US2993183A

US2993183A - Transformer structures

Info

Publication number: US2993183A
Application number: US693305A
Authority: US
Inventors: Harold R Moore; William D Albright; Jr Paul Evans
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1957-10-30
Filing date: 1957-10-30
Publication date: 1961-07-18
Anticipated expiration: 1978-07-18
Also published as: GB865019A; DE1087263B; JPS369465B1; FR1214298A; JPS368813B1

Description

July 18, 1961 Filed Oct. 30, 1957 H. R. MOORE ET AI.

TRANSFORMER STRUCTURES Fig.|.

5 Sheets-Sheet 1 July 18, 1961 H. R. MOORE ETAL TRANSFORMER STRUCTURES 5 Sheets-Sheet 2 Filed Oct. 50, 1957 Fig.3.

Fig.2.

Fig. 7.

llllll lllllilllllllllIl-rll'l! lilllllnilloz- INVENTORS Poui Evons,dr. Harold R. Moore WITNESSES and William D. Albrighr.

July 18, 19 H. R. MOORE ET AL 2,993,183

TRANSFORMER STRUCTURES Filed Oct. 30, 1957 3 Sheets-Sheet 3 lllllll Breakdown Voltage Fig.8.

I l I IlIllll I III I I l I I I I I l l I I I .OOI .Ol

Rudii 01 Edge of Coil Fig.9..

9 I I 1 I I United States Patent 2,993,183 TRANSFORMER STRUCTURES Harold R. Moore and William D. Albright, Sharon, Pa.,

and Paul Evans, Jr., Niles, Ohio, assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporatlon of Pennsylvania Filed Oct. 30, 1957, Ser. No. 693,305 28 Claims. (Cl. 336-60) The invention relates generally, to inductive apparatus and, more particularly, to transformers and the insulation provided between windings and between windings and ground and the means provided for preventing the concentration of electrical stresses to keep the corona level low and for distributing surge voltages.

In the building of transformers, it has always been a problem to adequately insulate coils or windings to withstand the high voltage stresses that may occur between them. Along with the problem of providing the required insulating materials comes a requirement for keeping the corona level low and for distributing surge voltages to prevent possible breakdown of certain coils. Further, when liquid dielectrics are employed for cooling, other problems \arise such as the accumulation of oil pockets in the insulation which may be ionized reducing the insulation value to that of the insulating material between the oil pocket and the ground or core iron.

The object of the present invention is to provide in a transformer utilizing a dielectric liquid for cooling, for so insulating the coils which may be subjected to high electrical stresses with solid insulation so shaped and arranged that there are no spaces or pockets in the insulation in which the dielectric liquid may collect.

It is also an object of the invention to provide for so distributing electrical stresses tending to concentrate on the inside and outside edges of the coils of a transformer that the corona level is kept low.

A further object of the invention is to provide for maintaining a low corona level around the edges of static plates provided for distributing voltage stresses to which coils may be subjected as a result of surge voltages or the like when in service.

Another object of the invention is to provide for spacing the layers of conductors of a coil to provide a duct through the coil where the voltage between the layers of conductors is low and therefore presents no insulation problems and enables the filling of the space between coils, where high electrical stresses occur, with solid insulation so tightly interfitted that there are no spaces for the accumulation of the dielectric liquid.

A further object of the invention is to provide a static plate, for application to a transformer coil to distribute excessive voltages, which has a low corona level around its inside and outside edges and which is adequately insulated.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the features of construction, combination of elements and arrangement of parts, which will be exemplified in the construction hereinafter set forth, and the scope of the application of which will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings, in which:

FIGURE 1 is a view, in section, of portions of high and low voltage transformer coils showing details of the features of the invention;

FIG. 2 is a view, partly in section and partly in top plan, showing how the coils are disposed relative to one another; s

vention only to the high voltage coils.

FIG. 3 is a view, in section, of a portion of a static plate showing details of the structure;

FIG. 4 is a top plan view of a static plate;

FIG. 5 is a view, taken along the line VV of FIG. 4.

FIG. 6 is a view, in section, through a core shield utilized for wrapping the legs of the core iron;

FIG. 7 is a view, in section, of a high voltage coil showing details of the features of the invention;

FIG. 8 is a curve showing the relation between the breakdown voltage between standard coils and coils insulated in accordance with the teaching of this invention;

FIG. 9 is a view, partly in section and partly in elevation, showing details on how leads are brought out of the transformer winding in accordance with the teaching of the invention;

FIG. 10 is a top plan view of the coil showing diagrammatically where the transposition of conductors in thetwo-layer structure is made and where the leads are brought out;

FIG. 11 is a view, in side elevation, of a shaped insulating member; and,

FIG. 12 is a view, in section, taken along the line XIIXII of FIG. 11.

Referring now to the drawings and FIGS. 1 and 2, in particular, a transformer of shell form comprising low voltage coils 15 and high voltage coils 16 disposed in cooperative relationship is illustrated. The invention will be described in detail as applied to the high voltage coils or windings, since the insulating of the high voltage windings is usually the more difficult problem in the building of transformers. However, it has been found in building transformers in accordance with the teachings of this invention, that when the voltage of the low voltage coils reaches a high value for example around 90,000 volts that it is desirable to construct the low voltage coils in accordance with the practices that will be described hereinafter for building the high voltage coils.

