EP1575061A2 - Means for electrical insulation of medium and high voltage components - Google Patents

Abstract

Especially in high-voltage switchgear so far electrical insulation of nitrogen (N ₂ ) or sulfur hexafluoride (SF ₆ ) are used. According to the invention, a material (3) with suitable dielectric properties is used as insulation means, in which material gas-containing pores having a largely monodisperse distribution are located. The material may be a polymer, in particular a polymer foam. But also possible beds of discrete hollow material particles (40) whose inner cavity corresponds to a pore.

Description

The invention relates to means for electrical insulation of medium and high voltage components according to the Preamble of claim 1.

Switchgear and switchgear in the medium and high voltage range need to reduce the size and thus the Costs are electrically insulated with special insulating media, since the ambient air for achieving a compact Construction has an insufficient dielectric strength. This will be a cheap, environmentally friendly, hard Flammable insulating medium with the highest possible dielectric Strength needed. In addition, complicated designs to be able to isolate, the insulating medium should be as possible in liquid or gaseous form can be used. The concentration should be as low as possible for cost and weight reasons be.

In most cases, in switching devices and switchgear in particular for high-voltage insulation, a gas filling with the insulating gas sulfur hexafluoride (SF ₆ ) is used, with the about 3 times higher than the insulation with air or dry nitrogen breakthrough field strength can be achieved. As a result, a significant reduction in size and thus a significant cost savings compared to the use of nitrogen can be achieved. SF ₆ not only has excellent electrical properties but is also non-toxic and chemically very stable. However, the latter characteristic, together with the high absorption coefficient in the infrared spectral range, leads to an extremely high global warming potential of approximately 22,000 CO ₂ equivalents.

By increasing the pressure, it is also possible to improve the dielectric strength in nitrogen-insulated systems, but the expenditure for ensuring the operational safety at higher pressure leads to considerable additional costs compared with the use of SF ₆ . Alternatives to SF _6, a highly potent greenhouse gas with comparable electrical and chemical properties, but a lower global warming potential and low toxicity, are still lacking.

In the case of transformers, often cleaned ones are used for isolation purposes Mineral oil, so-called transformer oil used, but what the disadvantage of flammability, environmental damage at leakage of insulating agent and the disadvantage of high Weight and high cost. Alternative liquid insulation with less flammability and better environmental compatibility such as. Silicone oil is very expensive and therefore account for a majority of applications.

The use of insulating, hardenable casting compounds such as Synthetic resins are complex, expensive and because of the fading in the Curing often does not work or can only be used to a limited extent.

The solid state insulation materials used for cables Plastic base, e.g. crosslinked polyethylene (PE) are in switchgear because of the complicated geometry and the Use of moving parts and high costs also so far not in use.

In the literature is also already proposed as Electric insulation means to use plastic foams for which reference is made in particular to US 2002/0094443 A1 becomes. For example, in "Transaction on Electrical Insulation", Vol. 24 (1989), pp. 239 become polyurethane resin foams proposed as insulation, the structure of a such foam in "Proc. of Nordic Insulation", June 14-16 (1999), pp. 261-268 specifically shown on page 266 in FIG and is reproduced below as Figure 1.

However, foams of this type have not proven themselves in practice, as it may lead to discharges, which the effectiveness of the foam.

Further notes on the use of foams as electrical Insulating agents are described in DE 101 17 017 A1, the CH 572 269 A5, EP 0 713 897 B1, US Pat. No. 4,273,806 and US Pat US Pat. No. 5,468,314. Finally, it is still on the reference A. Roth "High Voltage Technology", 4th edition (Springer Verlag Wien 159), especially page 132, at the - similar to the above already mentioned Publications - the correlations of glow charge voltage and pore size. Total occupied but the multitude of references with very different ones Indicate the fact that concrete parameters of electrical Purpose suitable foams not reproducible in detail were examined.

Starting from the latter prior art, it is the task of Invention, an improved means for electrical insulation to accomplish. Especially for use with high voltage switches the parameters for foams should be considered as relevant Isolation can be specified.

The object is achieved by the features of claim 1 solved. Advantageous developments are in specified in the dependent claims. In particular, the claims. 7 to 9 include the realization of the invention as such microporous foam mentioned, while the claims 10 et seq. As Alternative insulation by means of a bed of microporous Material particles includes.

The invention is based on the use of insulating materials, the a variety of gas-filled, thin-walled closed Contain cavities to which a substantial part of the the insulation of applied electrical potential drops. According to the invention, the cavities lie in the area between 10 and up to a maximum of 30 microns and are called as Micropores designated, the dimensions of which along the by the application of certain electric field a predetermined Do not exceed the maximum value. The cavities can Pores of a closed cell foam or a Be filling of hollow bodies.

