated with oil, in particular transformer oil. It is particularly advantageous for the chemical fibers to be impregnated with oil, in particular transformer oil. The insulation element then contains oil, in particular transformer oil, and is thus directly usable for a corresponding electrical application. For many electrical applications, and in particular for use in transformers and specifically in HVDC converter transformers, the insulation element impregnated with oil, in particular transformer oil, preferably has an electrical conductivity of at least 1*10 -17< S / m, more preferably of at least 1*10 -15< S / m, even more preferably of at least 1*10 -13< S / m, even more preferably of at least 1*10 -10< S / m, and most preferably of at least 1*10 -9< S / m. Preferably, the maximum electrical conductivity is 1*10 -7< S / m, more preferably 1*10 -8< S / m.The electrical conductivity in oil is measured according to the standard IEC 60093, 2nd edition, January 1, 1980. The measurement is evaluated using the so-called Kuechler method (A. Kuechler; High-Voltage Engineering: Fundamentals - Technology - Applications; 3rd edition, 2009, ISBN 978-3-540-78412-8; Chapter 4.2.2.3). By using electrically conductive particles with an electrically non-conductive core and an electrically conductive or semi-conductive coating surrounding the core, the electrical conductivity of the insulation element can be precisely adjusted to these advantageous values.

[0019] The insulating element can have one or more layers, each containing the chemical fibers, the electrically conductive particles, and the cationic polymer. The individual layers are advantageously bonded to one another. If the insulating element has at least two layers, the electrically conductive particles and / or the cationic polymer are preferably homogeneously distributed in at least one layer, more preferably in each layer. The homogeneous distribution is preferably present across the thickness of at least one layer, more preferably across the thickness of each layer.

[0020] Preferably, the amount of cationic polymer used in the production of the insulating element is such that the cationic polymer amounts to 0.1-15 wt.%, more preferably 1-15 wt.%, even more preferably 2-15 wt.%, even more preferably 3-15 wt.%, and most preferably 4-15 wt.%, based on the total weight of the insulating element in the dry state. This results in particularly good manufacturability of the insulating element when using the stated amounts of cationic polymer.

[0021] For the purposes of this invention, the term "dry state" refers to the insulation element containing 1 wt% or less water based on the total weight of the insulation element. The water content is measured according to IEC 60814, 2nd edition, August 29, 1997. A Metrohm 774 sample furnace combined with an 831 KF Coulometer is used for the water measurement.

[0022] Preferably, the electrically conductive particles make up 1-30 wt.%, more preferably 4-30 wt.%, even more preferably 6-30 wt.%, even more preferably 8-30 wt.%, even more preferably 11-30 wt.%, even more preferably 12-30 wt.%, even more preferably 12-28 wt.%, even more preferably 12-26 wt.%, most preferably 18-26 wt.%, of the total weight of the insulating element in the dry state. These particle quantities allow a defined adjustment of the electrical conductivity in oil, particularly in the ranges of 1*10 -17< S / m to 1*10 -8< S / m, 1*10 -16< S / m to 1*10 -8< S / m, 1*10 -15< S / m to 1*10 -8< S / m, 1*10 -13< S / m to 1*10 -8< S / m, 1*10 -10< S / m to 1*10 -8< S / m and 1*10 -9< S / m to 1*10 -8< S / m, which are particularly important in transformer applications. At at least 11 wt.%, in particular at least 12 wt.-% of electrically conductive particles, the electrical conductivity is mainly determined by the electrically conductive particles and no longer by the chemical fibers, which can be explained by the percolation effect of the particles. The electrical conductivity of the insulating element can then be precisely adjusted by selecting the appropriate particles and is essentially independent of the particle concentration. From this particle dosage onwards, the electrical conductivity of the insulating element is also largely independent of temperature. If the particle concentration is in a range of 11 - 30 wt.%, preferably in a range of 12 - 30 wt.%, more preferably in a range of 12 - 28 wt.%, even more preferably in a range of 12 - 26 wt.%, most preferably in a range of 18 - 26 wt.-%, based on the total weight of the insulation element in the dry state, a clearly defined electrical conductivity of the insulation element that is largely independent of the particle concentration and the temperature can be achieved with a minimal amount of particles. If the particle concentration is in a range of 11 - 30 wt.%, preferably in a range of 12 - 30 wt.%, more preferably in a range of 12 - 28 wt.%, even more preferably in a range of 12 - 26 wt.%, most preferably in a range of 18 - 26 wt.%, based on the total weight of the insulation element in the dry state, and the cationic polymer is in a range of 2 - 5 wt.%, preferably in a range of 2 - 4 wt.%, even more preferably in a range of 3 - 4 wt.%, based on the total weight of the insulation element in the dry state.-%, a clearly defined electrical conductivity of the insulation element that is largely independent of particle concentration and temperature can be achieved particularly well with minimal particle expenditure. Since particles are usually a particularly expensive component of the insulation element, the overall cost of the insulation element can be reduced in this way.

