Abstract:
A generator rotor core ( 54 ) carrying superconducting windings ( 60 ) and having a shield ( 426 ) over the superconducting windings ( 60 ) to prevent external magnetic fields from impinging the windings. Axial shield edges ( 430/434 ) mate with corresponding features of the rotor core ( 54 ) or with structures affixed to or supported by the core ( 54 ) to support the shield ( 426 ).

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates generally to superconducting generators and specifically to a non-magnetic rotor shield for protecting superconducting rotor windings and optimizing the production of magnetic flux generated by the rotor windings. 
       BACKGROUND OF THE INVENTION 
       [0002]    An electric generator transforms rotational energy into electrical energy according to generator action principles of a dynamoelectric machine. The turning torque supplied to a rotating rotor by a combustion or steam-driven turbine is converted to alternating current (AC) electricity, typically three-phase AC, in a stationary stator that surrounds the rotor. The generator is a mechanically massive and electrically complex structure, supplying output power up to 2,222 MVA at voltages up to 27 kilovolts. Electrical generators are the primary power producers in an electrical power system. 
         [0003]    As shown in the cross-sectional view of  FIG. 1 , a conventional electric generator  10  comprises a substantially cylindrical rotor  12  supporting axial field windings or rotor windings  13 . A direct current (DC) supplied to the rotor windings  13  produces a constant magnetic flux field that rotates with the rotating rotor within a stationary armature or stator  14 . One end  15  of the rotor  12  is drivingly coupled to a steam or gas driven turbine (not shown in  FIG. 1 ) for providing rotational energy to turn the rotor  12 . The opposing end  16  is coupled to an exciter (not shown) for supplying the direct current to the rotor windings  13 . An alternating current is generated in the stationary stator windings as the rotor&#39;s magnetic flux field crosses the stator windings. Rotor rotation subjects the rotor  12  and the rotor windings  13  to radial centrifugal forces that may result in radial distortion of these components. 
         [0004]    The stator  14 , a shell-like structure, encloses the rotor and comprises a core  17  further comprising a plurality of thin, high-permeability circumferential slotted laminations  17 A placed in a side-by-side orientation and insulated from each other to reduce eddy current losses. Stator coils are wound within the inwardly directed slots. The AC electricity induced in the stator windings by action of the rotor&#39;s rotating magnetic field flows to terminals  19  mounted on the generator frame for connection to an external electrical load. Three-phase alternating current is produced by a generator comprising three independent stator windings spaced at 120° around the stator shell. Single-phase alternating current is supplied by a stator having a single stator winding. 
         [0005]    The rotor  12  and the stator  14  are enclosed within a frame  20 . Each rotor end comprises a bearing journal (not shown) for cooperating with bearings  30  attached to the frame  20 . 
         [0006]    A generator cooling system removes heat produced by current flow through the generator conductors, including the direct current flow through the rotor windings  13  and the alternating current induced in the stator coils. Additional heat sources include mechanical losses, such as windage caused by the spinning rotor, and friction at the bearings  30 . The rotor  12  carries a blower  32  for forcing cooling fluid through the generator elements. Coolers  36  receive and cool the cooling fluid to release the heat absorbed from the generator components. The cooling fluid is then recirculated. back through the generator components. 
         [0007]    To increase generator output and efficiency and reduce generator size and weight, conventional copper rotor windings are replaced by superconducting windings (filaments) that exhibit effectively no resistance to current flow when maintained below the material&#39;s critical temperature (T C ). Superconductivity is a phenomenon observed in several metals and ceramic materials when the material is cooled to temperatures ranging from near absolute zero (0° K or −273° C.) to a liquid nitrogen temperature of about 77° K or −196° C. The critical temperature for aluminum is about 1.19° K and for YBa 2 Cu 3 O 7  (yttrium-barium-copper-oxide) is about 90° K. Yttrium-barium-copper-oxide (one example of a high temperature superconducting (HTS) material) is commonly used for the rotor windings of a superconducting generator. 
