EP3146551B1 - Electrical switching apparatus for medium and high voltage - Google Patents

Description

The invention relates to an electrical switching device, in particular for medium-voltage and / or high-voltage applications, comprising at least two contactable conductor elements that can be spaced apart by a movement device and a housing that defines a switching chamber and consists of an insulator that at least partially surrounds the conductor elements.

In medium and / or high voltage applications, generally speaking at voltages that are greater than 1 kV, the high voltages require more complex switching devices that can withstand the electrical fields that occur, are as resistant to degradation effects as possible, and also skip processes outside the actual switching chamber should avoid.

A classic example of this are the vacuum circuit breakers (VCB), which are core components in power transmission and distribution, especially in their switching systems. They cover a large part of the medium-voltage switching applications, i.e. switching applications for example in the range from 1 kV to 52 kV, as well as a relevant part in low-voltage systems. Their use in high-voltage transmission systems, for example at voltages greater than 52 kV, is also increasing. While a VCB is closed most of the time, thus providing contacting of the conductor elements, its main task is the interruption of currents in AC systems under nominal conditions, in particular for switching nominal currents on and off, or preferably for interrupting currents under fault conditions to break short circuits and protect the systems. Other applications include pure switching of load currents using contacting conductor elements, which is mostly used in low and medium voltage systems.

The vacuum interruptor (VI, also vacuum interrupter) is the core element of a VCB. A vacuum interrupter usually has a pair of contacts which are formed by corresponding conductor elements, at least one of which can be moved by means of a movement device in order to be able to bring about the open and closed states of the switching device. Usually, one conductor element is moved axially with respect to the other fixed conductor element. The contacts can be made on current-conducting bolts, in particular made of metal, which provide both current and heat conduction and the mechanical means for holding and / or moving the contacts.

A VI further comprises a vacuum-tight housing and the movement device mentioned and can also comprise a metal bellows which is connected on one side to the housing and on the other side to the moving conductor element, in particular the moving bolt. The housing is essentially formed by an insulating component, that is to say an insulator, for example a ceramic tube which is connected to the conductor elements via connecting elements, metal caps or the like being used, for example, which terminate the insulating component in the axial direction to form the switching chamber . A permanent high vacuum of less than 10 ^-8 Pa prevails within the sound chamber, which can be assured, for example, for operating periods of at least 30 years by appropriate design of the housing and the caps. The vacuum is necessary to assure the "make-break operations" and to ensure the insulation properties of the switching device in the open state.

If the switching device is in an open state, the nominal voltage of the system must be isolated on the one hand, and on the other hand also surge voltages of high amplitudes, for example can be triggered by a lightning strike in the system. If the switching device changes from the closed to the open state, and consequently the contacts of the conductor elements are spaced apart, nominal currents or short-circuit currents must be interrupted, which lead to the appearance of transient voltage peaks across the VI, which are significantly higher than the nominal AC voltages of the system.

High voltages in vacuum systems usually generate free electrons through field emission processes if the electric field strength is sufficiently high. The acceleration of the electrons in the high electrical fields increases the kinetic energy of these electrons, for example up to energies that exceed a few tens or even hundreds of KeV. The interaction of these high-energy electrons with the housing structures leads to the production of high-energy X-rays, which can leave the vacuum interrupter. While under normal conditions the fault current inside the vacuum interrupter is minimal and does not generate any significant X-ray radiation components, circumstances can arise, for example when temporary high-amplitude voltage peaks occur in which the resulting X-ray radiation generates free electrons on and / or near the outer surface of the insulator. These electrons can be accelerated by the electrical fields on and near the insulator surface, disturb the electrical field distribution in sensitive areas and lead to gas breakdown, which leads to an error in the operation of the vacuum interrupter.

Even in cases where there is no detectable X-ray radiation, e.g. in low and medium voltage applications, the high electrical fields in critical areas of the vacuum interrupter, e.g. at the connection of the insulator and the metal caps by soldering (brazing), can lead to the emission of electrons, which leads to a significant amount of field emission. These electrons can also locally disturb the electric field and others Field strengthening and / or lead to the multiplication of charges by electron avalanches, which in turn can result in the loss of the insulation strength and / or the voltage resistance of the vacuum interrupter.

