Advances in Wound-Field Generator Conductors for Large-Scale Direct-Drive Wind Energy Systems: A Review

Direct-drive offshore wind turbines require generator technologies that deliver high torque density, efficiency, reliability and controllability while reducing generator mass, dependence on critical rare-earth materials and associated lifecycle costs. Although permanent magnet synchronous generators remain the performance benchmark for large-scale direct-drive wind power generation, their reliance on rare-earth permanent magnets, thermal sensitivity and fixed excitation motivate continued investigation into electrically excited and wound-field alternatives. This review presents a focused synthesis of wound-field generator technologies for large-scale direct-drive wind power generation (LSDDWPG), with particular emphasis on stator-mounted wound-field and flux-modulation generators (SMWFGs or WFFMGs) and the integration of advanced conductor materials. Major conventional and emerging generator topologies are comparatively assessed in terms of torque density, efficiency, thermal performance, reliability, control requirements and scalability. The review further examines carbon-nanotube-, graphene- and high-temperature-superconducting conductor technologies, while distinguishing between their material-level potential and demonstrated integration into electrical machines. The findings indicate that WFFMGs offer a promising rare-earth-free and brushless pathway through controllable excitation and magnetic-gearing effects, although field-winding losses, thermal loading, power-factor limitations, torque ripple and structural complexity remain important constraints. High-temperature superconducting (HTS) windings have progressed to megawatt-scale concepts and full-scale wind-turbine demonstration, whereas carbon-nanotube (CNT) windings remain at an early feasibility stage and graphene-based conductors have not yet been demonstrated in generator windings at a relevant power scale. By integrating generator topology, advanced conductor development and system-level performance evaluation, this review identifies the principal technological trade-offs, research gaps and future directions for developing scalable, high-performance and material-resilient generators for LSDDWPG.