EP2907144A1

EP2907144A1 - Resistance covering for a d.c. insulation system

Info

Publication number: EP2907144A1
Application number: EP14700861.9A
Authority: EP
Inventors: Steffen Lang
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2013-03-18
Filing date: 2014-01-15
Publication date: 2015-08-19
Also published as: US20160027549A1; WO2014146802A1; DE102013204706A1

Abstract

The invention relates to a resistance covering for a d.c. insulation system, comprising a matrix material with particles embedded therein, said particles having an aspect ratio greater than 1. The matrix material is flexible to such an extent that the particles align depending on an electric field strength. The particles can align in the electric field and thus a breakdown voltage of the resistance covering is increased. The invention also relates to a d.c. insulation system comprising said resistance covering.

Description

description

Resistance Cover for a DC Insulation System The present invention relates to a resistance cover for a DC insulation system. The invention also relates to a DC insulation system with the resistance coating.

Insulating systems for DC applications are usually based on a gaseous or a solid dielectric. If these insulation systems are subjected to direct current and exposed to a stationary electric field, the electric field distribution is determined only by the resistive properties of the insulation system. Decisive for the resistive properties is mainly the surface resistance of the dielectric. If the insulating system is under the influence of a rectified electric field, a charge carrier accumulation forms at the interface between solid dielectric and gaseous dielectric. Here, the charge accumulation can also by

Dirt particles on the surface of the dielectric are caused. As a result, the field distribution at the surface of the dielectric is adversely affected so that local field peaks occur, which can lead to flashovers. A conductive surface of the dielectric, for example in the form of a conductive resistance coating, can dissipate these charge carrier accumulations and thus avoid field overshoot. Recent developments call for ever more compact design of electrical systems in low, medium and high voltage engineering. Due to the ever-decreasing distances between the conductors, ever higher field strengths occur. However, from a field strength of 30 V / mm, non-linear effects in the conductive resistance coating can occur, and the current density no longer increases linearly with the field strength. The resistance coating then no longer behaves ohmsch. The excessive current density leads to this Heating and worst case overheating of the resistance coating, which can be damaged.

It is therefore an object of the invention to provide an improved resistance coating, which behaves ohmic even at high field strengths of more than 30 V / mm and can be used for different applications.

A resistance lining for a DC insulation system is proposed, which comprises a matrix material with embedded particles having an aspect ratio greater than one. In this case, the matrix material is so flexible that the particles align themselves as a function of an electric field strength.

The aspect ratio may preferably be greater than 2 and more preferably greater than 15. The aspect ratio here means the ratio of an expansion of a particle in a first spatial direction to an expansion of the particle in a second spatial direction. In particular, particles with an aspect ratio greater than 1, preferably greater than 2 and particularly preferably greater than 15 have a preferred direction along which they align. If the particles in the matrix material of the resistive lining can be aligned as a function of the electric field strength, an ohmic behavior of the resistive lining can be achieved at high field strengths of, for example, more than 30 V / mm, preferably more than 100 V / mm and particularly preferably more than 500 V / mm can be guaranteed or maintained. "Ohmic behavior" means that the current density of the resistive pad increases linearly with the electric field strength. Conductive effects between the particles are responsible for the ohmic behavior of the proposed resistor pad.

Thus, the grain boundaries in the individual particles as well as the particle transitions form potential barriers below the Breakthrough voltage can not be tunneled through. The conduction mechanism in this region results from a leakage between the particles, which can be described, for example, by the Pool-Frenkel effect or the Richardson Schottky mechanism.

At high voltages greater than the breakdown voltage, the electrons can overcome the potential barrier and the

