Patent Publication Number: US-8124878-B2

Title: Apparatus for electrical screening of a high-voltage bushing

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to an apparatus for electrical screening of a bushing output of a high-voltage line into the outer wall of an electrical high-voltage device, with the screen comprising an electrically conductive screen interior and an outer insulating layer which surrounds the screen interior. 
     With electrical high-voltage devices, such as power transformers for example, it must be guaranteed that there is adequate electrical screening in the region of the electrical connection and the transition of the electrical line into the high-voltage device in order to allow electrical connection to a high-voltage network. Particularly in the case of high-voltage direct current (HVDC transmission) equipment, it is absolutely essential that the bushing and the wall of the electrical high-voltage device facing the bushing are adequately electrically screened. Adequately electrically screened means a reduction in the electrical field strength which occurs on the surface of the screen and in the surrounding space. 
     According to the prior art, for reasons of high-voltage screening, the edge transitions and corners between the bushing of the high-voltage line and the wall of the tank perpendicular thereto are screened in such a way that a so-called screen wire is arranged around the bushing outlet within the electrical high-voltage device. Traditionally, this screen wire is provided with an electrically conducting screen interior which is electrically insulated on the surface, in particular by a paper insulation. In variance with this, differently formed screening rings, which have a rectangular electrically conducting screen interior, are also disclosed in the prior art. 
     U.S. Pat. No. 4,296,274, for example, describes a high-voltage bushing, which has a weather-tight screen. According to this invention, an advantageous guiding of the electrical field is achieved by additionally thickening the appropriate transition region of the bushing. 
     Likewise WO 00/19453 describes a high-voltage bushing with a high-voltage conductor in an insulator equipped with insulating screens, which has a flange on one end through which the high-voltage line is fed, an insulation medium being provided between flange and high-voltage conductor. In addition, a screen electrode is arranged around the insulator which extends from the flange in an axial direction over part of the length of the insulator, the number of insulating screens per unit length being different in the individual sections. 
     A disadvantage of all the screening solutions for high-voltage bushings disclosed in the prior art is that the previous solutions are either structurally elaborate or do not guarantee complete screening of the almost right-angled bushing output. In particular, the electrical screening of the almost right-angled corners and edges of the bushing output relative to the tank wall is not fully guaranteed by the solutions provided in the prior art. 
     BRIEF SUMMARY OF THE INVENTION 
     The object of the present invention is therefore to provide an electrical screen, which guarantees complete screening of the corners and edges of a bushing output in an electrical high-voltage device. 
     This object is achieved by an apparatus for electrical screening of an almost right-angled bushing output of a high-voltage line into an outer wall of an electrical high-voltage device. The screen comprises an electrically conductive screen interior and an outer insulating layer which surrounds the screen interior. The screen is arranged in the form of a curved circular segment around the almost right-angled bushing output and the limbs are arranged almost parallel to the alignment of the bushing and to the alignment of the tank wall. According to the invention, the object is achieved in that the screen is designed in the form of a curved circular segment around the almost right-angled outlet of the bushing. This guarantees that the field characteristic of the electrical field is changed in the vicinity of the almost right-angled bushing output. Traditionally, the field characteristic of the electrical field has a high degree of complexity in the vicinity of corners and edges, and has previously only been inadequately ensured by an electrical screen. However, by means of the apparatus according to the invention, it is possible to influence the characteristic of the electrical field in the vicinity of the corners and edges so that an almost complete screening of the corners and edges is guaranteed in the vicinity of the bushing output. The embodiment of the electrical screen according to the invention in the form of a curved circular segment with two long limbs influences the characteristic of the electrical field in the area surrounding the bushing output in such a way that the housing of the electrical high-voltage device is completely electrically screened in the area of the bushing output. 
     In an advantageous embodiment of the apparatus according to the invention, the electrical screen is positioned around the almost right-angled bushing output so that the radius of the curved circular segment runs through the corner of the almost right-angled bushing output. As a result of this, the screen is positioned symmetrically with respect to the two edges of the almost right-angled bushing output. 
     Furthermore, in an advantageous embodiment of the apparatus according to the invention, it is provided that the form of the screen corresponds to an idealized field characteristic of the electrical field in the area around the bushing output. 
     Advantageously, it is provided that the radius of the curved circular segment of the screen is less than four times the length of the limbs. This guarantees screening of the appropriate corners and edges of the bushing output from all sides. Likewise, the radius preferably has a length of 10 mm to 500 mm, preferably 100 mm. In an advantageous embodiment of the apparatus, it is provided that the screen is screened by an additional barrier system. These barrier systems, which are already used with line bushings, are likewise combined with the screen according to the invention in the form of a screening barrier system around the output of the bushing. An effective electrical screening of the corners and edges of the bushing output with respect to the outer wall of the electrical high-voltage device is guaranteed, particularly for HVDC transmission applications, because of the high transmission voltages possible. Advantageously, the barrier system is mechanically connected to the barrier system of the bushing and/or the barrier system of the electrical high-voltage device. 
     It is seen to be an advantage that the screen is formed around the bushing output as a circumferential one-piece element. This means that the screen can be produced cost effectively and results in a dimensionally stable effect of the screen. Alternatively, it is provided that individual screen segments are arranged in the form of a circle around the bushing output. The same applies to an appropriate barrier system for additional screening. Advantageously, the screen is connected to the electrical ground. 
     Furthermore, it is possible to use this electrical screen simultaneously as a magnetic screen if the screen is made from conductive material. Local heating caused by eddy currents, which can lead to considerable heating and possibly damage to the electrical high-voltage device, would then be prevented. Advantageously, the screen consists at least partially of conductive material which is partially interspersed with non-conductive material. 
     Furthermore, in an advantageous embodiment of the apparatus, the screen consists of a carrier material, which is at least partially sheathed, in particular coated or sheathed, with a conductive layer. 
     Further advantageous embodiments of the invention can be found in the dependent claims. The invention is explained in more detail in the drawing with reference to the exemplary embodiments. In the drawing: 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  shows a schematic section drawing of the screen; 
         FIG. 2  shows a schematic section drawing of the screen around a line bushing with additional barrier systems. 
     
