Blocking of Distance Relays Zone3 under Load Encroachment Conditions- A New Approach Using Phasor Measurements Technique

Cascaded tripping of power lines due to mal-operation of zone 3 distance relays was one of the main causes of many previous blackouts worldwide. Encroachment of load into the zone 3 characteristics is one of the main cause for the maloperation of the relays. By improving the operation of zone 3 of distance relays it is possible to prevent such mal-operations so that cascaded line tripping can be avoided. This paper proposes a new algorithm that utilizes Synchronized Phasor Measurements (SPM) to control the operation of distance protection zone 3. Based on the proposed algorithm, several improvements in operation of zone 3 can be achieved. For instance, the relay can differentiate between actual system faults and load encroachment. In addition to this, the tripping decision is not affected by the value of fault resistances from zero to High Impedance Faults (HIF). These improvements would in turn help prevent blackouts due to mal-operation of distance relay zone3. KeywordsDistance Relay Zone3; Mal-Operation of Zone 3; Cascade Tripping; Fault Resistance; N-x criterion