Multiple terminals bushing assembly

Field of the Invention

The present invention relates to a multiple terminals bushing assembly for electrical transformer.

Background of the Invention

Electrical transformers are used to distribute electricity from the point of generation or substations for domestic home or business and thereby stepping down the voltage.

Transformers require insulative conductors on both the high and low voltage sides. The transformer case or tank is usually metal and must be safely insulated. A bushing is a hollow isolator, carrying high and low voltage leads through a wall portion of the tank.

Low voltage bushings of tank-type transformers are generally made up of molded epoxy bushing or multi-piece assemblies with porcelain isolators. Mounting space of a bushing assembly is restricted by outer dimensions, creepage distance and clearance distance between the terminals.

Traditionally, the bushing compartment of electrical

transformers, especially distribution transformers, is arranged on the top portion of the transformer tank. The tank is hermetically sealed by placing a sealing gasket between the top portion, usually a flat cover, and a pot-shaped casing portion. The bushing compartment comprises

electrically insulated conductors extending through an opening of the tank and having substantial vertical oriented outer terminals. Electrical connection of the terminals is easily possible with the bushing compartment arranged on the top of the tank.

However, in applications where additional high current connection is required the available space on the top portion of the tank is limiting factor. The required space for multiple bushings per phase will increase the outer

dimensions of the transformer. It is common practice to place the bushings on the cover of the transformer in single or multiple rows making the connection to the control unit grid difficult. A further disadvantage is, that such an low voltage bushing arrangement is not easy to adapt on various modifications for specific applications. Thus, it would be desirable to provide a multiple terminals bushing assembly which requires less mounting space and which is easy to adept on specific applications.

Summary of the Invention

Against the foregoing background, it is an object of the present invention to provide a multi terminals bushing assembly with small overall installed size.

In accordance with the present invention there is provided a multiple terminals bushing assembly for an electrical transformer, comprising:

an insulating support, comprising:

a pair of closely spaced press plates made of an insulating material and arranged in an opening of the transformer casing, whereby each of the press plates having corresponding openings which are adapted for connector elements passing through the openings; a ring-shaped gasket arranged in each opening and in contact with each of the press plates and the curved surface area of a corresponding connector element ; a clamping arrangement for squeezing the gasket.

In other words, the bushing assembly according to the invention has two press plates which serve as an insulation support for the connector elements. In comparison to epoxy molded bushings the space required for large amount of bushings is reduced. Susceptibility to variations and

modifications for specific applications e.g. simple

implementation of additional connector elements and easy rearrangement for specific applications, various connector elements having different cross-sections and shapes

implemented in one bushing assembly. Low cost is achieved by the unsophisticated machining and assembly technology and by the availability of the raw materials on the market.

Preferably the ring-shaped gasket is arranged in a ring- shaped recess provided in at least one of the press plates and serve as a seal to prevent fluid leakage around the connector elements.

In a preferred embodiment the clamping arrangement comprises a first set of bolts to hold together the press plates during the pre-assembly of the transformer's active part.

The first set of bolts are arranged in counterbored holes provided in the upper press plate averted form the

transformer casing to leave clearance between the bolt head and clamping frame and screwed in threaded blind holes in the lower press plate to prevent the fluid flow through the hole.

In order to distribute the clamping force uniformly the clamping arrangement further comprises a clamping frame and second set of bolts, whereby the pair of press plates is sandwiched between the clamping frame and the transformer casing . For leakage-free mounting, a sealing plate is arranged between the transformer casing and the opposing press plate.

For the assembling process it is desirable that the second set of bolts to be introduced form the exterior of the transformer casing in order to pull the clamping frame towards the tank and squeeze the sealing gaskets around the connector elements.

In a preferred embodiment the second bolts are flange-type bolts and sealing between the tank wall and the bolt is accomplished by a gasket, whereby a washer limits the squeezing of the gasket.

Preferably the curved surface area of each of the connector elements is coated with a sealant compound.

Brief Description of the Drawings

In the accompanying drawing,

Fig. la is a front view of a terminal assembly of an

electrical transformer having multiple low voltage bushings per phase;

Fig. lb is a top view of Fig la;

Fig. 2 is an enlarged view of two different type of

connector elements;

Fig. 3 is an in enlarged view according to section "A-A and "B-B" in Fig. lb;

Fig. 4 top view of a steel clamping frame; Fig. 5 is a top view of a sealing plate;

Fig. 6 is an enlarged front view of "Detail X" in Fig la;

Fig. 7 is a partial sectional drawing of a front view of an application of the invention, whereby the low voltage bushing assembly is mounted on a bushing compartment which is attached on the side wall of a transformer tank;

Fig. 8 is a side view of Fig 7.

