Analysis And Control Of The Output Performance Of DFIG Under Unbalanced Grid Voltage

Doubly Fed Induction Generator (DFIG) is currently the most popular generator typeadopted in wind power generation. Considering the grid-connected requirements of windpower generation and its distribution characteristics, more and more researches have beenfocused on the control and operation of DFIG under unbalanced network condition. Thispaper focuses on the control and operation of DFIG under unbalanced grid voltage,meanwhile, creates an adaptive model of DFIG under unbalanced grid voltage.Firstly, after a comprehensive analysis of the background and research status of thesubject, this dissertation analyzes the components and working principle of the DFIG,builds the mathematical model and control strategy of DFIG under normal voltagecondition. Meanwhile, a detailed study has been put on the proportional resonant (PR) andthe Space Vector Pulse Width Modulation (SVPWM) strategy. The correctness of theDFIG model and the effectiveness of the control strategies can be verified by theMATLAB/Simulink simulation.Secondly, a research has been put on the basic issues of DFIG under unbalanced gridvoltage, which mainly introduces the calculation methods of voltage unbalance factor andthe method of symmetrical components used to analyze the unbalanced voltage. Then thepaper builds the dynamic equivalent circuits of DFIG, which are mainly used to analyzethe impact of the unbalanced grid voltage on DFIG and draw the curves between thevoltage unbalance factor and the output characteristics of DFIG. The curves can beverified by a dynamic model simulation.In addition, this dissertation gives a method which can be used to extract the positiveand negative sequence component from the unbalanced grid. The instantaneous powertheory is applied to build the mathematical models of grid side converter (GSC) and rotorside converter (RSC) in the stationary αβ reference frame under unbalanced grid voltage.Compared with the traditional control methods based on dq reference frame， themathematical model of DFIG system in αβ reference frame can be used to track thealternating current component directly by using the proportional-resonant controller, which can effectively reducing the difficulty of control system. Based on the previousanalysis, the paper utilizes proportional resonant method to develop an unbalanced controlstrategy which selects some appropriate control objectives and takes the GSC and RSC asthe control object. In order to reduce the negative effects of unbalanced voltage further, thepaper builds an adaptive model of DFIG under unbalanced conditions and optimizes thecontrol strategy which can enhance the ability to resist disturbance by achievingcoordination of different control objectives of the GSC and RSC. Finally, the simulation inMATLAB/Simulink which takes a1.5MW DFIG system as an object can verify thefeasibility and effectiveness of the control strategy.