The disclosure concerns a bushing and a method for producing such bushing.
Bushings are known, for example, as electrical condenser bushings used in medium and high voltage electrical installations for leading a conductor through a grounded barrier (e.g., a metal wall of a transformer, tank or substation housing or similar).
Known bushings can include flanges made of metal, in particular of aluminium and can be expensive. In particular, the manufacturing process can be rather complicated. First, a core comprising an electrical field grading insulation and a test tap connection point is produced, e.g., by moulding and finished by machining. The expensive metal flange is produced separately and afterwards fixed to the core, such as with an adhesive, and an electrical connection with the test tap connection point established.
A bushing is disclosed comprising: a rotationally symmetrical core surrounding an axial duct for accommodating a conductor; a grading insulation surrounding the axial duct; a plurality of conductor layers which are electrically insulated from each other, an innermost of the conductor layers being electrically conductively connected to a contact element arranged in the axial duct for establishing contact with the conductor; a shell having an insulating matrix material with, at a circumference, a test tap with a test contact electrically conductively connected to an outermost of the conductor layers of the grading insulation; a grounding contact electrically connectable to the test contact and with a flange laterally projecting and circumferentially surrounding the shell, which flange has axial through holes for accommodating bolts for mounting the bushing, a first side of the flange being adjacent to a mounting side of the bushing, wherein the flange is in one piece with the shell and is formed at least in part of the insulating matrix material and at least one metal insert at least partially embedded in the insulating matrix material adjacent to the through holes such that the at least one metal insert forms at least part of a surface of the flange surrounding the through holes on a second side of the flange opposite the first side of the flange; and at least one electrically conductive connection element which connects the grounding contact with the at least one metal insert.
A method of producing a bushing is disclosed having a rotationally symmetrical core surrounding an axial duct for accommodating a conductor, a grading insulation surrounding the axial duct, a plurality of conductor layers which are electrically insulated from each other, a shell having an insulating matrix material with a test contact connected to an outermost of the conductor layers of the grading insulation, a flange in one piece with the shell formed at least in part of the insulating matrix material, with at least one metal insert at least partially embedded in the insulating matrix material adjacent to through holes such that the at least one metal insert forms at least part of a surface of the flange surrounding the through holes on a second side of the flange opposite a mounting side of the bushing, and at least one electrically conductive connection element which connects a grounding contact with the at least one metal insert, the method comprising: placing at least components of the grading insulation, the test contact, the at least one metal insert and the at least one connection element in a mould; filling the mould with the insulating matrix material in a liquid form; allowing the insulating matrix material to harden; and removing the bushing from the mould.
The disclosure will be explained with reference to the following figures which show an exemplary embodiment, wherein:
An exemplary bushing according to the disclosure can be robust and can be reliably fixed to a wall while also establishing a desired electrically conductive connection of a grounding contact with the bushing.
A particularly simple and cost-efficient way of producing a bushing is also disclosed where, after positioning elements of a grading insulation and other components in a mould, the bushing, including the flange, can be completed in substantially one shot by filling of the mould with insulating matrix material in liquid form.
According to the disclosure, an exemplary flange is in one piece with the core, comprising the same insulating matrix material. No machining is required, and production costs can be considerably reduced. However, the surface of a plastic flange can be too soft to withstand high mechanical loads exerted on it by bolts used to mount the bushing to, e.g., any kind of plane, a housing or a wall. Also, while the metal flange of known generic bushings can be employed for grounding the test contact of the test tap incorporated in the housing, establishing an electrically conductive connection between the grounding contact and the wall via the bolts used for mounting the bushing to the same, such a connection is not automatically present in a bushing with a flange comprising an electrically insulating material.
