Robust multi-loop control of a floating wind turbine

A principal challenge facing the control of floating offshore wind turbines (FOWTs) is the problem of instability, or "negative damping," when using blade pitch feedback to control generator speed. This closed-loop instability can be attributed to non-minimum phase zeros in the transfer function from blade pitch to generator speed. Standard approaches to improving stability and performance include robust tuning of control gains and introducing multiple feedback loops to respond to platform motion. Combining these approaches is nontrivial because multiple control loops complicate the impact of coupling in the system dynamics. The single-loop approach to analyzing stability robustness neglects inter-loop coupling, while a simplistic multi-loop approach is highly sensitive to dimensional scaling and overestimates the robustness of the single-loop controller. This work proposes a sensitivity representation that separates some of the natural FOWT dynamic coupling into a parallel feedback loop in the sensitivity function loop to address both of these concerns. The modified robustness measure is used with a simplified linear FOWT model to optimize scheduled multi-loop control parameters in an automated tuning procedure. This controller is implemented for the 10-MW Ultraflexible Smart FLoating Offshore Wind Turbine (USFLOWT) and compared against conventional single- and multi-loop controllers tuned using frequency-domain analysis and high-fidelity OpenFAST simulations. The multi-loop robust controller shows the highest overall performance in generator speed regulation and tower load reduction, though consideration of power quality, actuator usage, and other structural loading leads to additional trade-offs.