An algorithm to secure the zone 3 operation of distance relay

Abstract

The insecure third zone operation of distance relay due to its inability to distinguish between symmetrical faults and system stresses has led to severe blackouts. An algorithm to enhance the security of the zone 3 operation of distance relay using only the local information is proposed in this paper. The proposed scheme takes advantage of the fact that a system unbalance exists during the post fault window of a three phase fault which is absent during the system stressed conditions. The ratio of the negative sequence impedance to positive sequence impedance monitored during the system stressed conditions is nearly equal to zero whereas during the symmetrical fault conditions this ratio becomes non-zero. The proposed methodology is tested on the WSCC three generator, nine bus system modeled in ATP-EMTP environment for symmetrical faults during power swing, load encroachment and voltage stressed conditions. The results obtained prove the reliability of the scheme.