Patent Publication Number: US-2023142063-A1

Title: Liquid/fluid cooling systems for high power-density (hpd) transformers

Description:
I. FIELD OF THE INVENTION 
     The present subject matter relates generally to cooling systems and to high-power electronics power systems, and more particularly to high power-density (HPD) transformers in high-power electrical power systems. The subject matter further relates to liquid cooling systems for HPD transformers. 
     II. BACKGROUND OF THE INVENTION 
     Transformer Overview: A transformer is a passive electrical device that transfers electrical energy from one electrical circuit (the “source”) to one or more other electrical circuits (the “load(s)”), without any current transfer between the source and load(s). Energy is transferred via electrical field transmission only. A transformer has at least two separate wire coils, each typically wrapped around one or more ferrous (magnetic) metal cores. A varying current in any one source coil of the transformer produces a varying magnetic field (flux), which, in turn, induces a varying electromotive force across any other load coils wound around the same core. If the load coils are connected to electrical loads, current flows through the load coils. Electrical energy can be transferred between the (possibly many) coils, without a current-conducting metallic connection between the source and load circuits. This enables complete physical isolation of the source current and the load current(s). 
     Transformers are used in electric power applications for increasing alternating voltages at low current (Step Up Transformers) or decreasing the alternating voltages at high current (Step Down Transformer). 
     High Power Systems Overview: Medium-to-high power systems may provide electricity for large industrial plants, factories, large vehicles (such as large ships and airplanes), office buildings, apartment blocks, or entire cities. Power conversion systems, or power converters, transform electric power in medium and high power electronic distributed power buses and grids, for example: converting higher voltages to lower voltages; converting lower voltages to higher voltages; converting electricity from one alternating current frequency to another; or converting from direct current to alternating current, or alternating current to direct current. 
     Electrical power systems generally consists of generation, transmission, distribution and end use. Power is supplied by an electric generator or generators, or by renewable energy systems such as solar power. En route to its final load (devices which use the electric power), the power is typically received and transmitted on by one or more power converters. For example, a generator-side converter can receive alternating current (AC) power from the generator via a stator bus and can convert the AC power to a suitable output frequency, such as the grid frequency. The AC power is provided to the electrical grid via a line bus. 
     Low, medium, and high voltages are not rigidly defined, but for example the term “low voltage” may refer to voltages less than or equal to 1.5 kV, “medium voltage” may refer to voltages greater than 1.5 kV and less than 100 kV, and high voltage may refer to voltages at 100 kV and above. 
     High Power-Density (HPD) Power Systems—Power Converters for Ships and Other Environments With Compact Space Requirements: Certain environments, such as military and commercial ships, and also aircraft, place a premium on the utilization of space. As a result, ships require power converters which are more compact than those which may be employed in land-based environments. It is also desirable to reduce the weight of power conversion systems for maritime applications. Reductions in power converter volume and power converter weight lead to both improved power density and less drag on a ship. 
     The present system and method is particularly though not exclusively suited for microgrids such as those found on ships and airplanes. (Grids and microgrids are generally referred to in this document as “power systems.”) Power systems for ships and airplanes, as well as power systems suited for other compact physical spaces, benefit from being as physically small and compact as possible. For a given power level, the smaller the physical size the higher the resulting power density throughout the power system. High power densities in turn entail the generation of large amounts of undesired heat which needs to be dissipated. 
     One example of a high power-density (HPD) system is a Power Electronic Building Block (PEBB) Least Replaceable Unit (LRU), which is a structural and functional element of a power converter, and may be any power processor that converts any input electrical power to the desired voltage, current, and frequency output. PEBBs are intended for use as part of a modular and scalable power converter architecture typically employing multiple interconnected PEBBs. 
     A PEBB typically incorporates power devices, gate drives, transformers, and other components into a building block with a configurable and clearly defined functionality. 
     For reasons of energy-efficiency and effective ship-board space utilization, then, it is desirable to provide for PEBB LRUs with compact elements, high power densities, with the resulting high heat. Such PEBB LRUs, as well as other compact, high power systems, may entail the use of transformers which are both physically compact, and which step up lower voltages to higher voltages. Such high-power transformers may entail the use of 1:1 winding ratios, or may entail the use of K:N winding ratios, where N is a value equal or greater than K. The low volumes are particularly prone to heat dissipation, and the and high winding ratios result in still more heat generation and corresponding need for heat dissipation. 
     Heat Dissipation Overview: The power processing limits of power converters and power electronics building blocks (PEBBs) for power converters are largely determined by the thermal management of the high-frequency transformers employed in such systems. “Thermal management” is another way of referring to the heat dissipation abilities for the transformer, which in turns largely determines the volume and weight, and therefore the power density and specific power of the power converter. 
     Legacy power converters and power electronics building blocks (PEBB) have relied on air-cooled transformers. The heat generated from the transformer(s) during power converter operation is comprised of the Ft loss in the primary and secondary coils (coils loss), and the heat/power loss in the magnetic core (core loss). 
     The heat loss distribution among coils loss and core loss varies based on a specific design and materials used. Typical transformer cooling systems employ air cooling, which may for example include fans which force air around and through a transformer. This in turns requires ample space for air flow, as well as high air velocities, both of which may work against the goal of maintaining low overall volume for a power converter. 
     Given the aforementioned deficiencies, such as for example volumetric power density challenges, what is needed is a compact cooling system for medium-to-high voltage HPD electrical transformers. What is further needed is a cooling system which employs liquid cooling for efficient heat transfer. What is further needed is a cooling system which provides for structural integration with a power transformer, or which provides for a broad contact area between one or more liquid coolants and the electrically active, heat-generating elements of a power transformer. 
     III. BRIEF SUMMARY OF THE INVENTION 
     This present system and method advances the air-cooled converter with a liquid-cooled thermal management solution to provide for improved volumetric power density, especially but not exclusively for space-constrained pulse load power converter applications, including for example and without limitation onboard a military or commercial ship, or onboard an airplane. 
     Liquid vs. Fluid: In general/common usage, the terms “liquid” and “fluid” are generally or loosely equivalent. In chemistry and physics, a “fluid” is anything that flows (including both liquids and gases), while a “liquid” is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure. In this document the terms “liquid” and “fluid” both refer to flowing, nearly non-compressible materials with nearly constant volume. (So both terms broadly mean “a fluid which is a liquid but is not a gas”). However, in this document, “liquid” and “fluid” are further assigned distinct meanings: 
     (A) “Liquid” refers to a coolant liquid material  240  (see  FIG.  2   ), which in some embodiments may be water, that is run through a cold plate  160  (see  FIG.  1   ); 
     (B) “Fluid” refers to a heat-transfer liquid material  740  (see  FIG.  7   ), which may in some embodiments be an oil, for use in direct contact with a transformer  120  inside a heat management enclosure  710 . 
     The usage of “liquid”  240  (for example, water) vs. “fluid”  740  (for example, oil) is for convenience of reading only, to aid the reader in distinguishing the different types/applications for different liquids which provide cooling and/or which transfer and remove heat. 
     It will be noted that the consistent usage in this document of “coolant” in one context vs. “heat-transfer” in the other is also for convenience and for ease of reader comprehension. Other literature in the relevant arts may use such terms as “coolant” or “heat-transfer substance” equivalently, or with other significations. 
    
    
     
       IV. BRIEF DESCRIPTION OF THE DRAWINGS 
       Advantageous designs of embodiment of the present invention result from independent and dependent claims, the description, and the drawings. In the following, preferred examples of embodiments of the invention are explained in detail with the aid of the attached drawings. The drawings, which are incorporated herein and form part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the relevant art(s) to make and use the invention. 
         FIG.  1    illustrates an exemplary transformer with integrated cold plates according to an embodiment of the present system and method. 
         FIG.  2    illustrates an exemplary liquid cooling system according to an embodiment of the present system and method. 
         FIG.  3 A  illustrates an exemplary transformer with integrated cold plates according to an embodiment of the present system and method. 
         FIG.  3 B  illustrates an exemplary transformer with integrated cold plates according to an embodiment of the present system and method. 
         FIG.  4    illustrates an exemplary transformer with integrated cold plates according to an embodiment of the present system and method. 
         FIG.  5    illustrates an exemplary power converter with multiple power electronic building blocks, each power electronic building block having an exemplary liquid cooling system. 
         FIG.  6 A  illustrates an exemplary transformer coil according to the present system and method. 
         FIG.  6 B  illustrates an exemplary transformer coil according to the present system and method. 
         FIG.  7    illustrates some elements of an exemplary fluid-immersed transformer according to an embodiment of the present system and method. 
         FIG.  8    illustrates some elements of exemplary fluid-immersed transformers according to embodiments of the present system and method. 
         FIG.  9    illustrates an exemplary application of a liquid or fluid cooled transformer integrated into a hybrid power electronics building block which may be used in a power converter. 
     
