DESC:FIELD OF THE INVENTION

[001] The present invention relates to bushings used to connect electrical systems and more particularly to transformer bushings.

BACKGROUND OF THE INVENTION

[002] Transformers are essential components of the power distribution system, and bushing is an essential part of distribution transformers, ensuring its reliable and safe operation.

[003] The purpose of an electrical bushing is to transmit electrical power in or out of enclosures, i.e., barriers, of an electrical apparatus such as transformers, circuit breakers, shunt reactors, and power capacitors. Transformer bushing is an insulating structure that facilitates the passage of an energized, current-carrying conductor through the grounded tank of the transformer. The conductor may be built in to the bushing, or the bushing may be built with the provision for a separate conductor to be drawn through its center. The bushings used for the low voltage windings of a transformer are often solid type with a porcelain or epoxy insulator.

[004] All materials carrying an electric current generate an electric field and when an energized conductor is near any material at ground earth potential, it can cause very high field strengths to be formed, especially where the field lines are forced to curve sharply around the grounded material such as transformer tanks. The bushing controls the shape and strength of the field and reduces the electrical stresses in the insulating material.

[005] A typical bushing design includes a conductor usually of copper or aluminium surrounded by insulation except at the terminal ends. The terminal ends are provided with threads which allows the bushing to be connected to a power cable at one end and the transformer windings at the other end. The terminal end towards winding includes a base connector where the lugs from the transformer windings are connected. The bushing further includes metal flanges surrounding the insulation part to install the bushing on the transformer tank wall. The air-to-oil type of bushing is primarily used for discussed in this invention, where one end of the bushing is in air and the second end is in the oil of the transformer.

[006] The insulation part is made of porcelain or epoxy with flanges over it to protect flashover. The bushing must be designed to withstand the electrical field strength produced in the insulation, when any grounded material is present. As the strength of the electrical field increases, leakage paths may develop within the insulation. If the energy of the leakage path overcomes the dielectric strength of the insulation, it may puncture the insulation and allow the electrical energy to conduct to the nearest earthed material causing burning and arcing.

[007] Bushing failures are often credited as one of the top causes of transformer failures so the condition of the bushings is of high interest to transformer asset owners. Typical bushing failure modes include moisture ingress, electrical flashover, bushing misapplication, corrosive sulphur, broken connection between ground sleeve and flange.

[008] Bushings, including the weather sheds, have been made from porcelain or cast epoxy. Typically, these weather sheds are designed so that water rolls off the sheds keeping the underside of the sheds substantially dry. However, a significant portion of the insulator surface can become wet or degraded by environmental pollution. The resulting weakening of the dielectric field can cause leakage and flashover conditions, thereby resulting in failure of the bushing.

[009] An additional drawback of porcelain or cast epoxy bushings is that they are relatively brittle and, therefore, are subject to damage from external conditions that can cause them to shatter. To provide an optimal solution and a safe and reliable housing for the bushing conductor, the porcelain and cast epoxy insulators are produced with a relatively thick wall (i.e., about 1 inch). The increased thickness further increases the weight of the bushings, and increases the cost of the bushings.

[0010] Mostly bushing with polymeric sheds are used nowadays. The conductor is enveloped in a polymeric shed insulation. However there is chance of leakage of oil from transformer tank to outside from the space between the polymeric shed and the conductor surface. This calls for use of seals as additional components. Another disadvantage with present polymeric bushings is that, the base connector is made as a separate casting joined to the central conductor through a gasket threaded joint. Due to this when there is a maintenance activity has to be performed, the bushing has to be removed. Also the small value of linear expansion of porcelain has to be accommodated by using flexible seals and substantial metal fittings, both of which present manufacturing and operational problems.

[0011] In view of the limitations inherent in the available bushings, there exists a need for an improved bushing, which overcomes the disadvantages of the prior art and which can be used in a simple, cost effective, reliable, secure and environmental friendly manner.

[0012] The present invention fulfils this need and provides further advantages as described in the following summary.

SUMMARY OF THE INVENTION

[001] In view of the foregoing disadvantages inherent in the prior arts, the general purpose of the present invention is to provide an improved combination of convenience and utility, to include the advantages of the prior art, and to overcome the drawbacks inherent therein.

[002] A primary objective of the present invention is to provide a bushing for an electrical apparatus which is simple and cost effective.

[003] In one aspect, the present invention provides a bushing for an electrical apparatus that comprises a conducting stud having a first end connected to an external conductor and a second end connected to an electrical component of the electrical apparatus, the conducting end having a plurality of threads at both the ends, a shell of polymeric insulating material mounted on the conducting stud, such that the shell includes a plurality of sheds and covers at least a portion of a length of the conducting stud, a mounting flange mounted over the shell, such that the mounting flange is adapted to mount the bushing on a wall of the electrical apparatus enclosure and a base connector detachably mounted on the second end of the conducting stud for connecting the conducting stud with the electrical component of the electrical apparatus.

