Cable set for HVDC cables

Description

[0001] The present invention relates to a cable assembly for cables that can be used for high-voltage direct current (HVDC) transmission of electrical energy, wherein the cable assembly has an electrically insulating layer, a method for producing an electrically insulating layer of such a cable assembly, and its use.

[0002] With the rapidly increasing global energy demand, it is urgently necessary to be able to transport large amounts of energy over long distances with minimal loss. Currently, the transmission of high-voltage electrical energy over long distances is achieved using lossy alternating current cables (HVAC). These technologies have been used for many decades, are well-researched, and are widespread globally.

[0003] The continuous increase in power densities and transmission rates in the international energy market has created a need for energy transmission methods that generate significantly lower power losses than alternating current (AC) transmission. Therefore, there is an increasing search for direct current (DC) transmission methods, as these achieve lower power losses. However, the technology itself, and especially the materials used, are subject to stringent requirements.

[0004] In light of the potential global expansion of HVDC networks, significant efforts have been made in recent years to develop high-performance and reliable direct current systems. While insights and technologies gained in alternating current transmission have proven valuable for this development, they are not universally transferable.

[0005] In particular, the insulating bodies of cable assemblies do not yet sufficiently meet the requirements for high-performance insulation when exposed to high-voltage direct current fields, with regard to the materials used in them.

[0006] The direct current (DC) field polarizes the insulating material over a very long period with the same orientation. Space charges are injected into the insulation system of the cable and cable accessories from the conductive layers of the main conductor (core) and the outer conductor (ground potential). Unlike in alternating current (AC) systems, these space charges are not extinguished by polarization reversal, but accumulate over time and penetrate further into the material. There, defects in the material, such as impurities, inclusions, or gas bubbles, lead to a local concentration of space charges, which can become so large that the insulation system fails and partial discharges occur. These effects are amplified with increasing voltage.While the resulting problem is already critical at current voltages of 320 or 525 kV, significantly greater problems with failing insulation layers are to be expected at network voltages of 840 kV and 1100 kV, which are currently in the testing phase. Particularly in cable accessories, which include cable joints, cable terminations, cable connectors, and cable penetrations that represent spatial breaks in the cable network, the risk of severe partial discharges is high if regular dissipation of space charges is not ensured or measures are not taken to prevent their accumulation.

[0007] The insulating bodies of direct current high-voltage transmission systems (hereinafter: HVDC systems) should have a high insulation resistance under normal conditions, but this can be reduced to a lower insulation resistance if space charges need to be dissipated.

[0008] In WO 00 / 74191 A1, a method for the geometric control of the electric field in cable joints or cable terminations of HVDC systems is described, in which a resistive field-controlling layer of a cable has contact with a stress cone which, if necessary, can distribute the electric field more evenly over a wide area at the conductive cable ends and thus reduce or avoid local overloads (e.g. field peaks, field surges, space charges).

[0009] Geometric field control elements for cable joints or cable terminations are also known from patent literature for alternating current high-voltage systems (hereinafter: AC systems) and are described, for example, in DE 197 46 313 A1. For a long time, they represented a proven means of controlling field enhancements between the end of the cable's insulation layer and the exposed electrical conductor in the cable joint, but they do not prevent the problems described above that can occur in the insulation body itself.

[0010] Cable accessories are often predominantly made of elastomers. These allow, for example, cable joints to be attached to the cable ends to be joined using a push-fit or shrink-fit method. During installation, the component must expand by 15 to 35% of its external dimensions, while remaining fully deformable, to successfully complete the installation process. Cable accessories consisting of many different materials and rigid components can be compromised during this installation process due to the differing expansion rates of the individual components and the displacement of components during expansion. The interfaces between the individual layers of the typically multi-layered cable accessories are also affected by the expansion process, as well as by high operating temperatures of up to 95°C, impairing their effectiveness.For example, air inclusions between a conductive layer and an insulating layer can cause partial discharges.

[0011] Therefore, there has been no shortage of attempts to provide cable assemblies for high-voltage direct current transmission that consist essentially of an elastomeric material and in which effective field control is not achieved via geometric components.

[0012] A high-voltage direct current (HVDC) cable joint is known from EP 2 026 438 A1. This joint comprises an electrically insulating body into which a field-controlling layer is at least partially incorporated, the field-controlling layer exhibiting a thickening in certain areas. The electrically insulating body consists of an elastomer, which can be EPDM (ethylene propylene diene monomer) or a silicone elastomer. A field-controlling material with non-linear field-controlling properties, consisting of ZnO or SiC particles, is incorporated into the field-controlling layer, the matrix of which can also be composed of EPDM or silicone elastomer.

[0013] A multi-layered HVDC cable joint is also known from WO 2016 / 096276 A1. It comprises both an insulating layer and a field-controlling layer, the latter containing a non-linear field-controlling particulate material. This material consists either of known spherical ZnO microvaristor particles doped with various metal oxides such as Sb₂O₃, Bi₂O₃, Cr₂O₃, or Co₃O₄ and subsequently sintered, or of silicon carbide (SiC) particles. The insulating layer and the matrix of the field-controlling layer can be composed of different elastomers.

[0014] The aforementioned ZnO microvaristor particles have a high specific gravity and are therefore difficult to process in the elastomeric precursor compounds to be crosslinked. This is because they sink in the uncrosslinked mass during processing and cannot be evenly distributed, making gradient formation in the resulting field-controlling layer almost unavoidable. Furthermore, the component filled in this way also has a higher weight. While SiC particles have a lower specific gravity, their field-controlling properties are significantly less adjustable than those of ZnO microvaristor particles. Moreover, the patent specifications described above require both a purely insulating layer and an additional field-controlling layer to ensure the desired insulating and field-controlling properties.However, as previously described, interfaces between different layers in cable assemblies are generally prone to weaknesses, and these can lead to performance reductions with regard to electrical insulating properties. Therefore, it would be advantageous to have cable assemblies available that exhibit such weaknesses in their construction and processability only to a limited extent.