In practicing this invention, it has been found that when the voltage applied to the low voltage coils is below 90,000 volts that it is good practice to build them in the standard way and apply the teachings of the in- However, this will be a matter for the designer when all the information on the transformer to be built has been submitted.

In building transformers utilizing a liquid dielectric, such as oil, for dissipating the heat generated in the coils, ducts are provided for circulating the oil as close to the conductors as possible in order to accumulate the heat and carry it away by convection. In the present structure the coils 16, which are of the well-known pancake type, are modified in accordance with the teaching of the invention disclosed in application Serial No. 672,287, filed iuly 16, 1957, and assigned to the same assignee. As taught in the above-identified patent application, the pancake coils are made from layers of conductors 14 spaced apart to provide a duct 17 through the coil. This practice is possible since the voltage between the layers of conductors is substantially negligible and therefore introduces no insulation problem.

The high voltage coils shown generally at 18, .19 and 20 are inclined relative to one another. This is common practice, since the pancake coils 18 and 19 are connected in series circuit relationship at the lower end as illustrated in FIG. 1 and therefore the voltage stress between them at the ends where they are connected is very low and the coils may be readily insulated even if located close to one another. At the upper ends of the coils, as illustrated in the same figure, the voltage is high. Therefore the coils are tilted relative to one another to provide a wider space for solid insulation. The tilting of the coils will be in proportion to the voltage gradient between them.

Considering now the section of the coil, shown gen erally at 18, it will be noted that as described hereinbefore it includes two layers of conductors 14. In this particular embodiment of the invention, only two layers of conductors have been shown in the interest of simplicity in illustration. However, it is to be understood that as many layers of conductors as required to reduce the flow of eddy currents across the wide side of the conductors to the value required may be employed. In some instances, as many as four layers of conductors 14 have been utilized. When four layers of coils are employed they are grouped in pairs and the pairs spaced apart to provide the duct.

It will be noted that the inclining of the coil 19 to the coil 18 also disposes it in an inclined position relative to the coil which stands substantially vertical. The coil 19 will be connected to the coil 20 at the upper end, thereby arranging them in series circuit relationship. Consequently, the voltage gradient between coils 19 and 20 increases from the upper end of the coil to the lower end as shown in FIG. 1.

As described hereinbefore, the coil 18 comprises two layers of conductors 14. It has been found that in addition to the eddy current flow across the wide side of the conductors 14, there are circulatory eddy currents which flow around the coil and which are caused by the build up of induced leakage voltages in the coil. In order to overcome the circulatory eddy currents in the process of winding the layers of coils, the conductors are transposed from one layer of conductors to the other at the center thereof, as shown at 21. This practice is very effective in reducing the circulatory eddy currents and prevents the building up of eddy current voltage.

The filling of the space between the pancake coils, for example, between coils 18 and 19 and 19 and 20 becomes a very important problem. It may also be pointed out that the filling of the space between the low and high voltage coils 15 and 16, respectively, must also be taken care of in the same manner as the filling of the space between the high voltage coils.

The insulating members to be described herinafter will generally be made of some cellulose material that has been treated to remove any contents that would be considered deleterious matter in insulation and reduce its resistance to breakdown under voltage stresses. Any of the wellknown insulating materials commonly employed in the art, such as pressboard, kraftboard, or other similar sheets may be utilized for preparing the members to be described hereinafter.

Consider, for example, the section of the pancake coil 18 shown in FIG. 1. This coil will have pressboard sheets, such as 22, applied, which sheets are substantially the shape of the pancake coil. Outside of this insulating member 22 will be U-shaped members 23 and beveled plates 24. The U-shaped insulating members 23 will usually be made from a corrugated member which is shaped in a die to conform to the curvature of the pancake coils or layers of conductors. The corrugated pressboard used for the shaping of the members 23 will not be corrugated with deep convolutions but only with substantially square corrugations. Such corrugated material suitable for making members such as 23 is known in the art and is readily available. The use of corrugated materials makes it possible to form with greater accuracy the insulating members 23 to fit the coil. In view of the importance of this, very expensive and carefully designed dies are utilized for forming the members. In some instances, they are formed while being subjected to temperatures of the order of 200 C.

After the member 23 has been formed, as described, it will be machined to bevel the arms, as shown in FIG. 1. The straight insulating members 24 have their ends beveled to match the beveling on the U-shaped members such as 23. When the arms of the U-shaped member 23 are beveled and the ends of the straight member 24 are beveled, they will overlap or interfit accurately so that there are no ridges and the mass of insulating material adjacent the member can be applied without leaving any substantial spaces between the insulating members.

It will be noted that a great number of U-shaped members 23 are prepared with different spacings between the legs. This enables the nesting of one U-shaped member in the other and the interfitting or overlapping of the ends of the beveled side members. When the U-shaped members and plate members are properly shaped and beveled, they can be interfitted so that the joints are disposed in stepped relationship and there is no straight continuous joint extending across the insulation at any point. Further, by making a variety of insulating members principally U-shaped and straight, and sometimes L-shaped, the space between the coils 18 and 19 can be completely filled with solid insulation. Further, the space between the low voltage coils 15 and the high voltage coils 16 may also be filled with solid insulation.

It will be noted that the corners of the U-shaped members 23 are shown slightly rounded. This is slightly exaggerated for purposes of illustration. However, in order to avoid sharp corners, the corners of the U-shaped members 23 are slightly rounded but when they are all rounded on the same radius, the fit tightly together leaving no space.

