AC Fault Ride-through Strategy For Large-scale Wind Power Based On Flexible DC Transmission

With the increasing proportion of wind power in the power system,the power grid puts strict requirements on the operation of wind power grid connection,one of which is the fault ride-through ability.Modular Multilevel Converter(MMC) as a new type of topology voltage source converter has attracted much attention in recent years,and MMC-HVDC has become an ideal grid-connecting method for large-scale long-distance wind power transmission.In view of the problems encountered in the research of wind farms connected to MMC-HVDC,the paper works on wind turbines,DC transmission systems,and fault ride-through technology when AC fault occurred on the wind farm side.Firstly,the mathematical model and control principle of doubly-fed induction generator(DFIG) are analyzed.Based on the research on the mathematical model and control strategy of the wind turbine and generator,the single machine equivalent model of large-scale DFIG is established by MATLAB/Siumulink.Then,the working principle and controller design of MMC and MMC-HVDC transmission system are studied.The control strategy of wind farm side converter and grid side converter is designed for wind farm through MMC-HVDC two-terminal system,and the model is established by MATLAB/Siumulink.Finally,the transient characteristics of the system under the AC fault on the wind farm are analyzed.Based on this,a fault ride-through strategy is proposed.The simulation results show that the voltage drop amplitude is reduced and the overvoltage phenomenon after fault elimination is suppressed.Therefore,the fault ride-through ability of system is improved under that strategy.