With the deregulation of electric utility industry, generation and transmission and distribution agencies have a competitive environment. Critical lines must be operated securely as its outage will result in widespread disruption and result in loss of income. The inclusion of series capacitor in ac transmission lines improves the line loadability and flexibility thereby making best possible use of transmission assets. However the possibility of Subsynchronous Resonance (SSR) due to series capacitors is seen with caution and its use is limited. SSR conditions may also occur due interaction of PSS, HVDC and FACTS controllers.

Subsynchronous Resonance problems result from the interaction between an electrical mode of the series compensated network and a mechanical shaft mode of a turbine-generator group. TG shaft failures reported at Mohave Power Station are related to SSR. FACTS devices can be effectively used for damping of subsynchronous oscillations. The possibility of SSR phenomenon is evaluated by system study in the linearised domain by eigenvalue analysis. A Single Machine Infinite Bus (SMIB) system comprising of generator, transmission network with series compensation is studied. The inclusion of TCSC shows improvement in containing growing oscillations. The closed loop controller of TCSC is investigated and optimized to provide maximum damping of torsional modes. The dynamic performance of the system is studied for small disturbance using MATLAB. For large disturbance PSCAD/ EMTDC is used for obtaining dynamic response. The controller gains obtained by optimisation provide adequate damping to torsional oscillations.