Cooling apparatus for a gas insulated transformer
BACKGROUND OF THE INVENTION 
     The present invention relates to a cooling apparatus for a gas insulated transformer wherein coils and an iron-core are housed in a tank in which an electrically insulating gas is sealed. 
     In a conventional oil filled transformer, coils and an iron core are housed in a tank which is filled with an electrically insulating oil. The insulating oil serves to insulate and cool the coils, the iron core and so on. However, use of such an oil filled transformer is not desirable from the viewpoint of safety. A nonflammable transformer is thus desired, an example of which is a gas insulated transformer. In a gas insulated transformer of this type, an electrically insulating gas such as SF 6  gas is sealed in a tank housing coils and an iron core therein to insulate them. The coils and iron core are also cooled upon contact with a volatile cooling medium in the liquid phase. 
     The cooling medium evaporates by extracting heat from the coils. The vapor of the cooling medium is coexistent with a noncondensable insulating gas in the tank. If the noncondensable insulating gas is mixed in the vapor of the cooling medium, even in a small amount, condensation heat transfer coefficient of the vapor of the cooling medium is significantly lowered. In a gas insulated transformer of the vaporization cooled type which is cooled with the cooling medium, the coils are cooled by being sprayed with the cooling medium in the liquid phase, and the vapor of the cooling medium generated by this cooling process is condensed by a cooling unit. However, as mentioned earlier, mixing of the insulating gas renders condensation of the vapor of the cooling medium difficult. For this reason, the temperature of the cooling medium for cooling the coils and the like is raised, resulting in degradation of the cooling efficiency and an increase in the internal pressure of the tank. In order to prevent these problems, a cooling unit of large capacity must be mounted, and the overall apparatus becomes bulky, costly and heavy. 
     A gas insulated transformer, of separate cooling type has also been proposed wherein a duct for flowing a cooling medium therethrough is incorporated within the coils so as to cool the coils, instead of spraying them with the cooling medium. However, in order to obtain satisfactory cooling effects, a cooling duct of small wall thickness must be incorporated throughout the height (about 1 to 2 m) of the coils, which makes the manufacture of the apparatus difficult. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a cooling apparatus for a gas insulated transformer, which is capable of efficiently condensing the vapor of a cooling medium generated upon cooling the coils. 
     It is another object of the present invention to provide a cooling apparatus for a gas insulated transformer, which is capable of cooling coils and an iron core with high efficiency. 
     It is still another object of the present invention to provide a cooling apparatus for a gas insulated transformer, which is simple in construction, easy to manufacture, light in weight and compact in size. 
     In order to achieve these and other objects, there is provided according to the present invention a cooling apparatus for a gas insulated transformer comprising a tank in which an insulating gas is sealed; an iron core disposed inside said tank; coils wound around said iron core inside said tank; first cooling means for supplying a cooling medium to said coils for cooling said coils, so that the heat of the coil converts part of the cooling medium into a vapor; second cooling means disposed inside said tank, for spraying the cooling medium into an interior of said tank so as to condense the vapor of the cooling medium converted by the heat of the coils, the temperature of the cooling medium sprayed, by said second cooling means being not higher than the temperature of the cooling medium supplied by said first cooling means; a feed device for feeding the cooling medium to said first and second cooling means; and a cooling device for cooling the cooling medium fed to said first and second cooling means. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a view showing a cooling apparatus for a gas insulated transformer according to the first embodiment of the present invention; 
     FIG. 2 is a view showing a cooling apparatus for a gas insulated transformer according to the second embodiment of the present invention; and 
     FIG. 3 is a view showing a cooling apparatus for a gas insulated transformer according to the third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a cooling apparatus for a gas insulated transformer according to the first embodiment of the present invention. In this gas insulated transformer, an iron core 1 and coils 2 are housed in a tank 4 which is filled with an electrically insulating gas such as SF 6  gas. The coils 2 wound on the iron core 1 are housed within a reservoir 3 having an outlet 3a at its upper end. A cooling unit 5 is arranged at the bottom of the tank 4. A plurality of cooling pipes 6 are assembled in the cooling unit 5. A cooling medium 9 in the liquid phase such as Refrigerant R113 (otherwise known as C 2  F 3  Cl 3 , or trifluourotrichloroethane) is held in the cooling unit 5. The lower end of the cooling unit 5 communicates with an inlet 3c at the bottom of the reservoir 3 through a pipe 8a. A pump 7a is mounted in the pipe 8a. After the cooling medium 9 in the tank 4 is collected in the cooling unit 5, it is pumped into the reservoir 3 by the pump 7a. The coils 2 are housed in the reservoir 3 so as to define small gaps 3b between itself and the walls of the reservoir 3. The cooling medium 9 which is pumped into the reservoir 3, as indicated by an arrow 20a, flows through the gaps 3b and overflows through the outlet 3a at the upper end of the reservoir 3. The cooling medium 9 is then collected into the cooling unit 5 at the bottom of the tank 4. 
