Patent Publication Number: US-2022216004-A1

Title: 110 kV Three-phase Dry-type Transformer and Assembly Method Therefor

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims the priority to a Chinese patent application with the filing No. CN201910366006.3 filed with the State Intellectual Property Office on Apr. 30, 2019 and entitled “110 kV three-phase dry-type transformer” the content of which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present application relates to the technical field of power transformers, in particular to a 110 kV three-phase dry-type transformer and an assembly method therefor. 
     BACKGROUND ART 
     At present, the existing three-phase dry-type transformers are mostly applied for a low voltage level, for example 35 kV. When applied for a high voltage level, the existing three-phase dry-type transformers have insufficient insulation capacity and cannot meet the requirement for the insulating voltage resistance capability at a high voltage level. 
     SUMMARY 
     An object of an exemplary embodiment of the present application comprises, for example, providing a three-phase dry-type transformer for meeting the requirement for the insulating voltage resistance capability of the three-phase dry-type transformer at a high voltage level, for example, a 110 kV voltage level. 
     An object of an exemplary embodiment of the present application comprises, for example, providing an assembly method for assembling single-phase dry-type transformers into the above three-phase dry-type transformer. 
     In order to achieve the above objects, an exemplary embodiment of the present application provides the technical solutions as follows. 
     An embodiment of the present application provides a three-phase dry-type transformer comprising three single-phase dry-type transformers. Each single-phase dry-type transformer comprises an incoming line side and an outgoing line side. The incoming line side comprises an incoming-line-side iron core column, a first low-voltage coil and an incoming-line-side high-voltage coil, which are provided at intervals in sequence. The outgoing line side comprises an outgoing-line-side iron core column, a second low-voltage coil and an outgoing-line-side high-voltage coil, which are provided at intervals in sequence, and wherein in an exemplary configuration of the single-phase dry-type transformer, the outgoing-line-side iron core column and the above incoming-line-side iron core column are provided opposite to each other. The incoming-line-side high-voltage coil comprises a first incoming-line-side high-voltage coil and a second incoming-line-side high-voltage coil connected in parallel with the first incoming-line-side high-voltage coil. The outgoing-line-side high-voltage coil comprises a first outgoing-line-side high-voltage coil and a second outgoing-line-side high-voltage coil connected in parallel with the first outgoing-line-side high-voltage coil. A first end for connecting in parallel the first incoming-line-side high-voltage coil and the second incoming-line-side high-voltage coil is provided in the middle of or substantially at the middle of the incoming-line-side high-voltage coil, to be used as a first incoming line end, and optionally, a second end for connecting in parallel the first incoming-line-side high-voltage coil and the second incoming-line-side high-voltage coil is provided at an end of the incoming-line-side high-voltage coil, to be used as a first outgoing line end. A first end for connecting in parallel the first outgoing-line-side high-voltage coil and the second outgoing-line-side high-voltage coil is provided in the middle of or substantially at the middle of the outgoing-line-side high-voltage coil, to be used as a second incoming line end, and optionally, a second end for connecting in parallel the first outgoing-line-side high-voltage coil and the second outgoing-line-side high-voltage coil is provided at an end of the outgoing-line-side high-voltage coil, to be used as a second outgoing line end. In this single-phase dry-type transformer, the first outgoing line end and the second incoming line end are connected. The three single-phase dry-type transformers form the three-phase dry-type transformer through connection of their corresponding three second outgoing line ends. 
     The beneficial effects achieved by the three-phase dry-type transformer provided by the embodiment comprise: by providing the incoming line end of the three-phase dry-type transformer in the middle of or substantially at the middle of the incoming-line-side high-voltage coil of each single-phase dry-type transformer, for example, the lightning surge voltage resistance capability of the incoming line end of each single-phase dry-type transformer is improved, that is, for example, the voltage resistance capability of the high voltage coil of each single-phase dry-type transformer is improved, and then the voltage resistance capability of, for example, the combined three-phase dry-type transformer is improved. The three-phase dry-type transformer meets the requirement for the insulating voltage resistance capability of the three-phase dry-type transformer at a high voltage level, for example, a 110 kV voltage level, thus solving the problem in the prior art that the three-phase dry-type transformers have insufficient insulation capacity at a high voltage level, for example, a 110 kV voltage level. Further, this three-phase dry-type transformer contains no flammable transformer oil or SF6 gas resulting in greenhouse effect, and has a strong resistance capacity to short-circuit. This three-phase dry-type transformer is a flame-retardant and environmentally friendly power transformer. 
