Potential Performance of Fully Superconducting Generators for Large Direct-Drive Wind Turbines

Fully superconducting generators (FSCGs) for wind turbine applications are usually synchronous machines. A high torque density should be obtained by increasing both the main field and the armature current loading. However, there are tendencies that the main field can be low but the armature current loading can be high, such as permanent magnet superconducting generators. Despite Jc(B,T) characteristics and AC loss issues, simply using superconducting armature may not lead to a desired design result but deteriorate the performance with high armature reaction. This paper implements the vector-phasor diagram of non-salient synchronous generators for revealing the potential performance of only increasing the armature current loading. Three commonly used control strategies are compared. The results show that increasing the armature-field flux to a certain level will deteriorate the generator performance with all three control strategies. Comparatively, the voltage control for equaling EMF and voltage amplitudes provides the best overall performance: high torque production capability, high power factor, small power angle and a wide range of armature current loading. The results obtained from the vector-phasor diagrams are verified by finite element models with an FSCG design.