[0023] An optimal percolation effect can be observed when the electrically conductive particles are platelet-shaped. The required quantity of electrically conductive particles in the insulation element can therefore be reduced by designing the particles in platelet-shaped form.

[0024] The term "man-made fibers" is a collective term for fibers that are manufactured industrially, particularly those manufactured industrially using chemical processes. Man-made fibers include fibers made from natural polymers, fibers made from synthetic polymers, and inorganic man-made fibers. Fibers made from natural polymers include, for example, polylactide fibers. Fibers made from natural polymers can also be cellulose-based and are then, for example, regenerated fibers or cellulose ester fibers. Regenerated fibers include, for example, viscose, modal, lyocell, or cupro. Cellulose ester fibers include, for example, cellulose acetate fibers or cellulose triacetate fibers. Fibers made from synthetic polymers are generally obtained from petroleum or coal.Chemical fibers made from synthetic polymers include polyamide fibers, polyester fibers, polyurethane fibers, polyvinyl fibers, polyolefin fibers, fluoro fibers, polyethersulfone fibers, polyacrylonitrile fibers, melamine resin fibers, or aramid fibers. Polyolefin fibers include polyethylene fibers or polypropylene fibers. Aramid fibers include meta-aramid fibers or para-aramid fibers. Inorganic chemical fibers include glass fibers or ceramic fibers.

[0025] The chemical fibers advantageously make up at least 10 wt.%, more advantageously at least 20 wt.%, more advantageously at least 40 wt.%, more advantageously at least 60 wt.%, more advantageously at least 80 wt.%, most advantageously 90 wt.%, based on the total weight of the insulation element in the dry state. The insulation element can thus be produced cost-effectively and is easily impregnated with transformer oil. A high proportion of chemical fibers, particularly synthetic fibers such as aramid fibers, leads to improved temperature resistance of the insulation element, especially during extended use. The service life of the insulation element can thus be increased.

[0026] The electrically conductive or semiconductive coating advantageously completely surrounds the core of the particles. It is advantageously based on an inorganic material. The coating of the electrically conductive particles preferably contains a metal, more preferably a metal oxide. Particles comprising a metal-containing coating, in particular a metal oxide-containing coating, allow for a more precisely defined adjustment of the electrical conductivity in the insulating element. Furthermore, such a particle is easier to produce and more durable. The metal oxide can be, for example, tin oxide, zinc oxide, antimony oxide, titanium dioxide, zirconium dioxide, indium dioxide, silicon dioxide, or, for example, a mixture of individual metal oxides mentioned. The metal oxide is preferably doped with a foreign atom. By appropriately selecting and dosing the foreign atom, the conductivity of the insulating element can be precisely adjusted to a desired value.Possible doping atoms include gallium, aluminum, indium, thallium, germanium, tin, phosphorus, arsenic, antimony, selenium, tellurium and / or fluorine.

[0027] The size of the electrically conductive particles is advantageously at most 200 µm (micrometers), more advantageously at most 100 µm, and even more advantageously at most 60 µm. The particle size is also preferably at least 2 µm. It has been shown that with particles in these size ranges, in combination with the cationic polymer, the electrical conductivity can be adjusted particularly precisely. The thickness of the electrically conductive particles, especially if the electrically conductive particles are platelet-shaped particles, is preferably in the range of 0.3 to 4 µm, in particular in the range of 0.5 to 3 µm.

