An improvement in coordination of distance and over current relays for DERs connected power networks

Distance relays are set in coordination with the over current relays for protection of transmission power lines. Depending upon the location of faults on the transmission lines, faults are cleared by the distance relays from zone-1 or zone-2. If the distance relays fails due to any relay failure reasons. Then over current relays acts at backup relays to clear the same fault with some time delay. These time delays are known as coordination time margins. The coordination time margins with respect to backup over current relays are at minimum at starting of zone-1 and zone-2 of distance relays and increases in forward direction of zones of distance relays. In this research article, the zone -2 of distance relays are optimised in such a manner that desired coordination time margin remains valid for all the fault location in zone-1 and zone-2 of distance relays. Integration of decentralised distributed energy resources may cause the change in the fault levels which may lead to violation of coordination time interval among the relays. In order to handle this problem, the non-standard time inverse over current relays are defined for better zone-2 selection of distance relays. Where the user can select non-standard time inverse over current relay characteristics, which have flexible characteristics to maintain the desired coordination time margin among the over current and distance relays for all possible fault locations on zone-1 and zone-2 of distance relays.