     Optionally, the first incoming line end is formed by connecting line ends of the first incoming-line-side high-voltage coil and of the second incoming-line-side high-voltage coil, which line ends are adjacent to each other in proximity to a position substantially the middle of the incoming-line-side high-voltage coil. 
     Optionally, the second incoming line end is formed by connecting line ends of the first outgoing-line-side high-voltage coil and of the second outgoing-line-side high-voltage coil, which line ends are adjacent to each other in proximity to a position substantially the middle of the outgoing-line-side high-voltage coil. 
     Optionally, the single-phase dry-type transformer further comprises a first no-load voltage-regulating tap terminal and a second no-load voltage-regulating tap terminal, wherein the first no-load voltage-regulating tap terminal is provided on the first incoming-line-side high-voltage coil, and the second no-load voltage-regulating tap terminal is provided on the second incoming-line-side high-voltage coil. 
     With the three-phase dry-type transformer in the optional configuration, after the first no-load voltage-regulating tap terminal and the second no-load voltage-regulating tap terminal are regulated, a rated high voltage at which the transformer operates can be changed within a no-load regulation range. 
     Optionally, the single-phase dry-type transformer further comprises an on-load voltage-regulating tap switch, a first on-load voltage-regulating tap terminal and a second on-load voltage-regulating tap terminal. The first on-load voltage-regulating tap terminal is provided on the first outgoing-line-side high-voltage coil, and the second on-load voltage-regulating tap terminal is provided on the second outgoing-line-side high-voltage coil. The first on-load voltage-regulating tap terminal comprises a first common end and a plurality of first tap ends. The second on-load voltage-regulating tap terminal comprises a second common end and a plurality of second tap ends. The first common end is connected in parallel with the second common end, and each of the plurality of first tap ends is connected in parallel with a corresponding tap end of the plurality of second tap ends, and they are connected with the on-load voltage-regulating tap switch after the parallel connection. In this optional configuration, the three-phase dry transformer can achieve nine different ranges of on-load voltage regulations. 
     Optionally, at least one of the first incoming line end, the second incoming line end, the first outgoing line end, and the second outgoing line end is provided with a capacitive screen line. Regarding the three-phase dry-type transformer in this optional configuration, the capacitive screen line can increase the series capacitance at the head and end of the coil, improve the lightning surge voltage distribution, and thus increase the lightning surge voltage resistance capability of the coil. 
     Optionally, at least one of the first incoming line end, the second incoming line end, the first outgoing line end, and the second outgoing line end is provided with a voltage equalizing cover. Regarding the three-phase dry-type transformer in this optional configuration, the voltage equalizing cover can improve the electric field distribution outside the high voltage coil, and improve the voltage resistance capability of the high voltage coil. 
     Optionally, a first insulating cylinder is provided between the first low-voltage coil and the incoming-line-side high-voltage coil, and/or a second insulating cylinder is provided between the second low-voltage coil and the outgoing-line-side high-voltage coil. 
     Optionally, a material of the insulating cylinder is an epoxy resin plus a glass fiber. 
     Optionally, an insulating partition is provided between the incoming-line-side high-voltage coil and the outgoing-line-side high-voltage coil. 
     Optionally, at least one of the first low-voltage coil, the second low-voltage coil, the incoming-line-side high-voltage coil, and the outgoing-line-side high*voltage coil is an epoxy resin vacuum-cast solid coil which is a copper conductor (i.e. a solid coil formed of conductive copper by epoxy resin vacuum casting). 
