Patent Publication Number: US-8111123-B2

Title: Disc wound transformer with improved cooling

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. provisional patent application No. 61/241,684 filed on Sep. 11, 2009, which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to transformers and more particularly to transformers with a disc wound coil. 
     As is well known, a transformer converts electricity at one voltage to electricity as another voltage, either of higher or lower value. A transformer achieves this voltage conversion using a primary coil and a secondary coil, each of which is wound on a ferromagnetic core and comprise a number of turns of an electrical conductor. The primary coil is connected to a source of voltage and the secondary coil is connected to a load. The ratio of turns in the primary coil to the turns in the secondary coil (“turns ratio”) is the same as the ratio of the voltage of the source to the voltage of the load. Two main winding techniques are used to form coils, namely layer winding and disc winding. The type of winding technique that is utilized to form a coil is primarily determined by the number of turns in the coil and the current in the coil. For high voltage windings with a large number of required turns, the disc winding technique is typically used, whereas for low voltage windings with a smaller number of required turns, the layer winding technique is typically used. 
     In the disc winding technique, the conductor turns required for a coil are wound in a plurality of discs serially disposed along the axial length of the coil. In each disc, the turns are wound in a radial direction, one on top of the other, i.e., one turn per layer. The discs are connected in a series circuit relation and are typically wound alternately from inside to outside and from outside to inside so that the discs can be formed from the same conductor. An example of such alternate winding is shown in U.S. Pat. No. 5,167,063. 
     A transformer with disc windings may be dry, i.e., cooled by air as opposed to a liquid dielectric. In such a dry transformer, the disc windings may be coated with, or cast in, a dielectric resin using vacuum chambers, gelling ovens etc. If the disc windings are cast in a solid dielectric resin, cooling issues are raised. In order to address these issues, U.S. patent application Ser. No. 11/494,087 to Pauley et al. (which is assigned to the assignee of this application and is hereby incorporated by reference) discloses using pre-formed cooling ducts to provide cooling. The present invention is directed toward improvements in the construction, installation and use of such pre-formed cooling ducts in a cast resin transformer having disc windings. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a method of manufacturing a transformer. In accordance with the method, a disc-wound coil is formed using a plurality of pre-formed cooling ducts. A first conductor layer is formed that includes a plurality of disc windings arranged in an axial direction of the disc-wound coil. Each of the disc windings includes a conductor wound into a plurality of concentric turns. A spacer layer is formed over the first conductor layer. The spacer layer includes a plurality of spacers. A second conductor layer is formed over the spacer layer. The second conductor layer includes a plurality of disc windings arranged in an axial direction of the disc-wound coil. Each of the disc windings includes a conductor wound into a plurality of concentric turns. The spacer layer is formed such that when the second conductor layer is formed, a plurality of axially-extending passages is formed between the first and second conductor layers. The pre-formed cooling ducts are slid into the axially-extending passages so as to be disposed between the first and second conductor layers. 
