Research On The Low Voltage Ride Through Technology Of Direct-driven Permanent Magnet Wind Power System

With its high efficiency in energy conversion,high reliability,simple control method in grid-side,the direct-driven permanent magnet synchronous generator has gained more and more popularity in wind power system.On the circumstance of the proportion of wind power becoming higher,it will lead adverse effect to grid if the wind turbine off-grid during the fault of grid.According to the national standard of China,“the technical rule for connecting wind farm to power system”,wind farms are required to keep on-line operation and to provide reactive power during the fault until grid recovered.With wind power penetration in power systems increasing gradually,in order to ensure the security and stability of the power grid,the technical rule for connecting wind farm to power system will be tightened.Over the coming years,the wind farm will be required to have low voltage ride through capability under asymmetric voltage fault.In this thesis,2MW permanent magnet direct drive wind turbine is the object.Theoretical study is mainly on the low voltage ride through technology under symmetric and asymmetric voltage fault,which is verified to be feasible by simulation.First of all,the mathematical model of direct drive permanent magnet wind power system is established,including the mathematical model of wind power generator,the direct drive permanent magnet wind turbine,the machine side converter and the grid side converter.The model of wind turbine is constructed and simulated,which provides theoretical support for the low voltage ride through.Secondly,the low voltage ride through technology under symmetric voltage fault is studied.The power dissipative crowbar and the power storage crowbar are analyzed amply,and the advantages and disadvantages of the two different measures are compared through simulation.According to the demand of reactive power when grid voltage dips,a reactive power control strategy in grid side converter is studied.Thirdly,an improved control method is studied,which is based on storing the active power surplus in the turbine-generator mechanical system intertia.Finally,the low voltage ride through technology under asymmetric voltage fault is studied.The formation mechanism of active power double frequency components under unbalanced grid voltage level is analyzed,and the influence of active power ripple on the electromagnetic torque of generator and on the voltage of DC bus is discussed.A vector-oriented control strategy based on positive and negative voltage-oriented respectively(PN-VOC)is studied.Another method is studied,which is based on positive voltage-oriented(P-VOC).The advantages and disadvantages of the two different measures are compared through simulation.Simulation results show that both of the two measures can suppress the double frequency components of the DC bus voltage,and verify that the two control methods are effective.