Research On Low Voltage Ride Through Of Direct Driven Permanent Magnet Synchronous Generation

In order to develop human society,the demand for energy must increase rapidly,resulting in the depletion of non-renewable energy and serious damage to the environment.For the sake of human destiny community,all countries are committed to green wind energy utilization.direct drive permanent magnet synchronous wind turbine has been greatly promoted and developed by all countries due to its simple structure,high power generation efficiency,high operational reliability and other characteristics,and its supporting safe and stable grid-connected operation control has also become a research hotspot.In order to study the grid-connected operation characteristics of direct-drive permanent magnet synchronous wind turbine under grid failure,a voltage sag generator based on transformer is designed.in view of the shortcomings of the existing transformer vsg design scheme,such as limited by switching devices and inaccurate control of voltage sag time,a three-phase vsg design scheme based on three-phase autotransformer and used by IGBT bidirectional switch and contactor is proposed.The experimental results show that the vsg design scheme can simulate all kinds of voltage drop faults,which provides favorable conditions for the verification of low voltage ride-through capability of wind turbines.The grid-connected operation characteristics of direct-drive permanent magnet wind power generation system under grid failure are analyzed.due to the grid voltage drop of the access point and the unequal power between the wind turbine side and the grid side,the voltage on the DC bus of the converter instantly rises above the rated value.A power coordinated control method is proposed.the generator-side converter controls the DC bus voltage to be constant,changes the generator electromagnetic torque,preferentially uses the rotor of the fan to bear the unbalanced energy,and changes the pitch angle of the pitch system to reduce the wind energy capture of the fan.The network side converter adopts decoupling control of active and reactive power,inputs maximum reactive power to the power grid,and reduces the voltage drop amplitude of the access point of the power grid;The simulation verifies that thecontrol method can effectively reduce the output of the energy storage system and improve the low-voltage ride-through capability of the system in the whole wind range.In order to deal with more severe power grid faults and further improve the low-voltage ride-through capability of the fan,a super capacitor energy storage system is added to the DC bus to store the unbalanced energy in the DC fan itself.Analyze that power balance relation of the direct drive permanent magnet wind pow generation system based on the super capacitor energy storage,establishes the model of the super capacitor energy storage system and the bi-directional DC /DC convert model,and gives the control strategy and the main parameter design.The simulation results show that the unbalanced power on the bus is stored in the energy storage system when the grid voltage drops,which improves the low voltage ride-through depth of the system.