A High Voltage-high Current Transformer Bushing Having A Cast Resin Insulating Housing And A Hollow Central Conductor Containing Fluid Coolant

Abstract

A high voltage-high current bushing for connecting the output from an electrical generator to the primary or low potential winding of a step up transformer. The bushing comprises a hollow high current carrying member surrounded by insulating material having a ground shield embedded therein. The hollow high current carrying member contains liquid coolant material which increases the thermal time constant of the bushing and tends to eliminate hot spots in the current carrying member.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to high voltage-high current electrical bushings particularly of the type used for connecting the output from an electrical generator to the primary or input winding of a step-up transformer.

2. Description of the Prior Art

In the prior art, bushings used for connecting the output of an electrical generator to the primary or input winding of a step-up transformer usually comprise a central conductor member surrounded by a porcelain insulator with gasketed joints at each end of the porcelain insulator. This type of bushing is satisfactory but it is expensive to manufacture and hard to maintain in good working order. Some reasons why such a bushing requires extensive maintenance is because the porcelain members crack and the gasketed joints at the ends of the porcelain develop leaks. This invention eliminates the objections to the prior art bushings by providing a bushing which is simple to manufacture and easy to maintain. In the bushing of the present invention there are no gasketed joints between the central conductor and the outer insulating coating. The coolant material is maintained in a central chamber fabricated with all welded joints which lessens the possibility of leaks which occur in the prior art devices having gasketed joints. The outer insulating coating of the present invention is highly resistant to cracks.

SUMMARY OF THE INVENTION

This invention provides an improved and economical bushing for connecting the output of an electrical generator to the input or primary winding of a step-up transformer. The bushing comprises a hollow metallic current carrying member having an insulating body cast or molded thereabout. The insulation body may be made of a suitable resin, such as filled epoxy, or other suitable castable or moldable material. A stress grading and ground shield is cast or molded in the insulating body portion and spaced from the current carrying member. A flange is provided on the insulation body portion for mounting the insulator on a transformer casing. The insulator is usually gasket mounted on the transformer casing. Connections are provided to the shield for grounding the shield to the transformer casing. Walls are provided near the ends of the hollow current carrying member to provide a reservoir or chamber inside the hollow current carrying member for retaining liquid coolant which increases the time constant and tends to eliminate or lessen hot spots in the current carrying member. Means are provided near each end of the hollow current carrying member, beyond the insulation body portion, for connecting electrical conductors to the hollow current carrying member.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a perspective view, with parts broken away, showing a bushing as provided by this invention connecting the output of an electrical generator to the input or primary winding of a step-up transformer;

FIG. 2 is a longitudinal sectional view of a bushing as provided by this invention; and

FIG. 3 is a top view of the bushing shown in FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Like reference characters refer to like elements throughout the various figures of the drawings.

Referring in detail to FIG. 1, this figure shows the environment in which the bushing provided by this invention would normally be used.

FIG. 1 is a partially perspective view showing a step-up transformer 10 having bushings 12 as provided by this invention mounted thereon. Although only one bushing 12 is shown in FIG. 1 the transformer is equipped with three bushings 12 as provided by this invention. FIG. 1 also illustrates an electrical generator 14, with the output from each phase of the generator connected to one of the bushings 12 on the transformer 10 through isolated phase duct 16. The output or high voltage from the transformer 10 is connected to a transmission line through three high-voltage bushings 18. As seen in FIG. 1 the bushings 12 are protected from weather by a cover 20.

The output from the electrical generator 14 is usually in the order of 23,000 volts at 10,000 amperes or higher. This output is connected through the isolated phase duct 16 to the bushings 12 which are connected to the low voltage or primary winding of the step-up transformer 10. This voltage is stepped up by the transformer 10 and applied to a transmission line (not shown) for distribution to the customers. It is therefore seen that the bushing 12 must function as a relatively high voltage bushing which carries high currents. The high currents carried by the bushing will necessarily cause high heating of the current carrying elements of the bushing. Because of this heating, sufficient means must be built into the bushing to handle this high heat without damaging the bushing and the associated transformer.

