Research On Control Technologies Of DFIG-Based WECS Under Grid Fault Condition

The doubly-fed induction generator(DFIG) based wind energy conversion system(WECS) is the most popular wind power conversion system.The low voltage ride-through(LVRT) capability is important for DFIG to improve the operation reliability of itself and stability of the network.The studies of this dissertation focus on the transient response of DFIG influenced by the grid voltage sags and recovery process,and the control strategies of DFIG under grid faults.Firstly,the mathematical models of DFIG and grid-side converter(GSC) are established in stationary ABC reference frame and synchronous dq reference frame.The influences of the transformer configuration,line impedance,fault distance and circuit breakers on voltage sags and recovery process experienced at the generator terminals are analyzed.Then the change of stator flux is discussed based on the above analysis.The dynamic response of the rotor circuit is also studied.Secondly,this thesis analyzes the improved control strategies of the rotor-side converter(RSC) under voltage sags,and simulation models are built based on the Matlab/Simulink software.The control performance of three kinds of improved control strategies are compared.Then,a modified control strategy is designed to reduce the over current and over torque at the same time during grid faults.A simulation study is carried out to verify the effectiveness of the modified control strategy.At last,on the basis of the mathematical model of DFIG and GSC under unbalanced grid voltage condition,this dissertation analyzes the priority of different control objectives under asymmetrical grid faults.A control scheme of RSC and GSC is designed by means of Proportional-Integral-Resonant Sliding mode controller.A simulation study is carried out to verify the effectiveness of the designed control scheme under grid faults.