Research On Low Voltage Ride Through Of Direct Drive Permanent Magnet Wind Turbine

Wind power has become the most important part of new energy power generation in recent years,and the proportion of wind power in the power grid has increased year by year.Wind power off-grid operation during grid failure will affect the stability of the grid.At present,many countries have formulated corresponding grid fault operation standards,put forward detailed requirements on the running time and descent depth of grid faults,and output reactive power to the large grid to support voltage recovery during voltage sag.The direct-drive permanent magnet wind power generator set occupies an important position in the research of wind power generation because of its simple structure,convenient maintenance and high conversion efficiency.The main research focus is on the direct drive low voltage ride-through technology,and the two fault states are analyzed and compared.MATLAB simulation verified the correctness of theoretical research.Firstly,the mathematical model and basic control methods of the direct-drive permanent magnet wind power generation system are introduced and analyzed,including the basic characteristics of wind turbines,the principle of wind energy capture,and the mathematical model of direct-drive permanent magnet generators.The mathematical model of the machine-side converter and the zero-d-axis vector control principle are studied.Based on the theoretical study of the power transfer characteristics between the machine side and the grid side PWM converter,a mathematical model of the intermediate link is established.It provides basic mathematical model and theoretical basis for model construction and simulation of low-voltage wire technology for direct-drive permanent magnet wind power generation system.Secondly,the influence of grid voltage sag on the operation of wind power system when symmetrical fault occurs occurs.For the DC link unbalanced power,the traditional protection method is introduced.Aiming at the deficiencies of the existing methods,an improved low-voltage traverse technology based on machine-side power feedforward control and Crowbar unloading circuit is proposed,which consumes DC side redundant unbalanced power.In addition,when the grid voltage is restored,the grid-side converter starts the reactive powercompensation mode,and provides reactive power to support the grid voltage recovery.The correctness of the control strategy is verified by simulation.Finally,based on the asymmetric faults that occur in most faults in the power grid,the operation of the asymmetric grid is studied.The positive sequence and negative sequence separation methods are proposed on the network side,and then analyzed one by one using the symmetrical component method.The existing positive and negative sequence separation method is improved,the loading and unloading circuits on the DC side are added,and excess energy is consumed,thereby improving the positive and negative sequence separation methods.The method combines the control strategy with the unloading circuit to improve the LVRT capability of the direct drive permanent magnet wind power system under asymmetric faults.The correctness of the proposed strategy is verified by simulation.