Combined optimal distance to overcurrent relay coordination

Slow operational speed of overcurrent relay, forces application of distance relay as main protection device for subtransmission system. Overcurrent relays are used as backup protection to main distance protection of sub transmission system. Overcurrent relay provides backup protection to zone-2 operation of distance relay. Differential evolutionary (DE) algorithm is used for optimal coordination of distance to overcurrent relay operation. Coordination time interval between zone-2 operations of distance to overcurrent relay backup operation time is minimized. The coordination is set at five critical points. Critical points are identified at which coordination time interval between zone-2 operations of distance to overcurrent relay backup operation time is minimum.