A Low Voltage Ride Through Strategy Of DFIG Based On Model Predictive Control

With more and more attention being paid to green environmental protection and low-carbon living,clean energy such as wind power and photovoltaic are widely used.With the development of wind power technology,the penetration rate of wind power in China has increased,and the phenomenon of abandoning wind and light has improved in recent years.In the context of large-scale onshore and offshore wind power access to the power grid,the impact of wind power on the power grid cannot be ignored,especially when the power grid faults,how to reduce the impact on the power grid and support grid recovery have become new research central issues.The Doubly Fed Induction Generator(DFIG)is a mainstream model in wind turbines due to its small capacity,low cost,and variable speed constant frequency operation.Because of the special structure of DFIG,the fluctuation of the grid voltage will affect the operation of the DFIG.This will result in rotor over-current,DC bus over-voltage,and electromagnetic torque oscillations,which will cause the DFIG to leave the grid or even be damage.In order to avoid such problems during failures,there are two main types of solutions.The first type of strategy is to add hardware protection to DFIG,such as crowbars,DC bus choppers,STATCOM,and SVC.However,adding hardware protection will increase the cost of the DFIG or cause DFIG to lose control and fail to provide reactive power.The second method is to improve the control strategy of DFIG,using its own controller to improve the fault ride-through capability.Among them,demagnetization control has become a research focus because it can cope with symmetrical and asymmetrical faults,reduce rotor current,and avoid converter saturation.During grid faults,the demagnetization control eliminate the induced voltage by injecting a rotor current opposite to the free and negative flux.The conventional demagnetization control is based on fixed scaling factors,whose control performance can’t be guaranteed for different fault conditions.In this way,the demagnetization current cannot be flexibly adjusted in different fault conditions,and when the fault fault is deep,the demagnetization current is too large,and it is difficult to achieve a good demagnetization effect.An improved demagnetization control scheme based on Explicit Model Predictive Control(E-MPC)is proposed in this paper,in order to make full use of the capacity of the DFIG to achieve Low Voltage Ride Through(LVRT).First,a model of DFIG is established in this paper to study the dynamic behavior of DFIG during faults.Based on the mathematical model of the DFIG,the control rate of the demagnetizing current is dynamically adjusted according to the fault condition,and the capacity of the rotor converter is fully utilized to improve the LVRT capability of DFIG.The demagnetizing current is restricted according to the capacity of the rotor converter.Besides,the control parameters of E-MPC are derived offline and very efficient for online control.In addition,the proposed E-MPC structure is simple and easy to be implemented.The mechanism of proposed E-MPC is presented in detail and verified in Matlab/Simulink.The results show that the proposed control scheme has a good performance and can improve the LVRT capability of DFIG under various fault conditions,especially unbalanced faults.