[0028] The particle size and particle size distribution can be determined using various standard methods. However, the invention preferably uses the laser diffraction method in a standard procedure using a Malvern Mastersizer 2000, APA2000 (product of Malvern Instruments Ltd., UK). This method has the advantage that particle size and particle size distribution can be determined simultaneously under standard conditions.

[0029] The particle size and thickness of individual particles can also be determined using SEM (Scanning Electron Microscope) images. These allow the particle size and geometric particle thickness to be determined through direct measurement. To determine average values, at least 1,000 particles are evaluated individually, and the results are averaged.

[0030] The core of the electrically conductive particles is advantageously a mineral material. The core preferably comprises natural or synthetic mica. However, the core can also comprise calcium carbonate, chalk, talc, bentonite, kaolin, glass, titanium dioxide, silicon dioxide (SiO 2 ), sericite, or aluminum oxide (Al 2 O 3 ). Electrically conductive particles containing mica exhibit greater durability and are easier to coat.

[0031] In a particularly preferred embodiment, the electrically conductive particles are the product from Merck, Darmstadt, Germany, with the trade name Minatec®< 51 CM and / or the product from Merck, Darmstadt, Germany, with the trade name Minatec®< 31 CM. The electrically conductive particles can also be the products from Merck, Darmstadt, Germany, with the trade name Minatec®< 40 CM, Minatec®< 60 CM, or Minatec®< 42 CM.

[0032] Polyethylenimine (PEI) and / or cationic starch are preferred cationic polymers. However, the cationic polymer can also be polyacrylamide (PAM), polydiallyldimethylammonium chloride (PDADMAC), polyvinyl alcohol, polyester epoxy resin, polyvinylamine (PVAm), polyethylene oxide (PEO), dicyandiamide formaldehyde (DCD), polyamidoamine (PAMAM), polyaminoamide epichlorohydrin (PAE), or polyamide epoxy resin.

[0033] For the purposes of this invention, the term "dry" refers to chemical fibers containing 1% or less water by weight based on the total weight of the chemical fibers, i.e., the fiber stock. The water content is measured using ISO 4119, 2nd edition, June 1, 1995.

[0034] In addition to the chemical fibers, the insulation element can also contain any number of other materials. For example, in addition to the chemical fibers, the insulation element can also contain non-fiber materials such as polyester resin, amylose, amylopectin, starch, algin, pectin, carrageenan, locust bean gum, xanthan gum, guar gum, agar, furcellaran, carboxymethylcellulose (CMC), and / or tamarind extract. Of course, in addition to the chemical fibers, the insulation element can also contain a natural fiber obtained from wood and / or annual plants, particularly cotton. The natural fiber can be pulp, particularly cellulose. Alternatively or additionally, the natural fiber can also be based on abaca, jute, hemp, sisal, and / or recycled paper.

[0035] The insulation element is preferably in the form of paper, cardboard or paperboard.

[0036] The invention also relates to an electrical component for the high-voltage range with an insulating element designed as described. The electrical component can, in particular, be a transformer, such as a high-voltage DC / DC converter transformer.

[0037] The present invention also provides a method for producing an insulation element configured as described. The method comprises at least the following steps: Mixing the chemical fibers with electrically conductive particles having an electrically non-conductive core and an electrically conductive or semi-conductive sheath surrounding the core; and dewatering the chemical fibers mixed with the electrically conductive particles.

[0038] The process also includes the step of adding a cationic polymer to the chemical fibers prior to dewatering. The addition of the cationic polymer can be carried out before, after, or during the mixing of the chemical fibers with the electrically conductive particles. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Preferred embodiments of the invention are described below with reference to the drawings, which are for illustrative purposes only and are not to be construed as limiting. In the drawings: Fig. 1 is a schematic cross-sectional view of a first insulation element according to the invention, with a relatively low dosage of electrically conductive particles; Fig. 2 is a schematic cross-sectional view of a second insulation element according to the invention, with a Fig. 1increased dosage of electrically conductive particles; Fig. 3 a, b, c schematic cross-sectional views of electrically conductive particles; and Fig. 4 a graph showing the relationship between the electrical conductivity of an insulating element according to the invention and its content of electrically conductive particles. DESCRIPTION OF PREFERRED EMBODIMENTS

[0040] The following describes a method for producing a preferred embodiment of an insulation element 1 according to the invention for electrically insulating an electrotechnical component in the high-voltage range. Figures 1 and 2 schematic cross-sectional views are shown of such insulation elements 1, each with a different dosage of electrically conductive particles 3.