     Also provided in accordance with the present invention is a transformer that includes a disc-wound coil having a first conductor layer that includes a plurality of disc windings arranged in an axial direction of the disc-wound coil. Each of the disc windings includes a conductor wound into a plurality of concentric turns. A second conductor layer is disposed over the first conductor layer. The second conductor layer includes a plurality of disc windings arranged in an axial direction of the disc-wound coil. Each of the disc windings includes a conductor wound into a plurality of concentric turns. A spacer layer is disposed between the first and second conductor layers. The spacer layer includes a plurality of spacers arranged so as to form a plurality of axially-extending passages between the first and second conductor layers. A plurality of cooling ducts is disposed inside the axially-extending passages, respectively, thereby being positioned between the first and second conductor layers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
         FIG. 1  is a schematic sectional view of a transformer embodied in accordance with the present invention; 
         FIG. 2  shows a perspective view of a coil of the transformer, with a portion of the coil cut away to show a cross-section of a portion of the coil; 
         FIG. 3  shows an end view of the coil; 
         FIG. 4  shows a plurality of coaxial pairs of disc windings of the coil; 
         FIG. 5  shows an end view of the coaxial pairs of the disc windings; 
         FIG. 6  shows a wiring schematic of the transformer; 
         FIG. 7  shows a perspective view of a first cooling duct constructed in accordance with a first embodiment of the present invention; 
         FIG. 8  shows an elevational view of a second cooling duct embodied in accordance with a second embodiment of the present invention; 
         FIG. 9  shows an end view of the second cooling duct; 
         FIG. 10  shows an elevational view of a plug adapted for insertion into an end of the first cooling duct or the second cooling duct; 
         FIG. 11  shows a side perspective view of the coil of the transformer being formed on a winding mandrel in a first manufacturing method of the present invention; 
         FIG. 12  shows an end perspective view of a portion of the coil being formed on the mandrel in the first manufacturing method; 
         FIG. 13  shows a schematic view of an insert partially inserted inside the first cooling duct; 
         FIG. 14  shows an end view of the coil being formed in a second manufacturing method of the present invention, wherein a spacer tape is wrapped over a first conductor layer; 
         FIG. 15  shows an end view of the coil being formed in the second manufacturing method, wherein a second conductor layer is wrapped over spacers of the spacer tape; and 
         FIG. 16  shows a schematic view of a cooling duct being inserted into the coil during the second manufacturing method. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     It should be noted that in the detailed description that follows, identical components have the same reference numerals, regardless of whether they are shown in different embodiments of the present invention. It should also be noted that in order to clearly and concisely disclose the present invention, the drawings may not necessarily be to scale and certain features of the invention may be shown in somewhat schematic form. 
     Referring now to  FIG. 1 , there is shown an interior view of a three phase transformer  10  containing a coil embodied in accordance with the present invention. The transformer  10  comprises three coil assemblies  12  (one for each phase) mounted to a core  18  and enclosed within a ventilated outer housing  20 . The core  18  is comprised of ferromagnetic metal and is generally rectangular in shape. The core  18  includes a pair of outer legs  22  extending between a pair of yokes  24 . An inner leg  26  also extends between the yokes  24  and is disposed between and is substantially evenly spaced from the outer legs  22 . The coil assemblies  12  are mounted to and disposed around the outer legs  22  and the inner leg  26 , respectively. Each coil assembly  12  comprises a high voltage coil  30  and a low voltage coil, each of which is cylindrical in shape. If the transformer  10  is a step-down transformer, the high voltage coil  30  is the primary coil and the low voltage coil is the secondary coil. Alternately, if the transformer  10  is a step-up transformer, the high voltage coil  30  is the secondary coil and the low voltage coil is the high voltage coil. In each coil assembly  12 , the high voltage coil  30  and the low voltage coil may be mounted concentrically, with the low voltage coil being disposed within and radially inward from the high voltage coil  30 , as shown in  FIG. 1 . Alternately, the high voltage coil  30  and the low voltage coil may be mounted so as to be axially separated, with the low voltage coil being mounted above or below the high voltage coil  30 . 
     The transformer  10  is a distribution transformer and may have a kVA rating in a range of from about 112.5 kVA to about 15,000 kVA. The voltage of the high voltage coil may be in a range of from about 600 V to about 35 kV and the voltage of the low voltage coil may be in a range of from about 120 V to about 15 kV. 
     Although the transformer  10  is shown and described as being a three phase distribution transformer, it should be appreciated that the present invention is not limited to three phase transformers or distribution transformers. The present invention may utilized in single phase transformers and transformers other than distribution transformers. 
