Research On Relevant Issues Of Low Voltage Ride-through Techniques For Doubly Fed Induction Generator

In recent years, the proportion of wind power capacity has increased year by year. Large-scale of the wind turbines’ operating unconnected to the grid will aggravate the grid fault when it occurs. As a result, many countries have issued the interconnection requirements for wind farms called low voltage ride-through(LVRT). The doubly fed induction generator(DFIG) has various advantages such as high energy utilization rate, variable speed, low mechanical stress and low rating and cost of power convertor, so it is one of the most widely used wind turbines. But at the same time, due to the low rating of its power converter, DFIG is very sensitive to grid fault especially voltage sag. Thus the LVRT requirements are particularly high for DFIG.Based on the introduction of basic principle of DFIG, this dissertation establishes the mathematical model of DFIG in the three-phase static coordinate and two-phase rotating coordinate systems. Grid voltage oriented vector control strategy of the grid-side convertor(GSC) and stator flux oriented vector control strategy of the rotor-side convertor(RSC) are analyzed, respectively. The simulation model of DIFG is set up.This dissertation focuses on the analysis of the dynamic characteristics of DFIG under grid faults. Base on the stator flux and the rotor open circuit voltage, the influence of all kinds of grid fault on the dynamic characteristics of DFIG is discussed. Different dynamic responses of DFIG under the same fault with different sag degree, under symmetrical and asymmetric fault and under fault with and without the phase angle jump are analyzed and compared. The dynamic responses under grid faults are derived theoretically, and simulation is proceeded for the theory above.The widely used LVRT strategies based on the Crowbar circuit are also studied in this dissertation. The setting range of the resistance in the Crowbar circuit is mainly considered. The influences of the resistance on the electromagnetic torque and the absorption effect of reactive power of DFIG in steady state after fault are derived. Simulation results verify the reliability of the result, which provides a reference for the optimization of Crowbar circuit resistance selection.