Analysis of Sub-Synchronous Band Oscillation in a DFIG System With Non-Smooth Bifurcation

With the interconnection of large-scale wind power, the sub-synchronous oscillation (SSO) which is mainly analyzed by the local small-signal method occurs in many power grids. This paper presents the phenomenon of the sub-synchronous band oscillation (SSBO) of the doubly-fed induction generator (DFIG) system due to limits, which belongs to global dynamics, and analyzes its relationship with certain limits from the viewpoint of non-smooth bifurcation, which can serve as one of the mathematical foundations of the SSBO. Specifically, the concept of non-smooth bifurcation, a power system interconnected with the DFIG and the related current limit (CL) of d-axis and q-axis in the outer power control loop (OPCL) of the DFIG's rotor side converter (RSC) are introduced first. Secondly, the SSBO phenomenon induced by the CL of d-axis and q-axis is presented. And then, the influence of one parameter on the SSBO is analyzed, such as the short circuit capacity, the fault grounding resistance and the PI parameters of the OPCL, which reveals the non-smooth bifurcation with respect to relative one parameter. Finally, on the plane, the non-smooth bifurcation characteristics of PI controller parameters in the OPCL are given, showing that the non-smooth bifurcation/SSBO may be associated with saturation of different limits.