Research on Direct Speed Control Strategy for Maximum Power Point Tracking (MPPT) of Wind Turbines

Abstract. Maximum Power Point Tracking (MPPT) below the rated wind speed is a key technology to improve the wind energy utilization efficiency of wind turbines. The existing commercial optimal torque control strategy is essentially an indirect speed control method, which has such problems as lacking optimal speed tracking error term, having non-adjustable convergence speed, and invalid control logic in the constant speed zone. To address the above shortcomings, this paper proposes a dynamic optimal speed estimation method based on the generator electromagnetic torque by analyzing the aerodynamic torque characteristics of the wind rotor under different wind speeds and the torque-speed operation trajectory of the generator at each maximum power tracking point, in accordance with the principle of the balance between aerodynamic torque and generator torque at the maximum power point. Taking this optimal speed as the control target, a direct speed controller based on speed deviation feedback is designed to achieve unified control of both constant speed zone and variable speed zone. A MATLAB/Simulink simulation model was built based on a 1.5 MW variable-speed variable-pitch wind turbine, and simulation verification was carried out under the conditions of gradient wind and turbulent wind in the minimum speed zone, variable speed zone and maximum speed zone, followed by field tests in wind farms. The results show that the proposed direct speed control strategy can stably realize MPPT under full operating conditions, effectively avoid torque and power oscillation, and keep the wind energy utilization coefficient stable at around 0.49. Moreover, it features simple control logic, can adapt to smooth switching of different wind speed conditions, and has good engineering application prospects.