Research On Low Voltage Ride-through Control Strategies Of Doubly-fed Induction Wind Power Generator

Recent years, interest has risen in the search and development of using renewablealternative energy sources. Among the renewable alternatives, wind power has beendeveloping rapidly. As the penetration of wind power in electric power systemcontinues to increase, it becomes very important to improve the fault ride-throughcapability of grid-connected wind turbines. Variable speed generation systems showhigher energy capturing and better converting efficiency than the fixed speed systems.The doubly-fed induction generator (DFIG) provides flexibility of operation insub-synchronous and super-synchronous speeds both in the generating and motoringmodes, thus making DFIG suitable for variable speed generating system such as windpower.At first, the fundamental characteristics of wind turbine are discussed in thisthesis，then introduced the current technologies of wind power around the world. Theoperation principle of the variable-speed-constant-frequency (VSCF) wind energygeneration system is analyzed according to the model of the wind turbine. Themathematical model of the DFIG is established, a stator-flux-oriented control schemefor power control is derived. The structure of the pluse-width-modulation (PWM)converter is given, and the part of the DC of Dual-PWM is described throughanalyzing the power transformation theory. The control strategy for the grid-sideconverter is also introduced. A DFIG experimental system based on DSP wasestablished.The dynamic electromagnetic behavior of the DFIG in the case of grid voltage dipsis analyzed. And an improved vector control strategy of the DFIG is proposed toreduce the rotor over-current during non-serious voltage dips, which considers thedynamics of the stator flux deduced by the voltage dip when designing the rotorcurrent controller. Results from simulation prove that the improved vector controlstrategy is effective.Traditional control strategy of pulse width modulation (PWM) rectifier possessesgood dynamic and steady state performances while voltages of power network aresymmetrical, however due to evident harmonic power in asymmetrical power networkthe quality of output voltage and input current of PWM rectifier are seriously affected.After analyzing the features of asymmetrical power network by symmetrical component method, two vector-oriented control strategies, which respectivelyorientate positive-and negative-sequence voltages under positive-andnegative-sequence synchronous coordinate transforms, for PWM rectifier areproposed to eliminate the fluctuating component in power transmission, thus the DCbus voltage can be stabilized while unsymmetrical fault occurs in power network.Based on this control strategy and let negative-sequence input current equals to zero,another control strategy can be obtained to make the input current sinusoidal whileasymmetrical fault happens in power network. Results from simulation andexperiments prove that the proposed two control strategies are effective.