Abstract:
Disclosed is a method of manufacturing a continuous disk winding. A high-temperature superconducting wire is lapped using Kapton films to insulate the high-temperature superconducting wire. The high-temperature superconducting wire is wound on a bobbin by a predetermined number of turns to form a layer of windings. An annular disk having a slit formed therein is fitted onto the bobbin. The slit is formed along the circumference of the disk and to be inclined from the inner side of the disk towards the outer side thereof. The high-temperature superconducting wire is inserted into the slit of the disk to pass through the annular disk smoothly along the inclined slit and wound again by the predetermined number of turns to form a next layer of windings. The above steps of fitting an annular disk, and inserting and winding the high-temperature superconducting wire are repeated to form multiple layers of windings.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method of manufacturing a high-temperature superconducting winding, and, more particularly, to a method of manufacturing a continuous disk winding for high-voltage superconducting transformers that has special features of no joint and low loss, which are advantages of a layer winding, while being formed in the shape of a disk winding, which is advantageous in voltage distribution and insulation. 
   2. Description of the Related Art 
   Transformers are being recognized as the first application that can be put to practical use among currently developed high-temperature superconducting power equipment. Most high-temperature superconducting transformers recently researched and developed domestically and abroad are based on high voltage and large capacity. Depending upon the shape of winding, a high-temperature superconducting winding constituting such high-temperature superconducting transformers is generally classified as a solenoid type layer winding as shown in  FIG. 1A  or as a disk winding, which is widely adopted as a winding using a high-temperature superconducting wire made of a general tape formed in double pan cake type, as shown in  FIG. 1B . The winding shape of most high-temperature superconducting transformers currently developed all over the world is the layer winding. On the other hand, the disk winding has not been much developed. This is because alternating current loss is less when the winding of the same high-temperature superconducting transformer is manufactured in the shape of the layer winding than when the winding of the same high-temperature superconducting transformer is manufactured in the shape of the disk winding. 
   For a superconducting wire used in developing the high-temperature superconducting power equipment, alternating current loss is generated due to an alternating magnetic field. Especially, the superconducting wire is very weak with relation to the magnetic field applied perpendicular to the surface of the superconducting wire. For this reason, it is a general trend to adopt the high-temperature superconducting winding manufactured in the layer shape in order to reduce loss and prevent decrease of efficiency. 
   In the case of the high-voltage transformer, the layer winding reduces the alternating current loss generated in the superconducting wire. However, the layer winding is disadvantageous in the terms of insulation. Furthermore, the capacitance of the high-voltage transformer is small, and therefore, the high-voltage transformer is vulnerable to voltage spikes. For this reason, the high-voltage transformer using a copper wire uses the disk winding. 
   The disk winding is more advantageous, in terms of voltage distribution and insulation, than the layer winding as the terminal voltage of the transformer is increased. In a transformer, to which high voltage is applied as in a power-transmission transformer, it is generally preferable to adopt the disk winding. In the case that the disk winding is applied to the high-temperature superconducting transformer, however, it is necessary to electrically connect the disks with each other, and a great loss is generated in such electrical connections. Consequently, the stability of the superconducting winding is reduced, and it results in high costs to cool the high-temperature superconducting transformer. 
   In spite of the above-mentioned drawbacks, however, the adoption of the disk winding as the winding for high-temperature superconducting transformers is urgently requested considering high voltage, which is a current trend of developing the high-temperature superconducting power equipment. 
   SUMMARY OF THE INVENTION 
   Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a method of manufacturing a continuous disk winding for high-voltage superconducting transformers that has special features of no joint and low loss, which are advantages of a layer winding, while being formed in the shape of a disk winding, which is advantageous in voltage distribution and insulation. 
   The continuous disk winding is manufactured by winding a high-temperature superconducting wire in the shape of a disk such that the winding is continuously carried out without joining, thereby reducing the reduction in efficiency of the wire due to the joining and reducing loss of the wire. The continuous disk winding is more advantageous, in terms of insulation, than the layer winding, and therefore, the continuous disk winding can be applied to a high-voltage transformer. Furthermore, the continuous disk winding has technical characteristics advantageous in controlling voltage stress. Consequently, development of the above-described continuous disk winding is strongly requested. 
   In accordance with the present invention, the above and other objects can be accomplished by the provision of a method of manufacturing a continuous disk winding for high-voltage superconducting transformers, comprising the steps of: lapping a high-temperature superconducting wire using Kapton films to apply multiple layers of insulation around the high-temperature superconducting wire; fixing one end of the high-temperature superconducting wire to a current inlet terminal of a bobbin, to the lower end of which is attached a fixing plate, which has the current inlet terminal, and winding the high-temperature superconducting wire on the bobbin by a predetermined number of turns; fitting a disk having a slit formed therein onto the bobbin to form high-temperature superconducting wire layers; inserting the high-temperature superconducting wire into a front end of the slit of the fitted disk and moving the high-temperature superconducting wire to a rear end of the slit such that the high-temperature superconducting wire can be wound on the next layer by the predetermined number of turns; fitting a plurality of disks, each of which has a slit formed therein, onto the bobbin at predetermined intervals to form a plurality of high-temperature superconducting wire layers, and repeatedly winding the high-temperature superconducting wire for each disk such that the high-temperature superconducting wire is wound from one end of the bobbin to the other end of the bobbin; and after the winding operation is completed, fixing the other end of the high-temperature superconducting wire to a current inlet terminal of another fixing plate attached to the upper end of the bobbin. 
