Method of making preformed insulation for high voltage transformer

Nov. 15, 1955 h. d. isenberg 2,723,933

METHOD OF MAKING PREFORMED INSULATION

FOR HIGH VOLTAGE TRANSFORMER

Filed Jan. 31, 1950 4 Sheets-Sheet 1

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Nov. 15, 1955 h. d. isenberg 2,723,933

METHOD OF MAKING PREFORMED INSULATION

FOR HIGH VOLTAGE TRANSFORMER

Filed Jan. 31, 1950 4 Sheets-Sheet 2

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Nov. 15, 1955 h. d. isenberg 2,723,933

METHOD OF MAKING PREFORMED INSULATION

FOR HIGH VOLTAGE TRANSFORMER

Filed Jan. 31, 1950 4 Sheets-Sheet 3

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INVENTOR.

Hans D. Isenberof

BY

Nov. 15, 1955 h. d. isenberg 2,723,933

METHOD OF MAKING PREFORMED INSULATION

FOR HIGH VOLTAGE TRANSFORMER

Filed Jan. 31, 1950 4 Sheets-Sheet 4

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United States Patent Office

2,723,933

Patented Nov. 15, 1955

2,723,933

METHOD OF MAKING PREFORMED INSULATION FOR HIGH VOLTAGE TRANSFORMER

Hans D. Isenberg, Wilmette, III.

Application January 31, 1950, Serial No. 141,559

Claims. (Cl. 154—80)

The present invention relates to a new and improved insulated high voltage power transformer and also to a method of manufacturing certain insulation employed therein.

In the manufacture of high voltage transformers three important factors must be considered, insulation stresses, mechanical stresses and thermal limits. For transformers involving very high voltages and particularly those above 138,000 volts or 10,000 kva., it is common practice for manufacturers to employ the well-known shell form of construction as contrasted with core form transformers, although as will become apparent from the following description, the present invention is well adapted for application to either type of transformer. For high voltage transformers the basic shell form transformer design comprises windings which are arranged in flat groups surrounded and supported by the core laminations. The combination of broad flat coils and relatively few coils gives a high coil-to-coil and low coil-toground capacitance, which results in an almost “straight line” distribution of surge voltage throughout the windings, with the result that insulation stresses are kept to a minimum. The mechanical forces in transformers are due to electromagnetic reactions between the high and low voltage coils and are proportional to the ampere turns in the windings. Because there are relatively few turns and the high and low voltage windings are interleaved in shell type transformers, these forces are minimized. It has been customary heretofore to employ socalled “pancake” type coils for such transformers in which the conductor making up the coil is wound in a spiral in the form of a pancake and the total winding comprises a number of such pancake coils connected in series. The coils are suitably insulated from each other by insulating barriers. The thermal aspects are taken care of by oil cooling occasioned by permitting oil to flow freely between the various coils.

It has been discovered that lightning surges are much more apt to creep over insulating barriers than to puncture them and this has led to a new principle of insulating high voltage transformer windings. The individual coil groups are completely surrounded by suitable insulation and it has been customary to use molded insulated sections of channel-shaped or angle-shaped material, which for a circular winding, are pre-designed to comprise segments of a circle. The dies for making such insulated segments are very expensive and when it is appreciated that transformers may take many forms and be of different sizes, the expense of such special dies is great. Furthermore, the step of inserting the insulated pieces which are usually made from pressboard is laborious and causes the direct labor cost for manufacturing a transformer to be appreciably increased. The channel-shaped members are used to fit over the edges of one or more of the coil sections, since it will be readily understood that at these edges the maximum voltage stresses will occur and corona is more likely to be formed at those places.

It would be desirable to provide a high voltage trans10 former in which continuous pre-formed insulation could be employed for insulating the edges of the various coil sections rather than a large amount of sectional insulation which requires a great deal of labor to apply, and which is expensive to manufacture initially.

Accordingly, it is an object of the present invention to provide a new and improved high voltage transformer winding.

It is another object of the present invention to provide continuous pre-formed channel and angle insulation sections for insulating the coils of high voltage transformers.

It is a further object of the present invention to provide a new and improved process of manufacturing preformed insulation, particularly well adapted for use in insulating high voltage transformers.

Still another object of the present invention resides in providing new and improved plastic strips of different configurations for insulating the edges of high voltage transformer windings, including means for reducing the effect of corona.

Further objects and advantages of the present invention will become apparent as the following description proceeds and the features of novelty which characterize the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.