The invention was based on the finding that compliance an upper limit in the size distribution of the pores crucial is. Only so can the occurrence of discharges in otherwise in low probability, but statistically absolutely existing large pores are prevented for example, in pores> 100 microns, which in the state of Technique a gradual deterioration of the insulation properties is explainable. In the invention, a largely monodisperse distribution with defined pore size <30 microns are present.

Figur 1

eine Schaumstruktur gemäß dem Stand der Technik,

Figur 2

eine graphische Darstellung der Porengrößenverteilung beim als Isolationsmittel verwendeten Schaum gemäß Figur 1,

Figur 3

den Aufbau der neuen geschlossenporigen Isolation im Schnitt,

Figur 4

eine mikroskopische Hohlkugel zum Aufbau einer zu Figur 3 gleichwirkenden Isolation, und

Figur 5

eine graphische Darstellung zum Vergleich der Erfindung mit nach dem Stand der Technik als Isolationsmittel üblicherweise verwendeten Stickstoff oder SF₆.

Further advantages and details of the invention will become apparent from the following description of exemplary embodiments with reference to the drawings in conjunction with the claims. It show in a schematic representation

FIG. 1

a foam structure according to the prior art,

FIG. 2

1 is a graphic representation of the pore size distribution in the foam used as the insulating agent according to FIG. 1,

FIG. 3

the structure of the new closed-cell insulation on average,

FIG. 4

a microscopic hollow sphere for the construction of an equivalent to Figure 3 insulation, and

FIG. 5

a graphical representation for comparison of the invention with commonly used in the art as insulation means nitrogen or SF. ₆

High-voltage switches use insulating materials. Such materials can be realized in such a way that they contain filled, thin-walled closed cavities with a length D _pore . The dimension along the determined by the application of the electric field determines a maximum value D _max , which must not be exceeded.

Foams are state of the art, especially for thermal insulation known. Partially they are also used for electrical Isolation purposes proposed.

In the prior art with foam pores 11 of the average size d = 100 μm corresponding to FIG. 1, a typical Gaussian normal distribution is obtained according to FIG. Plotted there is the relative size fraction in% as a function of the absolute pore size d, wherein according to characteristic curve 21, the maximum of the distribution is 100 microns. However, this means that statistically, individual larger pores, for example of 150 microns, may be present. When it comes to a single discharge, the foam wall is destroyed and creates a larger cavity. In this way, the insulation properties deteriorate and it can suddenly come to an overvoltage through an avalanche effect.

The characteristic curve 22 describes a distribution with a maximum which lies between 10 and 20 μm. In particular, no pore sizes> 30 microns are present here. In the embodiments described with reference to FIGS. 3 and 4, precisely defined pore sizes are present. Typically, the value d _{pore is} in the range of 10 microns to a maximum of 30 microns.

In FIG. 3, 1 denotes a live conductor, 2 a grounded conductor, 3 a closed-cell insulation between the ladders 2 and 3. The closed-cell insulation 3 has gas-filled cavities 4, where 5 is the extent of the cavity 4 along the electric field and 6 the extent of the cavity 4 perpendicular to the electric field.

The direction of the electric field is illustrated by the double arrow 7 and the distance between the mutually insulated conductors 1 and 2 by the double arrow 8. By the dimensions 5 and 6, therefore, a pore diameter d _{pore is} defined, which has a scattering width according to the Gaussian distribution 22 according to FIG. 2, in particular when using a foam.

The length of a pore from FIG. 3 can be equated with the pore diameter d _pore without necessarily implying a spherical geometry of the cavities. Rather, cavities are also conceivable whose dimensions along the electric field in the direction of the double arrow are smaller than perpendicular to it. As already mentioned, the pore diameters advantageously satisfy a size distribution f (d) with a maximum at 10-20 μm, the distribution curve 23 assuming the value 0 above d _max = 30 μm .

A restriction to small pore sizes with values <150 μm is known from the literature of JP 11-9437 A1, however only with regard to heat-insulating foams with requirements in terms of mechanical stability. For foams for electrical insulation purposes, such as e.g. in the US 2002/94443 A1, a restriction is very on small pores <50 microns previously unknown. Foams with medium Pore sizes of 50 microns and 100 microns and a far above are reaching pore size distribution function described in the cited publications, wherein Here are the outstanding properties of micropores in the Range 10 to 30 microns are not recognized. As below in the Detail and quantitatively described, but are just materials With pore sizes <30 microns a particularly advantageous Substance class with surprising properties in terms of Dielectric strength is.

