US3376530A - Axially spaced transformer pancake coils having static plate - Google Patents

Description

April 2, 1968 H. G. FISCHER AXIALLY SPACED TRANSFORMER PANCAKE COILS HAVING STATIC PLATE Filed April 27,

2 Sheets-Sheet 1 INVENTOR Heinz G. Fischer WITNESSES d X/ QZM ATTORNEY H. G. FISCHER 3,376,530 NS FORMER PANCAKE COILS HAVING STATI( ril 2, 1968 J PLATE AXIALLY SPACED' TRA SheetsShee1.

Filed April 27, 1966 FIG. 2

FIG.4

FIG. 5

FIG; 3

PRIOR ART United States Patent AXIALLY SPACED TRANSFORMER PANCAKE (SOILS HAVING STATlC PLATE Heinz G. Fischer, Muncie, Ind., assignor to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Apr. 27, 1966, Ser. No. 545,649

5 Claims. (Cl. 336-70) ABSTRACT OF THE DISCLOSURE Electrical transformers of the type which utilize a plurality of axially spaced pancake coils connected to form a winding thereof, and one or more static plate members disposed and electrically connected to more uniformly distribute surge potentials across the winding.

This invention relates in general to electrical inductive apparatus, such as transformers, and more particularly to new and improved winding and shielding structures for reducing the magnitude of electrical stress concentrations and for improving the distribution of surge potentials across the windings of electrical transformers.

Electrical transformers of the type which utilize a plurality of disc or pancake type coils, stacked in spaced side-by-side relation to provide electrical clearance between the coils and cooling ducts for cooling the coils, and which are electrically connected to form the windings of the transformer, commonly employ a static plate or shield which is electrically connected to the line terminal, and which is disposed adjacent the line end of the stack of pancake coils. The static plate is usually formed of an insulating washer member having rounded edges and the same general outline dimensions as the pancake coils, and has a thin layer or coating of an electrically conductive material, such as copper or aluminum, disposed on the surfaces of the washer member. The electrically conductive surface of the static plate is electrically connected to the electrical line terminal. The function of the static plate is to improve the distribution of surge potentials across the line end of the electrical winding.

The distribution of a surge potential across a pancake coil, and across the electrical winding, is determined by the capacitance between the individual turns of the pancake coils, the capacitance between adjacent coils, and the capacitance between the coils to ground. The distribution of surge potential across a pancake coilor across an electrical winding, is determined by the formula where a is the distribution constant, C is the capacitance of the electrical turns of the pancake coils to ground, and C is the through series capacitance of the electrical winding. The smaller the distribution constant a, the more linear the surge voltage distribution across the coils and windings, which results in less stress concentration between the initial turns of the line end coil, and less stress concentration between the coil-coil spaces at the line end of the winding. Thus, as shown in the formula for the distribution constant a, the distribution of surge potentials across a winding may be improved by increasing the series capacitance of the winding. Unfortunately, the through series capacitance of the winding is inherently low due to the serially connected turn-to-turn capacitances, Which add similarly to parallel connected resistors, making the through series capacitance of each coil due to the turn-to-turn capacitance smaller than the smallest value of turn-to-turn capacitance. The problem is fur- "ice ther aggravated in high voltage transformers which utilize graded insulation, as the line end coils are generally constructed to have a larger opening for receiving the associated magnetic core, and have a smaller outside diameter in order to provide greater clearances between the high voltage coils and the grounded magnetic core than bet-ween the coils and ground which are disposed nearer the neutral end of the winding. Thus, the capacitance between the physically smaller line end coils is less than the capacitance between the coils which have a larger surface area, as the capacitance between coils is directly proportional to the surface area of the smaller of any two adjacent coils.

Since capacitive reactance is inversely proportional to capacitance, surge potentials applied to the line end of the winding will concentrate on the first few turns,

of the line end coil, and across the coil-coil space between the line end coils. Further, as the distribution of potential changes from capacitive to inductive, oscillatory voltages are produced as charging currents flow through the capacitance and inductance of the windings, with the magnitude of the voltage oscillations being proportional to the difference between the initial or capacitive voltage distribution, and the final or inductive voltage distribution across the winding. Increasing the coil-coil space between the line end coils to withstand these concentrations of surge potentials is partially self-defeating, as this further reduces the capacitance between these coils which results in an even poorer impulse distribution. Further, adding additional insulation between the coils and ground to protect the apparatus from large oscillatory voltages increases the mean length of the winding turns and the mean length of the magnetic circuit, which increases copper and iron requirements, and deleteriously affects the cost, size, regulation and efiiciency of the electrical apparatus.