Usually when insulating members present sharp corners or edges, there is a concentration of electrical stresses at these sharp corners or edges. In the operation of inductive apparatus where such concentration of electrical stresses occur there is apt to be corona which, in the course of time, will damage or erode the insulation. In this structure, by providing rounded corners on the insulating members, the concentration of electrical stresses is kept below the corona level.

The locating of the oil duct between layers of conductors of the pancake coil rather than between the coils has enabled the building of a much more effective insulation between the coils where high voltage gradients are established. When the duct for the liquid dielectric oroil was located between the coils, it was necessary to provide almost as much insulation between each coil and the duct as is provided between the coils in the present structure. This results in a substantial reduction in the overall dimension of the coil stack. When the overall dimensions of the coil stack are reduced the window in the core may also be reduced in size. When the core is reduced in size, the total amount of iron used in the core is reduced. The reduction of the iron in the core permits a certain reduction in the mean turn length of the coils which results in a reduction in the weight of copper required for a transformer of the same capacity over that of a transformer constructed in accordance with the old principles. A table giving detailed comparison of the weights of coils and cores of transformers of a selected capacity built according to the teachings of the present invention and in accordance with previous practice follows:

Table I Old Insula- Solid Insulation tion Kva., Capacity 8, 500 8,500 Basic Impulse Level, Kv 825 825 Core and Coil Weight, Lhs 32, 000 26, 500 Case and Fittings Weight, Lbs 17,000 15, 000 011 Weight, Lbs 23,700 13, 950

Total Weight, Lbs 72, 700 65, 450

Tank Length, Inches 102 89 Tank Width, Inches 57 47% Tank Height, Inches 166 149 Iron Loss, Watts i9, 850 15, 800 Copper Loss, Watts 59, 000 55, 600

Total Loss, Watts 78, 850 71, 400

Impedance, Percent 22. 6 13. 75

In a structure built in accordance with the teachings of this invention, the insulation is more highly stressed than in the older designs. Consequently, at the edges of the coils 18, 19 and 20 the voltage gradient may be come quite high which will result in stressing the solid insulating material that covers these edges.

Referring now to FIG. 9 it will be noted that the leads shown generally at 53 are brought out through the channels 54. In order to protect the leads at the ends of the ducts 17 where the concentration of electrical stresses is high, the insulating members 24 between the coils are extended beyond the coils to provide the channels 54 for the leads '53.

Further the leads are provided with added insulation 55 from a point near the junction with the coil to the top of the channels 54. This gives the added insulation required to Withstand any added electrical stresses.

In order to keep the corona level low at both the inside and outside edges of the coils 18, 19 and 20, members which in this application will be designated edge strips, shown generally at 25, are provided. In this particular embodiment of the invention edge strips 25 are disposed on both the inside and outside edges of the layers of conductors 14. Refer now to FIGS. 1 and 7, the edge strips shown generally at 25 comprise a conductor 26 carrying suitable insulation 27. The conductor 26 will be circular in cross section and relatively large in diameter. In the embodiment of the invention illustrated in FIG. 1 the conductor assembly is covered only with paper insulation. The diameter of the conductor and amount of paper insulation will to some extent be determined by the size of the conductors employed in building up the layers of conductors 14. Generally the edge strip should not extend beyond the sides of the layers of conductors. As shown in both FIGS. 1 and 7, the edge strips 25 applied to adjacent layers of conductors 14 are spaced to conform to the spacing of the layers of conductors 14. Thus they do not obstruct the flow of oil through the duct 17.

The edge strips 25 may be retained in position in any suitable manner. In this embodiment of the invention they are held in position on the inside and outside edges of the layers of conductors 14 by tape 28 which is wrapped around the edge strip, shown generally at 2'5, and the adjacent turn of the layers of conductors 14.

The edge strips 25 perform two functions. They reduce the voltage gradient at the edges of the coils and serve as rounded outer and inside edges for the coils to prevent the concentration of electrical stresses. Since the voltage gradient at the edge of the strip is much less than at the edges of the coil, the pancake coil is kept below r the corona level. The edge strip, shown generally at 25, FIG. 7, differs from the edge strip shown in FIG. 1 in that the conductor 26 carries a layer of enamel 2.9. Enameled wire suitable for these edge strips is available to the trade. In making the edge strip 25 shown in FIG. 7, the enameled wire will be Wrapped with paper insulation as in the modification, as shown in FIG. 1.

In the modification of the edge strip 25, shown in FIG. 7, the use of an enameled conductor introduces material with a high specific inductive capacity at the point of highest stress and reduces the voltage gradient in the first layer of paper tape applied to the conductor. Therefore, the corona level at the surface of an edge strip such as shown in FIG. 7 will be lower than for the modification. just illustrated in FIG. 1. In addition to reducing the corona level at the surface of the edge strip, the copper conductor to which the enamel is applied is usually shaved eliminating feathering and presenting a much smoother surface.

The curve 3t), shown in FIG. 8, discloses the breakdown voltage between coils in percent of theoretical maximum. The vertical line 31 at the point where it meets the curve represents the breakdown point between coils of the old design without edge strips while the vertical line 32 where it meets the curve represents the breakdown voltage between coils constructed in accordance with the present invention.

