OVERCURRENT (OC) and distance relays are commonly used in transmission and sub transmission protection systems [1]. To consider comprehensive coordination, all the distance and overcurrent relays when one of them is considered to be the main relay and the other is backup, must be coordinated.
When both main and backup (M/B) relay are distance relay the impedances of the three zones of relays are calculated considering all conditions of the interconnected network such as connection and disconnection of generators and lines [2].