[004] In another aspect of the present invention, the electrical apparatus is a transformer and the bushing connect the external conductor to a coil of the transformer.

[005] In yet another aspect of the present invention, the bushing is installed on an opening of the electrical apparatus enclosure.

[006] In one aspect of the present invention, the shell is molded on the conducting stud.

[007] In another aspect of the present invention, the conducting stud includes serrations over its external surface which allows the shell to snugly fit over the conducting stud.
[008] In yet another aspect of the present invention, the mounting flange is a thick plate comprising a hole to allow the conducting stud to pass through it and a plurality of holes over it to align the mounting flange with corresponding holes on an enclosure wall of the electrical device using nuts and bolts.

[009] In one aspect of the present invention, the mounting flange further includes an annular groove on its surface to dispose a resilient gasket to provides sealing between the mounting flange and the enclosure wall on tightening of the nuts and bolts.
[0010]

[0011] In another aspect of the present invention, the shell comprises a first portion and a second portion, such that the first portion includes a plurality of annular sheds on its external surface and the second portion having a longitudinal section with a plurality of serration over its external surface.

[0012] In yet another aspect of the present invention, the first portion is located outside the electrical device enclosure and the second section is located inside the electrical device enclosure.

[0013] In a further aspect of the present invention, the second portion further includes a pair of stoppers, such that the mounting flange when slipped over the shell abuts about the stopper and defines the position of the mounting flange in the bushing.

[0014] In one aspect of the present invention, the base connector is a thick plate that comprises a central hole for fitting on the conducting stud, a vertical slit extending from the central hole to a top edge forming two slit flanges each on one side of the vertical slit, the slit flanges facing opposite to each other and includes holes to allow a bolt to pass through it which when tightened brings the opposite slit flanges towards each other for gripping the conductor stud and a plurality of lug connection holes to electrically connect the electrical component of the electrical apparatus with the bushing.

[0015] In another aspect of the present invention, the base connector is detached from the second end of the conducting stud by untightening the bolt.

[0016] In yet another aspect of the present invention, the bolt is an Allen bolt.

[0017] These together with other aspects of the invention, along with the various features of novelty that characterize the invention, are pointed out with particularity in the description annexed hereto and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and the specific objects attained by its uses, reference should be made to the accompanying drawings and descriptive matter in which there are illustrated exemplary embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The advantages and features of the present invention will become better understood with reference to the following more detailed description taken in conjunction with the accompanying drawings in which:

[0019] FIG. 1 illustrates a schematic view of a bushing, according to one embodiment of the present invention;

[0020] FIG. 2 illustrates a cross sectional view of the bushing, according to one embodiment of the present invention; and

[0021] FIG. 3 illustrates a schematic view of the bushing with the mounting flange cut section, according to one embodiment of the present invention.

[0022] Like reference numerals refer to like parts throughout the several views of the drawings.

DETAILED DESCRIPTION OF THE DRAWINGS

[0023] In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without these specific details.

[0024] As used herein, the term ‘plurality’ refers to the presence of more than one of the referenced item and the terms ‘a’, ‘an’, and ‘at least’ do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item.

[0025] Reference herein to “one embodiment” or “another embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Further, the diagrams representing one or more embodiments of the invention do not inherently indicate any particular order nor imply any limitations in the invention.

[0026] Referring to FIG. 1, which illustrates a schematic view of a bushing 10 for an electrical apparatus, according to one embodiment of the present invention. The bushing 10 comprises a conducting stud 12 having a first end 12A connected to an external conductor and a second end 12B connected to an electrical component of the electrical apparatus (not shown), the conducting stud 12 having a plurality of threads at both the ends, a shell 14 of polymeric insulating material mounted on the conducting stud 12, such that the shell includes a plurality of sheds and covers at least a portion of a length of the conducting stud 12, a mounting flange 16 mounted over the shell 14, such that the mounting flange 16 is adapted to mount the bushing 10 on a wall of the electrical apparatus enclosure and a base connector 18 detachably mounted on the second end 12B of the conducting stud for connecting the conducting stud 12 with the electrical component of the electrical apparatus.

[0027] In one embodiment of the present invention, the bushing 10 is installed on an opening in a wall of the electrical apparatus enclosure. In one preferred embodiment of the present invention, the electrical apparatus is a transformer and the bushing 10 connect the external conductor to a coil of the transformer.

