Research On Variable-speed Control For Fixed-pitch Variable-speed Wind Turbines

Because of the simple structure, reliable performance and without the complex pitch control system, fixed-pitch stall-type wind turbines are widely used in small and medium-sized wind power generation system. However, because the wind turbine is operated in constant speed, the power coefficient is relatively low. Although the traditional pitch-controlled variable speed wind turbines have high power coefficient, but it need complex pitch control system, which increases the cost and reduces the reliability. Therefore this paper focuses on the control of fixed-pitch variable speed wind turbines, which simplify structure and reduce the cost. Fixed-pitch variable speed wind turbines can be operated not only for MPPT in low wind speed but also for constant power control in high wind speed with the variable-speed control system, which makes the fixed-pitch variable speed wind turbines have the same power characteristic with the pitch-controlled variable speed wind turbines.In the first, this paper studied the operating states of fixed-pitch variable speed wind turbines under the conditions of different wind velocity, and then the relations between rotational speed reference and wind velocity were found. In the second, the dynamics of the wind turbines with permanent magnet synchronous generator were researched in detail and their small-signal linear model was established, on the basis of which the stability of system is studied.Because of the influences of the non-linear relation among the aerodynamic torque, rotational speed and wind speed, in high wind speed, the speed system of fixed-pitch variable speed wind turbines has already turned into an unstable non-linear system. So in this paper, the use of disturbance observer and Internal Model PID Controller for design of speed control system was presented. Then experiments were done to analyze the property of speed controller of fixed-pitch variable speed wind turbines, which show that the fixed-pitch variable speed wind turbines can be operated stably in all operating wind speed and the speed control system have good at tracking changing wind velocity, which indicated that fixed-pitch variable speed wind turbines can be operated not only for MPPT in low wind speed but also for constant power control in high wind speed.