The Research On Low Voltage Ride Through Tecnology Of PMSG Unit

In recent years,the wind power is becoming the fastest and most competitive technology of the new energy development with the advantages of mature technology,low cost and large-scale development and utilization.The interaction between the wind farm and the grid can not be ignored.When the grid voltage drops,the wind turbine will be a large area off the grid and have a serious impact on the stability of the grid if the wind turbine does not have the ability of Low Voltage Ride Through.Permanent magnet synchronous generator(PMSG)has become main models of new wind farm.How to avoid the mismatch of the active power between the generator-side and the grid-side during the voltage drops of the grid point,and quickly stabilize the dc link voltage,which is the key problem of PMSG to achieve LVRT.This paper mainly aims to improve the LVRT capability of PMSG.The operation mechanism of the permanent magnet synchronous generator is analyzed and the mathematical model of the main part of the dq rotating coordinate system is constructed.The control strategy of the whole power converter is deeply researched and analyzed.The zero-d-axis control method based on the Field-oriented control(FOC)is used for the generator-side converter.The voltage-oriented control(VOC)is employed for the grid-side converter.In order to improve the voltage regulation and energy transfer capability of the grid-side converter and the system energy imbalance is mitigated during the grid fault,and fast control the DC link voltage,the paper proposes a LVRT control strategy based on control mode switch.The strategy changes the control mode between in normal and fault of grid-voltage,respectively adopt the gird-side converter or generator-side converter to control the DC-Link voltage;an improved feed-forward compensation strategy is proposed to speed up the DC-Link control,simplify the feed-forward calculation and reduces the peak of the DC bus voltage.When the grid voltage drops deeply,for the capacity constraint of grid-side converter and dynamic reactive power support required by Grid-code,the active output capacity of grid-side converter will be reduced,it cannot suppress the dc-link voltage swells relies solely on the grid-side converter.Therefore,an improved strategy of the dc-link voltage control based on coordinate control generator-side and grid-side converter's active power is proposed.This method can quickly suppress the DC link voltage fluctuations and can improve the ability of LVRT of the PMSG.The paper introduces two kinds of common control methods under asymmetric drop.Thepositive and negative sequence component separation is the key link to realize the control of the grid-connected converters under unbalanced voltage conditions.In this paper,the existing methods of positive and negative sequence separation methods are summarized,and four methods are selected,which are direct filter method,delay signal cancellation method,total differential method,and orthogonal filter method.The paper compared to the normal voltage drop,including harmonic voltage drop and frequency offset voltage drop in three scenarios of separation performance.The advantages and disadvantages of each method are simulated and analyzed.Based on the active coordination control,the negative sequence voltage feed forward method is adopted,and the second order generalized integrator method(SOGI)is used as the positive and negative sequence separation method to suppress the unbalanced component and optimize the control effect under unbalanced voltage conditions.The control target of grid-side current sinusoidal is achieved.The effectiveness and superiority of this method is verified by MATLAB / Simulink simulation.