The present invention is at least partially concerned with a process of manufacturing insulation which comprises extruding a continuous hollow strip of rectangular configuration formed of a suitable extrudable plastic insulating material such as vinyl formal resin. This continuous hollow strip may be cut in any one of several different ways to produce either wide or narrow channel members, or angle members of continuous strip material. These cut continuous strips of material are then spooled at a suitable spooling stage before the plastic material has fully set so that it is caused to set in a predetermined configuration, such as a circular configuration if it is to be used with circular windings. It will be understood that it might also be spooled on forms which have other than a circular shape so as to set in this configuration. The set spooled strip material in the form of angles and channels of various configurations is used in the manufacture of transformers for encasing the outer peripheral edges or corners of the pancake windings, as well as the inside edges. No securing means for such strips is required since the legs of the channels, if channel-shaped strips are used, or one of the flanges of the angle member, if angle members are used, are clamped between the windings during the transformer assembly operation. To reduce corona effects, the plastic strip is preferably coated with a lacquer having a dielectric constant equal to that of the transformer oil. A substantially improved transformer is provided and applicant’s invention is also concerned with an improved transformer employing the insulation manufactured by the process of the present invention.

For a better understanding of the present invention reference may be had to the accompanying drawings in which:

.Fig. 1 is a perspective, and somewhat schematic view, with a portion cut away, of a shell type transformer with which the present invention may be employed;

Fig. 2 is a sectional view taken along line 2—2 of Fig. 1 assuming Fig. 1 shows the complete structure;

Fig. 3 is an enlarged sectional view taken along line 3—3 of Fig. 2 showing details of one embodiment of the present invention;

Fig. 4 is a perspective view of a portion of a pancake coil as employed in the transformer of Fig. 1;

Fig. 5 is a somewhat schematic sectional view of a core

2,723,933 ύ· type transformer to show a further application of the present invention;

Fig. 6 is a schematic diagram illustrating the successive steps in carrying out the process of the present invention and showing at each stage certain apparatus s there employed or the product produced at that stage;

Fig. 7 is a plan view of angle-shaped sectional insulation employed heretofore for insulating the outside edges of high voltage pancake coils;

Fig. 8 is a sectional view taken on line 8—8 of Fig. 7; 10

Fig. 9 is a sectional view similar to Fig. 7 showing sectional preformed angle-shaped insulation employed heretofore for insulating the inside edges of high voltage pancake coils;

Fig. 10 is a sectional view similar to Figs. 8 and 9 15 showing preformed sectional channel-shaped insulation for insulating the outside edges of high voltage pancake coils;

Fig. 11 is the plan view of one section of the insulation of Fig. 10;

Fig. 12 is a sectional view taken on line 12—12 of Fig. 11;

Fig. 13 is a plan view similar to Fig. 11 of one section of channel-shaped insulation for insulating the inside edges of high voltage transformer coil sections, and

Fig. 14 is a sectional view taken on line 14—14 of Fig. 13.

Referring now to the drawings, there is illustrated in Fig. 1 in somewhat schematic manner a representative shell type transformer generally designated at 15 comprising a rectangular shaped laminated core 17 having a center leg 17a encircled by transformer windings 18 which include interleaved high voltage and low voltage sections described in greater detail hereinafter. Preferably, a sleeve 20 of insulating material encircles the center leg 17a of the core 17 to insulate the windings 18 from the core. As illustrated, suitable channel supports 21 and 22 are disposed along the flat sides of the core 17 for mounting the transformer in a suitable oil container or the like. Preferably also, the channel supports are separated from the core by suitable spacer members such as wood spacers 23. By way of example only, windings 18 have been illustrated as circular windings. it should be understood, however, that noncircular windings might equally well be used, it being common practice in some high voltage transformers to employ somewhat rectangular shaped windings with rounded corners. The present invention is applicable regardless of the shape of the transformer windings 18.

Referring more specifically to Figs. 3 and 4 of the drawings, the former illustrating a partial section through the windings, and the same reference numerals being employed to designate corresponding parts in Figs. 1 to 4 of the drawings, it may be noted that the windings are disposed in the “windows” 25 defined between the outer core legs of the core 17 and the center leg 17c. A portion of the sleeve 20 surrounding the center leg 17α is indicated in Fig. 3. As illustrated, the windings 18 include high voltage winding sections 26 and low voltage winding sections 27 disposed in interleaved relationship. As is best shown in Fig. 4, each low voltage winding section 26 comprises a plurality of pancake coils 26« each formed from a continuous length of suitable copper conductor 29 with an impregnated insulating material, such as 30. The pancake coils are formed by winding the conductor 29 in a spiral as clearly shown in Fig. 4 of the drawings. This winding may be accomplished in a manner to provide a space between each winding turn for the free passage of oil. As illustrated, the low voltage coils 26« may be wound on a suitable annular insulating member 31 so as to insulate the coil section from an adjacent coil section. The high voltage pancake coils 27a are, as far as the present invention is concerned, substantially identical with the low voltage pancake coils 26«.