Current density within the resistor pad increases disproportionately to the field strength. This nonlinear, and in particular exponential, behavior of the current density can be characterized by the non-linearity exponent alpha and the breakdown voltage, where the breakdown voltage is the voltage at which the electrons can overcome the potential barriers at the grain boundaries and particle transitions The breakdown voltage is thus proportional to the number of particles, and thus to the potential barriers of the grain boundaries and particle transitions, ie if the field strength increases so far that the breakdown voltage is exceeded, the electrons can tunnel between the individual particles and the particles The current density of the resistive lining no longer increases linearly and in particular exponentially The nonlinearity exponent is defined by the slope of the respectively logarithmically applied current density-field-strength characteristic curve In the case of a linear, ohmic characteristic curve In the case of a non-linear resistance behavior, "alpha" is greater than 1. With increasing field strength, additional charges can be shifted within the particles and the particles are polarized. If the matrix material is so flexible that the particles can move, they align with each other according to their polarization. In doing so, the distance and, as a result, the potential barrier between individual particles are increased. The breakdown voltage shifts to higher field strengths, and the resistance pad exhibits greater voltages than the original throughputs. Breaking voltage on a resistive behavior. With the resistance lining, an ohmic resistance behavior can thus be ensured even at high voltages or field strengths, and it can be ensured that the resulting current density does not rise disproportionately, even at high field strengths, but only linearly. This in turn can ensure that the power loss resulting from the current density also increases only linearly with increasing field strength, whereby the resulting Joule heating, which is proportional to the power loss, also does not increase disproportionately. As a result, the resistance coating is not exposed to an inadmissibly high temperature and as a result thereof is not thermally destroyed. Thus, therefore, can be derived by the resistance coating advantageously an electrical charge at interfaces, for example between a solid and a gaseous dielectric, without having to take constructive measures that take up a lot of space and at the same time ensure that the resistance coating is not unduly hot.

By "resistance coating" is meant herein also a resistance layer which may, but does not have to be formed cohesively with an insulator or any other component.

The resistance coating can be used in different DC insulation systems with field strengths greater than 30 V / mm, preferably greater than 100 V / mm and particularly preferably greater than 500 V / mm. For example, the resistive lining may be used in high voltage direct current (HVDC) transmission or in high voltage DC isolation systems such as transformers and their feedthroughs. The use in electronic components, where high field strengths occur, such as in printed circuit boards, is possible. For example, in printed circuit boards of semiconductor technology, for example in processors or chips, field strengths greater than 30 V / mm, preferably greater than 100 V / mm and particularly preferably greater than 500 V / mm occur when conductors are arranged by the miniaturization at a small distance from each other.

For the necessary flexibility of the matrix material, in one embodiment, the matrix material is an elastomer. The

Elastomer has a glass transition temperature which is smaller than a specified operating temperature of the resistive lining. An operating temperature range here refers to the temperatures which can occur in operation in the component equipped with the resistance lining. The operating temperature range thus includes the temperatures to which the resistance coating may be exposed. For example, the matrix material may be elastic in an operating temperature range of -200 to 500 degrees Celsius, preferably -20 to 120 degrees Celsius, and more preferably 40 to 70 degrees Celsius. The glass transition temperature is thus preferably smaller than the lower limit of the operating temperature range. The resistance lining can accordingly be designed for an operating temperature range of -200 to 500 degrees Celsius, preferably -20 to 120 degrees Celsius, and more preferably 40 to 70 degrees Celsius.

In a further embodiment, the matrix material is elastic. The matrix material of the resistor layer is preferably to be chosen such that it is elastic at the intended operating temperatures. The particles can thus move in the matrix material and align depending on the field strength. After removing the electric field, the particles return to their original orientation.

The matrix material is a variety of elastomers. Examples include rubbers such as natural rubber (NR), acrylonitrile-butadiene rubber (NBR), styrene-butadiene rubber (SBR), chloroprene rubber (CR), butadiene rubber (BR) and ethylene-propylene-diene rubber

(EPDM) or poly (organo) siloxane rubber (silicone rubber). Further exemplified elastomers are har- ze, such as polymethylsiloxane resin, polymethylphenylsiloxane resin, epoxy resin, alkyd resin or polyesterimide resin. The matrix material may also contain a blend with various elastomers.

In a further embodiment, the matrix material has a Shore A hardness of from 10 to 90, preferably from 20 to 80 and particularly preferably from 30 to 50. In this case, the Shore hardness is related to the matrix material without embedded particles. The matrix material can furthermore have a loss modulus G ", which is smaller than a storage modulus G '.

Rubbers, such as silicone rubber, are more elastic than resins, such as polyesterimide resin. For example, the Shore hardnesses A of silicone rubbers are in the range from 35 to 50. Elastic

In contrast, polyesterimide resins have a Shore A hardness greater than 45, in particular between 50 and 80, for example between 60 and 80. The elasticity of the matrix material influences how fast the particles align with changing field strength or how fast the particles relax, i. return to their original position. For example, the particles in a silicone rubber can align themselves directly with the increasing field strength, while particles, for example, in one

Polyesterimide resin can be aligned with the increasing field strength with a time delay, or if the matrix is stiff enough, it will not align at all. Similarly, particles relax faster in, for example, the silicone rubber than in, for example, the polyesterimide resin.

