Insulation structure for electrical apparatus

[57] ABSTRACT

Insulation structure having filler members which keep the overall insulation structure tight when heated. The filler members are located within the insulation structure at various places and in sufficient quantities, with the shortest dimension of the filler member oriented in the direction in which expansion is desired. As the insulation structure is heated, the shortest dimension of the filler member increases to tighten the insulation structure. The filler member is constructed from a material which tends to reduce its surface area when heated without changing its volume.

Claims, 7 Drawing Figures

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PATENTED JAN 2 9 1874

3,789,337

SHEET 1 OF 2

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PATENTED JAM 2 9 1974

SHEET 2 OF 2

3,789^337

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3,789,337

INSULATION STRUCTURE FOR ELECTRICAL APPARATUS

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates, in general, to electrical apparatus and, more specifically, to electrical apparatus insulation structures.

2. Description of the Prior Art

Insulation structures for electrical apparatus physically separate the electrical conductors to prevent current transfer between adjacent electrical conductors and other conducting members of the apparatus. The arrangement and composition of insulation structures depends largely upon the type and rating of the electrical apparatus. In nearly all types of electrical apparatus, the insulation structure must support the electrical conductors and/or the windings they form. Insufficient support may result in damage to the electrical apparatus and possible failure.

In electro-mechanical apparatus, such as motors and generators, the insulation structures are usually associated with the winding structures of the rotor and stator. In electrical inductive apparatus, such as power transformers, the insulation structures are usually associated with the winding structures which are positioned around the magnetic core. In all cases, good mechanical integrity of the insulation structure is desirable for proper apparatus performance and reliability.

The need for mechanically tight insulation structures is particularly desirable in power transformers where conductor movement may be caused by excessive forces, such as encountered when the transformer is subjected to a short-circuit load. Although the need is great, the ability to produce tight insulation structures in transformers is complicated by the insulation materials normally used.

In nearly all high power transformers, cellulosic materials comprise the largest amount of the insulation structure. Kraft paper, crepe paper, Nomex, wood, and press-board are some of the more generally used materials. Since cellulosic materials are hydroscopic, they are normally assembled into the insulation structure while containing a certain amount of water. In subsequent processing, water is removed to enhance the electrical insulating characteristics of the cellulosic materials. Unfortunately, when the moisture is removed from the cellulosic materials, they tend to shrink and the entire insulation structure becomes loose.

A loose insulation structure not only permits movement of the conductors during short circuit conditions, but also permits movement due to thermal cycling and shipment vibrations. Therefore, various methods have been employed to reduce the effects of insulation shrinkage in transformers. A present method involves the precompression of the insulating material at specified loads, together with subsequent tightening of the structure to produce a rigid winding structure. While this method is adequate, the time and labor involved therein is considerable.

Therefore, it is desirable, and it is an object of this invention, to provide an insulation structure for electrical apparatus which does not become loose when heated to remove the moisture contained in the insulating material.

SUMMARY OF THE INVENTION

There is disclosed herein new and useful electrical insulation apparatus which permits the heating of the insulating materials without a loosening of the insulation structure. Filler members are inserted in the insulation structure at various locations and in sufficient quantities. The filler members are constructed of a material which has a tendency to reduce its surface area when heated without any substantial change in its volume. As a consequence thereof, the smallest dimension of the filler material tends to increase when heated. The filler members are positioned in the insulation structure with the dimension which is not greater than its other two dimensions oriented in the direction in which expansion is desired. As the cellulosic insulation shrinks with the application of heat, the filler member enlarges in a direction which prevents the insulation structure from becoming loose. If the other materials of the insulation structure do not shrink as much as the filler member expands, compressive forces are developed which may be used to brace the insulation structures to each other. The ratio of filler material to insulation can be adjusted so that the filler material expansion always exceeds the insulation shrinkage.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and uses of this invention will become more apparent when considered in view of the following detailed description and drawings, in which:

FIG. 1 is a view of a three-phase power transformer constructed according to the teachings of this invention;

FIG. 2 is a partial top view of a transformer illustrating an insulation structure constructed according to the teachings of this invention;

FIG. 2A is a view of a vertical spacing member constructed according to the teachings of this invention;

FIG. 3 is a sectional elevational view taken along the line III—III of FIG. 1;

FIG. 4 is a partial elevational view of a disk-type transformer constructed according to the teachings of this invention;

FIG. 5 is a sectional view of a disk-type transformer constructed according to the teachings of this invention; and

FIG. 5 A is an enlarged view of a portion of the transformer shown in FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Throughout the following description, similar reference characters refer to similar members in all figures of the drawings.