The static plate, disposed adjacent the outside surface of the line end coil, and electrically connected to the line terminal, aids in increasing the series capacitance of the line end coil, which distributes surge stresses more uniformly across this coil. The arrangement of the static plate adjacent the outside surface of the line end coil of an electrical winding, however, provides several areas near the line end of the winding which are very highly stressed, with some of the solid insulating members, such as washers, channels, and angles, being stressed more in creep than in puncture. It would, therefore, be desirable to provide a shielding and winding arrangement which will distribute surge potentials more uniformly across the line end of an electrical winding, and also reduce the high concentration of puncture and creep stresses near the line end of the winding, both during the time of surges and during'steady state operation.

Accordingly, it is an object of this invention to provide a new and improvedv winding and shielding structure for electrical inductive apparatus. I

Another object of the invention is to provide a new and improved winding and shielding structure for electrical transformers which more uniformly distributes surge potentials across the winding. A further object of the invention is to provide a new and improved winding and shielding structure for electrical transformers, which reduces creep and. puncture stress concentrations near the line end of the winding. Still another object of the invention is to provide a new and improved winding and shielding structure for electrical transformers, which is more effective in'ur'iiforinly distributing surge potentials across an electrical winding, and at the same time reduces the magnitude of electrical stress concentrations at the highly stressed line end of the winding.

v Briefly, the invention accomplishes the above-cited objects by changing the location of the static shield or plate relative to the pancake or disc type coils which make up the electrical winding. In general, instead of disposing the static plate adjacent the outer surfaces of the line end coil, it is disposed intermediate the line end coil and the immediately adjacent coil. This construction has the efiectbf substantially reducing stress concentrations at certain locations near the line end of the winding, and at the same time more uniformly distribute surge potentials across the line end of the winding. Thus, the static plate performs its primary function more effectively, and at the same time substantially reduces creep and puncture stresses at the line end of the winding.

4 Further objects and advantages of the invention will become apparent from the following detailed description, taken in connection with the accompanying drawings, in which:

FIGURE 1 is a plan view, in section, of an electrical transformer constructed according to the teachings of one embodiment of the invention;

FIG. 1A is an elevational view of a static plate member which may be used in the transformer structure shown in FIG. 1;

FIG. 1B is an enlarged cross-sectional view of the static platemember shown in FIG. 1A, taken along the line "1B--1B; 4

FIG. 1C is an enlarged cross-sectional view of another static plate arrangement which may be used;

, FIG. 2 is a fragmentary view of the transformer shown in FIG. 1, illustrating the capacitive relationships between'two of the line end coils and the static plate;

FIG. 3 is a fragmentary view, illustrating the capacitive relationships of a prior art winding and shielding arrangefnent, 'for comparison with FIG. 2;

7 FIG. 4 is a cross-sectional view of a winding and shieldin'g arrangement constructed according to another embodim ntor the invention; and

FIG. 5 is a cross-sectional view of a winding and shieldingarrangement constructed according to still another embodiment of the invention. 7 V Referring now to the drawings, and FIG. 1 in particular, there is illustrated a plan view, in section, of a transfoi-mer'l't) constructed according to the teachings of one embodiment of the invention. In general, transformer 10' is of the shell-form type, comprising magnetic core members 12 and 14, and a plurality of disc or pancake type coils such as pancake coil 16, all disposed about a common center line 9. All of the figures will show this same center line 9, in order to illustrate the placement of the various fragmentary cross-sectional views in an actual transformer structure. Transformer 10' may be of the single 'or polyphase type, and may be of the auto-transformer or' of the isolated winding type. For purposes of example, transformer 10 is shown in FIG. 1, as being of the isolated winding type. One phase of transformer ltl is illustrated, with magnetic core members 12 and 14 being shown" with an irregular line to indicate that other similar phases may be added if desired.

