Study On Low Voltage Ride Through Based On Rotor Circuit Overcurrent And Overvoltage Synergy Suppression For Doubly Fed Induction Generator

As a form of development and utilization of renewable energy,wind energy has developed rapidly in recent years.As the proportion of power generation in wind power generation continues increasing,in order to maintain safe and stable operation of the power system,wind turbines are required to have low voltage ride-through(LVRT)capability,to ensure that the wind turbines do not operate off-grid during voltage dips,and can provide power support.As the mainstream model of wind power generation,double fed induction generator(DFIG)adopts the structure that the stator side is directly connected to the grid,and the rotor side is connected to the grid through the small-capacity inverter.The operating characteristics are greatly affected by the grid disturbance.At present,the research on the LVRT of the DFIG is mainly based on the crowbar hardware protection circuit,so that the DFIG does not operate off-grid,but this method will make it lose controllability and cannot provide reactive power to the grid support.Therefore,reducing the number of crowbar hardware protection circuit actions under grid faults and using the converter's own capacity to improve the controllability of the DFIG during low voltage ride-through has become the focus of current research.Aiming at this problem,this paper proposes LVRT for doubly-fed wind turbine based on rotor circuit overcurrent and overvoltage synergy which maximizes the operating range of low-voltage ride-through for the DFIG and improves its controllability.Mathematical model of the double fed induction generator is established firstly,and the equivalent circuit of the rotor side of the DFIG is established by its mathematical model.The rotor side converter overvoltage and overcurrent is further analyzed under the condition of grid voltage symmetrical drop.By analyzing the existing problems of the existing low voltage ride through control strategy,the control strategy of the overcurrent and overvoltage cooperative suppression of the rotor circuit is proposed.The principle of inputting crowbar hardware protection circuit and virtual resistance control strategy is analyzed and summarized,and the relationship between the overcurrent and overvoltage suppression synergy of the rotor circuit and the virtual resistance value range is determined.A novel LVRT control strategy based on dynamic virtual resistance.The low voltage ride-through control strategy of the DFIG based on dynamic virtual resistance is proposed,which expands the uninterrupted operation range of the DFIG.According to the different output power and different voltage drop degrees,the virtual resistance minimum and maximum values are determined according to the rotor side voltage constraint conditions and current constraint conditions respectively,and the virtual resistance value dynamically changing with the voltage drop depth is finally determined.Finally,the correctness of the control strategy is verified by simulation with DIgSILENT simulation software.Finally aiming at the shortcomings of slower rotor current decay in dynamic virtual resistance control,a dynamic virtual impedance control strategy based on rotor transient current is proposed.The impedance angle of the virtual impedance increases with the decay of the rotor transient current,so that the virtual impedance exhibits a virtual resistance property at the initial stage of the fault,and exhibits a virtual inductance property at a later stage.The simulation results show that the control strategy combines the advantages of the virtual resistance and the inductance control strategy,and has the effect of suppressing the transient current of the rotor and accelerating its attenuation.