For the purpose of separating the high voltage and the low voltage winding sections 26 and 27, there are provided main insulating barriers 33 in the form of annular insulating plates which fit over the cylindrical core sleeve 29. As illustrated, the annular insulating disk 31 associated with each low voltage pancake coil 26 forms with the low voltage coil an assembly unit. Suitable spacer plates, not shown, maintain the desired spacing between the coils through which oil is permitted to circulate freely for proper cooling action.

The high voltage coils 27α are indicated as being arranged in spaced relationship between the two main insulating barriers 33 shown in Fig. 3 of the drawings and include in addition to the pancake coils 27α a so-called “static” plate 36. This “static” plate 36 comprises an electrostatic plate vzhich is placed next to the line end turns of the high voltage windings to distribute the impulse voltage stresses evenly over the first coil section. This “static” plate is preferably shaped just like the pancake coils 27α and moreover is similarly insulated so that it adds no complications, produces no voltage hazard, and does not impede oil circulation. The high voltage coils 27α are suitably spaced by spacer members 37, only two of which are shown, and vzhich define oil pas25 sageways for free oil circulation. In addition insulating barriers may be interposed between each coil 27« to provide the desired insulation between these coils. It will be understood that adjacent coils are suitably interconnected so that all the coils 27α of the high voltage 30 winding sections are connected in series and similarly all coils 26α of the low voltage winding sections are connected in series.

It will be understood by those skilled in the art that the maximum voltage stresses in the transformer windings 35 will occur at the edges along the inside and outside of the pancake coils by virtue of the small radius of the corners defining these edges and corona and flash-over are most likely to occur here. In accordance with the present invention, each of the coils 26α and 27α has its inside and 40 outside edges insulated vzith suitable channel or angleshaped continuous strips of plastic material.

In order better to understand the present invention, reference may be had to Figs. 7 to 14 of the drawings which show angle and channel-shaped insulating members here45 tofore employed for insulating the edges of transformer pancake coils. Figs. 7 and 8, for example, illustrate a plurality of curved segments 40 of angle-shaped insulating material which interfit to provide a circle of angle-shaped cross section for use in insulating the outside edges of 50 pancake coils. These segments 40, as heretofore constructed, are preferably made of pressboard and require very expensive dies to make the same. They furthermore can be used for only one size of winding which moreover must have a circular configuration.

For insulating the inside edges of such pancake coils, it was heretofore necessary to employ circular segments such as 41 disclosed in Fig. 9 which, when assembled provide a circle of insulating material having an angle-shaped cross section. In addition to the high cost of manufactured ing the insulating segments such as 41 and 42, there is also a considerable labor cost involved in applying these segments to the pancake coils which they are to insulate. Also, heretofore, if it was desired to employ channelshaped insulation for insulating the outer edges of pancake 65 windings of high voltage transformers, then segments 42 as shown in Figs. 10, 11 and 12 of the drawings were employed. These segments 42 as are best shown in Fig. 11 are provided with preformed ends for proper interfitting relationship with adjacent segments to provide a complete 70 circle of channel-shaped cross section. The preformed ends of the segment are indicated as 42« and 426 respectively.

Heretofore for insulating the inside edges of high voltage pancake type transformer coils with channel-shaped 7® members, segments such as 43 shown in Figs. 13 and 14

2,723,933 of the drawings were required, which, like the segments 42, have preformed ends 43α and 436 for proper interfitting relationship with the adjacent segments to provide a continuous channel-shaped insulating circle for insulating the inside edges of transformer windings. It will be understood that the segments 40 and 41 are also preferably provided with interfitting end portions, not shown in the drawings.

From an examination of the configurations of Figs. 7 to 14 inclusive, it becomes apparent at once that for insulating a single transformer at least four separate dies are required and four separate insulating segments must be kept available and manually applied during the assembly of the transformer. For circular windings of different diameter, obviously different dies are required. Furthermore, if the windings are not circular, the situation is even more complicated since to provide an angle-shaped or channel-shaped member for insulating the inner and outer edges, a single die will not be sufficient since the segments will differ at various portions of the configuration particularly in the conventional windings which may be somewhat rectangular in configuration with rounded corners.

Before considering further the edge insulation of the present invention shown in Fig. 3 of the drawings, reference will now be had to Fig. 6 of the drawings showing the process of constructing the insulation of the present invention. As shown, a plastic extruding stage generally designated at 50 is provided which comprises a suitable extrusion machine capable of producing a continuous hollow strip of plastic insulating material of rectangular configuration. Adjacent the stage 50 there is indicated somewhat schematically a die 51 which might be employed in the stage, comprising an outer rectangular member 52 and inner rectangular member 53 between which is defined a forming space 54 for forming a continuous hollow rectangular strip such as 55 shown adjacent a second stage 56 and referred to as a cutting stage. This hollow rectangular strip 55 may be formed of any suitable plastic insulating material such as vinyl formal resin or the like.