If a single cavity 4 from the foam 3 of FIG. 3 is considered as a pore, this pore in principle realizes a microscopic hollow sphere. This is illustrated in detail with reference to FIG. 4: 40 here mean the hollow sphere with a gas-filled cavity 41. The hollow sphere 40 consisting of an insulator 42 has a pore diameter d _pore and a wall thickness d _wall . Because of the microscopic training is also referred to below as "hollow spheres" or "microballoons". A plurality of hollow spheres 40 are embedded in an insulating material 43, in particular gas.

For a material with a relative permittivity ε _r , the thickness of the wall d _wall enclosing the cavities satisfies a minimum condition for increased dielectric strength of the insulation according to the invention d wall / ε r << d pore ,

Preferably, the wall thickness is smaller than the pore radius d wall < d pore / 2, and further advantages arise when the wall thickness is typically 20% of half the pore diameter, ie the pore radius.

In this consideration, it was assumed that the insulating material 13, in which the micropores are embedded, with respect to the electrically effective thickness d _{int er} / ε _{int he} with the average distance d _inter (wall-to-wall) between two hollow spheres and the relative Permittivity ε _inter embedding can be neglected compared to the wall of the hollow spheres. If this is not the case, then the condition mentioned in equation (1) is to be modified as follows: d wall / ε r + d int he / ε int he << d pore

This condition is directly related to the fact that gas-filled pores a substantial proportion of the insulator applied electrical potential drops. As the wall thickness the pores, their diameter and their distance from each other determine the volume ratios to each other, it can also a corresponding condition for the volume ratios be derived.

The insulating material described above can be used to fill insulating cavity as closed-cell foam or in the form of a bed. It may vary depending on the application made of a plastic, glass or another glassy Material such as glass ceramic exist: have plastics compared to glassy materials the advantage of weight savings, but at temperatures above 100 ° C rapidly decreasing mechanical stability and dielectric strength. method for producing corresponding closed-cell foams correspond to the state of the art and are exemplary described in the two applications mentioned above.

In the case of plastics, a wide variety of criteria can be decisive for the selection: For reasons of environmental compatibility and safety in the event of fire, the insulation can preferably be made of a plastic which contains neither halogens nor sulfur or nitrogen. For particularly high dielectric strength, however, a halogen-containing plastic or a polyimide can be selected. An example of this is a newly developed material for thermal insulation based on polyimide (SOLREX®) in the form of microscopic hollow spheres; In this form, the material can be poured like a liquid and thus fills all accessible cavities analogous to liquid or gaseous insulating materials. If necessary, the material can be cured by moderate heat, so that a positive, closed-cell, foam-like insulating body with a low density of <0.1 g / cm ³ is formed. Similar materials are offered eg by Akzonobel eg under the name Expancel®.

Because particles with diameters of typically 10 μm are respirable It is advantageous when using the micro microbeads or "microballoons" with a liquid binder temporarily, i. during processing, or permanently to bind. For temporary binding are water and low boiling organic compounds such as e.g. Suitable alcohols, which subsequently eliminated by evaporation again become. For permanent bonding are liquid insulating agents such as. Transformer or silicone oils suitable, but also curable liquids, e.g. Synthetic resins or silicone rubber.

For the gas filling of the pores or "microballoons" advantageously dry air, nitrogen, carbon dioxide or a mixture of carbon dioxide and one of the above gases can be used. Although the present invention describes an isolation system which avoids the use of SF ₆ or halogenated hydrocarbons, the use of SF ₆ or similar insulating gases as a filling of the closed cavities should not be ruled out, since this further increases the dielectric strength the same volume or a reduction in the amount of insulating gas can be achieved for a given insulation potential.

If the against each other is referred to insulating circuit with d _gap the distance and the electric potential difference between the conductors with Δ V, then gives the average field strength in the range Δ between the conductors to E _av = .DELTA.V / d _gap.

In the following, it will be shown which advantage in insulation resistance results from the use of a microfoam instead of a gas insulation or a coarse-pored foam: With a nitrogen filling of the cavities a dielectric strength of the insulator comparable to the pure SF ₆ -insulation is achieved, if the pore size is set to values of typically 10 microns lowers. Thus, nitrogen can be used as insulating medium even at atmospheric pressure, without having to do without a compact design. Compared to conventional foam materials with pore sizes of typically ≥ 50 microns up to 1 mm, the breakdown field strength increases by a factor of 4-10 in this very small pore size, since the field strength necessary for the use of partial discharges increases with decreasing pore size. The possibility to do without SF ₆ results immediately in a great potential for environmentally friendly electrical insulation systems.