       FIGS. 2-3  show one of the high voltage coils  30 , which are constructed in accordance with the present invention. Each high voltage coil  30  has a plurality of conductor layers, which comprise at least an inner or first conductor layer  32  and an outer or second conductor layer  34 . Each of the first and second conductor layers  32 ,  34  comprises a plurality of disc windings  36 . The disc windings  36  in the first conductor layer  32  may be coaxially disposed inside the disc windings  36  in the second conductor layer  34 , respectively, so as to form coaxial pairs  37  of disc windings  36  that are arranged along a longitudinal axis of the high voltage coil  30 , as shown in  FIG. 4 . A plurality of pre-formed cooling ducts  40  or  42  are disposed around the circumference of the high voltage coil  30  in a spaced-apart manner. The cooling ducts  40 ,  42  are positioned between the first and second conductor layers  32 ,  34 . The cooling ducts  40 ,  42  and the first and second conductor layers  32 ,  34  are encapsulated in an encasement  44  comprised of a solid dielectric insulating resin  45 . 
     In  FIGS. 2 and 3 , the cooling ducts  40  or  42  are shown generically for purposes of ease of illustration. The structure of the cooling ducts  40  is shown in  FIG. 7 , while the structure of the cooling ducts  42  is shown in  FIGS. 8 and 9 . Both cooling ducts  40 ,  42  are described in more detail in the paragraphs that follow. The number of cooling ducts shown in  FIGS. 2 and 3  should not be construed as limiting the scope of the present invention. A greater or lesser number of cooling ducts  40 ,  42  may be utilized. 
     Referring now to  FIGS. 4 and 5 , each disc winding  36  comprises a plurality of concentric layers of a conductor  46 . The conductor  46  is composed of a metal such as copper or aluminum and may be in the form of a wire with an elliptical or rectangular cross-section. Alternately, and as shown, the conductor  46  may be in the form of a foil, wherein the conductor  46  is thin and rectangular, with a width as wide as the disc winding  36  it forms. In the embodiments shown and described, it has been found particularly useful to use foil conductors, more specifically foil conductors having a width to thickness ratio of greater than 20:1, more particularly from about 250:1 to about 25:1, more particularly from about 200:1 to about 50:1, still more particularly about 150:1. In one particular embodiment, the foil conductor is between about 0.008 to about 0.02 inches thick and between about 1 and 2 inches wide, more particularly about 0.01 inches thick and about 1.5 inches wide. In each disc winding  36 , the turns of the conductor  46  are wound in a radial direction, one on top of the other, i.e., one turn per layer. A layer of insulating material is disposed between each layer or turn of the conductor  46 . In this manner, there are alternating layers of the conductor  46  and the insulating material. The insulating material may be comprised of a polyimide film, such as is sold under the trademark Nomex®; a polyamide film, such as is sold under the trademark Kapton®, or a polyester film, such as is sold under the trademark Mylar®. 
     The disc windings  36  may be connected together in the manner shown in  FIG. 6 . As shown, the first conductor layer  32  comprises disc windings  36   a - 36   h  and the second conductor layer  34  comprises disc windings  36   i - 36   p . In the first conductor layer  32 , the disc windings  36   a - 36   d  are serially connected together and the disc windings  36   e - 36   h  are serially connected together. The disc winding  36   d  is not connected to the adjacent disc winding  36   e . In this manner, the first conductor layer  32  has two groups of serially-connected disc windings  36 , wherein the two groups are not directly connected together. In the second conductor layer  34 , there are four groups of disc windings  36  that are not connected together, wherein each group consists of a pair of connected-together disc windings  36 . The four pairs are:  36   i  and  36   j ,  36   k  and  36   l ,  36   m  and  36   n  and  36   o  and  36   p . Main taps  50 ,  52  are connected to the disc windings  36   i ,  36   p , respectively of the second conductor layer  34 . Nominal taps  54  are connected to different disc windings  36 , respectively. Connecting together different pairs of the nominal taps  54  changes the turns ratio of the transformer  10 . For example, connecting together the nominal taps  54   a  and  54   b  serially connects together all of the disc windings  36  in both the first and second conductor layers  32 ,  34 . The main taps  50 ,  52  are located toward ends of the high voltage coil  30 , respectively, while the nominal taps are located toward the center of the high voltage coil  30 . The main taps  50 ,  52  and nominal taps are located in the dome  82  of the high voltage coil  30 . 
