SYSTEM AND METHOD FOR COOLING TRANSFORMERS

Abstract: A system and method are provided for cooling transformers utilizing a fluid to air heat exchanger to cool dielectric fluid flowing through the transformer. The system includes multiple vertical cooling tubes in fluid communication with the transformer to cool the dielectric fluid. The tubes are configured to create vertical air passages such that the system utilizes natural convection air flow and thermal siphoning to cool the fluid.

Description: SYSTEM AND METHOD FOR COOLING TRANSFORMERS FIELD OF THE INVENTION This present invention relates generally to the field of transformer cooling and, more particularly, to a transformer dielectric fluid cooler using a fluid to air heat exchanger. BACKGROUND OF THE INVENTION The power transmission industry utilizes transformers in a variety of settings such as using large power transformers to step up voltage at large power plants for subsequent transmission and using smaller distribution transformers to step down voltage feeding residential and industrial communities. Transformers can have rated outputs in the thousands of volts and can generate substantial heat during operation. If this heat is not properly dissipated, the heat can damage the transformer reducing its life expectancy or even rendering it inoperable. Most large power transformers are immersed in dielectric fluid to insulate and cool the components. Typical dielectric insulating fluids include standard mineral oils, high temperature mineral oils and high temperature synthetic fluids. During operation, transformers can be sufficiently cooled by natural circulation of dielectric fluid or oil in the transformer and air surrounding the transformer (ONAN) . Using natural circulation of oil and air, the fluid circulates through the transformer core and exterior heat exchangers or coolers by natural thermal convection. The cooler outside the transformer is designed to allow natural convection of air. Typically, large heat exchangers are required because of the relative inefficiency of natural convection. To overcome the limitation of the (ONAN) type of cooling, natural circulation of oil and forced air systems are used (ONAF) . The dielectric fluid circulates through the transformer core and exterior coolers by natural thermal convection. The heat exchanger or cooler outside the transformer is designed to accommodate fans that force air over the cooler. This improves the cooling characteristics of the heat exchangers thereby reducing the number of exchangers required to achieve the same amount of cooling. This also leads to smaller overall dimensions of the transformer/cooler combination . To increase the heat capacity even further, forced oil and force air (OFAF) cooling is used. The oil is forced through the transformer using a pump. Increasing fluid velocity in the transformer allows for greater heat transfer between the heat exchanger material and the fluid. As with ONAF, air is forced over the heat exchanger thereby increasing the cooling efficiency of the heat exchanger. The radiator is the most common heat exchanger used for cooling dielectric fluids in transformers. Radiators take on a variety of shapes and configurations. One type of cooler is a tube type radiator with carbon steel tubes welded into a pipe header. Another more common type of cooler is made out of carbon steel panels stacked to form a radiator unit. Theses radiators are used in a bank consisting of several individual radiator sections and they can be bolted directly on the side of the transformer. During operation, the dielectric fluid flows from the transformer into the steel panels by entering at the top of the radiator and exiting at the bottom. To achieve cooling, air flows vertically across the radiator panels and pulls the heat away from fluid. As discussed above, natural convection of the air may be insufficient to achieve the desired cooling characteristics. Thus, fans are installed to facilitate air flow across the radiator panels to improve cooling. However, the fans are typically installed such that air flows horizontally across the panels and thus, fails to take advantage of the natural thermal flow properties of the surrounding air. Further, the current radiator technology requires a large amount of space to cool the dielectric fluid. The individual radiator panels become less efficient as heat transfer mechanisms as the required amount of cooling increases. Panel type radiators are generally constructed from thin material and have a welded seam around the periphery of each panel. This seam is a prime location for corrosion and the design needs to address this concern. Additionally, the carbon steel and other materials used in construction of radiators are relatively poor heat conductors. Because the radiators are filled with dielectric fluid, the amount of dielectric fluid increases as the number and size of the radiators increase. As will be appreciated by those skilled in the art, this not only adds to the cost of the transformer but it also increases the weight and affects the center of gravity of the transformer requiring additional structural stiffening of the transformer tank. In the ONAF mode, the cooling system utilizes small