Calculation of optimum directional overcurrent relays settings: the parallel lines case

A simple and practical criterion is proposed to consider the transient changes due to nonsimultaneous operation of directional overcurrent relays in parallel lines. The criterion consists of using two different time coordination intervals: one for relay coordination pairs that do not belong to parallel lines and another for pairs belonging to parallel lines. The method can be implemented in programs that use the optimization technique for the calculation of relay settings. Numerical results showing the performance of the method are also presented.