Design challenges of direct-drive permanent magnet superconducting wind turbine generators

In recent years, permanent magnet superconducting (PMSC) generators have become a candidate for applying superconducting (SC) generators in large direct-drive wind turbines. This configuration keeps the SC armature winding and its cooling system stationary and eliminates rotational cooling couplings. However, the low excitation by permanent magnets may lead to poor power factors if the armature current is high. Furthermore, the permanent magnets are prone to demagnetization when the armature reaction is strong. This paper investigates the design challenges regarding the power factor, demagnetization and short circuit characteristics by analyzing two PMSC generator designs. The results show that the power factor cannot be as high as 0.9 and a low power factor such as 0.6 can take advantage of the high current carrying capability of the SC armature winding. However, this low power factor will cause demagnetization. The armature current may cause quenching of the SC wires during a three-phase short circuit. Demagnetization of the permanent magnets during the short circuit is strong and could be an intrinsic weakness of a PMSC generator.