A probabilistic approach to setting distance relays in transmission networks

The setting of back-up zones in distance relays has to deal with fault current infeeds between the relay and remote faults. Traditional approaches to this problem are conservative, sometimes leaving portions of a power system without sensitive or selective back-up protection. In this paper a probabilistic model for the relay errors resulting from the variation of the infeeds is developed. Other measurement inaccuracies are also considered in order to build a probabilistic approach to setting all zones of distance relays. The developed concepts are associated with automatic tools for computer aided protection coordination and applied to the Portuguese transmission grid.