DFIGs’ Low-frequency Damping Effects Based on Responses of Generic Component

There exist arguments that whether the DFIGs contribute to low-frequency-oscillation of power systems. The controversial issue is hard to explain by traditional analytical tools like the modal analysis or MIMO methods, since their numerical results cannot provide fundamental physical significances about the coupling between DFIGs and the power system. Based on the idea of modular transfer-function modelling, the paper proposes a general theory named as rationales of "responses of generic component (RGC)", in order to have a complete explanation of potential damping effects induced by DFIGs (and other similar converter-interfaced generators) in intuitive frequency-domain forms. By using the complex torque coefficient method, the RGC-based rationales only rely on mathematical models and parameters of the DFIG, and thus are independent from the adjacent power systems. Analysis based on calculation of the DFIG's RGC proves that, even if internal dynamics of the DFIG itself are poorly damped, DFIGs do not actively "participate" in electromechanical oscillations when in the normal maximum-power-tracking mode. In fact, only when additional active-power control like droop control is activated would the DFIG induce effective damping. Also, the proposed rationales clearly show the possibility of negative damping effects introduced by DFIGs. When a rudimentary power system is studied, observations and conclusions of both modal analysis and time-domain simulation could be well predicted by the RGC-based rationales.