Similar challenges exist on the inner surfaces of the vacuum interrupter while an additional problem must be solved. Due to the interruption of the current (nominal current as well as short-circuit current), parts of the contact material are evaporated and distributed as hot metal vapor within the switching chamber. This metal vapor can settle on the insulator surface and builds up a conductive metal layer over time. This metal layer, even if it is only weakly conductive, can also disrupt the electrical field outside and inside the vacuum interrupter and therefore deteriorate the voltage resistance of the vacuum interrupter over time. In this context, it was proposed to provide a shielding element in the contact area of the conductor elements, which may also be made of metal, for trapping free metal particles of the conductor elements, but which also has an influence on the field distribution within the switching chamber, but also on the insulator.

For the reasons mentioned, the insulator, which is usually made of ceramic, must be able to withstand high voltages across its surface, even if X-rays and free electrons are present or, in some cases, even if the insulator is contaminated by dust particles that electrostatically attached to the outer surface of the insulator. After the insulator significantly adds to the cost of a vacuum interrupter (or other switching device) and also negatively affects the cost of other structural elements of the vacuum interrupter (or other switching device), it is necessary to optimize the insulator for maximum dielectric strength with a minimum size.

This problem has been solved so far by selecting the inner and outer geometry of the vacuum interrupter in such a way that the expected electric field strengths do not exceed empirically derived limits for a specific geometry of the vacuum interrupter. Since these limitations cannot be predicted precisely, especially for triple point areas and sharp metal edges, the design of vacuum interrupters not only depends on calculations for the electrical field during the development process, but also requires a large amount of empirical optimization. This also relates to the build-up of metallic layers from the inner surfaces of the insulator, which, as already mentioned, are usually to be avoided today by using shield structures (shield elements) within the switching chamber. Nevertheless, the deposits of metal vapor and their influence on the dielectric strength of the vacuum interrupter cannot be predicted quantitatively in a sufficiently precise manner today. It should also be noted that the design processes mentioned all lead to a reduction in the insulation properties of the outer structure of the vacuum interrupter well below the dielectric strength of air or other gases which surround the vacuum interrupter, so that isolator sizes (length, diameter) are required which are necessary with regard to the Costs and space are not optimal. The addition of shielding elements with regard to the metal vapors leads to distortion of the electrical fields occurring during operation at the insulator, which can lead to strong fields at certain points and thus to an overload of the insulator, which is caused by the charges that build up there. However, other causes, as has already been shown, lead to such local high fields on the insulator of the housing of the vacuum interrupter, the problems described here also applying to other switching devices in addition to the vacuum interrupter mentioned as an example.

The document JP 2004 265801 A discloses a switching device according to the preamble of claim 1.

The invention is therefore based on the object of specifying a switching device with a housing comprising an insulator which, despite being simple to implement, reduces distortions in the electrical field in the region of the switching device due to surface charges.

To solve this problem, an electrical switching device of the type mentioned is provided, which is characterized in that the housing has a resistive coating of a matrix material filled with a filler at least on one side, preferably on the outside, the surface resistance of the coating between 10 ⁸ and 10 ¹² Ω at the operating field strength and the coating is conductively connected to the conductor elements, in particular by conductive caps closing the housing at the end and holding the conductor elements, the surface resistance being varied along the direction of extension of the conductor elements, the variation of the surface resistance being along the direction of extension is achieved by using different fillers and / or by varying the concentration of the single filler.

The property spectrum of the coating is preferably further improved in that the non-linear exponent describing the slope in the current-voltage characteristic of the coating is less than 6. The invention presented here is based on a special coating, which is preferably applied to the outside of the insulator and can be applied before or during the manufacturing process of the housing, for example as a glazing process of the housing made of ceramic, or spraying on at the end of the manufacturing process or other suitable application processes so that a well-defined coating is created.

Suitable measures for setting the desired surface resistance can already be taken during production, after a skillful choice of the grain size of the filler or of a conductive material or a conductive coating of particles from which the filler consists can reduce the surface resistance, with an appropriate doping also an increase in the sheet resistance can be achieved.

