| With the increasing proportion of wind power as a new energy source in the power grid,the characteristics of double-Fed wind Generators(DFIG),as the main wind power generators,have attracted wide attention from researchers.It has the characteristics of variable excitation current frequency to change the generator speed to achieve variable speed constant frequency,high utilization rate of wind energy,backto-back bidirectional converter,etc.,which is used as distributed power supply in distribution network.Due to the access of DFIG,the power grid has changed from the original single power supply network to multiple power sources at the same time.When short-circuit fault occurs in the system,a series of transient processes occur in DFIG and are directly fed into the power grid.During this period,the magnitude and direction of short-circuit current have a great influence on the location of the fault and the control strategy of DFIG itself.This poses a great challenge to the safe and stable operation of the power grid,and also results in that the original three-stage current velocity break protection is no longer applicable,and causes large errors in fault calculation and relay protection setting of the power grid including DFIG.Therefore,this paper first establishes a DFIG transient mathematical model,analyzes the transient behavior characteristics,and on the basis of in-depth analysis of DFIG short-circuit fault characteristics of voltage outer loop and current inner loop control,uses the space vector method to simplify the double-fed wind turbine transient equivalent model,and then proposes an improved scheme for DFIG control strategy.It includes active and reactive decoupling control on the machine side to control gridconnected power factor,phase-locking on the network side based on SOGI and adding resonance controller to suppress harmonics,and studying the control scheme of DFIG on-grid operation on the basis of low voltage crossing control algorithm of voltage drop to realize reactive power support.By building a simulation model in MATLAB/SIMULINK,The transient output characteristics including LVRT control strategy are studied to explore the fault crossing and non-off-grid operation control under various working conditions.The improved algorithm is compared with the traditional vector control algorithm,and the advantages of the improved algorithm in fault crossing are summarized.Secondly,the fault current is derived based on the consideration of DFIG control strategy and a calculation method of short-circuit current of distribution network including DFIG is presented.According to DFIG,the short circuit location is the fault point and different equivalent power sources are used to replace it upstream and downstream.The faults are divided into symmetric and asymmetric fault types.The calculation sequence component model is established based on the fault boundary conditions,which is decomposed into normal network and fault component network.Then,it is substituted into the sequence component model of DFIG short-circuit calculation to obtain the calculated value of short-circuit current,and then injected into the network to obtain the sequence component of voltage.Then,according to this voltage value,the current at the fault point is corrected,and the short-circuit current at the outlet of each DFIG after the fault can be iteratively obtained.Finally,on the basis of theoretical analysis,combined with MATLAB/SIMULINK simulation software to build a model and write the algorithm program,through a number of DFIG access to the standard power system examples of symmetric and asymmetric faults to test,the results are within the allowable error range,thus verifying the correctness of the model and the proposed algorithm. |
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