In building edge strips, it has been pointed out that they are generally made of approximately the same width as the layers of conductors 14 in order to avoid obstructing the duct. In building edge strips 25 for pancake coils insulated conductors of the same width as conductors 14 have been tested and found to be entirely satisfactory. Enameled wire from .162 inch in diameter to .258 inch in diameter carrying the well-known rope cement paper to a thickness of .098 to .198 inch has been found satisfactory. These dimensions are given to present a clear concept of size of the edge strips that have been made and applied to the inner and outer edges of pancake coils successfully but are not intended to be interpreted as limiting.

In order to protect the pancake coils from surge voltages, static plates shown generally at 33 are employed. Since in the transformer structure disclosed and described herein the insulation is stressed more highly than in the usual construction, the edges of the static plate shown generally at 25 will be subjected to a greater concentration of electrical stresses and it is desirable to provide means for keeping the concentration of electrical stresses at the edges of the static plate below corona level. In the embodiments of the invention illustrated in FIGS. 1 and 3 to 5 inclusive, provision is provided for keeping the concentration of electrical stresses below corona level.

In the embodiment of the static plate illustrated in FIGS. 4 and 5, the static plate comprises washers 34 of some suitable insulating material such as pressboard to which is applied washers 35 of a metallic foil commonly used in static plates. In a structure of this kind the concentration of electrical stresses at the inner and outer edges of the foil will be high and in order to keep these stresses at the edges of the foil below corona level, a conductor having a high resistance is employed. In the embodiment of the invention illustrated in FIGS. 4 and 5, a coronox tape 36 is applied to both the inner and outer edges of the foil.

The coronox tape 36 may be applied by cutting strips and cementing it to the washer 34 in position to make electrical contact with the foil 35. When the foil and coronox tape have been assembled, another insulating washer 37 will be applied over the foil 35 and cemented to the washer 34 forming a unitary structure. U-shaped insulating members 38 will be prepared and fitted over the edges of the static plate. These U-shaped insulating members 38 will be specifically formed to conform to the curvature of the static plate and the arms will be beveled, as described, in connection with the U-shaped insulating members 23. Other insulating members 39 which are substantially flat will be beveled to interfit with the beveled edges of the U-shaped insulating members 38.

A modified static plate is shown in FIG. 3. It. comprises a washer of insulating material 40 rounded at the edges and coated with a suitable film 41 of a conducting material such as copper. This fihn or layer of copper 41 may be applied in any well-known manner such, for example, as spraying. The electrode comprising the washer 40 of insulating material and the film of copper 41 has rounded edges both inside and outside which cooperate in preventing the concentration of electrical stresses.

It has been found that in some instances, it is desirable to go further in an effort to distribute the stresses and keep the corona level low. In such case, a conductor 42 somewhat similar to the conductor employed in edge strip 25 shown in FIGS. 1 and 7 may be employed. The conductor edge strip 42 will comprise a conductor wrapped with paper tape. The conductor edge strip 42 will be applied to both edges of the electrode 40. In the embodiment of the invention that has been described and found to be satisfactory in tests, the round copper conductor was about .102 inch in diameter and the thick- 7 ness of paper insulation applied was approximately .088 inch. It prevented the concentration of electrical stresses and the occurrence of corona.

In constructing the static plate illustrated in FIG. 3 the electrode 40 and edge strip 42 have been disposed in proper relative positions and

washers

43 and 44 of suitable insulating material applied to the sides of the members and cemented thereto. U-shaped members 45 of insulating material shaped to conform to the curvature of the static plate are applied. These members will be shaped as described hereinbefore and are illustrated in FIGS. 11 and 12. Flat washers 46 with beveled edges are then applied to the U-shaped insulating members 45 and cemented in position. The static plate as illustrated in FIG. 3 comprises the electrode 40 and edge strips 4-2 which are completely enclosed by the solid insulating

members

43, 44, 45 and 4-6. The static plate will then be applied to an end winding of the high voltage coils as shown in FIG. 1. In this particular illustration, the static plate of the type shown in FIG. 3 is applied to the end winding shown generally at 18. When the static plate is so constructed, it will prevent the concentration of electrical stresses that might cause corona at its edges.

In the construction of transformers the edges of the core adjacent to the line coils are sharp and cannot conveniently be rounded. These edges are subjected to high intensity fields and if not protected might cause corona. As shown, a core shield shown generally at 47 in FIG. 6 is wrapped around the core. This core shield comprises a layer of copper foil 48 disposed between two insulating

sheets

49 and 50. These insulating

sheets

49 and 50 are made from paper or other suitable material. The insulating

materials

49 and 50 will be brought together at the edges so as to completely enclose the copper foil. In order to prevent any concentration of electrical stresses at the edges coronox tape 51 may be applied as described in the making of the static plate in FIG. 5. This coronox tape 51 will extend around the edges and will cooperate'in preventing the concentration of stresses and thereby provide alow corona level. The core shield 47 may be overlapped at its meeting edges or just brought into engageemnt as long as the copper foil does not meet. There always should be a gap in the core shield to prevent excessive eddy currents. This is provided for in this structure by enclosing the foil. In addition to the foregoing, a shield 52 similar to the core shield 47 may be disposed between the low voltage winding and the high voltage winding 16.

