Study On The Control Strategy Of Low Voltage Rid Through Of Doubly Fed Induction Generator Based With Wind Turbine

Cascading trip-off of a large number of wind turbine generators has brought many new problems.The power system losses a lot of active power,which harms the security and stability of power system seriously.In order to avoid doubly fed induction generator based wind turbines(DFIG)from tripping-off caused by over-current/over-voltage of rotor-side converter(R-VSC)during voltage drops,the active crowbar protection is set by-pass the rotor-side converter,protects the whole system and implement low voltage ride through(LVRT).As the crowbar circuit is put into operation for a long time(above 200 ms generally)in the voltage sag of grid,DFIG tripping-off from grid for the sake of over-speed may happen.For this,low voltage ride-through control strategy for DFIG is proposed to prevent and suppress the over-speed wind turbine generator cascading trip-off accident.In this paper,the main research object is the popular doubly-fed induction wind turbine,and the mathematical model is used to further analyze the mathematical and control system model of doubly-fed induction wind power generation system based on power control and rotor excitation control of grid side converter.The electromechanical transient process of DFIG equipped with Crowbar has been analyzed in this paper.The mechanism of DFIG over-speed tripping-off from grid has been revealed.Based on the simulation system of DFIG interconnected with grid in the platform of PSCAD/EMTDC,the transient over-speed process of DFIG caused by unbalance of rotor torques of wind generator is simulated under lightly and heavily loaded condition.As a result,it shows that over-speed degree for rotor is influenced by wind speed condition obviously and the event of DFIG tripping off grid is most likely to happen under heavily loaded condition.In order to avoid DFIG tripping off grid,initiative cut-off criterion of Crowbar circuit and R-VSC restarting control strategy are designed.The new LVRT scheme is improved on the basis of Crowbar protection strategy and R-VSC restarting control strategy.The system of DFIG-LVRT is established under PSCAD/EMTDC,and the improved method is verified by the simulations.