   Preferably, the high-temperature superconducting wire is a BSCCO (Bi—Sr—Ca—Cu—O)-2333 based wire or a YBCO coated conductor based wire, which is a second-generation superconducting wire. 
   Preferably, the thickness of each disk, which is used to separate the high-temperature superconducting wire located between the respective disks from each other, is equal to the thickness of the high-temperature superconducting wire. 
   Preferably, the number of the disks and the number of turns of the high-temperature superconducting wire are changed depending upon the capacity of the high-voltage superconducting transformer. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
       FIGS. 1A and 1B  are views respectively illustrating methods of winding high-temperature superconducting wires; 
       FIG. 2  is a view illustrating a continuous disk winding for high-voltage superconducting transformers according to the present invention; 
       FIG. 3  is a view illustrating a method of insulating a high-temperature superconducting wire according to the present invention; 
       FIG. 4A  is a view illustrating the distribution of a magnetic field in a continuous disk winding according to the present invention; 
       FIG. 4B  is a view illustrating the distribution of a magnetic field in a conventional disk winding; 
       FIG. 5  is a graph illustrating the relationship between current and a magnetic field for estimating critical current according to the present invention; 
       FIG. 6  is a graph illustrating the comparison between alternating current loss of a continuous disk winding according to the present invention and alternating current loss of a conventional disk winding; 
       FIG. 7  is a perspective view illustrating a continuous disk winding for high-voltage superconducting transformers according to the present invention; 
       FIG. 8A  is a perspective view illustrating the shape of a disk according to the present invention; 
       FIG. 8B  is a view illustrating a method of manufacturing a continuous disk winding for high-voltage superconducting transformers according to a preferred embodiment of the present invention; and 
       FIG. 9  is a graph illustrating the results of a test for measuring critical current of a continuous disk winding according to the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Now, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
     FIG. 2  is a view illustrating a continuous disk winding for high-voltage superconducting transformers according to the present invention,  FIG. 3  is a view illustrating a method of insulating a high-temperature superconducting wire according to the present invention,  FIG. 4A  is a view illustrating the distribution of a magnetic field in a continuous disk winding according to the present invention,  FIG. 4B  is a view illustrating the distribution of a magnetic field in a conventional disk winding,  FIG. 5  is a graph illustrating the relationship between current and a magnetic field for estimating critical current according to the present invention,  FIG. 6  is a graph illustrating the comparison between alternating current loss of a continuous disk winding according to the present invention and alternating current loss of a conventional disk winding,  FIG. 7  is a perspective view illustrating a continuous disk winding for high-voltage superconducting transformers according to the present invention,  FIG. 8A  is a perspective view illustrating the shape of a disk according to the present invention,  FIG. 8B  is a view illustrating a method of manufacturing a continuous disk winding for high-voltage superconducting transformers according to a preferred embodiment of the present invention, and  FIG. 9  is a graph illustrating the results of a test for measuring critical current of a continuous disk winding according to the present invention. 
   A continuous disk winding for high-voltage superconducting transformers according to the present invention is illustrated in  FIG. 2 . 
   A high-temperature superconducting (HTS) wire  10  is threefold insulated by Kapton films  11 , as shown in  FIG. 3 . 
   Referring to  FIG. 7 , one end of the high-temperature superconducting wire  10 , which is threefold insulated by Kapton films  11 , is fixed to a current inlet terminal  23  of a bobbin, to the lower end of which is attached a fixing plate  22 , to which the current inlet terminal  23  is coupled by means of bolts  24 , and then, the high-temperature superconducting wire  10  is wound on the bobbin by the predetermined number of turns. The bobbin  21  is hollow from one end to the other end. 
   In order to form high-temperature superconducting wire layers, a disk  25  including a slit  26 , which has a front end  261  and a rear end  262 , is fitted onto the bobbin  21 . The high-temperature superconducting wire  10 , which has been wound by the predetermined number of turns, is inserted into the front end  261  of the slit  26 , and is then moved to the rear end  262  of the slit  26 . Subsequently, the high-temperature superconducting wire  10  is wound on the next layer by the predetermined number of turns. 
   As described above, a plurality of disks  25 , each of which includes a slit  26 , are fitted on the bobbin  21  at predetermined intervals to form a plurality of high-temperature superconducting wire layers, and the high-temperature superconducting wire  10  is wound by the same number of turns for each disk  25 . In this way, the high-temperature superconducting wire  10  is wound from one end of the bobbin  21  to the other end of the bobbin  21 . 
   After the high-temperature superconducting wire  10  is wound from one end of the bobbin  21  to the other end of the bobbin  21 , another fixing plate  22 , to which an current inlet terminal  23  is coupled by means of bolts  24 , is attached to the upper end of the bobbin  21 , and then the other end of the high-temperature superconducting wire  10  is fixed to the current inlet terminal  23 . 
   The number of the disks  25  and the number of turns of the high-temperature superconducting wire  10  are changed depending upon the capacity of a high-voltage superconducting transformer. According to the present invention, a BSCCO (Bi—Sr—Ca—Cu—O)-2333 based wire, which is currently commercialized and widely used, or a YBCO coated conductor based wire, which is a second-generation superconducting wire, may be selectively used as the high-temperature superconducting wire  10 . Preferably, the high-temperature superconducting wire  10  has the following specification, which is indicated in Table 1 below. 
   