     Optionally, the single-phase dry-type transformer further comprises a plurality of solid insulating members. The plurality of solid insulating members are respectively configured to support and fix the first low-voltage coil, the second low-voltage coil, the incoming-line-side high-voltage coil, and the outgoing-line-side high-voltage coil. Regarding the three-phase dry-type transformer in this optional configuration, the plurality of solid insulating members enable the low voltage coils and the high voltage coils to be fixed by them, and thus not loosen due to the transportation process or a complicated relation. 
     An embodiment of the present application further provides an assembly method which can be used to assemble three single-phase dry-type transformers to obtain the above three-phase dry-type transformer, especially a 110 kV three-phase dry-type transformer. The method comprises providing three single-phase dry-type transformers, each single-phase dry-type transformer comprising an incoming line side and an outgoing line side, wherein the incoming line side comprises an incoming-line-side high-voltage coil that provides a first incoming line end and a first outgoing line end, the first incoming line end is located at substantially the middle of the incoming-line-side high-voltage coil, and the outgoing line side comprises an outgoing-line-side high-voltage coil that provides a second incoming line end and a second outgoing line end, and the second incoming line end is located at substantially the middle of the outgoing-line-side high-voltage coil; connecting the first outgoing line end and the second incoming line end of each single-phase dry-type transformer so that the incoming-line-side high-voltage coil and the outgoing-line-side high-voltage coil are connected in series; and making the three single-phase dry-type transformers connected and assembled through their corresponding three second outgoing line ends after the series connection to form the three-phase dry-type transformer. 
     The assembly method provided in the embodiment obtains a three-phase dry-type transformer that can effectively operate at a high voltage level, especially a 110 kV voltage level, by assembling three specially designed single-phase dry-type transformers. Specifically, the incoming line end of the three-phase dry-type transformer obtained by the assembly method is provided in the middle of or substantially at the middle of the incoming-line-side high-voltage coil of each single-phase dry-type transformer, thus, for example, the lightning surge voltage resistance capability of the incoming line end of each single-phase dry-type transformer is improved, that is, for example, the voltage resistance capability of the high voltage coil of each single-phase dry-type transformer is improved, and then the voltage resistance capability of, for example, the assembled three-phase dry-type transformer is improved. 
     Other features and advantages of the embodiments of the present application will be described in the following description, and partly become obvious from the description, or can be understood by implementing the embodiments of the present application. The objects and other advantages of the present application can be implemented and obtained through the structure specifically indicated in the drafted description and the drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       In order to more clearly illustrate the technical solutions of the exemplary embodiments of the present application, the drawings required to be used in the exemplary embodiments of the present application will be briefly introduced below. It should be understood that the following drawings show only some embodiments of the present application and therefore should not be considered as a limitation to the scope, and those skilled in the art may obtain other related drawings in the light of the drawings without any inventive labor. 
         FIG. 1  is a schematic view of a three-phase composition of a three-phase dry-type transformer provided by an embodiment of the present application; 
         FIG. 2  is a structural schematic view of a single-phase dry-type transformer provided by an embodiment of the present application; 
         FIG. 3  is a connection schematic view of an on-load voltage-regulating tap terminal provided by an embodiment of the present application; 
         FIG. 4  is a schematic view of a capacitive screen line provided by an embodiment of the present application; 
         FIG. 5  is a structural schematic view of a voltage equalizing cover provided by an embodiment of the present application; 
         FIG. 6  is a structural schematic view of an outer profile of a single-phase dry-type transformer provided by an embodiment of the present application; and 
         FIG. 7  is a schematic view of a flowchart of an assembly method provided by an embodiment of the present application. 
