| With the increasing installed capacity of doubly fed induction generator (DFIG),the adverse impact of DFIG on the grid when the grid fault occurs is more and moresevere, which requires DFIG having the ability of low voltage ride through (LVRT).In this paper, the transient characteristics of DFIG during the grid voltage sag areanalysed and a LVRT control strategy is proposed.Firstly, the basic structure and operating principle of DFIG is introduced in thispaper. After gaining sophisticated understanding of DFIG, the mathematical model ofthree-phase static coordinates and two-phase synchronous rotating coordinates isbuild. The rotor voltage and current, active and reactive power can get decouplingcontrol by implementing stator flux oriented vector control on DFIG.Secondly, the transient characteristics of DFIG under voltage sag are studied inthis paper and the electromagnetic relationship is gained by applying the method ofspace vector on DFIG. A detailed analysis of dynamic properties of stator flux androtor flux during the grid voltage sag is carried out and the reason of over-voltage andover-current’s signing on the rotor side is pointed out, laying the basis for researchLVRT control strategy of DFIG.A precise mathematical model of the doubly-fed machine is studied in this paper.This precise mathematical model can describe the transient characteristics of DFIGdue to it takes into account the changes of the stator excitation current under the gridfault. Stator flux oriented vector control method is applied on DFIG based on thisprecise model and a LVRT control strategy is gained.Finally, a simulation model of DFIG is establishment by using MATLAB/Simulink. The simulation of characteristics of DFIG using traditional control strategyand LVRT control strategy is carried out. The simulation results show that overcurrent on rotor side is depressed by applying the proposed LVRT control strategy incontrast with using the traditional control strategy, thus proves the validity of theproposed LVRT control strategy. |
