Research On LVRT Reactive Power Support Capability Of Doubly-fed Tnduction Generator

With the increase of wind power capacity, it is important to handle the relationshipbetween wind turbines and grid properly. Therefore, the research of LVRT technologyunder power system fault is of urgent and practical significance. The paper studied LVRTcapacity of doubly-fed wind turbine from the starting point of friendliness of the grid andthe angle of reactive support.Firstly, this paper introduced several common VSCF wind turbines, which focused onanalyzing the structure and control strategy of doubly-fed wind turbine and it hasestablished the mathematical model of DFIG and GSC in the three phase static coordinatesystem. Then, it derived the mathematical model of them in rotating coordinate system byusing the basic coordinate transformation. Meanwhile, in order to achieve the secureoperation of doubly-fed wind turbine, the GSC and RSC adopted vector control whichbased on the SFO and GVO.Secondly, this paper analyzed the working status and power flow relations of DFIG. Itobtained the reactive power limit of a single doubly-fed wind turbine based on itsequivalent model and characteristics of the reactive power, determined the scope of reactivepower capacity of wind farm, and carried on optimal allocation of reactive power capacityof it. Simultaneously, the paper analyzed control objectives of LVRT, common types of gridfault, and Crowbar circuit. When grid voltage dropped symmetrically, it deduced thetransient process of DFIG and reactive power support capability. In addition, when itdropped asymmetrically, positive and negative sequence symmetrical component method isused respectively to set up the mathematical model o f wind power turbine. It applied jointcontrol policy of GSC and RSC to achieve LVRT and meet system various requirements.Finally,50KW doubly-fed wind turbine was built. According to the simulation results,it verified the control strategy above. Simultaneously, when grid voltage droppedasymmetrically taken place, DFIG can provide much more reactive power support to grid,which operates in super synchronous generator state than sub synchronous state.