More "specifically, transformer 10 includes high and low voltage windings 18 and 20, respectively, disposed in inductive relation with magnetic core members 12 and 14. Magnetic core members 12 and 14 each include a plurality of stacked metallic laminations, formed of grain oriented magnetic silicon steel, such as laminations 22, which are arranged about a window or opening 24 for receiving the electrical windings 18 and 20.

The low voltage winding is formed of a plurality of pancake coils, with four pancake coils 26, 28, and 32 being shown for purposes of example. Pancake coils 26,

28, 30 and 32 are each formed of a plurality of spirally wound turns 34 of electrical conductor, with the pancake coils each having two main opposed surfaces, and an opening, such as opening 36 in pancake coil 26, which joins the major surfaces, and which has the function of receiving the magnetic core members 12 and 14. The pancake coils of the low voltage winding are stacked in spaced side-by-side relationship, with the openings 36 being in alignment, and they are serially connected to form a complete winding having terminals 38 and 40. The pancake coils of the low voltage winding 20 are shown connected start-start and finish-finish, but they may be connected finish-start if desired. As utilized in this specification and which is well known in the art, the start of a pancake coil is the end of its inner turn, and the finish of a pancake coil is the end of its outer turn, regardless of where the electrical line first enters the coil. Solid insulation, shown generally at 42 in the lower half of FIG. 1, which may comprise washer members, angles and channels of pressboard, or other suitable electrical insulating material, is suitably disposed to form coil cooling ducts 44 immediately adjacent each pancake coil 26, 28, 30 and 32, and the solid insulation also forms the electrical barrier between adjacent pancake coils, and between the pancake coils and the grounded magnetic core members 12 and 14. The solid insulation is not shown in the upper half of FIG. 1, for purposes of clarity.

The high voltage winding 18 is disposed in spaced relation relative to the low voltage winding 20, and is also formed of a plurality of spaced pancake-type coils, with six pancake coils 1 6, 46, 48, 50, 52 and 54 being shown for purposes of example. Pancake coils 16, 46, 48, 50, 52 and 54 each have two major opposed surfaces, which are connected by an opening, such as opening 56 in pancake coil 16, with the pancake coils being stacked in spaced sideside relation with their openings in alignment, similar to the pancake coils of the low voltage winding 20. The pancake coils 16, 46, 48, 50, 52 and 54 are serially connected to form the high voltage winding 18, which has a line terminal L connected to one end of the stacked pancake coils, and a terminal 60, which may be the neutral or grounded end of the winding.

In high voltage transformers, the pancake coils of the high voltage winding may be constructed to utilize graded insulation. For example, as shown in FIG. 1, the pancake coils at the line end of the winding near the line terminal L, are constructed with a larger window or opening for receiving the magnetic core members 12 and 14, and have a smaller outside diameter, than do the pancake coils located nearer the grounded or neutral end 60 of the winding 18. Also, certain of the pancake coils of the high voltage winding 18 may be formed such that as the potential difference increases between them, the spacing between them will increase accordingly. For example, pancake coil 16 of high voltage winding 18 may be formed in a substantially flat manner, while the immediately adjacent pancake coil 46 may be formed such that it is relatively close to pancake coil 16- at their inner diameters, with the spacing between the coils increasing as the turns of the coils spiral outwardly. This construction takes advantage of the fact that the coils have very little potential difference at the ends where they are electrically interconnected, to reduce the physical length of the coil stack.

Like the pancake coils of the low voltage winding 20, the pancake coils of the high voltage winding 18 cooperate with the members which form the solid insulation 42 to provide coil ducts, such as ducts 62, disposed immediately adjacent the major surfaces of the pancake coils.

The magnetic core-coil structure of transformer 10 is disposed within a suitable enclosure or tank shown generally at 64, and the tank 64 may contain a suitable insulating dielectric fluid, such as oil or SE which circulates through the coil ducts and suitable external heat exchanger means (not shown), to remove heat from the pancake coils and the dielectric fluid.

