Research On Control Of Flexible DC Transmission System For Doubly-fed Offshore Wind Farms

In recent years,the application of flexible direct current transmission technology(VSCHVDC)in offshore wind farms has attracted more and more attention due to its advantages such as flexible operation control and good power quality etc..As one of the mainstream wind turbines,doubly-fed wind turbines(DFIG)have been widely used in wind power generation.In order to better combine DFIG and VSC-HVDC,this paper is based on the doubly-fed offshore wind farm multi-terminal flexible DC transmission system,and conducts research on the control of the multi-converter under normal and faulty operation of the system.First,the topological structure of the flexible DC transmission system of the double-fed offshore wind farm used in this paper is described.The stator-side converter(SSC)and the rotor-side converter(RSC)of DFIG adopt voltage source converter(VSC).Compared with the traditional solution,this system has the advantages of not requiring a power frequency transformer,small footprint,low loss,etc.,and because the stator side of the DFIG is isolated from the grid,the output AC power can be variable frequency and voltage,and the control is simpler and more flexible.Second,the control of the stator and rotor side converters of the DFIG in the flexible DC transmission system of the doubly-fed offshore wind farm is carried out.By modeling DFIG and adopting a control strategy based on stator voltage orientation to control the doubly-fed wind turbine,the effectiveness of the control strategy is verified by MATLAB/Simulink simulation analysis and prototype experiment.Third,considering the advantages of the VIENNA rectifier with low current harmonics and low voltage stress of power devices,in order to further improve system performance and reduce costs,a scheme of using VIENNA rectifier as the stator-side converter of DFIG is proposed.,And research on the control strategy of the topology structure.The simulation verification is carried out on the MATLAB/Simulink platform,and the validity of the proposed scheme is verified.Fourth,because the voltage drop of the DC grid will threaten the safe operation of the double-fed offshore wind farm flexible DC transmission system,this paper proposes a low voltage ride-through(LVRT)scheme based on supercapacitor energy storage devices.This scheme uses the fast charge and discharge characteristics of supercapacitors to ensure the transient stability of the flexible DC transmission system.The scheme provides the control methods of the DFIG stator-side converter,rotor-side converter,DC grid-side converter and energy storage device-side converter during the low voltage ride-through process.The simulation verification is carried out through the MATLAB/Simulink platform,and the simulation results show that the formulated control scheme can meet the expected control requirements.Finally,considering that the voltage increase of the DC grid caused by the fault of the converter at the transmitting end station will threaten the safe operation of the flexible DC transmission system of the doubly-fed offshore wind farm,this paper proposes a high voltage based on super capacitor energy storage device the solution for the traversal.The program gives the DFIG stator-side converter,rotor-side converter,DC grid-side converter,AC gridtesting converter,and super capacitor-side converter during the failure process of the converter at the receiving end station the control method of the device.The simulation results verify the correctness of the proposed control strategy.