FIG. 5 shows the breakdown field strength or breakdown voltage U _{D of} a foam according to FIG. 1 as a function of the pore size d in a double logarithmic plot. The breakdown voltage U _{D of} this material is compared with the breakdown field strength of sulfur hexafluoride (SF ₆ ) and of nitrogen (N ₂ ), both of which have constant values. The characteristic curves for SF ₆ are denoted by 31 and for N ₂ by 32, both of which form abscissa-parallel straight lines. The characteristic of the microfoam designated 33 has a characteristic decreasing with the pore size d. The boundary condition is a 1 mm gap of a switching arrangement.

From the figure 5 results from a value d <30 microns for the Foam an improvement over known gas insulation and further, at values below 30 μm, an increasing, significantly improved breakdown field strength over the previously used funds. Here is the breakdown voltage coupled with the pore size insofar as that the product of breakthrough field strength and pore size remains constant.

From the comparison of the characteristic curves 31 to 33 results in the advantageous usability of the new insulation means, namely foams or beds, in particular as a replacement for SF ₆ .

• hohe Durchbruchfeldstärke, die mit sinkendem Porendurchmesser ansteigt und bei 30 µm bereits 3fach höher als die des trockenen Stickstoffs und damit vergleichbar der von SF₆ ist
• Formschlüssigkeit
• schaumstoffartiger Isolierkörper mit mikroskopischer geschlossenporiger Porenstruktur
• geringe Dichte/verringerte Kosten
• gute Umweltverträglichkeit
• einfache Verarbeitbarkeit (Gießen und bei moderater Temperatur aushärten)
• bei Auswahl geeigneter Isolationsmaterialien schwer entflammbar.

In summary, it should be noted that the particular advantages of the present invention are in particular the following properties:

• high breakdown field strength, which increases with decreasing pore diameter and at 30 μm is already 3 times higher than that of dry nitrogen and thus comparable to SF ₆
• positiveness
• Foam-like insulating body with microscopic closed-pore structure
• low density / reduced costs
• good environmental compatibility
• easy processability (casting and curing at moderate temperature)
• with selection of suitable insulation materials hardly inflammable.

Claims (16)

Means for electrical insulation of medium and high voltage components, switching devices and / or switchgear, or other high voltage electrical equipment, wherein an insulating material with suitable dielectric properties and having in the material, gas-containing pores, which have a size distribution with predeterminable parameters is used, characterized in that the pores (40) have a largely monodisperse size distribution and claim at least 10% of the total volume, wherein the size distribution (f (d)) of the pore diameter (d _pore ) (40) has a maximum between 10 .mu.m and 20 .mu.m and wherein the size distribution (f (d)) assumes the value 0 above d _max = 30 μm. Insulation according to Claim 1, characterized in that the insulating material (42, 43) is a polymer. Insulation according to Claim 1, characterized in that the insulating material (42, 43) is glass or ceramic. Insulation according to claim 1, characterized in that the pore filling gas consists of a sulfur hexafluoride-free (SF ₆ ) gas or gas mixture. Insulation according to claim 1, characterized in that the pore filling gas consists of dry nitrogen (N ₂ ) or dry air. Insulation according to claim 1, characterized in that the pore filling gas consists of a low-boiling hydrocarbon. Insulation according to one of claims 1 to 6, characterized in that a closed-pore microporous foam (3) is used as the insulating material with a maximum pore diameter of 30 microns. Insulation according to Claim 7, characterized in that , given the pore dimensions (d _pore ), an electric field of predetermined size is not exceeded. Insulation according to claim 7 or 8, characterized in that the foam is a closed-cell polymer foam (3). Insulation according to one of claims 1 to 6, characterized in that a bed of discrete, porous material particles (40) is used. Insulation according to claim 10, characterized in that the bed of monodisperse closed material hollow bodies ("microspheres"), wherein the internal volume of a pore (40). Insulation according to claim 8 or claim 10, characterized in that the expansion of the individual hollow material body (40) of pore diameter (d _pore ) and double-sided wall thickness (2 * d _wall ) is at most 30 microns. Insulation according to claim 12, characterized in that for the wall thickness of the hollow body (40): d wall / ε r << d pore , where ε _{r mean} the relative electrical permittivity of the material, d _pore the inner (pore) diameter of the hollow body (40) and d _wall the wall thickness of the hollow body (40). Insulation according to claim 13, characterized in that the wall thickness is smaller than the (pore) radius 1 / 2d _{pon of} the hollow body, preferably at about 10% (d _wall = 1/10 r _Pore ). Insulation according to one of claims 10 to 14, characterized in that the bulk material of the hollow body (40) made of plastic, glass or a glassy material, in particular glass ceramic, consists. Insulation according to one of the preceding claims, characterized in that the breakdown voltage is coupled to the pore size (d _pore + 2d _wall ).

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