Study And Design Of Fixed-pitch Variable-speed Non-grid-connected Wind Power Control System

The rotational speed of variable speed wind turbines can be adjusted according to the change of wind speed, which allows the turbine to operate with the optimum power efficiency and highly improves the working efficiency of the whole system. According to different wind turbine aerodynamic power regulation methods, there are two basic plant types, the fixed-pitch type and the variable-pitch type. The latter one is equipped with both rotational speed and turbine blade control systems, which enable it the best power characteristic and it is has been widely used in medium and large-scaled wind farms nowadays. But for small-scaled ones, e.g. the non-grid-connected wind power system, it increases the whole costs by implying the complicated parts. Differs from pitch-controlled types, fixed-pitch variable speed wind turbine adjusts its aerodynamic power by fully control the shaft speed and it can achieve the comparable power characteristic in theory. Besides that, it takes the advantage of being simple, reliable, and much cheaper, which makes it very suitable for medium and small-scaled wind farms.This paper focuses on the design of control system based on fixed-pitch variable speed wind power system, and the aim is to achieve the optimal control with energy efficiency criterion. Two different topologies have been studied in detail.Firstly, generator-side converter control based on reactive-rectify topology has been discussed. By modifying the generator-side converter as equivalent resistance at the generator terminal, the mathematical equation of the plant is established. Because it is a highly nonlinear system, but with smooth nonlinearities, a possible optimal control design solution can be the feedback linearization control. The controller design has been discussed in detail, and the simulation results prove that the proposed controller manages to ensure maximum power extraction on low wind condition.Secondly, generator-side converter control based on active-rectify topology has been discussed. The steady state and dynamic characteristics of the turbine show that the plant is unstable under indirect speed control strategy on high wind condition, so direct speed control strategy is introduced, which is used to keep the aerodynamic power at a constant value on high wind condition. Vector control is applied to PMSG, the design method of current and speed controller is given, and the instability of the system can be eliminated by aerodynamic torque feed forward technique. Simulation and experiments are done at last, which show that the fixed-pitch variable speed wind turbine can be operated stably in all operating wind speed and the speed controller have good dynamic characteristics, which indicated that the fixed-pitch variable speed wind turbine can be operated not only for MPPT in low wind speed but also for constant power control in high wind speed.