In order to more uniformly distribute surge potentials across the line end of winding 18, which may be applied to the line terminal L, such as caused by lighting surges transmitted through the associated electrical system, or by 7 switching surges, a static plate member 70 is disposed in spaced relation with the line coil 16 and electrically connected to the line terminal L. However, instead of disposing the static plate 70 adjacent the outer surface of the line coil 16, i.e., the surface facing away from the remaining pancake coils of the high voltage winding 18, as taught by the prior art, a static plate 70 is disposed between the line coil 16 and the immediately adjacent coil 46. As will be more fully explained hereinafter, this structure reduces the concentration of electrical stresses at certain areas adjacent the line end coils, and provides a more favorable distribution of surge potentials across the line end pancake coils of the winding.

The static plate 70 may be formed in many ways, with US. Patent 2,993,183, issued July 18, 1961, and assigned to the same assignee as the present application, teaching acceptable static plate structures. In general, as shown in FIGS. 1A and 1B, the static plate 70 may be formed of an insulating washer member 71 having rounded edges and an opening for receiving the magnetic core. The external surfaces of the washer member are completely covered with a metallic foil or metallic coating 73 which may be in strip form, or it may be sprayed on. Metals such as aluminum or copper may be utilized. Thus, the static plate, like the pancake coils, has two major opposed surfaces connected by the opening in the static plate, and connected by the outer edge of the static plate.

FIG. 1C is a cross-sectional view of another static plate structure 76' which may be used for static plate 70. In this arrangement, two similar washer members 75 and 77, having a configuration similar to that shown in FIG. 1A, are disposed in adjacent contacting relationship, with a thin layer or coating 79 of electrically conductive material being disposed between them. The electrically conductive material should preferably terminate before reaching the inner and outer edges of the washer members 75 and 77. The static plate 70' is completed by disposing a round electrical conductor 81, such as copper, around the outer and inner edges of the washer members 75 and 77, with the electrical conductor 81 having tightly wound cellulosic insulation 83 disposed thereon. The inner and outer edges of the static plate 70' are then fitted with tight channel members 85, which are formed of an electrical insulating material such as pressboard.

FIGS. 2 and 3 illustrate fragmentary cross-sectional views of the first two line end coils 16 and 46 of the transformer shown in FIG. 1, with FIG. 2 illustrating the disposition of the static shield 70 according to the teachings of the invention, and FIG. 3 illustrating a static shield 72 disposed according to the teachings of the prior art. Since the pancake coils are all symmetrical about center line 9, only one half of the pancake coils are shown in FIGS. 2 and 3 for purposes of simplicity.

Referring first to the prior art structureof FIG. 3, it will be noted that the pancake coils provide a capacitive structure which includes capacitance C, between adjacent turns 74 in each pancake coil, capacitance C between the conductor turns 74 of adjacent pancake coils, and capacitance C between turns 74 of pancake coil 16 and the static plate 72. The capacitance C, between adjacent turns in each coil actually reduces the through series capacitance of the coils and of the winding, as serially connected capacitors add in a manner similar to parallel connected resistors. The capacitance C between adjacent coils and the capacitance C between pancake coil 16 and the static plate 72, being etTectively connected in parallel with the pancake coils, add in a manner similar to series connected resistors, thus increasing the through series capacitance of the winding. Thus, the static plate 72 increases the series capacitance of pancake coil 16, aiding in more uniformly distributing surge potentials across coil 16, and reducing the concentration of surge potentials on the first few turns of the line coil.

While the static plate 72, disposed as shown adjacent the outer surface of the line coil 16, is of definite value in aiding the distribution of surge potentials, it should be noted that the capacitance between each turn of pancake coil 16 and the static plate 72, and the capacitance between each turn of pancake coil 16 and coil 46 are connected in series, which reduces the effective capacitance between coil 46 and the static plate 72 to a value smaller than the smaller of two serially connected capacitances, thus decreasing the magnitude of the electrical charge which may be delivered to pancake coil 46.

Further, the static plate 72 does not effectively reduce the magnitudes of certain areas of high electrical stress. In fact, the location of the static plate 72 adjacent the outside surface of the line end coil 16 actually increases puncture and creep stresses in certain areas. It is well known that pressboard and other solid insulating materials commonly utilized in transformer construction, have much higher strength in puncture than in creep. Thus, the placement of the solid insulating members, such as angles, channels, and washers, should be such as to substantially follow the equipotential lines surrounding the electrical coils, and thus utilize the solid insulating members most etficiently. Anything which upsets the expected location of the equipotential lines, and stresses the solid insulation in creep instead of puncture, will lower the insulating value of the system and should therefore be avoided.