     Spray equipment 11 having a plurality of spray nozzles 12 is arranged above the coils 2 and the iron core 1 inside the tank 4. A cooling unit 10 communicates through a pipe 8c with the spray equipment 11. The cooling unit 10 has a plurality of cooling pipes 6 assembled therein. The lower part of the cooling unit 10 communicates with the bottom of the cooling unit 5 through a pipe 8b. A pump 7b is mounted in the pipe 8b. The cooling medium 9 inside the cooling unit 5 is pumped into the cooling unit 10 by the pump 7b as indicated by an arrow 20b, is passed through the cooling unit 10, and is then supplied to the spray equipment 11. The cooling medium 9 is then sprayed from spray nozzles 12 into the interior of the tank 4. 
     A cooling tower 14 for cooling the cooling water 16 stored therein is disposed outside the tank 4. The cooling tower 14 communicates with the cooling pipes 6 inside the cooling unit 10 through a pipe 15b. The cooling pipes 6 inside the cooling unit 10 communicate with those in the cooling unit 5 through a pipe 15c. The cooling pipes 6 inside the cooling unit 5 communicate with the cooling tower 14 through a pipe 15a. A pump 7c is mounted in the pipe 15a to return the cooling water 16 to the cooling tower 14 after it is passed through the cooling pipes 6 inside the cooling units 5 and 10, as shown by dotted arrows 21. While the cooling water 16 is passed through the cooling pipes 6 inside the cooling unit 10, it cools the cooling medium 9 to be supplied to the spray equipment 11 through the pipes 8b and 8c. When the cooling water 16 is passed through the cooling pipes 6 of the cooling unit 5, it cools the cooling medium 9 stored in the cooling unit 5. 
     The mode of operation of the gas insulated transformer of the configuration as described above will now be described. The cooling medium 9 cooled by the cooling unit 5 is supplied to the reservoir 3 through the inlet 3c at its bottom by the pump 7a, and it then overflows from the outlet 3a after being passed through the gaps 3b. While the cooling medium 9 flows through the gaps 3b to overflow from the outlet 3a, it is brought into contact with the coils 2 to extract heat generated therein upon current flow, thereby cooling them. The cooling medium 9 which has cooled the coils 2 in this manner is partially evaporated; the vapor becomes coexistent in the interior of the tank 4 and the remaining portion of the cooling medium 9 overflows from the outlet 3a of the reservoir 3 and is collected in the cooling unit 5 at the bottom of the tank 4. 
     The cooling medium 9 which is collected in the cooling unit 5 is also supplied to the cooling unit 10 by the pump 7b. The cooling medium 9 supplied to the cooling unit 10 is cooled thereby to a lower temperature than that cooled by the cooling unit 5, since the cooling unit 10 is disposed upstream of the cooling unit 5 along the direction of flow of the cooling water 16. The cooling medium cooled by the cooling unit 10 is supplied to the spray equipment 11 which sprays it into the interior of the tank 4 from the spray nozzles 12. The cooling medium 9 evaporates upon contact with the coils, and the vapor of the cooling medium in the interior of the tank 4 is brought into direct contact with mist 13 of the cooling medium sprayed from the spray nozzles 12. Since the vapor directly contacts the mist 13 and the surface area of the mist 13 is extremely large, the vapor of the cooling medium condenses efficiently and is collected in the cooling unit 5 at the bottom of the tank 4 in liquid phase. Since the vapor of the cooling medium 9 is efficiently condensed, a cooling unit of large capacity need not be incorporated, so that the overall apparatus may be rendered simple in construction, compact in size and light in weight. Furthermore, since the cooling medium 9 which cools the coils 2 is only present in the gaps 3b inside the reservoir 3, the amount of the cooling medium 9 required is small, which also results in a light-weight and low-cost apparatus. Spraying also serves to cool the iron core. 
     FIG. 2 shows a cooling apparatus for a gas insulated transformer according to the second embodiment of the present invention. Unlike the first embodiment where the coils 2 is cooled by passing the cooling medium 9 through the gaps 3b in the reservoir 3, the coils 2 are cooled in the second embodiment by dripping the cooling medium from a position above the coils 2. The same reference numerals as in FIG. 1 denote the same parts in FIG. 2, and a detailed description thereof will be omitted. The coils 2 are set on a suitable frame 16. Dripping equipment 17 for dripping the cooling medium 9 is arranged below the spray equipment 11 and above the coils 2 at a small distance from the upper ends thereof. The bottom of the cooling unit 5 communicates with the dripping equipment 17 through the pipe 8a. The cooling medium 9 stored in the cooling unit 5 is supplied to the dripping equipment 17 by the pump 7a mounted in the pipe 8a, and drips onto the coils 2 from the dripping equipment 17. The cooling medium 9 dripped onto the coils 2 contacts the coils 2 and the iron core 1 to cool them. The cooling medium is partially evaporated by heat generated by the coils 2, and the remaining portion thereof is collected in the cooling unit 5 at the bottom of the tank 4. As in the first embodiment, the vapor of the cooling medium is brought into direct contact with mist 13 of the cooling medium 9 sprayed from spray nozzles 12, condenses into liquid, drips into the cooling unit 5, and is collected therein. 