Referring now to the drawings, and FIG. 1 in particular, there is shown a power transformer having a laminated magnetic core 10 and winding structures 12, 14 and 16. An end frame having a top support 18, a bottom support 20, and side braces 22 and 24, is positioned around the magnetic core 10. The insulating barriers 26 are located between adjacent winding structures and between the winding structures 12 and 16 and the side braces 22 and 24, respectively.

Each winding structure includes a plurality of turns of an electrical conductor with insulation disposed between adjacent turns. Normal practice is for each winding structure to include at least a high voltage or pri3,789,337 mary winding and a low voltage or secondary winding. The lead groups 28, 30 and 32 provide means for connecting the winding structures 12, 14 and 16, respectively, to other components of the transformer, such as bushings mounted on the transformer casing. The insulation structure of the winding structure is shown in detail in FIG. 2.

FIG. 2 is a partial top view of the transformer shown in FIG. 1 illustrating the detail of the insulation structure with the lead groups eliminated from the figure in the interest of clarity. The winding is formed by a plurality of turns or turn groups 34 which are disposed concentrically around the magnetic core leg. Each turn group 34 may include one or more insulation conductors of the strap or sheet type. Various turn groups 34 may be interconnected to form separate windings, such as a high voltage winding and a low voltage winding. Cellulosic insulation covers the conductors and/or turn groups 34.

The turn groups 34 are separated by vertical spacing members 36 which provide channels for the flow of the fluid coolant of the transformer. The vertical spacing members 36 include a material which increases its dimension in the radial direction. In the embodiment shown, each vertical spacing member comprises a bracing member 38 and a filler member 40. The bracing member 38 is constructed of a conventional material, such as pressboard. The filler member 40 is constructed of a material which increases its dimension in the radial direction when heated.

The filler member 40 has a configuration which permits the desired increase in the radial direction, More exactly, the dimension in the radial direction is not greater than the other two dimensions of the filler member before being heated.

FIG. 2A illustrates a vertical spacing member 36 removed from the insulation structure of FIG. 2. The radial dimension of the filler member 40 is designated r, the axial dimension is designated a, and the tangential dimension is designated t. Proper performance of the filler member may also be realized when the radial r and the tangential t dimensions are equal to each other and each is less than the axial a dimension. If sufficient bracing can be achieved and if the required dimensions of the filler member can be maintained, it is within the scope of this invention that the bracing member 38 may be eliminated.

The filer member 40 is constructed of a material which tends to decrease its surface area when heated, without substantially decreasing its volume. This produces the effect of decreasing the longest dimension and increasing the shorter dimensions, or shortest dimension, depending upon the original configuration. Suitable materials include the thermoplastics of “teflon” or polytetrafluoroethylene, polyvinyl chloride, polyvinylidene chloride, polyvinyl fluoride, polyethylene, polypropylene, isotatic polystyrene, polyphenylene oxide, polycarbonate, and nylon. Thermosetting materials, such as silicone rubber, may also be used.

When the insulation structure is heated to remove the moisture from the cellulosic insulation, the cellulosic insulation shrinks. However, the filler member increases in its size in the radial direction and keeps the insulation structure tight. The materials disclosed will continue to increase their size in the radial direction throughout the life of the transformer, thus providing a tight insulation structure for as long as it is desirable.

An expanding member 42, constructed and dimensioned as disclosed above, may be placed between the insulation structure and the wedge block 44. The expanding member 42 also increases its demension in the radial direction to tighten the insulation structure.

The expanding materials disclosed herein are prepared by prestretching the material along one or two of its dimensions. Since the volume remains constant, one dimension must decrease in size during the prestretching operation. Consequently, the dimension which decreases in size during the prestretching operation is the dimension which increases in size when heated.

A sectional view of the transformer shown in FIG. 1 and taken along the line III—III is illustrated in FIG. 3. The laminations of the magnetic core 10 are secured by the top support 18 and by the bottom support 20 of the end frame. The top support 18 includes the overlapping portions 46 and 48 which are pressed together and welded to hold the laminations. Insulating members are usually placed between the support portions and the magnetic core to provide an air gap between these members for reducing flux in the end frame structure, to reduce the effects of irregularities in the end frame portions, and for other reasons. As shown in FIG. 3, the insulating members include the spacing members 50 and the filler members 52. The filler members 52 are constructed and dimensioned similar to the filler members already described. They may be in the form of strips, sheets, or other configurations. The dimension which is oriented in the direction which is perpendicular to the adjacent faces of the magnetic core and the end frame portions is not greater than either of its other two dimensions. The filler members 52 are constructed of similar materials as described concerning the filler members 40.