A well-known example of a material combination that is suitable for such a coating is shown by DE 198 39 285 C1 described. Although this is about a smoldering protective tape, it has been shown that the combination there of a carrier material and an inorganic filler which has tin oxide is also suitable for producing a coating in the context of the present invention in order to achieve the desired properties of the coating .

As already mentioned, variables influencing the resistance / conductivity of the coating are, in addition to their thickness, the amount of doping, the concentration of the filler, the conductivity of the filler itself and the particle size of the filler. Overall, the coating is generally conductive, even if there is a high resistance, but this means that a fault current is deliberately impressed into the switching device in order to optimize its electrical field distribution under operating conditions. The conductive coating of the present invention causes surface charges to dissipate that would otherwise accumulate on the insulator and result in distortion of the electrical field. With a skilful choice of properties, as already indicated, an extremely stable, corrosion-resistant and reproducible conductive layer with a desired surface resistance is created.

ergibt, worin I der Strom ist, U die Spannung, K eine geometrieabhängige Konstante und α der Nichtlinearitätsexponent.The coating according to the invention thus allows the field distribution on the surface of the insulator to be homogenized. The coating is ohmic as much as possible, which means that it has as little dependency on the applied voltage (and thus on the applied electric field). As already explained, it is particularly preferred if the non-linear exponent describing the slope in the current-voltage characteristic of the coating is less than 6. This occurs, for example, for the already mentioned tin oxide, SnO ₂ , but also for the silicon carbide, SiC, which is also mentioned, and consequently also for the corresponding fillers. The non-linearity exponent mentioned, which is usually referred to as α, is known in connection with voltage-dependent resistors (varistors). In the case of varistors it is known from the voltage-current characteristic curve that the resistance decreases with increasing voltage, which is described by the non-linearity exponent α, as can be seen from the defining equation $$\mathrm{I.}={\mathit{KU}}^{\alpha }$$

gives, where I is the current, U is the voltage, K is a geometry-dependent constant and α is the non-linearity exponent.

Known combinations of coating materials use materials whose varistor properties are more pronounced, for example fillers with zinc oxide, ZnO. This class of materials has highlighted switching characteristics, so it shows a strong non-linear behavior above a certain threshold value of the electric field. Within the scope of the application of the present invention, this would lead to a drastic disturbance of the field distribution as soon as even a portion of the coating exceeds this threshold value, which itself can already lead to a malfunction of the switching device. Coatings that use graphite as part of the filler are also rather unsuitable for the application described here, since this is the disadvantage there is that the resistance to corrosion, in particular the resistance to partial discharge erosion, is significantly worse than in the materials described by the present invention; furthermore, the conductivity of such a coating would be significantly too high, so that the Joule heating occurring within the conductive coating would be too high.

In contrast to these examples, the soft characteristics of the material composition, as claimed by the present invention, serve to gradually reduce the surface charges that would otherwise accumulate and / or lead to electron avalanches near the surface, so that the coating according to the invention provides a strong one Distortion of the electrical field distribution is avoided. Electrons that are released by X-rays, charge accumulation or electron avalanches are thus quickly removed from the surface of the insulator, so that field distortions are largely avoided. As a result, the electrical field strength on the surface of the switching device, and consequently of the housing, becomes extremely homogeneous, which in turn results in a reduction in size, in particular the length, and other geometrical requirements for the switching device. The switching device can be implemented inexpensively.

As has been explained, material combinations are used here that are not only easy to process, but can also be adjusted to certain desired surface resistance values by simple modifications. As already mentioned, it is preferred if the filler is or comprises SnO ₂ or silicon carbide SiC. If the conductivity properties of these substances are to be adapted by doping, a preferred embodiment of the invention provides that the filler is or comprises tin oxide doped with antimony and / or silicon carbide doped with aluminum. For example a doping of 0 to 15 mol% of antimony (Sb) in tin oxide (SnO ₂ ) can be provided.

At this point it should also be noted that these preferred material combinations are particularly suitable for operating field strengths in the range of the isolator from 100 to 1200 V / mm.