    
    
     Regarding text in the Figures: Any text in the figures is provided for convenience as an aid to understanding, to provide a reader with a verbal reminder as to the nature of some elements. Such text should not be construed a limiting, and different elements may be known or understood by additional or alternative labels, nomenclature, or alternative embodiments, as described within the written disclosure. For a more complete description of the elements illustrated, the reader is referred to the reference numbers shown in the drawings and to discussion in the disclosure associated with those reference numbers, as well as to other discussion in the disclosure where reference numbers may be omitted. 
     Specific functional or operational values shown in the figures (for example, voltage values, power values, structural dimensions, and other numerical values) should be construed as exemplary only and not as limiting, unless described as limiting in the written disclosure. 
     V. DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     While the present invention is described herein with illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. 
     The following detailed description is merely exemplary in nature and is not intended to limit the system, configurations, and methods taught, nor to limit the elements or steps of the system, configurations, and methods taught, nor to limit the applications of the present systems, methods, and configurations as disclosed herein. Further, there is no intention for the scope to be bound or limited to or by any theory presented in the preceding background or summary, nor in the following detailed description. Those skilled in the art with access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the invention would be of significant utility. 
     Throughout the application, description of various embodiments may use “comprising” language, indicating that the system and method may include certain elements or steps which are described; but that the system and method may also include other elements or steps which are not described, or which may be described in conjunction with other embodiments, or which may be shown in the figures only, or those which are well known in the art as necessary to the function of power systems. However, it will be understood by one of skilled in the art, that in some specific instances, an embodiment can alternatively be described using the language “consisting essentially of” or “consisting of.” 
     For purposes of better understanding the present teachings and in no way limiting the scope of the teachings, it will be clear to one of skilled in the art that the use of the singular includes the plural unless specifically stated otherwise. Therefore, the terms “a,” “an” and “at least one” are used interchangeably in this application. 
     Headings used in this detailed description are present only to assist in making this document readable and easy to navigate, and should not be construed as defining or limiting. 
     The system and method is not limited to the embodiments described below, which are exemplary only. Rather, the full scope of the system and method is recited in the claims which follow. It will be further understood that the appended claims are themselves disclosure, and the full scope of the system and method may include elements which are recited in the claims only. 
     VI. EXEMPLARY LIQUID COOLING ELEMENTS AND SYSTEMS FOR TRANSFORMERS 
     Persons skilled in the art will appreciate that heat dissipation is necessary for effective operations of electric systems, including heat dissipation for transformers. Legacy power converters may employ air-cooling, for example cooling fans, or possibly only the general flow of air in the ambient environment, in some cases supplemented by vents and heat sinks. 
     As noted above, emerging power converters for compact environments may have a high power density (HPD) and a specific power which is significantly increased (as compared with the power density/specific power of a legacy power converters). The increased power density and specific power generates more volumetric or gravimetric heat (or heat per unit volume or weight) and higher temperature than is generated by legacy power converters. Further, such systems may employ the HPD transformers  120  for conversion of currents to high frequencies (HF), which further increases the generation of heat. 
     To dissipate the heat generated by HPD-HF transformers in HPD converters, a liquid-based cooling system may be employed for the HPD transformers  120 , either in combination with air-based cooling or to substantially replace air based cooling for the transformers  120 . 
     In particular, the combination of higher voltages, multiple transformer windings, and the more compressed size of the HPD power converter as a whole may result in intense heat generation by the HPD-HF transformers  120 . The heat generated from the HPD-HF transformers  120  during operation is comprised of the Ft loss (current-squared (I 2 ) time loss) in the primary and secondary coils (coils loss), and the heat/power loss in the magnetic core (core loss) of the transformers  120 . 
     In some embodiments, the present system and method introduces dedicated liquid based cooling for HPD-HF transformers  120 . 
     Thus, in some embodiments, the present system advances air-cooling with a liquid-cooled thermal management solution to further improve volumetric power density for especially space constrained pulse load converter applications (for example, onboard a military or commercial ship). The heat loss distribution among coils-loss and core-loss varies based on a specific design and materials used. To reduce volume and increase power density, embodiments of the present system and method employ a liquid-cooled HPD-HF transformer  100 , with a liquid-cooled solution for the thermal management of the transformer coil components  135 . 2 ,  135 . 1  and transformer core  145 . 
     Exemplary HPD-HF Transformer with Integrated cold plates:  FIG.  1    provides a schematic illustration of an exemplary HPD-HF transformer  120  with integrated cold plates  160  of a liquid cooling system  200  (see  FIG.  2   ) (in  FIG.  1   , the combined transformer  120  proper (that is, the coils plus magnetic core) with integrated cold plates  160 , hereinafter “TICP”, is labeled with reference number  100 . 1 ), according to one embodiment of the present system and method. In an embodiment, TICP forms structurally integrated units, that is, with some elements  160 ,  165  of the liquid cooling system  200  embedded within the structure of HPD-HF transformer  120 . 
     Coil Components (Coils) and Cores: In an embodiment, the HPD-HF transformer  120  includes one, two, or more primary (high voltage/HV) coil components  135 . 1 ; one, two, or more secondary (low voltage/LV) coil components  135 . 2 ; one, two, three or more magnetic (ferrous) cores  145  (three shown in the figure), and electrical connections  185  (only one shown in the figure). One or more of the cores  145  may have one or more internal core element(s)  147  which may run through a gap or gaps in the coils  135  and/or gaps in the cold plates  160 . The internal core element(s)  147  may only provide structural support, or be made of ferrous materials to provide for additional magnetic coupling/inductance between the coils  135 . 
     The metallic, electrically conducing elements  610  of the primary coils  135 . 1  and secondary coils  135 . 2  are not actually illustrated in  FIG.  1    (see instead  FIGS.  6 A and  6 B ); rather, illustrated in the figure are the exteriors of the solid coil components  135 . 1 ,  135 . 2 , which may for example be made of silicon, resins, epoxies, ceramics, glass, or other non-electrically-conducting substance or materials  620 . 1 ,  620 . 2  (see  FIGS.  6 A and  6 B  for cross-sectional view of the coils  135 ), with the electrically conducting (typically metallic) coils  610  embedded within. In this document, for brevity, the “coil components”  135  are typically referred to in brief simply as “coils”  135 . In one exemplary embodiment, the coil wires  610  may be Litz wire. 
     The epoxy, resin, glass, ceramic material or similar bonding/enclosure material is referred to generally as the “coil support material”  620 . 1 ,  620 . 2 , and is an effective heat conductor. Heat generated by the electrically conducting elements (such as wires, filaments, or foils)  610  of the coils  135  is readily transferred into the surrounding epoxy, resin, or ceramic material  620 . 1 ,  620 . 2  of the coils components  135 . 1 ,  135 . 2 . 
     In the exemplary embodiment shown, three cold plates  160  (also referred to as “heat exchange plates”  160 ) are physically inserted or sandwiched between, and in substantial contact with, all of the coil components  135 . 1 ,  135 . 2  and the core(s)  145 . In an embodiment, the cold plates are made of a non-ferrous (non-magnetic) metal. In an alternative embodiment, the cold plates  160  may be made of other non-ferrous, non-metallic materials which are suitable for conducting heat. 
     Running through each cold plate  160  are one or more internal liquid transport channels  260  (or “coolant channels”  260 ) (not illustrated in  FIG.  1   , see  FIG.  2   ) suitable for conducting a coolant liquid  240  (see  FIG.  2   ) such as deionized water, chilled water, or processed water with or without petro-chemical additives, or other coolant liquids  240 . 
     Two or more coolant tubes  165  (or “coolant pipes”  165 ) are connected to the cold plates  160  at coolant ports  175 , which are fluid input/output ports along an exterior surface of the cold plates  160 . The coolant ports  175  may have valves or other fluid control mechanisms not shown in the figure. The coolant tubes  165  transfer the liquid coolant  240  into and out of the interior coolant channels  260  of the cold plates  160 . Transport of the coolant liquid  240  through the interior coolant channel(s)  260  of the cold plates  160  conducts heat away from transformer  120 , and serves to maintain the transformer  120  at a safe operating temperature during power conversion. 
     In an alternative embodiment, exterior coolant tubes  165  or coolant pipes  165  may be integrated into cold plates  160 , bonded or otherwise attached to coolant ports  175 , and/or be integrated extensions of interior coolant channels  260 . 
