Study of torsional interactions in multi-terminal DC systems through small signal stability analysis

Turbo-generators can be subjected to negatively damped subsynchronous frequency oscillations caused by the interactions between the generator and the external network. This phenomena is termed as subsynchronous resonance (SSR) and it is well known that series compensated AC lines are the major contributors. In recent years, it has been established that HVDC systems with converter controllers can also cause unfavourable torsional interactions. Report the development of the mathematical model based on small signal stability analysis for the study of torsional interactions in HVDC systems. The formulation is general enough to include multi-terminal DC (MTDC) systems. The case study of two terminal and three terminal systems is presented to illustrate the effect of converter controller gains on the damping of subsynchronous frequency oscillations