     
       
             
           
             
             
             
           
             
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               Specification of High-temperature Superconducting Wire 
             
           
        
         
             
                 
               Specification 
               Value 
             
             
                 
                 
             
           
        
         
             
                 
               Thickness 
               0.5 
               mm 
             
             
                 
               Width 
               5 
               mm 
             
             
                 
               Critical Tensile Stress 
               265 
               MPa 
             
             
                 
               Minimum Bending Diameter 
               50 
               mm 
             
             
                 
               Critical Current 
               126 
               A 
             
             
                 
                 
             
           
        
       
     
   
   In order to apply the high-temperature superconducting wire  10  indicated in Table 1 to the high-voltage superconducting transformer, the high-temperature superconducting wire  10  is threefold insulated by the Kapton films in the same manner as shown in  FIG. 3 , and then a continuous disk winding is manufactured using the insulated high-temperature superconducting wire  10 . The specification of the continuous disk winding is indicated in detail in Table 2 below. 
   
     
       
             
           
             
             
             
           
             
             
             
             
           
         
             
               TABLE 2 
             
           
           
             
                 
             
             
               Specification of Continuous Disk Winding 
             
           
        
         
             
                 
               Specification 
               Value 
             
             
                 
                 
             
           
        
         
             
                 
               Length of HTS Wire 
               61.78 
               m 
             
             
                 
               Inner Diameter 
               170 
               mm 
             
             
                 
               Outer Diameter 
               180 
               mm 
             
             
                 
               Height of Winding 
               225 
               mm 
             
             
                 
               Number of Disks 
               23 
             
             
                 
               Number of Turns 
               115 
             
             
                 
                 
             
           
        
       
     
