The Research On Control Strategy For Improving LVRT Capability Of Doubly-fed Wind Power Generator

In recent years, with the wind power market continues to expand, the number of wind farm and wind farm capacity are rapidly increasing. Besides, because the proportion of wind power over power system is increasing year by year, it is growing more and more impact on power system. To prevent all turbines disconnection from inducing power impulse to the grid, it is a necessary demand that on-grid wind power generator must possess low voltage ride through (LVRT) capability. Taking single doubly-fed induction generator (DFIG) as a research object, this paper encloses the dynamic behavior of DFIG during the grid voltage sag and control strategy for low voltage ride through operation.At first, this paper analyses the operation principle of DFIG on the basis of referring to the standard of low-voltage ride through (LVRT) drifted by American Wind Energy Association (AWEA), by means of building different simulation models of grid voltage sag, the dynamic responses of terminal voltage, stator current, rotor current, active power output and reactive power output, voltage at DC side and electromagnetic torque during the fault are simulated respectively. Through comparing the variations of these variables under several typical faults, corresponding control strategies are investigated.On the basis of understanding transient physical processes of grid voltage sag, in order to keep DFIG connecting with power system, rotor current weakening excitation control strategy has been proposed during power grid fault. The improved excitation control strategy, which aiming at the transient characteristic of the generator electromagnetic variable, can control the rotor over-current effectively, through controlling the excitation current to offset the direct current component and negative sequence component of the stator flux. Finally, effectiveness of this control strategy has been verified by Matlab/Simulink simulation which provides a theoretical basis in improving low voltage ride through ability of doubly fed wind turbines.