Patent Publication Number: US-9837202-B2

Title: Stationary induction apparatus

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a stationary induction apparatus, and particularly to a stationary induction apparatus including an electrostatic shield. 
     Description of the Background Art 
     When an impulse voltage such as a lightning surge intrudes into a stationary induction apparatus such as a transformer or a reactor, the potential distribution within a winding becomes steep as compared with the potential distribution proportional to the turn number, and then, it oscillates around the potential distribution proportional to the turn number. This phenomenon is referred to as potential oscillation. When the amplitude of the potential oscillation is relatively large, a large potential difference occurs between the electric wires located adjacent to each other within the winding, and between the windings located adjacent to each other, which may cause a dielectric breakdown. When an electrostatic shield is arranged adjacent to the winding, a capacitance between the windings becomes larger than the capacitance between the winding and the ground, so that the amplitude of the potential oscillation is reduced. 
     As a prior art document, Japanese Utility Model Laying-Open No. 60-113614 discloses a transformer including an electrostatic shield. In the transformer disclosed in PTD 1, an electrostatic shield is provided at both ends of the winding in the central axis direction. Each of the ends of the electrostatic shield on the outer circumferential side and the inner circumferential side is formed of a curved surface. 
     The electrostatic shield of the transformer disclosed in Japanese Utility Model Laying-Open No. 60-113614 includes, on the side opposite to the side adjacent to a coil, an electric-field concentrating area at each of its ends on the outer and inner circumferential sides. When the electrostatic shield is configured to have a relatively large radius of curvature at each of its ends on the outer and inner circumferential sides in order to suppress the electric field concentration at each of the ends of the electrostatic shield on the outer and inner circumferential sides, the electrostatic shield is increased in thickness, so that the stationary induction apparatus is increased in size. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a stationary induction apparatus capable of suppressing concentration of an electric field at each of ends of the electrostatic shield on the outer circumferential side and the inner circumferential side while suppressing thickening of the electrostatic shield. 
     A stationary induction apparatus according to the present invention includes: an iron core; a plurality of windings wound around the iron core as a central axis and arranged so as to be coaxial with each other; and a plurality of electrostatic shields each formed in an annular shape and each arranged adjacent to an end of a corresponding one of the plurality of windings in a direction along the central axis. Each of the plurality of electrostatic shields includes an insulator portion and a conductor portion that is disposed annularly around the central axis on an inside of the insulator portion. The conductor portion includes a flat portion formed in an annular shape and extending in a circumferential direction of the central axis, and a pair of protruding portions protruding to an opposite side to each of the windings in the direction along the central axis, the pair of protruding portions each being arranged adjacent to a corresponding one of an outer circumferential end and an inner circumferential end of the flat portion. The insulator portion is provided with a first housing portion housing the flat portion and a pair of second housing portions each housing a corresponding one of the pair of protruding portions. Each of the pair of second housing portions has an inner surface located on the opposite side to each of the windings in the direction along the central axis, the inner surface being formed in a semicircular shape in a cross-sectional view. Each of the pair of protruding portions includes a protruding end portion located along the inner surface of a corresponding one of the pair of second housing portions, and a center portion located adjacent to the protruding end portion on a side of each of the windings in the direction along the central axis. In each of the pair of protruding portions, the protruding end portion and the center portion are electrically connected to each other and are equal in electric potential to each other. 
     According to the present invention, it becomes possible to suppress concentration of an electric field at each of ends of the electrostatic shield on the outer circumferential side and the inner circumferential side while suppressing thickening of the electrostatic shield. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing an external appearance of a stationary induction apparatus according to the first embodiment of the present invention. 
         FIG. 2  is a cross-sectional view of the stationary induction apparatus according to the first embodiment of the present invention, which is seen from the direction indicated by an arrow line II-II in  FIG. 1 . 
         FIG. 3  is a cross-sectional view of the stationary induction apparatus according to the first embodiment of the present invention, which is seen from the direction indicated by an arrow line in  FIG. 2 . 
         FIG. 4  is an exploded perspective view of the stationary induction apparatus according to the first embodiment of the present invention, which is seen from the direction indicated by an arrow IV in  FIG. 3 . 
         FIG. 5  is a cross-sectional view of the stationary induction apparatus according to the first embodiment of the present invention, which shows a V area in  FIG. 3  in an enlarged manner. 
         FIG. 6  is a cross-sectional view of the stationary induction apparatus according to the first embodiment of the present invention, which shows a VI area in  FIG. 5  in an enlarged manner. 
         FIG. 7  is a cross-sectional view of a stationary induction apparatus according to a modification of the first embodiment of the present invention. 
         FIG. 8  is a cross-sectional view of a stationary induction apparatus according to the second embodiment of the present invention. 
         FIG. 9  is a cross-sectional view of the stationary induction apparatus according to the second embodiment of the present invention, which shows an IX area in  FIG. 8  in an enlarged manner. 
