IEEE Std C37.112-2018 pdf download – IEEE Standard for Inverse- Time Characteristics Equations for Overcurrent Relays.
The characteristics of a microprocessor-based protective relay conform to this standard when they are implemented according to Equation (3), where t([) is given by Equation (2) and the operate time values corresponding to values in the range of 1.5 to 20 multiples of the pickup current are within the conformance bands shown in Figure 2, Figure 3, and Figure 4. Note that in each of these figures, the reset characteristic curve is shown on the left side of the plot, and the trip characteristic curve is shown on the right side of the plot. The upper and lower limits of the conformance bands are 1.15 and 0.85 times the characteristic defined in Table 1. The conformance band for the optional reset characteristic extends from 0.05 to 0.9 multiples of pickup current. The conformance bands are templates for classifying the shape of standard inverse time- current characteristics and are not tolerance bands for accuracy or repeatability.
The normalized flux versus M multiples of pickup can be determined by setting t, equal to the total reset time with zero current and then substituting values of operating time corresponding to various multiples of pickup current. Plots of normalized flux for an extremely inverse, a very inverse, and a moderately inverse induction-type overcurrent relay are shown in Figure A. T and the resulting characteristics are compared in Figure A.2. The plot shows the electromchanical technique uses specific degrees of saturation to produce the familiar time-current characteristic and shows the following order. The extremely inverse relay saturates at four multiples of pickup, the very inverse at two multiples (half the previous value), and the moderately | inverse at pickup (again, halfthe previous value). The derivation shows that the induction characteristic, were it not for the deliberate saturation, is the straight line log-log characteristic of a fuse. However, the curve is formed by saturating the electromagnet at a specific multiple of pickup current. It has also been shown that saturation is the means that, in effect, incorporates the definite time component to form a practical curve for coordination. Therefore, adding a constant definite time term to Equation (A.9) forms the induction characteristic equation.