Research On Fault Ride-through Technology Of Doubly Fed Induction Generator

With the wind power generation technology growing mature gradually, the characteristics of China's wind power development shows regional and clustering.Because of the direct connection with power grid of the doubly fed induction generator(DFIG) stator winding network and the weak control ability of its converter of the rotor over current, when power grid faults occur, converters lose control over DFIG, causing wind farm separated from the power grid. Therefore, aiming at the control system,transient mechanism analysis and fault ride through(FRT) technology of DFIG, this paper launched the following researches:In view of the fact that PI parameters acquired by the existing tuning methods can't get good dynamic responses, immune genetic algorithm(IGA) was used to optimize PI parameters of the DFIG control system. The DFIG control targets were regarded as the objective function of PI parameters optimization, and setting values of pole placement method are used as the initial values to improve the optimization efficiency. As characteristics of the DFIG control system include relative independence between control loops, close connection in circuit and large variation range of PI parameters, it is necessary to make adaptive improvements on encoding, crossover and mutation operator,and immune operator, finally forming the improved immune genetic algorithm(IIGA).The simulation model was built in MATLAB/Simulink, and then making use of the PI parameters obtained by the pole placement method and IIGA to carry out the simulation experiments respectively. The results demonstrated that the PI parameters gained by IIGA have better dynamic performance.Aiming at the problem that the transient process of DFIG is difficult to analyze, the mathematical model in the two phase static coordinate system was used to derive the expressions of the stator flux linkage in the voltage sag and recovery process during symmetrical and asymmetrical grid voltage faults, and then comparing their similarities and differences. Because stator and rotor flux linkages had different frequency components, space vector diagram method was introduced to deduce their spatial position relations and variation rules. On this basis, changing laws of rotor flux in the whole fault process, extremal conditions of the rotor current and impacts of Crowbar resistance on the magnetic flux and current amplitude were analyzed and calculated byusing the method of space vector synthesis and 3D graphics. Accordingly, it was proposed that resistance selection and switching control strategy of Crowbar circuit with parallel resistance. Finally, building the simulation model for conducting the DFIG electromagnetic transient process, and verified the rationality of theoretical analysis and improved schemes.As the domestic accidents that wind turbine are separated from the power grid in large-scale are more frequently appearing, which are caused by the voltage swelling, it was summarized that some typical accidents in recent years and its reasons and existing foreign high voltage ride through(HVRT) technical standards. Expressions of flux and current were deduced in the voltage swelling process by using mathematical methods of voltage sag. It was discovered that both of them have the similar transient variations, but in the limits of their respective standards, the amplitude of flux transient DC component is relatively small when grid voltage swells, so it was easier to achieve FRT when using the same control strategy. Consequently, a control strategy of coordination between the rotor side converter(RSC) and grid side converter(GSC) was proposed in consideration of both the control performance and ride through capability during fault.