         FIG. 10  is a cross-sectional view of a stationary induction apparatus according to a modification of the second embodiment of the present invention. 
         FIG. 11  is a perspective view showing an external appearance of a stationary induction apparatus according to the third embodiment of the present invention. 
         FIG. 12  is a partial cross-sectional view of the stationary induction apparatus according to the third embodiment of the present invention. 
         FIG. 13  is a cross-sectional view of the stationary induction apparatus according to the third embodiment of the present invention, which shows a XIII area in  FIG. 12  in an enlarged manner. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A stationary induction apparatus according to each embodiment of the present invention will be hereinafter described with reference to the accompanying drawings. In the following description of each embodiment, the same or corresponding components are designated by the same reference characters, and description thereof will not be repeated. 
     First Embodiment 
       FIG. 1  is a perspective view showing an external appearance of a stationary induction apparatus according to the first embodiment of the present invention.  FIG. 2  is a cross-sectional view of the stationary induction apparatus according to the first embodiment of the present invention, which is seen from the direction indicated by an arrow line II-II in  FIG. 1 .  FIG. 3  is a cross-sectional view of the stationary induction apparatus according to the first embodiment of the present invention, which is seen from the direction indicated by an arrow line in  FIG. 2 .  FIG. 4  is an exploded perspective view of the stationary induction apparatus according to the first embodiment of the present invention, which is seen from the direction indicated by an arrow IV in  FIG. 3 .  FIG. 5  is a cross-sectional view of the stationary induction apparatus according to the first embodiment of the present invention, which shows a V area in  FIG. 3  in an enlarged manner.  FIG. 6  is a cross-sectional view of the stationary induction apparatus according to the first embodiment of the present invention, which shows a VI area in  FIG. 5  in an enlarged manner. It is to be noted that  FIG. 1  does not show an electrostatic shield, and  FIG. 4  does not show an iron core. 
     As shown in  FIGS. 1 to 6 , a stationary induction apparatus  100  according to the first embodiment of the present invention is a core-type transformer. Stationary induction apparatus  100  includes: an iron core  110 ; and a low-voltage winding  120  and a high-voltage winding  130  that are wound around a main leg portion of iron core  110  as a central axis and that are arranged so as to be coaxial with each other. 
     Stationary induction apparatus  100  further includes a tank (not shown). The tank is filled with insulating oil or insulating gas that serves as an insulating medium and a cooling medium. Insulating oil is mineral oil, ester oil, or silicone oil, for example. Insulating gas is SF 6  gas or dry air, for example. Iron core  110 , low-voltage winding  120 , and high-voltage winding  130  are housed in the tank. 
     High-voltage winding  130  is located on the outside of low-voltage winding  120 . High-voltage winding  130  is formed by stacking a plurality of disc-shaped windings in the direction along the central axis. The plurality of disc-shaped windings are formed by winding a flat wire  140  in a disc shape. Flat wire  140  includes: a wire portion  141  having a cross section formed in an approximately rectangular shape; and an insulating coating portion  142  coating wire portion  141 . Although not shown, low-voltage winding  120  is similar in configuration to high-voltage winding  130 . 
     Stationary induction apparatus  100  further includes four electrostatic shields  150  each formed in an annular shape. Each of four electrostatic shields  150  is arranged adjacent to a corresponding one of ends of low-voltage winding  120  and high-voltage winding  130  in the direction along the central axis. Each of four electrostatic shields  150  is electrically connected to a line end of adjacent low-voltage winding  120  or a line end of adjacent high-voltage winding  130 , and is approximately equal in electric potential thereto. In addition to the purpose of reducing the amplitude of the potential oscillation, each of four electrostatic shields  150  is provided for the purpose of suppressing concentration of an electric field at each of the ends of low-voltage winding  120  and high-voltage winding  130  in the direction along the central axis. 
     Each of four electrostatic shields  150  includes an insulator portion and a conductor portion that is disposed annularly around the central axis on the inside of the insulator portion. The conductor portion includes: a flat portion  153  formed in an annular shape and extending in the circumferential direction of the central axis; and a pair of protruding portions protruding to the opposite side to each winding in the direction along the central axis. The pair of protruding portions each are arranged adjacent to a corresponding one of the outer circumferential end and the inner circumferential end of flat portion  153 . Flat portion  153  and the pair of protruding portions are electrically connected to each other. 
     The conductor portion in each of four electrostatic shields  150  is provided with slits at one or more portions so as to extend discontinuously in the circumferential direction. These slits can prevent a flow of a current that circulates through the entire circumference of each electrostatic shield  150 . In the present embodiment, slits are not provided in the insulator portion in each of four electrostatic shields  150 , but slits only have to be provided at positions in each electrostatic shield  150 , which are located to correspond to the slits in the conductor portion. 