[0041] Chemical fibers 2 are used to produce the inventive insulation element 1 of this exemplary embodiment. Para-aramid fibers, such as Twaron®< 1094 from Teijin (Kasinostraße 19-21, 42103 Wuppertal, Germany), can be used as chemical fibers. The chemical fibers 2 can be ground so that they are available in a ground form for further processing. They have a drainage resistance of 5 SR (Schopper-Riegler) to 80 SR. The average length-weighted fiber length of the starting material, i.e., the chemical fibers 2, is preferably in a range between 0.3 mm and 6.0 mm, but more preferably in a range between 0.3 mm and 2.2 mm. The drainage resistance is determined according to ISO 5267-1, 2nd edition, March 1, 1999. The fiber length is determined according to the standard TAPPI T271, pm-91, 1991.

[0042] The ground chemical fibers 2 are suspended in water. An additive in the form of a cationic polymer 4, such as a cationic starch, a cationic polyethyleneimine, cationic polyacrylamide, cationic polydiallyldimethylammonium chloride (polyDADMAC), or a cationic polyvinyl alcohol, is added to the water-suspended chemical fibers 2. The additive should be added at a consistency of 0.01 to 10 wt.%, preferably 0.1-10 wt.%, but ideally 0.5 to 1.0 wt.%. The cationic polymer 4 enables a homogeneously distributed, low electrical conductivity to be achieved within the insulation element 1.

[0043] In the preferred embodiment, a cationic starch (ROQUETTE FRERES, 62080 LESTREM, France, VECTOR®< SC 20157) is used in an amount corresponding to a proportion of at least 0.2%, more preferably at least 0.5% by weight, even more preferably at least 1.0% by weight of dry substance, based on the total weight of the insulating element 1 in the dry state. Preferably, the cationic starch is used in an amount corresponding to a maximum proportion of 8% by weight, more preferably a maximum proportion of 10% by weight, even more preferably a maximum proportion of 15% by weight, of dry substance, based on the total weight of the insulating element 1 in the dry state.

[0044] In the further course of the manufacturing process, electrically conductive particles 3 with an electrically non-conductive core and an electrically conductive or semi-conductive coating surrounding the core are added to the suspension of chemical fibers 2 and cationic polymer. The electrically non-conductive core of these particles 3 is a mineral filler, such as calcium carbonate, chalk, talc, bentonite, kaolin, titanium dioxide, or, particularly preferred, mica. The conductive or semi-conductive coating layer preferably comprises an oxide of tin, zinc, indium, titanium, zirconium, silicon, and / or antimony. The coating layer preferably makes up 20-60 wt.% of the total mass of an individual conductive particle 3. The mineral filler of the particle 3 accordingly has a weight fraction of 40-80 wt.% based on the total mass of the conductive particle 3.

[0045] The size of the conductive particles 3 is 2 - 200 µm, but preferably 100 µm or less, more preferably even 60 µm or less, but advantageously at least 2 µm. The mineral filler itself preferably has an ash content of 99.5%. The particle size and particle size distribution can be determined using various standard methods. However, according to the invention, the laser diffraction method is preferably used in a standard procedure using a Malvern Mastersizer 2000, APA2000 (product of Malvern Instruments Ltd., UK). This method has the advantage that the particle size and particle size distribution can be determined simultaneously under standard conditions.

[0046] The particle size and thickness of individual particles can also be determined using SEM (Scanning Electron Microscope) images. These allow the particle size and geometric particle thickness to be determined through direct measurement. To determine average values, at least 1,000 particles are evaluated individually, and the results are averaged.

[0047] In the preferred embodiment, the conductive particles 3 are formed by the product Minatec ®< 31 CM ( Figure 3a ) or Minatec ®< 51 CM ( Figure 3a) from Merck, Darmstadt. In the case of Minatec ®< 31 CM, the conductive particles 3 have a particle size in the range from 2 µm to 15 µm. In the case of Minatec ®< 51 CM, the conductive particles 3 have a particle size in the range from 10 µm to 60 µm. The proportion of the coating layer 6 consisting of antimony oxide is 38 - 54 wt.% for Minatec ®< 31 CM and 21 - 36 wt.% for Minatec ®< 51 CM, and that of the non-conductive core 5 consisting of mica is thus 46 - 62 wt.% for Minatec ®< 31 CM and 64 - 79 wt.% for Minatec ®< 51 CM.