[0015] The object of the present invention is therefore to provide an HVDC cable assembly that does not have the described disadvantages with regard to processability and effectiveness of insulating and field-controlling elements and can be manufactured economically using the known elastomeric matrix materials.

[0016] Another object of the invention is to provide a method for producing an electrically insulating layer of an HVDC cable assembly which simultaneously has electrically insulating and field-controlling properties.

[0017] Furthermore, an additional object of the invention is to demonstrate the use of an electrically insulating layer produced as before.

[0018] The object of the present invention is solved by a cable assembly for HVDC cables, comprising a molded body which is multilayered and has an electrically insulating layer comprising an elastomer and a field-controlling particulate filler, wherein the field-controlling particulate filler is core-shell particles containing metal oxide.

[0019] Furthermore, the object of the invention is also achieved by a method for producing an electrically insulating layer of an HVDC cable assembly, wherein An unsolidified elastomer precursor composition, a crosslinking agent, a field-controlling particulate filler containing metal oxide core-shell particles, and optionally other additives are homogeneously mixed together to obtain an insulating layer precursor composition. The insulating layer precursor composition is introduced into a hollow body which has a cavity with an external shape corresponding to the shape of the electrically insulating layer of a cable assembly. The insulating layer precursor composition is crosslinked and solidified by standing or by applying heat and / or actinic radiation, and the insulating layer formed is removed from the hollow body.

[0020] Furthermore, the object of the invention is also achieved by using the pre-produced electrically insulating layer as an insulating layer in an HVDC cable assembly, wherein it is a cable sleeve, a cable end closure, a cable plug or a cable gland.

[0021] HVDC cable accessories, whether cable joints, cable terminations, cable connectors, or cable glands, are generally multi-layered molded bodies and, viewed from the cable surface or the inside of the cable accessory, usually feature an insulating body and an outer conductive layer (ground potential). In the case of geometric field control, electrically conductive deflectors are also frequently incorporated.

[0022] In the cable assemblies described above according to EP 2 026 438 A1 and WO 2016 / 096276 A1, the field-controlling function of the deflectors is taken over by a field-controlling layer that is applied directly to the insulating body (the insulating layer) and contains field-controlling particulate materials.

[0023] In contrast to these prior art embodiments, the HVDC cable assembly according to the invention comprises only at least one electrically insulating layer and an outer conductive layer, wherein the electrically insulating layer comprises an elastomer and a field-controlling particulate filler, the latter being core-shell particles containing metal oxide. Surprisingly, the inventors of the present invention have found that the incorporation of electrically conductive deflectors or a separate field-controlling layer in addition to an insulating layer in HVDC cable assemblies can be dispensed with if the insulating layer itself contains a field-controlling particulate filler of a specific composition in a homogeneous distribution. Therefore, in addition to the electrically insulating layer according to the invention, the cable assembly according to the invention preferably has neither field-controlling electrically conductive components (e.g.,In addition to deflectors, a separate field-controlling layer is also included. Other components of an HVDC cable assembly, such as additional mechanical protective layers, moisture barriers, etc., which are standard practice, will neither be described nor explained in detail here, because their integration into an HVDC cable assembly corresponds to the general expertise of a specialist and they can therefore be added professionally as needed.

[0024] Therefore, only the electrically insulating layer of an HVDC cable assembly composed according to the invention will be described in more detail below.

[0025] The electrically insulating layer of the HVDC cable assembly according to the invention essentially comprises an elastomeric material and a field-controlling particulate filler, which consists of core-shell particles containing metal oxide. According to the invention, the field-controlling particulate material is homogeneously distributed within the elastomeric material.

[0026] The elastomeric material is preferably an elastomer that is commonly used for the insulating bodies or insulating layers of cable accessories, i.e. a suitable silicone rubber, but also a polyurethane or EPDM.

[0027] Silicone rubber is preferably used because this material offers significant advantages in terms of its elongation, elasticity, tensile strength, and gas permeability, particularly for HVDC cable accessories that are to be fitted onto the corresponding HVDC cables using a push-fit technique. Furthermore, electrically insulating silicone-based layers compensate for temperature fluctuations and unevenness on the cable conductor surface more effectively than, for example, electrically insulating layers based on EPDM.

[0028] In particular, the silicone rubber is a low-temperature (from room temperature to < 200°C, two-component) cross-linked silicone rubber, which is called RTV2 silicone; a high-temperature cross-linked silicone rubber (from approx. 110°C, two-component, or from approx. 160°C, one-component), which is called HTV silicone; or a liquid-cured silicone rubber (from approx. 110°C, two-component), which is called LSR silicone.

[0029] Suitable materials are already used for cable accessories, including in the HVDC sector, and are commercially available, for example from companies such as Wacker Chemie, Momentive or Dow Corning, Inc.

[0030] These are reactive silicone compounds, which are mostly polymerized using platinum complexes as catalysts. Organosilicon compounds can be added to the starting materials as crosslinkers for polymerization, and, if necessary, auxiliary materials such as inert fillers, color pigments, reinforcing fillers, or other special additives can be included. An overview of suitable silicone rubber compounds can be found in J. Ackermann, V. Damrath, Chemie und Technologie der Silicone II, Chemie in unserer Zeit, 23rd year 1989, no. 3, pp. 86-99, VCH Verlagsgesellschaft mbH, Weinheim. , or also B. Pachaly, F. Achenbach et al., Silicone; from Winnacker / Küchler: Chemical Engineering: Processes and Products, Volume 5, pp. 1095-1213, Weinheim, WILEY VCH, 2005 , to be taken.

