Small-signal stability of power system integrated with ancillary-controlled large-scale DFIG-based wind farm

In this study, small-signal stability of the power system integrated with ancillary-controlled large-scale doubly fed induction generator (DFIG) based wind farm (WF) is studied. A model which considers grid code requirements and ancillary controllers is presented to indicate important DFIG dynamics for the study. The ancillary control of the WF which aims to improve power system voltage/frequency stability is evaluated from the small-signal stability perspective. It is shown that the ancillary controller deteriorates power system low-frequency oscillations and/or induces new lightly damped oscillation modes especially in a weak grid. The potential risks may restrict the WF to fulfil gird code requirements and threaten small-signal stability of the power system. Meanwhile, this study reveals that wind power penetration level and WF connection impedance are two main factors which affect dynamic interactions between the WF and the power system and thus affect system small-signal stability. The principle of the influence on the small-signal stability with different grid weakness and ancillary control schemes is evaluated in this study by eigenvalue analysis and verified with time-domain simulations.