[0048] Alternatively or additionally, the use of conductive particles 3 is possible, each comprising two mica particles, each having a conductive layer, in particular a coating layer 6, and additionally connected to one another via a quartz or talc particle 7. The conductive layer 6 of these particles 3 ideally comprises an oxide of antimony. The electrically conductive particles of the products of Merck, Darmstadt, with the trade name Minatec ®< 40 CM ( Figure 3b ), Minatec ®< 60 CM ( Figure 3b ) have a quartz particle 7. The electrically conductive particles of the product from Merck, Darmstadt with the trade name Minatec ®< 42 CM ( Figure 3c ) contain a talcum particle 7.

[0049] In a further production step, the chemical fibers 2 suspended in water and mixed with the cationic polymer 4 and the electrically conductive particles 3 are dewatered using a sieve. In a subsequent pressing process, the water remaining in the chemical fibers 2 is separated.

[0050] By dewatering the suspension, whose solids content comprises 60-94% chemical fibers 2, 1.0-4.0% cationic polymer 4, and 5.0-39.0% electrically conductive particles 3, a single-layer structure is formed. The insulation element 1 is ultimately preferably formed from fewer than 10, more preferably fewer than 8, and most preferably from one to seven of these individual layers. These layers can be formed into a thick cardboard or paperboard by winding them on a format roller to a thickness of 50 mm, which refers to the wet state with a water content of 50-90%.

[0051] During the dewatering process, the fiber material is dewatered not only by pressure but also by thermal heating to 50 to 160 °C.

[0052] After dewatering, the fiber material 2 mixed with a cationic polymer 4 and conductive particles 3 is in the form of paper, cardboard, or paperboard and preferably has a basis weight of 10 g / m 2 - 12,000 g / m 2 . The cardboard preferably has a basis weight of 225 to less than 600 g / m 2 . The paperboard preferably has a basis weight of 600 - 12,000 g / m 2 . The paper preferably has a basis weight of less than 225 g / m 2 .

[0053] In a further manufacturing step, the flat, dewatered fiber material 2 can be bonded with an adhesive to achieve a thickness of up to 500 mm. The adhesive can be based, for example, on a polyester resin, a casein, or a micro- or nanoscale cellulose. It is also possible to convert the fiber material 2, mixed with electrically conductive particles 3 and the cationic polymer 4, into any desired three-dimensionally structured shape during the pressing process and to thermally dry it following this molding process.

[0054] The amount of electrically conductive particles 3 is selected such that the dewatered insulation element 1 contains 1-30 wt.%, preferably 4-30 wt.%, more preferably 6-30 wt.%, even more preferably 8-30 wt.%, even more preferably 11-30 wt.%, even more preferably 12-30 wt.%, even more preferably 12-28 wt.%, even more preferably 12-26 wt.%, most preferably 18-26 wt.%, of the total weight of the insulation element in the dry state. The upper amount of electrically conductive particles 3 in the stated ranges is not 30 wt.%, but preferably 28 wt.%, more preferably 26 wt.%.These particle quantities allow a defined adjustment of the electrical conductivity in oil, particularly in the ranges of 1*10 -17< S / m to 1*10 -8< S / m, 1*10 -16< S / m to 1*10 -8< S / m, 1*10 -15< S / m to 1*10 -8< S / m, 1*10 -13< S / m to 1*10 -8< S / m, 1*10 -10< S / m to 1*10 -8< S / m and 1*10 -9< S / m to 1*10 -8< S / m, which are particularly important for transformer applications. Experiments whose results are presented in the . Figure 4have shown that the electrical conductivity in oil of the insulation element 1 is hardly increased at less than 12 wt.% of particles 3 compared to the case without electrically conductive particles 3. From a content of electrically conductive particles 3 of approx. 8 wt.% and up to a content of approx. 18 wt.%, the electrical conductivity of the dry and oil-impregnated insulation element 1 increases from a value in the range between 1*10 -17< S / m and 1*10 -10< S / m to a value in the range between 1*10 -10< S / m and 1*10 -8< S / m, in particular up to a value in the range between 1*10 -9< S / m - 1*10 -8< S / m. With a further increase in the concentration of electrically conductive particles 3 in the insulation element 1 to at least 20 wt.%, in particular to at least 24 wt.%, the electrical conductivity of the insulation element 1 remains constant in the range between 1*10 -10< S / m and 1*10 -8< S / m, in particular 1*10 -9< S / m and 1*10 -8< S / m. Figure 4 The electrical conductivities in oil shown were measured on insulation element 1, which is made of paper on one side and cardboard on the other. For both paper and cardboard, the electrical conductivities in oil were measured according to the standard IEC 60093, 2nd edition, January 1, 1980, and at temperatures of 23 °C and 90 °C. The measurements were evaluated using the so-called Kuechler method (A. Kuechler; High-Voltage Engineering: Fundamentals - Technology - Applications; 3rd edition, 2009, ISBN 978-3-540-78412-8; Chapter 4.2.2.3).