         [0008]    Since the superconducting materials exhibit substantially no electrical resistance when maintained at or below their critical temperature, these materials can carry a substantial electric current for a long duration with insignificant energy losses. To maintain the superconducting conductors at or below their critical temperature, coolant flow paths carrying coolant supplied from a cryogenic cooler are disposed adjacent or proximate the windings. Typical coolants comprise liquid helium, liquid nitrogen and liquid neon. 
         [0009]    Disadvantageously, the HTS rotor windings are sensitive to mechanical bending and tensile stresses that can cause premature degradation and winding failure (e.g., an open circuit). For example, bends formed in the HTS rotor windings to circumscribe the cylindrical rotor core induce winding stresses. Normal rotor torque, transient fault condition torques and over-speed forces induce additional stress forces in the rotor windings. These over-speed and fault conditions substantially increase the centrifugal force loads on the rotor coil windings beyond the loads experienced during normal operating conditions. 
         [0010]    The co-pending commonly-owned application entitled Superconducting Coil Support Structures (Attorney docket number 2006P13505US) describes and claims HTS winding support structures that support the windings against these loads. This application is incorporated by reference herein. The support structures also limit heat transfer from the “warm” (i.e., approximately room temperature) rotor core to the “cold” (i.e., cryogenically cooled) HTS windings. In addition to conductive thermal paths in the support elements, it is desired to maintain the HTS rotor windings in a vacuum condition to limit radiative heat transfer from the rotor core to the superconducting windings. 
         [0011]    AC electricity available at the stator terminals is supplied to an electrical power grid through a transmission and distribution system. Grid fault currents, e.g. caused by a lightning-induced current spike on the grid, are coupled to the stator through the intervening transmission and distribution lines. The grid fault currents generate a stator fault current and an attendant strong transient magnetic flux that is magnetically coupled to the HTS rotor windings. This flux can generate a significant torque on the rotor core and the HTS winding, potentially damaging the HTS winding and its support structures. The transient magnetic fields can also be caused by system or internal short circuits, transmission switching operations, synchronizing operations, transient voltages on the transmission system and loss of synchronism between the generator and the grid. In addition to the undesired mechanical forces produced by these transient torques, any magnetic field coupled into the rotor windings causes undesired alternating current (AC) losses in the HTS conductors. 
         [0012]    Although rotor winding support structures can be designed to allow the HTS conductors to withstand the additional torque introduced by these transient magnetic fields, such support structures increase the support mass and may introduce additional undesired thermal paths between the warm rotor core and the cold HTS windings. 
         [0013]    Typically however, the rotor windings are shielded to prevent transient magnetic fields from reaching the rotor HTS windings. An electromagnetic shield, comprising copper or aluminum for example, encloses the HTS rotor windings to prevent magnetic flux from coupling to the rotor, thereby avoiding the consequent torques induced on the HTS windings. The shield is also referred to as a non-magnetic shield since it is constructed from non-magnetic material. 
         [0014]    For relatively small electric generators the shield comprises a thin tubular or cylindrical structure surrounding the rotor core and the HTS windings and attached to the rotor core end faces. However, it is a substantial challenge to manufacture, assemble and balance a large and continuous cylindrical shield structure with the precision and tolerances required for a large electrical generator. According to one embodiment, a tubular shield having a relatively thin wall surface is supported by the rotor shaft with a tight clearance between the rotor and the shield. Gravity loading deforms the thin tube into an elliptical shape and interface contact is made at the top and bottom surfaces of the rotor shaft. Further, the considerable rotor weight tends to cause rotor sag. These effects lead to fretting damage due to relative motion (albeit a small displacement) at the interface of the rotor core and the non-magnetic shield. Alternatively, the tube shield has relatively thick wall surface with a larger gap between the shield and the rotor. Little or no fretting damage occurs in this configuration, but the shield must be sufficiently thick to support its own weight. 