The matrix material can be selected from the group comprising elastomers, thermosets, thermoplastics and glass. The various coating methods for producing the coating can be selected accordingly. The matrix material can therefore be organic, for example as a polymer, or inorganic, for example as glass, in which the filler is introduced. It is expedient if the filler concentration is 10 to 90% by weight, in particular 40 to 60% by weight. The preferred range from 40 to 60% by weight corresponds to a volume fraction of about 20 to 30% by volume when tin oxide is used on mica platelets.

The thickness of the coating also influences how high the surface conductivity of the coating is; In addition, thicker coatings tend to have more stable surface resistance properties with certain material combinations. In the context of the present invention, coating thicknesses of 100 μm to 500 μm have proven to be expedient.

The filler can consist of particles with a grain size of 100 nm to 300 μm, preferably 1 μm to 50 μm. If inorganic particles in the micrometer range, for example silicon carbide, are used, a carrier material is not absolutely necessary, although it can also be expedient, in particular if a filler comprising tin oxide SnO _{2 is} used if the particles are platelets made of a carrier material, in particular mica , are those with the resistance material defining the resistance properties, in particular tin oxide SnO ₂ or silicon carbide SiC, are coated, preferably with a layer thickness in the range from 10 to 100 nm. Accordingly, mica platelets (mica platelets) can be used which are coated with a layer of semiconducting material, in particular tin oxide. An alternative to using such platelets is quartz flour. Especially when using the platelets, the aspect ratio also plays a role in the properties of the coating. For example, in the case of platelets, an aspect ratio less than or equal to five can be set for width to height. If a filler with an emphasized aspect ratio, for example platelets, is used, it is, as already explained at the beginning, particularly advantageously possible to reach a range in which the surface resistance no longer depends significantly on the concentration of the filler, which reduces the reproducibility of the filler Coating increased.

A further possibility for adapting the surface resistance, here specifically for increasing the conductivity, is a surface treatment of the particles, it being possible, for example, for the particles to be coated on the outside with an electrically conductive layer, in particular titanium oxide TiO ₂ . Particularly with smaller grain sizes and / or lower concentrations, such a conductive coating, preferably with titanium oxide, can be expedient in order to produce the desired conductivity properties and thus surface resistances.

According to the invention, the use of background knowledge for local variation of the surface resistance leads to improved results, so that, for example, in Areas in which it is known that, for example due to other components of the switching device, high fields occur anyway, a lower surface resistance can be selected so that charges are distributed faster than in areas of smaller operating field strengths. After switching devices are usually designed symmetrically around the direction of extension of the conductor elements (and therefore also the direction of movement of the at least one movable conductor element), the invention provides that the surface resistance is varied along the direction of extension of the conductor elements, in particular depending on a change in the electrical field under operating conditions along the direction of extension of the conductor elements. Such a variation of the resistance along the direction of extension is achieved by using different fillers and / or by varying the concentration of a single filler, for which suitable manufacturing techniques are already known in the prior art. The variation of the surface resistance along the direction of extension can additionally be achieved by varying the thickness of the coating. For example, a certain course of the sheet resistance can be realized over the length of the switching device, be it by changing the thickness of the coating, by using different fillers with different conductivities, the respective concentration of which changes along the length of the switching device, or by varying the Concentration of the single filler over the length of the switching device.

An adjustment can thus be made with regard to prior knowledge about the distribution of the electric field when the switching device is in operation.

The switching device can in particular be designed as a vacuum interrupter. It is now further provided that the vacuum interrupter in the contacting area of the conductor elements has a shielding element which influences the electrical field on the insulator and is arranged within the switching chamber and / or held between two housing parts of the housing Intercepting free metal particles of the conductor elements, the shield element (which can also be referred to as a vapor shield) frequently also causes field distortion, which can be significantly homogenized or compensated for by the use of the coating in the context of the present invention, and its effects, for example charge accumulations , can be avoided. For example, with such shielding elements, the operating field strength may weaken in the area of the shielding element itself, that is to say behind or next to the shielding element, while larger operating field strengths may occur on the insulator after the length of the shielding element. This knowledge can also be used to vary the surface resistance depending on the location, as has just been explained.