     In the present embodiment, the insulator portion is formed of: a first insulator portion  151  located on the winding side in the direction along the central axis; and a second insulator portion  152  located on the opposite side to the winding in the direction along the central axis. First insulator portion  151  and second insulator portion  152  are bonded to each other with an adhesive that is applied over the entire surfaces of these insulator portions that face each other. 
     First insulator portion  151  and second insulator portion  152  each has an approximately rectangular outer shape in a cross-sectional view, but may have a curved portion in a cross-sectional view. It is to be noted that first insulator portion  151  and second insulator portion  152  each having a rectangular outer shape can be more easily produced and can more easily hold electrostatic shield  150 . 
     The insulator portion is provided with a first housing portion  152   a  housing flat portion  153  and a pair of second housing portions  152   c  each housing a corresponding one of the pair of protruding portions. In the present embodiment, second insulator portion  152  is provided with an annular groove portion serving as first housing portion  152   a  and the pair of second housing portions  152   c . First housing portion  152   a  is filled with flat portion  153 . 
     Each of the pair of second housing portions  152   c  has an inner surface located on the opposite side to the winding in the direction along the central axis. This inner surface is formed in a semicircular shape in a cross-sectional view. In addition, the semicircular shape also includes a semi-elliptical shape close to a semicircular shape. 
     Each of the pair of protruding portions in the conductor portion includes: a protruding end portion  154  located along the inner surface of a corresponding one of the pair of second housing portions  152   c ; and a center portion  155  located adjacent to protruding end portion  154  on the winding side in the direction along the central axis. In the present embodiment, in a cross-sectional view, protruding end portion  154  is formed in an arc shape while center portion  155  is formed in a semicircular shape. In each of the pair of protruding portions, center portion  155  is sandwiched between protruding end portion  154  and first insulator portion  151 . 
     As shown in  FIG. 6 , the inner surface of each of the pair of second housing portions  152   c  that is located on the opposite side to the winding in the direction along the central axis is covered by protruding end portion  154  such that three-quarters or more of a semicircle of the inner surface is covered in a cross-sectional view. In other words, in a cross-sectional view, an angle θ of less than 45° is formed between: a straight line connecting a center point C on the inner surface of each of the pair of second housing portions  152   c  and a tip end  154   t  of protruding end portion  154 ; and a boundary line between first insulator portion  151  and second insulator portion  152 . 
     Each of the pair of second housing portions  152   c  has a space  10  in which protruding end portion  154  and center portion  155  are not located. This space  10  is filled with insulating oil or insulating gas within the tank. In other words, the conductor portion is surrounded by first insulator portion  151 , second insulator portion  152 , and the insulating oil or the insulating gas within space  10 . In addition, each of first insulator portion  151  and second insulator portion  152  is higher in dielectric strength than each of the insulating oil and the insulating gas. 
     In each of the pair of protruding portions, protruding end portion  154  and center portion  155  are electrically connected to each other and are approximately equal in electric potential to each other. Specifically, at a connection portion (not shown), protruding end portion  154  and center portion  155  are connected to a line end of adjacent low-voltage winding  120  or a line end of adjacent high-voltage winding  130 . 
     Each of first insulator portion  151  and second insulator portion  152  is formed of a pressboard or compressed laminated wood. First insulator portion  151  and second insulator portion  152  may be made of the same material or may be made of different materials. In the present embodiment, the material forming second insulator portion  152  is less in relative permittivity than the material forming first insulator portion  151 . 
     Flat portion  153  and protruding end portion  154  each are formed of a metal wire mesh, metal foil, a conductive tape, or a conductive coating material. Center portion  155  is formed of a bare wire, a covered electric wire, or a conductive coating material. In the present embodiment, flat portion  153  and protruding end portion  154  each are formed of a conductive coating material. Center portion  155  is formed of a shield wire made of a bare wire or a covered electric wire. 
     In the case where protruding end portion  154  is formed of a conductive coating material, if the conductive coating material overflows second housing portion  152   c , an electric field concentrates on the portion across which the conductive coating material overflows, thereby causing a weak point therein in terms of insulation. Accordingly, it is necessary to prevent an overflow of the conductive coating material from second housing portion  152   c . Thus, in the present embodiment, a portion not covered by protruding end portion  154  is intentionally provided in the inner surface of each of the pair of second housing portions  152   c , as shown in  FIG. 6 . 
     In order to reduce the amplitude of the potential oscillation, electrostatic shield  150  needs to be equal in electric potential to low-voltage winding  120  or high-voltage winding  130  that is located adjacent to electrostatic shield  150  when an impulse voltage intrudes into stationary induction apparatus  100 . If the conductor portion has relatively high electrical resistivity, the following performance of the electric potential in electrostatic shield  150  may slow down, with the result that the potential oscillation may be unable to be sufficiently suppressed. Accordingly, it is preferable that the surface resistivity of the conductor portion is 10 Ω/sq or more and 50 Ω/sq or less. 
