title: Direct Drive Wind Generators with Superconducting Excitation in the Multi-MW Class
Creators: name: Köster, Robin
Publisher: name: UNSPECIFIED
Year: 2023

description: The main objective of this thesis is the evaluation of superconducting windings for the use in large-power direct drive wind generators. State-of-the-art synchronous wind generators feature rare-earth permanent magnets for the rotor field excitation. Currently, the investigation of alternative excitation technologies is triggered by the generally high and volatile price of rare-earth elements and the market dominance of the People’s Republic of China. When economically competitive, superconducting windings offer the potentials of reduced generator active mass, higher efficiency and simplified up-scaling of the generator power. The effort for the cryogenic cooling generally limits the application of the superconductor technology to rated powers larger than ⪆5MW. In this thesis, direct drive generators with superconducting field winding and normal conducting three-phase AC stator winding are considered. Analytical and field-numerical, electromagnetic models for this generator type are developed and complemented by a thermal model. Different magnetic generator topologies, i.e. a three-phase AC winding in slots in combination with (i) a ferromagnetic rotor, (ii) non-magnetic rotor poles, (iii) a non-magnetic rotor as well as (iv) a three-phase AC air gap winding with ferromagnetic rotor, and different superconductors, i.e. second generation high-temperature superconductors with / without artificial pinning and MgB₂, are compared. Exemplary designs for direct drive generators with superconducting excitation are derived based on parameter studies and multi-objective, numerical optimizations. For comparison, permanent magnet synchronous generators for gearless drive trains are analysed, since no detailed parameter studies on commercially available permanent magnet excited generators are publicly available. The theoretical investigation of the operating behaviour under single-, two- and three-phase short circuits and under partial stator feeding as well as the calculated loss in the cryogenic section reveal no deal-breaker, which could hamper the commercialization of superconducting generators. Regarding the use in field windings, MgB₂ conductors are technically and economically inferior to high-temperature superconductor tapes of the second generation (GdBCO, EuBCO). The much lower conductor cost of MgB₂ wires is overcompensated by the necessarily lower operating temperature and the much lower in-field critical current. For high-temperature superconducting excited generators, the all-iron topology, i.e. with a stator copper winding in slots and ferromagnetic rotor poles and yoke, yields the lowest overall component costs (generator active parts, converter and cryogenic cooling system). An increase in gravimetric power density by ≈33% compared to permanent magnet excited generators can be achieved with this topology. The superconducting field winding allows rated power factors ⪆ 0.97, which is by ≈0.2 higher compared to typical gearless permanent magnet excited generators. However, a price reduction of the high-temperature superconductor to about 1/3 of the current value is required in order to achieve economical competitiveness with respect to the PM excitation.