Research On Harmonics Of DFIG Wind Power System Based On Dual-PWM Converter

Doubly fed wind turbines,based on doubly fed induction generators(DFIG),is the mainstream power generation equipment for wind power market of China.The back-to-back dual-PWM converter as the most widely used AC excitation converter of the DFIG system,is an energy exchange interface between the rotor side of DFIG and the power grid,also the main harmonic source in DFIG wind power system.With the rapid development of wind power industry at home and abroad as well as the increasing capacity of wind turbine installed capacity,wind power harmonics standards will become more and more strictly.Therefore,it is of great theoretical significance to study harmonics of dual-PWM converter based DFIG wind power system.According to the status and characteristics of wind power development,in this paper,the research statuses of harmonics for wind power system as well as the power electronic converter have been reviewed,the features and problems of existing research have also been analyzed.This paper points out that the research on the harmonics generation mechanism of DFIG wind power system is still quite scarce.It is necessary to establish a targeted harmonic research system.Firstly,the harmonics generation mechanism of DFIG wind power system has been analyzed and studied under balanced grid voltage.Considering structure characteristics and modulation theory,low frequency harmonics of DC side and grid side for the back-to-back dual-PWM converter have been analyzed based on power-balancing theory.The high frequency harmonics analysis model of rotor current has been established using double Fourier transform algorithm.Secondly,on the basis of harmonic analysis under balanced grid voltage state,harmonic generation mechanism of DFIG wind power system under unbalanced grid voltage state has been analyzed.The voltage and current of power grid have been modeled based on the symmetric component method and the spatial coordinate transformation.Combining the power balance theory of PWM converter and the iterative recursion method,the analytical expressions of the low frequency harmonics in the grid side and DC side of the converter as well as the stator and rotor side of the DFIG have been derived.The mathematical model of the closed-loop switching function of the converter is established via double Fourier transform algorithm,and the high frequency harmonic modeling of grid side,DC side and rotor side has been realized with combination of switching function and iterative recursion method.Finally,a reliable real-time digital simulation model of back-to-back dual-PWM converter based DFIG wind power system has been built based on RTDS platform.On the basis of this model,the harmonic signals of balanced and unbalanced grid voltage state are obtained.The simulation results are in good agreement with the theoretical results,which verifies the accuracy and credibility of the theoretical analysis.