     In electrostatic shield  150  according to the present embodiment, protruding end portion  154  and center portion  155  are electrically connected to each other and are approximately equal in electric potential to each other. Accordingly, it becomes possible to suppress concentration of an electric field on the contact portion between protruding end portion  154  and center portion  155 , and also possible to suppress concentration of an electric field in space  10 . Furthermore, since electrostatic shield  150  is approximately equal in electric potential to low-voltage winding  120  or high-voltage winding  130  that is located adjacent to electrostatic shield  150 , it becomes possible to suppress concentration of an electric field on the contact portion between center portion  155  and first insulator portion  151 . 
     The following is an explanation about the reason why electrostatic shield  150  according to the present embodiment can suppress concentration of an electric field at each of the ends of electrostatic shield  150  on the outer circumferential side and the inner circumferential side while suppressing the electrostatic shield from thickening as compared with the conventional electrostatic shield. 
     In general, the strength of the electric field generated in the conductor portion is decreased as the distance from the conductor portion applied with a high voltage is increased. The smaller the radius of curvature is set at each of the ends of the conductor portion on the outer and inner circumferential sides, the greater the effect is achieved for reducing the electric field strength by the distance from the conductor portion. In the conventional electrostatic shield, it was difficult to provide a thick insulating coating for covering the entire surface of the conductor portion in terms of production. Accordingly, in order to suppress concentration of an electric field generated in the vicinity of the surface of each of the ends of the electrostatic shield on the outer and inner circumferential sides, the radius of curvature of each of the ends of the conductor portion on the outer and inner circumferential sides has been set to be relatively large. 
     In electrostatic shield  150  according to the present embodiment, the conductor portion is covered by first insulator portion  151  and second insulator portion  152  that are higher in dielectric strength than each of insulating oil and insulating gas. Thus, in electrostatic shield  150  according to the present embodiment, the radius of curvature of each of the ends of the conductor portion on the outer and inner circumferential sides can be set to be smaller than those in the conventional electrostatic shield. Specifically, the radius of curvature of protruding end portion  154  can be set to be relatively small. This can increase the effect of reducing the electric field strength achieved by the distance from the conductor portion, so that the electric field strength at each of the ends of electrostatic shield  150  on the outer and inner circumferential sides can be reduced. 
     As described above, in stationary induction apparatus  100  according to the present embodiment, electrostatic shield  150  can suppress concentration of an electric field at each of the ends of electrostatic shield  150  on the outer and inner circumferential sides, and also can reduce the amplitude of the potential oscillation. Furthermore, it is not necessary to thicken electrostatic shield  150 . In other words, stationary induction apparatus  100  can suppress concentration of an electric field at each of the ends of electrostatic shield  150  on the outer and inner circumferential sides while suppressing thickening of electrostatic shield  150 . 
     In stationary induction apparatus  100  according to the present embodiment, the material forming second insulator portion  152  is less in relative permittivity than the material forming first insulator portion  151 , thereby further increasing the effect of reducing the electric field strength achieved by the distance from the conductor portion. Consequently, it becomes possible to further reduce the electric field strength at each of the ends of electrostatic shield  150  on the outer and inner circumferential sides. 
     In stationary induction apparatus  100  according to the present embodiment, in a cross-sectional view, center portion  155  is formed in a semicircular shape, but may be formed in a circular shape.  FIG. 7  is a cross-sectional view of a stationary induction apparatus according to a modification of the first embodiment of the present invention.  FIG. 7  shows a cross-sectional view seen from the same direction as that in  FIG. 5 . 
     As shown in  FIG. 7 , in an electrostatic shield  150   x  of a stationary induction apparatus according to the modification of the first embodiment of the present invention, a pair of protruding portions in the conductor portion each includes: a protruding end portion  154  located along the above-described inner surface of a corresponding one of the pair of second housing portions  152   c ; and a center portion  155   x  located adjacent to protruding end portion  154  on the winding side in the direction along the above-described central axis. In the present modification, in a cross-sectional view, protruding end portion  154  is formed in an arc shape and center portion  155   x  is formed in a circular shape. In each of the pair of protruding portions, center portion  155   x  is sandwiched between protruding end portion  154  and first insulator portion  151 . 
     In the present modification, center portion  155   x  can be formed by a shield wire having a round shape in a cross-sectional view, so that electrostatic shield  150   x  can be readily produced. Also in the stationary induction apparatus according to the present modification, electrostatic shield  150   x  can suppress concentration of an electric field at each of the ends of electrostatic shield  150   x  on the outer and inner circumferential sides and also can reduce the amplitude of the potential oscillation. Furthermore, it is not necessary to thicken electrostatic shield  150   x . In other words, in the stationary induction apparatus according to the modification, it becomes possible to suppress concentration of an electric field at each of the ends of electrostatic shield  150   x  on the outer and inner circumferential sides while suppressing thickening of electrostatic shield  150   x.    