         [0015]    The rotor core and the surrounding non-magnetic shield independently vibrate at a different resonant frequency with a different vibration pattern. These effects create additional dynamic loads on the rotor core and the HTS windings. The cumulative effect of the interface contact forces and the vibration forces create extremely high stresses on the rotating non-magnetic shield. 
         [0016]    If the rotor core and the HTS rotor windings are enclosed in a vacuum vessel (comprising stainless steel for example) additional design difficulties arise. If the shield and the vacuum vessel are both cylindrical with the vacuum vessel nested within the shield they are preferably joined to maintain the vacuum condition. Joining the dissimilar metals of the vacuum vessel and the shield is problematic. Further, the disadvantages associated with the large generator shield discussed above are exacerbated by the addition of the vacuum vessel. 
         [0017]    It is known by those skilled in the art that the rotor must be balanced to minimize undesired rotor torques. During the balancing process balancing weights are added to the rotor body at various locations along its axial length to balance the rotor at its operating speed. Effective balancing requires access to the entire rotor body surface to permit placement of the balancing weights as desired to effect a balanced condition. A shield that covers the entire rotor requires performing the balancing operation prior to placement of the shield over the rotor. But such a process increases production cycle time and process costs. Also, this pre-shield installation balancing operation is conducted with the rotor at ambient temperature, but the rotor operates at cryogenic temperatures. Undoubtedly, the lower temperature affects the rotor&#39;s balance. Thus it is preferable to balance the rotor under cryogenic operating conditions with access to the entire rotor surface to place balance weights as required. 
       BRIEF SUMMARY OF THE INVENTION 
       [0018]    One embodiment of the invention comprises a rotor for an electric generator. The rotor comprises a rotor core defining a first and a second axially extending flat surface region, each of the first and the second flat surface regions bounded by first and second opposing edges; a superconducting winding circumscribing the rotor core, axial segments of the superconducting winding disposed within the first and the second flat surface regions; a plurality of first and a plurality of second core extension elements mating with the respective first edge of the first and the second flat surface regions; a first arcuate shield extending axially along the rotor core enclosing the axial segment within the first flat surface region, the first shield having a first axial edge mating with a first edge of the plurality of first core extension elements and a second axial edge mating with the second edge of the first flat surface region and a second arcuate shield extending axially along the rotor core enclosing the axial segment within the second flat surface region, the second shield having a first axial edge mating with a first edge of the plurality of second extension elements and a second axial edge mating with the second edge of the second flat surface region. 
         [0019]    In another embodiment the invention comprises a rotor shield for a rotor core of an electric generator comprising a superconducting winding. The rotor shield comprises an arcuate sheet of electrically conductive non-magnetic material for shielding the superconducting winding and each one of a first and a second axial edge of the arcuate sheet having a first dovetail feature for mating with a second dovetail feature of a respective first and a second core element. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The foregoing and other features of the invention will be apparent from the following more particular description of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different figures. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
           [0021]      FIG. 1  is a cross-sectional view of a prior art electric generator; 
           [0022]      FIG. 2  is an illustration of a rotor for use in a superconducting dynamoelectric machine according to the teachings of the present invention; 
           [0023]      FIGS. 3 ,  4 ,  5 A,  5 B and  6  are perspective views of a rotor for use with a shield of the present invention. 
           [0024]      FIGS. 7 and 8  are views of a non-magnetic shield constructed according to the teachings of the present invention. 
           [0025]      FIGS. 9 and 10  are perspective views of a rotor including the non-magnetic shield of the present invention. 
           [0026]      FIG. 11  is an illustration of another embodiment of a non-magnetic shield of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0027]    Before describing in detail the particular non-magnetic rotor winding shield in accordance with the present invention, it should be observed that the present invention resides primarily in a novel and non-obvious combination of hardware elements and method steps. Accordingly, these elements and steps have been represented by conventional elements and steps in the drawings, showing only those specific details that are pertinent to the present invention so as not to obscure the disclosure with details that will be readily apparent to those skilled in the art having the benefit of the description herein. 
