Research On Low Voltage Ride Through Technology Of Permanent Magnet Direct Drive Wind Power System Based On VSG

The permanent magnet direct-drive synchronous wind turbine has the advantages of gearless box,low mechanical loss,high operating efficiency,low maintenance cost and high reliability.It has become the mainstream model in the field of wind power.Virtual Synchronous Generator(VSG)technology can make distributed power supply have better grid-connected characteristics.In the permanent magnet direct-drive wind power grid-connected system,virtual synchronous generator technology can be applied to simulate the output of synchronous generator.Features,the converter has the ability to better support the grid frequency and voltage.In this paper,the wind turbine model,mechanical transmission mechanism model and permanent magnet synchronous generator mathematical model of permanent magnet direct drive wind power system were established in detail.The control method of maximum wind power tracking was studied based on the wind turbine model.The VSG algorithm model was established based on the second-order mathematical model of synchronous generator,and its basic characteristics were analyzed in detail.The small-signal model of VSG algorithm was established.The influence of parameters on the stability and dynamic response of virtual synchronous generator was analyzed according to the system root locus map obtained by different inertia coefficients and damping coefficients.The correctness of theoretical analysis was verified by simulation..Based on the research of VSG algorithm,the VSG algorithm of the net-side converter of permanent magnet direct-drive wind power system was proposed to realize the maximum power tracking of wind energy.The machine-side converter controls the electromagnetic torque of permanent magnet synchronous generator to maintain the stability of DC bus voltage.The control strategy was validated by simulation to verify the effectiveness of the overall control strategy of the machine-side converter,grid-side converter and wind power system.In order to improve the low voltage ride-through capability of permanent magnet direct drive wind power system,the influence of grid voltage drop on wind power system was analyzed in detail.Based on the VSG control strategy,the grid side converter with priority to meet the reactive current output was proposed/reactive power coordinated control,and the use of generator rotor energy storage to achieve low voltage ride through control strategy,and theoretical analysis shows that the generator rotor speed fluctuation range of 4%-8%,will not exceed the safe operating range of the rotor speed.The simulation model of permanent magnet direct-drive wind power system was established in MATLAB/Simulink.The proposed low-voltage ride-through control strategy was simulated.The results show that the proposed control strategy can keep the DC bus voltage stable when the grid voltage drops.The reactive current is output to the grid in time,and the rotor speed was also within the safe operating range.Finally,the back-to-back converter experimental platform was built.The effectiveness of the control strategy of the permanent magnet direct-drive wind power grid-connected system based on VSG algorithm was verified by experiments.