Fig. 1

eine erfindungsgemäße Schaltvorrichtung gemäß einem ersten Ausführungsbeispiel,

Fig. 2

ein möglicher Verlauf des Flächenwiderstands entlang der Erstreckungsrichtung der Leiterelemente, und

Fig. 3

eine erfindungsgemäße Schaltvorrichtung gemäß einem zweiten Ausführungsbeispiel.

Further advantages and details of the present invention result from the exemplary embodiments described below and from the drawing. Show:

Fig. 1

a switching device according to the invention according to a first embodiment,

Fig. 2

a possible course of the sheet resistance along the direction of extension of the conductor elements, and

Fig. 3

a switching device according to the invention according to a second embodiment.

Fig. 1 shows in the form of a schematic diagram a first embodiment of a switching device 1 according to the invention, here a vacuum interrupter. A housing 3 composed here of two tubular ceramic parts, that is to say insulators 2, is closed off by metal caps 4 and defines a switching chamber 5 into which two conductor elements 6 with contacts 7, for example designed as bolts, are guided. This in Fig. 1 the lower of the conductor elements 6 is designed to be movable according to the arrow 8 and the indicated movement device 9 and can be displaced in the direction of extension 10 of the conductor elements 6, which also forms the axis of symmetry of the switching device 1, in order to bring the contacts 7 into contact or at a distance, an open state of the switching device 1 being shown here. Due to the mobility of the lower conductor element 6, it is coupled to the metal cap 4 via a metal bellows 11; The metal caps 4 are thus conductively connected to the conductor elements 6 on both sides.

A vacuum prevails within the switching chamber 5, in the present case with a pressure of <10 ^-8 pa.

For example, in order to prevent metal vapors generated when the switching device 1 is opened from coming onto the inner surface of the insulator 2, here ceramic, a metal shielding element 12 (steam shield) is provided in the contacting area in the switching chamber 5. However, this shield element 12 now also provides for a distortion of the electric field, so that a lower electric field would be present in operation in an area 13 behind the screen elements than in areas 14, where, for example, charges can accumulate and thus can cause further field distortions that could question the functionality of the switching device 1. In order to counteract this, the outside of the insulator 2 (and therefore the housing 3 in the region of the insulator 2) is provided with a resistive coating 15 which covers the entire outer surface of the insulator 2 and makes conductive contact with the caps 4 on both sides of the switching device 1, for example by a solder connection or the like. The resistive but conductive coating 15 therefore provides a conductive connection between the conductor elements 6, so that a small fault current arises, but is not essential due to the high resistance of the coating 15, in the present case in the range of 10 ¹⁰ Ω contributes to field alignment and the removal of surface charges. Too high fields are also unproblematic for these properties, because the nonlinearity exponent describing the slope in the current-voltage characteristic of the coating 15 is clearly smaller than 6, in the present case it is in the range from 4 to 4.5. Breakdowns are avoided even with transient voltage peaks.

The coating 15 consists of a material composition which initially comprises a carrier material, in the present case glass, in which a filler is provided. The filler is 50% by weight. The filler is tin oxide, SnO ₂ , which is applied as a resistance material to mica platelets which have an aspect ratio of width to height of less than 5 and have sizes in the range from 1 to 50 μm. The thickness of the layer of resistance material on the plate is between 10 and 100 nm, the total thickness of the coating 15 here being 250 μm.

Exemplary embodiments are conceivable in which the resistance material is still doped, in the example described here of tin oxide (SnO ₂ ) with antimony (Sb), the doping here being able to be achieved with 0 to 15 mol%. Another embodiment provides that titanium oxide, TiO ₂ , is additionally applied to the platelets if the conductivity is to be increased.

According to the invention, prior knowledge is incorporated in order to implement a variation of the surface resistance as a function of the position in the extension direction 10, that is to say the longitudinal direction of the switching device 1, so that, for example, a higher surface resistance can be present in the area 13 behind the shield element 12 than in FIG the areas 14. This is in Fig. 2 shown schematically, the surface resistance R _?? against the position 1 in the direction of extension 10 and the areas 13 and 14. It can be seen that the course 16 of the surface resistance in area 13 shows an increase.

