Subsynchronous Interactions Induced by DFIGs in Power Systems Without Series Compensated Lines

This study examines the subsynchronous interactions (SSIs) induced by grid-connected doubly fed induction generators (DFIGs) in a power system without series-compensated transmission lines. The investigation is based on a closed-loop interconnected dynamicmodel, wherein the DFIG and the rest of the power system are modeled as two interconnected open-loop subsystems. Our analysis reveals that when an open-loop subsynchronous oscillation (SSO) mode of the DFIG is close to an open-loop SSO mode of the rest of the power system on the complex plane, the resulting open-loop SSO modal resonance will likely degrade the damping of one closed-loop SSO mode. When the modal resonance is strong, the DFIG may cause poorly or even negatively damped SSOs in the power system. A modified IEEE first benchmark power system (FBMPS) with two DFIGs and without series compensation is used to demonstrate and validate the analysis and conclusions made in this paper. Case studies are presented to show that under the condition of open-loop SSO modal resonance, strong SSIs between the two DFIGs occur and are poorly damped. Open-loop SSO modal resonance between a DFIG and the torsional dynamics of a synchronous generator is shown to cause growing torsional SSIs in the modified IEEE FBMPS.