Methodology for modeling overcurrent relays with non-standard curves by using logarithmic-linear correction

Studies for coordinating overcurrent protection devices require accurate mathematical models of the time overcurrent curves, so that coordination and selectivity can be achieved properly. Standard time overcurrent curves defined by ANSI and IEC, for example, have well-defined equations. On the other hand, for electromechanical relays and/or non-standard curves, which are still present in many electrical systems, it is not possible to draw a general equation of their time overcurrent curve. In this context, this paper proposes a simple and easy applicable method for modeling electromechanical relays and/or non-standard time overcurrent curves for overcurrent protection coordination studies. The results demonstrate the accuracy of the methodology and its advantages when compared with traditional methods for modeling time inverse overcurrent characteristics.