         [0028]    The following embodiments are not intended to define limits as to the structures or methods of the invention, but only to provide exemplary constructions. The embodiments are permissive rather than mandatory and illustrative rather than exhaustive. 
         [0029]    A rotor shield of the present invention overcomes the various limitations described above. The shield of the present invention effectively prevents stator-originating transient time-varying magnetic fields from impinging rotor HTS windings, but allows access to substantially all of the rotor body for the placement of balancing weights. Also, the shield and the rotor body cooperate to transfer transient loads (generated during fault conditions) and steady state loads imposed on the shield to the rotor body. 
         [0030]    It is known that the shield provides a shielding function only for magnetic fields that vary in time at the surface of the rotor. Since the shield rotates with the rotor flux (which is generated by a DC current) there is no time-varying rotor flux component and thus the shield does not impede the main rotor flux. The shape of the rotor pole tends to reduce the magnetomotive force (MMF) drop of the rotor pole to a low level and thereby ensures that the rotor leakage flux remains at a relatively low level. 
         [0031]    However, the shield shields the HTS windings from stator magnetic flux components that are time-varying from the perspective of the rotor. As is known, a time-varying field generates a time-varying voltage on the surface of the conductive shield responsive to the change of magnetic flux with time. A current flows within the shield responsive to this voltage and generates a time-varying magnetic field that counters the external time-varying magnetic field. Thus the time-varying field is prevented from reaching the rotor core and the HTS windings. 
         [0032]      FIG. 2  illustrates a superconducting rotor  50  defining a longitudinal axis  52  and comprising a generally cylindrically-shaped core  54  and coaxially aligned rotor end segments  55  and  57  each attached to an end surface of the core  54 . A material of the core  54  exhibits a high magnetic permeability, e.g. a ferromagnetic material such as iron, for increasing the magnetic flux generated by the rotor windings. 
         [0033]    The superconducting rotor  50  further comprises a generally longitudinally-extending, racetrack-shaped superconducting (HTS) coil or winding  60  comprising axial segments  60 A connected by radial segments  60 B, the latter extending through openings  55 A and  57 A defined between end surfaces of the core  54  and the respective end segments  55  and  57 . Non-magnetic shields  70 A and  70 B of the present invention are each supported by the rotor core  54  and enclose the superconducting coil segments  60 A. 
         [0034]    The end segment  57  further comprises a cryogenic transfer coupling  68  that supplies cooling fluid (cryogenic fluid) from a cryogenic cooler (not shown) to closed coolant flow paths or channels in the superconducting coil  60  to maintain the superconducting coil  60  at or below its critical temperature. From the channels, the coolant returns to the transfer coupling  68  then to the cooler for lowering the coolant temperature. The coolant is then circulated back to the coolant flow paths. 
         [0035]    The rotor  50  for use with the magnetic shield of the present invention is illustrated in greater detail in  FIG. 3 , absent the rotor end segments  55  and  57 . The rotor core  54  comprises oppositely-disposed axially-extending flat surface regions  404 . The flat surfaces balance the stiffness of the rotor to avoid excessive dynamic forces. 
         [0036]      FIG. 4  illustrates the rotor core  54  and the superconducting winding segment  60 A supported by the aforementioned HTS winding support structures attached to the flat surface regions  404 . As illustrated in  FIG. 5A , a plurality of blocks  412  (also referred to as core extensions and comprising a ferromagnetic material such as steel) are disposed in a side-by-side configuration axially along one exposed edge of each flat surface region  404 , with a spacer  413  intermediate two adjacent blocks. Typically, the blocks  412  are installed after the superconducting winding  60  is attached to the core  54 . In one embodiment the blocks comprise a dovetail surface  412 A that mates with a corresponding dovetail groove in the rotor core  54 . See  FIG. 5B . 