The invention disclosed herein is specifically suited to the making of shell type transformers such as shown in FIG. 2. However, it may also be applied to core type transformers as effectively, but the application may be more expensive. However, there are certain types of cylindrical windings in core type transformers where the transformer structure disclosed herein may be as effectively and economically applied as in the case of shell type transformers. The structure described hereinbefore does result in reduced clearance between live points and ground. This can be efiected by the insulation design and arrangement described hereinbefore and as pointed out. j

' When there are reduced clearances, the core openings will be smaller requiring less copper, tank dimension will become smaller and less oil will be required. The overall picture is that the transformer will be smaller, less expensive and may be operated with reduced losses.

Since certain changes may be made in the above construction and different embodiments of the invention could be made without departing from the spirit and scope thereof, it is thought that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

We claim as our invention:

l. In a transformer employing a dielectric liquid, in combination, a pluralityof coils, each coil comprising 8. layers of spirally wound conductors disposed side by side in spaced relationship providing cooling ducts through the coils, the voltage between said layers of each coil being substantially negligible, the coils being disposed in spaced relationship, means disposed in close association with the inner and outer edges of the coils for maintaining a low corona level, and solid insulation filling the space between adjacent coils to insulate them from one another and prevent the occurrence of pockets of dielectric liquid between the coils which would be subject to ionization when subjected to high voltage stresses.

2. In a high voltage transformer provided with high and low voltage coils and a cooling dielectric, in combination, the high and low voltage coils being disposed in cooperative spaced relationship, each high voltage coil comprising a plurality of layers of spirally wound conductors disposed side by side in spaced relationship to provide a duct in the coil for circulation of the dielectric liquid, solid insulation filling the spaces between the coils to prevent the occurrence of pockets of dielectric liquid outside the coils where they would be subject to the concentration of high electrical stresses and possible ionization and means for keeping the corona level low disposed along the inner and outer edges of the coils.

3. In a high voltage transformer provided with high and low voltage coils and a cooling dielectric liquid, in combination, the high and low voltage coils being disposed in cooperative spaced relationship, each coil comprising a plurality of layers of spirally wound conductors disposed side by side in spaced relationship to provide a duct in the coil for the circulation of the cooling dielectric, the voltage between the layers of each coil being substantially negligible, solid insulation filling the spaces between the coils to prevent the occurrence of pockets of dielectric liquid outside of the coils where they would be subjected to the concentration of electrical stresses and possible ionization, and means associated with the inner and outer edges of the high voltage coils for maintaining the corona level along the edges of the coils at a low level.

4. In a high voltage transformer provided with high and low voltage coils and a liquid dielectric, in combination, the high and low voltage coils being disposed in cooperative spaced relationship, each coil comprising a plurality of layers of spirally wound conductors disposed side by side in spaced relationship to provide a duct in the coil for the circulation of the dielectric liquid, solid insulation filling the spaces between the coils to prevent the occurrence of pockets of dielectric liquid outside of the coils where such pockets would be subject to the concentration of electrical stresses and possible ionization, and an edge strip applied to the inner and outer edges of a layer of conductors to distribute electrical stresses and thereby 'keep the corona level low where the edge strip is applied.

5. In a high voltage transformer provided with high and low voltage coils and a liquid dielectric for cooling, in combination, a plurality of high and low voltage coils being disposed in cooperative spaced relationship, each coil comprising a plurality of layers of spirally wound conductors disposed side by side in spaced relationship to provide a duct in the coil for the circulation of the dielectric liquid, solid insulation filling the spaces between the coils to prevent the occurrence of pockets of dielectric liquid outside the coils where they would be subject to the concentration of electrical stresses and possibleionization, and edge strips each comprising an insulated conductor disposed in close proximity to the inside and outside edges of the layers of conductors to keep the corona level low.

6. In a high voltage transformer provided with a cooling dielectric liquid, in combination, high and low voltage coils disposed in cooperative spaced relationship, each high voltage coil comprising a plurality of layers of conductors disposed side by side in spaced relationship to provide a duct in the coil for circulation of the dielectric liquid, and solid insulation filling the spaces between the coils, an edge strip applied to the inside and outside edges of each layer of conductors of each high voltage winding to prevent the concentration of electrical stresses and thereby keep the corona level low, each edge strip comprising an insulated conductor and means for retaining the edge strip in position on the edges of the layers of conductors.

7. In a high voltage transformer provided with a cooling dielectric liquid, in combination, high and low voltage coils disposed in cooperative relationship, each high and low voltage coil comprising a plurality of layers of spirally wound conductors disposed in spaced relationship to provide a cooling duct in each coil for the circulation of the dielectric liquid, the high and low voltage coils being disposed in spaced relationship, solid insulation filling the spaces between the coils to prevent flashover between them, and means applied to the inner and outer edges of the layers of conductors cooperative to effect a distribution of electrical stresses along the edges of the layers of conductors to keep the corona level low.

8. In a high voltage transformer provided with a dielectric liquid for cooling, in combination, high and low voltage coil stacks disposed in cooperative relationship, each high voltage coil comprising a plurality of layers of spirally wound conductors disposed in spaced relationship to provide a duct in each coil for circulation of the dielectric liquid, the high voltage coils being disposed in spaced relationship, solid insulation filling the spaces between the high voltage coils to prevent flashover between the conductors of the respective coils, an insulated conductor applied to the edges of each layer of conductors to prevent the concentration of electrical stresses and thereby keep the corona level low, and static plates applied to the opposite ends of the high voltage coil stacks to effect a distribution of surge voltages across the conductors of the end coils.