     Persons skilled in the art will appreciate that thin layers of various additional heat conducting materials (not shown in  FIG.  1   ), such as thermal binding materials or glues, may be used to help bond or adhere the surfaces of the coils  135 . 2 ,  135 . 1  and the core  145  with the surfaces of the cold plates  160 . Such thermal heat conducting materials may also help to maintain efficient and uniform heat transfer. In some embodiments, such thermal heating conducting materials may be applied in thin layers, on the order of 1 mm or less; in alternative embodiments, thicker layers may be used; in alternative embodiments, no additional heat conducting materials or glues are used for bonding. Instead, the coils  135 . 2 ,  135 . 1  and cold plates  160  may be bound together and maintained in thermal contact via mechanical means such as screws (not shown in the figure), clamps (not shown), bolts (now shown), or via the containment and pressure exerted by the surrounding cores  145 . 
     In an alternative embodiment, some or all of the coils  135 . 2 ,  135 . 1  and cold plates  160  may be bound together and maintained in permanent thermal contact during manufacturing process by: (i) using heat, compression, application of surface solvents, or similar means to melt, partially melt, or chemically soften a thin surface layer and/or the edges of the coils  135 . 2 ,  135 . 1  and/or cold plates; (ii) mechanically pressing together the coils  135 . 2 ,  135 . 1  and/or cold plates; and (iii) then allowing the surfaces/edges so treated to physically harden and bond together at a molecular level. 
     In alternative embodiments, combinations of two or more of screws, clamps, bolts, the cores  145 , thermal heat conducting materials and glues, and chemical/heat bonding may be used to maintain contact, pressure, and the necessary degree of thermal conductivity between the coils  135 . 2 ,  135 . 1  and the cold plates  160 . 
     In further discussion in this document, a “cold plate” is sometimes abbreviated by a capital “C”, the secondary/low-voltage coils by “S”, and the primary/high-voltage coils by “P”. Various embodiments of the present system and method may be referred to, in brief, according to the stacking order of these elements. For example, in the exemplary embodiment  100 . 1  of  FIG.  1   , the stacking order is “C-S-P-C-P-S-C”. 
     Major (Larger) Surfaces: It will be apparent from the figures that in some embodiments, and in some geometries, some flat surfaces  190  or substantially flat surfaces  190  of the cold plates  160 , the coils  135 , and/or the core(s)  145  may be the largest surfaces of those elements, or may be one of two opposing largest surfaces. Such largest surfaces  190  are referred to in this document as “major surfaces”  190 , and it will be apparent that such major surfaces  190  are particularly well-suited for heat transfer to the cold plates  160 . Smaller, minor surfaces  195  may facilitate heat transfer as well. In alternative embodiments, some transformer components  135 ,  145  may be approximately or substantially cubical, in which case the area distinction between major surfaces  190  and minor surfaces  195  may be insignificant. 
     Exemplary Liquid Cooling System (LCS) 
     Persons skilled in the relevant arts will appreciate that a full or functionally complete liquid cooling system  200  for transformer  120  will include not only the liquid cooling elements (LCS)  160 ,  165  and coolant liquids (s)  240  of cooling system  100 . 1 , but also various additional elements, some or all of which may be external and possibly remote from transformer  120 . 
       FIG.  2    illustrates an exemplary liquid cooling system (LCS)  200  for a HPD-HF transformer  120  according to the present system and method. Exemplary LCS  200  and other LCS systems consistent with the scope of the appended claims may employ, among other elements: 
     (i) the exemplary transformer with integrated cold plates (TICP)  100  discussed above in conjunction with  FIG.  1    (and see also  FIGS.  3 ,  4 ,  5   , and  FIG.  6   ); 
     (ii) the exemplary fluid immersed transformers (FIT)  700  (see  FIGS.  7 ,  8   , and also  FIG.  6   , for further discussion); 
     (iii) any transformers consistent with the scope of the appended claims. 
     The LCS  200  includes one or more cold plates  160 , already discussed above, which are bonded to, embedded within, or otherwise in close structural contact with/thermally coupled with elements of the HPD-HF transformer  120 . 
     Cold plates: It will be noted that while, in the figures in this document, the cold plates  160  are illustrated as substantially cuboid with six substantially flat, mutually orthogonal surfaces, other shapes are possible. In some embodiments of the present system and method, it may prove advantageous for the cold plates to be molded to non-cuboid shapes, or into modified cuboid shapes with various extensions, to better conform to, increase surface contact, or increase current or magnetic interactions with other elements  135 . 2 ,  135 . 1 ,  145  of the transformer  100 . In some embodiments of the present system and method—and possibly to reduce the weight of the cold plates  160 , provide for additional thermal conductivity, or to provide supplemental air-cooling for the cold plates  160 —it may prove advantageous for the cold plates to have textured surfaces, including for example and without limitation: ridges, bumps, grooves, other textures or variations in the surface height, or to have non-linear (curved) portions. 
     In some embodiments, the cold plates  160  may be made of a single metal, a single metal alloy, a single ceramic material, a single polymer material, carbon-based material, or other single non-ferrous, non-electrically conducting, but heat-conducting material. In alternative embodiments, the cold plates  160  may be made from two or more materials, for example separate, different materials may be used for a first side of the cold plate, a second side of the cold plate, and possibly a third material for the lining of the interior channels  260  (discussed below). In an embodiment, surfaces of the cold plates which are exposed to the air may have an insulating material attached to the exposed surfaces to prevent heat leakage, to prevent fires or burns, and/or to maintain maximum heat transfer through the coolant channels  260 . 
     Coolant channels: Interior to the cold plates  160  are one or more coolant channels  260  which conduct coolant liquid  240  through the cold plates  160 . It will be noted that the number, geometric configuration, relative width and/or diameter (in relation to the size of the cold plate), and arrangement of the coolant channels  260  illustrated in  FIG.  2    is exemplary only; many alternative arrangements of coolant channels  260  may be employed within the scope of the present system and method. The coolant channels  260  can be bulk/macro channels (with diameters on the order of the shortest-width  310  (see  FIG.  3 A ) of the cold plates  160 , or may be micro-channels with diameters substantially smaller than the shorter width  310  of the cold plates  160 . 
     It will be further noted that while the coolant channels  260  are described in this document as “interior” to the cold plates  160 , “interior channels” are construed to include cooling channels for which the metallic surface of the cold plates  160  may extend or protrude partially above a flat, smooth, or ridged surface of the cold plates  160 ; and/or cooling pipes (for example, metal pipes) carrying the coolant liquid  240  which are bonded to the flat, smooth, or ridged surfaces of the cold plates  160 . 
     Coolant tubes: Feeding liquid coolant  240  into and out of the coolant channels  260  within the cold plates  160  are one or more inflow coolant tubes  165 . 1  and one or more outflow coolant tubes  165 . 2 . Here again, the single inflow coolant tube  165 . 1  and single outflow coolant tube  165 . 2  illustrated in  FIG.  2    are exemplary only, and (as in  FIG.  1   ) greater numbers of coolant tubes  165  may be employed. While both  FIGS.  1  and  2    illustrate coolant tubes  165  as entering/exiting the cold plate  160  along a narrow side surface  185 , this is exemplary only. In alternative embodiments, one or more coolant tubes  165  may be attached, and provide for coolant inflow or outflow, along a larger planar surface  190  of a cold plate  160 . 
     Exemplary cooling system  200  may also include, for example and without limitation: more or fewer cooling plates  160 ; one or more alternative or additional cold plates  160  bonded to exterior surface(s) of the magnetic cores  145  (see  FIG.  4   ); other alternative geometries; and other variations within the scope of the appended claims. 
     Exemplary cooling system  200  may also include a pumping/heat-exchange sub-system  210 , referred to in the appended claims simply as “pumping system ( 210 )”, and which may also be referred to as a “regulatory system”  210 , “filtering system  210 ”, “coolant conditioning system”  210 , and other similar terms. Pumping/heat-exchange sub-system  210  may include for example and without limitation: 
     (i) Liquid pumps  215 . In an embodiment, a first coolant pump  215  provides pressure to drive cold coolant  240  into the cold plate  160  via input coolant tube(s)  165 . 1 ; while a second coolant pump  215  pressures either or both of a used coolant and a fresh coolant (from bypass/mixing valve  220 ) into a coolant conditioning unit  225 . The two pumps shown are exemplary only. Other pumps may be employed as well, for example a fresh coolant input pump (not shown in the figure) to draw fresh cold coolant liquid  240  either directly into coolant tubes  165  or into a heat exchanger  235 ; 
     (ii) a heat exchanger  235  to remove heat from hot coolant  240  for transfer to an environmental heat sink  295 ; and/or to provide cooling for liquid coolant  240  via an environmental cold source  201 ; 
     (iii) a bypass/mixing valve  220  which may recycle some of heated coolant  240  by mixing it with fresh coolant liquid  240 , or by alternately using cold coolant  240  and recycled hot liquid  240 ; 
     (iv) a coolant conditioning unit  225  which may clean or filter coolant liquid  240  to remove metal or non-metal particles, dirt and extraneous chemicals, or which may provide chemical additives (such as antifreezes) to the coolant liquid  240 ; 
     (v) a coolant reservoir  230  which provides a short-term reserve or coolant buffer for coolant liquid  240 . 