The flange 3, which can project laterally and circumferentially surround the outer shell 2, can have a plurality of axial through holes 8 equally distributed along its circumference (e.g., twelve of them as shown in
On the sleeve of the flange 3 a test tap 13 is provided which projects radially outward from the cylindrical shell 2. The tast tap 13 comprises a test contact shaped as a contact pin 14 which can be electrically conductively connected to the outermost conductor layer of the grading insulation 5. The contact pin 14 is coaxially surrounded at a distance by a contact ring 15 which serves as a grounding contact. An insulating ring 16 separates the contact ring 15 from the contact pin 14 to prevent flashovers. In an exemplary embodiment, every second one (or every one) of the metal inserts 9 can be electrically conductively connected to the contact ring 15 by a connection element, e.g., a connection wire 17. The connection wires 17 each comprise an electrically conductive material, such as metal, e.g., aluminium or copper. They can have equal lengths and cross sections so as to have, for example, approximately (e.g., ±1% or ±10%) equal electrical resistances.
The test tap 13 can be covered by a metal cap which electrically conductively connects the contact pin 14 to the contact ring 15 so the outermost conductor layer of the grading insulation 5 is grounded. When the cap is removed the contact pin 14 is electrically disconnected from the contact ring 15 and can be connected to an input of a measuring instrument, e.g., a voltmeter, and electrical measurements carried out for diagnostic and other purposes.
The bushing as described can be produced according to an exemplary method as follows:
The winding for the electrical field grading insulation 5 can be wound from a web of porous material, with the sheets of metal foil inserted between subsequent turns (or the porous material comprises metal coatings or a net shaped or meshed material is used), and then fixed inside a mould together with the diverse metal parts (e.g., the sleeve 7, the metal inserts 9, the contact pin 14, the grounding contact 15, the connection wires 17 and other electrical connections). Then the mould is filled with liquid insulating matrix material (e.g., a plastic material, such as a polymer or mixture of polymers or resin, preferably with an admixture of inorganic filler material like silica, alumina, dolomite, wollastonite or boron nitride or the like as described above. This step can be carried out in different ways (e.g., involving vacuum casting or, for example, automated pressure gelation or injection moulding). The matrix material forms the shell 2 and the flange 3 and at the same time impregnates the porous material of the winding to complete the formation of the grading insulation 5. Afterwards the matrix material can be left to harden. When it has solidified sufficiently, the mould can be opened and the bushing removed.
There are many ways of modifying the above-described design and method within the scope of the disclosure. For example, the metal inserts can be of various shapes (e.g., each may comprise a ring extending through the through hole to the first face of the flange and carrying a second rim at the end proximal to the same). It is also possible to replace the plurality of metal inserts by a single metal insert (e.g., in the shape of a ring essentially following the circumference of the flange and forming an annular surface interrupted by the through holes). Connection cables or massive connection pieces may be used instead of connection wires and various materials can be employed, provided that they offer sufficient electrical conductivity. The grounding contact can be connected to the at least one contact piece at three or more positions which are for example substantially (essentially) equally distributed over the circumference of the flange, as in this way an approximately axially symmetrical distribution of currents and electromagnetic fields can be achieved. The shape of the core can be, at least in part, frustoconical. The flange can be reinforced by ribs or reinforcing materials, e.g., glass fibers.
It will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the invention is indicated by the appended claims rather than the foregoing description and all changes that come within the meaning and range and equivalence thereof are intended to be embraced therein.
1 core
2 shell
3 flange
4 wall
5 grading insulation
6 duct
7 sleeve
8 through hole
9 metal insert
10 ring
11 rim
12 bolt
13 test tap
14 contact pin
15 contact ring
16 insulating ring
17 connection wire
This application claims priority as a continuation application under 35 U.S.C. §120 to PCT/CH2006/000726 filed as an International Application on Dec. 20, 2006 designating the U.S., the entire content of which is hereby incorporated by reference in its entirety.
Number | Name | Date | Kind |
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3861777 | Clark | Jan 1975 | A |
4965407 | Hamm | Oct 1990 | A |
6610933 | Baker et al. | Aug 2003 | B2 |
20020104679 | Krol et al. | Aug 2002 | A1 |
Number | Date | Country |
---|---|---|
0 075 471 | Mar 1983 | EP |
620822 | Mar 1949 | GB |
Number | Date | Country | |
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20090288878 A1 | Nov 2009 | US |
Number | Date | Country | |
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Parent | PCT/CH2006/000726 | Dec 2006 | US |
Child | 12487336 | US |