fans that minimize the stress placed on the radiator. The small fans do not move large volumes of air and, thus, if the design requires a significant amount of air cooling, many fans are required. Using many fans creates other problems in the cooling system such as additional wiring, poor air distribution and increased maintenance and electrical losses. When the radiator operates under natural fluid flow, as described earlier, fluid flow is generated because of the relative density differences between the hot oil and cold oil. As in any fluid system, this system follows a curve of flow versus resistance where, as the resistance or pressure drop of the system increases, the fluid flow decreases. When a single radiator is considered, the outermost panels (farthest away from the tank) will see less oil flow because the oil has to travel further to reach the outer panels and therefore encounters more resistance. Thus, the process of cooling the oil is not maximized and results in inefficient cooling of the dielectric fluid. Although the prior art discloses various systems for cooling dielectric fluids flowing through transformers, there exists a need for a heat transfer device that cools dielectric fluids that is more efficient, smaller and lighter than traditional transformer cooling devices. The present invention fills these and other needs, and overcomes the shortcomings of the prior art. SUMMARY OF THE INVENTION Generally described, a cooling system for cooling a dielectric insulating fluid flowing through transformers is provided. The system includes one or more cooling tubes oriented to have a top opening and bottom opening. Each tube has a plurality of radially projecting interior fins and exterior fins extending longitudinally therealong. Further, the tubes are interconnected along the longitudinal axis thereof by coupling portions of the exterior fins to form a bundle configuration. One or more distributor headers are provided that are in fluid communication between the transformer and the top openings of the cooling tubes. Additionally, one or more collector headers are provided that are in fluid communication between the transformer and the bottom openings of the cooling tubes. Finally, a plurality of vertical air channels are formed by the exterior fins of the bundle configuration. In another aspect of the invention, a method for cooling dielectric insulating fluid flowing through transformers is provided. In accordance with the method, dielectric insulating fluid is circulated from a transformer through one or more vertical tubes and an air stream is circulated through vertical air channels formed by the interconnection of the tubes to cool the dielectric insulating fluid. A system and method are provided for cooling transformers utilizing a fluid to air heat exchanger to cool dielectric fluid flowing through the transformer. The system includes multiple aluminum cooling tubes in fluid communication with the transformer to cool the dielectric fluid. The tubes are configured to create vertical air passages such that the system utilizes counter current natural convection air flow to cool the fluid. BRIEF DESCRIPTION OF THE DRAWINGS These and other objectives and advantages of the present invention will be more readily apparent from the following detailed description of the drawings of the preferred embodiment of the invention, in which: Fig. 1 is a perspective view of a preferred transformer cooling system placed in relation to an electric transformer; Fig. 2 is a side elevational view ot the preferred transformer cooling system of Fig. 1 placed in relation to a schematically shown electric transformer; Fig. 3 is an enlarged, fragmentary end elevation view of the present invention, taken along line 3-3 in the direction of the arrow shown in Fig. 2; Fig. 4 is an enlarged cross-sectional view of the cooling tubes of the present invention, taken along line 4-4 in the direction of the arrow shown in Fig. 3; and Fig. 5 is a side elevational view of the preferred transformer cooling system of Fig. 2 with portions of the electric transformer broken away to further illustrate the dielectric fluid flow and air flow during operation of the present invention. DETAILED DESCRIPTION OF THE INVENTION Turning now to the drawings in greater detail, and initially to Figs. 1 and 2, a cooling system of the present invention is designated generally by the reference numeral 10. The cooling system 10 is designed to cool a dielectric insulating fluid used to thermally and electrically insulate the windings and other internal parts of an electric transformer using a fluid to air heat exchanger. In a preferred embodiment, the cooling system 10 is comprised of multiple cooling tubes 12 in fluid communication with an electric transformer 14 via one or more distributor headers 16 and collector headers 18. The distributor headers 16 connect the upper vertical portion of transformer 14 to the upper portions of the cooling tubes 12. The collector headers 18, symmetrically identical to the distributor headers 16, connect the lower vertical portion of the transformer 14 to the lower portions of the cooling tubes 12. As would be understood, numerous hardware configurations are available to connect the cooling