Research On Low Voltage Ride Through Technology In Direct Drive Permanent Magnet Wind Power System

In recent years,wind power has become the most important component of new energy power generation,and the penetration rate in the power grid continues to rise,wind turbines out of operation during the grid failure will endanger the reliability of the grid.Nowadays,many countries have issued relevant operational standards in the event of grid faults,requiring wind turbines not to run off the grid when voltage sags,and providing reactive power support to the grid until normal operation is restored and crossing grid faults period.Direct-drive PMSG with its simple structure,low maintenance costs,high conversion efficiency,high reliability,has higher and higher proportion in the wind power market.This dissertation mainly studies the low voltage ride-through technology in direct-driven PMSG system,analyzes and discusses the low-voltage protection technology of wind power system under symmetrical faults and asymmetrical faults respectively.The correctness of theoretical research is verified by MATLAB simulation.First of all,the mathematical model and basic control method of direct-driven permanent magnet wind power system are introduced in general,including the basic characteristics of wind turbine,the mechanism of wind energy into electric energy,and the mathematical model of direct-driven permanent magnet generator.The mathematic model of PWM generator-side converter is studied and the traditional control strategy of rotor field vector orientation i_d=0 is introduced.By analyzing the energy transfer characteristics between the dual PWM converters,the mathematical model of the intermediate DC link is analyzed.The mathematic model of grid-side converter and the basic control strategy of grid voltage-oriented vector are introduced.In order to study the low voltage ride-through technology of direct-drive permanent magnet wind power system,the basic mathematical model and theoretical basis are provided.Secondly,the influence of grid voltage drop on the operation of wind power system is discussed when symmetrical fault occurs.Aiming at the unbalanced power appearing in the DC link,the traditional protection methods are introduced.Based on the deficiencies of the existing methods,an improved low voltage ride through technique based on the crowbar unloading circuit with linear ADRC is proposed,consuming excess unbalanced power on the DC link side.And when the grid-side inverter is in fault,it runs in STATCCOM's reactive power control strategy and provides reactive power support to the grid restoration.The LVRT capability of the wind power system is improved and simulation results show the effectiveness of the control strategy.Finally,the operating characteristics of the direct-drive permanent-magnet wind power system under asymmetrical faults are analyzed.The principle and conventional separation methods of positive and negative sequence components of asymmetric faults are discussed in detail.An improved positive-negative sequence separation method is proposed and applied to grid-side inverter positive and negative sequence voltage directional vector control strategy to suppress the DC bus voltage fluctuations.And crowbar unloading circuit is added in the DC link to consume extra energy.The two methods are combined to improve the LVRT capability of the wind power system under asymmetric faults,and the simulation proves the effectiveness of the protection method.