9. In a high voltage transformer provided with a dielectric liquidfor cooling, in combination, high and low voltage coils disposed in cooperative relationship, each high voltage coil comprising a plurality of layers of spirally wound conductors disposed in spaced relationship to provide a duct in each coil for circulation of the dielectric liquid, the high voltage coils being disposed in spaced relationship, solid insulation filling the spaces between the high voltage coils to prevent fiashover between the conductors of the respective coils, an insulated conductor applied to the inner and outer edges of each layer of conductors, to prevent the concentration of electrical stresses and thereby keep the corona level low, a static plate applied to the end of a high voltage transformer coil to effect adistribution of surge voltages across the conductors of the coil, and means disposed around the edges of the static plate to keep the corona level low.

10. In a high voltage transformer provided with a dielectric liquid for cooling, in combination, high and low voltage coils disposed in cooperative relationship, each high voltage coil comprising a plurality of layers of spirally wound conductors disposed in spaced relationship to provide a duct in each coil for circulation of the dielectric liquid, the high voltage coils being disposed in spaced relationship, solid insulation filling the spaces between the high voltage coils to prevent fiashover between the conductors of the respective coils, an insulated conductor applied to the inner and outer edges of each layer of conductors to prevent the concentration of electrical stresses and thereby keep the corona level low, an insulated static plate applied to the opposite ends of the high voltage coils to effect a distribution of applied voltages across the coils to prevent an excessive concentration of electrical stresses, and means extending around the edges of the static plate cooperative therewith to maintain a low corona level.

ll. In a high voltage transformer provided with a dielectric liquid for cooling, in combination, high and low voltage coils disposed in cooperative relationship, a high voltage coil comprising a plurality of layers of spirally wound conductors disposed in spaced relationship to provide a cooling duct in the coil for circulation of the dielectric liquid, the high voltage coils being disposed in spaced relationship, solid insulation filling the spaces between the high voltage coils to prevent flashover between the conductors of the respective coils, an insulated conductor disposed in cooperative relationship with the inner and outer edges of the coil to prevent the concentration of electrical stresses and thereby keep the corona level low and a static plate applied to the end of a high voltage coil to effect a distribution of surge voltages across the conductors of the coil, the static plate comprising an insulated conductive metallic member and a high resistance conductor disposed around the edges of the metallic member to keep the corona level low.

12. In a high voltage transformer, in combination, high voltage and low voltage coils disposed in cooperative spaced relationship, solid insulation disposed between the high voltage coils to completely fill the space, a static plate applied to a high voltage coil to effect a distribution of impressed voltages, the static plate comprising a washer shaped insulating member, a conducting layer applied to the washer shaped insulating member, an insulated conductor applied to the edges of the washer shaped member and applied conducting layer, and solid insulation applied to enclose the insulating washer, conducting layer and the conductor extending around the edges of the washer and conducting layer.

13. In a high voltage transformer provided with a cooling liquid dielectric, in combination, high and low voltage coil stacks disposed in cooperative relationship, each coil of the high voltage coil stack comprising a plurality of layers of spirally wound conductors disposed in spaced relationship to provide a duct for the liquid dielectric, the coils of the high voltage winding being disposed in spaced relationship, solid insulation disposed between the coils of the high voltage coil stack to completely fill the space, and means applied to the inner and outer edges of each layer of conductors to prevent the concentration of electrical stresses and keep the corona level low, said means being of substantially the same width as the layers of conductors to avoid blocking the duct, and insulated leads connected to the layers of conductors and brought out adjacent to the end of the duct.

14. In a high voltage transformer, in combination, high and low voltage coils disposed in cooperative relationship, each high voltage coil comprising a plurality of layers of conductors disposed in spaced relationship to provide a duct between them, the high voltage coils being disposed in spaced relationship, layers of solid insulation disposed between the high voltage coils, the solid insulation comprising layers of solid insulation beveled at the ends disposed between the coils, U-shaped insulating members having beveled arms disposed to fit over the coils and overlap the beveled ends of the layers of insulation, the U-shaped insulating members being shaped to conform to the curvature of the high voltage coils, the edges of the joints between the layers of insulation and the arms of the U-shaped members being arranged in stepped order to increase the insulating value of the insulating members.

15. In a high voltage transformer employing a dielectrio liquid, comprising stacks of high voltage and low voltage coils disposed in cooperative relationship, each coil comprising a plurality of layers of conductors disposed in spaced relationship providing a duct between them for the circulation of the dielectric liquid, an insulating conductor extending around the inside and outside edges of the layers of conductors to effect the distribution of electrical stress and maintain a low corona level, the coils of the stacks of coils being disposed in spaced relationship, layers of solid insulation having beveled edges Ill disposed between the coils, U-shaped insulating members of corrugated material shaped to conform to the curvature of the edges of the coils and having beveled arms disposed in interfitting relationship with the layers of insulating material between the coils, the layers of insulating material and the U-shaped insulating members being so disposed as to completely fill the space between the coils, static plates applied to the opposite ends of the stacks of coils, means applied to the edges of the static plates for maintaining a low corona level, and leads attached to the conductors of the layers of conductors extending out through the ducts, and added insulation applied to the leads for a distance below the edges of the layers of conductors and for a distance beyond the edges of the layers of conductors to prevent breakdown under electrical stress.

