Abstract:
A method for cooling a transformer and transformer apparatuses are implemented. A transformer apparatus, according to one embodiment, comprises: a conduction cooled electrical transformer ( 240 ) mounted to a cold plate ( 252 ); and a heat pipe ( 262 ) for supplementing cooling of the transformer ( 240 ), the heat pipe ( 262 ) comprising a first end ( 303 ), a second end ( 305 ), and a sealed low pressure cavity containing an amount of a fluid ( 311 ), wherein the first end ( 303 ) of the heat pipe ( 262 ) is located in a hot region of the transformer ( 240 ), the second end ( 305 ) is maintained colder than the first end ( 303 ) by contact with the cold plate ( 252 ), and heat produced at the first end ( 303 ) by operation of the transformer ( 240 ) is moved to the second end ( 305 ) by a closed loop vapor cycle in the sealed low pressure cavity using the fluid ( 311 ) of the heat pipe ( 262 ).

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a cooling method and apparatus, and more particularly, to a cooling method and apparatus for a transformer, using a heat pipe. 
     2. Description of the Related Art 
     Electrical transformers are ubiquitous devices used for energy transfer and conversion. During operation, transformers heat up due to a plurality of factors. For example, current flowing through the transformer windings causes resistive heating of the transformer conductors, and consequently, heat dissipates from the conductors. Induced eddy currents may circulate within the core of the transformer, causing resistive heating. The heat produced by eddy currents in the core radiates out to other components of the transformer. Residual DC currents in the transformer also lead to transformer heating. Hence, transformer heating accompanies transformer operation. The amount of heat dissipated during transformer operation depends on the transformer size, weight, etc., and on the magnitude of voltages handled by the transformer. 
     Transformers typically have a large thermal resistance across their body. Due to large thermal resistances, conduction cooled transformers mounted on a cold plate often exhibit a large temperature delta across the distance from the cold plate to the hottest location on the transformer. 
     Disclosed embodiments of this application address these and other issues by utilizing a heat pipe supplemented cooling for a transformer. In accordance with the present invention, heat pipes are introduced in a transformer so that cooling is effectively applied to both sides (or multiple locations) of the transformer, thereby effectively reducing the thermal resistance across the transformer to the cold plate used to cool the transformer. This results in a cooler operating transformer, and/or a smaller and lighter transformer performing the same functions as larger transformers. According to one embodiment of the present invention, one or more heat pipes are embedded/introduced at various locations inside a conduction cooled transformer. Heat pipes efficiently move heat from transformer hotspots to a cold plate or other cold side of the transformer assembly. Heat pipes are evacuated devices that contain an amount of fluid. Heat dissipated from an operating transformer vaporizes fluid inside the heat pipes, and the formed vapors transport heat to regions of the heat pipes that are kept colder than the operating transformer. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a method for cooling a transformer and to transformer apparatuses. According to a first aspect of the present invention, a transformer apparatus comprises: a conduction cooled electrical transformer mounted to a cold plate; and a heat pipe for supplementing cooling of the transformer, the heat pipe comprising a first end, a second end, and a sealed low pressure cavity containing an amount of a fluid, wherein the first end of the heat pipe is located in a hot region of the transformer, the second end is maintained colder than the first end by contact with the cold plate, and heat produced at the first end by operation of the transformer is moved to the second end by a closed loop vapor cycle in the sealed low pressure cavity using the fluid of the heat pipe. 
     According to a second aspect of the present invention, a transformer apparatus comprises: an electrical transformer mounted to a cold plate; and a heat pipe for supplementing cooling of the transformer, the heat pipe comprising a first end, a second end, and a sealed low pressure cavity containing an amount of a fluid, wherein the first end of the heat pipe is immovably embedded in a hot region of the transformer, the second end is maintained colder than the first end by contact with the cold plate, and heat produced at the first end by operation of the transformer is moved to the second end by a closed loop vapor cycle in the sealed low pressure cavity using the fluid of the heat pipe. 
