Effects of the curve type of overcurrent functions and the location of analyzed faults on the optimal coordination of directional overcurrent protections

The time-current curve type and the considered fault locations influence the optimal coordination of directional overcurrent protections. It is shown that curve type can determine the feasibility of having solutions and the feasibility of obtaining reasonably low trip times. It is also shown that constraints for faults near to the main relay location must be complemented with those of other fault locations. In general, the use of Extremely Inverse curves implies a faster protection in the analyzed cases. This is because this coordination depends on the ratio between the currents seen by each relay, and Extremely Inverse curves have the greatest inclination in time-current graphs. The backup action for faults near to the remote line end of main relays can be very slow with Extremely Inverse curves in some cases; therefore, an analysis of benefits and drawbacks of using each curve type should be performed in each analyzed case.