    
    
     DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a schematic section drawing of the structure of the screen  10  according to the invention in principle. The dotted line represents the axis of rotation of the screen  10 . The screen interior  11  is insulated, at least with respect to the inside of the electrical high-voltage device, with an additional insulation layer  12 . Traditionally, this is a paper insulation. An insulation medium, in particular an oil, is provided within the electrical high-voltage device in the area around the screen  10 . The screen  10 , which is arranged almost at right angles around a bushing output (not shown), has two limbs  23  which are part of the curved circular segment. The length of the limbs is defined with regard to the transition of the curved circular segment to the straight limbs  23  in each case. The radius  22  defines the form of the curved circular segment of the screen  10  and runs through the corner (not shown) of the line bushing output  21  (not shown). 
       FIG. 2  shows a screen  10  with an additional barrier system  14  around a bushing output  21  for an electrical high-voltage line. The dotted line represents the axis of rotation of the screen  10 , the circular bushing  15  and the circular output cable  16 . An output cable  16  with a paper insulation  17 , which enables the high-voltage device to be electrically connected to an external high-voltage network, in particular an HVDC transmission network, is arranged within the bushing  15 . The screen  10  is arranged with respect to the almost right-angled bushing output  21  in such a way that a symmetrical covering of both angles of the bushing  15  and the tank wall  20  around the corner point of the almost right-angled bushing output  21  is guaranteed. In conjunction with the circulating oil within the electrical high-voltage device and the bushing  15 , the additional barrier system  14  around the screen  10  guarantees an optimum electrical screening of the corners of the bushing output  21 . Advantageously, the barrier system  14  of the screen  10  can be connected to the barrier system  14  within the bushing  15  and/or the electrical high-voltage device so that a continuous barrier system  14  is produced from the bushing output  21  right into the bushing  15 .