     
    
    
     Reference signs:  1 —single-phase dry-type transformer;  201 —incoming-line-side iron core column;  202 —first low-voltage coil;  203 —incoming-line-side high-voltage coil;  2031 —first incoming line end;  2032 —first outgoing line end;  2033 —first incoming-line-side high-voltage coil;  2034 —second incoming-line-side high-voltage coil;  301 —outgoing-line-side iron core column;  302 —second low-voltage coil;  303 —outgoing-line-side high-voltage coil;  3031 —second incoming line end;  3032 —second outgoing line end;  3033 —first outgoing-line-side high-voltage coil;  3034 —second outgoing-line-side high-voltage coil;  40 —first no-load voltage-regulating tap terminal;  50 —second no-load voltage-regulating tap terminal;  60 —on-load voltage-regulating tap switch;  70 —first on-load voltage-regulating tap terminal;  701 —first common end;  80 —second on-load voltage-regulating tap terminal;  801 —second common end;  90 —capacitive screen line;  91 —voltage equalizing cover;  92 —first insulating cylinder;  93 —second insulating cylinder;  94 —insulating partition;  95 —solid insulating member. 
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     In order to make the objects, technical solution and advantages of the embodiments of the present application clearer, the technical solutions of the embodiments of the present application will be clearly and completely described in the following with reference to the accompanying drawings, and it is apparent that the described embodiments are some but not all of the embodiments of the present application. The components of the embodiments of the present application, which are generally described and illustrated in the figures herein, may be arranged and designed in a variety of different configurations. 
     Therefore, the detailed description of the embodiments of the present application set forth in the accompanying drawings is not intended to limit the claimed scope of the present application, but illustrate only selected embodiments of the present application. All other embodiments, obtained by those skilled in the art in light of the embodiments of the present application without inventive efforts, will fall within the claimed scope of the present application. 
     It should be noted that similar reference numerals and letters indicate similar items in the following figures, and therefore, once an item is defined in a drawing, it is not necessary to further define or explain it in the subsequent drawings. 
     It should be noted that, in the embodiments of the present application, terms such as “first” and “second” are used only for distinguishing the description, and should not be understood as indicating or implying to have importance in relativity. 
     According to an exemplary embodiment of the present application, a three-phase dry-type transformer capable of effectively operating at a high voltage level, for example, a 110 kV voltage level, is provided. 
     Referring to  FIG. 1 , the three-phase dry-type transformer comprises three single-phase dry-type transformers  1 . Each single-phase dry-type transformer  1  comprises an incoming line side and an outgoing line side. 
     Specifically, referring to  FIG. 2 , the incoming line side comprises an incoming-line-side iron core column  201 , a first low-voltage coil  202  and an incoming-line-side high-voltage coil  203 , wherein the incoming-line-side high-voltage coil  203  comprises a first incoming-line-side high-voltage coil  2033  and a second incoming-line-side high-voltage coil  2034  which are connected in parallel; and the outgoing line side comprises an outgoing-line-side iron core column  301 , a second low-voltage coil  302  and an outgoing-line-side high-voltage coil  303 , wherein the outgoing-line-side high-voltage coil  303  comprises a first outgoing-line-side high-voltage coil  3033  and a second outgoing-line-side high voltage coil  3034  which are connected in parallel. 
     In the configuration shown in  FIG. 2 , the incoming-line-side iron core column  201  and the outgoing-line-side iron core column  301  are provided opposite to each other. Further, the incoming-line-side iron core column  201 , the first low-voltage coil  202  and the incoming-line-side high-voltage coil  203  are provided in sequence at an interval, and similarly, the outgoing-line-side iron core column  301 , the second low-voltage coil  302  and the outgoing-line-side high-voltage coil  303  are provided in sequence at an interval. Those skilled in the art can understand that this configuration is only exemplary instead of restricting. 
     A first end, i.e., a first parallel terminal, formed by connecting in parallel the first incoming-line-side high-voltage coil  2033  and the second incoming-line-side high-voltage coil  2034  is provided in the middle or substantially at the middle of the incoming-line-side high-voltage coil  203  as the first incoming line end  2031  (i.e., the middle incoming line end as shown in  FIG. 2 ). A second end, i.e., a second parallel terminal, formed by connecting in parallel the first incoming-line-side high-voltage coil  2033  and the second incoming-line-side high-voltage coil  2034  is provided at an end of the incoming-line-side high-voltage coil  203  as the first outgoing line end  2032 . 
