Secure third zone operation of distance relay using impedance prediction approach

The large reach setting of distance relays third zone along with its inability to distinguish system stresses from symmetrical faults is often the reason for its insecure operation. The distance relays mal-operation during such system stressed conditions has led to numerous major blackouts in the recent times. This paper proposes a new technique based on the impedance prediction approach to detect three phase faults during system stresses, namely power swing and load encroachment. The k step ahead predictor is used to predict the apparent impedance seen by the distance relay; which is compared with the actual impedance seen by the distance relay. The difference in the actual and predicted impedances is minimal during system stresses; whereas there is significant deviation observed when a symmetrical fault occurs. The proposed methodology is tested on the WSCC three generator, nine bus system modeled in ATP-EMTP environment and encouraging results are obtained.