     Second Embodiment 
     A stationary induction apparatus according to the second embodiment of the present invention will be hereinafter described. The stationary induction apparatus according to the present embodiment is different only in electrostatic shield configuration from the stationary induction apparatus according to the first embodiment. Thus, the same configurations as those of the stationary induction apparatus according to the first embodiment will be designated by the same reference characters, and the description thereof will not be repeated. 
       FIG. 8  is a cross-sectional view of a stationary induction apparatus according to the second embodiment of the present invention.  FIG. 8  is a view shown in the same cross section as that in  FIG. 3 .  FIG. 9  is a cross-sectional view of the stationary induction apparatus according to the second embodiment of the present invention, which shows an IX area in  FIG. 8  in an enlarged manner. 
     As shown in  FIGS. 8 and 9 , a stationary induction apparatus  200  according to the second embodiment of the present invention includes four electrostatic shields  250  each formed in an annular shape and each arranged adjacent to a corresponding one of ends of a low-voltage winding  120  and a high-voltage winding  130  in the direction along the above-described central axis. 
     Each of four electrostatic shields  250  is electrically connected to a line end of adjacent low-voltage winding  120  or a line end of adjacent high-voltage winding  130 , and are approximately equal in electric potential thereto. In addition to the purpose of reducing the amplitude of the potential oscillation, each of four electrostatic shields  250  is disposed for the purpose of suppressing concentration of an electric field at a corresponding one of the ends of low-voltage winding  120  and high-voltage winding  130  in the direction along the central axis. 
     Each of four electrostatic shields  250  includes an insulator portion  252  and a conductor portion that is disposed annularly around the central axis on the inside of insulator portion  252 . The conductor portion includes: a flat portion  253  formed in an annular shape and extending in the circumferential direction of the central axis; and a pair of protruding portions each arranged adjacent to a corresponding one of the outer circumferential end and the inner circumferential end of flat portion  253 . The pair of protruding portions protrudes to the opposite side to the winding in the direction along the central axis. In the present embodiment, flat portion  253  and the pair of protruding portions are formed as an integrated component, but flat portion  253  and the pair of protruding portions may be formed separately from each other. 
     Insulator portion  252  has an approximately rectangular outer shape in a cross-sectional view, but may have a curved portion in a cross-sectional view. It is to be noted that insulator portion  252  having a rectangular outer shape can be more simply and readily produced and can more easily hold electrostatic shield  250 . In the present embodiment, insulator portion  252  is integrally formed. However, the insulator portion may be formed of the first insulator portion and the second insulator portion similar to electrostatic shield  150  according to the first embodiment. 
     Insulator portion  252  is provided with a first housing portion  252   a  housing flat portion  253  and a pair of second housing portions  252   c  each housing a corresponding one of the pair of protruding portions. In the present embodiment, insulator portion  252  is provided with an annular hole portion serving as first housing portion  252   a  and the pair of second housing portions  252   c.    
     First housing portion  252   a  is filled with flat portion  253 . Each of the pair of second housing portions  252   c  is filled with a corresponding one of the pair of protruding portions. In other words, the conductor portion is covered by insulator portion  252 . Accordingly, in the present embodiment, each of the pair of second housing portions  252   c  does not include a space in which a conductor portion is not located. In addition, insulator portion  252  is higher in dielectric strength than each of insulating oil and insulating gas. 
     Each of the pair of second housing portions  252   c  has an inner surface located on the opposite side to the winding in the direction along the central axis. This inner surface is formed in a semicircular shape in a cross-sectional view. In addition, the semicircular shape also includes a semi-elliptical shape close to a semicircular shape. 
     Each of the pair of protruding portions in the conductor portion includes: a protruding end portion  254  located along the inner surface of a corresponding one of the pair of second housing portions  252   c ; and a center portion  255  located adjacent to protruding end portion  154  on the winding side in the direction along the central axis. In the present embodiment, in a cross-sectional view, protruding end portion  254  is formed in a semi-arc shape while center portion  255  is formed in a semicircular shape. In each of the pair of protruding portions, center portion  255  is sandwiched between protruding end portion  254  and insulator portion  252 . In the present embodiment, protruding end portion  254  and center portion  255  are formed as an integrated component, but protruding end portion  254  and center portion  255  may be formed separately from each other. 
     Insulator portion  252  is made of thermosetting resin such as epoxy resin. The conductor portion is made of metal such as copper, stainless steel or aluminum, or an alloy thereof. Electrostatic shield  250  is formed, for example, by the insert casting method. 
