US2972723A - Through type transformer - Google Patents

Description

Feb. 21, 1961 M. R. SABISTON THROUGH TYPE TRANSFORMER Filed Oct. '7, 1957 INVENTUR MALCOLM R. SABlS ON THROUGH TYPE TRANSFORMER Malcolm R. Sabiston, Leaside, Ontario, Canada, assignor to Sangamo Electric Company, Springfield, Ill.

Filed Oct. 7, 1957, Ser. No. 688,584

Claims priority, application Canada May 2, 1957 13 Claims. (Cl. 336175) This invention relates to improvements in through type current transformers. Transformers of this type employ a primary in the form of a straight conductor or bus bar of high current capacity rating which extends through and is insulated from the secondary windings. The insulation for the bar conventionally comprises a straight tube or bushing of insulating material.

In the past, when such transformers were required to withstand relatively low impulse voltages without fiash over, in order to satisfy acceptable standards, such bushings were found adequate. However, when the flash over impulse levels of such transformers were increased it was found that the conventional bushing was inadequate to prevent electrical breakdown or flash over under applied impulse voltages if the bushing were kept to a practical length. That is, it was found that the conventional bushing did not give adequate control of the electrostatic stresses around the transformer and, under the application of high impulse voltages, electrostatic stress concentration occurred causing electrical breakdown of the air dielectric and consequent flash over.

It is therefore the object of this invention to provide a transformer of the type referred to in which the electrostatic stresses around the transformer will be positively controlled to provide a substantially uniform distribution to eliminate the problem of electrical breakdown and flash over presently experienced.

More particularly, it is the object to provide an insulating transformer bushing of extremely compact form which will be capable of preventing electrical breakdown or flash over under the application of very high impulse voltages to thereby enable transformers of high flash over impulse levels to be constructed much more compactly than previously possible.

Another important object is to provide a bushing as aforesaid of simple and economical construction.

The principal feature of the invention resides in embedding, in a bushing, tubular high voltage and ground conductors or foils so disposed and arranged that the ends of the conductors define an annular electrical stress distributing throat or nozzle disposed adjacent to each end of and internally embedded within the bushing to distribute electrical stresses outwardly from the center of the bushing.

Another important feature resides in contouring the ends of the bushing to provide a substantially uniform electrical stress gradient over the ends of the bushing, the bushing when installed in a through type current transformer controlling the electrical stresses around the transformerto provide a uniform stress distribution eliminating high stress concentrations which cause electrical breakdown.

More particularly according to the preferred form of the invention, the bushingcomprises a tube or bushing preferably cast of epoxy resin in the shape of a dumbbell, the bushing having embedded therein a tubular high voltage foil disposed immediately adjacent to the inner periphery of the bushing throughout the major nited States Patent an I 1C portion of its length and turned outwardly at the bush ing ends, and a tubular ground foil disposed immediately adjacent to the outer periphery of the bushing through the central bushing portion and flared outwardly at the bushing ends, the ends of the foils defining therebetween the above-mentioned annular throats or nozzles. Preferably the ends of the high voltage foil are turned rearwardly as well as outwardly and the ends of both high voltage and ground foils are provided with beads to eliminate sharp edges.

These and other objects and features will become apparent from the following description taken in conjunction with the accompanying drawings, in which:

Figure 1 is a perspective view of a through type current transformer embodying the invention;

Figure 2 is a top plan view of the transformer bushing;

Figure 3 is a side-elevational view of the bushing of Figure 2;

Figure 4 is an end elevational view of the bushing of Figures 2 and 3;

Figure 5 is an enlarged fragmentary part vertical section, part diagrammatic view of the end portion of the bushing, illustrating the electrical stress distribution.

With reference to Figure 1, the transformer generally comprises a frame 1 having mounting feet 2, and the frame carries a secondary winding formation comprising series connected secondary windings 3. The output from the secondary is taken off through secondary terminals 4. The primary of the transformer comprises simply a straight conductor bar 5 which extends through the transformer and is insulated therefrom by a bushing generally designated as 6. Transformers of this type are commonly used, for instance, where the current ratings are of the range of 1000/5 amps. to 10,000/5 amps.