     Referring now to  FIG. 7 , there is shown one of the cooling ducts  40 , which is constructed in accordance with a first embodiment of the present invention. Each cooling duct  40  has a generally elliptical cross-section, with open ends and spaced-apart generally planar front and rear walls  60 ,  62  joined together by a pair of spaced-apart curved side walls  64 . It has been found particularly useful to provide each cooling duct  40  with a linear dimension, x, that is about three times the width, d, of the cooling duct  52 . Each cooling duct  40  is comprised of a fiber reinforced plastic in which fibers, such as fiberglass fibers, are impregnated with a thermoset resin, such as a polyester resin, a vinyl ester resin, or an epoxy resin. In one embodiment, the cooling ducts  40  are each formed using a pultrusion process, wherein the fibers are drawn through one or more baths of the thermoset resin and are then pulled through a heated die where the thermoset resin is cured. The fibers may be aligned as either unidirectional roving or a multi-directional mat. In this embodiment, the thermoset resin may be a polyester resin. In another embodiment, the cooling ducts  40  are each formed using a tape comprised of fiber glass impregnated with an F-class epoxy resin (suitable for use above 150° C.) that is about 70% cured. The tape is wound around a mold and then fully cured under the application of heat. 
     Referring now to  FIGS. 8 and 9 , there is shown one of the cooling ducts  42 , which is constructed in accordance with a second embodiment of the present invention. The cooling duct  42  may have the same construction as the cooling duct  40 , except a support pipe  66  is secured between the front and rear walls  60 ,  62 . More specifically, the cooling duct  42  may be constructed by securing the support pipe  66  inside the cooling duct  40 . The support pipe  66  is comprised of the same material as the cooling duct  40  (i.e., fiber-reinforced plastic that is formed by pultrusion or tape wrapping) and is constructed to be rigid. The support pipe  66  is cylindrical in shape and has a hollow interior. The support pipe  66  is sufficiently shorter than the cooling duct  40  so that gaps are formed between ends of the support pipe  66  and ends of the cooling duct  40 , respectively, when the support pipe  66  is secured between the front and rear walls  60 ,  62 . A high strength adhesive, such as a two-part epoxy adhesive, may be used to secure the support pipe  66  between the front and rear walls  60 ,  62 . The support pipes  66  help strengthen the cooling ducts  42  and prevents the cooling ducts  42  from collapsing or deforming when a vacuum is applied to the cooling ducts  42  during the resin casting process. 
     The cooling ducts  40 ,  42  are installed after the first conductor layer  32  is formed. Depending on the manufacturing method utilized, the cooling ducts  40 ,  42  may be installed before or after the second conductor layer  34  is wound. 
     Referring now to  FIGS. 11 and 12 , there is shown one of the high voltage coils  30  being manufactured in accordance with a first manufacturing method of the present invention. Initially, a first insulating layer (not shown) is formed over a winding mandrel  72  of a winding machine. The first insulating layer may be formed on an inner mold mounted to the mandrel  72  or may be formed directly on the mandrel  72 , depending on the mold that is used during the resin casting process. The winding mandrel  72  may be rotated by an electric motor. Rotation of the mandrel  72  is used to wind the conductor  46  and insulating material over the mandrel  72  to form layers of the high voltage coil  30 , as described below. The first insulating layer comprises a sheet or web of screen material  70 , which is comprised of glass fibers woven into a grid with rectangular openings. More specifically, the screen material  70  has spaced-apart longitudinally arranged glass fibers that adjoin spaced-apart laterally arranged glass fibers at intersections that form the corners of the rectangular openings. The glass fibers may be impregnated with an insulating resin, such as an epoxy. A mound or button of insulating material may be joined to each intersection and protrudes above the web and may also protrude below the web. The buttons have a rounded shape and may be formed by building up the insulating resin at the intersections. The screen material  70  may have the construction and arrangement of the screen material disclosed in U.S. patent application Ser. No. 10/858,039 (Publication No. 2005/0275496), which is hereby incorporated by reference. The web of screen material  70  is wound around the winding mandrel  72  to form a cylinder and opposing longitudinal edges of the web are held together, at least temporarily with a glass fiber tape. 