     Persons skilled in the relevant arts will recognize that exemplary cooling system  200 , and in particular pumping/heat-exchange system  210 , may include other elements not shown in  FIG.  2   , including for example and without limitation: valves; temperature sensing devices; pressure sensing devices; additional chemical or coolant reservoirs; internal processing and memory to control the cooling system  220  via software and firmware; internal electrical systems to power the pumps  215 , valves, processor, and memory; and input and output control/data ports for external monitoring of the cooling system  200 . Such additional elements will, in some embodiments, be structurally part of a PEBB LRU and/or a power converter which contains one or more transformers with liquid cooling elements  100 ,  700 . 
     In an embodiment of the present system and method, multiple elements of pumping/heat-exchange subsystem  210  may be commonly housed and structurally combined in a sub-system enclosure  280 , providing for convenient mounting and modularity. In an embodiment of the present system and method, the pumping system  210  elements within the enclosure  280  may provide pressure, conditioned coolant, and heated coolant removal for multiple transformers  100  (the one, single transformer  100  shown in  FIG.  2    being exemplary only, and not limiting.) 
     No Air-Based Cooling or Limited Air-Based Cooling: It will be apparent from the above description and figures, as well as further description and figures below, that the liquid-based transformer cooling of the present system and method removes all or substantially most of the heat generated by the transformer  120  via heat transfer through the solid material components of the transformer, the cold plates  160 , and the liquid coolant  240 . In some embodiments, heat transfer via the ambient air immediately surrounding transformer  120  may be essentially negligible. 
     In alternative embodiments, heat transfer from the transformer  120  via the proximately surrounding air may provide some amount of substantive or beneficial additional cooling; but the dominant mode for heat removal is still primarily via: (i) the flow of heat from the transformer coils  135  and core(s)  145  into the cold plates  160 , and then (ii) from the cold plates  160  to an external environmental heat sink  295  via the liquid coolant  240  running through the cold plate(s)  160 . 
     Cold Source and Environmental Heat Sink: The exemplary environmental cold source  201  and the exemplary environmental heat sink  295  illustrated in  FIG.  2    are understood to be elements of the larger environment apart from the exemplary cooling system  200 . For example, for a ship-based power converter application, both the environmental cold source  201  and the environmental heat sink  295  may be an overall shipboard process water or chill water supply system, water in the sea, ocean, or river in which a ship travels. For another example, for an airplane-based power converter application, both the environmental cold source  201  and the environmental heat sink  295  may be the air external to the aircraft. 
       FIG.  3 A  illustrates an exemplary HPD-HF transformer  120  with integrated cold plates (TICP) of a liquid cooling system  200  (see  FIG.  2   ). (In  FIG.  3 A , the combined transformer  120  with integrated cold plates (TICP) is labeled with reference number  100 . 2 ). Some elements of exemplary TICP  100 . 2  are the same or substantially similar to elements of the exemplary TICP  100 . 1  of  FIG.  1    and/or cooling system  200  of  FIG.  2   ; to avoid redundancy, some details of those elements already described in  FIG.  1    and/or  FIG.  2    are not repeated here. 
     In  FIG.  3 A , the transformer with integrated cooling plates (TIPC)  100 . 2  is shown in cross-sectional view, and with some elements omitted as compared to the embodiment  100 . 1  of  FIG.  1   . In the cross-sectional view of  FIG.  3 A  an internal core element  147  is not present. 
     In TICP  100 . 2 , only a single primary coil  135 . 1  is employed, which is sandwiched between two secondary coils  135 . 2 . Two cold plates  160  are employed on the outer surfaces of secondary coils  135 . 2 . The cold plates  160  are in thermal contact with the magnetic core  145  as well. Coolant pipes  160  with interior channels  260  running through the cold plates  160  (or heat exchange plates  160 ) are shown as well. In TLCS  100 . 2 , the stacking order is “C-S-P-S-C”. 
       FIG.  3 B  illustrates another exemplary HPD-HF transformer  120  with integrated cold plates (TICP)  100 . 3  of a liquid cooling system  200 . (In  FIG.  3 B , the combined transformer  120  proper with integrated cold plates ( 160 ) (TICP) is labeled with reference number  100 . 3 ). 
     In  FIG.  3 B , the transformer with integrated cooling plates (TIPC)  100 . 3  is shown in cross-sectional view, and with some elements omitted as compared to the embodiment  100 . 1  of  FIG.  1   . In the cross-sectional view of  FIG.  3 B  an internal core element  147  is not present. 
     Some elements of exemplary TICP  100 . 3  are the same or substantially similar to elements of the exemplary TICP  100 . 1  of  FIG.  1    and/or cooling system  200  of  FIG.  2   , and details of those elements already described in  FIG.  1    and/or  FIG.  2    are not repeated here. 
     In  FIG.  3 B , the TICP  100 . 3  is shown in cross-sectional view, and with some elements omitted as compared to the embodiment  100 . 1  of  FIG.  1   . In TICP  100 . 3 , only a single primary coil  135 . 1  and a single secondary coil  135 . 2  are employed, with a single cold plate/heat exchange plate thermally coupled between them. The cold plate  160  is in limited thermal contact with the magnetic core  145  as well. In TICP  100 . 3 , the stacking order is “S-C-P”. 
       FIG.  4    illustrates another exemplary HPD-HF transformer  120  with integrated cold plates (TICP)  100 . 4  of a liquid cooling system  200 . Some elements of exemplary TICP  100 . 4  are the same or substantially similar to elements of the exemplary TICP  100 . 1  of  FIG.  1    and/or cooling system  200  of  FIG.  2   , and details of those elements already described in  FIG.  1    and/or  FIG.  2    are not repeated here. In the cross-sectional view of  FIG.  4    an internal core element  147  is present (so that two single cold plates  160 ′ and  160 ″, as well as each single coil  135 , appears to be split into two parts). 
     In  FIG.  4   , the TICP  100 . 4  is shown in and with some elements omitted as compared to the embodiment  100 . 1  of  FIG.  1   . In TICP  100 . 4 , only a single primary coil  135 . 1  and two secondary coils  135 . 2  are employed. Four cold plates/heat exchangers  160  are employed: (i) Two cold plates  160 ′,  160 ″ are physically and thermally coupled with two respective surfaces of the two secondary coils  135 . 2 , and are in interior physical/thermal contact with the magnetic core  145  as well; and (ii) Two cold plates  160  are attached to and thermally coupled with exterior surfaces of the ferrous core  145 . In TICP  100 . 4 , the stacking order is “C-F-C-S-P-S-C-F-C”. 
     The arrangements and ordering of elements of primary coils  135 . 1 , secondary coils  135 . 2 , cold plates  160 , and ferrous core parts  145 ,  147  illustrated and discussed in embodiments above are exemplary only and not limiting. Other arrangements may readily be envisioned within the scope of the appended claims, as discussed further below. 
     Generalized Transformer Component Arrangements: As will apparent to persons skilled in the relevant arts, and based on the above discussion and associated figures, various embodiments of the liquid cooled transformer ( 100 ) may include, for example and without limitation, and alone or in some cases in combination: 
     (i) A first major surface  190  (a largest surface or one of two largest surfaces) of the cold plate  160  being in contact with and thermally coupled with a second major surface  190  of the at least one of (i) the core  145  and (ii) one of the coil components  135 . 1 ,  135 . 2 . The first and second major surfaces  190  are so mutually shaped as to facilitate extended surface contact and thereby the effective transfer of heat between the first major surface  190  and the second major surface  190 . (See for example  FIGS.  1 ,  3 A /B,  4 , and  5 .) (Note that in  FIG.  1   , the contact major surfaces of the core  145  and the coil components  135  are not labelled with a references number, as these contact surfaces are obscured from direct view.) 
     (ii) The liquid cooled transformer ( 160 ) as described in list item (i) immediately above, where the first major surface  190  and the second major surface  190  are flat surfaces. (See for example  FIGS.  1 ,  3 A /B,  4 , and  5 .) 
     (iii) A single cold plate  160  has a first major surface  190  and a second opposing major surface  190 , each of the two opposing major surfaces  190  in contact with a major surface  190  from a different one of the transformer elements from among the core ( 145 ) and the two coil components ( 135 . 1 ,  135 . 2 ). (See for example  FIGS.  1 ,  3 A /B,  4 , and  5 .) 
     (iv) A single cold plate  160  is physically situated between, in physical contact along its major surfaces  190  with, and thermally coupled along those major surfaces  190 , with at least one of: (a) both the core  145  and one of the coil components  135 . 1 ,  135 . 2  (see  FIGS.  1 ,  3   ), or else (b) with two coil components  135 . 1  (see  FIGS.  1 ,  4 ,  5   ). 
     (v) At least two separate cold plates ( 160 ), where the at least two cold plates ( 160 ) are configured and arranged to be in physical contact with and in thermal contact with at least two different, non-adjoining transformer elements from among the core ( 145 ), the first coil component  135 . 1 , and the second coil component  135 . 2 . (See FIG. 
     (vi) At least two separate cold plates ( 160 ), where the at least two cold plates ( 160 ) are configured and arranged to be in physical contact with and in thermal contact with at least three different transformer elements from among the core ( 145 ), the first coil component ( 135 . 1 ), and the second coil component ( 135 . 2 ). (See  FIGS.  1 ,  3 ,  5   .). 
     (vii) Two or more primary coils  135 . 1 , and/or two or more secondary coils  135 . 2  (see  FIGS.  1 ,  3  and  5   ) may be employed. Such embodiments will typically but not necessarily employ two or more separate cold plates  160 , which are sandwiched between various coils  135 . 
