SUPERCONDUCTIVE ARMATURE WINDING FOR AN ELECTRICAL MACHINE

Abstract: An armature winding (10) for an electrical machine is formed of superconductive cable (16). To shield the superconducting wire from large AC magnetic fields and to minimize the mechanical forces and torques on the conductor components, the superconducting armature winding is placed in a slotted stator core (22). The superconductive cable is formed of multi-filamentary superconducting wire tape with an aspect ratio close to unity or is alternatively formed of continuous cables of superconducting wire. Magnetic wedges (26) disposed in openings of the slots shield the slot-embedded SC sinding from AC field components.

Description: SUPERCONDUCTIVE ARMATURE WINDING FOR AN ELECTRICAL MACHINE BACKGROUND OF THE INVENTION This invention relates to turbomachinery and, more particularly, to stator armature windings for an electrical machine using superconductive materials. hi conventional generators, a significant portion of losses are attributed to I2R losses in the two main generator windings, namely the field winding on the rotor and the armature winding on the stator. The development of superconductor (SC) technology, in particular of high temperature superconductors (HTSC), has provided a conductor medium that, when implemented successfully, has the potential for significantly reducing, if not completely eliminating, the associated I2R losses in the main generator windings. The successful introduction of SC technology into generators hinges on the solution of issues of cooling the superconductors, providing adequate mechanical support, and shielding the superconducting wires from alternating magnetic fields to minimize parasitic eddy currents. The critical current density (Jc) below which the superconducting materials retain their superconducting capability is strongly reduced if the superconductor is placed into a large magnetic field. Since the current density (Jc) decreases with increasing magnetic flux density, it becomes important to shield the superconductor effectively from magnetic fields. hi conventional generators, the armature winding is located in stator slots, and steel teeth guide the main magnetic flux from the airgap to the stator yoke. In this configuration, the armature conductors are not exposed to the main magnetic flux but only to the significantly smaller slot leakage flux. The magnetic torque acts on the stator teeth that transfer it to the core and the stator frame. The small slot leakage field causes eddy current losses in the conductors of the armature winding and gives rise to forces acting on the slot-embedded conductors, which are manageable with present slot-support methods. Several concepts for superconducting synchronous generators have been proposed and implemented to date, an example of which is disclosed in U.S. Patent No. 5,548,168, the contents of which are incorporated by reference herein. During steady state operation, the rotor field winding of a synchronous generator carries DC current and is exposed only to the relatively low magnetic leakage field. Therefore, the field winding has been traditionally the first focus for applying SC technology to generators. The field winding is assembled from superconductors to eliminate excitation I2R losses and to provide a source for magnetic airgap fields that are, in all concepts for SC generators presented to date, considerably higher than in conventional generators. The winding is cooled by liquid helium in the case of low temperature superconductors (LTSC) and liquid nitrogen in the case of a HTSC. Time- varying fields during load imbalance or transients such as during load shedding are shielded from the SC rotor winding by an electrically conductive shield around the rotor. Most existing concepts for superconducting generators proposed in literature and patents to date are based on a tooth-less stator core that consists of a steel yoke or flux shield and an "airgap" armature winding (see e.g., "Panel Discussion on the Impact of Superconducting Technologies on Future Power Systems and Equipment - Superconducting Generators" by D. Lambrecht, Study Committee 11, CIGRE, 1990 Session). With this configuration, the armature winding is located in the main magnetic flux path and exposed to magnetic fields of full airgap flux density levels of 2 Tesla or more. The large magnetic airgap is magnetized by the high ampere-turn capability of the superconducting field winding. In addition, the magnitudes of airgap flux density levels are further increased above the ones used in conventional generators to achieve higher power densities and reduced overall generator size. The airgap armature winding is typically assembled from copper conductors that are supported by a nonmagnetic structure. These concepts have several inherent problems. The armature winding is exposed to the full airgap flux densities resulting in large AC losses in the copper conductors. Since the armature is located in the main airgap field, the full rated magnetic torque is acting directly on the armature winding, and radial forces are also significantly larger than in conventional generators. This requires that the nonmagnetic supporting structure of the armature winding be designed for both rated torque and large radial forces. These problems that are associated with the High Power Density superconducting generator even when a conventional copper armature winding is employed have been addressed by the Low Power Density concept in the noted U.S. patent. Taking the next step and replacing the conventional copper conductor with a superconducting wire in the airgap armature winding is complicated by the fact that superconductors are not yet capable of