     The first conductor layer  32  is formed (wound) over the first insulating layer from two or more lengths of the conductor  46 . The glass fiber tape holding the first insulating layer together may be removed as the first conductor layer  32  is being formed, or the glass fiber tape may be left in place. In forming the disc windings  36 , the conductor  46  can be continuously wound or may be provided with “drop-downs”. If the conductor  46  is continuously wound, the conductor  46  is wound in alternating directions, i.e., inside to outside and then outside to inside, etc. If the conductor  46  is provided with drop-downs, the conductor  46  is wound in one direction, i.e., inside to outside. A drop-down is a bend that is formed at the completion of a disc winding  36  to bring the conductor  46  from the outside back to the inside to begin a subsequent disc winding  36 . 
     After the first conductor layer  32  has been formed, a second insulating layer  74  comprised of a sheet or web of the screen material  70  is formed over the first conductor layer  32 . Opposing longitudinal edges of the web are held together, at least temporarily with a glass fiber tape. Next, a layer  76  of cooling ducts  40 ,  42  is disposed over the second insulating layer  74 , as will be described more fully below. A third insulating layer  78  comprised of a sheet or web of the screen material  70  is then formed over the layer of cooling ducts  40 ,  42 . 
     The second conductor layer  34  is formed over the installed layer  76  of the cooling ducts  40 ,  42  from a plurality of lengths of the conductor  46 . After the second conductor layer  34  has been formed, a fourth insulating layer (not shown) comprised of a sheet or web of the screen material  70  is formed over the second conductor layer  34 . The partially-formed coil  30  is then ready to be impregnated with the insulating resin  45 , which is described in more detail below. 
     When the disc windings  36  are formed between the first and second insulating layers comprised of the grid material with buttons, as described above, the disc windings  36  are held between the buttons so as to form insulation gaps between the disc windings  36  and the grids of the screen material disposed on opposing sides of the disc windings  36 . Such insulation gaps are also formed on the opposing sides of the cooling ducts  40 ,  42 . Such insulation gaps are filled by the insulating resin  45  during the encapsulation of the coils with insulating resin  64 . 
     Returning now to the formation of layer  76  of the cooling ducts  40 ,  42 , each cooling duct  40 ,  42  is wrapped with a layer of glass tissue along its entire length before installation. In addition, before installation, each cooling duct  40 ,  42  is wrapped at each end with tape comprised of a compressible material, such as a closed cell silicone foam or silicone rubber. The compressible tape is wrapped at each end of the cooling duct  40 ,  42  so as to extend about 3 centimeters down from the end. Each cooling duct  40 ,  42  can further be wrapped at each end with the screen material  70  used to form the insulating layers. This further wrapping extends about 10 cm down from each end. After being wrapped as described above, the cooling ducts  40 ,  42  are disposed around the circumference of the partially formed coil  30 , over the second insulating layer  74 . The cooling ducts  40 ,  42  are substantially evenly spaced apart, except for an enlarged spacing or gap  80 , wherein the dome  82  is formed during the encapsulation process. The cooling ducts  40 ,  42  are initially held in place by a plurality of bands  84  of a glass fiber tape that are disposed around the layer  76  of cooling ducts  40 ,  42 . As shown, the cooling ducts  40 ,  42  extend longitudinally between first and second ends of the partially-formed coil. 
