| With rapid development of wind power generation industry at home and abroad in recent years, large-scale wind power integration brought electric dispatching difficulties and stability problem of the power system. Transmission system operators (TSOs) of countries or regions have issued their grid codes which require wind turbines to remain connected during grid faults. The doubly fed induction generator (DFIG) system is sensitive to the grid disturbance and even fault tripping, due to the stator is directly connected to the grid. Therefore it is necessary and important for DFIG system to research the control technologies of the grid fault ride-through (FRT).This thesis focuses on the research of DFIG characteristics and converter control technologies during grid voltage faults. Through theoretical analysis, control method improvement, simulation and experimental verification, a comprehensive, accurate and thorough research work is accomplished. It aims at obtaining some valuable conclusions and achievements for both scientific research and engineering application.1. The DFIG dynamic modeling is established and expressed in the three-phase stator stationary reference frame, two-phase stator stationary reference frame and two-phase rotating reference frame, respectively. Using the ABC classification scheme and symmetrical component method, the grid voltage dip fault types and characteristics at the wind turbine connecting point in the actual power system are analyzed. Under the conditions of asymmetrical grid voltage dips, the flux dynamic response and rotor overvoltage induced by grid voltage dip and recovery are qualitatively described and quantitatively analyzed. It focuses on the rotor voltage response caused by the transient stator flux by the theoretical calculation and simulation research. The method to analyze DFIG dynamic responses respectively by the means of positive and negative components under asymmetrical grid voltage dips is proposed. The dynamic response processing is visually described by the stator flux vector locus and rotor voltage vector locus. The research conclusion indicates that the transient components of stator flux and rotor overvoltage are main influencing factors to the control performance of DFIG system. This work provides a beneficial basis for the hardware parameters design and control strategy improvement. 2. Based on the stator flux or stator voltage oriented vector controls, a rotor side converter (RSC) control which is added in the rotor electromotive force (EMF) feedforward compensation enhances the continuous operation ability of DFIG under grid voltage small amplitude disturbance. Under symmetrical grid voltage dip, an improved PI+Resonant (PI-R) control for RSC with the active flux damping is presented. The damping rate of the transient stator-flux is accelerated and flux oscillation is eliminated by means of the improved control to the rotor current. Then the rotor overvoltage and overcurrent are reduced, and stator reactive compensation current and electromagnetic torque is well controlled during grid fault. PI-R control can achieve zero steady-state error to both the fundamental dc reference and the resonant ac reference. Hence the control system interference rejection to EMF is improved, and regulation accuracy of the proposed active flux damping scheme is guaranteed. The parameters design principles on PI-R regulator are given based on frequency domain characteristics analysis. Simulation and experimental results validate the theoretical analysis and the feasibility of the improved control scheme. In addition, the capacity of voltage and current output for RSC during low voltage ride-through (LVRT) is analyzed. It reveals the software control limitation for coverter to achieve serious grid voltage dip ride-through. Thus it is necessity for DFIG system to apply some hardware protection measures to realize LVRT.3. The control strategy of DFIG system under asymmetrical grid voltage dip is emphasized. Dynamic modeling of DFIG system applied on unbalanced voltage conditions is established. Under unbalanced grid voltage, a coordinated control target between RSC and grid side converter (GSC) is realized, which based on two vector control schemes, viz., dual PI current controls implemented in the respective positive and negative synchronous reference frames (SRFs), and PI-R current control in the positive SRF. Under asymmetrical grid voltage fault, an improved PI-R control for RSC with the active flux damping is presented. It not only realizes the steady-state control under unbalanced grid voltage, but also improves the transient performance during asymmetrical voltage dip. The oscillations of stator flux and electromagnetic torque are well eliminated, and the rotor overvoltage and overcurrent are reduced. Simulation results validate the feasibility of the proposed control. In addition, the capacity of voltage and current output for RSC is also analyzed during LVRT.4. The LVRT control technology of MW-level DFIG system is investigated. Considering the hardware protection measures, improved RSC control algorithm and output capacity limitations, a comprehensive and practical LVRT control strategy is proposed, which can be applied in the symmetrical or asymmetrical grid voltage dips with different depth. The resistance value choice principles of rotor side active crowbar and dc-chopper are given. Reactive power control of DFIG system is discussed. Through GSC fast reactive power compensation and reasonable design of dc-chopper resistance, the reactive power control performance of DFIG wind turbine is improved. Simulation results prove the feasibility of the scheme. Based on the analysis of DFIG transient behaviors under the grid voltage dip and recovery, a test scheme to simulate the grid voltage dip and recovery by the stator asynchronous grid-connection is presented. The proposed test scheme can realize preliminary experiment on the LVRT control performance of MW-level DFIG converter. Simulation and experimental results of1.5MW DFIG system validate the feasibility of this test scheme. Finally, the wind farm LVRT experiment of1.5MW DFIG converter is accomplished, and the feasibility of the enhanced software and hardware control technology is validated. |
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