     (viii) A primary coil  135 . 1  and a secondary coil  135 . 2  may be placed in direct physical and thermal contact, with one or two cold plates  160  attached to the directly-physically coupled coils  135  for heat removal from both. 
     (ix) A single cold plate  160  may be configured for direct physical and thermal contact with two different coils  135  ( 135 . 1 / 135 . 1 ,  135 . 1 / 135 . 2 , or  135 . 2 / 135 . 2 ) and also with direct physical and thermal contact from one or more cores  145 , for heat removal from both the coils  135  and the cores. In some embodiments, this is accomplished by having at least one of the minor sides  195  of the cold plate  160  in contact with the core(s)  145 , while the facing major sides  190  of the cold plate  160  are in contact with the two different coils  135 . 
     In general, other geometric arrangements, as well as the shapes and relative sizes of the coils  135 , core(s)  145 , and cold plate(s)  160  may be envisioned and fall within the scope of the appended claims. For example, in an embodiment not illustrated, the core  145  may be fixed in place as a layer between two coils  135 . 1 ,  135 . 2 , forming a block structure, with multiple coolant plates  160  placed on two, three, or up to six sides of the resulting block. Also, while the maximum number of primary and secondary coils  135  illustrated in the figures is two of each type, more than two coils  135  of a type (low voltage and/or high voltage) may be employed if suitably electrically coupled. Additional cold plates  160  may then be employed as well as needed. 
     Exemplary Application, Power Converter:  FIG.  5    illustrates an exemplary power converter  500  employing a liquid cooling system  200  or employing a liquid immersed transformer  700  (see  FIG.  7    and associated discussion, below) according to the present system and method. The exemplary power converter  500  may include, for example and without limitation: 
     (i) Two or more power electronic building block least replacement units (PEBB)  510 ; each PEBB  510 . 1 ,  510 . 2  having its own TICP  100 , and also other power elements  515  such as bridge converters with power switches (not shown in detail in  FIG.  5   ). For further discussion of an exemplary PEBB  510 , specifically a hybrid PEBB (HPEBB), see  FIG.  9    below. The two PEBBs  510 . 1 ,  510 . 2  are electrically/current-linked by one or more power couplings  530 . The power converter  500  will also have at least source (or input) power connection and at least one load (or output) power connection, not illustrated in the figure. 
     (ii) At least one pumping/heat-exchange sub-system  210 . In the embodiment shown, a single pumping/heat-exchange sub-system  210  may provide coolant for all the cold plates of the power converter  500 . In an alternative embodiment not illustrated, two or more pumping/heat-exchange sub-systems  210  may be employed. 
     (iii) Other converter elements  520 , which may include for example and without limitation additional or supplemental cooling systems (such as a fan-based air cooling system); control systems and circuits; monitoring systems; and input and output power ports. 
     (iv) In addition to the cold plates  160 . 1  which in integral to the transformers  100 , additional cold plates  160 . 2  may be provided for additional system cooling. Shown in  FIG.  5    are four exemplary additional cold plates  160 . 2  which may for example be attached to the exteriors of the PEBBs  510 , but other cold plates  160  may be envisioned as well. Shown in  FIG.  5    is also one exemplary cold plate  160 . 3  attached to the exterior of the power converter  500 , but additional exterior cold plates  160 . 3  may be employed. 
     It will be noted that the transformers  100  of both PEBBs  510 . 1 ,  510 . 2  shown in  FIG.  5    employ a “C-S-P-S-C” stacking arrangement, but this is exemplary only and other stacking arrangements fall within the scope of the present system and appended claims. 
     Some elements of exemplary TICPs  100 ′,  100 ″ are the same or substantially similar to elements of the exemplary TICP  100 . 1  of  FIG.  1    and/or cooling system  200  of  FIG.  2   , and details of those elements already described in  FIG.  1    and/or  FIG.  2    are not repeated here. 
     Coil Components (Coils)  FIG.  6 A  provides a cross-sectional view of an embodiment of an exemplary solid coil component  135  (or simply “coil  135 ” in brief) of an exemplary transformer  100 , which may be either a low-voltage/secondary coil  135 . 2  or a high-voltage primary coil  135 . 1 . The conducting-wire/metal-film  610  may be arranged in any of a variety of flattened spiral surface arrangements on a planar interior surface of a coil support material  620 , so that the wire/metal film  610  is fully embedded within the coil support material  620  except for external electrical connections  640 . Consistent with the present system and method, other coiled or winding surface patterns (not illustrated) may be made as well for conducting-wire/metal-film  610  which leave the conducting filament  610  fully embedded within coil support material  620 , except for external electrical connections  640 .  FIG.  6 B  provides for two cross-sectional views (I, II) of another exemplary embodiment of a solid coil component  135  (or simply “coil  135 ” in brief) of the exemplary transformer  100 , which may be either a low-voltage/secondary coil  135 . 2  or a high-voltage primary coil  135 . 1 . In the cross-sectional embodiments illustrated, electrically conducting wire  610  or metal film  610  of the coil  135  may be wound around a flattened section  620 . 1  of the coil support material  620 ; and the wire  610  and flattened section are the further embedded within an enclosing block  620 . 2  of coil support material  620 . 
     In an alternative embodiment (not illustrated) the conducting-wire/metal-film  610  may be arranged in any of a variety of flattened spiral surface arrangements other flattened, winding surface patterns (suitable for magnetic induction due to current flow) on a narrow or micro-channel interior surface of coil support material  620 , and still fully embedded within the coil support material  620  except for external electrical connections  640 . Consistent with the present system and method, other geometric coiled or winding arrangements (not illustrated) may be made as well for conducting-wire/metal-film  610  which leave the filament fully embedded within coil support material  620 , except for external electrical connections  640 . 
     In an alternative embodiment (not illustrated), coil  135  may be constructed so that part of wire/filament  610  is embedded, wound and/or coiled, interior to coil support material  620 ; while a portion of wire/filament  610 , possibly with suitable electrical insulation, may be proximate to or partially or wholly exposed on one or more exterior surfaces of solid coil  135 . 
     It will be understood by persons skilled in the art that, however wire/filament  610  may be arranged or configured in relation to coil support material  610  so that: (i) coil support material  620  absorbs substantially all of the heat generated by wire/filament  610 ; and (ii) solid coil component  135  has at least one exposed exterior surface suitable for dissipating heat to a thermally coupled adjacent material (which may be either a cold plate  160  or another solid coil  135 ; or may be a surrounding heat-transfer fluid  740  such as oil  740 ). 
     Coil Materials: In an embodiment of the present system and method, the coil support material  620  may be silicon. In alternative embodiments, coil-support material(s)  620  may for include, for example and without limitation: resins, epoxies, ceramics, glass, or other non-electrically-conducting but thermally conducting substance or materials. The coils  135  may also include other materials, including for example and without limitation: (i) Polymer or polymer composites (used for insulation), for example, Epoxy or Bisphenol-A type epoxy, with 60 wt % of quartz filler added to it; and/or (ii) ceramics (e.g., alumina) used for insulation as an alternative to polymer or polymer composites. 
     In an embodiment, the coil support material  620  is selected to be able to readily sustain (without melting, fracture, burning, or other decay) temperatures of up to 200° C. which may be generated by the conducting (typically metallic) coils  610  embedded within. Conducting Material: In exemplary embodiments, coil  135  may be made from metals or metal alloys such as Litz wire, or other metals or metal alloys. 
     Exemplary HPD-HF Fluid-Immersed Transformers 
     In an embodiment of the present system and method, and as either an alternative to or as an addition to embodiments discussed above in this document, the entire HF transformer  120  may be structurally fixed and/or suspended within a substantially sealed container  710 . The entire container may be filled with a non-electrically conducting, but heat-conducting fluid  740 , such as a mineral oil (“the oil”), thereby immersing the transformer  120  in the oil  740  or other heat conducting fluid  740 . 
     In an embodiment, a selected oil  740  is the medium of heat transfer from transformer  120 . Oil  740  is both an very good thermal conductor and an excellent electrical insulator. Further, the use of a fluid  740  as the heat transfer medium, whether oil or another heat conducting fluid, ensures that the heat transfer medium has full contact with all exposed surfaces of the transformer  120 , for optimal heat removal. 
     As compared to air as a potential cooling medium, oil  740  has higher heat capacity and better thermal conductivity. Table 1 lists approximate, relative heat capacities and thermal conductivities for water, air, and oil (selected for ranges of operating temperatures and pressures that may be applicable for the present system and method). For simplicity, air is assigned a normalized heat capacity of 1. It will be noted that: (1) relative heat capacity and thermal conductivity will vary for different kinds of oils which may be used; and (2) water has significantly better heat capacity/thermal conductivity compared with oil, but water cannot be used as the fluid  740  for direct immersion of transformers  120  due to the electrical conductivity of water; however water is suitable for use in the cooling channels  260  of cold plates  160 . 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Relative Heat Capacities and Thermal Conductivities 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Relative 
                 Thermal Conductivity 
               
