Control and performance assessment of variable rotor resistance-based wind turbines regarding aerodynamic power fluctuations

This paper, first, deals with the analytical controller design of wind turbines with variable rotor-resistance control. Wind turbines with variable rotor-resistance control, known as Limited Variable Speed Wind Turbine (LVS-WT), provide a limited variation of the generator speed. In the LVS-WT, the rotor current and, consequently, the output power can be controlled by varying the rotor resistance. Then, modal and small signal analyses of the wind turbine are examined. It is found that for certain values of shaft stiffness, frequency of the mechanical modes coincides with the triple oscillation frequency appeared due to tower shadow effects. This in turn results in resonance phenomena magnifying the fluctuation of the generator power and electromagnetic torque. Next, the paper evaluates the impact of aerodynamic torque fluctuations on the dynamic response of the LVS-WT. In this way, analytical expressions for the fluctuations of the stator voltage and current, electromagnetic torque, and generator active power are proposed. These fluctuations arise because of the aerodynamic torque and rotor speed perturbations. The paper also investigates the effects of shaft stiffness, slope of power-slip curve, and rotor resistance on the wind turbine response. At the end, time domain simulations verify the results of theoretical analyses.