1. Introduction
The overcurrent relays (OCRs) have been most
commonly used as an efficient device to protect radial distribution systems. In such systems, the OCRs must operate rapidly to minimize fault duration and damage of equipment. Also, the appropriate time-lag between primary and back-up protections in OCRs must be ensured to avoid unnecessary operation or false trip. In general, the OCR taking its primary protection zone must operate firstly. Then, the OCR for back-up protection should operate only if primary protection fails to operate correctly and after waiting for the appropriate delay duration, which is referred to as the coordination time interval.