     According to a third aspect of the present invention, a method for cooling a transformer comprises: cooling the transformer by conduction; and enhancing cooling of the transformer using a heat pipe, the enhancing step including placing a first end of the heat pipe in a hotspot region of the transformer, maintaining a second end of the heat pipe colder than the first end, and moving heat from the first end to the second end by vaporizing a fluid in a sealed low pressure cavity of the heat pipe, transporting the vaporized fluid to the second end, condensing the vaporized fluid, and returning the condensed fluid to the first end by a capillary action along a wall of the heat pipe. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further aspects and advantages of the present invention will become apparent upon reading the following detailed description in conjunction with the accompanying drawings, which are given by way of illustration only and, thus, are not limitative of the present invention. In these drawings, similar elements are referred to using similar reference numbers, wherein: 
         FIG. 1  is a general block diagram of a system containing a transformer with heat pipe supplemented cooling according to an embodiment of the present invention; 
         FIG. 2  illustrates a transformer with heat pipe supplemented cooling according to an embodiment of the present invention; 
         FIG. 3A  illustrates a potted transformer with heat pipe supplemented cooling according to an embodiment of the present invention; 
         FIG. 3B  illustrates internal details of a potted transformer with heat pipe supplemented cooling according to an embodiment of the present invention illustrated in  FIG. 3A ; 
         FIG. 4  illustrates a non potted transformer with heat pipe supplemented cooling according to an embodiment of the present invention; 
         FIG. 5  illustrates an exemplary heat pipe used for heat pipe supplemented transformer cooling according to an embodiment of the present invention illustrated in  FIG. 2 ; and 
         FIG. 6  illustrates a transverse view of the heat pipe of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
     Aspects of the invention are more specifically set forth in the accompanying description with reference to the appended figures.  FIG. 1  is a general block diagram of a system containing a transformer with heat pipe supplemented cooling according to an embodiment of the present invention. The system  100  illustrated in  FIG. 1  includes the following components: a cold plate  252 ; a transformer  240 ; and heat pipes  262 _ 1 ,  262 _ 2 , . . . ,  262 _N. Operation of the system  100  in  FIG. 1  will become apparent from the following discussion. 
     System  100  may be associated with an aircraft, a ship, a laboratory facility, an industrial environment, a residential environment, etc. The cold plate  252  is at a lower temperature than the transformer  240 . The cold plate  252  may be any type of system maintained at a lower temperature. The cold plate  252  may be, for example, part of a refrigerant system, air cooled system, water cooled system, etc. 
     Transformer  240  heats up during its operation. The transformer  240  may be any type of transformer used, for example, in aerospace systems, in vehicles, in laboratory facilities, etc. Heat pipes  262 _ 1 ,  262 _ 2 , . . . ,  262 _N are located in various regions of the transformer  240 , and transport heat from the transformer  240  to the cold plate  252 . 
     Although the systems and elements in system  100  are shown as discrete units, it should be recognized that this illustration is for ease of explanation and that the associated functions of certain functional modules can be performed by one or more physical elements. 
       FIG. 2  illustrates a transformer with heat pipe supplemented cooling according to an embodiment of the present invention. 
     The transformer  240 A illustrated in  FIG. 2  includes a transformer core  256 , windings  260 , an optional epoxy filling  258 , and a mounting base  254 . The transformer core  256  may be, for example, a core made of a magnetic material, a conductive material, etc. The windings  260  may include, for example, wires, coils, electrical insulations, enameled wires, other types of conductors, etc. The transformer  240 A is cooled through mounting base  254 . The windings  260  may be protected by layers of epoxy  258 . Epoxy layers  258  protect the transformer in damp or dirty environments, for example. 
     During operation, the transformer  240 A heats up. The heating of the transformer is due to a plurality of factors. For example, the current flowing through the windings  260  causes resistive heating of the conductors. Induced eddy currents may circulate within the core  256 , causing resistive heating. 
     The transformer  240 A may be conduction cooled. A cold plate  252 A is connected to one end of the transformer  240 A, to cool the transformer. A cooling media is found on a side of the cold plate  252 A. Such cooling media may be liquid, air, etc. 
     One or more heat pipes  262  are integrated into the transformer assembly to move heat from the transformer hotspots. The heat pipes  262  are embedded or inserted into the transformer  240 A. The heat pipes are heat sunk to the cold plate, which is in turn cooled either by a cooled liquid loop, or by forced air, natural convection cooling, etc. The transformer is clamped to a metal block that is used to conduct the heat away from the core  256 . The metal block may be the cold plate. In one embodiment, the metal block is made of aluminum. The heat path out of the windings is typically through a thermal pad (to accommodate the tolerance variation of the windings) and into the base mounting block. From the base mounting block, the heat is transferred to the cold plate/metal block. Heat is then transferred to the heating and ventilation system. If the transformer is used in an aircraft, then the Environmental Control System (ECS) will finally disperse the heat. 