In a further embodiment, the particles are

platelet-shaped or rod-shaped. Particle mixtures with a mixture of platelet-shaped particles and rod-shaped particles are also possible. In this case, the particles can have an aspect ratio of from 10 to 1000, preferably from 10 to 100 and particularly preferably from 15 to 50. The aspect ratio for platelet particles refers to the ratio of each of length and width to thickness. at rod-shaped particles, the aspect ratio refers to the ratio of each of width and thickness to length. The aspect ratio and the resulting asymmetry in the particle dimensions influence the tendency of the particles to align. Thus, particles with a high aspect ratio have a greater tendency to align than particles with a smaller aspect ratio. In the case of platelet-shaped particles, for example, the particles in the resistance lining align themselves along the largest surface, ie the largest surface is oriented parallel to an interface between, for example, a solid and a gaseous dielectric. Similarly, rod-shaped particles can align along the length, ie the largest axis is oriented parallel to an interface between, for example, a solid and a gaseous dielectric.

In a further embodiment, the particles contain mica particles, silicon carbide particles (SiC particles), metal oxide particles, in particular aluminum oxide particles (A1 ₂ 0 ₃ particles), carbon nanotubes or mixtures thereof.

These particles are available in particular in the aforementioned aspect ratio.

In a further embodiment, a volume fraction of the particles is between 5 and 55% by volume, preferably between 6.5 and 40% by volume and particularly preferably between 15 and 30% by volume. Here, the volume fraction and data in% by volume refer to the total volume of the matrix material and the particles. These volume fractions of particles correspond to a matrix material with a density of 1 g / cm ³ and

platelet-shaped particles with a density of 3.5 g / cm ³ an aspect ratio of 20. If the particle content is too high, the freedom of movement of the individual particles are limited and they can no longer align themselves in the matrix material. Therefore, the particle content is chosen so that the particles can align in the matrix material. If the particle content is too low, the particles can not contact each other, creating no guiding paths be formed and the resistance pad has the resistivity of the matrix.

In a further embodiment, a volume fraction and / or aspect ratio of the particles is selected such that the percolation threshold is exceeded. In this case, the percolation threshold denotes the volume fraction of particles, beyond which the particles can contact and form guide paths in the matrix material. In this case, the volume fraction at which the percolation threshold is exceeded may depend on the aspect ratio of the particles.

In a further embodiment, the matrix material contains first particles having a first electrical conductivity or a first electrical resistance, and second

Particles having a second electrical conductivity or a second electrical resistance, wherein the first electrical conductivity or the first electrical resistance of the second electrical conductivity or the second electrical resistance is different. Thus, in particular the electrical conductivity or the electrical resistance of the resistance lining can be adjusted by a proportion by weight of the first and second particles.

In this case, the proportion by weight is based on the total weight of the first and second particles. With a mixture of first and second particles, the electrical conductivity and thus the power loss of the resistor pad can be adjusted. Due to the weight proportions of the first and second particles, the resistance lining can thus be optimally adapted to the desired DC insulation system. In this case, not only a particle mixture with first and second particles but also particle mixtures with a plurality of particles can be used. In order to easily adapt the electrical conductivity or the electrical resistance of the resistor pad, the particles contain at least one dopable semiconductor material whose doping is the electrical conductivity or the determined electrical resistance of the particles. In this case, the particles may be coated with the dopable semiconductor material. Furthermore, depending on the doping, the dopable semiconductor material may have an electrical square resistance in the range from 1 × 10 3 to 1 × 10 5 Ω. In this case, indications of square resistances mean that the surface resistance was measured at a field strength of 100 V / mm. By doping the semiconductor material particles with different electrical conductivities or resistances can be provided. The electrical conductivity or the resistance of the resistive lining is correspondingly easily adjustable via the particles contained therein and can be easily adapted to the requirements in different Gleichstromisoliersystemen.