     In forming the layer  76 , either the cooling ducts  40  or the cooling ducts  42  may be used. If the cooling ducts  42  are used, the support pipes  66  provide the cooling ducts  42  with support during the resin casting process. Plugs  90  are simply inserted into the ends of each cooling duct  42 , respectively, and then the partially-formed coil  30  is encapsulated in the insulating resin  45 , as will be described more fully below. The plugs  90  keep the insulating resin  45  from flowing into the cooling ducts  42  during the resin casting process. Each plug  90  is composed of a resilient material, such as silicone rubber, and is dimensioned to frictionally fit within the gap formed between the end of the support pipe  66  and the end of the cooling duct  42 . More specifically, as shown in  FIG. 10 , each plug  90  has a body that is tapered inwardly (i.e., downwardly) and has ribs  92  disposed around the periphery of the body to ensure a positive seal with inner surfaces of the cooling duct  42 . After the resin casting process, the plugs  90  are removed from the cooling ducts  42 . 
     If the cooling ducts  40  are used, inserts  100  (shown in  FIG. 13 ) are used with them. The inserts  100  are formed from a high temperature plastic, such as polyphenylene sulfide, polyamideimide, polyimide, polyaramide, polyphthalamide or polyether ether ketone (PEEK). Each insert  100  has a cross-section that is elliptical and is sized so that the insert  100  can be facilely inserted into one of the cooling ducts  40 . The inserts  100  may be solid or hollow. If the inserts  100  are hollow, they have sufficient wall thickness so as to not be deformable. Each insert  100  is sufficiently shorter than the cooling duct  40  so that gaps are formed between ends of the insert  100  and ends of the cooling duct  40 , respectively, when the insert  100  is disposed inside the cooling duct  40 . The gaps are sized to receive the plugs  90 . For each insert  100 , one of the plugs  90  may be secured to an end of the insert  100  by a mechanical fastener (such as a screw or a bolt) and/or a high strength adhesive. Alternately, the inserts  100  may be separate from the plugs  90 . 
     The inserts  100  are inserted inside the cooling ducts  40 , respectively, either before or after the cooling ducts  40  are installed in the partially formed coil  30 . After the inserts  100  are inserted into the cooling ducts  40 , plugs  90  are inserted into the ends of the cooling ducts  40 . If plugs  90  are attached to ends of the inserts  100 , as described above, the attached plugs  90  are inserted into first ends of the cooling ducts  40  at the time the inserts  100  are inserted. In this manner, plugs  90  only need to be inserted into second ends of the cooling ducts  40 . If the plugs  90  are not attached to the inserts  100 , plugs  90  are inserted into both first and second ends of the cooling ducts  40 . During the resin casting process, the inserts  100  internally support the cooling ducts  40  and prevent the cooling ducts  40  from collapsing or deforming when a vacuum is applied to the cooling ducts  40 . After the resin casting process, the plugs  90  and the inserts  100  are removed from the cooling ducts  40 . 
     Referring now to  FIGS. 14 &amp; 15 , there is shown one of the high voltage coils  30  being manufactured in accordance with a second manufacturing method of the present invention. In the second manufacturing method, the first insulating layer is formed in the same manner as in the first manufacturing method described above. Next, the coaxial pairs  37  of windings  36  are formed, wherein each coaxial pair  37  comprises an inner disc winding  36  of the first conductor layer  32  coaxially disposed inside an outer disc winding  36  of the second conductor layer  34 . The coaxial pairs  37  of windings  36  may be formed serially, with one coaxial pair  37  being completely formed and then an adjacent coaxial pair  37  being completely formed and then another and so on. Alternately, the entire first conductor layer  32  may be formed first and then the second conductor layer  34  may be formed over the same. 