[0031] According to the invention, core-shell particles containing metal oxides are used as a particulate field-controlling filler. A prerequisite for the field-controlling properties of these particles is that either the core or the shell, or both the core and the shell, of these particles contain at least one electrically conductive or electrically semiconducting metal oxide.

[0032] Electrically conductive or electrically semiconducting metal oxides can be doped metal oxides, metal suboxides, or oxygen-deficient metal oxides.

[0033] Metals used for electrically conductive or semiconducting metal oxides, i.e. oxides, mixed oxides or oxide mixtures, include in particular zinc, tin, germanium, titanium, gallium, indium, antimony, silicon, tungsten, molybdenum, lead, cadmium, calcium, strontium, barium, copper and rhenium.

[0034] The metal oxides can be doped individually or as a phase-pure mixed oxide with one or more from the group consisting of antimony, indium, tungsten, molybdenum, chromium, cobalt, manganese, iron, cadmium, gallium, germanium, tin, vanadium, niobium, tantalum, cerium, scandium, lanthanum, yttrium, bismuth, titanium, copper, calcium, strontium, barium, aluminium, arsenic, phosphorus, nitrogen, boron, fluorine and chlorine.

[0035] The metal in the metal oxide and the dopants are usually not identical. However, in special cases, the same metal centers with different oxidation states can be present in a doped metal oxide or mixed metal oxide.

[0036] Particularly preferred metal oxides used are tin oxide, zinc oxide, indium oxide, and / or titanium oxide, especially titanium dioxide. Preferably used dopants include aluminum, indium, fluorine, tungsten, tin, and / or antimony, chromium, cobalt, vanadium, niobium, and tantalum. Additives such as bismuth, cerium, boron, chromium, silicon, strontium, barium, or calcium can be added to further adjust the material properties.

[0037] The proportion of dopants in the electrically conductive or semiconducting core or in the electrically conductive or semiconducting shell of the core-shell particles can range from 0.01 to 30 wt.%, based on the weight of the core or shell, respectively. Particularly preferred are tin oxide doped with antimony, tin oxide doped with tungsten, indium oxide doped with tin, zinc oxide doped with aluminum, or tin oxide doped with fluorine and / or phosphorus. Titanium oxide doped with niobium or tantalum, molybdenum or tungsten, and other transition elements is also advantageously used as a semiconducting material. The percentage of doping determines the strength of the electrically conductive or semiconducting properties. The lower the proportion of the dopant, the lower the expected electrical conductivity.Thus, the same metal oxide / doping element combination can be used to vary the electrical conductivity of the core or shell from strongly semiconducting to highly electrically conductive. The nonlinear electrical properties of the materials can be adjusted via the other additives mentioned above and the annealing conditions.

[0038] The use of core-shell particles according to the invention allows for finely tuned adjustment of the electrically conductive properties of the particulate filler material in the electrically insulating layer of the HVDC cable assembly. The metal oxides described above allow for the adjustment of the electrically conductive properties in the core or in the shell. However, the core or shell can also consist of dielectric material if the other part of the particle consists of an electrically conductive or semiconducting material.For example, the use of core-shell particles is advantageous, in which the core consists of a dielectric material, which is selected from, for example, SiO2, TiO2, Al2O3, glass or synthetic or naturally occurring aluminosilicates such as mullite, pearlite, pumice, fly ash, or layered silicates such as natural or synthetic mica, talc, sericite or mixtures of at least two of these, while the shell contains an electrically conductive or semiconducting metal oxide as described above.Similarly, electrically conductive cores consisting of particles of the previously described electrically conductive or semiconducting metal oxides can be surrounded by a shell of a dielectric material such as SiO2, Al2O3, TiO2, or polymers such as PVDF (polyvinyl fluoride) or polymeric functional siloxanes, which gives the core-shell particles intrinsic electrical conductivity without the core-shell particles themselves needing to be electrically conductive as a powder.

[0039] Furthermore, the structure of the core-shell particles used according to the invention also allows for the targeted control of the specific gravity of the individual particles, which is particularly advantageous for the processing properties during the production of the insulating layer or the insulating body of the HVDC cable assembly and, for example, enables the homogeneous distribution of the core-shell particles in the elastomer. Thus, according to the invention, core-shell particles are preferably selected whose specific gravity is less than 5 g / cm³, and in particular less than 4 g / cm³.Compared to the ZnO microvaristors used in the prior art, these offer the advantage that their tendency to settle in the uncured (crosslinked) insulating layer precursor composition during the manufacturing process is significantly lower. This allows them to be incorporated homogeneously, and this homogeneous distribution is maintained even after the crosslinking of the uncrosslinked insulating layer precursor composition during the cable assembly manufacturing process. Furthermore, the weight of the finished component is significantly reduced compared to a component filled with the same volume fraction of ZnO microvaristors.

[0040] The advantageous electrically conductive properties of the core-shell particles used according to the invention can be particularly well combined with an advantageous specific gravity if the core of the particles consists of a dielectric material and the shell contains at least one electrically conductive or semiconducting metal oxide. Here, particulate cores with low specific gravity can be used, which are commercially available and chemically inert, for example particles made of SiO₂, TiO₂, Al₂O₃, glass, or, more preferably, synthetic or naturally occurring aluminosilicates such as mullite, pearlite, pumice, fly ash, or layered silicates such as natural or synthetic mica, talc, sericite, or mixtures of at least two of these.If these are used as cores of the core-shell particles, a wide range of variations is possible for shaping the material and electrically conductive properties of the shell, without an excessively high specific weight of the final core-shell particles leading to undesirable processing properties in the production of the insulation layers of cable accessories.

