Research On Dynamic Characteristics And Ride-Through Method Under Sudden Voltage Change Of Doubly-Fed Wind Power System Connected To DC Grid

To promote the energy revolution and build a clean,low-carbon,safe and efficient energy system is an important direction for future development of China in the field of energy.Continued increase in the grid-connected capacity of renewable energy,including wind power,is an effective way to achieve clean energy substitution.On the one hand,the city has the demand of DC transformation of distribution network in the development;On the other hand,the construction of offshore DC power grid can greatly meet the economic gridconnected requirements of large-scale offshore wind farms.Therefore,improving the safe and reliable operation capability of DC grid-connected wind turbines connected to the DC power grid is of great practical significance for the development and utilization of wind power resources.This thesis studies the topology of the doubly-fed wind power generation system connected to DC grid based on diode uncontrolled rectifier(DFIG-DRUDC system)and the doubly-fed wind power generation system connected to DC grid based on voltage source converter(DFIG-VSC-DC system)separately through the rectification link.Based on the flexible regulation of frequency in DFIG-DRU-DC system and the flexible regulation of voltage and frequency in DFIG-VSC-DC system to meet various steady-state or transient operation control objectives,the fault characteristics of DC single-pole short-circuit fault and dynamic characteristics of doubly-fed wind turbine under the two topologies are analyzed respectively.Subsequently,the idea of DC low voltage ride-through control based on the principles of wide frequency control and wide voltagefrequency control is proposed respectively,and the basic principle of avoiding impact current in the stator and rotor windings is expounded.Then,the dynamic characteristics of DFIG-DRU-DC system and the DC low voltage ride-through method are studied.It is pointed out that the voltage amplitude at the AC side of the diode rectifier operating in the current continuous mode has a fixed constraint relationship with the DC voltage,resulting in the synchronous drop of the generator terminal voltage caused by the DC grid voltage drop.Through comparison and analysis,it is concluded that the adjustable frequency can avoid the problem of stator flux transient DC component caused by the non-adjustable frequency rigidity,and then the DC low voltage ride-through control method that the rotor side converter directly controls the stator frequency to change with the DC voltage in the same proportion is proposed,which makes the flux stable and controllable after the machine terminal voltage drop to achieve a smooth electromagnetic transient transition process,avoid the impact current of stator and rotor windings,ensure the maximum wind energy tracking control while achieving the continuous grid-connected operation of wind turbines,and improve the ability of wind turbines to withstand external faults.Finally,a simulation model is built in MATLAB/Simulink to verify the feasibility and correctness of the DC low voltage ride-through control strategy based on the wide frequency control principle.Furthermore,an experimental platform is built to verify the correctness of RSC’s improved vector control based on wide frequency control principle to realize frequency and active power control,and the feasibility of the proposed DC low voltage ride-through method based on wide frequency control to avoid impact current in stator and rotor windings.Due to the inherent problems of uncontrollable stator voltage amplitude,high distortion of stator voltage waveform and two-dimensional control of frequency and active power only through RSC in the DFIG-DRU-DC system.This thesis further studies the dynamic characteristics of DFIG-VSC-DC system and the DC low voltage ride-through problem.Through VSC and RSC,the fourdimensional flexible operation control of frequency,active power,stator voltage and reactive power can be realized,which not only enhances the steady-state operation control capability of the system;Moreover,the wide voltage-frequency coordinated control of the converter is realized through the constant modulation ratio control that the VSC controls AC voltage changing with the same proportion of the medium-voltage DC voltage and the voltage-frequency control that the RSC controls frequency changing with the same proportion of the machine terminal voltage,which enhances the control ability of the stator flux linkage,and makes the DFIG better realize the DC low voltage ride-through process.Next,in order to improve the long-term safe and stable operation capability of the system,a DC unloading circuit is added at the DC side of the RSC to solve the problem of a sharp increase in the output power of the rotor caused by the frequency reduction after the voltage drop of the DC power grid.Finally,a simulation model is built in MATLAB/Simulink to verify the feasibility and correctness of the DC low voltage ride-through control strategy based on the wide voltage-frequency control principle.