Stabilization of Floating Wind Turbines

Abstract

Wind turbines mounted on floating platforms are, under a specific set of circumstances, prone to a phenomena called control-induced resonance. The phenomena materializes as persistent oscillations in the platform’s surge and pitch displacement and occurs when pitch-control algorithms without modifications for floating operation are used. This thesis examines the physical underpinnings of the resonance problem by a novel modeling strategy. A systems approach is adopted where the floating wind turbine is conceived as an interconnection of subsystems. New aerodynamic models suitable for a systems approach are proposed, validated and examined. Notably, a vortex theoretical approach is shown to be fruitful both in the steady and unsteady loading regimes. A new type of dynamic inflow model based on vortex transport and frequency domain identification is proposed and compared favorably to existing strategies. The vortex lifting law is used to generate a succinct model for loads that fits into the systems representation and replaces the commonly used table-data approach. A simplified dynamic model suitable for control analysis is derived and validated against a standard tool in windturbine analysis. The system model developed in the first part of the thesis is used for a smallsignal stability analysis invoking passivity tools in the second part. It is shown that floating wind turbines are stable under quite general conditions as long as no pitch control is applied. If collective blade pitch is used for the sole purpose of setpoint control of the angular velocity, destabilization is shown to be inevitable. An examination of the energy flow within the wind turbine system motivates a novel "energy shaping" control strategy. Existing control strategies are also examined in the new paradigm. A case study of a representative 5MW wind turbine is performed. The resonance phenomena is demonstrated under realistic conditions. A number of control strategies are tested and compared. The "energy shaping" control strategy is shown to perform well and eliminates the control-induced resonance. Justification for the results are given based on the theory developed in the thesis.