| Under the background of the global energy crisis and green development,the distributed new energy generation system represented by wind power is mostly connected to the grid through power electronic devices.Its randomness and volatility have brought great challenges to the power system.The stator of a doubly fed induction generator(DFIG)is directly connected to the grid,while the rotor is connected to the grid through a back-to-back converter,resulting in the DFIG system exhibiting low inertia and weak damping,making it difficult to maintain frequency stability at the grid connection point.At the same time,due to the small capacity of the converter,when the voltage is caused by a power grid fault,the overcurrent generated by the DFIG stator and rotor will damage the hardware equipment,causing the DFIG system to be disconnected from the power grid.In severe cases,it can lead to large-scale disconnection of wind turbines and even the collapse of the power system.At this point,the DFIG system needs to have a certain level of low-voltage traversal capability to ensure grid connected operation during power grid failures.(1)In response to the issues of low inertia,weak damping,and poor frequency stability in DFIG,this thesis adopts Virtual Synchronous Generator(VSG)control technology at the grid side converter.By simulating the electromechanical transient equation and voltage frequency regulation characteristics of the synchronous generator in the converter control section,the excellent characteristics of inertia damping and other characteristics of the synchronous generator are transplanted into the DFIG system.Considering the asymmetric voltage drop in the power grid leading to overcurrent in the DFIG stator and rotor,this thesis adopts amplitude limiting control and negative sequence voltage control to assist in improving VSG technology.Through simulation and comparative experiments,it has been proven that the improved VSG control can significantly improve the low-voltage crossing ability of DFIG compared to traditional VSG control.When the single-phase voltage drop in the power grid is within 20%,the improved VSG control technology can effectively suppress the stator and rotor overcurrent caused by negative sequence components,while stabilizing the frequency of the grid connection point,and improving the low-voltage crossing ability of the DFIG system from two dimensions.(2)When a three-phase symmetrical fault occurs in the power grid,the voltage drop amplitude is relatively deep,and it is difficult to ensure the safe grid connection of DFIG by improving the control strategy.In response to this issue,this thesis proposes an additional Crowbar protection circuit on the rotor side to achieve lowvoltage crossing of the DFIG system during three-phase symmetrical faults.Through simulation,two working conditions of symmetrical voltage drop of 30% and 80% of three-phase voltage were simulated.Compared with traditional vector control rotor side Crowbar protection,it has a good suppression effect on DFIG stator rotor overcurrent and DC bus overvoltage caused by power transmission imbalance.Finally,the exit time of the Crowbar protection circuit was optimized through simulation analysis.Exiting the Crowbar circuit within the first half cycle of the fault can avoid the superposition of transient processes with fault removal,and provide a large amount of reactive power support to the system after fault removal to help quickly recover the system voltage.After optimizing the exit time,the Crowbar protection circuit on the rotor side effectively improves the low-voltage crossing ability of the DFIG system during deep three-phase voltage drops. |
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