For example, the semiconductor material may be a metal oxide such as tin oxide (SnO ₂ ), zinc oxide (ZnO), zinc stannate (ZnSnO ₃ ), titanium dioxide (TiO ₂ ), lead oxide (PbO) or silicon carbide (SiC). Suitable doping elements are antimony (Sb), indium (In) or cadmium (Cd). Tin oxide (SnO ₂ ) doped with antimony (Sb) is preferably used. Depending on the doping, the use of the dopable semiconductor material makes it possible to realize different electrical square resistances in the range from 1 × 10 3 to 1 × 10 5 Ω, preferably in the range from 1 × 10 10 to 1 × 10 5 Ω. In order to provide a particle having a high square resistance in the range of 10 10 to 1 10 10 Ω, the particles may additionally be coated with an electrically insulating layer, such as titanium dioxide (TiO ₂ ).

In a further embodiment, the resistance coating is set such that it behaves ohmically at field strengths, in particular greater than 30 V / mm, preferably greater than 100 V / mm and particularly preferably greater than 500 V / mm. This means that the current density of the resistive lining increases linearly with the increasing field strength. Furthermore, the resistance coating can be adjusted so that it is in a first field strength range, in particular greater than 30 V / mm, preferably greater than 100 V / mm and especially preferably larger than 500 V / mm ohmic behaves and in a second field strength range, in particular greater than 30 V / mm, preferably greater than 100 V / mm and more preferably greater than 500 V / mm not ohmic behavior. Thus, a resistance lining can be provided which, for example, has an ohmic behavior only in the field strength region relevant for the respective DC insulation system. For adjusting the resistance lining, as described above, the matrix material and / or the particles can be selected accordingly. For example, the field strength, from which the resistance lining behaves ohmically, can be set by the flexibility of the matrix material at different temperatures. In addition, by setting the resistivity of the resistive lining, such as by selecting the mixing ratio of the particles, a predetermined power dissipation can be set in a predetermined field strength range.

It is further proposed a Gleichstromisoliersystem with the above-described resistance coating. Field strengths greater than 30 V / mm, preferably greater than 100 V / mm and particularly preferably greater than 500 V / mm can occur in the region of the resistive lining. In one embodiment, the DC insulation system comprises a first conductor and a second conductor, between which, for example, electric field strengths greater than 30 V / mm, preferably greater than 100 V / mm and particularly preferably greater than 500 V / mm can be generated during operation of the DC insulation system.

In a further embodiment, the DC insulation system comprises a first conductor and a second conductor, wherein the resistance lining is arranged between the two conductors. In particular, between the first and the second conductor at least one insulator may be provided with the resistance pad, which extends at least partially between the first and the second conductor. The resistance pad preferably extends from the first to the second conductor. The additional space between the first and second conductors may be filled with a gaseous dielectric such as air be. The insulator can thus form a solid dielectric with interfaces to a gaseous dielectric. Preferably, the resistance coating is arranged at such interfaces of the insulator, which adjoin a gaseous dielectric, such as air. The coating of the insulator with the resistance coating can be done, for example, by spraying, knife coating, brushing, dipping or the like. Thus, the resistive coating can be applied as a lacquer on the interfaces of the insulator, which contains the matrix material, the particles and optionally a solvent.

In the following, embodiments of the invention will be explained in more detail with reference to the accompanying schematic drawing. Showing:

Figure 1 is a DC insulation system with two conductors, between which an insulator is arranged; Figure 2 shows the Gleichstromisoliersystem according to Figure 1, in which the insulator has a resistance pad;

FIG. 3 shows a printed circuit board as a DC insulation system with the resistance lining;

Figure 4 shows a course of the square resistance against the

Field strength for resistance coatings with rigid matrix material and different particle proportions; FIG. 5 schematically shows a resistance covering with a flexible matrix material and particles embedded in it at field strengths of less than 30 V / mm;

FIG. 6 is a schematic view of the resistance lining of FIG. 5

Field strengths greater than 30 V / mm; FIG. 7 shows the profile of the square resistance versus the field strength for resistance coatings which have different elastomers as the matrix material; and FIG. 8 shows the profile of the square resistance versus the field strength for resistive linings with elastomers which are tougher than those of the resistive linings of FIG. 7. Identical or functionally identical elements are given the same reference numerals in the figures unless otherwise stated.

Figure 1 shows a DC insulation system 1 with a first conductor 2, which carries a direct current, and a second conductor 3, which is at ground potential as neutral. Between the two conductors 2, 3 is an electric field E, which is greater than 30 V / mm, preferably greater than 100 V / mm and more preferably greater than 500 V / mm.