[0028] The conducting stud 12 is a straight cylindrical central conductor made of any electrically conducting material such as Aluminum, copper etc. The conducting stud 12 includes threads at its opposite ends 12A and 12B and are used to connect an external conductor and the internal electrical component of the electrical apparatus respectively. In one preferred embodiment of the present invention, the shell 14 is molded between the two ends 12A and 12B of the conducting stud. The shell 14 surrounds the stud 12 serves to support the stud 12 on the wall of the transformer enclosure and to electrically insulate the stud 12 from the wall. The shell 14 is made of silicone rubber which makes the surface hydrophobic. In one embodiment of the present invention, the conducting stud 12 includes serrations over its external surface which allows the shell 14 to snugly fit over the conducting stud.

[0029] Due to the moulding of the shell 14 on the stud 12, the gap between the stud 12 outer surface and inner surface of the shell 14 is eliminated. This prevents chances of any leakage of oil from the electrical apparatus enclosure such as transformer tank to outside. It also eliminates need of additional sealing arrangement which is used in transformer bushings of prior art.

[0030] In one embodiment of the present invention, the shell 14 is made of a thermosetting polyester material and a thin film-like layer of adhesive material, preferably an epoxy bonding agent applied between the shell and the stud which forms a high-quality leak-proof bond.

[0031] During the molding process, the polymeric material of the shell 14 fills over the surface of the stud 12 and upon hardening and curing, mechanically locks with the stud 12 and forms a mechanically-strong connection between the stud 12 and the shell 14 that precludes any relative motion between these parts and provides additional protection for the bond between these parts.

[0032] Referring to FIG. 2 that illustrates a cross sectional view of the bushing 10, according to one embodiment of the present invention. As seen from the figure, the shell 14 comprises the shell comprises a first portion 14A and a second portion 14B, such that the first portion 14A includes a plurality of annular sheds 20 on its external surface and the second portion 14B having a longitudinal section with a plurality of serrations 22 over its external surface.

[0033] The annular sheds 20 cover a portion of the shell 14, serving in a conventional manner to impart improved electrical tracking resistance to the bushing 10 in this region. The first portion 14A including the shed 20 is located outside the electrical apparatus enclosure towards left side of the wall 24 and the second portion 14B is located inside the electrical apparatus enclosure towards right side of the wall 24.
[0034] Referring to FIG. 3 that illustrates a schematic view of the bushing 10 with the mounting flange 16 cut section, according to one embodiment of the present invention. In one embodiment of the present invention, the second portion 14B of the shell further includes at least one stopper 26, such that the mounting flange 16 when slipped over the shell abuts about the stopper 26 and defines the position of the mounting flange on the bushing 10. The stopper 26 has an upward annular protrusion around the shell. In another embodiment of the present invention, the shell 14 may include two stoppers 26, such that the mounting flange 16 when slipped over the shell 14, the width of the mounting flange 16 is between the two stoppers 26. The serrations 22 also add to the grip between the mounting flange 16 internal surface and the external surface of the shell 14, thereby making it leak proof.

[0035] In one embodiment of the present invention, the mounting flange 16 is a thick plate that comprises a hole to allow the conducting stud 12 to pass through it and a plurality of holes 28 over it to align the mounting flange with corresponding holes on the wall 24 of the electrical apparatus enclosure using nuts and bolts 30. In one embodiment of the present invention, the mounting flange 16 further includes an annular groove (not shown) on its surface to dispose a resilient gasket 32 to provides sealing between the mounting flange 16 and the wall 24 on tightening of the nuts and bolts 30. The mounting flange 16 is an integral part of the bushing 10 which is mounted over the shell 14 and is located adjacent the transformer wall 24.

[0036] When mounting flange 16 is tightened on the wall 24 with the use of nuts and bolts 30, the right-hand end face of it presses against the wall 24, thereby clamping the wall 24 between the nut and the mounting flange 16 and thus holding the bushing 10 securely in place in the opening of the wall. This clamping action compresses the gasket 32 and thus maintains a good seal between the mounting flange 16 and the wall 24.

[0037] In one embodiment of the present invention, the base connector 18 is a thick plate that comprises a central hole 34 for fitting on the conducting stud 12, a vertical slit 36 extending from the central hole 34 to a top edge forming two slit flanges 38 each on one side of the vertical slit, the slit flanges 38 facing opposite to each other and includes holes to allow a bolt 40 to pass through it which when tightened brings the opposite slit flanges 38 towards each other for gripping the conductor stud 12 and a plurality of lug connection holes 42 to electrically connect the electrical component of the electrical apparatus with the bushing 10. The dimensions and shape of the central hole 34 corresponds to the size and shape of the stud 12.

[0038] In one preferred embodiment of the present invention, the bolt 40 is an Allen bolt. The base connector 18 may be detached from the second end 12B of the conducting stud by untightening the bolt 40.