More specifically, the static plate 72, being disposed adjacent one end of the high voltage winding, adjacent the outer surface of the line end coil 16, and being substantially at the same potential as the line terminal L, provides a high stress concentration near the outer edge of the static plate 72, shown generally at 76. Also, the edge of the outer turn 74 of the line coil 16 produces a highly stressed area, shown generally at 78. High creep stresses are created during surge potentials between the first and second line coils, and especially between the outer edge of the second coil 46, and the outer edge of the line coil 16, shown generally at 80, due to the fact that the capacitance of the second coil 46 receives a smaller charge than the capacitance of the line coil 16, due to the close proximity of the static plate 72 adjacent the line coil 16, which increases the capacitance of the line coil to the static plate, and the series charging path from the static plate through capacitance C through the turns of pancake coil 16, and through capacitance C which reduces the capacitance of the second coil 46 to the static plate 72. The difference in the capacitances of the two coils creates a large voltage gradient between them, as they each receive a substantially different charge, which creates high creep stresses, and also produces large transient oscillatory voltages as the voltage distribution changes from capacitive to inductive.

Disposing the static plate between the first two coils from the line end of the winding, such as static plate 70 shown in FIGS. 1 and 2 reduces the magnitude of the stress concentrations produced by the arrangement shown in FIG. 3, reduces the creep stresses between the first tWo line end coils, increases the through series capacitance of the first two line end coils, which enforces a more favorable distribution of surge potentials across these coils and across the coil-coil spaces between the coils, allows a reduction in the coil-coil space between the line end coils, and has the advantage of reducing the space required be tween the high and low voltage windings 18 and 20, respectively, as shown in FIG. 1.

As illustrated in FIG. 2, the capacitance C, between the adjacent turns of each coil remains the same as the capacitance C, in the prior art arrangement. The capacitance C between the line end coil 16 and the static plate 70, also remains substantially unchanged, as in general, the static plate 70 will be disposed to maintain the same spacing with the line coil 16 as in the prior art arrangement. The capacitance between the static plate 72 and coil 46 has been replaced with the capacitance C between the static plate 70 and the coil 46. The value of capacitance C,,, is greater than the value of capacitance which it replaced. This may be readily observed from the formula for capacitance:

Where C is the capacitance in micromicrofarads, A is the area of one of the capacitor plates or major coil surfaces in square inches, d is the distance between the capacitor plates, or between the major coil surfaces and the static plate, and K is the dielectric constant of the insulating medium which separates the plates of the capacitor. Therefore, in the arrangement shown in FIG. 2, the capacitive relationship between pancake coil 16 and static plate 70 is similar to the capacitive relationship between pancake coil 46 and static plate 16. Therefore, coils 16 and 46 will receive similar charges, which substantially reduces the stress in the coil-coil space between the coils 16 and 46. The static plate 70 in FIG. 2 causing pancake coils 16 and 46 to have similar capacitive relationships relative to the static plate, and thus receive similar charges, regulates the voltage distribution in the second coil better than the line coil '16 does in the prior art structure shown in FIG. 3. Thus, the arrangement shown in FIG. 2 substantially reduces the stresses between the pancake coils 16 and 46, including a very substantial reduction in the creep stresses in this area.

Increasing the magnitude of the effective through series capacitance of coils 16 and 46 offsets the lower value of capacitance between these coils, as compared to the capacitance between the remaining coils of the winding, due to the smaller physical size of the line end coils occasioned by the grading of the insulation from the coils to ground based upon the potential of the coils. Therefore, the arrangement shown in FIG. 2 promotes a more uniform distribution of surge potentials across the, entire winding structure.

