Research On Low Voltage Ride Through Technology Of Direct Drive Wind Power System

Direct-drive wind power generation system is widely used in wind power industry because of its simple structure,high energy conversion and utilization,and support for grid connection.However,when the power grid falls due to a fault,the wind turbine needs to continue to be connected to the grid without off-grid operation.Therefore,during the grid fault,It is very important to study how to realize the discharge of excess energy from the DC side accumulation for the safe operation of wind power generation systems.The direct-drive wind turbine is taken as the research object in this paper.Based on the composition and mathematical model of the permanent magnet synchronous wind power generation system,the low voltage ride through(LVRT)technology of the direct drive wind power generation system under the grid symmetric fault drop and asymmetric fault drop is proposed.The proposed low voltage ride through technology was verified by simulation and experiment.Firstly,the basic characteristics of the wind turbine and the maximum power tracking are discussed in detail.Then the mathematical model of the direct-drive permanent magnet synchronous generator is built and the space vector modulation technology is analyzed to obtain the vector control strategy of the machine side and grid side converter.The mathematical model of dual PWM converter is built and its steady state control strategy is analyzed.In order to eliminate the influence of symmetrical drop faults in the grid when the symmetrical drop fault occurs,combined with the traditional energy-consuming Crowbar control protection circuit,an improved energy-saving Crowbar low voltage ride through technology is proposed.The simulation results show that this method can operate safely and stably,and can effectively reduce the output of active power on the system network side to prevent the impact of overvoltage on the direct current(DC)bus side.Compared with the traditional low voltage ride through control strategy,the Crowbar method based on energy storage can effectively compensate the undervoltage state of the DC busvoltage.Thereby the system's low voltage ride through capability is significantly improved.When an asymmetric drop fault occurs in the power grid,the traditional strategy will cause the DC bus side voltage to oscillate at 2 times the power frequency.The influence of active power fluctuation on generator electromagnetic torque and DC bus voltage is analyzed.A direct vector control strategy based on positive and negative sequence voltage of power grid is proposed.The simulation results show that the voltage oscillation on the DC side can be effectively suppressed,and the unloading Crowbar circuit is combined to effectively suppress the overvoltage on the DC side,and the effectiveness of the control strategy is verified.Finally,a small constant frequency variable speed wind power experimental platform was built and experimentally verified.The experimental results show that the access unloading protection circuit can make the DC bus side voltage have better stability.The effectiveness of vector control strategy and the low voltage ride through control strategy is demonstrated.