Design and Analysis of Gain-Scheduled Collective Pitch Control for Down Regulation in Floating Offshore Wind Turbines

As Floating Offshore Wind Turbines (FOWTs) take on a growing share in global renewable energy generation, they will eventuallyj be required to support the grid through ancillary services. This will be achieved through down-regulating (DR) the FOWT by varying the output electrical power through Active Power Control (APC). The FOWT control schemes for power maximization, derived from land-based wind-turbine (LWT) have bandwidth limitations due to the existence of right-half-plane zeros (RHPZs) in Region 3. Conventionally this problem is solved by detuning the land-based controllers but this significantly impacts the power quality. This paper proposes a novel gain-schedule design methodology for Collective Pitch Control (CPC), based on linear model dynamics in Region 3 for DR operation of FOWT. Parallel compensation through feedback of nacelle fore-aft velocity in generator torque is used to address the RHPZs and to achieve control bandwidth similar to LWT counterparts. Two operational strategies (OS) for power command tracking are compared for distributing demanded power, i.e., constant generator torque or angular speed references. System performance is evaluated through both linear and nonlinear analysis in Region 3 considering up to 20% DR. OpenFAST toolbox in MATLAB/SIMULINK is used under varying wind conditions, using RegA and RegD standard power signals by the PJM regional transmission organization for nonlinear analysis. Results demonstrate the proposed solutions' effectiveness in FOWTs for achieving power quality alongside system stability.