Optimal Coordination of DOCRs With OCRs and Fuses

Coordinating directional overcurrent relays (DOCRs) need to trace the current direction on each branch. On the opposite side, overcurrent relays (OCRs) can see the current from both directions, which means that there is a challenge to achieve the selectivity criteria between primary and backup OCRs. This chapter shows how to coordinate OCRs of a simple radial network and OCRs/DOCRs of a simple double‐line system. It also shows that the problem complexity exponentially increases as the network size increases regardless of whether the network contains fuses or not. The protection devices have inverse‐time characteristic curves, but they are not the same and thus we cannot mimic the operating time of fuses by the equations used for overcurrent relays. The main problem faced in the coordination scheme is that there is no general characteristic curve equation that can be used to calculate the operating time of fuses at different fault currents.