An insulator 4 spaces the two conductors 2, 3 from each other. In this case, the insulator 4 extends partially in a space 5 between the two conductors 2, 3. The further space 5 is filled with a gaseous dielectric, such as air. This forms on the insulator 4 interfaces 6, 7, which form a transition between the insulator 4 as a solid dielectric and the gaseous dielectric. At these interfaces 6, 7 dirt particles 8 can accumulate, which can lead to Feldüberhöhungen and thermal destruction of the insulator 4. In order to avoid such damage, the insulator 4 may be coated with a resistance coating 9.

The configuration of FIG. 2 illustrates the use of the resistance lining 9 in the DC insulation system 1 of FIG. 1. Here, the insulator 4 is coated with the resistance coating 9. This is arranged at the interfaces 6, 7 (shown only by way of example for the interface 7) of the insulator 4, which adjoin the gaseous dielectric, such as air. Due to the resistance coating 9 field peaks caused by dirt particles 8 can be avoided. Thus, the insulator can be protected against electrical damage caused by (partial) discharges, in particular at field strengths greater than 30 V / mm, preferably greater than 100 V / mm and particularly preferably greater than 500 V / mm.

FIG. 3 shows a printed circuit board 10 with the resistance coating 9 as a further example of a DC insulation system 1 with field strengths of, for example, greater than 30 V / mm, preferably greater than 100 V / mm and particularly preferably greater than 500 V / mm.

The printed circuit board 10 of FIG. 3 comprises a substrate onto which a printed conductor structure 11 with printed conductors 12, for example, is printed. In order to be able to build such printed circuit boards 10 as minimized as possible, the printed conductors 12 must be provided in a high density on the substrate, without influencing the functionality. However, the closer the printed conductors 12 are arranged to one another, the higher the electric field strengths E between the printed conductors 12. Thus, the electric field strength E between printed conductors 12 can exceed 30 V / mm, preferably more than 100 V / mm and particularly preferably more than 500 V. / mm increase. In order to homogenize such field strengths E over the entire distance of the two conductors, the resistance coating 9 is provided on the insulating substrate in region 13 between the conductor tracks 12 shown by way of example in FIG.

FIG. 4 shows a profile of the square resistance R against the electric field strength E for resistive linings 9 with rigid matrix material 22 (see FIGS. 5 and 6) and different mixing ratios of first particles 23 with a first, high resistance (in the present case also "high-coherence filler"). and particles 24 with a second, lower In this case, the square resistance R is given in ohms and the field strength E in V / mm In the illustrated curves 14 to 18, the particle content of the high-resistance filler continues to increase, at the same time the particle fraction of the low-resistance Filler in the same ratio (eg. In steps of 25%) is reduced.

The course 14 shows the behavior of the square resistance R against the field strength E in the case of a resistance coating 9 which has a matrix material 22 (for example 78% by volume) and a low-resistance particle fraction (for example 22% by volume). This shows at low field strengths E below 10 V / mm a constant square resistance R of about l * 10el0 Ω. From a field strength E of about 10 V / mm, the square resistance R decreases. The resistance pad 9 thus shows from about 10 V / mm a non-ohmic behavior, the square resistor R decreases with increasing field strength E and accordingly increases the current density. The course 15 shows the behavior of the square resistance R against the field strength E in the case of a resistance coating 9 in which a particle fraction of the low-resistance filler of 25% by weight has been replaced by a high-resistance filler. Due to the increased particle content, the square resistance R increases up to an electric field strength E, from which the behavior deviates from the ohmic behavior. The courses 16, 17, 18 show an analogous behavior, with the low-resistance particles 24 being replaced step by step (for example in 25% steps) by high-resistance particles 23 in the case of the resistive linings 9 investigated.

Further shown in FIG. 4 is the working range of the tested resistor pads 9. Thus, the current that can be measured in the resistor pad 9 is too low in the range 19 with low field strengths E and high square resistance values R for the measurement. In a region 21 with low square resistance values R and high field strengths E, heating and thermal destruction of the resistance lining 9 occurs. In a region 20 with high square resistance values R and high field strengths E, on the other hand, discharges or partial discharges occur in air, which can likewise lead to damage to the resistance lining 9.

FIG. 5 schematically shows a resistance lining 9 with a flexible matrix material 22 and particles embedded therein

23, 24 at field strengths E less than 30 V / mm. The matrix material 22 is in particular an elastic material which has a Shore hardness A of, for example, 10 to 80. For this purpose, elastomers, such as silicone rubbers or

Polyersterimide resins.