     Similarly, a first end formed by connecting in parallel the first outgoing-line-side high-voltage coil  3033  and the second outgoing-line-side high-voltage coil  3034  is provided in the middle of or substantially at the middle of the outgoing-line-side high-voltage coil  303  as a second incoming line end  3031 . A second end formed by connecting in parallel the first outgoing-line-side high-voltage coil  3033  and the second outgoing-line-side high-voltage coil  3034  is provided at one end of the outgoing-line-side high-voltage coil  303 , to be used as a second outgoing line end  3032 . 
     As shown in  FIG. 2 , in the single-phase dry-type transformer of the embodiment, the first outgoing line end  2032  is made to be connected with the second incoming line end  3031 , so that the incoming-line-side high-voltage coil and the outgoing-line-side high-voltage coil are connected with each other in series. 
     Based on the single-phase dry-type transformer in the above configuration, such three single-phase dry-type transformers  1  are assembled into a three-phase dry-type transformer through connection of their corresponding three second outgoing line ends  3032 , the three-phase dry-type transformer being capable of operating normally at a high voltage level, for example, a 110 kV voltage level. 
     To facilitate the understanding, the single-phase dry-type transformer with a reference sign A (referred to as an A-phase transformer for short), the single-phase dry transformer with a reference sign B (referred to as a B-phase transformer for short), and the single-phase transformer with a reference sign C (referred to as a C-phase transformer for short) are shown in  FIG. 1 . For distinguishing here, the first incoming line end  2031  and the second outgoing line end  3032  of each single-phase dry-type transformer are distinguished by different reference signs. Specifically, the incoming line end of the A-phase transformer is indicated by A 1 , and the outgoing line end thereof is indicated by N 1 ; the incoming line end of the B-phase transformer is indicated by B 1 , and the outgoing line end thereof is indicated by N 2 ; the incoming line end of the C-phase transformer is indicated by C 1 , and the outgoing line end thereof is indicated by N 3 . As can be seen from  FIG. 1 , the three-phase dry-type transformer consists of three single-phase dry-type transformers connected though N 1 , N 2  and N 3 . Each single-phase dry-type transformer may further comprise an on-load switch  60  (please see  FIG. 3 ) to be provided at a corresponding outgoing line end. 
     Turning to  FIG. 2  again, the incoming-line-side iron core column  201  and the outgoing-line-side iron core column  301  can be combined to form an iron core. Optionally, the iron core is of a square shape. The right portion of the iron core is the incoming-line-side core column  201 , and the incoming-line-side core column  201  is provided outside with a first low-voltage coil  202  and an incoming-line-side high-voltage coil  203  respectively. The left portion of the iron core is outgoing-line-side core column  301 , and the outgoing-line-side core column  301  is provided outside with a second low-voltage coil  302  and an outgoing-line-side high-voltage coil  303  respectively. 
     In the configuration shown in  FIG. 2 , the first ends configured to connect in parallel the first incoming-line-side high-voltage coil  2033  and the second incoming-line-side high-voltage coil  2034  may be adjacent line ends with a closer distance after the first incoming-line-side high-voltage coil  2033  and the second incoming-line-side high-voltage coil  2034  are connected in parallel; the second ends configured to connect in parallel the first incoming-line-side high-voltage coil  2033  and the second incoming-line-side high-voltage coil  2034  may be adjacent line ends with a far distance after the first incoming-line-side high-voltage coil  2033  and the second incoming-line-side high-voltage coil  2034  are connected in parallel. The first outgoing-line-side high-voltage coil  3033  and the second outgoing-line-side high-voltage coil  3034  of the outgoing line side portion may be configured in a similar fashion. 
     It should be noted that the forgoing first incoming-line-side high-voltage coil  2033  and second incoming-line-side high-voltage coil  2034  may have consistent sizes, and turns; and the first outgoing-line-side high-voltage coil  3033  and the second outgoing-line-side high-voltage coil  3034  may also have consistent sizes, and turns. 
