A Method to Examine the Impact of Grid Connection of the DFIGs on Power System Electromechanical Oscillation Modes

Grid connection of a DFIG for power generation changes the power system load flow and introduces dynamic interactions with the synchronous generators (SGs). A method is proposed in this paper for the separate examination of the impact of those two affecting factors, the change of load flow and dynamic interaction brought about by the DFIG, on the electromechanical oscillation modes of a power system. The method is developed on the establishment of a "closed-loop control system" type of state space model of the power system where the DFIG is treated as the feedback controller. The established model unambiguously indicates that the impact of the DFIG can be examined separately: (1) With the DFIG being modelled as a constant power injection into the system, the impact of the load flow change brought about by the DFIG on the oscillation modes is determined; (2) The impact of the dynamic interactions introduced by the DFIG is estimated by calculating the contribution of the damping torque from the DFIG with its full dynamics being considered. Thus a more detailed examination on and deeper insight into the impact of grid connection of the DFIGs on power system small-signal angular stability is obtained by using the proposed method.