Platelet-shaped particles 23, 24 are embedded in the matrix material 22. The particles 23, 24 are designed as coated particles 23, 24 with an aspect ratio of 10 to 100. For example, platelet-shaped particles 23, 24, such as mica particles, which have a thickness of a few hundred nanometers, for example 350 nm, and a width or length of a few micrometers, for example 6.5 .mu.m, are suitable. Also suitable are rod-shaped particles 23,

24, such as carbon nanotubes, for example, have a width and thickness of a few nanometers and a length of a few hundred nanometers.

Furthermore, the particles 23, 24 are preferably coated with a doped semiconductor material, such as tin oxide. For example, antimony is suitable as doping element. Depending on the doping of the semiconductor material with which the particles 23, 24 are coated, other electrical results

Conductivities or resistances for the particles 23, 24. Thus, the resistance coating 9 can have different particles 23, 24 or a particle mixture, via which the resistance or the conductivity of the resistance coating 9 can simply be adapted to the respective application.

The particles 23, 24 are further arranged in a plurality of particle layers 26. The particles 23, 24 are along their larger dimension, ie at platelet-shaped particles 23, 24 along the larger surface and rod-shaped particles 23, 24 along the major axis aligned. In addition, the particles 23, 24 of adjacent layers 26 overlap at least partially.

In FIG. 5, the resistance coating 9 is exposed to low field strengths E of, for example, less than 30 V / mm. FIG. 6 schematically shows the resistance coating 9 at field strengths E, for example greater than 30 V / mm, preferably greater than 100 V / mm and particularly preferably greater than 500 V / mm.

For illustrative purposes, in FIGS. 5 and 6, a particle 24 is shown which aligns at higher field strengths. In comparison to FIG. 5, the particle 24 in FIG. 6 is more strongly polarized, ie. H. the charge shift within the particle 24 is enhanced. At high field intensities E greater than 30 V / mm, preferably greater than 100 V / mm and particularly preferably greater than 500 V / mm and given distance 27 in a non-flexible matrix material 22, the electrons could overcome the potential barrier and the current density of the resistive layer 9 would increase disproportionately ,

However, if the matrix material 22 is so flexible that the particle 24 can move, this aligns with the adjacent particles 23 in accordance with its polarization. For by applying a constant voltage U ₂ >> Ui to the resistance coating 9, the particles 23, 24 are polarized. Depending on the aspect ratio of the particles 23, 24, the conductivity of the particles 23, 24 and the adjacent

Field strength acts on the torque of the particles 23, 24. In a flexible matrix material 22, the torque of the particles 23, 24 hardly counteracts a force and the particles 23, 24 can align themselves in the field. This flexibility of the matrix material 22 and the resulting mobility of the particles 23, 24 is indicated in FIGS. 5 and 6 with the springs 28 between particles 24 and the adjacent particles 23. The orientation of the particle 24 increases the distance 27 to adjacent particles 23 and the resulting potential barrier. The electrons can no longer tunnel, and a leakage current will flow, which results in an ohmic resistance behavior. The breakdown voltage of the resistor pad 9 thus shifts towards higher field strengths E, and the resistor pad 9 has an ohmic behavior even with field strengths E greater than 30 V / mm, preferably greater than 100 V / mm and particularly preferably greater than 500 V / mm.

FIG. 7 shows the course of the square resistance R against the field strength E for resistance coverings 9 which comprise different elastomers as matrix material 22.

Based on the total volume, the tested resistive linings 9 contain a volume fraction of 22% by volume of particles 23, 24 with a square resistance R of 1 * 10 2 ohms. The composition of the elastomers 22, in which the particles 23, 24 are embedded, based on silicone rubber, which has a Shore A hardness between 37 and 45. The course 29 represents the behavior of the resistance covering 2, which contains a silicone rubber with Shore hardness A 45, at room temperature. The course 31 represents the behavior of the resistance covering 2, which contains another silicone rubber with Shore hardness A 37, at room temperature The course 32 represents the behavior of the resistance covering 2, which contains a further silicone rubber having a Shore hardness A 45, at room temperature. The different resistance values R result here from the different starting monomers which are contained in the matrix material 22.