     With the three-phase dry-type transformer provided by the exemplary embodiment, by providing the incoming line end of the three-phase transformer at a position substantially the middle of the incoming-line-side high-voltage coil of each single-phase transformer, for example, the lightning surge voltage resistance capability of the incoming line end of each single-phase transformer is improved, that is, for example, the voltage resistance capability of the high voltage coil of each single-phase transformer is improved, and then the voltage resistance capability of the combined three-phase dry-type transformer is improved, which meets the requirement for the insulating voltage resistance capability of the three-phase dry-type transformer at a high voltage level, for example, a 110 kV voltage level. This three-phase dry-type transformer solves the problem in the prior art that the three-phase dry-type transformer has insufficient insulation performance, and this three-phase dry-type transformer contains no flammable transformer oil or SF6 gas resulting in the greenhouse effect, and has a strong resistance capacity to short-circuit. This three-phase dry-type transformer is a flame-retardant and environmentally friendly power transformer. 
     In an optional example of the embodiment, and as further shown in  FIG. 2 , the single-phase dry-type transformer  1  may further comprise a first no-load voltage-regulating tap terminal  40  and a second no-load voltage-regulating tap terminal  50 . Optionally, the first no-load voltage-regulating tap terminal  40  is provided on the first incoming-line-side high-voltage coil  2033 , and optionally, the second no-load voltage-regulating tap terminal  50  is provided on the second incoming-line-side high-voltage coil  2034 . 
     With the single-phase dry-type transformer  1  in this optional configuration, after the first no-load voltage-regulating tap terminal  40  and the second no-load voltage-regulating tap terminal  50  are provided, a rated high voltage at which the transformer operates can be changed within a no-load regulation range by correspondingly regulating the first no-load voltage-regulating tap terminal  40  and the second no-load voltage-regulating tap terminal  50  to the same gear in case of power-down of the transformer. 
     In an optional embodiment of the embodiment, referring again to  FIGS. 2 and 3 , the single-phase dry-type transformer  1  may further comprise an on-load voltage-regulating tap switch  60 , a first on-load voltage-regulating tap terminal  70  and a second on-load voltage-regulating tap terminal  80 . Optionally, the first on-load voltage-regulating tap terminal  70  is provided on the first outgoing-line-side high-voltage coil  3033 , and optionally, the second on-load voltage-regulating tap terminal  80  is provided on the second outgoing-line-side high-voltage coil  3034 . The first on-load voltage-regulating tap terminal  70  may comprise a first common end  701  and a plurality of first tap ends (see detailed description with respect to  FIG. 3  below). The second on-load voltage-regulating tap terminal  80  may comprise a second common end  801  and a plurality of second tap ends (see detailed description with respect to  FIG. 3  below). The first common end  701  is connected in parallel with the second common end  801 , and each of the plurality of first tap ends is connected in parallel with a corresponding second tap end of the plurality of second tap ends, and they are connected with the on-load voltage-regulating tap switch  60  after their respective parallel connections. 
     Specifically, the first common end  701  is connected in parallel with the second common end  801 , forming a common parallelly-connected terminal, and the common parallelly-connected terminal is connected to the on-load voltage-regulating tap switch  60 ; each of the plurality of first tap ends is connected in parallel with a corresponding second tap end of the plurality of second tap ends, forming a plurality of tap parallelly-connected terminals, and the plurality of tap parallelly-connected terminals are connected with the on-load voltage-regulating tap switch  60  respectively. The on-load voltage-regulating tap switch  60  can selectively connect the common parallelly-connected terminal to the plurality of tap parallelly-connected terminals within a desired on-load voltage regulation range. 
     More specifically, referring to  FIG. 3  again, in the single-phase dry-type transformer  1 , the first outgoing-line-side high-voltage coil  3033  is provided thereon with a first common end  701  of the first on-load voltage-regulating tap terminal  70  (denoted by X 1  hereinafter) and the first tap ends a 1  to a 9 ; the second outgoing-line-side high-voltage coil  3034  is provided thereon with the second common end  801  of the second on-load voltage-regulating tap terminal  80  (hereinafter denoted by X 2  hereinafter) and the second tap ends b 1  to b 9 . The first common end  701  and the second common end  801  are connected in parallel, forming a common parallelly-connected terminal X 0 , and then the common parallelly-connected terminal X 0  is connected to the on-load voltage-regulating tap switch  60 . The first tap end aj and the second tap end bj are connected in parallel, where j=1, 2, . . . 9, forming tap parallelly-connected terminals  1 - 9 , and then the tap parallelly-connected terminals  1 - 9  are respectively connected to the on-load voltage-regulating tap switch  60 . The on-load voltage-regulating tap switch  60  can selectively connect the parallelly connected common terminal X 0  to any one of the parallelly-connected terminals  1  to  9  of respective tap terminals, thereby realizing nine different ranges of on-load voltage regulations. 