carrying AC currents in strong magnetic fields without incurring high AC losses, leading to a loss in superconductivity. Therefore, following this paradigm of an airgap armature winding there has been limited success to date to use superconductors in the armature winding. In recent years, SC wires have been implemented in AC power cable prototypes by various cable manufacturers. In these cables, the electrical line-ground insulation is either at room temperature (warm dielectric) or cryogenic temperatures (cold dielectric), and the conductor is assembled from HTSC wire. The conductors in these AC power cables are exposed only to the small self-field, which is sufficiently small for today's superconducting materials. Several concepts also exist for cable-wound generators in which the stator winding is assembled of low- or high- voltage cables with conventional copper conductors. It is thus desirable to provide a superconducting armature winding that is assembled by placement into stator slots much like in conventional generators. The stator teeth serve to shield the SC winding from magnetic AC fields resulting in minimization of AC losses, forces and torques acting on the supeconducting wires. It would also be beneficial to manufacture the winding from continuous cables of superconducting wires or alternatively from multi-filamentary wires of aspect ratios close to unity. It may further be beneficial to employ magnetic wedges to further shield the SC conductors from AC magnetic fields. BRIEF SUMMARY OF THE INVENTION In an exemplary embodiment of the invention, a superconducting synchronous generator includes a rotor and a stator. The stator comprises a plurality of stator slots and armature windings respectively disposed in the stator slots. The armature windings are formed of superconductive cable. The superconductive cable may comprise multi-filamentary superconductive wire tape with an aspect ratio greater than one or alternatively with an aspect ratio of about one. In an alternative arrangement, the superconductive cable comprises continuous cables of superconducting wire. The superconductive cable may include a substantially concentrically layered construction including a cryo-refrigeration coolant passage, a superconductive material and insulation. The insulation may be thermal insulation disposed over electrical insulation or the opposite with electrical insulation disposed over thermal insulation. The stator may further include stator teeth defining the stator slots, where the stator teeth shield the superconductive cable from a majority of magnetic fields generated during operation of the generator. The armature windings of superconductive cable may be toroidal windings, and the stator may further include magnetic slot wedges respectively disposed in openings of the slots. In another exemplary embodiment of the invention, an armature winding for an electrical machine is formed of superconductive cable. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a schematic illustration of a slot-embedded superconducting armature winding with a cold dielectric; FIGURE 2 is a schematic illustration of a slot-embedded superconducting armature winding with a warm dielectric; FIGURE 3 illustrates a toroidial slot-embedded SC armature winding for an example of a two-pole generator; FIGURE 4 shows a slot-embedded superconducting cable made from superconducting tape of large aspect ratio; FIGURE 5 shows slot-embedded superconducting cable made from superconducting tape of an aspect ratio close to unity; and FIGURE 6 shows a magnetic slot wedge disposed in the opening of the stator slot. DETAILED DESCRIPTION OF THE INVENTION Referring to FIGURES 1 and 2, an SC armature winding 10 is shown disposed in a conventional stator slot 12. The SC armature winding 10 is formed in a substantially concentric layered construction including a cryo-refrigeration coolant passage 14 for receiving coolant, a superconductor 16 and insulation 18, 20. The conductors 16 of the SC stator winding are placed in the stator slots 12 similar to the winding arrangement of conventional generators. The main magnetic flux is guided tlirough a toothed stator core 22 that shields the SC wire from large AC flux densities. Since the main magnetic field is guided through the laminated core structure, the magnetic forces, torques, and additional AC losses are limited to values that are due to only the slot leakage field, but not the main magnetic field. Therefore, the forces and torques acting on the SC wires are comparable to the ones in conventional machines, and the SC conductors can be supported by conventional structures. With this structure, SC wire is exposed to an AC field that is limited to the slot leakage field. Moreover, since the AC field is small, the critical current density of the superconducting wire has to be reduced only modestly. Still further, the AC losses induced in the SC wire by the slot leakage field are minimal, and the full rated torque is transmitted to the magnetic yoke not by the armature winding, but rather by the magnetic teeth for better reliability. The superconductor may be arranged in several different configurations in the slot, either with a cold dielectric (thermal insulation 20 around the electrical insulation 18, as shown in FIGURE 1), or a warm dielectric (electrical insulation 18 around the thermal insulation 20, as shown in FIGURE 2), and the conductors may have either rectangular or round or other shaped configurations. It is preferable that the SC wires within a turn or coil will be arranged in accordance with any of the well-known techniques that reduce or eliminate circulating currents among the wires. One such technique uses the "Roebel" arrangement, for