Simulation of unified static VAr compensator and power system stabilizer for arresting subsynchronous resonance

In the literature, co-ordination of static VAr compensators (SVCs) and power system stabilizers (PSSs) using generator speed deviation or modal speeds as stabilizing signals is shown to damp the system oscillations. Though such schemes are able to damp the SSR modes for small disturbances, they are unable to damp transient SSR due to large disturbances. In this paper, improvement in the control aspect of the SVC at the midpoint of the transmission line is suggested. This scheme attempts different auxiliary signals that include line current, computed internal frequency and bus angle deviations. A system of configuration similar to IEEE First Bench Mark model is considered, eigenvalue analysis has been carried out, and results indicate that bus angle deviation signal as auxiliary control signal for SVC was able to damp most of the modes leaving some of them still oscillatory. The main feature of the proposed work is to use combination of deviation in speed and electrical power output of the generator as input signals to PSS which operates simultaneously along with SVC. Such simultaneous PSS and SVC scheme is found to improve the damping under large disturbances i.e. the growth of system oscillations is arrested. The simulations are carried out on PSCAD. The efficacy of controllers to damp SSR under steady-state and faulted conditions where one of the torsional modes gets excited is presented and discussed