Research On Grid-connected Control Of Doubly-fed Wind Turbine Generator

Wind energy has a potential development,which is friendly to the environment and one of the most competitive clean energy.However,the wind turbine installed capacity is increasing and faults occur frequently.The current shock induced by megawatt wind power system cannot be ignored.The wind turbines must be safe and stable operation and have a Low Voltage Ride Through(LVRT)capability.This thesis focuses on the study of Idle-load grid connection and LVRT control of Doubly Fed Induction Generator(DFIG)wind power system.First,the mathematical model and simulation model of the wind turbine are established.Based on the working principle of the doubly fed variable speed constant frequency wind power generator,the mathematical model of the doubly-fed machine in the three-phase stationary coordinate system and the two-phase rotating coordinate system is analyzed.Control strategies of the rotor side and network side control strategy for the double PWM converter are analyzed.Secondly,based on the analysis of the working principle of the Idle-load grid connection of the doubly-fed wind turbine,a grid-connected control strategy based on stator flux-oriented vector control is proposed.The strategy is double closed loop control.The outer loop is voltage fuzzy PI controller and the inner loop is current PI controller.The simulation model based on the above theory is established in Simulink.The simulation results show that the proposed control strategy can realize the grid connection control of doubly fed variable speed constant frequency wind turbine.Finally,based on the research of the control strategy of dual PWM converter,an improved control strategy for realizing low voltage crossing control is proposed.The expert PID controller is used in this strategy.The rotor side converter considers the influence of the stator excitation current.The grid side converter adds the feedforward term according to the power balance.The whole model of doubly-fed wind turbine system is established in Simulink.Through analog voltage drop,low voltage ride through control is realized.The simulation results show that the control strategy limits the overcurrent and overvoltage on the rotor side to a certain extent,and reduces the shock of the DC bus,and verifies the feasibility of the proposed strategy.