Abstract:
A method of operating a high speed machine, wherein the method includes providing at least one heteropolar generator that includes a stator, a rotor, and at least one stationary superconducting field coil therein. The method also includes coupling at least one cryogenic refrigeration system to the at least one stationary superconducting field coil, wherein the at least one cryogenic refrigeration system is coupled in flow communication with only the at least one stationary superconducting field coils to facilitate reducing an operating temperature of the at least one stationary superconducting field coil.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates generally to electric machines, and more particularly, to methods and apparatus for operating an electric machine. 
   At least some known high speed machines, such as a Lundell generator, benefit from eliminating active electrical components, such as field coils, from the rotor. More specifically, removing such components may improve the reliability of, and provide for, a more robust rotor design that does not require a mechanical support structure for such components. These machines have stationary field coils, and are generally fabricated from conventional conductors, such as, for example, copper or aluminum. Additionally, these stationary field coils are cooled by the same cooling medium as the rest of the machine. 
   It is well known that high speed machines, such as, for example generators or motors, generate heat that must be dissipated. To disseminate the heat, at least some known generators are equipped with cooling systems, such as, for example air, gas, and/or liquid ventilation systems. Typically, such ventilation systems cool both the stator and the rotor. It may be difficult to optimize the cooling of a generator that includes a stator, a rotor, with a ventilation system that provides cooling for both a rotor and a stator that are thermally coupled. However, the addition of a separate and independent cryogenic cooling system for cooling a stationary superconducting field coil would facilitate optimizing the cooling of the generator since it thermally decouples the field coil from the rest of the machine. 
   BRIEF DESCRIPTION OF THE INVENTION 
   In one aspect, a method of operating a high speed machine is provided. The method includes providing at least one heteropolar generator that includes a stator, a rotor, and at least one stationary superconducting field coil therein. The method also includes coupling at least one cryogenic refrigeration system to the at least one stationary superconducting field coil, wherein the at least one cryogenic refrigeration system is coupled in flow communication with only the at least one stationary superconducting field coils to facilitate reducing an operating temperature of the at least one stationary superconducting field coil. 
   In another aspect, a high speed machine is provided. The machine includes at least one heteropolar generator including at least one of a stator, a rotor, and at least one stationary superconducting field coil therein. The machine also includes at least one cryogenic refrigeration system in flow communication with the at least one stationary superconducting field coil to facilitate reducing an operating temperature of the at least one stationary superconducting field coil. 
   In a further embodiment, a power generation system is provided. The system includes at least one prime mover and at least one heteropolar generator rotatably coupled to the at least one prime mover, wherein the at least one generator includes a stator, a rotor, and at least one stationary superconducting field coil therein. The system also includes at least one cryogenic refrigeration system coupled in flow communication with only the at least one stationary superconducting field coil to facilitate reducing an operating temperature of the at least one stationary superconducting field coil. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of an exemplary superconducting generator system coupled to an exemplary power plant. 
       FIG. 2  is an enlarged cross-sectional view of an exemplary superconducting alternator that may be used with the generator system shown in  FIG. 1 . 
       FIG. 3  is a schematic diagram of an alternative embodiment of a superconducting generator. 
       FIG. 4  is an enlarged cross-sectional view of an exemplary superconducting alternator that may be used with the generator system shown in  FIG. 3 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  is a schematic diagram of an exemplary superconducting generator system  10  coupled to an exemplary power plant  12  configuration. In the exemplary embodiment, superconducting generator system  10  is a superconducting alternator  14  that includes at least one of a stator  16  and a rotor  18  that are coupled together such that rotor  18  is substantially co-axially aligned with respect to stator  16 .  FIG. 2  is an enlarged cross-sectional view of an exemplary superconducting alternator  14  that may be used with generator system  10 . In the exemplary embodiment, superconducting alternator  14  is a heteropolar superconducting alternator and includes at least two stationary superconducting field coils  20 . 
   In the exemplary embodiment, field coil  20  is a stationary high-temperature super-conducting (HTS) field coil  20  separate from rotor  18 . Separating HTS field coil  20  from rotor  18  permits the use of smaller size HTS field coils  20  and enables a higher ampere turn capability of HTS field coil  20 , thereby resulting in higher engineering current densities and increases in the rating capability of alternator  14 . HTS field coil  20  includes a plurality of HTS wires  22  wherein at least one HTS wire is a high temperature super-conducting conductor wire  22 , such as, for example, but not limited to a BSCCO 2223 (Bi 2 Sr 2 Ca 2 Cu 3 O 10 ) conductor wire laminated with a solid epoxy impregnated winding composite. For example, a series of BSCCO 2223 wires may be laminated, bonded together and wound into a solid epoxy impregnated coil. In another embodiment, each HTS field coil  20  is fabricated from at least one of YBCO (YBa 2 Cu 3 O 7 ) or MgB2. In alternative embodiments, each HTS field coil  20  is fabricated from any suitable material that enables alternator  14  to function as described herein. 
   Each HTS field coil  20  may be formed with a race-track shape, a cylindrical shape, or any other shape that is suitable for a particular rotor design. More specifically, although the dimensions of HTS field coil  20  are dependent on the dimensions of rotor  18 , the removal of the field coils from rotor  18  facilitates a robust rotor design. Furthermore, the robust rotor design allows for a smaller bearing (not shown) span and a smaller and lighter stator  16 . In the exemplary embodiment, each HTS field coil  20  is wound around the centerline of the rotor  18 . Generally, each HTS field coil  20  circumscribes the rotor. The HTS field coil  20  define a substantially resistance free electrical flow path around rotor  18  and between the magnetic poles of rotor  18  and thereby improve efficiency by facilitating the elimination of field I 2 R losses. 
