Trade-offs in the Design of Multi-Loop Controllers for Floating Wind Turbines

Feedback control of land-based wind turbines is well-established in both industry and academia, however, the same control strategies do not necessarily perform well when applied to floating offshore wind turbines (FOWTs). Multi-loop feedback has been investigated to address the challenges of FOWT control, but the various proposed auxiliary feedback architectures are seldom compared under a unified study. Four multi-loop control approaches are analyzed in this work and evaluated for their ability to improve FOWT performance metrics, including power quality, generator-speed tracking, and structural loading. Each control law is analyzed in the context of closed-loop system stability using root locus methods and tuned using a frequency-based stability margin. The controllers are evaluated using the nonlinear aero-servo-hydro-elastic simulation tool OpenFAST to validate performance benefits compared to a single-loop baseline controller.