Referring to FIG. 2 which shows a longitudinal sectional view of the bushing 12 provided by this invention, it is seen that the bushing 12 provided by this invention comprises a large cross section current carrying member 21. This current carrying member 21 may be rectangular, square, circular or triangular in cross section and is made from an aluminum extrusion. However it is understood that any other type of good current conducting metal and any other hollow cross section could be used for making the current carrying member 21. In one common size bushing the current carrying member 21 is approximately 10 inches square and approximately 44 inches long. The current carrying member 21 has a serrated or rough portion 22 provided thereon near its upper end. The remainder of the length of the current carrying member is coated with a mold release material 23 and a resin insulator 24 is cast onto the current carrying member 21. The serrated or roughened portion 22 of the current carrying member firmly bonds with the cast insulating member 24; however, the mold release material 23 over the remaining portion of the current carrying member permits the current carrying member to move with respect to the resin insulating member 24 over most of the length of the current carrying member 21 when the current carrying member 21 expands or contracts due to thermal cycling. A stress grading or ground shield 28 is cast into the insulating member 24 and the shield 28 is spaced from the current carrying member 21. The shield 28 is connected by means of a conductor 30 to an insert 32 for grounding the shield 28 to the top wall 34 of the transformer 10 when the bushing 12 is mounted on the transformer 10. As seen in FIG. 2 the ends 36 and 38 of the current carrying member 21 extend beyond the ends of the insulating member 24. The ends 36 and 38 are provided with a plurality of holes 40 which are used to attach leads 50 from the generator to the bushing by means of bolts 42 and nuts 44 and to connect the low-voltage winding of the transformer 10 to the lower end of the bushing by means of lead 51 and bolts 46 and nuts 48. A plurality of holes 40 may be provided on all four sides of the rectangular current carrying member 21 at each end thereof to provide for the convenience of attaching leads such as 50 or 51 to any of the four sides of the current carrying member 20. This convenience of connection is often important since it facilitates easy connection of the heavy stiff leads 50 and 51 to the current carrying member 21 regardless of the direction from which the leads 50 and 51 approach the current carrying member 21.

The terminal portions 36 and 38 of the current carrying member 21 are coated with a good tin-silver or other good conductive coating such as indicated at 54 to provide a good low-resistance electrical connection between the leads 50 and 51 and the current carrying member 21.

The bushing 12 is attached to the top wall 34 of the transformer 10 by means of a plurality of bolts such as 56, which extend through bolt holes 58 in the flange portion 59 of the bushing 12 and engage threaded holes 60 in the tank wall 34. As indicated in FIG. 1 metallic inserts 32 are provided in the flange portion 54 of the bushing which receives the bolts 56 to provide ground connection between the ground shield 28 and the top wall 34 of the transformer 10.

A fluid tight seal is provided between the flange portion 59 of the bushing 12 and the tank wall 34 by means of a gasket 61 which is clamped tightly between the wall 34 of the transformer 10 and the flange portion 59 of the bushing 12 when the bushing is mounted on the wall 34 of the transformer 10.

The upper end of the hollow rectangular current carrying member 21 is closed by a metallic plate 62. The plate 62 is welded, brazed or otherwise joined to the inside walls of the current carrying member 21 to provide a fluid tight connection between the plate 62 and the four sidewalls of the current carrying member 21. Likewise a wall member 64 is provided near the lower end of the current carrying member 21 to provide a fluid tight connection with the four sidewalls of the current carrying member 21. Each of the walls 62 and 64 is provided with a removable screw threaded plug 66. The chamber or compartment provided by the walls 62 and 64 is filled with a good coolant material to increase the thermal time constant of the bushing and to reduce the hot spot temperature of the current carrying member 21. The removable plugs 66 facilitate draining and filling of the compartment 68 provided by the walls 62 and 64 and the sidewalls of the current carrying member 21 with a good grade of fluid coolant 70, such as mineral oil or the like.

A plurality of weather sheds 72 are cast or molded into the insulating member 24 when the insulating member 24 is cast or molded about the current carrying member 21.

FIG. 3 is a top view of the insulator shown in longitudinal sectional view in FIG. 2.

The bushing provided by this invention comprises a substantial improvement over the bushings of the prior art for connecting the output from a high voltage-high current electrical generator to the primary or low-voltage winding of a step-up transformer. The bushing is of substantially lower cost than the prior art bushings and the simple construction of the bushing requires less maintenance than the prior art bushings. A typical bushing constructed according to the teachings of this invention requires approximately 300 pounds of resin to form the insulating portion 24, 90 pounds of aluminum to provide the current carrying member 21, 10 pounds of aluminum to provide the shield 28, and approximately 110 pounds, or 13 gallons of coolant to fill the compartment 68, therefore, the bushing 12 filled with dielectric totals approximately 510 pounds of material.

It is also seen that because of the large area of the current carrying member 21 in a bushing of this type that the deleterious effects of heating by the high currents carried by the bushing may be maintained at a minimum.

Claims

We claim as our invention:

1. A high-current bushing comprising an elongated hollow metallic current carrying member having four sidewalls and first and second ends, resin insulating material cast around said current carrying member, said resin insulating material terminating short of said first and second ends of said current carrying member, shield means cast in said resin insulating material in spaced relationship to said current carrying member, terminal means connected to said shield means, said cast resin insulating material comprising a flange for mounting said bushing, a pair of substantially transverse wall members welded inside said hollow current carrying member with a fluid tight joint, said transverse wall members being spaced from each other with a substantially transverse wall member adjacent each of said first and second ends of said current carrying member, said pair of substantially transverse wall members providing a liquid tight chamber between said transverse wall members for retaining coolant inside said elongated hollow metallic current carrying member, and terminal means located on the sidewalls of the respective ends of said current carrying member which extend beyond said resin insulating material adjacent the ends of said current carrying member for connecting electrical conductors to the sidewalls of said current carrying member.