The Research On Control Strategy For Doubly-fed Wind Power Generator Under Unbalanced Grid Voltage Conditions

In recent years, with the wind power generation continues to expand, the number and capacity of wind farm are rapidly increasing. Besides, because the proportion of wind power in power system is increasing year by year, the impact on power system becomes increasingly significant. To prevent power disturb to the grid because of all generators disconnection, it is a necessary that on-grid wind power generator possess ride-through capability under unbalanced grid voltage conditions. Taking doubly-fed induction generator (DFIG) as a research object, in order to solve problems of DFIG wind power generation system under conventional vector control strategy during unbalanced grid voltage conditions, an unbalanced control strategy based on rotor current is proposed in this paper to enhance ride- through capability of DFIG wind power generation system under unbalanced grid voltage conditions.Firstly, according to DFIG mathematical model under normal grid voltage, a conventional vector control strategy consisted of controlling rotor side converter and grid side converter is designed. The dynamic responses of stator reactive power, electromagnetic torque and rotor current under conventional vector control strategy during unbalanced grid voltage conditions is simulated by using Matlab/Simulink. By analyzing the dynamic response of rotor current, the reason why the stator reactive power and electromagnetic torque have pulsating components with frequency 2ωsunder unbalanced grid voltage conditions is explained.Secondly, based on DFIG mathematical model under unbalanced grid voltage conditions, an unbalanced control scheme based on rotor current is presented in positive and negative synchronously coordinates that makes the positive and negative sequence components of rotor current to regulate independently is presented to eliminate the pulsations in the stator reactive power and the torque. The principle of finding the positive and negative sequence references of rotor current is discussed. At last, simulation results using Matlab/Simulink is presented to validate the proposed control scheme and to show the enhanced ride-through capability of DFIG wind power generation system during unbalanced voltage supply.