Design and Implementation of an Algorithm for Diagnosis of Load Encroachment in EHV Lines

Zone-3 mal-operation of distance relays was one of the important cause for many of blackouts in worldwide. The one of the root cause for the mal-operation is encroachment of load into the Zone-3 characteristics. Such zone-3 mal-operations can be prevented by improvising the zone-3 characteristics which eventually avoids cascaded line tripping. This paper proposes and validates a new algorithm based on the total DC component and Phase angle of the positive sequence impedance to prevent its operation during the load encroachment conditions. Based on the proposed algorithm, improvement in the operation of Zone-3 element can be achieved. The relay can differentiate between actual system faults and load encroachment and thus ensure the security of the system. The proposed algorithm supplements the Zone-3 operation by introducing the ability to identify a load encroachment condition, and thus can avert the possibility of blackouts.