In addition to correcting the effective through series capacitance of the line end coils, the disposition of the static plate 70 between the first two line end coils also provides other advantages. For example, the magnitude of the electrical stress at highly stressed areas 76, 78 and 80 of the prior art arrangement of FIG. 3, is substantially reduced by the construction f.,FIG-. 2, wherein the static plate is disposed intermediate pancake 16 and pancake coil 46. Specifically, the electrical stress at location 76 shown in FIG. 3 is reduced by the amount of the voltage drop across the line end coil 16. In FIG. 3, the electrical stress at point 76 is due to the full line potential applied to the static plate 72. In FIG. 2, the electrical stress at this point is due to the potential of the inner turn of the line end coil 16, which is equal to the line potential minus the voltage drop across the pancake coil 16. The reduction in electrical stress in this area, also has the advantage of reducing the electrical clearance required between the high voltage winding 18 and the adjacent low voltage winding 20.

The electrical stress at point 78 shown in FIG. 3 is also reduced by the construction shown in FIG. 2, due to the fact that the static plate 70 is at substantially the same potential as the outer turns of pancake coil 16, thus setting up an area of zero potential gradient at this point. It should be noted that the static plate 70 has a slightly greater outside diameter than pancake coil 16, and a slightly smaller inside diameter, thus providing a larger major surface area than pancake coil 16, which more effectively shields the edges of the inner and outer turns of pancake coil 16. While the static plate 70 has a larger major surface area than the line coil -16, in structures utilizing graded insulation, the major surface area of the static plate is preferably less than the major surface area of the second coil from the line end. The static plate 70 has rounded edges, which provides a smooth equipotential surface which further aids the shielding of the corners of the inner and outer turns of pancake coil 16.

An additional benefit of the construction shown in FIG. 2, is the reduction in creep stresses in the general area between the outer edges of pancake coils 16 and 46, as shown in FIG. 3. By disposing the static plate between the first two line end coils, the capacitive relationships of the two coils to the static plate are similar, unlike the prior art structure shown in FIG. 3, thus allowing the capacitances of the coils 16 and 46 to receive similar charges. Since the potential across a capacitance is proportional to charge, the generation of large potential differences between the coils is precluded, which results in substantially reducing electrical stresses, which stress the solid insulating members in creep instead of puncture. Charging the capacitances of coils 16 and 46 to substantially the same potential also reduces the magnitude of voltage oscillations, compared with those which are produced by the different charge potentials on the line end coils by the arrangement shown in FIG. 3.

Increasing the through series capacitance of pancake coils 16 and 46, and reducing the puncture and creep stresses between these coils by providing similar capacitive relationships between these coils and the static plate, provides a lower stress across the coil-coil space which allows pancake coils 16 and 46 to be placed closer together, which further increases the beneficial circuit effect of the capacitance between these coils, and further aids in a more favorable voltage distribution across the winding.

In some instances, depending upon the voltage rating of the transformer and the design parameters of the electrical pancake coils, it may be desirable to dispose additional static plate members between some of the other pancake coils near the line end of the high Voltage winding. FIG. 4 shows the high voltage winding 18 of FIG. 1, illustrating the placement of additional static plate members.

More specifically, FIG. 4 illustrates the high voltage...

winding 18 of transformer 10 of FIG. 1, with only the cross section of the pancake coils on one side of center line 9 being shown for purposes of simplicity. In this embodiment of the invention, in addition to the static plate member 70 disposed between pancake coils 16 and 46, a static plate may be disposed between pancake coils 46 and 48, a static plate member 92 may be disposed between pancake coils 48 and 50, and a static plate member 94 may be disposed between pancake coils 50 and 52. The actual number of static plate members utilized will depend upon the particular requirements of the high voltage winding. As illustrated in FIG. 4, static plate member 90 is electrically connected to the interconnecting conductor 91 between pancake coils 16 and 46, static plate member 92 is electrically connected to the interconnecting conductor 93 between pancake coils 46 and 48, and static plate member 94 is electrically connected to the interconnecting conductor 95 between pancake coils 48 and 50. The static plate members 90, 92 and 94 are disposed adjacent the end of the pancake coil to which it is electrically connected. Since the capacitance between adjacent pancake coils increases from the line end of the winding 18 to its opposite end at terminal 60, due to the increase in the physical size of the pancake coils, the differences between the inner and outer diameters of static plate members 90, 92 and 94 should be successively reduced from static plate to static plate, starting with the largest static plate at the line end of the winding, in order to reduce the surface area of the static plates which form capacitor plates and reduce the value of the added capacitance in proportion to the increase in capacitance between the coils due to their increased physical size. A substantially linear capacitive voltage distribution across the coils of the winding would be most desirable, with the capacitive voltage distribution curve approaching the inductive voltage distribution curve. When this condition is achieved, voltage oscillations, produced when a surge voltage changes from capacitive to induc- :tive distribution, are minimized, which reduces the amount of insulation required between adjacent pancake coils, and between the pancake coils and ground. The surface area of each static plate member and the number of static plate members utilized may be tailored to accommodate the requirements of the winding, to provide the desired capacitive voltage distribution characteristic.

