Research On Low Voltage Ride Through Technology For Doubly Fed Wind Power System

Doubly-Fed Induction Generator,DFIG is currently one of the mains-tream of wind power generation in the world.However,due to the direct suspension of the stator side of DFIG,it makes wind turbine generator system very sensitive to the voltage drop fault of the power grid.Thus,Doubly-Fed wind farm urgently needs to solove the problem of Low Voltage Ride Through,LVRT in the process of grid connection.This thesis is devoted to the research of low voltage ride through technology of megawatt DFIG which respectively starts from inputting Crowbar hardware protection circuit and proposing nonlinear control strategy to make it meet the standard of grid connection at low voltage failure.The main contents of the full text are as follows:Firstly,the development history and research status of doubly-fed wind power generation as well as technical standard of LVRT for doubly-fed wind power generation at home and abroadare analyzed so as to summarize current mainstream control strategy.The establishment of mathematical model for wind turbine,DFIG and grid side converter wind turbine lays a theoretical foundation for studying the method of LVRT.Then,in order to realize Low Voltage Ride Through of DFIG unit when grid voltage falls sharply,the calculation formula of the fault current on the rotor side is derived by starting from transient mathematical model of DFIG unit under voltage drop fault.Considering the two aspects of rotor current limitation and DC busbar voltage limitation,one kind of tuning method for Crowbar resistance which conforms to the engineering practice is proposed in order to solve the problem that overcurrent and DC busbar Overvoltage occur on the rotor side after inputing Crowbar hardware protection circuit.The example and simulation analysis indicates that the tuning method for resistance menthioned can significantly improve the LVRT level of wind power generation system under large voltage drop.Meanwhile,based on nonlinear control theory,the nonlinear state feedback expression of the system is derived by adopting method of state feedback linearization and coordinate transformation in order to realizeDFIG’s low voltage ride through when the grid voltage falls slightly.The method of using linear optimization control system design proposes low voltage ride through of for grid side converter based on input output feedback linearization to achieve linearization of the nonlinear system and complete the design of the related controllers.The simulation results verify the correctness of the method and the nonlinear controller on the net side designed can play a good control role which has enured the LVRT level of wind power generation system under small voltage drop.Through the theoretical deduction and relevant simulation verification for the related control strategy proposed in this thesis,the simulation results are consistent with the theoretical analysis,the effectiveness of the low voltage ride through technology proposed in this thesis is verified.