     In each coaxial pair  37  of disc windings  36 , an inner disc winding  36  in the first conductor layer  32  is formed first. Next, the disc winding  36  is wrapped with one turn of a spacer tape  110  that comprises a plurality of spaced-apart spacers  112  secured to a piece of insulating tape  114  comprised of an insulating material, such as polyimide, polyamide, or polyester. Each spacer  112  has a rectangular cross-section and may be composed of a fiber reinforced plastic in which fibers, such as fiberglass fibers, are impregnated with a thermoset resin, such as a polyester resin, a vinyl ester resin, or an epoxy resin. The spacers  112  are secured to the tape  114  by an adhesive and extend longitudinally along the width of the tape  114 . In the embodiment where the conductor  46  forming the disc windings  36  is comprised of foil, the lengths of the spacers  112  and the width of the tape  114  are about the same as the width of the conductor  46 . The spacers  112  are spaced apart by a distance that is slightly greater than the long width (dimension x) of the cooling ducts  40 ,  42 . In addition, the dimension of the spacers  112  in a direction perpendicular to the tape  114  is slightly greater than the small width (dimension d) of the cooling ducts  40 ,  42 . In this manner, the spacers  112  form spaces that can accommodate the cooling ducts  40 ,  42 , as will be described more fully below. The spacer tape  110  is wrapped onto the disc winding  36  to form a single turn such that the tape  114  adjoins the disc winding  36  and the spacers  112  extend radially outward like spokes. Ends of each piece of spacer tape  110  may be fastened together (such as by adhesive tape) to form a loop that is disposed radially outward from the disc winding  36 . The loop may be secured to the radially inward disc winding  36 . In lieu of a separate piece of the spacer tape  110  being used to form the single turn, the spacer tape  110  may be part of a long length of the insulating tape  114  that is used to form an outer disc winding  36  over the spacers  112 . In this embodiment, the spacers  112  are secured to only a portion of the long length of the insulating tape  114  and only one end of the tape  114  is secured to the radially inward disc winding  36 . After the portion of the tape  114  with the spacers  112  secured thereto is disposed around the circumference of the radially inward disc winding  36 , the tape  114  continues to be wound over the spacers  112  (together with the conductor  46 ) to form the radially outer disc winding  36 . During this winding, the tension of the winding machine keeps the insulating tape  114  (and the conductor  46 ) in position. 
     After the inner disc winding  36  in the first conductor layer  32  has been wrapped with a piece of spacer tape  110 , an outer disc winding  36  in the second conductor layer  34  is formed over the loop of the spacer tape  110  so as to be supported on the spacers  112  and spaced from the inner disc winding  36 . An initial layer of the insulating material directly contacts the spacers  112 . Thereafter, alternating layers of the conductor  46  and the insulating material are wound over the loop of the spacer tape  110  to form the outer disc winding  36 . When the outer disc winding  36  is complete, the inner and outer disc windings  36  are separated by a series of circumferentially arranged spaces  120  separated by the spacers  112 , as shown in  FIG. 5 . 
     The spacer tape  110  is wound on each disc winding  36  of the first conductor layer  32  in the same manner so that the spacers  112  and spaces  120  in the coaxial pairs of disc windings  36  are aligned along the axial length of the high voltage coil  30 . In this manner, when the formation of the coaxial pairs of disc windings  36  is complete, the aligned spaces  120  form a series of passages  122  (shown in  FIG. 16 ) extending axially through the partially formed high voltage coil  30 . 
     After the coaxial pairs of disc windings  36  have been formed, an outer insulating layer (not shown) comprised of a sheet or web of the screen material is formed over the second conductor layer  34 . The cooling ducts  40 ,  42  are then inserted into the passages  122 , respectively, so that ends of the cooling ducts  40 ,  42  are substantially aligned with ends of the partially formed high voltage coil  30 , respectively. As in the first manufacturing method, before each cooling duct  40 ,  42  is inserted, it is wrapped with a layer of glass tissue along its entire length and then each of its ends is wrapped with tape comprised of a compressible material, such as a closed cell silicone foam or silicone rubber. Also as in the first manufacturing method, each cooling duct  40 ,  42  can further be wrapped at each end with the screen material used to form the insulating layers. 