Transformers of these ratings may be required to operate at up to 15,000 volts, and the requirements are such that the transformer should be capable of withstanding a full wave impulse in the neighborhood of 100,000 volts without flash over. It has been found that by employing a bushing such as illustrated in Figures 2 to 5, and hereinafter more fully described, the flash over impulse level requirements can be met with a relatively short bushing, enabling the construction of a compact transformer.

Referring to Figures 2 to 5, it will be seen that the bushing 6 in the embodiment of the invention herein illustrated is in the form of a dumb-bell having a central waist portion 7 and enlarged end portions 8. Embedded in the bushing 6, which preferably is formed of a suitable epoxy resin, is an inner tubular metal foil 9 disposed immediately adjacent to the inner surface 10 of the bushing throughout substantially the entire length of the bushing. The ends 11 of the tubular foil 9 are turned outwardly and rearwardly as indicated particularly in Figure 5, and these ends 11 terminate in a bead 12 which may be provided, for instance, by wrapping the ends around a suitable wire to eliminate any sharp edges.

Leading from the inner foil 9 is a lead 13 for connection to the primary conductor bar 5 as illustrated in Figure 1. Also embedded in the bushing 6 is an outer tubular conducting foil 14 which surrounds the inner foil and terminates in a bead 15 at each end inwardly from the end of the enlarged bushing end 8. Preferably the end portions 16 of the outer tubular foil are flared outwardly as shown in Figure 5 as the enlarged bushing end portions 8 are reached.

The outer foil 14 is disposed adjacent to the outer surface of the bushing 6 throughout the central waist portion 7, but the end portions 8 of the bushing are thickened and contoured to provide a substantial mass of resin overlying the outer foil ends. A suitable lead 17 connected to the outer foil 14 is provided for connecting the outer foil to the transformer frame or ground as' illustrated in Figure 1.

It will be seen particularly from Figure 4 that the ends 12 and 15 of the inner and outer foils 9 and 14 define therebetween at each end of the bushing an annular throat or nozzle 18, and the conductor foils form a control of the electrical stress distribution throughout the bushing so that the lines of electrical forces designated at 19 are constrained to pass through the throats 18 at the ends of the bushing. With this arrangement, the electrical stresses are distributed outwardly from the central waist portion of the bushing to the throats 18, and with the bushing installed in the transformer, the electrical stresses are thus distributed outwardly from the transformer frame. As a result, high stress concentrations immediately adjacent to the transformer frame and secondary winding formation are eliminated and by elimination of these high stress concentrations, ionization in this vicinity is eliminated and electrical breakdown prevented.

As will be seen from Figure 4, the end portions 8 of the bushing are convexly contoured'above the throat 18 at each end of the bushing, and the throat is embedded to a substantial depth within the bushing end. By the provision of a substantial mass of resin or dielectric above each of the annular throats 18, the electrical stresses which are constrained to pass through thethroat are distributed over a substantial surface area of the bushing end, and a relatively uniform surface gradient of electrical stresses over a substantial surface area of the bushing is achieved, the amount of the dielectric providing a control of the degree of distribution of the electrical stresses.

With the arrangement illustrated, that is, the bushing of dumb-bell form, the contour of the bushing is such that the electrical stress lines emerge from the bushing substantially normal to the surface thereof for optimum distribution. This shaping enables the dimensions of the bushing to be reduced to a minimum and also, of course, enables the amount of resin required to be also reduced to a minimum.