     In the optional example of the embodiment, at least one of the first incoming line end  2031 , the second incoming line end  3031 , the first outgoing line end  2032 , and the second outgoing line end  3032  may be provided thereon with a capacitive screen line  90 . Referring to  FIG. 4 , a configuration in which a capacitive screen line  90  is provided at the incoming line side portion is schematically illustrated. The optional configuration process of the capacitive screen line  90  is to wind the capacitive screen line an opposite direction when winding the coil. In the course of using the transformer, the capacitive screen line  90  does not have coil load current to flow through. The capacitive screen line  90  can for example increase the series capacitance at the head and end of the coil, improve the lightning surge voltage distribution, and thus increase the lightning surge voltage resistance capability of the coil. 
     In the optional example of the embodiment, referring to  FIG. 5 , at least one of the first incoming line end  2031 , the second incoming line end  3031 , the first outgoing line end  2032 , and the second outgoing line end  3032  may be provided with a voltage equalizing cover  91 . The voltage equalizing cover  91  may be connected with the first incoming line end  2031  by a bolt, and the voltage equalizing cover may also be connected with other wiring terminals in the same manner. The outer shell of the voltage equalizing cover  91  may be made of copper foil of about 1 mm, but a thinner or thicker copper foil is also possible. The voltage equalizing cover  91  may improve the electric field distribution outside the high voltage coil, and thus improve the voltage resistance capability of the high voltage coil. 
     In the optional example of the embodiment, as can be seen from the structural schematic view of an outer profile of the single-phase dry-type transformer  1  in  FIG. 6 , a first insulating cylinder  92  is provided between the first low-voltage coil  202  and the incoming-line-side high-voltage coil  203 , and/or a second insulating cylinder  93  is provided between the second low-voltage coil  302  and the outgoing-line-side high-voltage coil  303 . An insulating partition  94  may be provided between the incoming-line-side high-voltage coil  203  and the outgoing-line-side high-voltage coil  303 . A material of the insulating cylinder  92 , the insulating cylinder  93 , and the insulating partition  94  may be an epoxy resin plus a glass fiber. 
     Further, the single-phase dry-type transformer  1  may further comprise a plurality of solid insulating members  95 . The plurality of solid insulating members  95  may be configured to support and/or fix the first low-voltage coil  202 , the second low-voltage coil  302 , the incoming-line-side high-voltage coil  203 , and the outgoing-line-side high-voltage coil  303  (a fixed insulating member  95  on the top and a fixed insulating member at the bottom are schematically shown in the figure only). There is air capable of insulating and dissipating heat in the surrounding environment of the low voltage coil and the high voltage coil. 
     An embodiment of the present application further provides an assembly method for a three-phase dry-type transformer, especially for a 110 kV three-phase dry-type transformer. 
     Specifically, referring to  FIG. 7 , the method comprises: step S 1 , providing three single-phase dry-type transformers, each single-phase dry-type transformer  1  comprising an incoming line side and an outgoing line side, wherein the incoming line side comprises an incoming-line-side high-voltage coil  203  that provides a first incoming line end  2031  and a first outgoing line end  2032 , the first incoming line end  2031  is located at substantially the middle of the incoming-line-side high-voltage coil  2031 , and the outgoing line side comprises an outgoing-line-side high-voltage coil  303  that provides a second incoming line end  3031  and a second outgoing line end  3032 , and the second incoming line end  3031  is located at substantially the middle of the outgoing-line-side high-voltage coil  303 ; step S 2 , making the first outgoing line end  2032  and the second incoming line end  3031  of each single-phase dry-type transformer  1  connected with each other so that the incoming-line-side high-voltage coil  203  and the outgoing-line-side high-voltage coil  303  are connected with each other in series; and step S 3 , making the three single-phase dry-type transformers  1  connected and assembled through their corresponding three second outgoing line ends  3032  after the series connection to form a three-phase dry-type transformer. 
