The invention pertains to the field of high-voltage or medium-voltage engineering, particularly to electrical insulating and connecting techniques for grounded high-voltage apparatuses. The invention is based on a dielectric bushing and an electrical high-voltage apparatus according to the preambles of the independent claims.
The invention refers to the state of the art, as is known from WO 02/065486 A1. This publication discloses a high-voltage insulator, e.g., of porcelain or composite material, with a coating of field control material (FGM). The field control coating consists of varistor powder, e.g. of doped zinc oxide (ZnO) that is embedded in a polymer matrix. The FGM coating serves for homogenizing the field distribution on the insulator surface and is distributed such that part of the material is in electric contact with the ground electrode as well as with the high-voltage electrode. In this case, the FGM coating may only cover the insulator length partially and be concentrated in the field-stressed electrode regions. The FGM coating may be applied on the insulator surface, incorporated into a screening at this location or screened relative to the outside by means of a weather-proof, electrically insulating protective layer. A homogenization of the capacitive field stress can be realized with alternating horizontal strips or bands of FGM coating and insulating material.
In porcelain insulators, the FGM coating may be applied in the form of a glazing or a coat of paint, mixed into a paste or into clay, or applied on the porcelain insulator and fired such that a glazing or a ceramic layer is formed. Alternatively, the matrix for the FGM coating may consist of a polymer, an adhesive, a casting mass or a mastic or a gel.
EP 1 042 756 discloses a glass-fiber reinforced insulating tube that is impregnated with a resin on the inside surface and, if so required, on the outside surface, wherein said resin contains a particulate filler with varistor properties, particularly zinc oxide. The glass-fiber reinforced plastic (GFK) tube can be manufactured by winding up a glass-fiber netting, at least the outer layers of which are impregnated with the varistor-filled resin.
Various types of electrical bushings are disclosed in Chapter 3.13, “Electrical Bushings” by L. B. Wagenaar, pp. 3-171-3-184 in the book “The Electric Power Engineering Handbook” by L. L. Grigsby, CRC Press and IEEE Press, Boca Raton (2001). FIG. 3.151, in particular, shows a bushing with a grounded screening electrode that is arranged within the insulating tube. Due to the screening electrode, a field control is achieved in the region of the grounded installation or mounting flange such that the highly field-stressed zone is relieved at the transition from the flange to the insulator. Interior screening electrodes of this type are absolutely imperative in compressed gas-insulated bushings, e.g. in SF6-insulated or air-insulated bushings, particularly for high-voltage applications. Interior screening electrodes are also known for solids-insulated bushings. However, the screening electrodes lead to large diameters of the bushings. In addition, screening electrodes only make it possible to achieve relatively inhomogeneous field controls in comparison with capacitor bushings with oil-impregnated or resin-impregnated paper. This needs to be compensated with larger structural heights of the bushings.
The brochure “SF6-air bushings, type GGA”, Technical Guide, Mar. 30, 1996 by ABB Power Technology Products AB discloses dielectric bushings that are equipped with internal screening electrodes on the grounded flange and, for higher voltage levels, with additional screening electrodes on the flange on the voltage side.
DE 198 44 409 discloses an insulator that is suitable, in particular, for dielectric bushings. The insulator conventionally comprises an insulator body of porcelain or composite material and a screening of porcelain or silicone. The screening has a variable insulating screen density. A customary screening electrode is also provided between the insulator body and the conductor in order to relieve the field stress in an insulator end region. This publication proposes to arrange a larger number of insulating screens in the highly field-stressed region where the screening electrode ends. The field stress is relieved in an improved fashion in the end region of the screening electrode due to the increased insulating screen density.
The present invention is based on the objective of disclosing an improved dielectric bushing, as well as an electrical high-voltage apparatus and an electrical switchgear with such a bushing. According to the invention, this objective is attained with the characteristics of the independent claims.