which longstanding patents by Ringland (Allis Chalmers) and Willyoung (General Electric) are typical. In the specific application to SC cables, it is preferable that the wires will be wound in a spiral fashion to accomplish the cancellation of circulating currents The SC slot-embedded conductors may be connected in any of the typical connection schemes, such as individual bars or single- or multi-turn coils connected into a toroidal winding or a single- and multi-layer winding that is assembled from equal or concentric coils connected in a wave- or lap- winding pattern. The concept of SC slot embedded conductors also applies to salient pole stator windings and helical armature windings. With reference to FIGURE 3, a toroidal winding 23 consists of turns that extend around the yoke 24 of the stator core 22. The drawback of a toroidal winding in conventional generators is that they need approximately twice the coil length per induced voltage, resulting in twice the I2R losses of coils. This drawback of twice the I2R losses is eliminated by the use of SC wires as shown in FIGURE 3, and the advantage is compactness of the toroidal winding placed in a few slots/pole/phase. The slot-content of such a winding may implement the concepts of FIGURES 1 and 2. The cryogenic cooling paths 14 for the superconductor may be implemented in several configurations. In one configuration, each circuit of the armature winding forms a continuous cryogenic loop. In this case, each cryogenic circuit contains the same conductors as each electrical circuit. In another configuration, the cryogenic and electrical circuits may consist of different connection schemes. The cryogenic circuit may consist of a parallel connection of either individual coils or bars or groups of these. The superconducting armature winding is assembled from a continuous superconducting cable. The cable is assembled from layers of superconducting wire surrounded by a continuously extruded insulation system. In this concept, the superconducting wire extends continuously between the two terminals of each phase, or sections thereof. This approach minimizes the splices of superconducting wire that are required compared to a winding assembled from individual bars or coils. In different embodiments of the superconducting cable winding, the coolant may circulate either as an integral component of the continuous cable or around the extruded cable as part of the slot-containment of the cable. In the latter case, one or several cables can be immersed in the same coolant circuit within a slot. The concept of a SC cable winding applies to all winding configurations and connection schemes, including single- and multi-layer windings, wave- and lap- windings, toroidal windings, salient-pole windings, helical windings. Present prototypes of SC cables are built from multifilamentary SC wire tape with a high aspect ratio, i.e., a tape width that is several times the tape thickness. The conductor section 16 is preferably wound from such SC tape as indicated in FIGURE 4, where the individual tapes are twisted in the axial direction of the cable. The resulting self-field of the cable in air is indicated by the arrow A, and the self-field of slot-embedded cable is indicated by the arrow B. In this configuration, the magnetic leakage field A extends in peripheral direction of the cable and intersects the SC tape only over its thickness. Thereby, parasitic eddy currents are minimized. In the slot-embedded armature winding, the magnetic leakage field B is perpendicular to the axis of the cable. If a conventional cable of FIGURE 4 is used, the magnetic leakage field would be perpendicular to the width of the SC wire tapes in a large section of the SC region. This would result in excessive eddy current losses that are proportional to the square of the tape dimension that is perpendicular to the magnetic field. To minimize these eddy currents in a slot-embedded cable, a new configuration of wire tape is proposed, wherein the multi-filamentary SC wire has a cross section with an aspect ratio close to unity, such as wire strands of square or round cross sections as shown in FIGURE 5. When extending SC technology to AC applications, it is important to shield the SC wires from exposure to AC magnetic fields. In conventional machines, magnetic slot wedges are employed to reduce the stator slotting permeances. In the SC generator construction of the invention, referring to FIGURE 6, magnetic slot wedges 26 are disposed in openings of the slots 12. The magnetic wedges 26 serve the additional purpose of shielding the SC wire embedded in the stator slot from airgap field harmonics due to rotor MMF and permeance harmonics. Anisotropic wedges may be employed to reduce the slot leakage field passing through the wedge by aligning the magnetic preferential direction of the anisotropic wedge with the radial direction of the slot. Magnetic flux lines are shown in FIGURE 6. With the structure of the present invention, a superconducting stator armature winding can be assembled into conventional stator slots. The stator teeth serve to shield the SC winding from magnetic AC fields, resulting in minimization of AC losses, forces and torques acting on the superconducting wires. The winding is manufactured from continuous cables of superconducting wires or alternatively from multi-filamentary wires of aspect ratios close to unity. Magnetic wedges further shield the SC conductors from AC magnetic fields. While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. For example, the invention is applicable to various types of electrical machines beyond the synchronous type, including, but not limited to, DC motors and generators and induction motors, etc.