16. In a transformer which employs a dielectric liquid, in combination, stacks of high voltage and low voltage coils, each coil to be subjected to predetermined voltages comprising, layers of conductors disposed in spaced relation providing a duct through the coil for the circulation of the dielectric liquid, means provided on the edges of the coil cooperative to distribute electrical stresses to I maintain a low corona level, a lead connected to the conductors of the layers of conductors and extending out through the duct, added insulation applied to the lead from a point in the duct below the edges of the layers of conductors to a point outside of the duct, the coils of the stacks being disposed in spaced relationship, layers of solid insulation disposed between the coils, the layers of insulation adjacent the lead extending beyond the edges of the coils serving as additional insulation for the leads, the layers of solid insulation having their ends beveled, U-shaped insulating members presenting arms beveled to match the beveling of the layers of insulation between the coils, the U-shaped insulating members being fitted over the ends of the coils and interfitting with the beveled ends of the layers of insulation, the layers of insulation and the U-shaped members completely filling the space between the coils, the U-shaped members being made from corrugated insulating material and shaped to conform to the curvature of the coils to facilitate the filling of the space between the coils with solid insulation, the interfitting beveled ends of the layers of insulation in the U-shaped members presenting joints disposed in stepped relationship to give maximum strength to the insulation, means applied to the edges of the layers of conductors to effect the distribution of electrical stresses to maintain a low corona level, a static plate applied and extending across a coil to distribute voltage stresses in the coil, and means applied to the edges of the static plates to maintain a low corona level.

-17. In a high voltage transformer provided with a cooling dielectric, in combination, stacks of high voltage and low voltage coils, each coil of the high voltage and low voltage stacks comprising a plurality of layers of conductors disposed in spaced relationship to provide a duct for the cooling dielectric liquid, the conductors of each layer of conductors substantially midway from the edges of the layers of conductors being transposed to cooperate in the reduction of the flow of eddy currents in the coil, the coils of the stacks of coils being disposed in spaced relationship, layers of insulation disposed between the coils, the layers of insulation having beveled edges, U-shaped insulating members presenting beveled arms disposed in cooperative relationship with the layers of insulation between the coils to completely fill the space between the coils with solid insulation, the U-shaped insulating members with beveled edges being made from corrugated insulation and shaped to conform to the curvature of the coils to cooperate in filling the space between the coils with solid insulation, and beveled insulating members shaped for insertion in all spaces to eliect a complete filling of the space between the coils and providing solid insulation around the ends of the coils, leads con 12 nected to conductors in the layers of conductors, added insulation applied to the leads from a point below the edges of the coils to a point beyond the edges of the coils, the layers of solid insulation disposed between the coils being extended a predetermined distance beyond the edges of the coils to provide a channel for the insulated lead, the conductors of the layers of conductors at substantially the center of the coil being transposed from one layer to the other to cooperate in limiting eddy currents, means provided on the edges of the coils cooperative to maintain a low corona level, and static plates disposed at opposite ends of coil stacks to efifect a distribution of surge voltages across the coil, and means disposed around the edge of the static plates to maintain a low corona level.

18. In a high voltage transformer provided with a liquid dielectric for cooling, in combination, stacks of low voltage and high voltage coils disposed in cooperative relationship, each high voltage coil comprising a plurality of layers of conductors disposed in spaced relationship to provide ducts for the cooling dielectric liquid, the high voltage coils being disposed in spaced relationship, the coils intermediate the end high voltage coils being inclined away from the end coils and from one another, the inclination of the coils to one another being dependent upon the voltage gradient between coils, the greatest spacing occurring at the points of highest voltage between coils, layers of insulation disposed between the coils, the layers having beveled edges, and some of the layers being beveled in cross section to provide specially shaped insulation pieces for the complete filling of the space between the coils with solid insulation, U-shaped insulating members presenting beveled arms for interfitting with the beveled ends of the layers of insulation, the U-shaped insulating members being made from corrugated insulating material and shaped to conform to the curvature of the coils to cooperate in filling the space between the coils interfittiug with the layers of beveled insulation, the U-shaped insulating members fitting around the ends of the coils, means provided in cooperative relationship with the edges of the coils to prevent the concentration of electrical stresses on the edges of the coils and thereby maintain a low corona level, metallic static plates applied to the coils to effect a voltage distribution across the conductors, and means disposed in cooperative relationship with the edges of the static plates to maintain a low corona level.

19. In a transformer employing a dielectric liquid, in combination, a plurality of spirally wound pancake coils, each pancake coil comprising layers of conductors disposed side by side in spaced relationship providing cooling ducts through the pancake coils, the voltage between said layers of each pancake coil being substantially negligible, the pancake coils being disposed in spaced relationship, means disposed in close association with the edges of the pancake coils for maintaining a low corona level, and solid insulation filling the space between adjacent pancake coils to insulate them from one another and prevent the occurrence of pockets of dielectric liquid between the pancake coils which would be subject to ionization when subjected to high voltage stresses.

20. In a high voltage transformer provided with high' and low voltage coils and a cooling dielectric, in combination, the high and low voltage coils being disposed in 00- operative spaced relationship, each high voltage coil being would be subject to the concentration of high electricalstresses and possible ionization and means for keeping the corona level low along the edges of the coils.

