Superconducting generator field winding design for high fault tolerance

Development of rotating electrical machines with superconducting field windings is proceeding at numerous sites worldwide. The primary emphasis is on large turbine generators for application to power systems. The EPRI/Westinghouse 300 MVA superconducting generator program is directed towards demonstration of the technology in an actual utility environment for a long period of time. The concept of stability, in the case of superconducting generators, includes the traditional concepts of stability with respect to the electromechanical interactions and oscillations of the machine with the power system as well as the thermohydraulic stability of the cryogenic rotor and its helium supply system. Power system disturbances, such as faults, produce flow and pressure transients in the rotor cooling system. Depending upon the severity and time history of the disturbances, these transients may occasion normalization of the superconductor and destabilize the generator output through loss of field excitation. This paper addresses the question of designing the superconducting winding and its cryogenic cooling system for stability in the presence of large disturbances, a capability which has been called high fault tolerance.