Research On Harmonic Suppression Control Strategy Of Grid-Connected Doubly-Fed Wind Power Generation System

The grid-connected Doubly-Fed Induction Generator(DFIG) leads to stator currents contain large harmonic currents when grid voltage with harmonic fluctuations. To solve such problem, in the thesis harmonic suppression modules have been added on the basis of asynchronous double-fed back-to-back converter control system, so that the harmonic suppression is achieved.Firstly, the models of the DFIG and the three-phase converter are analyzed, and the traditional stator voltage oriented and grid voltage oriented vector control for the induction generator under normal conditions is introduced. Furthermore, under abnormal grid voltage conditions especially with low-order voltage harmonics, it is demonstrated that the harmonic voltage of the power grid will have a great influence on the output current of DFIG stator and grid side converter, which leads to the generation of the corresponding harmonic.Secondly, in order to obtain the grid phase information, the anti-Park transformation method is used in this thesis as the simulation shows the Phase-Locked-Loop(PLL) based on anti-Park transformation method is superior to the conventional method based on Proportion-Integral(PI) controller. Then the harmonic suppression algorithms have been studied:(1) Stator harmonic current suppression algorithm based on rotor-side converterdetecting the stator current and performing rotation axis transformation, extracting the stator’s 5th and 7th harmonic current components by a low-pass filter, and controlling corresponding current components to zero by PI controller.(2) Stator current harmonic suppression algorithm based on the resonant controller-detecting stator current and then controlling the specific harmonic currents through a resonant controller with the resonant frequency of 300 Hz under synchronous rotating coordinate system.(3) Utilizing the current loop harmonic compensation control based on the LCL filter and nested coordinate system.Finally, the simulation model of the whole system was set up by MATLAB / Simulink. The model includes operating characteristics analysis of converter’s rotor and stator control algorithms which were compared before and after the harmonic suppression algorithms are added. Experimental verification is carried out on the prototype system of the small power level 5kW, the results show that the proposed control strategy can restrain the grid-side and the stator-side current harmonic components and improve the output power quality of the wind power generation system in the case of large grid voltage harmonic distortions.