[0041] The core-shell particles used according to the invention can have different shapes, i.e., spherical, plate-shaped or needle-shaped particles as well as irregularly shaped particles or mixtures of two or more of these are suitable.

[0042] The shape of the core-shell particles largely depends on the shape of the respective particulate core material. For example, core-shell particles based on plate-shaped mica or talc as a core have a plate-like shape, since the shell of the core-shell particles firmly encloses the core and is usually formed as a largely uniform coating on the core, so that the shape of the core material also corresponds to the outer shape of the core-shell particles.

[0043] The particle size of the core-shell particles is in the range of 0.1 to 150 µm, preferably in the range of 0.5 to 100 µm, and particularly in the range of 1 to 80 µm. According to the invention, the particle size is considered to be the largest longitudinal dimension of the individual particles.

[0044] The particle size can be determined using conventional methods for particle size determination. A laser diffraction method is particularly preferred, as it advantageously allows for the determination of both the nominal particle size of individual particles and their percentage particle size distribution. Measuring instruments from various companies are available for this purpose, for example, a Mastersizer 3000 from Malvern Instruments Ltd. or an Accusizer 780 from Agilent Technologies. All particle size determinations carried out in the present invention are performed using the laser diffraction method with a Malvern Mastersizer 3000 instrument from Malvern Instruments Ltd., UK, under standard conditions of ISO / DIS 13320.

[0045] If the core-shell particles used are plate-shaped, the corresponding particles have a thickness in the range of 0.01 to 5 µm, particularly from 0.05 to 4.5 µm. The shape factor of the plate-shaped particles (ratio of diameter or particle size to thickness) is 2:1 to 2000:1, particularly 5:1 to 200:1.

[0046] If the core-shell particles have a dielectric core and a shell in the form of a coating of an electrically conductive or semiconducting metal oxide, the thickness of the shell (coating) on ​​the core is typically 10 to 200 nm, preferably 20 to 50 nm. The weight fraction of the shell can be 30 to 200 wt.%, preferably 50 to 150 wt.%, based on the weight of the core.

[0047] The core-shell particles used according to the invention for field control in the electrically insulating layer of an HVDC cable assembly exhibit, depending on their material composition, electrical conductivity, intrinsic electrical conductivity or semiconducting properties.

[0048] Since the insulating layer or insulating molded body containing the elastomer and the core-shell particles is intended to have overall electrically insulating properties, it is obvious that electrically conductive core-shell particles in particular must not be used in the layer (the molded body) in a concentration above the percolation threshold.

[0049] The percolation threshold of a system of electrically conductive particles in a dielectric matrix is ​​a narrow concentration range for the electrically conductive particles in which the electrically conductive properties of the matrix change abruptly, i.e., the overall system, through the formation of conduction pathways, with a small increase in the concentration of electrically conductive particles, abruptly reaches an electrical conductivity increased by orders of magnitude.

[0050] In order to adjust the insulation resistance of the electrically insulating layer in the HVDC cable assembly according to the invention to a value which, in the case of stress, is in the range of approximately 10⁸ to 10¹² Ohm*cm, electrically conductive core-shell particles in the elastomer can only be used in a low concentration significantly below the percolation threshold, namely in the range of 0.1 to 10 vol.%, preferably 0.5 to 6 vol.%, based on the volume of the insulating layer or the insulating body.

[0051] Higher concentrations are permitted and advantageous for intrinsically conductive or semiconducting core-shell particles, such as in the range of 0.1 to 25 vol.% for intrinsically conductive core-shell particles and in the range of 0.1 to 25 vol.%, in particular 0.5 to 15 vol.%, for semiconducting core-shell particles.

[0052] In general, the concentration of core-shell particles in the elastomer is 0.1 to 25 vol.%, preferably 0.5 to 20 vol.% and particularly 1 to 15 vol.%, based on the volume of the insulating layer or insulating body.

[0053] Provided no other solid particles are present in the elastomer, the concentration of the core-shell particles can be expressed as the pigment volume concentration. The latter represents the ratio of the total volume of pigments and / or fillers and / or other non-film-forming solid particles in a product to the total volume of the non-volatile components, expressed as a percentage.

[0054] According to the invention, semiconducting core-shell particles are particularly preferred, having a specific powder resistance in the range of 10⁶ < Ω*cm to 10¹² < Ω*cm. Preferably, the semiconducting core-shell particles have a specific powder resistance in the range of 10⁸ < Ω*cm to 10¹² < Ω*cm, the specified values ​​referring to an applied measuring voltage of 100 V. Like the electrically conductive or intrinsically conductive pigments mentioned above, they can be used individually or as mixtures of differently composed core-shell particles, each having, for example, a different specific powder resistance, so that fine-tuning of the desired conductivity properties in the insulating layer can be easily ensured by mixing core-shell particles.

[0055] The semiconducting core-shell particles are also preferably used in a concentration below the percolation threshold.

[0056] The electrical properties of the core-shell particles are characterized by the specific resistance of the powder. To measure the specific resistance of a pigment powder, an acrylic glass tube with an inner diameter of 2 cm is filled with a small amount of the pigment powder (approx. 0.5 to 3 g), which, according to the invention, is formed by the aforementioned core-shell particles, and compressed against a metal electrode using a 10 kg weight and a metal plunger. The specific resistance ρ is determined from the layer thickness L of the compressed powder according to the following relationship: ρ = R * π * d / 2 2 / L Ohm * cm .

