The Control Strategy Research On Low Voltage Ride Through Of Double Fed Induction Generators

Doubly-fed induction generator is the mainstream model in today’s small and medium-sized wind farms, which is variable speed and constant frequency. It has the advantages of high energy conversion efficiency and small capacity of excitation converter. As the wind power installed capacity is increasing, the interaction of wind generator and the grid is increasingly protruding. Researching on the transient characteristics of doubly-fed induction generator, analyzing its influence on the power grid and generator itsel, both are very important to help to ensure the reliable operation of wind generation system. Based on PSCAD/EMTDC platform, with doubly-fed generation system as the research subject, this paper elaborates on the wind generator transient response for during power grid voltage sag, and low voltage through uninterrupted operation issues, the main research content is as follows:For doubly-fed genrating system under synchronous rotating reference coordinates, models of a wind turbine aerodynamics, mechanical driven system, and converters are established. Based on these models, this paper analyzed double-fed induction transient process of wind power system during grid voltage sag, and compared the transient responses of doubly-fed induction generator with two typical cases of grid voltage sags.Using meter and the precise DFIG model during the stator exciting current transient process, based on the stator flux vector control principle, the control strategy of doubly-fed generator system when low voltage sag of power grid is proposed, including the control strategy of rotor side and network side. Ihe correctness and feasibility of the control strategy for the model are verified by simulation.This paper also analyzed the topology structure of the active IGBT type Crowbar device and its role during power grid plunge, conducted Crowbar switching on-off strategy simulation research when the grid value plunged sharply. By analyzing and comparing with unprotected unit operation characteristics, the proposed research verified the validity of the Crowbar protection, realized the low voltage through operation based on the Crowbar double-fed induction wind turbine during power grid failure. Finally this paper analyzed the grid asymmetric fault for doubly-fed induction generator, based on symmetrical component method, the doubly-fed induction generator mathematical model, positive and negative sequence of the stator output active as well as the reactive power and electromagnetic torque were deduced. Positive sequence current component tracking control strategy was put forward based on above methods; Simulation results demonstrated the superiority of the control strategy.