The additional static plate members 90, 92 and 94 shown in FIG. 4, will also provide the advantages of the static plate 70, hereinbefore described, in reducing the magnitude of electrical stress concentrations and creep stresses in the coil-coil spaces between the pancake coils.

The teachings of the invention may also be advantageously applied to multi-section coils, such as the commonly used double section coils in which each double section coil has two spaced pancake coils, with the space between the coils providing a low stress area for location of the coil cooling ducts. In other words, in a double section type coil and winding, there are two parallel paths between the line terminal and neutral or grounded terminal of the winding, with electrically similar pancake coils in each path being disposed in predetermined spaced relation to form the low stress cooling ducts for the coils. In the prior art, it is customary to dispose a static plate adjacent the outside surface of the line end double coil section. With this arrangement there is a possibility that voltage oscillations of undesirable magnitude may be created when surge voltages are applied to the transformer windings. The magnitude of the voltage oscillations using the double section coil arrangement may be substantially reduced by following the teachings of this invention.

More specifically, FIG. illustrates a high voltage winding 110 in section, wound about center line 9, with only one half of the winding being shown for simplicity. Instead of being formed of single section pancake coils, such as shown in transformer of FIG. 1, winding 110 of FIG. 5 is formed of double section coils, with each pancake coil of each double section coil being connected serially with a pancake coil in each of the other double section coils, and the two series paths being connected together at the line and neutral terminals of the winding. Thus, two series paths are provided through the winding, with the series paths being connected in parallel at the start and finish of the winding.

Specifically, winding 110 may have any desired number of double section coils, with four double section coils 112, 114, 116 and 118 being shown for purposes of example. Double section coil 112 includes pancake coils 120 and 122, constructed in a manner similar to the pancake coils of transformer 10 shown in FIG. 1, With pancake coils 120 and 122 being disposed in spaced sideby-side relation to form a coil cooling duct 121 between them. The outer turns of pancake coils 120 and 122 are connected in common to line terminal L through conductors 136 and 138 respectively. Each pancake coil of each double section coil is connected serially with a pancake coil in all of the other double section coils, utilizing startstart, finish-finish connections as shown in FIG. 5, or finish-start connections. The inner turn or start of pancake coil 120 may be connected to the inner turn or start of one of the pancake coils in double section coil 114, such as the inner turn of coil 124 via conductor 140, and the start of pancake coil 122 may be connected to the start of the remaining pancake coil 126 of double section coil 114, via conductor 142. In like manner, the finish or outer turn of pancake coil 124 in double section coil 114 may be connected to the outer turn of one of the pancake coils of double section coil 116, such as pancake coil 130 via conductor 144, and the finish of pancake coil 126 may be connected to the finish of pancake coil 128 via conductor 146. The start of pancake coil 128 of double section coil 116 may be connected to the start of one of the pancake coils of double section coil 118, such as pancake coil 132 via conductor 143, and the start of pancake coil 130 may be connected to the start of pancake coil 134 via conductor 152. The finish ends of pancake coils 132 and 134 of double section coil 118, are then connected in common to terminal 160 via conductors 15a and 156, to complete the parallel connection of the series paths through the winding.

In order to reduce the voltage oscillations produced by surge voltages applied to the line end of the winding 110, a static plate is disposed between the pancake coils 120 and 122, in the coil-coil space 121 formed between the spaced coils. The static plate 150 is electrically connected to the line terminal L via conductor 165. This construction has the advantage of providing a parallel charging path for the capacitances of pancake coils 120 and 122, charging both pancake coils of double coil section 112 at the same rate and to the same potential. This is very important in high voltage windings, as it substantially reduces the magnitude of voltage oscillations produced when a surge potential change from capacitive to inductive distribution. With the static plate disposed outside the double section coil, the capacitance of the immediately adjacent pancake coil to the static plate is larger than the capacitance of the other coil of the section to the static plate, causing the immediately adjacent pancake coil to charge to a greater potential than the other pancake coil, thus causing a potential gradient between these coils which produces oscillations which travel throughout the whole winding during the latter part of the surge.

