Research In Doubly-Fed Wind Power Excitation Control System

Energy issues become increasingly prominent day by day so it has profound significance to solve energy problems by developing renewable energy. The theme of this thesis is the control strategy of variable speed constant frequency (VSCF) doubly-fed wind power excitation system, carries with much in-depth theoretical analyses and simulation research.This thesis first analyzes the control strategy of doubly-fed wind power excitation system based on the ideal grid condition and deduces the mathematical model and power relations of excitation system. Then establishes the control model of Doubly-Fed Induction Generator (DFIG) based on stator voltage orient and got the grid converter control model based on grid voltage orient. Also this thesis achieves the decoupling control of generator output active power and reactive power. Besides, this thesis analyzes the characteristics of wind turbine and discusses the realization method of Maximum Wind Energy Tracking control. Finally, MATLAB simulation is carries out for all of above mentioned.Along with the increasing install capacity of wind power, there is higher requirement from power grid for wind power systems, in which the wind power systems is expected to operation uninterruptedly during poor grid condition. This thesis analyzes the dynamic response of DFIG during asymmetric failure of grid and during the recovery of the failure. The research results show that there are transient component fluctuation and steady-state component fluctuation in the wind power excitation system during asymmetric failure of grid. Further more this thesis also proposes a control method to accelerate the attenuation of transient components. Eventually the simulation results prove validity of the theoretical analysis and feasibility of the control method.Finally, this thesis manages to control the DFIG under the circumstance of network voltage steady-state asymmetry. Under the network voltage asymmetric conditions, the traditional PI current controller can not control positive and negative sequence components of the rotor current separately. Therefore, this thesis first analyzes the mathematical model and the power relationships of DFIG, considering of negative sequence components. And discusses the calculation of eliminate fluctuation of rotor reference current under various control targets. Moreover, the proportional-resonant (PR) controller is introduced to control rotor current in two-phase stationary reference frame. The use of reference frame is to avoid the time delay which is caused by the use of positive and negative sequence separation filter. At last, the simulation in MATLAB shows that the rotor current PR controller has a good dynamic performance and variety of control objectives can be achieved to realize the control of excitation system under unbalanced grid.