Active filtering based current injection method for multi modal SSR damping in an AC/DC system

The rapid expansion in power transmission for the integration of large-scale renewables complemented by infrastructure reinforcements in the form of series compensation poses the threat of subsynchronous resonance (SSR). However, existing and future high-voltage direct current (HVDC) links can be effectively used to mitigate this undesirable phenomenon. In this line, this paper presents an auxiliary control loop embedded in a voltage source converter (VSC) based HVDC system to damp torsional interactions - a form of SSR involving turbogenerators and series-compensated transmission lines. The proposed damping scheme employs modal filters to identify SSR upon occurrence and then injects currents at subsynchronous frequency to damp it. The SSR damper has been tested in the well-known IEEE First Benchmark Model, which has been upgraded with a VSC-HVDC link to form an integrated AC/DC system. Simulations have been performed in PSCAD, with eigenvalue analysis carried out in MATLAB for the small-signal stability assessment of the AC/DC system. Results show that the proposed scheme effectively damps SSR irrespectively of the torsional mode being excited.