Static plate 150 is illustrated as being utilized only be tween the pancake coils 120 and 122 of double section coil 112 in FIG. 5. However, in some instances, it may be desirable to dispose a static plate between any number, or all of the double section coils, depending upon the voltage rating of the winding and its electrical characteristics. It should be noted that the major surface area of one of the major opposed sides of the static plate 150 disposed between the coils connected to the line terminal L is larger than the surface area of one of the major sides of one of the pancake coils 120 and 122. Further, the area of one of the major sides of static plate member 150 is less than the area of one of the major sides of one of the pancake coils 124 and 126 of the second double section coil 114 from the line terminal L, when the winding uses the principles of graded insulation.

In summary, there has been disclosed new and improved static plate-pancake coil structures which aid in enforcing a more favorable distribution of surge potentials across the turns of the line end coils, and across the coil-coil spaces between the line end coils. The disclosed structures also aid in reducing the magnitude of oscillatory voltages produced as surge voltage distribution changes from capacity to inductive, it aids in reducing the magnitude of electrical stress concentrations at certain points in the winding, and it reduces the magnitude of creep stresses in the winding. Thus, the insulation required between adjacent pancake coils, and between the pancake coils and ground is reduced to a minimum, which reduces the mean lengths of the electrical and magnetic circuits, and takes maximum advantage of the insulating qualities of the solid insulation, stressing it in puncture instead of in creep.

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 matter contained in the foregoing description or shown in the accompanying drawings, shall be interpreted as illustrative, and not in a limiting sense.

I claim as my invention:

1. An electrical winding comprising a plurality of pancake coils each having two major opposed surfaces and an opening which extends between the major opposed surfaces, said pancake coils being disposed in space side-by-side relation with their openings in substantial alignment forming a stack of pancake coils having first and second ends,

a first electrical terminal adapted for connection to a source of electrical potential,

a second electrical terminal,

static plate means,

said plurality of pancake coils being electrically connected to provide two series paths between said first and second electrical terminals, the pancake coils of each series path being disposed in spaced adjacent relation with an electrically similar pancake coil of the other series path, providing a plurality of double section coils,

said static plate means being disposed between the major surfaces of the spaced pancake coils of at least the double section coil which is connected to said first terminal, and

means electrically connecting said static plate means to said first electrical terminal.

2. The electrical winding of claim 1 wherein said static plate means is a single static plate member disposed between the pancake coils which form the double section coil connected to said first terminal.

3. The electrical winding of claim 2 wherein said static plate member has two major opposed surfaces, the area of one of the major surfaces of said static plate member being larger than the area of one of the major opposed surfaces of the pancake coils connected to said first terminal.

4. The electrical winding of claim 1 wherein said static plate means includes a plurality of static plate members with one static plate member being disposed between the pancake coils of certain successive double section coils starting with the double section coil connected to said first electrical terminal.

5. The electrical winding of claim 2 wherein said static plate member has two major opposed surfaces, the area of one of the major surfaces of said static plate member being larger than the area of one of the major opposed surfaces of the pancake coils in the double section coil connected to said first terminal, and smaller than the area of one of the major opposed surfaces of the pancake coils in the double section coil disposed adjacent the double section coil connected to said first terminal.

References Cited UNITED STATES PATENTS 1,872,293 8/1932 Hodnette et a1 336-70 2,155,840 4/1939 Rorden 336-70 2,220,539 11/1940 Panov et a1 336-70 2,374,049 4/ 1945 Stephens 336-70 X 2,381,782 8/1945 Stephens 336-70 2,723,379 11/1955 Vogel 336-70 2,993,183 7/ 1961 Moore et a1. 336-70 3,183,460 5/1965 Bennon 336-70X LARAMIE E. ASKIN, Primary Examiner.

T. I. KOZMA, Assistant Examiner.

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