Detailed Description of Preferred Embodiments

Fig. la shows a front view of bushing assembly 1 of a tank- type electrical transformer having multiple low voltage bushings per phase. The transformer tank 6 has an opening 15. The bushing assembly 1 is arranged in this opening 15.

The bushing assembly 1 comprises multiple connector elements 2, 21 made of conductive material. The connector elements 2,21 are adapted for connection of electrical conductors.

The bushing assembly 1 further comprises one upper and one lower machined press plates 3, 4. Both press plates 3, 4 are closely spaced and made of an insulation material. Each press plate 3, 4 show through holes 12 (Fig. 3) for guiding the connector elements 2, 21 between the exterior 18 and the interior 19 of the transformer tank 6.

The press plates 3, 4 are sandwiched between a clamping frame 5 (Fig. 4) and the transformer tank 6. A sealing plate 8

(Fig. 5) is located between the lower press plate 4 and the tank 6. The clamping frame 5 and the sealing plate 8 have openings 22, 23. Fig. lb shows a top view of the bushing assembly 1. Each phase comprises six connector elements 2 and one connector element 21. The connector elements 2,21 of each phase arranged in the openings 22, 23 of the clamping frame 5 and sealing plate 8 (three rows, with two connector elements in each row) .

Fig. 2b shows two connector element types made of conductive material used for different ampacities. The electrical conductor is inserted into the ending of the tubular part of the connector element and electrically connected using crimp connection .

The clamping arrangement my bee more clearly understood with reference to Figure 3. Figure 3 shows partially in section the area around the through holes in the upper press plate and countersunk holes in the lower press plate 12 according to lines A-A and B-B in Figure 2. The clamping arrangement comprises first bolts 17 to pull together plates 3 and 4. The upper press plate has counterbored holes 10 for the head of bolt 17. The lower press plate 4 has threaded blind holes 11. With the bolts 17 the press plates 3, 4 are compressed and held in a closely spaced position. Fluid leakage is prevented by the sealing gaskets 7, which are provided in countersunk holes 16 in the lower plate 4, squeezed by pulling the plates 3, 4 together and by a sealing plate 8 between the

transformer tank 6 and the lower plate 4. Furthermore, the curved surface area of each of the connector elements 2, 21 is coated with a sealant compound. Retaining rings 20 are inserted in the groove on connector element 2 to prevent the slip out from the press plates. The final clamping is furnished by compressing the sealing gaskets 7 between the press plates 3, 4 around the connector elements with the steel clamping frame 5. The clamping bolts 13 are introduced from the outer side (exterior 18) . This facilitates the assembly process. To prevent fluid leakage through the clamping bolts 13, an o-ring 9 (Fig. 6) is placed between the flanged clamping bolt and the washer 14 (Fig. 6) . Fig. 7 and Fig. 8 show the bushing assembly 1 according to the invention located on the bottom part of a bushing compartment 24. The bushing compartment 24 is attached to a side wall 25 of a transformer tank. This design allows the arrangement of bushings in two levels: the high voltage side on the top of the transformer, the low voltage multiple terminals on the bottom portion 26 of a separate bushing compartment 24.

The two-level construction of a bushing assembly opens the possibility for arrangement of further connection means required in high current applications. Electrical terminals 2, 21 facing downwardly from the bottom portion 26 of the bushing compartment 24 are protected in a niche. The invention provides sufficient space for electrical connection required in additional high current applications.

The foregoing merely illustrates the principal of the invention. Those skilled in the art will be able to diverse numerous arrangements which, although not explicitly

described or shown herein, embody those principles and are within the scope of the invention.

List of used reference signs

1 bushing assembly

2 connector element

3 upper press plate

4 lower press plate

5 clamping frame

6 transformer tank

7 sealing gasket

8 sealing plate

9 O-Ring

10 counterbored hole

11 threaded blind hole

12 through hole

13 flanged clamping bolt

14 washer

15 opening in 6

16 countersunk holes

17 bolt

18 exterior

19 interior

20 retaining rings

21 connector element

22 opening in 5

23 opening in 8

24 bushing compartment

25 side wall of transformer tank

26 bottom portion of 24