   
   The total length of the high-temperature superconducting wire  10  used is approximately 61 m, the number of turns is 115, the outer diameter of the winding is 180 mm, and the height of the winding is 225 mm. A continuous disk winding and a conventional disk winding having the same number of turns as the continuous disk winding were modeled based on the design values indicated in Table 2 so as to analyze static magnetic fields of the continuous disk winding and the conventional disk winding. The static magnetic field analysis results are shown in  FIGS. 4A and 4B . 
   A method used for analysis was an axial-symmetry modeling method using a flexible element method. In comparison between the distribution of the magnetic field in the continuous disk winding and the distribution of the magnetic field in the conventional disk winding, it can be seen that the component of the magnetic field applied perpendicular to the surface of the high-temperature superconducting wire in the case of the conventional disk winding as shown in  FIG. 4B  is increased as compared to the case of the continuous disk winding as shown in  FIG. 4A . As the perpendicular component of the magnetic field is increased, the efficiency of the high-temperature superconducting wire is sharply decreased, and furthermore, alternating current loss is sharply increased. Consequently, it can be seen that the continuous disk winding is preferable. For example, when the current flowing through the coil is 70 A, the maximum value of magnetic field is 70 mT and the maximum value of magnetic field in the perpendicular direction is 68.5 mT in the case of the continuous disk winding. On the other hand, the maximum value of magnetic field is 68.5 mT and the maximum value of magnetic field in the perpendicular direction is 130 mT in the case of the conventional disk winding. As can be seen from the above comparison, as the magnetic field applied perpendicular to the disk winding is increased, critical current is sharply decreased, and alternating current loss is sharply increased. 
   In the case of the continuous disk winding designed based on the above-described static magnetic field analysis results, the critical current of the continuous disk winding is estimated with reference to the change of the critical current depending upon the magnetic flux density applied perpendicular to the high-temperature superconducting wire and the load lines of the designed continuous disk winding as shown in FIG.  5 . 
   Two lines shown in  FIG. 5  are load lines formed by the maximum magnetic flux density and the average magnetic flux density applied perpendicular to the high-temperature superconducting wire. Since the maximum perpendicular magnetic field is only partially applied to the high-temperature superconducting wire, the critical current of the real winding is present between two critical currents estimated by the load lines. When the average magnetic field is applied perpendicular to the high-temperature superconducting wire, the estimated value of the critical current is 112 A. When the maximum magnetic field is applied perpendicular to the high-temperature superconducting wire, on the other hand, the estimated value of the critical current is 73 A. Consequently, the critical current of between 73 A and 112 A is measured for the real winding. 
     FIG. 6  is a graph illustrating the comparison between alternating current losses generated depending upon currents applied to the two types of high-temperature superconducting wire as shown in  FIG. 4 . The calculated alternating current loss is a magnetization loss depending upon the applied current when alternating current of 60 Hz is applied in the case that the operation temperature is 77 K. It can be seen that the continuous disk winding generates the alternating current loss less than that of the conventional disk winding having the same number of turns as the continuous disk winding. The conventional disk winding, which is designed to compare with the continuous disk winding, is manufactured using only one disk. When the conventional disk winding is manufactured using a plurality of disks, on the other hand, resistance losses, which are generated at the connected parts of the disks, are further generated, and therefore, the losses are increased. 
   In the continuous disk winding manufactured based on the above-described design, the high-temperature superconducting wire is wound by five turns for each disk  25 , and the high-temperature superconducting wire is continuously wound for all of the disks, which is 23 in number. The total length of the high-temperature superconducting wire  10  used in the winding is approximately 61 m. The high-temperature superconducting wire  10  is threefold insulated before the high-temperature superconducting wire is used. For insulating tests, two sheets of Nomax tape are disposed between the turns to reinforce the insulation. Also, the thickness of each disk  25 , which is used to separate the high-temperature superconducting wire  10  located between the respective disks  25  from each other, is equal to the thickness of the high-temperature superconducting wire  10 . The material of the bobbin  21  is made of glass fiber reinforced plastic (GFRP), which has high mechanical characteristics and electrical insulation characteristics at low temperature.  FIG. 7  is a perspective view illustrating a continuous disk winding  20  for high-voltage superconducting transformers, which is manufactured as described above. The two current inlet terminals  23  are attached to the opposite fixing plates for measuring the critical current and the alternating current loss. 
     FIG. 8A  is a perspective view illustrating the shape of a disk according to the present invention. The disk  25  has a front end  261  formed at the inner circumferential surface thereof. From the front end  261 , the slit  26  extends along the disk  25  by an angular distance of 180 degrees with the result that a rear end  262  is formed in the disk  25 .  FIG. 8B  is a view illustrating a method of manufacturing a continuous disk winding for high-voltage superconducting transformers according to a preferred embodiment of the present invention. In order to form high-temperature superconducting wire layers, the disk  25  including the slit  26 , which has the front end  261  and the rear end  262 , is fitted onto the bobbin  21 . The high-temperature superconducting wire  10  is inserted into the front end  261  of the slit  26  as indicated by reference numeral  10   a , and is then moved to the rear end  262  of the slit  26  as indicated by reference numeral  10   b . Subsequently, the high-temperature superconducting wire  10  is wound on the next layer by the predetermined number of turns. The above winding method is repeatedly carried out. 
     FIG. 9  is a graph illustrating the results of a test for measuring critical current of a continuous disk winding according to the present invention. The critical current of the manufactured winding is approximately 97 A, which is within the range of the estimated values of the previously calculated critical current (73 A to 112 A). As a result, the method of manufacturing the continuous disk winding using the high-temperature superconducting wire can be applied to high-voltage superconducting transformers. 
   Consequently, the method of manufacturing the continuous disk winding according to the present invention can be applied particularly to a winding for high-voltage, high-temperature superconducting transformers, as a new method for accomplishing high voltage and large capacity thereof. 
   As apparent from the above description, the present invention provides a method of manufacturing a continuous disk winding for high-voltage superconducting transformers that has special features of no joint and low loss, which are advantages of a layer winding, while being formed in the shape of a disk winding, which is advantageous in voltage distribution and insulation. Consequently, the present invention has the effect of manufacturing a high-temperature superconducting winding without decrease of critical current and controlling voltage stress, whereby the continuous disk winding according to the present invention is applicable to high-voltage superconducting transformers. 
   Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.