tubes 12 to the transformer 14 and they are understood to be included within the teachings of this invention. Fig. 3 is a fragmentary end elevation view of the cooling system 10, taken along line 3-3 in the direction of the arrows shown in Fig. 2. Referring to Figs. 1, 2 and 3, in a preferred embodiment, each of the distributor headers 16 has a manifold 20 in fluid communication with the transformer. One or more airfoil extensions 22 extend outwardly from and are in fluid communication with the manifold 20. The bottom surface of each of the airfoil extensions 22 has a plurality of openings or apertures (not shown) that connect with the upper portions of the cooling tubes 12, and are in fluid communication therewith, to complete the connection to the transformer 14. The collector headers 18 are symmetrically identical to the distributor headers 16 and consist of a manifold 24 and corresponding airfoil extensions 26 in fluid communication with the lower portions of the cooling tubes 12. As described above, the cooling system includes multiple cooling tubes 12 in fluid communication with the transformer. Fig. 4 illustrates an enlarged cross-sectional view of the cooling tubes 12 taken along line 4-4 shown in Fig. 3. The cooling tubes 12 are preferably round and manufactured from extruded aluminum. Aluminum has advantageous properties in that it is a good heat conductor, is light weight and does not corrode. As would be understood, any material having these characteristics would be included within the teachings of this invention. As discussed subsequently in greater detail, each cooling tube 12 has a vertical fluid channel 28 and a plurality of spaced exterior fins that allow the multiple cooling tubes 12 to be connected to each other in such a way as to form vertical air channels 30 in a honeycomb type configuration. Preferably, each cooling tube 12 has a wall 32, six spaced, radially extending interior fins 34, six spaced, radially extending exterior cooling fins 36, three spaced radially extending exterior claw fins 38 and three spaced radially extending exterior ball fins 40 alternatingly spaced between the exterior cooling fins 36. All of the fins extend longitudinally along the surface of the cooling tube wall 32 of each cooling tube 12. The six interior fins 34 extend from the interior surface of the cooling tube wall 32, and are spaced evenly around the inner circumference of the cooling tube wall 32. The interior fins 34 extend radially inwardly toward the center of the cooling tube 12 for a distance of approximately half the radius of the cooling tube. These interior fins 34 aid in drawing the heat away from the dielectric fluid as it flows through the cooling tube 12. The twelve exterior fins 36, 38 and 40 extend longitudinally along the outer surface of cooling tube wall 32, and extend radially outwardly away from the cooling tube wall 32 for a distance of approximately the diameter of the cooling tube 12. Further, all of the interior and exterior fins 34, 36, 38, and 40 have longitudinal grooves or channels along the surfaces of the fins. This creates additional surface area to allow for greater cooling of the dielectric fluid flowing through the interior of cooling tube 12. The exterior claw fins 38 and exterior ball fins 40 are connected to the tube wall 32 and spaced evenly around the circumference of the tube wall 32 between the exterior cooling fins 36 in an alternating manner. The three exterior claw fins 38 and the three exterior ball fins 40 are positioned along the cooling tube wall 32 in a manner that when looking at the cooling tubes 12 in cross-section as in Fig. 4, these exterior fins 38 and 40 appear to be contiguous with the six interior fins 34, as if to project from the interior of the cooling tube 12 out through the cooling tube wall 32. Further, the fins are arranged such that when moving clockwise around the cooling tube wall 32, a cooling fin 36 is followed by a claw fin 38 which is followed by another cooling fin 36 which is then followed by a ball fin 40. The claw fins 38 and ball fins 40 have a claw section 42 and a ball section 44, respectively, attached to their terminating ends. The claw fins 38 and ball fins 40 are designed so that multiple tubes 12 can be connected together by mating the claw section 42 and ball section 44 to create a honeycomb type interlocking structure or bundle 46 for the cooling system. The bundle 46 created by connecting the cooling tubes 12 to one another creates vertical air channels or passages 30. The air channels 30 extend along the entire exterior surface of the cooling tube fins 36, 38 and 40 and allow air to travel vertically from the bottom of the cooling system 10 to the top using natural convection and thermal siphoning. Thermal siphoning or the "chimney effect" occurs when the air trapped in the confined space created by the cooling tubes and their associated exterior fins expands quickly in the vertical direction. The quick expansion of air in the upward vertical direction generates a higher air flow velocity that results in greater heat transfer properties. Thus, the chimney effect uses the natural properties of the flowing air and produces greater cooling of the dielectric fluid. As would be