Research On Zero Voltage Fault Ride Through Technology And Control Strategy Of Doubly-fed Induction Generators

For the purpose of achieving the ambitious goal of " 2030 carbon peak" and " 2060 carbon neutrality",China’s investment in new energy-related facilities and construction will continue to increase.As an important part of new energy,the proportion of wind power generation gradually increases with the rise of national wind turbine installed capacity.As one of the main wind turbine models,doubly-fed induction generator(DFIG)plays an extremely important role in wind energy utilization.Moreover,because of the stator of the DFIG,which is directly connected to the grid,and the capacity of the converter is only about one-third of the rated capacity of the fan.The voltage fluctuation during the grid fault may cause the voltage and current over-limit of the windings on the rotor side of the DFIG,which threatens the safe and stable operation of the wind turbine generators.With the increase of wind power permeability,the above phenomenon will become negligible.Ensuring the continuous operation of wind turbines without disconnection from the grid during the grid failure is an effective way to avoid a high percentage of new energy grid disconnection accidents.And the extreme situation in the low voltage faults-zero voltage fault will bring instantaneous impact of overvoltage and overcurrent which are several times of the safety threshold value to the DFIG,which will cause great damage in the wind turbine.In the face of large-depth voltage transient fault,the crowbar hardware circuit used in most field stations will make the generator-side converter of the DFIG out of control and absorb the reactive power from the grid side when the protection is started,which will result in further deterioration of the voltage of the point of common connection of grid and is unfavorable to voltage recovery.This passive protection mode is unfavorable to the safe and stable operation of power grid in grid connection region of high percentage of wind power.Therefore,this writing proposes a comprehensive fault ride-through scheme based on a high temperature superconducting fault current limiter with adaptive virtual impedance control and active dynamic reactive power support control strategy,which can realize uninterrupted operation under 100% voltage droop fault and actively provide dynamic reactive power support for the grid,helping recover the voltage of the grid connection point.Through simulation and comparison experiments based on MATLAB/Simulink platform,the rationality and effectiveness of the fault ride-through scheme mentioned herein are verified.The main research contents and work of this thesis include:Above all,the topological structure and control strategy of the DFIGs system were modeled and analyzed.The transient characteristics of the DFIG under 100% voltage transient fault were further analyzed by using the mathematical model under the established synchronous rotation coordinate system d-q,which provides theoretical basis for the indicator design of the fault ride-through scheme.Furthermore,this thesis briefly described and compared various hardware protection schemes in series and selected the high temperature superconducting fault current limiter as the hardware protection device in this design scheme.And detailed deduction and analysis were given for its action mechanism.What’s more,in allusion to rotor current and voltage over-limit problem,the rotor current and voltage suppression mechanism of DFIG in the period of virtual impedance versus fault was analyzed and its effectiveness was verified.With the purpose of preventing the unwanted control effect brought about by the setting of fixed impedance value,this thesis proposed an adaptive virtual impedance control strategy which can dynamically adjust the impedance value according to the transient conditions of the DFIG.After analyzing and deriving,the boundary of impedance value can be set,so as to be involved in the fault ridethrough control without changing the controller structure and without switching over to it.With the aim of taking advantage of the capacity of the wind turbine to provide reactive power support for the grid as much as possible after fully analyzing the reactive power output limits at both the stator side and the grid side,this thesis proposes a control strategy for actively dynamic reactive power support to maximize the ability to provide reactive power support for the grid during fault,and dynamically adjust the reactive power output value according to the grid-connected point voltage,so as to quickly restore the grid-connected point voltage while avoiding the occurrence of high-voltage faults caused by excessive reactive power.Finally,a simulation model was established based on MATLAB/Simulink and the rationality and effectiveness of the zero-voltage ride-through scheme put forward in this thesis was verified.Compared and realized,this model further proves the engineering operation value of the scheme put forward herein.