   Each HTS field coil  20  is coupled to a known cryogenic refrigeration system  30  though supply and return lines  32  that enable cryogenic liquid or gas to be supplied to HTS field coil  20 , and channel spent cryogenic liquid or gas from HTS field coil  20 . Typically, known cryogenic refrigeration system  30  requires a cryogenic transfer coupling, but however, because HTS field coil  20  is stationary, in the exemplary embodiment, no cryogenic transfer coupling is required. More specifically, refrigeration system  30  receives a cryogenic fluid in a liquid form or in a gas form from HTS field coil  20  and cools the cryogenic fluid such that the cryogenic fluid may be routed back to HTS field coil  20  for distribution within HTS field coil  20 . In one embodiment, fluid passages (not shown) for cryogenic cooling fluid are defined in the HTS field coil  20 . In another embodiment, cryogenic cooling passages are formed around an outside surface of HTS field coil  20 . 
   In the exemplary embodiment, cryogenic refrigeration system  30  only cools HTS field coil  20 . 
   In operation, cryogenic refrigeration system  30  supplies cryogenic fluid to superconducting HTS field coil  20 . The cryogenic liquid or gas cooled by refrigeration system  30  is routed to HTS field coil  20  through lines  32 . Cryogenic liquid or gas is then channeled through HTS field coil passages to cool HTS field coil  20 . The spent cryogenic liquid or gas is then returned through lines  32  routed to refrigeration system  30  wherein heat removed from HTS field coil  20  is released to a heat sink (not shown). The cooling fluid facilitates maintaining the low temperatures, e.g., 27° K., in the HTS field coil  20  needed to promote superconducting conditions, including the absence of electrical resistance in the coil. In the exemplary embodiment, the high temperature superconductors have an operating temperature between a range of about 20° K. to about 70° K. In contrast, components that exhibit superconducting properties while operating in a range of about 2° K to about 5° K. are known as low-temperature superconductors. 
   Rotor  18  is rotatably coupled to a prime mover  40  through shaft  42 . In one embodiment, prime mover  40  is a turbine assembly, such as but not limited to a gas turbine, a steam turbine, a hydro-turbine, and/or a wind turbine. In another embodiment, prime mover  40  is an internal combustion engine assembly. In the exemplary embodiment, power plant  12 , is illustrated as having a single shaft power train wherein prime mover  40  is coupled substantially coaxially with rotor  18 . In another embodiment, any portion of prime mover  40  may be coupled to rotor  18  via a power transmission device (not shown), such as, for example, a hydraulic coupling or a gear arrangement. Although prime mover  40  is illustrated and described herein as a single engine in the exemplary embodiment, it will be understood that prime mover  40  may be any suitable combination of engines capable of delivering rotary power to a shaft. For example, in combinations known as, but not limited to gas and steam turbines in simple cycle, combined cycle, tandem, cross compound, and dual-flow combinations. 
   In  FIG. 3  is a schematic diagram of an alternative embodiment of a superconducting generator system  100 . Superconducting generator system  100  is similar to superconducting generator system  10 , (shown in  FIG. 1 ) and components of superconducting generator system  100  that are identical to superconducting generator system  10  are identified in  FIG. 3  using the same reference numbers used in  FIG. 1 .  FIG. 4  is an enlarged cross-sectional view of an exemplary superconducting alternator  114  that may be used with the generator system shown in  FIG. 3 . 
   In the exemplary embodiment, superconducting generator system  100  is a superconducting alternator  114  that includes stator  16  and rotor  18  that are coupled together such that rotor  18  is substantially co-axially aligned with stator  16 . In the exemplary embodiment, superconducting alternator  114  is a heteropolar superconducting alternator that includes at least one stationary superconducting field coil  20 . 
   The above-described alternators are efficient, cost effective, and highly reliable. The heteropolar alternators include at least one stationary high-temperature superconducting field coil. The stationary nature of the high-temperature superconducting field coil allows for the elimination of a cryogenic transfer coupling. The removal of the field coils from the rotor improves efficiency by facilitating the elimination of field I 2 R losses, facilitates improving reliability, and provides for a robust rotor design. The removal also permits the use of a smaller size field coil which results in higher engineering current densities in the superconductor than in conventional conductors. Additionally, a smaller size field coil results in a smaller bearing span and a smaller and lower weight stator core and frame, therefore improving rotor dynamics. Finally, a higher ampere turn capability of the field coil increases the rating capability of the alternator thereby reducing the weight and volume of the alternator. The use of a single stationary high-temperature superconducting field coil further reduces the heat load, simplifies the mechanical supports, and further reduces the bearing span. As a result, the described alternators facilitates improving efficiency in a cost effective and reliable manner, 
   Exemplary embodiments of heteropolar alternators with at least one stationary high-temperature superconducting field coil coupled to a separate and independent cryogenic refrigeration system are described above in detail. The alternators are not limited to the specific embodiments described herein, but rather, components of the alternators may be utilized independently and separately from other components described herein. Each alternator component can also be used in combination with other alternator components. 
   While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.