The cutting stage 56 preferably comprises a plurality of pairs of knives designated as 60α, 60 b and 60c respectively, only one pair of knives being adapted to be rendered effective at any particular time. It will be apparent that if the knives 60α are rendered effective, the hollow continuous strip will be cut into two angle-shaped or L-shaped sections. On the other hand, if the knives 60Z&gt; are rendered effective, the strip will be cut in the two channelshaped or U-shaped sections with the channels having a narrow bight. If the knives 60c are rendered effective, the hollow strip of rectangular configuration is cut into two continuous channel-shaped strips in which the bight of the chanel is relatively wide. Thus, by the use of a particular set of knives 60α, 60b or 60c, either a plurality of continuous U-shaped strips or a plurality of continuous L-shaped strips may be produced.

In accordance with the present invention the continuous angle or channel strips leaving the cutting stage 56 are fed to a spooling stage 62 where the strip material is wound on suitable spools such as 64 and 65 of the proper diameter. In Fig. 6 the stage 62 illustrated indicates that the pah of knives 60ό have been rendered effective to produce channel-shaped strips 67 and 68. The strip 67 is wound on the smaller diameter spool 64 with the legs of the channel directed outwardly so as to be shaped for insulating the inside comers of circular pancake coils. The strip 68 is wound on the large diameter spool 65 with the bights of the channels directed outwardly as shown for insulating the outside edges of circular pancake coils. It will be understood that the same general conditions would prevail if the knives 60α or 60c of the cutting stage of Fig. 6 were rendered effective. This spooling operation occurs prior to setting of the plastic material which after spooling is then cooled so as to take a set and has the proper configuration for encircling or encasing the edges of pancake coils such as 26α or 27α described above. The spooled strip material is then shipped to transformer manufacturers and during manufacture of high voltage transformers is wound around the outside or inside edges of the pancake windings.

In certain cases an additional stage may be interposed between the cutting and spooling stage designated at 69 in Fig. 6 of the drawings. This stage involves coating the plastic strip prior to spooling with a lacquer having a dielectric constant equal to the dielectric constant of the oil employed in the transformer in which the strips are to be used. The purpose of this lacquer is to reduce corona effects as fully disclosed in copending Isenberg application Serial No. 132,368 filed December 10, 1949.

Referring now to Fig. 3 of the drawings, the channelshaped insulating strips 67 and 68 described above are shown as applied to the transformer 16 embodying the present invention. Similarly, channel-shaped strips 70 and 71 which result when the knives 60c are rendered effective are employed for insulating the inside and outside edges of the pancake coils respectively. The strips 73 of Fig. 3 are angle-shaped strips formed when knives 60α are rendered effective and these strips are spooled and cooled for encasing outside coil edges. It will be understood that similar strips for inside coil edges might also be employed.

Although the present invention has been described primarily in connection with shell type transformers, it will be understood that it might equally well be employed for insulating the pancake coils of core type transformers. Accordingly, in Fig. 5 there is illustrated somewhat schematically a portion of a core type transformer generally designated at 80 comprising a laminated core 81 surrounded by an insulating sleeve 82. Surrounding the insulating sleeve 82 are the low voltage coils schematically designated as 83 and preferably spaced from the sleeve 82 by suitable spacer members 84 to define oil cooling channels therebetween. A suitable insulating sleeve 85 is illustrated as separating the high and low voltage windings and preferably additional wood spacer members 84 are provided on either side of the barrier 85 to provide oil circulating passageways.

The high voltage coils are indicated as pancake coils 88 suitably spaced by spacer members 89. In accordance with the present invention, channel-shaped insulating strips 90 and 91 and angle-shaped insulating strips 92 and 93 are employed which are manufactured in accordance with the process illustrated in Fig. 6 of the drawings, continuous strips of preformed plastic insulating material being employed. During the assembly operation of the transformer, the insulation strips are applied as continuous preformed strips and are held in place as was mentioned above by having the legs or flanges thereof disposed between the coils. The windings 83 and 88 of the core type transformer 80 are illustrated as being disposed within the metal container 95 and suitably spaced therefrom by wood spacers 96.

In view of the detailed description included above, the present invention will be understood by those skilled in the art. Furthermore, it will be obvious that an improved transformer is provided greatly reduced in cost by virtue of the lower direct labor charges for applying edge insulation to pancake coils and also for initially manufacturing such insulation in either channel or angle-shaped configuration and whether for insulating inside or outside edges of such windings.

While there have been illustrated and described several embodiments of the present invention, it will be obvious to those skilled in the art that various changes and modifications thereof may be made and it is aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the present invention.

2,723,933