Study On Low Voltage Ride Through For Doubly Fed Induction Generator Based On Rotor Side Control

In recent years with the rapid development of China’s wind power industry, the State Grid Corporation have revised its grid code to ensure the stable operation of the power systems, especially under the low voltage condition. This thesis mainly carrys out theoretical analysis of the control strategies for steady-state operation of the motor as well as Low voltage ride through (LVRT) technique. Frist, this thesis introduces the classical crowbar control strategy, but its wake point is obvious as it will short-circuit the rotor by crowbar resistors. For this, the thesis carrys out low voltage ride through based on rotor side control, and then this thesis validated the theories by simulation.Firstly, the operation of DFIG wind generation has been analyzed. The dynamic mathematical models of DFIG and grid side converter (GSC) under regular grid voltage are built. On the basis of the mathematical models, the thesis investigates the stator flux oriented traditional vector control of the rotor side converter (RSC). In the meantime, the dual-loop grid voltage oriented traditional vector control of GSC is also analyzed.Then, the dynamic mathematical models of DFIG under low voltage condition are detailed analysis. The transient process of DFIG and the influence caused by the zero and negative sequence are also analysized by using these mathematical models. The classic crowbar control for the low voltage and the dual-dq rotor current control for the unbalance condition are introduced.Focus on the dynamic mathematical models in the transient process, novel rotor side control LVGT strategy is mainly researched. The flux damping control can shorten the transient process by changing the angle of the rotor current vector. The flux oriented control can limit the fault current of the rotor side as the rotor flux linkage is controlled to track the changing stator flux linkage. Also, a control strategy which will consider the reactive current injection of DFIG was proposed, the maximum limited reactive current expressions of the stator side of DFIG and the GCS are derived.The grid-connected inverter system with LCL filter is also analyzed. The active damping control with the virtual resistance and the state variable feedforward control are shown in this thesis. These strategies improve the stability of the GSC, and enhance the reliability of the wind power generation system under grid fault.This thesis adopts MATLAB to simulate the operation of the1.5MW DFIG wind generation system using different control strategies under the regular condition and the voltage dip. The response of DFIG using traditional vector control is investigated. Simulations results of DFIG using crowbar circuit and the dual-dq current control are verified. In order to overcome the weak point of these strategies, LVRT control strategies based on rotor side control are also investigated; to make the generater has better performance.