Investigation Of The AC-Excited Power Supply For A Variable-Speed Constant-Frequency Wind Energy Generation System With Doubly-Fed Induction Generator

With the growing rating of the wind energy generation system, capturing the maximum wind energy and improving the operation efficiency are becoming more and more important. The Variable-Speed Constant-Frequency(VSCF) wind energy generation system with Doubly-Fed Induction Generator(DFIG) has the advantages such as low cost, smaller rating of the excitation converter and very good performance. In this kind of wind energy generation system, the control and design of the AC-excited power supply is of great importance. This thesis deals with the control of the AC-excited power supply for the VSCF wind energy generation system with DFIG.An overview of the power electronic converters of interest to the field of the AC-excited power supply is given. Six kinds of converter topologies that may be used as an AC-excited power supply for the VSCF wind energy generation system are compared in detail. The conclusion is that the two-level voltage source back-to-back PWM converter is the most competitive one and that the multilevel and soft-switching technology may be the developing direction of the AC-excited power supply.A simplified model of the grid-side PWM converter used to analyze the impact of the load disturbance is proposed in this thesis. Based on the proposed model, the dynamic response problems of the grid-side PWM converter under the traditional control are deeply examined. Under the traditional control, only d-axis current component can be controlled freely, which will limit the anti-load-disturbance performance of the converter. An improved new current control scheme is put forward, in which the traditional PI regulator in the synchronous frame with the load current feed-forward compensation is used in the steady-state and the dynamic i_gq^*≠ 0 structure-variable control is employed in the transient state to effectively enhance the anti-load-disturbance performance of the grid-side converter. The validity of the proposed model and the effectiveness of the presented scheme are verified by the computer simulation study.The AC voltage sensorless control of the grid-side PWM converter has been investigated in the paper. A novel virtual line-flux-linkage oriented vector control scheme without AC voltage sensors is proposed. The proposed scheme can observe the initial value of the virtual line-flux-linkage precisely. As a result, the virtual line-flux-linkage vector can serve as the oriented vector in the vector control of the converter and the linevoltage sensor will be eliminated. The simulation and experimental results prove the validity of the proposed control scheme.If the grid voltage is considered constant both in magnitude and frequency, the model of DFIG may be simplified. Based on the simplified model, the rotor current controller for the stator-flux oriented vector control and the grid-voltage oriented vector control is studied. The principle and the realization scheme of the maximum wind energy tracking operation are investigated deeply. A grid-voltage oriented vector controller of DFIG is designed, which can make the system capture the maximum wind energy and decouple the active and the reactive power.A 15kW VSCF wind energy generation laboratory set-up with a DFIG and a back-to-back PWM converter is constructed. Systematic experimental study of VSCF generation operation has been carried out, and some valuable conclusions have been achieved.The full simulation model of the VSCF wind energy generation system with DFIG and back-to-back PWM converter is established, in which the grid voltage fault condition has been considered. The simulation model can be used to investigate the ride-through ability under grid faults, the new control scheme and other exploring research. The two kinds of traditional vector control are not fit for controlling the DFIG under the grid voltage dip fault since the precondition of the simplification is not true, an improved control schemes for these two kinds of traditional vector control are proposed and deeply investigated. The simulation study shows that each of the proposed schemes...