The Research Of Improve The Ability Of Lvrt For DFIG

Due to the DFIG relatively sensitive to the change of grid voltage,without corresponding measures,it will lead to large of DFIG out of the fault grid,or even further to the collapse of the power system.In order to improve the low voltage ride through ability for the DFIG,this thesis proposes several different measures of LVRT.Firstly,this thesis according to the structure and basic principle of the DFIG,established mathematical model in the ideal situation.On the basis of the basic knowledge of three-phase PWM converter,established the mathematical about it.Then,analyzed the technique of traditional low voltage ride through,which because only use SVC caused the grid recover slowly during the early period of the grid fault,voltage is over compensated during the later period and the over current of DFIG in rotor side,and also analyzed the problem about capacity limits of DVR.On this basis,this thesis presents a new scheme that use the SVC and DVR,and the combination can be used to solve the above mentioned problems.Aiming at mentioned problems for the use of traditional crowbar,understanding characteristic about using crowbar during the grid fault,then get the equivalent circuit model,so as to determine the parameter expression about wattless power of DFIG and analysised it,then design a new topological structure of crowbar,and finally put forward a new design algorithm that is not only applicable different voltage drop rate and is suitable for different types of grid fault,eventually reaching the goal of improving the ability of low voltage ride through capability for the DFIG.For asymmetric grid fault in wind power system of DFIG,based on the principle of positive and negative sequence decomposition,a compensation method based on positive sequence voltage component is put forward in this thesis.The topology and control strategy to implement the compensation method is researched.The reference value of positive sequence voltage is calculated.The problem of asymmetric grid fault is transformed to the symmetrical fault by using the method.