Control Of A DFIG-Based Wind Power Generation System Under Unbalanced Grid Voltage Condition

The main reason causing stator and rotor current unbalances,electromagnetic torque oscillations,and power pulsations in the DFIG is stator voltage unbalance: if the negative-sequence voltage at the generator's stator terminal can be eliminated and only balanced positive-sequence voltage is left during network unbalance,the adverse effects of unbalance upon the DFIG will be removed naturally.In this thesis,in order to eliminate mechanical problems associated with the torque oscillations,the main objective will be to achieve none oscillating torque accompanied by a sinusoidal current exchange with the grid.In order to achieve this,both rotor-side converter(RSC)and grid-side converter(GSC)control strategies are used,based on the appropriate active and reactive power reference generation strategy for the direct power control(DPC).A theoretical analysis of flux,torque and power disturbances created by the grid voltage unbalance in the DFIG is shown in thesis.This research focused on the direct power control(DPC)technique for a doubly fed induction generator(DFIG)-based wind-power generation system under unbalanced grid voltage conditions.The behavior of the DFIG system is tested under normal and unbalanced grid voltage conditions.This control method is implemented to validate the performance of the DFIG system using DPC,the digital simulation by MATLAB/SIMULINK of a 9-MW-DFIG-based wind farm is made.Later,different control solutions are provided.The simulation carried out for two cases.In the first case the system tests under normal operation with injection power with(pf =1)and 3MVar power at point common coupling(PCC).In the second case,the system is tested under unbalance three-phase grid voltage which is 10% of the nominal voltage.Finally,simulation and results validate the theoretical study and the simulation results clearly shown that this modified control technique mitigates the torque oscillations by generating active and reactive power references for the rotor side converter without the necessity of sequence component extraction and achieves sinusoidal stator currents exchange with the grid side converter.