[0039] The lug connection holes 42 may be used to connect lugs from the transformer winding with the bushing 10. The provision of slit 36 and Allen bolt 40 makes the base connector 18 detachable from the stud 12 when required. Traditionally the base connector 18 had been made as a separate casting joined to the stud through a gasketed threaded joint. One of the objects of the present invention, is to eliminate the need for such a joint, so the base connector 18 is made detachable from the stud by means of the Allen bolts 40. An additional advantage of this integral formation is the improved conductivity between the stud 12 and base connector 18 as compared to that available when a discrete joint is present between these parts. The base connector 18 of bushing is such that, it facilitates easy lifting and removal of core with coils from the tank without dismantling LV bushings. In other words, the base connector 18 is detachable from stud for easy removal of winding lugs during maintenance without removing bushing.

[0040] The advantages of the present invention includes:

1) the shell 14 molded under high pressure over the stud 12 and forming a joint with the stud that has a high resistance to leakage along the stud without use of any additional component or sealant.
2) Detachable base connector 18 which can be easily detached from the stud 12 for easy removal of winding lugs during maintenance without removing bushing.

[0041] It is to be noted that the bushing 10 of the present invention is described for transformers, however the same can be used for other electrical apparatus filled with oil such as circuit breakers, shunt reactors, power capacitors, and any similar devices.

[0042] Although a particular exemplary embodiment of the invention has been disclosed in detail for illustrative purposes, it will be recognized to those skilled in the art that variations or modifications of the disclosed invention, including the rearrangement in the configurations of the parts, changes in steps and their sequences may be possible. Accordingly, the invention is intended to embrace all such alternatives, modifications and variations as may fall within the spirit and scope of the present invention.

[0043] The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching.

[0044] The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the appended claims.
,CLAIMS:WE CLAIM:

1. A bushing for an electrical apparatus, comprising:
- a conducting stud having a first end connected to an external conductor and a second end connected to an electrical component of the electrical apparatus, the conducting stud having a plurality of threads at both the ends;
- a shell of polymeric insulating material mounted on the conducting stud, such that the shell includes a plurality of sheds and covers at least a portion of a length of the conducting stud;
- a mounting flange mounted over the shell, such that the mounting flange is adapted to mount the bushing on a wall of the electrical apparatus enclosure; and
- a base connector detachably mounted on the second end of the conducting stud for connecting the conducting stud with the electrical component of the electrical apparatus.

2. The bushing for an electrical apparatus according to claim 1, wherein the bushing is installed on an opening in a wall of the electrical apparatus enclosure.

3. The bushing for an electrical apparatus according to claim 1, wherein the electrical apparatus is a transformer.

4. The bushing for an electrical apparatus according to claim 3, wherein the bushing connects the external conductor to a coil of the transformer.

5. The bushing for an electrical apparatus according to claim 1, wherein the conducting stud includes serrations over its external surface which allows the shell to snugly fit over the conducting stud.

6. The bushing for an electrical apparatus according to claim 1, wherein the shell is molded on the conducting stud.

7. The bushing for an electrical apparatus according to claim 1, wherein the shell comprises a first portion and a second portion, such that the first portion includes a plurality of annular sheds on its external surface and the second portion having a longitudinal section with a plurality of serration over its external surface.

8. The bushing for an electrical apparatus according to claim 7, wherein the first portion is located outside the electrical device enclosure and the second section is located inside the electrical device enclosure.

9. The bushing for an electrical apparatus according to claim 7, wherein the second portion further includes at least one stopper, such that the mounting flange when slipped over the shell abuts about the stopper and defines the position of the mounting flange on the bushing.

10. The bushing for an electrical apparatus according to claim 1, wherein the mounting flange is a thick plate comprising:
- a hole to allow the conducting stud to pass through it; and
- a plurality of holes over it to align the mounting flange with corresponding holes on an enclosure wall of the electrical device using nuts and bolts.

11. The bushing for an electrical apparatus according to claim 10, wherein the mounting flange further includes an annular groove on its surface to dispose a resilient gasket to provides sealing between the mounting flange and the enclosure wall on tightening of the nuts and bolts.

12. The bushing for an electrical apparatus according to claim 1, wherein the base connector is a thick plate comprising:
- a central hole for fitting on the conducting stud;
- a vertical slit extending from the central hole to a top edge forming two slit flanges each on one side of the vertical slit, the slit flanges facing opposite to each other and includes holes to allow a bolt to pass through it which when tightened brings the opposite slit flanges towards each other for gripping the conductor stud; and
- a plurality of lug connection holes to electrically connect the electrical component of the electrical apparatus with the bushing.

13. The bushing for an electrical apparatus according to claim 12, wherein the bolt is an Allen bolt.

14. The bushing for an electrical apparatus according to claim 12, wherein the base connector is detached from the second end of the conducting stud by untightening the bolt.