Research On Low Voltage Ride Through Technology Of Wind Power System Based On Dynamic Voltage Restorer

With the over-exploitation of traditional energy sources,green,environmentally friendly,and renewable new energy sources such as wind energy have been developed by leaps and bounds,and key technologies such as the use of wind and grid-connected power generation have become the research hotspots of domestic and foreign scientific researchers.In the actual operation of wind turbines,because the Doubly-Fed-Induction Generator(DFIG)can better achieve the advantages of maximum wind energy tracking,doubly-fed induction generators are widely used in wind power generation systems.However,due to the use of small-capacity inverters in the doubly-fed wind power generation system,the anti-voltage drop capability of the system is weakened.Studies at home and abroad have shown that when the voltage drops to a certain value,if no technical measures are taken,the wind power system will have transient effects such as voltage sag while disconnecting from the grid.This situation may lead to the collapse of grid voltage and frequency,which will bring huge losses to production and life,which reduces the utilization rate of the doubly-fed wind power generation system in achieving wind power generation,thereby limiting this clean and green Energy application.Under the above background,improving the low voltage ride through control technology of the doublyfed wind power generation system has become the primary problem to be solved.Aiming at the above-mentioned Low Voltage Ride Though(LVRT)problem,the thesis starts with the series voltage sag compensation device of the wind power system,and mainly carries out the following research work:First,this article uses a series-type active real-time compensation device Dynamic Voltage Restorer(Dynamic Voltage Restorer,DVR)to assist the doubly-fed wind power system to achieve low voltage ride through.Starting from the principle of the dynamic voltage restorer,the system structure and function of the device are detailed analysis.The performance,advantages and disadvantages of the inverter circuit topology commonly used in dynamic voltage restorers are compared and analyzed,and a three-single-phase H-bridge inverter structure is determined to be adopted.The compensation capacity and compensation range of the three traditional compensation strategies commonly used by DVR under a certain energy storage capacity are compared and analyzed in detail,and the amplitude and phase of the compensation voltage under the three compensation methods are obtained respectively,and the full voltage is determined compensation strategy.The simulation results show that the doublyfed wind power generation system can maintain grid connection under the condition of voltage sag,and provide voltage support for the grid to obtain the best compensation effect.Next,in order to improve the response speed and dynamic performance of the compensation voltage,an improved detection method of instantaneous reactive power dq transform is proposed.The method is based on the iterative calculation of digital vector,and the required voltage vector is obtained by the angle bisector of two voltage vectors,then the virtual three-phase voltage is finally obtained from the three-phase symmetrical voltage relationship,and the amplitude and phase of the fundamental voltage vector are accurately extracted.Through the simulation results,the detection method has higher accuracy in the case of voltage sag and improves the real-time detection.In addition,in view of the delay problem of traditional low-pass filter,this paper proposes a time-free impulse filter based on predictive filter.The filter can capture the phase information of voltage signal effectively,and has high precision.The simulation results show that the filter can effectively suppress harmonic signals and extract the voltage sag characteristic more accurately.Finally,in order to realize the low voltage ride-through of the auxiliary wind turbine system of the dynamic voltage restorer when the grid voltage occurs a voltage sag,the DFIG-DVR mathematical model is established.In order to reduce the energy loss of the DVR DC energy storage device,taking the minimum active power provided by the DVR side as the objective function,a dual Q-P control strategy based on energy optimization is proposed.In order to simplify the subsequent calculations,the instantaneous space vector method is introduced to define the current and voltage expressions.Assuming that the load carried by the system is linearly balanced,by calculating the relative position of the grid-side voltage and the terminal voltage,the grid-side instantaneous reference power and DVR reference voltage are obtained.The simulation results show that the dual Q-P control strategy based on energy optimization proposed in this paper can not only achieve fault ride-through during three-phase symmetrical voltage sag,but also achieve smooth switching between islanding and grid-connected modes.