     Transformer hotspots include the top of the windings, the hop in a conduction cooled transformer design, locations on one end of the transformer at the transformer evaporator end, locations at transformer heat sink ends such as locations where heat sinks to a cold plate at a transformer condenser end, etc. As illustrated in  FIG. 2 , an exemplary heat pipe  262  may transport heat from a hot end  303  (evaporator end), to a cold end  305  (condenser end) placed in a cold support  266  that is kept cold by the cold plate  252 A. 
     The orientations of the heat pipes may affect the performance of the heat pipes. In a preferred embodiment, the condenser end  305  of the heat pipe is located above the evaporator end  303 . However, the cooling functions of the heat pipe can still be performed even if the condenser end is not above the evaporator end. 
     The heat pipe is a “heat transport” device that has a very low thermal resistance over a large distance. A heat pipe uses a closed loop vapor cycle with no moving parts. One end of the heat pipe, the evaporator end, is at a hot location of the transformer. Another end of the heat pipe, the condenser end, is mounted to the cold plate, which could be air cooled, liquid cooled, etc. 
     Since heat pipe cooling is applied to two sides or to multiple sides or locations of the transformer, the distance to the cooling media (for example, to the cold plate) is effectively reduced. With an effectively reduced distance to the cooling media, the temperature rise (delta T) of the transformer  240 A is significantly reduced. 
     Heat pipes transport the heat to the mounting base of the transformer, and can fit within the normal mounting footprint of the transformer. Heat pipes can also transport heat to a remote location farther away from the transformer, to help spread the thermal load over a larger area of the cold plate. Conduction cooled transformers may be packaged with or without potting material. Potted transformers are heavier than non-potted ones. If potted, the transformer assembly can be potted in a thermally conductive epoxy, to help transfer the heat load to the heat pipes and eliminate localized hotspots. 
     In one exemplary embodiment, heat pipes are subsequently attached to the packaged transformers. In another exemplary embodiment, heat pipes are packaged initially with the transformers. 
     One of the primary advantages of the present invention is that it is compatible with, and allows the use of standard transformer designs and manufacturing processes. According to the present invention, heat pipes may be added onto an existing transformer that is already cooled by a mounting base, and by thermally conductive epoxy, if epoxy is used. Because of insufficient cooling provided by a standard conduction cooling setup, the transformer may be overheating. The present invention provides supplemental/enhanced cooling by heat pipes, for conduction cooled transformer designs. The heat pipe supplemented cooling described in the current invention can be used to implement cooler operating transformers, lighter transformers that can retain and perform the same functions as heavier transformers, etc. 
       FIG. 3A  illustrates a potted transformer with heat pipe supplemented cooling according to an embodiment of the present invention. The transformer  240 B is potted with thermally conductive potting  274 . For example, the final assembly of the transformer may be potted with thermally conductive epoxy to improve thermal conductivity. The thermally conductive epoxy helps transfer the heat load to the heat pipes and eliminates localized hotspots. Two heat pipes  262 A and  262 B are illustrated, and more heat pipes may be present. 
       FIG. 3B  illustrates internal details of the potted transformer with heat pipe supplemented cooling according to an embodiment of the present invention illustrated in  FIG. 3A .  FIG. 3B  illustrates the same transformer as  FIG. 3A . Internal details of the transformer are visible, and the thermally conductive potting is hidden. Heat pipes  262 A and  262 B are embedded into the mounting base  254 A and into the potting. Clamps  271 A secure the position of the core  256 A. The top surface  273 A is cooled by contact with a cooling region such as a cold plate. The transformer  240 B may include an optional thermal pad  275 A. 
       FIG. 4  illustrates a non-potted transformer with heat pipe supplemented cooling according to an embodiment of the present invention. The transformer  240 C without thermally conductive potting, includes heat pipes, such as heat pipes  262 C and  262 D, embedded into the mounting base  254 B and into the clamps  271 B. Hence, the clamps are cooled by the heat pipes. Clamps  271 B secure the position of the core  256 B. The top surface  273 B is cooled by contact with a cooling region such as a cold plate. The transformer  240 C may include an optional thermal pad  275 B. 