               
                   
                 Heat Conductor 
                 Heat Capacity 
                 (Watt/(Meter * Kelvin) 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Air 
                 1 
                 ~0.02 → 0.05 
               
               
                   
                 Oils 
                 ~1.6 → 2.0 
                 ~0.1 → 0.2 
               
               
                   
                 Water 
                 ~4.2 
                 ~0.5 → 0.7 
               
               
                   
                   
               
            
           
         
       
     
       FIG.  7    illustrates of an exemplary HPD-HF transformer  120  which is immersed within a heat-transferring fluid (HTF)  740  (which may be oil  740 ), all contained within a heat management enclosure (HME)  710  with attached or integrated surface cold plates  160 , according to one embodiment of the present system and method. 
     In  FIG.  7   , the combined transformer  120  proper along with the HTF  740 , HME  710 , and cold plates  160 , hereinafter “fluid-immersed transformer” (FIT) is labeled with reference number  700 . 1 . FIT  700 . 1  may for example include a high-power-density, high frequency (HPD-HF) transformer  120  which may employed for pulse-load power conversion applications. 
     In an embodiment, the exemplary FIT  700 . 1  forms a structurally integrated unit, that is, with some elements  160 ,  165 ,  710 ,  740  of the liquid cooling system  200  are physically and/or thermally coupled with the structure of the HPD-HF transformer  120 . The FIT  700 . 1  is configured/assembled with two primary (high voltage/HV) coils  135 . 1  and two secondary (low voltage/LV) coils  135 . 2  in an S-P-P-S configuration. 
     Spatial gap for cooling fluid: In the embodiment shown in the figure, there is a spatial gap  730  between the larger planar surfaces of the two high voltage coils  135 . 1 . This spatial gap  730  enables the heat-transferring fluid (HTF)  740  to fill the spatial gap  730 , allowing for an increased rate of heat transfer between the high voltage coils  135 . 1  and the HTF  740 . In an alternative embodiment, the gap  730  is not present, or is instead filled with a non-electrically conducting material. 
     Heat Management Enclosure: The transformer  120  may be attached to one or more interior surfaces  755  of the enclosure  710 , or may be mechanically coupled to and suspended within the enclosure  710  via struts, brackets, or similar attachments  805  (see  FIG.  8   ). As may be seen in  FIG.  7   , the transformer  120  may be placed within the enclosure  710  so that it is substantially surrounded, on multiple transformer sides and/or on multiple transformer surfaces, by the HTF  740 . 
     The heat management enclosure (HME)  710  is sealed to prevent fluid leakage, with suitable fluid-sealed ports (not illustrated) for electrical connections to the transformer  120 . On one or more exterior walls/surfaces  755  of the HME  710  are one or more cold plates  160 , which are suitably bonded for effective thermal conductivity between the HME  710  and the cold plates  160 . While two cold plates  160  are shown in  FIG.  7   , additional cold plates  160  may be placed on other exterior surfaces  755  of HME  710  as well. In an embodiment, the cold plates  160  are shaped substantially the same as a face of the enclosure wall/skin (for example, with a rectangular shape). 
     In an alternative embodiment, other shapes (for example, circular or oval) may be employed instead for the cold-plates  160 . In an alternative embodiment, the HME  710  may have shapes other than cuboid (for example, spherical, ovoid, or with more than six exterior flat surfaces  755 ), with suitable shapes for the attached cold plates  160  to ensure effective thermal contact between the cold plates  160  and the HME  710 . 
     The HME  710  may be made of a material suitable to contain high temperature fluid  740  and to convey heat from the fluid  740  to the cold plates  160 . Such a material may include, for example and without limitation: a metal or metal alloy (preferably a non-ferrous metal) with suitable electrical isolation from the transformer  120 ; a ceramic material, a polymer material; a glass material; or a carbon-composite material. 
     In an alternative embodiment, the HME  710  and the cold plates may be formed, cast, or metallically-bonded to form a single, integrated structural unit. In such an embodiment, the cold plates  160  may also be viewed or understood as one or more thickened walls  755  of the heat management enclosure  710 , with coolant channels  260  running through the thickened wall(s) of the HME  710 . 
     During operation of the transformer, the heat generated from the winding coils  135 . 2 ,  135 . 1  and from the magnetic core body  145  will be conducted/transported to the HME skin/wall  755  via the HTF  740 . 
     Coolant fluids/liquids: It will be noted that two different cooling fluids/liquids may be employed in conjunction with exemplary fluid-immersed transformer  700 . 1 . For example, heat transfer fluid (HTF)  740  may be an oil or other complex hydrocarbon liquid which is effective for heat-conduction but is also an effective electrical insulator; while the liquid coolant  240  running through the cooling channels  260  of the cold plates  160  may be, for example and without limitation: tap-water, industrial-use water, chill water, de-ionized water, sea water, or water treated with suitable conditioning fluids such as antifreeze fluids, as well as possibly an oil coolant, an organic liquid coolant, or a silicone-based coolant. Other coolant liquids may be employed as well consistent with the scope of the appended claims. 
     In embodiments configured to employ a coolant liquid  240  which may potentially be corrosive (for example, salty sea water or ocean water used in ship-based power converters), suitable anticorrosive materials or linings may be employed for the interior surfaces of the coolant channels  260 . In an alternative embodiment, filtering elements (not illustrated) may be used to filter out potentially corrosive materials. 
     Other details of exemplary cold plates  160  and coolant channels  260  are discussed above in this document, and the details will not be repeated here. 
     Pumping System: A pumping/heat-exchange sub-system  210  (“pumping system  210 ” in the appended claims) may be required to provide the required flow-rates, pressure and liquid quality (filtration etc.) to remove the vast majority of the waste heat generated by the transformer  120 , and to reject the least amount of heat into the ambient environment. Such a pumping system may include, for example and without limitation: pumps, valves, heat exchanger, coolant conditioning components (e.g. filtration, de-gassing, de-ironing etc.), and a coolant  240  reservoir. A pumping system  210  the same or substantially similar to exemplary pumping/heat-exchange sub-system  210  of  FIG.  2    may be employed here as well, and a detailed discussion is therefore not repeated. 
     Circulation for the heat transfer fluid: In an alternative embodiment not illustrated, it may prove advantageous for heat transfer to provide for circulation of the HTF  740  within HME  710 . For this purpose, an internal fan or pumping system (not shown in  FIG.  7   ) may be included internally within HME  710 . In an alternative embodiment, a separate HTF pumping system may be situated externally to the enclosure  710 , with suitable pipes to circulate HTF  740  within the interior space of HTF  740 . 
     Additional Embodiments of a Fluid-Immersed Transformer:  FIG.  8    illustrates cross-sectional views of several alternative embodiments of exemplary fluid-immersed transformers (FITs)  700 , with transformers  120  which may be immersed within a heat-transferring fluid (HTF)  740 , all contained within a heat management enclosure (HME)  710  with attached cold plates  160 , according to alternative embodiments of the present system and method. 
     In  FIG.  8   , the FITs  740  are labeled  740 . 2  through  740 . 5 , respectively. FITs  740 . 2 - 740 . 5  shall be generally configured and arranged in ways similar to, and with similar arrangement and configuration as exemplary FIT  740 . 1  of  FIG.  7    above. Some details discussed above in conjunction with  FIG.  7   , as well as with other figures above, shall not be repeated here. 
     In  FIG.  8   , in one embodiment of the present system and method, FIT  700 . 2  includes one primary (high voltage/HV) coil  135 . 1  and one secondary (low voltage/LV) coil  135 . 2  with a fluid gap  730  in between in an S-G-P layout. The cooling channels  260  of the cold plate  160  are orthogonal to the plane of the cross-sectional view. 
     In an alternative embodiment, as illustrated by FIT  700 . 3 , the transformer  120  has one primary (high voltage/HV) coil  135 . 1  and one secondary (low voltage/LV) coil  135 . 2  in direct physical and thermal contact with each other (P-S configuration). Struts  805  or other mechanical connections may be employed to secure the transformer  120  to the interior walls of HME  710 . Heat is carried away via the HTF  740  on the sides, top, and bottom of the transformer  120 . The cold plates  160  have numerous micro-channel coolant channels  260 , which are orthogonal to the plane of the cross-sectional view. FIT  700 . 3  also illustrates exemplary transformer electrical connections  185 , which are not illustrated but are necessarily present for transformers  120 . 
     In an alternative embodiment, as illustrated by FIT  700 . 4 , the transformer  120  has two primary (high voltage/HV) coils  135 . 1  and one secondary (low voltage/LV) coil  135 . 2 , with two gaps  740  filled with HTF  740  in between the three coils  135 , for a P-G-S-G-P configuration. Coolant fluid is also present in the interior space of the enclosure  710  on the input and output sides of the transformer  120 . Heat is carried away via the HTF  740  on the sides, top, and bottom of the transformer  120 . The cold plates  160  have numerous micro-channel coolant channels  260  which are parallel to the plane of the cross-sectional view. 
