Study On The Characteristics Optimization Of Low Voltage Ride Through For Doubly-fed Induction Generators Based On Adaptive Control

As the proportion of grid-connected installed capacity of wind turbines increases year by year,its impact on the power grid is also increasing.In order to ensure the stable and reliable operation of the grid after the wind power integration,the wind farm must have the similar fault ride through ability with the traditional power plant and the ability to continue the grid connected operation after the grid fault.That is the capability of low voltage ride through(LVRT).Doubly fed induction generator(DFIG)is the main type of wind turbine.However,because the stator side directly connected to the power grid,it’s very sensitive to grid faults and there is a serious LVRT problem.Therefore,how to improve the LVRT capability of DFIG has become a hot topic that widespread concerned.This article takes this as the research theme and conducts an in-depth study on it using adaptive control theory.Firstly,starting from the basic physical model of DFIG,the mathematical model of DFIG with crowbar resistance in three-phase stationary coordinate system and two-phase synchronous rotating coordinate system is established by using mechanism analysis method.Based on this,the inner and outer loop control strategies of the stator side and rotor side of DFIG and the determination method of control parameters are also studied.Then the theoretical analysis and simulation analysis of the stable working state of DFIG are carried out.Secondly,this paper focuses on the analysis of the dynamic characteristics of DFIG when grid fails.Starting from the transient flux linkage of the stator after fault,the expression of rotor current under different conditions at different stages after fault is derived using the superposition principle.Based on this,two low-voltage ride-through optimization strategies are proposed including the strategy of rotor side converter phase modulation and the strategy of adaptive crowbar resistance.Considering the limitations of the strategy of rotor side converter phase modulation optimization,this paper focuses on the LVRT strategy of adaptive crowbar resistance protection.In order to reduce the reactive power absorbed by DFIG and improve the LVRT capability,this paper applies the method of multi-constraint single target to optimize the crowbar resistance value in real-time.Moreover,a function expression of the adaptive crowbar resistor and control flow chart is obtained.Then the LVRT strategy of adaptive crowbar resistance protection is verified through simulation experiments.Finally,based on the basic principle of the static synchronous compensator(STATCOM),this paper analysises its mathematical model and double closed-loop control strategy.On the basis of summarizing the limitations of adaptive crowbar resistance,a LVRT scheme based on the combination of STATCOM and adaptive crowbar resistance protection is proposed.The scheme was verified through simulation experiments.The results show that the current attenuation speed of stator and rotor is obviously faster when STATCOM and adaptive crowbar are used.After a period,the peak value of stator current decreases by 1.2pu,the peak value of rotor current also decreases by 1.0pu.The DC bus voltage is also more stable,and its fluctuation range is only about 5V.It effectively solves the problem of overcurrent and overvoltage of the converter.Therefore,the working conditions of the converter are improved,and the LVRT capability of the wind turbine is also improved.