Analysis And Control Of The Output Performance Of DFIG Under Harmonic Grid Voltage Conditions

With the continuous development of the wind power technologies, Doubly Fed Induction Generator(DFIG) with its many advantages has become the most widely used models. Considering both the structure of DFIG system and the non-ideal characteristics of the grid, the mathematical models under ideal grid conditions can not meet the actual control requirements. Therefore, the output characteristics and control strategies of DFIG wind turbines under harmonic conditions were studied in this paper, and an adaptive model of DFIG wind turbines was built under the harmonic conditions.Firstly, the components and working principle of the DFIG wind power systems was understood in depth. on this basis, a detailed analysis has been put on the function and the mathematical models of the main components of the DFIG system. And the Proportional Resonant(PR) control strategies of the grid side converter and rotor side converter under ideal grid voltage conditions were designed, and the MATLAB/Simulink simulation has been used to verify the correctness of the DFIG wind power system model and the effectiveness of the control strategies.Secondly, the impact of the grid voltage harmonics on the output characteristics of DFIG was studied on the basis of operating characteristics of DFIG wind turbine under grid voltage harmonic conditions. Then a steady state equivalent circuit of DFIG under the harmonic conditions was built in this paper, which is used to analyze the ripple of the output power and electromagnetic torque of DFIG and its realationship with the harmornic ratio under typical harmonic conditions. And the curve of the pulsation in the dynamic adjustament process was drawn. The results can be verified by using a dynamic simulation model.In addition, the mathematical models of grid side converter and rotor side converter under grid voltage harmonic conditions were built in this paper which targets to eliminate the effects of harmonic on the output characteristics of DFIG wind generation system. The reason of output power fluctuation and electromagnetic torque pulsation under such opereting conditions is analyzed and interpreted. Based on the control strategy of DFIG system as well as the output characteristics, the phase tracking link under ideal grid voltage conditions is improved in order to enhance the control accurancy of the system and an integral PR control strategy under grid voltage harmonic conditions with appropriate control objectives is proposed, which can efficiently smooth the instantaneous output power fluctuation and electromagnetic torque pulsation. Finally, the simulation studies on a 1.5MW doubly-fed wind generation system prove that the improved control strategy is feasible and effective.