D E S C R I P T I O N

A BUSHING AND A METHOD FOR PRODUCING THE SAME

Field of the invention

The invention concerns a bushing according to the generic part of claim 1 and a method for producing such bushing. Bushings of this type are known as electrical condenser bushings and 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.

Prior art

Bushings of the generic type with flanges made of metal, in particular of aluminium, are well known. They are, however, rather expensive. In particular, the manufacturing process is rather complicated. First, a core comprising an electrical field grading insulation and a test tap connection point must be produced, e.g., by moulding and finished by machining. The expensive metal flange must be produced separately and afterwards fixed to the core, usually with an adhesive, and an electrical connection with the test tap connection point established.

Summary of the invention

According to the invention the flange is in one piece with the core, comprising the same insulating matrix material. No machining is required. Production costs are considerably reduced. However, the surface of a plastic flange is usually too soft to withstand the 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 is 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.

Due to the features enumerated in the characterizing clause of claim 1, however, these problems are solved at the same time. The bushing according to the invention is robust and can be reliably fixed to the wall while also establishing the reguired electrically conductive connection of the grounding contact with the same, just as a known .generic bushing with a metal flange would.

The invention also concerns a particularly simple and cost- efficient way of producing a bushing as claimed where, after positioning elements of the grading insulation and other components in a mould, the bushing, including the flange, is essentially completed in one shot by filling of the mould with insulating matrix material.

Brief description of the drawings

The invention will be explained with reference to the following figures which show only an embodiment.

Fig. 1 shows a side view of a bushing according to the invention, and part of a wall the bushing is attached to,

Fig. 2 shows a cross section along II-II in Fig. 1,

Fig. 3 shows a longitudinal section along III-III in Fig. 1 and

Fig. 4 shows a portion of Fig. 3, enlarged and with additional details.

Description of the preferred embodiments

The bushing comprises a core 1 with a roughly cylindrical shell 2 surrounded by a flange 3 with a sleeve surrounding the shell 2 and an annular part for attaching the bushing to a plane grounded wall 4 or the like, e.g., the metal wall of a transformer or substation housing. The shell 2 and the flange 3 are in one piece and comprise an insulating matrix material, preferably a plastic material, e.g., a polymer or mixture of polymers or an insulating resin, preferably mixed with an anorganic filler material like silica, alumina or boron nitride. A plane first face of the flange 3 is in contact with the wall 4 or a housing. The shell 2 encapsulates an electrical field grading insulation 5 (s. Fig. 3) , which is configured as a winding made from a web of porous material, in particular paper or fiber fabric, impregnated with the matrix material, with a plurality of cylindrical coaxial or wound conductor layers configured as sheets of, e.g., metal foil, each placed between subsequent turns of the winding. The grading insulation 5 surrounds an axial duct 6 for accommodating a conductor rod (not shown) . At one end of the duct 6 a sleeve 7 is provided which serves as a contact element for contacting the conductor rod. The sleeve 7 consists of metal, e.g., aluminium, and is electrically conductively connected to the innermost conductor layer of the grading insulation 5.

The flange 3, which projects laterally and circumferentially surrounds the outer shell 2, has a plurality of axial through holes 8 equally distributed along its circumference, e.g., twelve of them as shown in Fig. 2. Every one of the through holes 8 is formed by a metal insert 9 partially embedded in the flange 3, each comprising (Fig. 4) a threaded ring 10 surrounding the through hole 8 and carrying at its end proximal to a second face of the flange 3 opposite the first face a laterally outward projecting rim 11 that exhibits an annular surface flush with the said second face of the flange 3. Every one of the rings 10 accommodates a threaded bolt 12 the head of which abuts against the annular surface of the rim 11 and which extends through the through hole 8 and into an aligned threaded hole in the wall 4. When the bolts 12 are tightened the pressure exerted by their heads on the second face of the flange 3 acts on the annular surfaces of the rims 11. As the said annular surfaces are formed by the metal inserts 9 which may comprise, e.g., aluminium or steel, they are hard and wear- resistant and considerable force may be applied for ensuring a reliable mechanical connection between the flange 3 and the wall 4 without risk of damage to the surface of the flange 3. The flange is typically annular, but any other design suitable for mounting the flange with its first side, which is to be mounted to the wall 4 or housing, can be used (e.g. the flange having an outer rectangular form).

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 is 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. Every second one of the metal inserts 9 is 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, usually metal, e.g., aluminium or copper. They have equal lengths and cross sections so as to exhibit equal electrical resistances.

Normally the test tap 13 is covered by a metal cap (not shown) 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 decribed is produced as follows:

First the winding for the electrical field grading insulation 5 is 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, i.e., 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, in particular a polymer or mixture of polymers or resin, preferably with an admixture of anorganic filler material like silica, alumina, dolomite, wollastonite or boron nitride as described above. This step can be carried out in

different ways, e.g., involving vacuum casting or, preferably, 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 is left to harden. When it has solidified sufficiently the mould is opened and the bushing removed.

There are many ways of modifying the above-described design within the scope of the invention. In particular, 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 in stead of connection wires and various materials can be employed, provided that they offer sufficient electrical conductivity. It is preferred that the grounding contact is connected to the at least one contact piece at three or more positions which are essentially equally distributed over the circumference of the flange as in this way an approximately axially symmetrical distribution of currents and electromagnetic fields is 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.

List of reference symbols

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