     FIG. 3 shows a cooling apparatus for a gas insulated transformer according to the third embodiment of the present invention. The third embodiment is different from the first embodiment (FIG. 1) in that a portion of a cooling medium for cooling coils 2 and another portion of the cooling medium for condensing the vapor of the cooling medium in a tank 4 are cooled by independent cooling units 19 and 10, respectively. The same reference numerals as in FIG. 1 denote the same parts in FIG. 3, and a detailed description thereof will be omitted. A collector 18 for collecting the cooling medium 9 is disposed at the bottom of the tank 4 in place of the cooling unit 5 (FIG. 1). The bottom of the collector 18 communicates with the end of the cooling unit 10 through the pipe 8b and communicates with one end of the cooling unit 19 through a pipe 8d. A plurality of cooling pipes 6 are assembled in the cooling unit 19. The other end of the cooling unit 19 communicates with the inlet 3c of the reservoir 3 through the pipe 8a. The cooling medium 9 in the collector 18 is supplied to the cooling unit 10 through the pipe 8b by the pump 7b mounted therein. The cooling medium 9 is also supplied to the reservoir 3 through the inlet 3c by the pump 7a mounted in the pipe 8a after passing through the cooling unit 19. The cooling water inlet of the cooling unit 19 communicates with the cooling water outlet of the cooling unit 10 through the pipe 15c. The cooling water outlet of the cooling unit 19 communicates with the cooling tower 14 through the pipe 15a. Therefore, cooling water 16 in the cooling tower 14 passes through the cooling pipes 6 of the cooling units 10 and 19, and is returned to the cooling tower 14 by the pump 7c. The cooling medium flowing through the cooling units 10 and 19 is cooled by the cooling water 16 circulated in this manner. If the capacity of the cooling unit 10 is the same as that of the cooling unit 19, the cooling medium supplied from the cooling unit 10 is cooled to a lower temperature than that supplied from the cooling unit 19 since the cooling unit 10 is disposed upstream of the cooling unit 19 along the direction of flow of the cooling water 16. 
     The cooling medium 9 which flows through the gaps 3b and contacts the coils 2 to cool them is cooled by the cooling unit 19. Meanwhile, the cooling medium 9 which is sprayed from the spray nozzles 12 and condenses the vapor of the cooling medium in the space inside the tank 4 is cooled by the cooling unit 10. Therefore, the temperatures of the cooling media may be set arbitrarily. The iron core is cooled by spraying. 
     According to the present invention, the vapor of the cooling medium, which has cooled the coils 2 and has evaporated, contacts the mist 13 of the cooling medium sprayed from the spray nozzles 12, condenses into liquid, and drips into the tank 4. Since the vapor of the cooling medium is efficiently condensed by the mist, it may be recovered with a high yield in liquid phase. Thus, the internal pressure of the tank 4 may not be inadvertently raised, and the temperature of the cooling medium may not be raised. Furthermore, since a cooling unit of large capacity need not be incorporated, the overall apparatus can be rendered compact in size, light in weight, and low in manufacturing cost. The temperature of the cooling medium which is sprayed into the tank 4 and condenses the vapor of the cooling medium is preferably set to be lower than that of the cooling medium for cooling the coils 2 for the purpose of improving the condensation efficiency of the vapor. However, even if the temperatures of both cooling media are set to be the same, the vapor of the cooling medium in the tank 4 can be condensed by the cooling medium sprayed from the spray nozzles. For this reason, the cooling unit 10 (FIG. 1), for example, need not always be incorporated in addition to the cooling unit 5. In this case, the cooling medium cooled by the cooling unit 5 can be directly supplied to the spray nozzles 12. If a cooling unit of larger capacity than the cooling unit 5 is used for the cooling unit 10, the temperature of the cooling medium sprayed from the spray nozzles 12 may be set to be lower than that of the cooling medium supplied to the reservoir 3, even if the direction of flow of the cooling water 16 is reversed from that indicated by dotted arrows 21 in FIG. 1. In order to increase the cooling capacity of the cooling unit 10, the number of cooling pipes 6 to be assembled in the cooling unit 10 may be increased.