Critical Evaluation of DC-Grid and Strong Grid-Forming Wind Generator Systems

The global transition to renewable energy has intensified challenges related to grid stability and material sustainability in wind power systems. This research addresses two critical issues: the declining grid strength due to inverter-based resource integration, and the dependency on rare-earth permanent magnets in wind generators. The research proposes and evaluates alternative system architectures and generator technologies through comprehensive theoretical analysis, numerical simulation, and experimental validation. The first part of this work develops novel strong grid-forming (SGF) wind energy systems based on synchronous machine interfaces. Three distinct configurations are investigated: a variable DC grid wind-cluster with motor-generator (MG) drive, a ground-mounted SGF (G-SGF) system, and a top-tower-mounted SGF (T-SGF) system. These systems demonstrate the capability to provide 24/7 grid strength, inertia, and reactive power support while maintaining maximum power point tracking (MPPT). A key finding reveals that the fundamental power factor of diode rectifier-connected generators deviates from the commonly assumed unity value, which is crucial for accurate system modeling. Stability analysis proves that overdamped operation is achieved through proper grid frequency feedback in the MG control. The second part presents the design of a non-permanent magnet alternative through a triple-three-phase wound rotor synchronous generator (T-WRSG) optimized for DC grid applications. A multi-stage optimization methodology combining the coordinate descent method and NSGA-II algorithms successfully reduces the synchronous reactance from 2.82 per unit (pu) to 0.94 pu for a 5-MW T-WRSG, enabling effective operation with passive diode rectifiers. Experimental validation using a 4.2-kW T-WRSG prototype confirms a measured synchronous reactance of 1.0 pu. The derived DC grid voltage versus speed function enables model-based MPPT control. This research contributes to the design of more resilient and sustainable wind energy systems by providing viable solutions for grid strength enhancement and rare-earth material independence. The integrated approach from theoretical design to practical implementation establishes a foundation for next-generation wind energy technologies suitable for large-scale offshore applications.

Grantor dc:publisher

Stellenbosch : Stellenbosch University

Year dc:date.issued

2026

Author dc:creator

• Dube, Lucky

Advisors dc:contributor.advisor

• Garner, Karen S.
• Kamper, Maarten J.

Language dc:language.iso

en

Repository record dc:identifier.uri

https://scholar.sun.ac.za/handle/10019.1/135787

OAI identifier oai:identifier

oai:scholar.sun.ac.za:10019.1/135787