     In an alternative embodiment, as illustrated by FIT  700 . 5 , the transformer  120  has two primary (high voltage/HV) coils  135 . 1  and one secondary (low voltage/LV) coil  135 . 2 , all in mutual physical and thermal contact, for a P-S-P configuration. Transformer  120  is suspended within the heat management enclosure (HME)  710  via struts  805  or other mechanical connections. Heat is carried away via the HTF  740  on the sides, top, and bottom of the transformer  120 . 
     Coolant fluid is also present in the interior space of the enclosure  710  on the input and output sides of the transformer  120 . Heat is carried away via the HTF  740  on the sides, top, and bottom of the transformer  120 . The cold plates  160  have numerous micro-channel coolant channels  260  which are parallel to the plane of the cross-sectional view. 
     Persons skilled in the relevant arts will appreciate that the embodiments of FITSs  700  of  FIGS.  7  and  8    are exemplary only, and that elements of the different exemplary embodiments may be combined in various ways. Other configurations/embodiments are possible as well within the scope of the present system and method, including for example and without limitation coils  135  arranged in configurations such as S-P-S, S-G-P-S, S-P-G-S, S-G-P-G-S, S-P-P-S, S-P-G-P-S, S-G-P-G-P-S-G, and other configurations as well. 
     Heat transfer: In FIT  700 , the transformer body  120  is immersed in a HTF  740 , such as oil. Heat generated by transformer  120  is thermally transported through oil  740 , to the skin/enclosure wall  755  of the HME  710 . This heat is then removed by cooling liquid  240  running through the coolant channels of cold plates  160 , which are in physical and thermal contact with the enclosure skin/wall  755 . 
     Additional FIT Embodiments: In some exemplary embodiments of the present system and method for the FIT  700 , the cold plates  160  (also referred to as “heat exchanger plates”  160 ) are configured in parallel with the plane of the coils  135 , as illustrated in figures above. In alternative embodiments, the cold plates  160  may be affixed to the exterior surfaces of the enclosure walls  755  along planes which are orthogonal to the plane of the coils  135 . In alternative embodiments, two or more cold plates  160  may be attached along different exterior walls of the  755  of the HME  710 , so that some cold plates  160  may be attached parallel to the plane of the coils  135 , and other cold plates  160  may be attached orthogonal to the plane of the coils  135 . In an alternative embodiment, a single cold plate  160  attached to one wall  755  of the enclosure  710  may be sufficient to cool the transformer  120 . 
     In an alternative embodiment, one or more cold plates  160  may be situated interior to HME  710 , with suitable coolant tubes  165  attached to run coolant liquid  240  through the cold plates. 
     In an alternative embodiment, an oil pumping system may be employed to circulate the oil  740  in the interior of HME  710 . 
     In an alternative embodiment, two or more transformers  120  may be contained within a single heat management enclosure  710 , with cold plates  160  affixed to the single HME  710  to remove the heat generated by all the transformers. 
     Persons skilled in the relevant arts will also appreciated that many particulars of any final design may vary depending on application specifics, including the power to be generated by a transformer, and the spatial constraints for the intended power converter application. Thus, such details as the number of coils  135 , the number and size of the core  145 , the size/weight/material/placement of cold plates  160 , the number and cross-section shape of coolant channels  260 , types of coolant fluid(s)  240 ,  740  to be employed, and many other specific design factors will be determined and optimized for particular applications. Laboratory and real-world testing of proposed design choices may be necessary to identify the optimum or near-optimum, specific structural, material, and configuration choices for a particular TICP  100  or FIT  700 . 
     It will be noted that, in addition to transformer components  135 ,  145  discussed in detail above, a transformer  120  may contain various additional components, such as current/electrical connectors  185 , and a variety of screws, nuts, bolts, clamps, braces, and other physical components, which may either generate heat (for example, the current/electrical connectors ( 185 )) or receive heat from the coils  135  and/or core(s)  145 . Heat-transfer fluid  740  may be in physical contact and thermally conductive with exposed portions of these additional transformer elements as well, thereby removing heat from the exposed surfaces of the additional physical components. 
     Cooling Liquids and Fluids 
     In various embodiments, the present system and method employs several ongoing stages of thermal conduction and thermal convection, facilitated by direct physical contact, to transfer heat from transformer components/elements  135 ,  145  to either of: 
     (i) cold plates  160  via conduction, and further by thermal convection via the coolant liquid  240  pumped through the cold plate  160 , or 
     (ii) convection in a heat-conducting fluid  740 , to the wall(s)  755  of a heat management enclosure  710  which contains the transformer  120  and the heat-conducting fluid  740 ; then further via heat conduction to a cold plate  160  attached to the wall(s)  755 ; and still further by thermal convection via the coolant liquid  240  pumped through the cold plate  160 . 
     In embodiments described above, the cooling liquids  240  and heat transfer fluids  740  employed have generally been characterized as liquids/fluids (such as water or water-based liquids, or most oils) which are normally in a liquid state at room temperatures; or more generally in a liquid state at temperature ranges above the freezing point of water. Such fluids may be readily stored and conveyed via tubes and pipes. For convenience, these coolants are referred to hereinafter as “room temperature coolants.” 
     Such room temperature coolants may have the advantages of (i) being in generous and convenient supply (for example, drawn in volume from sea water, river water, or ocean water for ship-based power converters, or even from rivers for compact land-based power converters); and/or (ii) being available in ready commercial supply (such as various oils), and/or (iii) ready and convenient storage in relatively lightweight reservoirs requiring limited or no heat insulation. Such coolants may also not require any special compressors. 
     In alternative embodiments, liquids/fluids  240 ,  740  may be employed (either in whole, or supplemental to “room-temperature” fluids) which are normally gaseous at room temperatures, and which must therefore be compressor-cooled or super-cooled to be used as liquids. Such super-cooled fluids may include for example and without limitation liquid nitrogen, liquid helium, liquid oxygen, liquid carbon dioxide, and various commercial refrigerants. Persons skilled in the art will recognize that the user of such liquids may require compressors, special storage reservoirs, and other elements not described elsewhere in this application. As such, embodiments of the present system and method with such compressor/super-cooled fluids may be heavier, and require more electricity for cooling, than embodiments employing room-temperature fluids; however, such embodiments may be useful for ultra-dense/compact power converters and for future power converters designed to generate still higher level of power (for example, with even higher voltage power-switches and higher transformer winding ratios) in very compact spaces. 
     Comparison with Air-Cooled Transformers 
     In comparison with air-cooled transformers, the present system and method (typically but not necessarily employing room-temperature coolants/fluids  240 ,  740 ) may provide certain benefits. These may include, for example and without limitation: 
     (i) Reduced volume (by approximately 35%), compared with air cooled HF solid state transformer; 
     (ii) Increased power density (˜1.5×), compared with air cooled HF solid state transformers; 
     (iii) Reduced heat rejection into the ambient environment (that is, reduced heating of the air, meant for activity and breathing by human personnel, of the room or facility housing a power converter), which additionally helps decrease the demands on air-conditioning/cooling for the human environment; and 
     (iv) Both the transformer with integrated cold plates (TICP)  100  and the fluid immersed transformer (FIT)  700  will be better-suited (as compared to air-cooled transformers) to fit into tight/constrained spaces on board military and commercial ships, with the reduced space providing beneficial usage for the “saved space” for other purposes. 
     Exemplary Application: HPEBB 
       FIG.  9    illustrates an exemplary power electronics building block (PEBB)  510  employing a liquid/fluid cooling system  200 ,  700  (see  FIGS.  2 ,  5  and  7    and associated discussion, above) according to the present system and method. More specifically,  FIG.  9    illustrates an exemplary hybrid power electronics building block (HPEBB)  510 . 1 . Note that PEBBs may also be referred to as “[hybrid] power electronic building block least replacement units” (PEBB LRU or HPEBB LRU”)  510 / 510 . 1 . 
     A legacy PEBB  510  typically employs power switches  915  of equal voltage ratings (for example, 1700 volts for 1000 volt nominal operation) throughout the PEBB  510 . A legacy PEBB also typically employs a high power transformer  120  with a 1-to-1 winding ratio. 
     Exemplary HPEBB LRU  510 . 1  employs both lower-voltage and higher-voltage switches  915  (hence the term “hybrid”), which may in some embodiments be Silicon Carbide (SiC) switches. For example, in an exemplary embodiment the lower voltage switches  915 . 1  may be 1700 volt-rated switches for 1000 volt nominal operation, while the high voltage switches  915 . 2  may be 10000 volt-rated switches for 6000 volt nominal operation. In general, the operational voltages may be A and B, where B&gt;A, and where A may be for example and without limitation 1000 volts or 2000 volts, or other voltages; and B may be for example and without limitation 2000 volts, 3000 volts, 6000 volts, or other voltages. The present systems and methods for liquid cooling are designed, in part, for cooling for HPEBB LRUs  510  and power converters  500  which benefit from the higher power switches  115 . 2  which are in development (or just emerging) at the time of the present application. 