[0057] Here, R represents the actual measured electrical resistance at a measuring voltage of 100 V and d the diameter of the pigment column.

[0058] The core-shell particles used according to the invention can be additionally provided on their surface with an organic or inorganic post-coating, which is intended to improve the incorporation properties of the core-shell particles into the elastomer. For example, the surface of the core-shell particles can be provided with a thin coating of organic silanes or amphiphilic surfactants. However, the surface coating constitutes only a proportion of at most 5 wt.%, based on the total weight of the core-shell particles, and has little or preferably no effect on the electrical conductivity of the core-shell particles.

[0059] Suitable electrically conductive, intrinsically conductive, or semiconducting core-shell particles are commercially available and are offered, for example, by Merck KGaA, Germany, in a wide selection under the name Iriotec®<. Of these, core-shell particles offered under the designations Iriotec®< 73xx (xx = 10, 15, 20, 25, 30, 40) and Iriotec®< 75xx (xx = 10, 50) have proven to be particularly suitable.

[0060] The insulating layer of the molded body of the HVDC cable assembly according to the invention has a specific through-resistance in the range of 10 8< to 10 13< Ohm*cm, in particular in the range of 10 10< to 10 12< Ohm*cm.

[0061] For values ​​below 10⁷ Ω·cm, the electrical conductivity of the insulating layer would be too high, resulting in significant power losses through the insulator even during normal operation. This would also lead to heating and damage to the insulating layer. Conversely, for resistances greater than 10¹³ Ω·cm, the insulating effect of the layer is as high as would be expected from the matrix alone. Any resulting space charges would then no longer be able to dissipate.

[0062] The voltage-dependent volume resistivity ρ of a layer is measured according to DIN IEC 60093 and DIN EN 61340-2-3: 2000 using a ring electrode (mean ring diameter d) on a 1 mm thick (L) flat test specimen. The test specimen is placed between two special measuring probes, and when a measuring voltage is applied, the resistance (R) is determined indirectly via the current flow through the test specimen and a defined measuring resistor (shunt) connected in series. The following equation applies to the volume resistivity: ρ = R * π * d / 2 2 / L Ohm * cm .

[0063] The present invention also relates to a method for producing an electrically insulating layer of a cable assembly, wherein An unsolidified elastomer precursor composition, a crosslinking agent, a field-controlling particulate filler containing metal oxide core-shell particles, and optionally other additives are homogeneously mixed together to obtain an insulating layer precursor composition. The insulating layer precursor composition is introduced into a hollow body which has a cavity with an external shape corresponding to the shape of the electrically insulating layer of a cable assembly. The insulating layer precursor composition is crosslinked and solidified by standing or by applying heat and / or actinic radiation, and the insulating layer formed is removed from the hollow body.

[0064] Electrically insulating layers for cable accessories, which consist of elastomers in the matrix, are typically manufactured using an injection molding process. For two-component raw material systems, this is a reactive injection molding process (RIM). In this process, suitable elastomer raw materials are mixed together, introduced in liquid form into a hollow body (the injection mold), and allowed to crosslink. After the crosslinking process, they can be removed from the mold and structurally completed.

[0065] The electrically insulating layers of the cable assembly according to the invention are also manufactured in this way. By applying the injection molding process, the electrically insulating layer of the cable assembly itself is formed as a molded body.

[0066] Preferably, silicone compounds belonging to the silicone resin types RTV2, LSR, or HTV silicones are used as starting materials for the elastomer. For this purpose, the corresponding reactive silicone compounds are mixed with crosslinking agents, which are usually also (short-chain) silicone compounds, and optionally with catalysts and other additives, such as inert fillers, in two-component systems. This mixture, in its liquid state, is then introduced into the cavity of an injection mold and allowed to crosslink under the respective conditions. The necessary conditions, such as temperature, pressure, and reaction time, are known to those skilled in the art and are selected according to the starting materials and the desired final elastomers. In contrast, the separate addition of a crosslinking agent is omitted in one-component systems.The crosslinking process can be accelerated by the supply of actinic radiation, for example by UV or gamma radiation.

[0067] If required, the cavity of the injection mold can also contain solid components in the form of inserts at a defined location, which are overmolded with the insulating layer precursor composition.

[0068] During the crosslinking process, the insulating layer precursor composition, which is a mixture of the reactive elastomer precursor composition with a crosslinker and optionally further additives (preferably at least with a catalyst that accelerates crosslinking), is solidified by crosslinking and thus transformed into the elastomeric solid, which can be removed from the mold as a shaped body at predetermined temperatures and after a defined standing time.

[0069] The entirety of the starting materials for the production of the insulating layer according to the invention is referred to herein as the insulating layer precursor composition. The main components of the starting materials for the production of the elastomer (excluding crosslinker and catalyst) are referred to as the elastomer precursor composition.

[0070] In the present invention, field-controlling particulate fillers in the form of core-shell particles are added to the usual starting materials for producing the elastomers before they are introduced into the cavity of the mold. These fillers are homogeneously distributed in this mixture and are maintained in this distribution even during the introduction into the mold. The resulting mixture is the insulating layer precursor composition.

[0071] The field-controlling particulate filler is present in the insulating layer precursor composition in an amount of 0.1 to 25%, based on the total volume of the insulating layer precursor composition. Accordingly, the proportion of the particulate filler in the resulting insulating layer or insulating body is also 0.1 to 25%, based on the volume of the insulating layer or insulating body.