     Optionally, the incoming-line-side high-voltage coil  203  comprises a first incoming-line-side high-voltage coil  2033  and a second incoming-line-side high-voltage coil  3024  connected in parallel with the first incoming-line-side high-voltage coil  2033 , and the outgoing-line-side high-voltage coil  303  comprises a first outgoing-line-side high-voltage coil  3033  and a second outgoing-line-side high-voltage coil  3034  connected in parallel with first outgoing-line-side high-voltage coil  3033 ; a first end configured to connect in parallel the first incoming-line-side high-voltage coil  2033  and the second incoming-line-side high-voltage coil  2034  is provided substantially at the middle of the incoming-line-side high-voltage coil  203  as a first incoming line end  2031 , and a second end configured to connect in parallel the first incoming-line-side high-voltage coil  2033  and the second incoming-line-side high-voltage coil  2034  is provided at an end of the incoming-line-side high-voltage coil  203  as a first outgoing line end  2032 ; and a first end configured to connect in parallel the first outgoing-line-side high-voltage coil  3033  and the second outgoing-line-side high-voltage coil  3034  is provided substantially at the middle of the outgoing-line-side high-voltage coil  303  as a second incoming line end  3031 , and a second end configured to connect in parallel the first outgoing-line-side high-voltage coil  3033  and the second outgoing-line-side high-voltage coil  3034  is provided at an end of the outgoing-line-side high-voltage coil  303  as a second outgoing line end  3032 . 
     In the optional example of the embodiment, the first incoming line end  2031  is formed by connecting terminals of the first incoming-line-side high-voltage coil  2033  and of the second incoming-line-side high-voltage coil  2034 , which terminals are adjacent to each other in proximity to substantially the middle of the incoming-line-side high-voltage coil  203 . 
     In the optional example of the embodiment, the second incoming line end  3031  is formed by connecting terminals of the first outgoing-line-side high-voltage coil  3033  and of the second outgoing-line-side high-voltage coil  3034 , which terminals are adjacent to each other in proximity to substantially the middle of the outgoing-line-side high-voltage coil  303 . 
     The assembly method provided in the embodiment obtains a three-phase dry-type transformer that can effectively operate at a high voltage level, for example a 110 kV voltage level, by assembling three specially designed single-phase dry-type transformers. The incoming line end of the three-phase dry-type transformer obtained by the assembly method is provided in the middle of or substantially at the middle of the incoming-line-side high-voltage coil of each single-phase dry-type transformer, for example, the lightning surge voltage resistance capability of the incoming line end of each single-phase dry-type transformer is improved, that is, for example, the voltage resistance capability of the high voltage coil of each single-phase dry-type transformer is improved, and then the voltage resistance capability of, for example, the assembled three-phase dry-type transformer is improved. 
     What described above are only specific embodiments of the present application, and the scope of protection of the present application is not limited thereto, and changes or substitutions that any technician familiar with the technique u the art would easily conceive of within the technical scope of the present application should be covered by the scope of the present application. Therefore, the scope of protection of the present application should be determined by the scope of the claims. 
     INDUSTRIAL APPLICABILITY 
     With the 110 kV three-phase dry-type transformer and the assembly method therefor provided by the embodiments of the present application, for example, by providing the incoming line end of the three-phase dry-type transformer substantially at the middle of the incoming-line-side high-voltage coil of each single-phase dry-type transformer, for example, the lightning surge voltage resistance capability of the incoming line end of each single-phase dry-type transformer is improved, that is, the voltage resistance capability of the high voltage coil of each single-phase dry-type transformer is improved, and then the voltage resistance capability of, for example, the combined three-phase dry-type transformer is improved.