     In the second manufacturing method, as in the first manufacturing method, either the cooling ducts  40  or the cooling ducts  42  may be used. The plugs  90  and inserts  100  are used in the same manner as described above for the first manufacturing method. 
     Once the high voltage coil  30  has been fully wound and the cooling ducts  40 ,  42  installed, the high voltage coil  30  is removed from the winding mandrel  72  and then encapsulated in the insulating resin  45  during the resin casting process. The coil  30  is first enclosed in a mold that includes generally cylindrical inner and outer molds. The inner mold is inserted into the open center of the coil  30  and the outer mold is disposed around the coil  30 . If the inner mold was mounted to the winding mandrel  72  and the coil  30  then wound over the inner mold, only the outer mold has to be disposed around the outside of coil  30 . The mold may be a vertical mold, i.e., the mold holds the coil  30  with the axis of the coil  30  extending vertically, or the mold may be a horizontal mold, i.e., the mold holds the coil  30  with the axis of the coil  30  extending horizontally. An example of a horizontal mold that may be utilized is disclosed in U.S. Pat. No. 6,223,421, which is hereby incorporated by reference. An example of a vertical mold that may be used is disclosed in U.S. Pat. No. 7,023,312, which is also hereby incorporated by reference. It should be appreciated that the support pipes  66  in the cooling ducts  42  and the temporary presence of the inserts  100  in the cooling ducts  40  provide sufficient support to the cooling ducts  40 ,  42 , respectively, to permit the coil  30  to be encapsulated with the insulating resin  45  in a horizontal mold, which was previously not possible. 
     The coil  30  and the mold are pre-heated in an oven to remove moisture from the insulating layers and the conductor layers. The coil  30  is then placed in a vacuum chamber. The vacuum chamber is evacuated to remove any remaining moisture and gases in the coil  30  and to eliminate any voids between adjacent turns in the disc windings  36 . The insulating resin  45 , which is flowable, is poured between the inner and outer molds to encapsulate the coil  30 . The vacuum is held for a predetermined time interval to allow the insulating resin  45  to impregnate the screen material of the insulating layers. The vacuum is then released. Pressure may then be applied to the resin-coated coil  30  to force the insulating resin  45  to impregnate any remaining voids in the insulating layers. The coil  30  is then removed from the vacuum chamber and placed in an oven to cure the insulating resin  45  to a solid. 
     The curing process in the oven is conventional and well known in the art. For example, the cure cycle may comprise a (1) gel portion for about 5 hours at about 85 degrees C., (2) a ramp up portion for about 2 hours where the temperature increases from about 85 degrees C. to about 140 degrees C., (3) a cure portion for about 6 hours at about 140 degrees C., and (4) a ramp down portion for about 4 hours to about 80 degrees C. Following curing, the inner and outer molds are removed. The plugs  90  may be easily removed with pliers or other gripping devices without damaging the surrounding insulating resin  45 . If inserts  100  are used, each insert  100  may be removed from its respective cooling duct  40  by inserting a bar or rod (not shown) through an end of the cooling duct  40  and pushing the insert  100  out of the cooling duct  40  through the other end 
     The insulating resin  45  may be an epoxy resin or a polyester resin. An epoxy resin has been found particularly suitable for use as the insulating resin  45 . The epoxy resin may be filled or unfilled. An example of an epoxy resin that may be used for the insulating resin  45  is disclosed in U.S. Pat. No. 6,852,415, which is hereby incorporated by reference. Another example of an epoxy resin that may be used for the insulating resin  45  is Rutapox VE-4883, which is commercially available from Bakelite AG of Iserlohn of Germany. 
     It is to be understood that the description of the foregoing exemplary embodiment(s) is (are) intended to be only illustrative, rather than exhaustive, of the present invention. Those of ordinary skill will be able to make certain additions, deletions, and/or modifications to the embodiment(s) of the disclosed subject matter without departing from the spirit of the invention or its scope, as defined by the appended claims.