[0072] According to the invention, silicone compounds are preferably selected as the elastomer precursor composition, which crosslink either at low temperature (RTV2), at higher temperature (HTV), or in the liquid state (LSR). Suitable classes of compounds are described, as mentioned above, in J. Ackermann, V. Damrath, Chemie und Technologie der Silicones II, Chemie in unserer Zeit, 23rd year 1989, No. 3, pp. 86-99, VCH Verlagsgesellschaft mbH, Weinheim ,or also in B. Pachaly, F. Achenbach et al., Silicone; from Winnacker / Küchler: Chemical Engineering: Processes and Products, Volume 5, pp. 1095-1213, Weinheim, WILEY VCH, 2005 , These are described in more detail and are also commercially available. The corresponding crosslinking agents, catalysts, and any other additives are also described in the cited literature.

[0073] The field-controlling particulate fillers in the form of core-shell particles used according to the invention have already been explained in detail. Reference is made here to these explanations. Of particular importance are core-shell particles which, in addition to the desired electrically conductive properties, have a specific gravity of < 5 g / cm³, preferably < 4 g / cm³, because these can be distributed homogeneously in the insulating layer precursor composition and can be maintained in this homogeneous distribution even during processing into an insulating layer / insulating body.

[0074] After the insulating layer / insulating body is removed from the hollow body (the injection mold in the injection molding process), the cable assembly, in the form of a cable joint, cable termination, cable connector, or cable gland, is fitted with an outer conductor (ground potential) and, if necessary, with other components. These components may also have been pre-integrated into the cable assembly in the injection mold. The cable assembly can then be mounted onto the HVDC cable. For example, in the case of a cable joint, this is done by connecting the conductive cable ends (inner conductors) with connectors and embedding this cable connection in the completed cable joint, advantageously using a push-fit technique or a heat-shrink process. A significantly simplified method also allows the insulating layer or...The insulating body is manufactured directly on-site over the already connected electrical conductors of the cable ends using an injection molding process in a hollow body (injection mold). An outer conductor can also be subsequently applied to the insulating layer of an already installed cable joint.

[0075] The present invention therefore also relates to an electrically insulating layer on a substrate, wherein the electrically insulating layer contains an elastomer and a field-controlling particulate filler, the latter being core-shell particles containing metal oxide and the elastomer being a silicone rubber.

[0076] According to the invention, the substrate is preferably an HVDC cable which has at least a partially external electrically conductive surface, i.e. an HVDC cable which is not provided with an insulating layer in some areas.

[0077] According to the present invention, the core-shell particles used have a specific gravity of < 5 g / cm³, preferably < 4 g / cm³. This comparatively low specific gravity enables a homogeneous distribution of the core-shell particles in the silicone rubber elastomer matrix.

[0078] Details regarding the materials used have already been discussed in detail above.

[0079] The electrically insulating layer according to the invention has a volume resistance in the range of 10⁸ to 10¹³ ohms*cm. With a volume resistance in this range, it is assumed that a directed dissipation of any space charges that may occur in HVDC cables is not possible, but rather that the electrically insulating layer causes a diffuse charge distribution and that any electrical charges that arise can be successively dissipated, so that dangerous and hidden charge accumulations and spontaneous discharges do not occur.

[0080] The present invention also relates to the use of an electrically insulating layer, as described above, as an insulating layer in a cable sleeve, a cable termination, a cable connector or a cable gland for HVDC cables.

[0081] In addition to the advantages already described above of successive charge dissipation in the event of space charges occurring during the operation of HVDC cables, the cable assembly according to the invention, in the form of a cable joint, etc., offers further advantages. For example, the formation of an electrically insulating layer containing no conductive material, as well as a separate dissipative layer, can be omitted if the electrically insulating layer is designed as described here. The use of silicone rubber as the preferred material for the elastomer matrix results in high elongation and elasticity with relatively low residual deformation (< 20%) of the cable assembly, combined with high tensile strength and good gas solubility.Since the core-shell particles used according to the invention are highly compatible with a silicone matrix and can be homogeneously incorporated into it, the desired conductivity, which in the prior art is achieved by separate field-controlling layers, can be generated in the inventive technical solution by a small amount of conductive material whose conductivity can be specifically adapted to the required conditions, within an electrically insulating layer. The cable assemblies according to the invention can therefore be manufactured highly efficiently, economically, and with the best possible adaptation to the specific requirements, used as HVDC cable assemblies, and applied to HVDC cables using the proven push-on or shrink-fit techniques.By form-fitting adaptation to the cable sections and diameters to be joined, the formation of air or foreign matter inclusions between the cable and cable assembly is largely avoided, thus minimizing power loss.

[0082] Figure 1 Figure 1 shows a schematic representation of a cable sleeve according to the present invention with an outer conductor layer (1), insulating layers (2) and (3) according to the invention, and an HVDC cable duct (4) with a conductive inner layer as a connecting element (5).

[0083] The present invention will be explained below by means of examples, but will not be limited to these. Examples: Examples 1 to 3: Production of core-shell particles as fillers

[0084] 100 g of ground and classified natural mica are suspended in 1900 ml of deionized water. In an acidic environment, the suspension is stirred at 75°C with a 50 wt% aqueous solution of SnCl₄, HCl, and a 35 wt% aqueous solution of SbCl₃, adding dropwise. The pH is maintained by the simultaneous, controlled addition of sodium hydroxide solution. After the entire volume of the solution has been added, the mixture is stirred for another 30 minutes at 75°C before a 50 wt% solution of titanium dioxide is added uniformly, with sodium hydroxide added at a constant pH (addition of titanium dioxide only in Example 3). The mixture is then cooled to room temperature with stirring and neutralized. The resulting pigment is filtered through a Büchner funnel, washed with water, dried at 140°C, and calcined for approximately 30 minutes at 800°C. Depending on the antimony content, the resulting pigment powder ranges from light grey to ochre yellow.In Examples 1-3, the specific powder resistivity of the pigments is varied by varying the antimony content in the tin oxide, as shown in Table 1. The resulting pigment particles have a core of natural, platelet-shaped mica (particle size < 15 µm) and a shell of antimony-doped tin oxide, optionally containing titanium oxide, which adheres firmly to it.

