Design Considerations and Short-Circuit Characteristics of Fully Superconducting Wind Turbine Generators

Compared with partially superconducting generators, fully superconducting generators (F-SCGs) can further increase the torque density in large direct-drive wind turbine applications. Design trends of F-SCGs intend to increase the electrical loading by applying superconducting wires and boost the current density in the armature winding to meet the critical current density with a safety margin. High currents may cause a low power factor and require the power electronic converter to have a much larger capacity. In an F-SCG, furthermore, torques could be too high, and field and armature currents may exceed the critical currents during a generator short circuit. This paper studies the design of a 20 MW F-SCG with consideration of the control strategy and the power factor, and then evaluates the short circuit characteristic of the F-SCG. The results analysis shows that a capacitive load control should be adopted to avoid a significant drop in the power factor and to make full use of the current-carrying capability of superconductors. An I_{d} = 0 control can also be used with a medium current level. During the short circuit, the negative side is that the phase currents exceed the critical currents and cause quenches. The positive side is that the field currents stay below the critical currents and the torques do not exceed the mechanical limitation of three times the rated torque.