       FIG. 5  an exemplary heat pipe  262 A used for heat pipe supplemented transformer cooling according to an embodiment of the present invention illustrated in  FIG. 2 . The heat pipe  262 A helps improve the thermal conductivity across a conduction-cooled transformer mounted to a cold plate, where the cold plate is cooled using liquid cooling means, air cooling means, or other cooling means. 
     Heat pipes are two-phase heat transfer devices that use vaporization of a small amount of liquid inside the heat pipe at a hot end of the heat pipe device, and condensation at the cold end of the heat pipe device, to transfer heat over a distance between the hot end and the cold end. Heat pipes are self-contained devices that have no moving parts to wear out; do not require energy to operate, and do not need maintenance and have extremely high effective thermal conductivity. Depending on transformer size and power, associated heat pipes can be designed to carry the needed amount of watts or kilowatts. Heat pipes are highly reliable, and typically last for years or decades. 
     Heat pipes may be located in a variety of positions, and can transfer heat to any areas. Hence, heat pipe  262 A may be located anywhere inside transformer  240 A, and may be designed to transfer heat to locations where heat dissipation is effectively managed. Heat pipes are low cost and can be manufactured in any configuration needed. 
       FIG. 5  and  FIG. 6  illustrate an exemplary heat pipe design. The heat pipe is an evacuated device that contains a small quantity of fluid  311 A. An evaporator end  303  of the heat pipe is heated by a heat flux, such as the heat produced by transformer  240 A operation. The heat flux initiates a passive evaporation and condensation cycle in the heat pipe. The inner surface of the heat pipe  262 A is lined with a capillary wicking material  321 , which contributes to fluid transport. Heat is absorbed at the evaporator section  303  and vaporizes some of the fluid  311 A. The fluid  311 A boils at any temperature above the fluid freezing point, because the heat pipe is an evacuated device. Hence, as soon as any heat reaches the evaporator end  303 , vaporization of the fluid  311 A occurs and the heat pipe begins to operate. 
     Vaporized fluid  311 B travels to the cold condenser end  305 , hence transporting heat to the condenser region  305 . The condenser end  305  is kept cold by a cold system or medium  301 . The vapor  311 B condenses at the condenser end  305  and releases heat to the cold system or medium  301 . The condensed working fluid  311 C is returned to the evaporator end  303 , by the capillary action of the wick structure  321 . 
     Exemplary working fluids  311 A used inside a heat pipe include water, acetone, and methanol. Other fluids may also be used. The heat pipe external casing  324  may be made of a variety of materials, such as, for example, copper, stainless steels, superalloys, refractory metals, etc. Other materials may also be used for the external casing. 
     The wick structure of heat pipe  252 A can be of various kinds. In exemplary embodiments, a grooved wick structure, a screen mesh wick structure, or a sintered powder wick structure may be used. Various materials may be used to make the wick  321 . The wick  321  may also be absent in an exemplary embodiment. 
     The geometry, structure, and surface size of the wick  321 , the pipe  262 A diameter and length, and the type of fluid  311 A, influence the heat flux capability of the heat pipe and the angles of operation of the heat pipe. For example, heat pipes may transport heat both in the direction of gravity, and against gravity. Hence, heat pipes may have many orientations, as needed, inside transformer  240 A. Heat pipes may be customized for transformer type and power, transformer spatial orientation, locations inside the transformer, etc. 
     Any type of heat pipes, including off-the-shelf heat pipes, may be used for transformer cooling as described in the current application. A few exemplary types of heat pipes, which may be used for transformer cooling as described in the current invention, are described in the scientific and promotional brochure “Heat Pipe Technology for a Changing World”, published by Thermacore/Modine Manufacturing Company, May 2006, the entire contents of which are hereby incorporated by reference. 
     According to the present invention, heat pipes are used to supplement transformer cooling for various size transformers. In a preferred embodiment, heat pipe supplemented cooling is implemented for transformers used in aerospace applications. 
     The heat pipe supplemented transformer can be used for a variety of applications. The heat pipes can be customized to a desired cooling rate for the transformer and to a desired weight. Aspects of the present invention are applicable to a wide variety of environments. Aspects of the present invention are particularly applicable in environments where weight savings are desired, such as in aerospace systems.