[0085] The measurement of the specific resistance of a pigment powder is carried out as previously described. Table 1 Example Mol% Sb Mol% Ti p [Ohm·cm] 1 8 0 28 2 1,0 0 2,5 x 10 6< 3 1,0 8 3,0 x 10 9< Examples 4 to 8: Production of silicone test specimens Production of silicone sheets from room temperature curing silicone (RTV2)

[0086] The quantities of core-shell particles from Examples 1 to 3, as specified in Table 2, are roughly premixed in a container with the respective proportions of component A of a commercial RTV2 silicone resin (manufacturer's material data: A:B = 9:1, viscosity of the mixture 3500 mPa*s at 23°C, Shore A hardness 45°). The mixture is then homogenized in a vacuum speed mixer (Hauschild) at a pressure reduced to 4 mbar and 1600 revolutions per minute for at least 2 minutes. Subsequently, the respective quantities of component B of the same RTV2 silicone resin are added, the components are again roughly premixed, and homogenized in the vacuum speed mixer for at least 1 minute at 4 mbar and 1600 revolutions per minute. The viscous mass is then quickly poured into a mold, observing the pot life, which defines the geometric dimensions of the test specimen. The silicone resin is cured in the mold for at least 30 minutes at 70°C.After the mold has cooled, it is opened, the test specimen is removed and stored in a dust-free environment.

[0087] The silicone test specimens are in the form of sheets measuring 100 mm x 100 mm and with thicknesses of 5 mm, 2 mm, and 1 mm, for mechanical or electrical testing. The quantity and type of materials used vary according to Table 2. Table 2: Examples RTV2, Component A [g] RTV2, Component B [g] Filler type Amount of filler [g] PVK* [%] 4 108,0 12,0 - 0,0 0,0 5 97,2 10,8 Example 2 12,0 3,1 6 75,6 8,4 Example 2 36,0 11,1 7 97,2 10,8 Example 1 12,0 2,8 8 97,2 10,8 Example 3 36,0 11,9 *PVC = Pigment Volume Concentration Examples 9 - 11: Production of silicone sheets from liquid reactive silicone compounds (LSR)

[0088] The quantity of component A of a commercial LSR silicone resin (manufacturer's material data: A:B = 1:1, viscosity of components 100 Pa*s at 20°C, Shore A hardness 40°) specified in Table 3 is weighed into a container with the respective quantity of core-shell particles from Examples 2 and 3 and the quantity of component B of the same LSR silicone resin specified in Table 3, and roughly mixed. Care should be taken to minimize air entrainment. The container is placed in a vacuum speed mixer (Hauschild) and the mixture is homogenized for at least 3 minutes at a pressure reduced to 4 mbar and 2000 revolutions per minute. The highly viscous mass is then quickly poured, observing the pot life, into a mold preheated to 60°C, which defines the geometric dimensions of the test specimen. The silicone resin is cured in the mold for at least 5 hours at 125°C.After the mold has cooled, the test specimen is removed and cured for a further 14 hours at 125 °C on a glass plate in the oven.

[0089] The silicone test specimens are in the form of sheets measuring 100 mm x 100 mm and with thicknesses of 5 mm, 2 mm, and 1 mm, for mechanical and electrical testing, respectively. Table 3 shows the variation in the amounts of silicone components and filler. Table 3: Example LSR silicone, component A [g] LSR silicone, component B [g] Filler type Amount of filler [g] PVK [%] 9 60,0 60,0 - 0,0 0,0 10 53,9 53,9 Example 2 12,2 3,1 11 54,3 54,3 Example 2 11,5 3,3 12 54,3 54,3 Example 3 11,3 3,3 13 43,2 43,2 Example 3 33,6 11,3 Measurement of Shore A hardness:

[0090] The hardness of elastomers is determined according to DIN ISO 7619-1 by gently pressing a steel indenter into the specimen for 15 seconds using spring force. The indenter used for Shore A hardness determination has a truncated cone geometry. Measurement of the strain properties:

[0091] The elongation at break and tensile strength are measured according to ISO 37 on the Instron 5967 test system at a crosshead speed of 200 mm / min using a 1 mm thick shoulder bar, as is standard for elastomers (DIN 53504 S2). Due to the lower crosslinking of the RTV2 material, it deviates from elastic behavior early and begins to behave irreversibly at approximately 30% elongation. The more highly crosslinked LSR material usually behaves elastically until shortly before breaking. Measurement of dielectric strength:

[0092] The dielectric strength is tested on 2 mm thick specimens (approx. 30 mm x 40 mm) using the Baur DTA 100 insulation tester. The specimen is clamped tightly between two disc-shaped electrodes according to ASTM D877, and the test cell is filled with silicone oil (AP 100, Aldrich) so that the specimen is completely covered to prevent pre-discharges through the air. The voltage is increased in 2 kV / s increments, and the voltage preceding the discharge is recorded.