Claims: WHAT IS CLAIMED IS: 1. A superconducting electrical machine including a rotor and a stator, the stator comprising a plurality of stator slots (12) and armature windings (10) respectively disposed in the stator slots, wherein the armature windings are formed of superconductive cable (16). 2. A superconducting electrical machine according to claim 1, wherein the superconductive cable (16) comprises multi-filamentary superconductive wire tape. 3. A superconducting electrical machine according to claim 2, wherein the multi- filamentary superconductive tape has an aspect ratio of about 1. 4. A superconducting electrical machine according to claim 1, wherein the superconductive cable (16) comprises continuous cables of superconducting wire. 5. A superconducting electrical machine according to claim 1, wherein the superconductive cable (16) comprises a substantially concentrically layered construction including a cryo-refrigeration coolant passage (14), a superconductive material (16) and insulation (18, 20). 6. A superconducting electrical machine according to claim 5, wherein the insulation comprises thermal insulation (20) disposed over electrical insulation (18). 7. A superconducting electrical machine according to claim 5, wherein the insulation comprises electrical insulation (18) disposed over thermal insulation (20). 8. A superconducting electrical machine according to claim 1, wherein the stator further comprises stator teeth (22) defining the stator slots (12), the stator teeth shielding the superconductive cable (16) from a majority of magnetic fields generated during operation of the electrical machine. 9. A superconducting electrical machine according to claim 1, wherein the armature windings of superconductive cable are toroidal windings (23). 10. A superconducting electrical machine" according to claim 1, wherein the stator further comprises magnetic slot wedges (26) respectively disposed in openings of the slots. 11. An armature winding (10) for an electrical machine, the armature winding being formed of superconductive cable (16). 12. An armature winding according to claim 11, wherein the superconductive cable (16) comprises multi-filamentary superconductive wire tape. 13. An armature winding according to claim 12, wherein the multi-filamentary superconductive tape has an aspect ratio of about 1. 14. An armature winding according to claim 11, wherein the superconductive cable (16) comprises continuous cables of superconducting wire. 15. An armature winding according to claim 14, wherein the superconductive cable comprises layers of superconducting wire (16) surrounded by a continuously extruded insulation system (18, 20). 16. An armature winding according to claim 11, wherein the superconductive cable comprises a substantially concentrically layered construction including a cryo- refrigeration coolant passage (14), a superconductive material (16) and insulation (18, 20). 17. An armature winding according to claim 16, wherein the insulation comprises thermal insulation (20) disposed over electrical insulation (18). 18. An armature winding according to claim 16, wherein the insulation comprises electrical insulation (18) disposed over thermal insulation (20). 19. A method of constructing an armature winding for an electrical machine, the method comprising forming the armature winding with superconductive cable.

Inventors: FOGARTY MICHAEL JAMES, GOTT BRIAN ERNEST BAXTER, WEBBER KONRAD ROMAN, FOGARTY, MICHAEL, JAMES, GOTT, BRIAN, ERNEST, BAXTER, WEBBER, KONRAD, ROMAN
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 2001
Main classification: H02K55
IPC codes: H02K3, H02K55