Research On Low Voltage Ride Through Stability Control Method Of Wind Turbine Generators

With the increase of wind power integration,the interaction between wind power and power grid is becoming more and more stronger.The operation and dynamic characteristics of power system are undergoing significant changes,which brings great challenges to the safe and stable operation of the power grid.Especially with the further weakening of the power grid,the voltage of the common couping point is more susceptible to the influence of output of wind turbine under the grid fault,which increase the risk of synchronous instability of the phase-locked synchronous wind turbine.Therefore,how to improve the synchronous operation ability under weak grid has been an urgent problem to be solved in the development of renewable energy power system.On the other hand,in the power system with high wind power penetration,a large amount of active power is lost when wind turbines enter the LVRT control due to the grid fault,which leads to the rapid drop of system frequency.Accordingly,how to take full advantage of the regulation capacity of wind farm to reduce the short-term active power shortage and improve the frequency support capacity of wind power during the LVRT period is also of great significance to ensure the grid to operate safely and steadily.This thesis focus on the synchronous control method and coordination control strategy during LVRT period.The main works can be summarized as follows:1)The mathematical model of grid connection based on phase-locked synchronous wind turbine is established.Then,the synchronization characteristics of wind power system in weak grid scenario are analyzed with the LVRT control method.On this basis,the effects of grid voltage drop,grid strength and output current of wind turbine converter on system synchronization stability are studied.2)A synchronous stability control strategy based on dynamic voltage feedback is proposed to improve the synchronization ability of wind turbine based on PLL during LVRT.The influence of PLL on the output characteristics of wind power is analyzed,then the transient synchronization equation of the system is deduced.The synchronization stability during LVRT is investigated by using equal area criterion.Based on analysis,a wind turbine synchronous stability control strategy using dynamic voltage deviation to adjust the current reference value is proposed to effectively improve the synchronous stability during LVRT period.3)For the high wind power penetration grid,the grid fault may cause large-scale wind turbines enter the LVRT control and further result in the system frequency drop.A LVRT control strategy for wind farm considering active and reactive power coordination is proposed.The coordination controller is designed for the wind farm layer,which fully consider the influence of wake effect and combine the reactive current constrains and the wind turbine’s operation constrains.By optimizing the current instruction of the wind turbines in the wind farm,the active power shortage is reduced and the frequency characteristics of the system are improved.