21. In a high voltage transformer provided with spirally wound high and low voltage pancake coils and a cooling dielectric liquid, in combination, the high and low voltage pancake coils being disposed in cooperative spaced relationship, each pancake coil comprising a plurality of layers of conductors disposed side by side in spaced relationship to provide a duct in the pancake coil for the circulation of the cooling dielectric, the voltage between the layers of each pancake coil being substantially negligible, solid insulation filling the spaces between the pancake coils to prevent the occurrence of pockets of dielectric liquid outside of the pancake coils where they would be subjected to the concentration of electrical stresses and possible ionization, and means associated with the edges of the high voltage pancake coils for maintaining the corona level along the edges of the pancake coils at a low level.

22. In a high voltage transformer provided with spirally wound high and low voltage pancake coils and a liquid dielectric, in combination, the high and low voltage pancake coils being disposed in cooperative spaced relationship, each pancake coil comprising a plurality of layers of conductors disposed side by side in spaced relationship to provide a duct in the pancake coil for the circulation of the dielectric liquid, solid insulation filling the spaces between the pancake coils to prevent the occurrence of pockets of dielectric liquid outside of the pancake coils where such pockets would be subject to the concentration of electrical stresses and possible ionization, and an edge strip applied to an edge of a layer of conductors to distribute electrical stresses and thereby keep the corona level low where the edge strip is applied.

23. In a high voltage transformer provided with high and low voltage pancake coils and a liquid dielectric for cooling, in combination, a plurality of high and low volt age spirally wound pancake coils being disposed in cooperative spaced relationship, each pancake coil comprising a plurality of layers of conductors disposed side by side in spaced relationship to provide a duct in the pancake coil for the circulation of the dielectric liquid, solid insulation filling the spaces between the pancake coils to prevent the occurrence of pockets of dielectric liquid outside the pancake coils where they would be subject to the concentration of electrical stresses and possible ionization and edge strips each comprising an insulated conductor disposed in close proximity to the inside and outside edges of the layers of conductors to keep the corona level low.

24. In a high voltage transformer provided with high and-low voltage spirally wound pancake coils disposed on a magnetic core and a cooling dielectric liquid, in combination, the high and low voltage coils being disposed in cooperative spaced relationship, each coil com prising a plurality of substantially fiat layers of conductors disposed in spaced relationship to provide a duct in the coil for the circulation of the cooling dielectric, the voltage between the layers of each coil being substantially negligible, solid insulation filling the spaces between the coils to prevent the occurrence of pockets of dielectric liquid outside of the coils where they would be subjected to the concentration of electrical stresses and possible ionization, means associated with the edges of the high voltage coils for maintaining the corona level along the edges of the coils at a low level, and means associated with the edges of said core to distribute the electrical stresses and keep the corona level low.

25. In a high voltage transformer provided with high and low voltage spirally wound pancake coils disposed on a magnetic core and a cooling dielectric liquid, in combination, the high and low voltage coils being disposed in cooperative spaced relationship, each coil comprising a plurality of substantially flat layers of conductors disposed in spaced relationship to provide a duct in the coil for the circulation of the cooling dielectric, the voltage between the layers of each coil being substantially negligible, solid insulation filling the spaces between the coils to prevent the occurrence of pockets of dielectric liquid outside of the coils where they would be subjected to the concentration of electrical stresses and possible ionization, means associated with the edges of the high voltage coils for maintaining the corona level along the edges of the coils at a low level, and means associated with the edges of said core to distribute the electrical stresses and keep the corona level low, said latter means comprising a layer of conductive material disposed between two layers of insulation.

26. In a transformer provided with a dielectric liquid for cooling, in combination, a plurality of pancake coils, each pancake coil comprising a plurality of spirally wound layers of conductors disposed side by side in spaced relationship providing ducts, the pancake coils being disposed in spaced relationship with respect to one another, and solid insulation filling the spaces between the pancake coils to prevent the occurrence of pockets of dielectric liquid between the pancake coils, said solid insulation comprising a plurality of insulating members fitted around each of said coils and having the meeting ends thereof beveled to substantially eliminate any liquid spaces between said insulating members.

27. In a transformer employing a dielectric liquid, in combination, a plurality of spirally wound pancake coils, each pancake coil comprising layers of conductors disposed side by side in spaced relationship providing cooling ducts through the pancake coils, the voltage between said layers of each pancake coil being substantially negligible, the pancake coils being disposed in spaced relationship, means disposed in close association with the edges of the pancake coils for maintaining a low corona level, and solid insulation filling the space between adjacent pancake coils to insulate them from one another and prevent the occurrence of pockets of dielectric liquid between the pancake coils which would be subject to ionization when subjected to high voltage stresses, said solid insulation comprising a plurality of insulating members fitted around each of said coils and having the meeting ends thereof beveled to substantially eliminate any liquid spaces between said insulating members.

28. In a high voltage transformer provided with spirally wound high and low voltage pancake coils and a cooling dielectric liquid, in combination, the high and low voltage pancake coils being disposed in cooperative spaced relationship, each pancake coil comprising a plurality of layers of conductors disposed side by side in spaced relationship to provide a duct in the pancake coil for the circulation of the cooling dielectric, the voltage between the layers of each pancake coil being substantially negligible, solid insulation filling the spaces between the pancake coils to prevent the occurrence of pockets of dielectric liquid outside of the pancake coils where they would be subjected to the concentration of electrical stresses and possible ionization, said solid insulation comprising a plurality of insulating members fitted around each of said coils and having the meeting ends thereof beveled to substantially eliminate any liquid spaces between said insulating members, and means associated with the edges of the high voltage pancake coils for maintaining the corona level along the edges of the pancake co-ils at a low level.

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