understood, other configurations of the cooling tubes 12 that utilize thermal siphoning of air to cool the dielectric fluid are within the teachings of this invention. Fig. 5 is a schematic view of the cooling system and associated transformer of Fig. 2, further illustrating the dielectric fluid and air flow of the present invention. During operation, heat produced by the transformer 14 causes the dielectric fluid 48 surrounding the transformer core 50 to convect up to the top of the transformer enclosure 52 and into the distributor headers 16. The manifolds 20 within the distributor headers 16 receive the fluid 48 and as the fluid starts to cool, it descends down through the individual cooling tubes 12 via the airfoil extensions 22. The dielectric fluid 48 cools rapidly in the cooling tubes 12 and descends down to the airfoil extensions 26 in the collection headers 18. The fluid 48 then flows into the manifold 24 of collector header 18 and then into the transformer enclosure 52, where it is heated and the cycle starts over again. As would be understood, several fluids can be utilized as dielectric fluids including mineral oils and high temperature synthetic fluids. During the cooling process, ambient air is drawn into the cooling system 10 in the direction shown by arrows 54 where it is first warmed slightly by the collection headers 18 at the lower portion of the cooling system 10. The air then passes upwardly through the vertical air channels 30 where it is heated, expands and speeds up. Finally, the air exits the cooling system 10 by flowing up past the distributor headers 16 to the ambient atmosphere in the direction shown by arrow 56, carrying the heat from the dielectric fluid 48 with it. As discussed above, natural convection of the vertical air flow achieves more efficient cooling of the dielectric fluid. As would be understood, other methods of vertical air flow through the cooling system are within the teachings of this invention. As with traditional transformer heat exchanger or cooling systems, forced air, forced fluid and the combination of the two can be used to achieve further cooling ot the dielectric fluid. In a natural oil, forced air (ONAF) configuration, fans 58 (Figs. 2, 3 & 5) are connected to the bottom of bundles 46. This provides increased vertical air flow over the cooling tubes 12, which in turn provides greater cooling of the dielectric fluid. Further cooling can be achieved by using a forced oil, natural air (OFAN) configuration. Pumps (not shown) are installed on the interior of the transformer to increase the flow of the fluid across the core 50 and through the cooling tubes 12. This also has the effect of increasing the cooling of the dielectric fluid. Combining forced oil, forced air (OFAF) provides even greater cooling characteristics of the cooling system. Constructed and operated as previously described, the cooling system utilizes multiple vertical aluminum cooling tubes to remove heat from dielectric fluid used in electric transformers. In a preferred configuration, the connected tubes form vertical air channels that utilize natural convection and thermal siphoning to cool the dielectric fluid flowing through the cooling tubes. Using thermal siphoning creates greater cooling of the fluid by using the natural thermal properties of the air flowing through the cooling system. Thus, in situations where greater cooling was achieved on conventional radiators by installing fans to force air horizontally across the radiator, the number of fans may be reduced or eliminated entirely with the cooling system of the present invention to achieve the same level of cooling seen with conventional radiator designs. Because of the higher cooling efficiencies experienced with the present invention, fewer cooling tubes are required to cool a transformer. Fewer cooling tubes results m a fluid to air heat exchanger structure that is significantly smaller and lighter than conventional transformer radiator designs. Size considerations are important in transformer designs, particularly for transformers used in areas with limited space, such as urban areas with limited land area for installing transformer substations. Smaller cooling systems also reduce the mechanical stress on the transformer. Further, less dielectric fluid is required to cool the transformer because of the fewer number of tubes. Another advantage with the present invention is that the cooling tubes are made of aluminum. This further reduces the weight of the radiator cooling system and prevents undue stress on the transformer structure. Aluminum is also a better heat conductor than carbon steel used in conventional transformer radiator designs. Weld seams in transformer radiators are primary locations for corrosion. The cooling system configured as discussed above utilizes approximately fifteen percent of the weld seams used in conventional transformer radiators and, thus, reduces the opportunities for deterioration of the cooling system structure. From the foregoing, it will be seen that this invention is one well adapted to attain all the ends and objects hereinabove set forth together with other advantages that are inherent to the structure. It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims.

