Design limitations for superconducting field magnet arrays on large hydrogenerators in the absence of shielding

This paper concentrates on the electrical dimensioning of an entirely new class of superconducting machines - the slow speed, multipolar hydrogenerator; with special reference to the design of field windings incorporating state-of-the-art Nb-Ti superconductor. The conceptual machine used to illustrate the limitations on the field array is a 60 Hz, 48 pole unit with a stator rating of 400 MVA; overall characteristics are compared with a conventional 615 MVA hydrogenerator. A specific superconducting cable shape is recommended for the field winding along with maximum charging rates, winding inductances, and inter-magnet attractive forces. A 230 kV stator winding is presented for investigating the large radial and attractive forces acting on the superconducting rotor as a function of load. Although the LHe refrigeration requirements and the abnormally low synchronous reactance of the example 400 MVA machine tend to obscure its commercial value, the limitations governing the field winding are applicable to turbogenerator type superconducting rotors.