This can be achieved by using two different fillers with different conductivity and varying their concentrations along the direction of extension 10 or by using a single filler and varying its concentration in the direction of extension 10. Furthermore, the variation of the surface resistance along the direction of extension 10 can additionally be achieved by varying the thickness of the coating 15.

Fig. 3 shows a second, slightly modified embodiment of a switching device 1 'according to the invention, again a vacuum interrupter. For the sake of simplicity, functionally identical components are provided with the same reference symbols.

As can be seen, the housing 3 again consists of two insulators 2, that is to say tubular ceramic parts, which in this case are spaced apart, however, since the shield element 12, which has a correspondingly larger radius, is held in the contacting area 13 between them. The coating 15 extends along the outside of the insulators 2 and is not only conductively connected to the caps 4, but of course also correspondingly to the (metal) shield element 12.

It should also be noted that silicon carbide (SiC) can also be used as an alternative for tin oxide, aluminum (Al) being preferred as the doping material if doping is also to be provided there.

Although the invention has been illustrated and described in detail by the preferred exemplary embodiment, the invention is not restricted by the disclosed examples and other variations can be derived therefrom by a person skilled in the art without departing from the scope of protection of the invention.

Claims (12)

1. Electric switching device (1, 1'), comprising at least two conductor elements (6) which can be placed at a distance from one another and contacted using a moving mechanism (9), and a housing (3) which defines a switching chamber (5), is made of an insulator (2) and at least partially surrounds the conductor elements (6), wherein at least one face of the housing (3) has a resistive coating (15) which is made of a matrix material filled with a filler, wherein the sheet resistance of the coating (15) is between 10⁸ and 10¹² ohm at operating field strength, and it is conductively connected to the conductor elements (6), wherein the sheet resistance is varied along the direction of extent (10) of the conductor elements (6), characterized in that the variation of the sheet resistance along the direction of extent (10) is achieved by using different fillers and/or by varying the concentration of the single filler.
2. Switching device according to Claim 1, characterized in that the non-linear exponent describing the upward gradient in the current/voltage characteristic of the coating (15) is less than 6.
3. Switching device according to Claim 1 or 2, characterized in that the filler is or comprises tin oxide SnO₂ or silicon carbide SiC.
4. Switching device according to Claim 3, characterized in that the filler is or comprises tin oxide doped with antimony and/or silicon carbide doped with aluminum.
5. Switching device according to one of the preceding claims, characterized in that the matrix material is selected from the group comprising elastomers, thermosetting plastics, thermoplastics and glass, and/or in that the filler concentration is 10 to 90% by weight, in particular 40-60% by weight.
6. Switching device according to one of the preceding claims, characterized in that the coating (15) has a thickness of 100 µm to 500 µm.
7. Switching device according to one of the preceding claims, characterized in that the filler consists of particles of a grain size of 100 nm to 300 µm, in particular 1 µm to 50 µm.
8. Switching device according to Claim 7, characterized in that the particles are platelets made of a base material, in particular mica, which are coated with the resistance material defining the resistance properties, in particular tin oxide SnO₂ or silicon carbide SiC, preferably with a layer thickness within the range of 10 to 100 nm, and/or the particles are outwardly covered by an electrically conductive layer, in particular titanium oxide TiO₂.
9. Switching device according to one of the preceding claims, characterized in that the sheet resistance is varied along the direction of extent (10) as a function of a change in the electric field along the direction of extent (10) of the conductor elements (6) under operating conditions.
10. Switching device according to one of the preceding claims, characterized in that the variation of the sheet resistance along the direction of extent (10) is additionally achieved by varying the thickness of the coating (15).
11. Switching device according to one of the preceding claims, characterized in that said switching device is embodied as a vacuum interrupter.
12. Switching device according to Claim 11 characterized in that, in the contact-making region of the conductor elements (6), the vacuum interrupter has a shielding element (12) which influences the electric field at the insulator, is arranged within the switching chamber (5) and/or is held between two housing parts of the housing (3) and is intended for catching free metal particles of the conductor elements (6).

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