An additional advantage also occurs from the bushing shape in that it allows the dimensions of the case which surrounds the bushing to be kept to a minimum resulting in a shorter magnetic path for the exciting magnetic flux and a gain in transformer accuracy. I

In summary, the use of the throat formation defined between the high voltage and ground foils to distribute the point of stress distribution outwardly from the central portion of the bushing, and according to the preferred form of the invention the use of the dumb-bell shaping enables the use in a through type transformer of an extremely short bushing without danger of providing electrical stress concentrations at any point in the air dielectric suflicient to cause ionization and consequent breakdown between the primary and secondary or transformer frame.

Additionally, the use of the high voltage foil disposed immediately adjacent to the inner periphery of the bushing eliminates any corona discharge occurring between the primary conductor bar and the bushing, thus eliminating an annoying source of radio-frequency interfer ence.

it will be appreciated that the above described bushing herein illustrated comprises the preferred form of the invention, but various modifications in details and shape may be made within the spirit of the invention and scope of the appended claims.

What I claim is:

l. A through type transformer comprising a secondary winding formation mounted on a frame, a tubular insulating bushing extending through said secondary winding formation and projecting on opposite sides thereof, a primary conductor extending through said bushing, said bushing having embedded therein an inner tubular conductor foil disposed immediately adjacent to the inner periphery of the bushing for a major portion of the bushing length and being turned outwardly at the ends of the bushing and curved rearwardly away from said bushing ends, said foil being connected to said primary conductor, and means to create an area substantially free of electrical stress in the vicinity of the intersection of said secondary winding and bushing, said means comprising an outer conductor foil embedded in said bushing and grounded to said frame, said outer foil surrounding said inner foil and being flared outwardly at the ends of the bushing, the out-turned ends of the inner tubular foil and the outwardly flared ends of said outer tubular foil defining therebetween at each end of the bushing an outwardly facing electrical stress distributing control throat embedded in the bushing.

2. A through type transformer comprising a frame, a secondary winding formation on said frame, a tubular bushing having a central reduced waist portion and enlarged ends presenting smoothly curved convex surface contours, said bushing being disposed with said waist portion extending through said secondary winding formation and said enlarged end portions disposed on opposite sides of said secondary winding formation, a primary conductor bar extending through said bushing, an inner tubular conductor connected to said primary conductor embedded in said bushing and disposed throughout the length of said waist portion and throughout the major portion of the length of said bushing end portions immediately adjacent to the inner periphery of said bushing, said inner tubular conductor being turned outwardly with respect to the axis of the bushing adjacent to the ends of said bushing, means connecting said tubular inner conductor to said primary conductor and means to create an area substantially free of electrical stress in the vicinity of the intersection of said secondary winding and bushing, said means comprising an outer tubular conductor embedded in said bushing and surrounding said inner conductor and grounded to said frame, said outer tubular conductor being flared outwardly in said enlarged bushing ends with the outwardly turned ends of said inner bushing and the outwardly flared ends of said outer bushing defining therebetween at each end of said bushing an annular electrical stress distributing control throat facing outwardly beneath smoothly curved convex bushing surfaces providing for emergence of electrical stress lines from the bushing substantially normal of said latter surfaces.

3. A device as claimed in claim 2 in which the ends of said inner and outer tubular conductors are headed.

4. A through type current transformer comprising a frame, a secondary winding formation wound on said frame, a tubular resin insulating bushing having a central Waist portion and an enlarged portion at each end of said waist portion, said bushing having a central passage therethrough of substantially uniform cross-section throughout the length of said bushing and said enlarged end portions having a smoothly curved convex surface contour curving radially outwardly from said waist portion to a point of maximum flare adjacent to the end faces of said bushing and thence curving smoothly inwardly to said end bushing faces, said bushing being disposed with said waist portion extending through said secondary winding formation and said enlarged end portions projecting on opposite sides of said secondary winding, a primary conductor bar extending through said bushing passage, a tubular inner conducting foil embedded in said bushing and disposed substantially immediately adjacent to the inner periphery of said bushing throughout the major portion of the length of said bushing and having the ends thereofcurved outwardly and then inwardly with respect to the end faces of said bushing to substantially follow the contour of the outer portions of said enlarged end portions, means connecting said foil to said primary conductor, and means to create an area substantially free of electrical stress in the vicinity of the intersection of said secondary winding and bushing, said means comprising an outer tubular conductor foil embedded in said bushing immediately adjacent to the surface of said waist portion and having the ends thereof flaring outwardly into said enlarged outer ends, and means connecting said outer foil to said transformer frame the ends of said inner and outer foils defining therebetween adjacent to each end of said bushing an outwardly facing annular control throat embedded beneath a convex bushing surface.