     In order to reduce the amplitude of the potential oscillation, electrostatic shield  250  needs to be equal in electric potential to low-voltage winding  120  or high-voltage winding  130  that is located adjacent to electrostatic shield  250  when an impulse voltage intrudes into stationary induction apparatus  200 . If the conductor portion has relatively high electrical resistivity, the following performance of the electric potential in electrostatic shield  250  may slow down, with the result that the potential oscillation may be unable to be sufficiently suppressed. Accordingly, it is preferable that the surface resistivity of the conductor portion is 10 Ω/sq or more and 50 Ω/sq or less. 
     In electrostatic shield  250  according to the present embodiment, the conductor portion is covered by insulator portion  252  that is higher in dielectric strength than each of insulating oil and insulating gas. Thus, in electrostatic shield  250  according to the present embodiment, the radius of curvature of each of the ends of the conductor portion on the outer and inner circumferential sides can be set to be smaller than those in the conventional electrostatic shield. Specifically, the radius of curvature of protruding end portion  254  can be set to be relatively small. This can increase the effect of reducing the electric field strength achieved by the distance from the conductor portion, so that the electric field strength at each of the ends of electrostatic shield  250  on the outer and inner circumferential sides can be reduced. 
     As described above, also in stationary induction apparatus  200  according to the second embodiment of the present invention, electrostatic shield  250  can suppress concentration of an electric field at each of the ends of electrostatic shield  250  on the outer and inner circumferential sides, and also can reduce the amplitude of the potential oscillation. Furthermore, it is not necessary to thicken electrostatic shield  250 . In other words, in stationary induction apparatus  200  according to the second embodiment of the present invention, it becomes possible to suppress concentration of an electric field at each of the ends of electrostatic shield  250  on the outer and inner circumferential sides while suppressing thickening of electrostatic shield  250 . 
     In stationary induction apparatus  200  according to the present embodiment, in a cross-sectional view, center portion  255  is formed in a semicircular shape, but may be formed in a circular shape.  FIG. 10  is a cross-sectional view of a stationary induction apparatus according to a modification of the second embodiment of the present invention.  FIG. 10  shows a cross-sectional view seen from the same direction as that in  FIG. 9 . 
     As shown in  FIG. 10 , in electrostatic shield  250   x  of the stationary induction apparatus according to the modification of the second embodiment of the present invention, each of the pair of protruding portions in the conductor portion also protrudes toward the winding in the direction along the above-described central axis. Thus, the inner surface of each of the pair of second housing portions  252   cx  is formed in a circular shape in a cross-sectional view. It is to be noted that a circular shape also includes an elliptical shape close to a circular shape. 
     Each of the pair of protruding portions in the conductor portion includes: a protruding end portion  254   x  located along the inner surface of a corresponding one of the pair of second housing portions  252   cx ; and a center portion  255   x  located adjacent to protruding end portion  254   x . In the present modification, in a cross-sectional view, protruding end portion  254   x  is formed in a circular annular shape while center portion  255   x  is formed in a circular shape. In each of the pair of protruding portions, center portion  255   x  is surrounded by protruding end portion  254   x.    
     As compared with electrostatic shield  250  of stationary induction apparatus  200  according to the second embodiment of the present invention, electrostatic shield  250   x  according to the present modification can suppress occurrence of peeling at the interface between the conductor portion and insulator portion  252 , which is located on the winding side in the direction along the above-described central axis. Thereby, the insulation reliability of electrostatic shield  250   x  can be improved. 
     Third Embodiment 
     A stationary induction apparatus according to the third embodiment of the present invention will be hereinafter described. The stationary induction apparatus according to the present embodiment is mainly different from the stationary induction apparatus according to the first embodiment in that it is a shell-type transformer. Thus, the description of the same configuration as that of the stationary induction apparatus according to the first embodiment will not be repeated. 
       FIG. 11  is a perspective view showing an external appearance of a stationary induction apparatus according to the third embodiment of the present invention.  FIG. 12  is a partial cross-sectional view of the stationary induction apparatus according to the third embodiment of the present invention.  FIG. 13  is a cross-sectional view of the stationary induction apparatus according to the third embodiment of the present invention, which shows a XIII area in  FIG. 12  in an enlarged manner.  FIG. 11  does not show an electrostatic shield.  FIG. 12  shows only an area above the iron core. 
     As shown in  FIGS. 11 to 13 , a stationary induction apparatus  300  according to the third embodiment of the present invention is a shell-type transformer. Stationary induction apparatus  300  includes: an iron core  310 ; and a low-voltage winding  320  and a high-voltage winding  330  that are wound around a main leg portion of iron core  310  as a central axis and that are arranged to be coaxial with each other. 
     Stationary induction apparatus  300  further includes a tank  360 . Tank  360  is filled with insulating oil or insulating gas serving as an insulating medium and a cooling medium. Insulating oil is mineral oil, ester oil, or silicone oil, for example. Insulating gas is SF 6  gas or dry air, for example. Iron core  310 , low-voltage winding  320  and high-voltage winding  330  are housed in tank  360 . 