The invention proposes a dielectric bushing, particularly a high-voltage bushing for an electrical high-voltage apparatus, that comprises an insulator part with a first installation flange and a second installation flange for installing the bushing, wherein a screening electrode required for the desired voltage level is omitted within the bushing in a field-stressed zone in the region of the first installation flange, and wherein a non-linear electric and/or dielectric field control element is instead provided in the field-stressed zone on the insulator part within the region of the first installation flange for field control purposes. The invention makes it possible to omit the screening electrode that, according to the previous technical knowledge, was necessarily present for a predetermined voltage level. This results in numerous advantages. The omission of the thus far required interior screening electrode makes it possible to realize the dielectric bushings in a thinner fashion, i.e. with a reduced diameter. The voltage limit, beginning at which a conical widening toward the grounded flange is more economical, can be shifted toward higher voltage levels. Cylindrical bushings can be manufactured more economically than conical bushings. The risk of electric sparkovers between adjacent bushings is reduced and adjacent phases can be spatially arranged closer to one another or closer to the ground. The relief of the field stress according to the invention by means of a field control material in the flange region also results in a superior field control in comparison with conventionally utilized screening electrodes. Consequently, the bushings can also have a shorter structural length. Under a pulsed stress, in particular, the E-field is no longer concentrated within the region of the screening electrode during the entire pulse duration, but is rather able to propagate and thereby to decay along the field control element in the form of a wave. In addition, the maximum field strengths are also reduced.
According to a first embodiment, the field control material is designed, with respect to its non-linear electric and/or dielectric properties, its geometric shape and its arrangement on the insulator part, for achieving a dielectric relief of the field-stressed zone without a screening electrode in all operating states, particularly for impulse voltages. Consequently, the field control element is also able to manage critical field stress states without a screening electrode or screening electrodes.
Claim 3 discloses design criteria for an electrical design of the field control material that makes it possible to realize an advantageous field control.
Claims 5 and 6 disclose design criteria for the geometric design of the field control element that make it possible to achieve an advantageous field control with a low material expenditure. Claim 6, in particular, defines a minimum required length of the field control element along the longitudinal direction of the insulator part. Due to this measure, the field stress, particularly under impulse voltages, propagates along the field control element in the form of a traveling wave and decays during this process to such a degree that no damaging field strengths can occur any longer once the distant end of the field control material is reached.
Claim 7 discloses how d.c. bushings can be easily manufactured with the field control element.
The embodiments according to claim 8 and claim 9 provide the advantage that, in particular, the highest field stresses can be managed with the field control material in the region of the grounded flange.
The embodiments according to claims 10 and 11 provide the advantage that both flange regions are protected from sparkovers or partial discharges independently of one another by the field control materials.
Claim 12 defines various radial positions for arranging the field control material on the insulator part. Claim 13 provides the advantage that a conventional GFK (glass-fiber reinforced plastic) tube or a conventional porcelain insulator can be replaced with a self-supporting FGM tube (field control material tube).
Claim 14 discloses advantageous material components for the field control element.
Claims 15 and 16 pertain to an electrical high-voltage apparatus and an electrical switchgear assembly comprising a bushing according to the invention with the above-described advantages.
Other embodiments, advantages and applications of the invention are disclosed in the dependent claims as well as in the following description and the figures.
a, 1b show cross sections through conventional high-voltage bushings according to prior art;
a-2d show cross sections through embodiments of a FGM bushing for a GFK tube with silicone screening, wherein
a shows a continuous FGM coating,
b shows a FGM coating on the grounded side,
c shows respectively independent coatings on the grounded side and the high-voltage side, and
d shows an interior and an exterior FGM coating;
a-3b show a cross section and a top view of embodiments of a FGM bushing for a porcelain insulator with an internal and an optional external FGM coating;
Identical components are identified by the same reference symbols in the figures.
a shows a conventional gas-insulated dielectric bushing 1, particularly a high-voltage bushing 1 for an electrical high-voltage apparatus. The bushing 1 comprises an insulator part 2; 2a, 2b with a first installation flange 4 on the grounded side that serves for installing the bushing 1 on a grounded housing 5 of a (not-shown) electrical apparatus and a second installation flange 8 on the voltage side that serves for installing the bushing 1 on a (not-shown) high-voltage section or high-voltage part. The interior of the insulator part 2; 2a, 2b contains a gas chamber 20 for an insulating gas 20g. The gas chamber 20 contains a dielectrically insulating gas 20g, e.g. air, compressed air, nitrogen, SF6 or a similar gas. It would also be conceivable to provide an insulating chamber 20 for accommodating an insulating liquid 20l. The gas-insulated bushing 1 consequently is realized in a hollow fashion, particularly in the form of a hollow cylinder with an axis 3a for receiving an electrical section 3 or at least an electric conductor 3 in the gas chamber 20. The bushing 1 usually serves for connecting the encapsulated electrical apparatus, that is connected to the ground potential 5, to a high-voltage or medium-voltage network. As is known, an interior screening electrode 6, 6a needs to be provided in order to relieve the field stress in the field-stressed zone 7, 7a on the lower grounded flange 4 and to reduce or prevent partial discharges and sparkovers. The screening electrode 6, 6a is typically in electric contact 46 with the grounded flange 4. It protrudes into the gas chamber 20 and is usually tapered upward in a conical fashion. It defines the diameter of the bushing 1 in the region of the grounded flange 4. The broken lines indicate another screening electrode 6, 6b that may be arranged in the field-stressed zone 7, 7b on the upper flange 8 on the voltage side. This additional electrode is also frequently tapered downward in a conical fashion and serves for the field control in the field-stressed zone 7, 7b.