         [0037]    As can be seen, the blocks  412  partially close the circumferential core gap formed by the flat surface regions  404 . The blocks  412  are functional elements of the core  50  (i.e., a material of the blocks  412  comprises a ferromagnetic material) and thus are formed from a core-like material. The blocks  412  also support the magnetic shield of the present invention as described further below. The blocks  412  can be installed beginning from either end of the core  50 . 
         [0038]    In lieu of individual blocks  412 , the circumferential gap can be closed by a single elongated piece (formed from ferromagnetic material) extending a length of the rotor core  50 . 
         [0039]      FIG. 6  illustrates the partially assembled rotor core  54 , including the superconducting winding segment  60 A, the blocks  412  and the spacers  413 , with end segments  55  and  57  affixed thereto according to known techniques. 
         [0040]      FIGS. 7  (a perspective view) and  FIG. 8  (an end view) illustrate one embodiment of a non-magnetic shield assembly  424  constructed according to the teachings of the present invention, comprising an arcuate shield  426  preferably constructed of aluminum (or another non-magnetic material). A plurality of adjacent sliding shoes  428  mate with the shield  426  at a dovetail interface along a shield edge surface  430 . A plurality of sliding shoes  432  similarly mate with an opposing edge surface  434  of the arcuate shield  426 . Each of the sliding shoes  428  and  432  is attached to the shield  426  by a plurality of fasteners, such as bolts  435  as indicated in  FIG. 8 . In one embodiment, adjacent sliding shoes  428  and adjacent sliding shoes  432  are spaced apart to avoid fretting damage to the shoes or a spacer member is inserted therebetween. 
         [0041]    A dovetail surface  432 A of the shoe  432  is received within a mating dovetail groove  438  in the rotor core  54 . See  FIGS. 8 and 9 . A dovetail surface  428 A of the oppositely disposed sliding shoe  428  is similarly attached (using a dovetail mating technique) to an exposed surface of each of the magnetic steel blocks  412 . 
         [0042]    To install the non-magnetic shield  426 , the sliding shoes  428  and  432  are affixed to the shield  426 . The surfaces  432 A and  428 A are aligned with respective mating grooves in the rotor core  54  and the magnetic steel blocks  412 . The non-magnetic shield assembly  424  is then slid axially along the rotor core  54  to cover and enclose the superconducting winding portion  60 A. 
         [0043]    Similarly, a second non-magnetic shield is affixed to the rotor core  54  to close the oppositely disposed flat surface region  404  and the superconducting winding portion  60 A (see  FIG. 2 ) affixed thereto. 
         [0044]    As illustrated in  FIG. 10 , an end cap  440  is attached to the rotor core  54  to close open ends formed when the non-magnetic shield assembly  424  is in place on the rotor core. Another end cap is similarly situated at the other end of the non-magnetic shield assembly  424 . As can also be seen in  FIG. 10 , the cooperating the non-magnetic shield assembly  424  and the end caps  440  completely enclose the super conducting winding portion  60 A. 
         [0045]      FIGS. 9 and 10  further illustrate bolts  450  for attaching the end segment  57  to the core  54 . 
         [0046]    Use of the non-magnetic shield assembly  424  in lieu of a shield that completely surrounds the rotor as known in the prior art, substantially reduces dynamic loads on the rotor core  54  and on the assembly  424  during both steady state and transient load conditions, while shielding the HTS winding  60  from transient magnetic fields. 
         [0047]    In another embodiment illustrated in  FIG. 11 , a magnetic shield  460  comprises a plurality of side-by-side curved elements or bands  462  extending axially along the rotor core  54 . The elements  462  may be spaced apart, but electrical conductivity must be maintained between the elements  462 . 
         [0048]    While the present invention has been described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalent elements may be substituted for the elements thereof without departing from the scope of the invention. The scope of the present invention further includes any combination of elements from the various embodiments set forth herein. In addition, modifications may be made to adapt a particular situation to the teachings of the present invention without departing from its essential scope. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.