[0055] The effect that the conductivity is hardly influenced by very small amounts of particles and only increases above a certain particle concentration, in order to then assume an approximately constant value above a certain particle concentration, can be seen in particular from the Figures 1 and 2can be explained: At a very low concentration of electrically conductive particles 3 in the insulation element 1, as in the Figure 1 As shown, the particles 3 are evenly embedded between the individual chemical fibers 2 and barely touch each other. The chemical fibers 2 therefore represent the determining factor for the electrical conductivity of the insulation element 1, so that a different electrical conductivity is measured depending on the type of chemical fibers 2. The electrical conductivity of the insulation element 1 thus roughly corresponds to the case of a corresponding insulation element 1 without electrically conductive particles 3.

[0056] From a certain concentration of electrically conductive particles 3 of approximately 8 wt.%, the particles 3 begin to touch each other ( Figure 2). The electrical conductivity of the insulating element 1 is thus increasingly determined by the particles 3. Above a certain concentration, the particles 3 form a multitude of chains of particles 3 in contact with one another, which extend from the top side of the insulating element 1 to its bottom side. Thus, a percolation effect occurs. A further increase in the particle concentration then no longer leads to a further increase in the electrical conductivity of the insulating element 1; instead, a type of saturation state is reached.

[0057] In order to achieve an electrical conductivity of the insulation element 1 which is desirable for many applications in the range between 1*10 -17< S / m and 1*10 -8< S / m, in particular in the range between 1*10 -10< S / m and 1*10 -8< , in particular in the range of 1*10 -9< S / m and 1*10 -8< , with the smallest possible amount of particles, a particle concentration of 1 wt.% to 30 wt.%, preferably 4 wt.% to 30 wt.%, more preferably 6 wt.%, more preferably 8 wt.% to 30 wt.%, even more preferably 11 wt.% to 30 wt.%, even more preferably 12 - 30 wt.%, even more preferably 12 - 28 wt.%, even more preferably 12 - 26 wt.%, most preferably 18 wt.% to 26 wt.%, should be selected.

[0058] Surprisingly, the experiments conducted also showed that the electrical conductivity of the insulating element 1 is largely independent of temperature starting at a particle concentration of more than approximately 12 wt.%, in particular at least approximately 18 wt.%. This can also be explained by the fact that, starting at this particle concentration, the electrical conductivity of the insulating element 1 is primarily determined by the particles 3 and no longer by the chemical fibers 2, which are more strongly influenced by temperature. Accordingly, starting at these particle concentrations, the electrical conductivity of the insulating element 1 is also largely independent of the type of chemical fibers 2 used.

[0059] A particularly preferred embodiment of an insulation element 1 has the following composition, based on the total weight of the insulation element 1 in the dry state: 8-18 wt% Minatec ®< 51 CM; 1-4 wt% cationic polymer 4; 0.5 - 1 wt% water; and 77.0 - 90.5 wt% man-made fibers 2.