b shows an example of a solid-insulated bushing 1 according to the state of the art. In this case, the insulator part 2, 2b is realized in the form of a resin body 2 that may be provided with an optional screening 2b and has a completely filled interior volume. The insulator part 2, 2b consequently contains in its interior an insulating chamber 20 for a solid insulating material 20s. The reference symbols 3b and 3c identify the supply terminals. The insulator part 2, 2b encompasses the conductor 3. In order to realize the field control, a screening electrode 6, 6a is again provided on the grounded flange 4 in the field-stressed zone 7, 7a and is connected thereto in an electrically conductive fashion by means of a contact 46.
a-2d and
According to
The field control element 9; 9a, 9b; 9i, 9o; 9s preferably has the following characteristics: non-linear electric varistor properties and, in particular, a critical field strength that characterizes a varistor switching behavior of the field control element 9; 9a, 9b; 9i, 9o; 9s and/or a high permittivity ε, for example, ε>30, preferably ε>40, in particular, ε>50.
It is advantageous that the field control element 9 is in electric contact with the first installation flange 4 and extends over a predetermined length l along the longitudinal extension x of the insulator part 2; 2a, 2b. It has a predetermined thickness d or thickness distribution d(l) as a function of the length l. Its length l is preferably greater or equal to the ratio between a maximum impulse voltage to be tested, particularly a lightning impulse voltage, and the critical electric field strength. This design consideration advantageously applies to all embodiments, in which the screening electrode 6a in the region of the grounded flange 7a is replaced with the field control element of 9; 9a; 9i, 9o.
According to
According to
According to
a and
According to
For d.c. applications, the field control element 9; 9i; 9s according to
One preferred material selection for the field control material 9; 9a, 9b; 9i, 9o; 9s comprises a matrix that is filled with micro-varistor particles and/or particles with high permittivity. For example, doped ZnO particles, TiO2 particles or SnO2 particles may be considered as micro-varistor particles. Examples of materials with high permittivity are BaTiO3 particles or TiO2 particles. If ZnO micro-varistor particles are used, they are typically sintered in the temperature range between 800° C. and 1200° C. After breaking up and, if so required, sieving the sintered product, the micro-varistor particles have a typical particle size of less than 125 μm. The matrix is chosen in dependence on the specific application and may comprise, for example, an epoxy, a silicone, an EPDM, a thermoplast, a thermoplastic elastomer or glass. The filling volume of the matrix with micro-varistor particles may lie, for example, between 20 vol. % and 60 vol. %.
The dielectric bushing l′ according to the invention is suitable, among other things, for use as a bushing l′ in an electrical high-voltage apparatus, particularly a disconnector, a life tank breaker, a vacuum circuit breaker, a dead tank breaker, a current transformer, a voltage transformer, a transformer, a power capacitor or a cable termination, or in an electrical switchgear assembly for high-voltage or medium-voltage levels. The invention also pertains to an electrical high-voltage apparatus, particularly a disconnector, a life tank breaker, a dead tank breaker, a current transformer, a voltage transformer, a transformer, a power capacitor or a cable termination, in which a dielectric bushing l′ of the previously described type is provided. The invention also claims an electrical switchgear assembly, particularly a high-voltage or medium-voltage switchgear assembly, that comprises such an electrical high-voltage apparatus.
Number | Date | Country | Kind |
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04405151 | Mar 2004 | EP | regional |
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20050199418 A1 | Sep 2005 | US |