     High-voltage winding  330  is arranged so as to be sandwiched between low-voltage windings  320  in the direction along the above-described central axis. High-voltage winding  330  is formed by stacking a plurality of disc-shaped windings in the axial direction along the central axis. The plurality of disc-shaped windings are formed by winding a flat wire  340  in a disc shape. Flat wire  340  includes a wire portion  341  having a cross section formed in an approximately rectangular shape and an insulating coating portion  342  coating wire portion  341 . Although not shown, low-voltage winding  320  is similar in configuration to high-voltage winding  330 . 
     Stationary induction apparatus  300  further includes a plurality of electrostatic shields  350  each formed in an annular shape. Each of the plurality of electrostatic shields  350  is arranged adjacent to a corresponding one of ends of low-voltage winding  320  and high-voltage winding  330  in the direction along the central axis.  FIG. 12  shows only one electrostatic shield  350  located adjacent to high-voltage winding  330 . 
     Each of four electrostatic shields  350  includes an insulator portion and a conductor portion that is disposed annularly around the central axis on the inside of the insulator portion. The conductor portion includes: a flat portion  353  formed in an annular shape and extending in the circumferential direction of the central axis; and a pair of protruding portions each arranged adjacent to a corresponding one of the outer circumferential end and the inner circumferential end of flat portion  353 . The pair of protruding portions protrudes to the opposite side to the winding in the direction along the central axis. Flat portion  353  and the pair of protruding portions are electrically connected to each other. 
     The conductor portion in each of four electrostatic shields  350  is provided with slits at one or more portions so as to extend discontinuously in the circumferential direction. These slits can prevent a flow of a current that circulates through the entire circumference of electrostatic shield  350 . In the present embodiment, slits are not provided in the insulator portion in each of four electrostatic shields  350 , but slits only have to be provided at positions in each electrostatic shield  350 , which are located to correspond to the slits in the conductor portion. 
     In the present embodiment, the insulator portion is formed of: a first insulator portion  351  located on the winding side in the direction along the above-described central axis; and a second insulator portion  352  located on the opposite side to the winding in the direction along the above-described central axis. First insulator portion  351  and second insulator portion  352  are bonded to each other with an adhesive applied over the entire surfaces of these insulator portions that face each other. 
     First insulator portion  351  and second insulator portion  352  each have an approximately rectangular outer shape in a cross-sectional view, but each may have a curved portion in a cross-sectional view. It is to be noted that first insulator portion  351  and second insulator portion  352  each having a rectangular outer shape can be more simply and readily produced and can more easily hold electrostatic shield  350 . 
     The insulator portion is provided with a first housing portion  352   a  housing flat portion  353  and a pair of second housing portions  352   c  each housing a corresponding one of the pair of protruding portions. In the present embodiment, second insulator portion  352  is provided with an annular groove portion serving as first housing portion  352   a  and the pair of second housing portions  352   c . First housing portion  352   a  is filled with flat portion  353 . 
     Each of the pair of second housing portions  352   c  has an inner surface located on the opposite side to the winding in the direction along the central axis. The inner surface is formed in a semicircular shape in a cross-sectional view. In addition, the semicircular shape also includes a semi-elliptical shape close to a semicircular shape. 
     Each of the pair of protruding portions in the conductor portion includes: a protruding end portion  354  located along the inner surface of a corresponding one of the pair of second housing portions  152   c ; and a center portion  355  located adjacent to protruding end portion  354  on the winding side in the direction along the central axis. In the present embodiment, in a cross-sectional view, protruding end portion  354  is formed in an arc shape while center portion  355  is formed in a semicircular shape. In each of the pair of protruding portions, center portion  355  is sandwiched between protruding end portion  354  and first insulator portion  351 . 
     The inner surface of each of the pair of second housing portions  352   c  that is located on the opposite side to the winding in the direction along the central axis is covered by protruding end portion  354  such that three-quarters or more of a semicircle of the inner surface is covered in a cross-sectional view. In other words, in a cross-sectional view, an angle of less than 45° is formed between: a straight line connecting the center point on the inner surface of each of the pair of second housing portions  352   c  and the tip end of protruding end portion  354 ; and a boundary line between first insulator portion  351  and second insulator portion  352 . 
     Each of the pair of second housing portions  352   c  has a space  10  in which protruding end portion  354  and center portion  355  are not located. This space  10  is filled with insulating oil or insulating gas within tank  360 . In other words, the conductor portion is surrounded by first insulator portion  351 , second insulator portion  352 , and the insulating oil or the insulating gas within space  10 . In addition, each of first insulator portion  351  and second insulator portion  352  is higher in dielectric strength than each of the insulating oil and the insulating gas. 
