On the significance of the plug setting in optimal time coordination of directional overcurrent relays

Summary Directional overcurrent relays give primary plus backup protection to electrical distribution networks. They are coordinated and set at the optimal values to make the total operating time of all the relays in the system minimum. The Time Multiplier Setting (TMS) is used practically to provide time grading to the distribution systems. The Plug Setting (PS) provides for different pick up values for the relays to operate. However, the Inverse Definite Minimum Time relay operating time mathematical expression depends on both these settings. Here, the Firefly algorithm is used to demonstrate that, keeping the PS as a variable and the TMS at the lower bound, gives the best results and the role of TMS for time grading is relegated. This is the novelty of this paper. It is stressed that researchers are minimizing operating time at the expense of downsizing the role of TMS as a time grader.