     Exemplary HPEBB LRU  510  also employs a high-power, high-frequency transformer  120  with a K:N winding ratio (N&gt;K; K=1, 2, . . . ; N=2, 3, . . . ). 
     In various embodiments, an HPEBB based converter  500  according to the present system and method may require fewer HPEBB LRUs  510  than the number of legacy PEBB LRUs which would be required in a legacy system (legacy PEBBs may be referred to as “PEBB  1000  LRUs”, and typically employ only power switches rated for 1000 volt nominal operation). Power converters  500  according to the present system and method may thereby reducing the total volume and weight of the power converter  500 , and increasing the power density and specific power of the converter  500 . In one embodiment of the present system and method, a power switch  915  is implemented as a MOSFET (metal-oxide semiconductor field effect transistor) in parallel with a diode, as illustrated in  FIG.  9   . In an alternative embodiment, a power switch  915  is implemented as in IGBT (insulated gate bipolar transistor) in parallel with a diode, as illustrated in  FIG.  9   . Persons skilled in the relevant arts will appreciate that a power switch  915  may be implemented as other combinations of one or more power transistors and other components, such as GaN (Gallium Nitride) wide band gap devices, JFET, IGCT (integrated gate-commutated thyristor), and diodes, within the scope of the present system and method. 
     In the exemplary HPEBB LRU of  510  of  FIG.  9   , a total of four bridge converters  910  are employed. In alternative embodiments, a total of two, three, or more than four bridge converters  910  may be used. It will also be understood that in the art, the bridge converters  910  are sometimes referred to by other terminologies, including for example and without limitation: power stage, power bridge, H-bridge converters, and full bridge converters. 
     The exemplary HPEBB LRU  510  also illustrates a K:N (N=2, 3, . . . ) high frequency (HF) transformer  120  configured to link the lower voltage elements  905 . 1  and the high voltage elements  905 . 2 . That is, exemplary HPEBB  510  couples the lower voltage elements  905 . 1  and the higher voltage elements  905 . 2  via a high frequency (HF) transformer  120  with a higher than unity (1:1) winding ratio such as K:N, such as for example and without limitation, a 1:3 ratio or a 1:6 ratio. 
     In some embodiments of the present system, an exemplary high power switching device  915 . 2  may consist of or may include 10 kV SiC MOSFETs which at the time of this application are under development by Cree (Cree, Inc., 4600 Silicon Drive, Durham, N.C., 27703). In exemplary embodiments, the bridge converters  910 . 1  on the low voltage side  905 . 1  may use 1.7 kV SiC MOSFET/IGBT devices, while the bridge converters  910 . 2  on the high voltage side  905 . 2  may use 10 kV SiC MOSFET/IGBT devices. In an exemplary embodiment with the HF solid state transformer  120  having a winding/turn ratio of 1:3 (one (1) on the low voltage side, and three (3) on the high voltage side), in which case multiple such HPEBBs  510  can be configured in a single power converter for a space and power-density efficient 1 kVdc-to-13.8k V AC power conversion. 
     In alternative embodiments, high power switches  115 . 2  may be implemented via other high power switches known or in development, including but not limited to high power MOSFETs and/or high power IGBTs. The higher turn ratio (for example 1:3) of the HF transformer  120  provides a voltage boost, and makes the hybrid PEBB ( 1000 / 6000 ) no longer voltage limited for higher medium voltage (MV) (&gt;12 kV) applications. 
     HPEBB heat management: The K:N high power density, high frequency transformer  120 , which provides for galvanic isolation between the low voltage components  905 . 1  and the high voltage components  905 . 2 , may generate a level of heat (and a rate of heat production) sufficient to benefit from the exemplary cooling systems  100 ,  200 ,  700  described throughout this document. Consequently, HPEBB LRU  510  may include cooling elements described throughout this document, including for example and without limitation: cooling fluids/liquids  240 ,  740 . 
     Power converter volume and weight: The hybrid PEBB LRU converter  510  volume and weight may increase somewhat due to the use of PEBB  6000  components  910 . 2 ,  915 . 2  on the primary or high voltage side  905 . 2 ; and may further increase due to additional cooling requirements. However, due to the reduced requirement for the total number of HPEBB LRUs  510  (as compared with legacy power converters), in various embodiments the total volume and weight of an exemplary 1 kVdc-13.8 kV 1 MW hybrid PEBB power converter  510  will be significantly reduced (as compared with the weight/volume of a legacy PEBB  1000  LRU-based power converter with the same voltage/power capacity). 
     Correspondingly, in various embodiments the present system provides for an HPEBB power converter  500 . 1  with a power density and specific power which is significantly increased via the use of HPEBB LRUs  510  which employ the cooling systems and methods of the present application (as compared with the power density/specific power of a legacy PEBB  1000  LRU-based power converter with the same total power capacity). 
     The exemplary HPEBB power converter  500  is typically contained in a cabinet or housing  525  (see  FIG.  5    above) which contains all the above elements, as well as others not shown in the figure but known in the art. The cabinet  525  may include, contain or have attached, for example and without limitation: various internal structural support elements (not shown), system buses, power buses, ports for connection with exterior elements and connection to exterior systems, vents for airflow, pipes or ducts for coolants associated with cooling system(s)  525 , exterior status display(s), electronics for feedback and control systems (including processors and memory), and other elements not shown in  FIG.  2   . 
     The cabinet or housing  525  may include the elements of one or more cooling systems  210 , as discussed in detail in this document. 
     Control Systems 
     In various exemplary embodiments, the present system and method may entail the use of or integration of control systems for regulation of switches, capacitors, cooling systems, valves, pumps, filters, and other factors requiring real-time control. Such control systems may entail the use of microprocessors, digital input/output elements, memory (such a random access memory (RAM) and various forms of non-volatile memory), display systems, audio input and/or audio signalling systems, and/or analogue control elements known in the art or to be developed. Such control systems may employ suitable-coded software, stored in memory, to control various aspects of system operations. 
     Where computer code is required for the present system and method, such as for control systems running on microprocessors, computer readable code can be disposed in any known computer usable medium including semiconductor, magnetic disk, optical disk (such as CD-ROM, DVD-ROM) and as a computer data signal embodied in a computer usable (e.g., readable) transmission medium (such as a carrier wave or any other medium including digital, optical, or analog-based medium). As such, the code can be transmitted over communication networks including the Internet and intranets. 
     It is understood that control functions or monitoring functions to be accomplished in conjunction with the systems and techniques described above can be represented in a core (such as a CPU core) that is embodied in program code and can be transformed to hardware via suitable circuits, wireless communications, and/or optical messaging. 
     VII. CONCLUSION 
     For maritime purposes, including Naval applications such as Naval Power System and Energy System (NPES) technologies, HPEBB bridge converters  910 , HPEBB LRUs  510 , and power converters  500  are being developed as part of a multi-function energy storage module (MFESM) effort. Emerging hybrid PEBB LRUs  510  in particular employ high-power transformers which can generate large amounts of heat in compact spaces. The liquid/fluid cooling systems  200  for HPD-HF transformers of the present system and method offer significant advantages in managing the heat generated by such systems. 
     Alternative embodiments, examples, and modifications which would still be encompassed by the disclosure may be made by those skilled in the art, particularly in light of the foregoing teachings. Further, it should be understood that the terminology used to describe the disclosure is intended to be in the nature of words of description rather than of limitation. 
     Those skilled in the art will also appreciate that various adaptations and modifications of the preferred and alternative embodiments described above can be configured without departing from the scope of the disclosure. Therefore, it is to be understood that, within the scope of the appended claims, the disclosure may be practiced other than as specifically described herein. 
     The present invention has been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed. 
     It should be noted that the simulation, synthesis, and/or manufacture of the various embodiments of this invention can be accomplished, in part, through the use of a variety of materials, including metals, non-metals, resins, epoxies, semi-conductors; glass, polymers, ferrous materials, non-ferrous materials, conductors, insulators; and water or water-based liquids for cooling, oils and other hydrocarbon-based fluids for cooling, some known in the art and some yet to be developed. 
     It is to be appreciated that the Detailed Description section (and not the Summary and Abstract sections) is primarily intended to be used to interpret the claims. The Summary and Abstract sections may set forth one or more but not all exemplary embodiments of the present invention as contemplated by the inventor(s), and thus, are not intended to limit the present invention and the appended claims in any way. 
     Further regarding the appended claims, any and all reference signs/numbers are provided to make the claims easier to understand, and are not to be treated as limiting the extent of the matter protected by the claims; their sole function is to provide for clear reference to elements in the disclosure and drawings.