     In each of the pair of protruding portions, protruding end portion  354  and center portion  355  are electrically connected to each other and are approximately equal in electric potential to each other. Specifically, at a connection portion (not shown), protruding end portion  354  and center portion  355  are connected to a line end of adjacent low-voltage winding  320  or a line end of adjacent high-voltage winding  330 . 
     Each of first insulator portion  351  and second insulator portion  352  is formed of a pressboard or compressed laminated wood. First insulator portion  351  and second insulator portion  352  may be made of the same material or may be made of different materials. In the present embodiment, the material forming second insulator portion  352  is less in relative permittivity than the material forming first insulator portion  351 . 
     Flat portion  353  and protruding end portion  354  each are formed of a metal wire mesh, metal foil, a conductive tape, or a conductive coating material. Center portion  355  is formed of a bare wire, a covered electric wire, or a conductive coating material. In the present embodiment, flat portion  353  and protruding end portion  354  each are formed of a conductive coating material. Center portion  355  is formed of a shield wire made of a bare wire or a covered electric wire. 
     In addition, in the case where protruding end portion  354  is formed of a conductive coating material, if the conductive coating material overflows second housing portion  352   c , an electric field concentrates on the portion across which the conductive coating material overflows, thereby causing a weak point therein in terms of insulation. Accordingly, it is necessary to prevent an overflow of the conductive coating material from second housing portion  352   c . Thus, in the present embodiment, a portion not covered by protruding end portion  354  is intentionally provided in the inner surface of each of the pair of second housing portions  352   c , as shown in  FIG. 13 . 
     In order to reduce the amplitude of the potential oscillation, electrostatic shield  350  needs to be equal in electric potential to low-voltage winding  320  or high-voltage winding  330  that is located adjacent to electrostatic shield  350  when an impulse voltage intrudes into stationary induction apparatus  300 . If the conductor portion has relatively high electrical resistivity, the following performance of the electric potential in electrostatic shield  350  may slow down, with the result that the potential oscillation may be unable to be sufficiently suppressed. Accordingly, it is preferable that the surface resistivity of the conductor portion is 10 Ω/sq or more and 50 Ω/sq or less. 
     In electrostatic shield  350  according to the present embodiment, protruding end portion  354  and center portion  355  are electrically connected to each other and are approximately equal in electric potential to each other. Accordingly, it becomes possible to suppress concentration of an electric field on the contact portion between protruding end portion  354  and center portion  355 , and also possible to suppress concentration of an electric field in space  10 . Furthermore, since electrostatic shield  350  is approximately equal in electric potential to low-voltage winding  320  or high-voltage winding  330  that is located adjacent to electrostatic shield  350 , it becomes possible to suppress concentration of an electric field on the contact portion between center portion  355  and first insulator portion  351 . 
     In electrostatic shield  350  according to the present embodiment, the conductor portion is covered by first insulator portion  351  and second insulator portion  352  that are higher in dielectric strength than each of insulating oil and insulating gas. Therefore, in electrostatic shield  350  according to the present embodiment, the radius of curvature of each of the ends of the conductor portion on the outer and inner circumferential sides can be set smaller than those in the conventional electrostatic shield. Specifically, the radius of curvature of protruding end portion  354  can be set relatively small. Thereby, since the effect of reducing the electric field strength achieved by the distance from the conductor portion is increased, the electric field strength at each of the ends of electrostatic shield  350  on the outer and inner circumferential sides can be reduced. 
     As described above, also in stationary induction apparatus  300  according to the present embodiment, electrostatic shield  350  can suppress concentration of an electric field at each of the ends of electrostatic shield  350  on the outer and inner circumferential sides and also can reduce the amplitude of the potential oscillation. Furthermore, it is not necessary to thicken electrostatic shield  350 . In other words, in stationary induction apparatus  300 , it becomes possible to suppress concentration of an electric field at each of the ends of electrostatic shield  350  on the outer and inner circumferential sides while suppressing thickening of electrostatic shield  350 . 
     In stationary induction apparatus  300  according to the present embodiment, the material forming second insulator portion  352  is less in relative permittivity than the material forming first insulator portion  351 , thereby further increasing the effect of reducing the electric field strength achieved by the distance from the conductor portion. Consequently, it becomes possible to further reduce the electric field strength at each of the ends of electrostatic shield  350  on the outer and inner circumferential sides. 
     In addition, the configuration of electrostatic shield  350  it not limited to those as described above, but may be the same as the configuration of the modification described in the first embodiment, the configuration of the second embodiment, or the configuration of the modification described in the second embodiment. 
     In the description of the embodiments set forth above, a core-type transformer and a shell-type transformer have been described as a stationary induction apparatus, but the stationary induction apparatus may be other types of stationary induction apparatuses such as a reactor. 
     Although the embodiments of the present invention have been described as above, it should be understood that the embodiments disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, and is intended to include any modifications within the meaning and scope equivalent to the terms of the claims.