FIG. 7 shows that resistive linings 9 with a flexible matrix material 22 have an ohmic behavior over a wide field strength range E of 10 to 500 V / mm. In addition, the curve 30 shows the behavior of the square resistance R against the field strength E, whereby not only the particles 23, 24 but also non-conductive beads are embedded in the matrix material 22 with a Shore hardness A of 45. As a result, the orientation of the particles 23, 24 in the matrix material 22 is suppressed. The curve 30 therefore shows a non-ohmic behavior even at some 10 V / mm. The ability of the particles 23, 24 to be able to align is thus crucial in order to achieve the desired ohmic behavior even at high field strengths.

FIG. 8 shows the course of the square resistance R against the field strength E for resistance coverings 9 with an elastomer that is tougher than the elastomers from FIG. 7.

Based on the total volume, the tested resistive linings 9 contain a volume fraction of 22% by volume of particles 23, 24 with a square resistance R of 1 * 10 2 ohms. The composition of the elastomer is based on a

Polyesterimide resin having a Shore hardness between 45 and 80. In this measurement, the courses were recorded at different times for the same resistor pad 9. Thus, the measurement of the course 33 of the square resistor R was started by applying the electric field. Here it can be seen that the ohmic behavior sets only at higher field strengths E in the range of 500 V / mm. The particles 23, 24 thus align themselves only slowly because the elastomer based on polyesterimide resin is tougher than elastomers based on silicone rubber.

After a period of 24 h, the same sample was measured again (course 34). It shows that the orientation of the particles 23, 24 was still partially present. The relaxation in the polyesterimide resin thus takes place more slowly. A re-measurement after 5 minutes with the same sample showed curve 35, which shows that the particles 23, 25 have not relaxed in such a short time and maintained their orientation. The course 36 and 37 were elevated with one Particle content recorded and show that the resistance coating 9 is not ohmic behavior from 500 V / mm, if the particles 23, 24 can not align.

Although the invention has been described herein with reference to various embodiments, it is not limited thereto, but variously modifiable.

Claims

claims

A resistive pad (9) for a DC insulation system (1) comprising a matrix material (22) having particles (23, 24) embedded therein having an aspect ratio greater than 1, said matrix material (22) being so flexible that Align the particles (23, 24) in response to an electric field strength (E).

2. Resistance layer according to claim 1, characterized in that the matrix material (22) is an elastomer.

3. Resistance coating according to claim 1 or 2, characterized in that the matrix material (22) has a Shore A hardness of 10 to 90.

4. Resistance coating according to one of claims 1 to 3, characterized in that the particles (23, 24) are platelet-shaped or rod-shaped.

5. Resistance coating according to one of claims 1 to 4, characterized in that the particles (23, 24) mica particles, silicon carbide particles, metal oxide particles, carbon nanotubes or mixtures thereof.

6. Resistance coating according to one of claims 1 to 5, characterized in that a volume fraction and / or

Aspect ratio of the particles (23, 24) is selected so that a percolation threshold is exceeded.

7. Resistance coating according to one of claims 1 to 6, characterized in that a volume fraction of the particles (23, 24) between 5 and 55 vol. % lies.

8. Resistance layer according to one of claims 1 to 7, characterized in that the matrix material (22) first particles (23) having a first electrical resistance, and second particles (24), the second electrical resistance stand, wherein the first electrical resistance is different from the second electrical resistance, and wherein the electrical resistance of the resistance lining (9) by a weight ratio of the first and second particles (23, 24) is set.

9. Resistance coating according to one of claims 1 to 8, characterized in that the particles (23, 24) contain at least one dopable semiconductor material whose doping determines the electrical resistance of the particles (23, 24).

10. Resistance layer according to claim 9, characterized in that the dopable semiconductor material has, depending on the doping an electrical resistance in the range of l * 10e3 to l * 10el5 Ω.

11. Resistance layer according to claim 9 or 10, characterized in that the dopable semiconductor material is a metal oxide.

12. Resistance covering according to one of claims 1 to 11, characterized in that the resistance lining (9) is such that it behaves ohmically in a first field strength range and does not behave ohms in a second field strength range.

13. DC insulation system (1) with a resistance lining (9) according to one of claims 1 to 12.

The DC isolation system of claim 13, further comprising a first conductor (2, 12) and a second conductor (3, 12), said resistance pad (9) being disposed between said first and second conductors (2, 3, 12).

15. Gleichstromisoliersystem according to claim 14, characterized in that between the first and the second conductor (2, 3, 12) at least one insulator (4) with the Widerstandsbe- Layer (9) is provided, which extends at least partially between the first and the second conductor (2, 3, 12).