Adaptive vs. conventional reach setting of digital distance relays

In contrast to the conventional approach of setting the reach of a distance relay based on off-line worst-case studies, the adaptive approach adjusts the reach of the relay on-line on the basis of the current power system state. In this paper, results of a simulation study are presented to quantitatively compare the two approaches, taking into account the effects of transmission line parameter uncertainties, mutual coupling between parallel lines, and variations in the system external to the protected line. It is shown that if the relay is to provide optimal selectivity, irrespective of the power system state and the operation mode of the line, it must be set adaptively.