A similar arrangement may be used for the bottom support 20 of the magnetic core. The support portions 54 and 56 enclose the magnetic core 10 with the spring members 58 and the filler members 60 therebetween. Construction and operation of the filler members 60 is similar to that described in connection with the support 18.

The winding structure 16 is axially supported from the yoke portions of the magnetic core 10. Insulating members 62 and filler members 64 are located between the ends of the winding structure 16 and the yoke portions of the magnetic core 10. The filler members 64 expand in the axial direction when heated to tighten the widing structure 16 in the axial direction. Similar filler members may be positioned between the winding structure 16 and brace members connected to the top and bottom supports.

The elevational view of the winding structure 16 shown in FIG. 3 illustrates a modification of the winding structure shown in FIG. 2. Instead of having filler members 40 included in each layer of vertical spacers, as in FIG. 2, FIG. 3 illustrates an embodiment wherein filler members 40 are positioned at only one layer in the winding structure. The number of layers requiring filler members depends on the size of the filler member, the amount of cellulosic insulation shrinkage, and on other factors. Although shown as a continuous strip from one end of the winding structure 16 to its other end, a plurality of shorter strips may be used without departing from the teachings of this invention. Additionally, the filler members 40 may be disposed by winding a suitably dimensioned strip around the wind

3,789,337 ing structure between its conductor turn groups in a concentric or spiral pattern.

Application of this invention to a transformer having disk-type coil sections is illustrated in FIG. 4. Disk coil sections 66 are concentrically positioned around the leg of the magnetic core 68 of the transformer. Each coil section 66 contains one or more insulated electrical conductors which are interconnected to the other coil sections by a suitable means. The coil sections 66 are separated from the magnetic core 68 by the insulating region 70 which comprises the insulation 72 and the filler member 74.

The filler member 74 may be in the form of axial strips, concentric sheets, spirally wound strips, or in other forms. Regardless of the form, the radial dimension of the filler member 74 is not greater than its other two dimensions. With the filler member constructed from a material similar to those herebefore mentioned, heating of the transformer will produce an expansion of the filler member 74 in the radial direction. Thus, any shrinkage of the other insulating materials is counteracted by the filler member 74 and the insulation structure remains tight.

Axial tightening of the insulation structure is accomplished by the filler members 76 which are positioned between the pressure ring 78 and the winding structure, and between the coil disks 66 of the winding structure. Radial spacers 80, which separate the coil disks 66, are usually constructed of pressboard material. They may be replaced completely by filler members, such as the filler members 76, or the filler members 76 may, as shown, be placed adjacent to only a few of the radial spacers 80. The number of locations of filler members 76 depends on the size, type and rating of the insulation and filler member materials, and upon other factors. As in the other embodiments described, the dimension of the filler member 76 in the axial direction must not be greater than either of its other two dimensions. The insulating material which is wrapped around the conductors of the coil disks 66 may include a layer of material acting as a filler member. With this arrangement, tightening in both the axial and radial directions will result when the filler member expands.

In high power, concentrically wound transformers, it is often the case that the high and low voltage windings are wound separately on separate winding cylinders and then slipped over each other during the assembly operation. FIG. 5 illustrates an embodiment of this invention used in securing the winding cylinders. The low voltage winding 82 is disposed around the magnetic core leg 84. The high voltage winding 86 is disposed concentrically with the low voltage winding 82, with spacing members 88 placed therebetween to provide coolant convection channels 89. FIG. 5A shows the winding cylinder 90 of the low voltage winding 86 braced to the insulation structure 92 around the low voltage winding 82 by the spacing member 88. The spacing member 88, as illustrated, includes a bracing member 94 and a filler member 96. It is within the contemplation of this invention that the spacing member 88 may comprise only a filler member. As hereinbefore disclosed, the dimensions and material of the filler member may be such as to exhibit a tendency to increase its radial dimension when heated. This tightens the insulation structure and, as applicable to other embodiments of this invention, if the other insulation members have a smaller dimensional change than the filler members, a force is exerted by the filler member which tends to hold the structures together.

There has been disclosed a new and useful arrangement for improving the insulation structures of electrical apparatus. Since numerous changes may be made in the abovedescribed apparatus and different embodiments of the invention may be made without departing from the spirit thereof, it is intended that all of the matter contained in the foregoing description, or shown in the accompanying drawings, shall be interpreted as illustrative rather than limiting.