[0093] Table 4 shows the corresponding measurement results. Table 4: Example 4 5 6 7 8 Filler from example - 2 2 1 3 Silicone type RTV2 RTV2 RTV2 RTV2 RTV2 PVK [%] 0,0 3,1 11,1 2,9 11,9 Shore A hardness 35 48 69 48 69 Elongation at break [%] (at tensile stress MPa) 115 (5,3) 120 (5,8) 128 (5,6) 126 (5,4) 142 (6,1) Dielectric strength [kV] 42,2 48,9 50,7 48,5 51,2 Example 9 10 11 12 13 Filler from example - 1 2 3 3 Silicone type LSR LSR LSR LSR LSR PVK [%] 0,0 3,1 3,3 3,3 11,3 Shore A hardness 33 47 53 55 70 Elongation at break [%] (at tensile stress MPa) 325 (2,9) 324 (3,0) 326 (3,0) 335 (3,3) 340 (3,9) Dielectric strength [kV] 31,2 39,8 41,2 43,4 52,8

[0094] The electrical strength, measured as the breakdown voltage, increases significantly with increasing filler content in the polymer composites shown, without compromising elasticity. Example 8, in particular, with a filler content of approximately 12 vol.%, also exhibits considerably higher mechanical strength than the unfilled silicone material. The electrical breakdown strength of the LSR silicone increases by up to 69% with increasing filler content, and the elasticity of the filled material also increases. Even with the less cross-linked RTV2 material, a higher filler content leads to an improvement in electrical strength of up to 21%. The best electrical properties are achieved with the semiconducting filler from Example 3.

Claims

1. Cable fitting for HVDC cables, comprising a moulding which has multilayer structure and which has an electrically insulating layer that comprises an elastomer and that comprises a field-controlling particulate filler in homogeneous distribution, wherein the field-controlling particulate filler is metal-oxide-containing core-shell particles and the core-shell particles have a density of < 5 g / cm3.

2. Cable fitting according to Claim 1, wherein it is a cable sleeve, a cable end seal, a cable plug or a cable bushing.

3. Cable fitting according to Claim 1 or 2, wherein the elastomer is a silicone rubber.

4. Cable fitting according to Claim 3, wherein the silicone rubber is a silicone rubber of RTV2 type, a silicone rubber of HTV type or a silicone rubber of LSR type.

5. Cable fitting according to one or more of Claims 1 to 4, wherein the core and / or the shell of the core-shell particles comprise(s) at least one electrically conductive or electrically semiconductive metal oxide.

6. Cable fitting according to Claim 5, wherein the electrically conductive or electrically semiconductive metal oxide is a doped metal oxide, a metal suboxide or an oxygen-deficient metal oxide.

7. Cable fitting according to Claim 6, wherein the metal oxide is selected from the group consisting of oxides, mixed oxides or oxide mixtures of zinc, tin, germanium, titanium, gallium, indium, antimony, silicon, tungsten, molybdenum, lead, cadmium, calcium, strontium, barium, copper and rhenium.

8. Cable fitting according to one or more of Claims 6 and 7, wherein the metal oxide has been doped with one or more of the elements antimony, indium, tungsten, molybdenum, chromium, cobalt, manganese, iron, cadmium, gallium, germanium, tin, vanadium, niobium, tantalum, cerium, scandium, lanthanum, yttrium, bismuth, titanium, copper, calcium, strontium, barium, aluminium, arsenic, phosphorus, nitrogen, boron, fluorine and chlorine.

9. Cable fitting according to one or more of Claims 5 to 8, wherein the core of the core-shell particles consists of a dielectric material and the shell comprises at least one electrically conductive or electrically semiconductive metal oxide.

10. Cable fitting according to one or more of Claims 1 to 9, wherein the electrically insulating layer comprises a quantity of from 0.1 to 25%, based on the volume of the electrically insulating layer, of the field-controlling particulate filler.

11. Cable fitting according to one or more of Claims 1 to 10, wherein the volume resistivity of the electrically insulating layer is in the range from 108 to 1013 ohm*cm.

12. Process for the production of an electrically insulating layer of a cable fitting according to one or more of Claims 1 to 11, characterized in that - an unhardened elastomer-precursor composition, a crosslinking agent and a field-controlling particulate filler which comprises metal-oxide-containing core-shell particles and has a density of < 5 g / cm3, and also optionally further additives, are homogeneously mixed with one another to give an insulation-layer-precursor composition, - the insulation-layer-precursor composition is introduced into a hollow body which has a cavity with an exterior shape corresponding to the shape of the electrically insulating layer of a cable fitting, and - the insulation-layer-precursor composition is hardened in a crosslinking manner by passage of time or introduction of heat and / or of high-energy radiation, and the resultant insulation layer is removed from the hollow body.

13. Process according to Claim 12, characterized in that the insulation-layer-precursor composition is introduced into the hollow body by means of an injection moulding process.

14. Process according to Claims 12 or 13, characterized in that the unhardened elastomer-precursor composition is a silicone composition of RTV2 type, a silicone composition of HTV type or a silicone composition of LSR type.

15. Process according to one or more of Claims 12 to 14, characterized in that, based on the volume of the insulation-layer-precursor composition, the quantity present therein of the field-controlling particulate filler is from 0.1 to 25%.

16. HVDC cable with an outer electrically insulating layer, wherein the HVDC cable has, at least to some extent, an outer electrically conductive surface, wherein the electrically insulating layer comprises an elastomer and a field-controlling particulate filler, wherein the field-controlling particulate filler is metal oxide-containing core-shell particles which have a density of < 5 g / cm3 and are distributed homogeneously in the elastomer and the elastomer is a silicone rubber.

17. HVDC cable according to Claim 16, wherein the volume resistivity of the electrically insulating layer is in the range from 108 to 1013 ohm*cm.