Claims: CLAIMS : 1. A cooling system for cooling a dielectric insulating fluid flowing through a transformer, the system comprising: a plurality of cooling tubes oriented in a substantially vertical manner, wherein each tube has a top opening and a bottom opening and, wherein the tubes are interconnected to form a bundle configuration; one or more distributor headers positioned between and in fluid communication with the transformer and the top openings of the cooling tubes; one or more collector headers positioned between and in fluid communication with the transformer and the bottom openings of the cooling tubes; and a plurality of vertical air channels formed by the interconnected bundle configuration. 2. The cooling system of claim 1, wherein the cooling tubes further comprise a plurality of outwardly radially projecting external fins extending longitudinally along an outer surface of the tubes. 3. The cooling system of claim 2, wherein the cooling tubes further comprise a plurality of inwardly radially projecting interior fins extending longitudinally along an inner surface of the tubes. 4. The cooling system of claim 3, wherein the fins have longitudinal grooves running parallel to the tubes. 5. The cooling system of claim 3, wherein the interior fins and exterior fins are evenly spaced apart. 6. The cooling system of claim 2, wherein the exterior fins comprise a plurality of exterior claw fins projecting radially outwardly from an exterior surface of the tube, each exterior claw fin having a claw at a terminal end thereof. 7. The cooling system of claim 6, wherein the exterior fins comprise a plurality of exterior ball fins projecting radially outwardly from an exterior surface of the tube, each exterior ball fin having a ball at a terminal end thereof. 8. The cooling system of claim 7, wherein the claw fins and the ball fins are designed to allow interconnection therebetween to form the bundle configuration. 9. The cooling system of claim 2, wherein the exterior fins comprise a plurality of exterior cooling fins projecting radially outwardly from an exterior surface of the tube. 10. The cooling system of claim 1, wherein the cooling tubes are made of aluminum. 11. The cooling system of claim 1, wherein each of the one or more distributor headers includes: a manifold in fluid communication with the transformer; and one or more airfoil extensions extending therefrom and in fluid communication therewith and, wherein each of the one or more airfoil extensions includes a bottom surface having a plurality of openings . 12. The cooling system of claim 11, wherein each of the openings of the airfoil extension is in fluid communication with the top openings of a corresponding cooling tube. 13. The cooling system of claim 1, wherein each of the one or more collector headers includes: a manifold in fluid communication with the transformer; and one or more airfoil extensions extending therefrom and in fluid communication therewith and, wherein each of the one or more airfoil extensions includes a top surface having a plurality of openings . 14. The cooling system of claim 13, wherein each of the openings of the airfoil extension is in fluid communication with the bottom openings of a corresponding cooling tube. 15. The cooling system of claim 1, wherein the cooling tubes in the bundle are connected in a honeycomb arrangement . 16. A method for cooling dielectric insulating fluid flowing through transformers, the method comprising: circulating dielectric insulating fluid downwardly from a transformer through one or more vertical tubes; and circulating an air stream upwardly through vertical air channels formed by the interconnection of the tubes to cool the dielectric insulating fluid. 17. The method of claim 16, wherein circulating an air stream through the vertical air channels occurs through natural thermal convection. 18. The method of claim 16, wherein circulating an air stream through the vertical air channels occurs through thermal siphoning. 19. The method of claim 16, wherein circulating an air stream through the vertical air channels occurs through forced air flow. 20. The method of claim 16, wherein circulating dielectric insulating fluid occurs through natural thermal convection. 21. The method of claim 16, wherein circulating dielectric insulating fluid occurs through forced dielectric insulating fluid flow.

Inventors: BRESCACIN LAURIE JOHN, CORKE CHRISTOPHER C, SHEERIN GEOFFREY THOMAS, BRESCACIN, LAURIE, JOHN, CORKE, CHRISTOPHER, C., SHEERIN, GEOFFREY, THOMAS
Applicants: UNIFIN INTERNATIONAL INC, UNIFIN INTERNATIONAL, INC.
Application year: 2001
Main classification: H01F27
IPC codes: H01F27