Improved Distribution System Overcurrent Protection using Tolerance-based Time-current Coordination

The availability of information on the accuracy of current sensing, relaying and protective device operation, along with the high accuracy sensing precision, and fast clearing times achieved by certain modern distribution protection devices, enable improved distribution overcurrent protection using tolerance-based time-current coordination rather than following the existing convention of using a specified coordinating time interval (CTI) or grading interval. This paper discusses the significant differences in tolerance-based time-current coordination performance between existing and certain modern protective devices and presents the methodology used to achieve improved overcurrent protection using multiple series protective devices on a distribution feeder.