Abstract: This paper present an approach for maintain system stability in case of power oscillation in power system network by using numerical relay for protection. Fault clearing time plays an important role for maintain system stability. Variation in load angles due any disturbance in huge interconnected network may cause power swing .By keeping fault clearing time as small as possible in case of, any large variation in load angles system stability can be enhance. Setting of fault clearing time is only possible in case of numerical relay haying very fast and accurate response .The proposed approach was tested for 3 phase fault in IEEE 9 bus system .Electromagnetic Transient program (EMPT) and Power System Computer Aided Design (PSCAD)were used for simulation purpose. The results show the variation in power swing with different fault clearing time.
Keywords: Critical Clearing Time (CCT), PSCAD/EMTDC, Transient Stability, IEEE 9-Bus system, Test system.