Damping subsynchronous oscillations using thyristor-controlled series capacitor (TCSC)

This paper investigates the feasibility of a Thyristor-Controlled Series Capacitor (TCSC) for mitigation of subsynchronous frequency oscillations in electric power systems. A thyristor-controlled series capacitor provides the required damping function based on its "Rapid Adjustment of Network Impedance (RANI)" characteristic, and therefore it is also referred to as a RANI element.; The studies are conducted on the first IEEE benchmark model for subsynchronous oscillation studies. The "complex torque method", and an eigen analysis method are used as the analytical techniques to investigate the impact of a RANI element on subsynchronous frequency oscillations, and to design its required controller for damping the oscillations. The analytical results are verified by digital time-domain simulation of the actual nonlinear system behaviour using the Electromagnetic Transients Program (EMTP).; The investigation results demonstrate that subsynchronous oscillations (SSO) can be effectively damped out through proper modulation of the thyristor firing angles of a RANI element. The required firing angle modulation for typical high voltage transmission lines (400 kV and above) is within {dollar}pm2spcirc{dollar}. The change in the angle is small and does not result in noticeable changes in the line steady-state power flow. The studies also show that a RANI element can effectively mitigate simultaneous multiple subsynchronous oscillations.