5. A bushing for a through type transformer comprising a tubular insulating member having a tubular inner foil for high voltage connection embedded therein and disposed throughout the major length of the member immediately adjacent to its inner periphery, the ends of said inner foil being turned outwardly into the body of said tubular member at the ends thereof, and an outer tubular foil, and conductor means connected to. said outer foil and leading through the outer wall of said tubular insulating member for connecting said outer foil to a transformer frame.

6. A bushing as claimed in claim 5 in which said tubular member comprises a cast epoxy resin.

7. A bushing as claimed in claim 5 in which the ends of said inner foil are additionally curved inwardly from the ends of said tubular member.

8. A bushing as claimed in claim 5 in which the ends of said inner and outer tubular members are provided with beaded edges.

9. A bushing as claimed in claim 5 in which said tubular member has a dumb-bell shape providing a reduced central waist portion and enlarged convexly contoured end portions and said outer tubular foil is disposed to lie adjacent to the outer periphery of said insulating member throughout said waist portion and has the ends thereof flared outwardly into said enlarged convexly contoured end portions.

10. A bushing as claimed in claim 5 in which said annular throats are embedded to an appreciable depth in the ends of said insulating member.

11. A bushing for a through type transformer comprising a hollow, dumbbell shaped insulating member of epoxy resin having a central waist portion and enlarged end portions, an inner tubular conductor foil for high voltage connection embedded in said tubular body and disposed immediately adjacent to the inner periphery of said'tubular body through the major portion of the body length and being turned outwardly of the body axis at opposite ends and an outer tubular conductor foil embedded in said tubular body to surround said inner foil, conductor means leading from said outer foil to the exterior of said insulating member for grounding said outer foil, said outer foil extending substantially parallel to said inner foil through said central waist portion and flaring outwardly into said enlarged end portions, the ends of said tubular foils defining therebetween at each end of said tubular member an internal annular throat across which is adapted to be developed full transformer voltage, said throat distributing electrical stresses outward- 1y from said waist portion, and the enlarged ends of said hollow member being contoured to present smoothly curved convex surfaces to provide a substantially uniform electrical stress gradient over the surfaces thereof.

12. A bushing for a through type transformer comprising a hollow dumbbell shaped insulating member adapted to receive a transformer primary conductor bar, said insulating member having a central waist portion adapted to be received within a transformer secondary winding, and enlarged, smoothly curved convex end portions, an inner tubular conductor embedded in said insulating member and disposed immediately adjacent to the inner periphery thereof and extending into said enlarged end portions, conductor means leading from said inner tubular conductor for connection to a primary conductor bar when disposed in said insulating member, an outer tubular conductor embedded in and extending into said enlarged end portions of said insulating member and surrounding said inner tubular conductor, and conductor means leading from said outer tubular conductor for connection to a transformer secondary winding frame.

13. A bushing as claimed in claim 12 in which the ends of said inner and outer tubular conductors extend outwardly of the axis of said insulating member in said enlarged end portions to define therebetween electrical stress distributing throat formations across which is adapted to be applied full transformer voltage.

References Cited in the file of this patent UNITED STATES PATENTS 1,651,234 Stolz Nov. 29, 1927 2,053,422 Calvert Sept. 8, 1936 2,214,415 Wehrlin Sept. 17, 1940 2,288,969 Kirkwood July 7, 1942 FOREIGN PATENTS 474,473 Canada June 12, 1942

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