[0060] The insulation element 1, for example, does not necessarily have to be a component of a transformer. The insulation element 1 can also be used to insulate other electrical components, such as compensation chokes or phase shifters. In the case of a transformer, it does not necessarily have to be an oil-filled transformer. The insulation element 1 according to the invention could, of course, also be used in gas-insulated transformers, for example. LIST OF REFERENCE SYMBOLS 1 Insulation element 5 core 2 Chemical fibers 6 Wrapping 3 Electrically conductive particles 7 Quartz or talc particles 4 Cationic polymer

Claims

1. An insulation element (1) with low electrical conductivity for the electrical insulation of an electrical engineering component in the high-voltage range, comprising - chemical fibres (2) comprising chemical fibres made of synthetic polymers or inorganic chemical fibres; - electrically conductive particles (3) having an electrically non-conductive core (5) and an electrically conductive or semiconductive sheath (6) surrounding the core (5); and - a cationic polymer (4).

2. The insulation element (1) according to claim 1, wherein the cationic polymer (4) is present in an amount of 0.1 - 15 wt.%, preferably 1 - 15 wt.%, more preferably 2 - 15 wt.%, even more preferably 3 - 15 wt.%, most preferably 4 - 15 wt.%, based on the total weight of the insulation element (1) in the dry state.

3. The insulation element (1) according to claim 1 or 2, wherein the electrically conductive particles (3) constitute 1 - 30 wt.%, preferably 4 - 30 wt.%, more preferably 6 - 30 wt.%, even more preferably 8 - 30 wt.%, most preferably 12 - 30 wt.%, based on the total weight of the insulation element (1) in the dry state.

4. The insulation element (1) according to one of the preceding claims, wherein the chemical fibres (2) constitute at least 60 wt.%, in particular at least 80 wt.%, based on the total weight of the insulation element (1) in the dry state.

5. The insulation element (1) according to one of the preceding claims, wherein the sheath (6) of the electrically conductive particles (3) contains a metal.

6. The insulation element (1) according to claim 5, wherein the sheath (6) contains a metal oxide which is in particular doped with a foreign atom.

7. The insulation element (1) according to one of the preceding claims, wherein the size of the electrically conductive particles (3) is in the range from 2 µm to 200 µm, in particular in the range from 2 µm to 100 µm.

8. The insulation element (1) according to one of the preceding claims, wherein the core (5) of the electrically conductive particles (3) comprises a natural or synthetic mica.

9. The insulation element (1) according to claim 8, wherein the electrically conductive particles (3) are conductive particles which have a particle size in the range from 2 µm to 15 µm, wherein the proportion of the sheath layer (6) consisting of antimony oxide is 38 - 54 wt.% and the proportion of the non-conductive core (5) consisting of mica is 46 - 62 wt.%, or conductive particles (3) which have a particle size in the range from 10 µm to 60 µm, wherein the proportion of the sheath layer (6) consisting of antimony oxide is 21 - 36 wt.%, and the proportion of the non-conductive core (5) consisting of mica is 64 - 79 wt.%.

10. The insulation element (1) according to one of the preceding claims, wherein the cationic polymer (4) is polyethyleneimine or cationic starch.

11. The insulation element (1) according to one of the preceding claims, wherein the chemical fibres (2) are para-aramid fibres.

12. The insulation element (1) according to one of the preceding claims, wherein the chemical fibres (2) are impregnated with oil, in particular transformer oil, and wherein the insulation element comprises an electrical conductivity of 1*10-17 S / m to 1*10-8 S / m, in particular of 1*10-10 S / m to 1*10-8 S / m.

13. An electrical engineering component for the high-voltage range, comprising an insulation element (1) according to one of the preceding claims.

14. The electrical engineering component according to claim 13, wherein the electrical engineering component is a transformer, in particular an HVDC converter transformer.

15. A method for producing an insulation element (1) according to one of claims 1 to 12, comprising at least the following method steps: - mixing chemical fibres (2) in the form of meta-aramid fibres or para-aramid fibres with electrically conductive particles (3) which comprise an electrically non-conductive core (5) and an electrically conductive or semiconductive sheath (6) surrounding the core (5); - admixing a cationic polymer (4) with the chemical fibres (2); and - dehydrating the chemical fibres (2) admixed with the electrically conductive particles (3) and the cationic polymer (4).