IEEE Std C37.112 pdf free download

IEEE Std C37.112 pdf free download.Inverse- Time Characteristics Equations for Overcurrent Relays.
3. The time-current equation
3.1 Coordination of inverse time-current characteristics
Coordination practice in distribution systems is influenced by the type of grounding used. Notably, in Europe and Japan the practice is to operate three-wire primary distribution systems, either impedance grounded or ungrounded, over relatively short distances. Since there arc no single-phase laterals protected by fuses, coordination can be achieved using definite-time characteristics. In North America the practice is to operate grounded four-wire distribution systems with loads served by single-phase laterals protected by fuses. As a result, coordination is obtained using inverse time-current characteristics suitable for fuse coordination. Figure 1 shows the close coordination of an extremely inverse induction characteristic with a high-voltage fuse.
The straight line Ft log-log plot of a fuse minimum melting time is often visualized as the basic time-current characteristic. However, a definite time must be added to emulate the maximum clearing time of the fuse. This illustrates the fundamental concept that whenever fixed clearing time is added to a straight line log-log plot, the result is a curve. For this reason, the best shape for a time-current characteristic for coordination purposes is the curve formed when a definite time is added to the straight line of a log-log plot.
3.2 The analytic equation
Equation (1) and Equation (2) define the reset time and pickup time of an inverse-time overcurrent curve as shown in Annex A. By applying the constants to these equations, a characteristic curve can be accurately defined. Equation (2) is similar to the I EC equation (see I EC 60255-151:2009, Measuring relays and protection equipment- -Part 151: Functional requirements for over/under current protection) except for the addition of constant B. The constant B defines the definite time component that is the result of core saturation of an induction type relay.
3.3 Time dial The time dial of an overcurrent relay is a control that permits the characteristic of the relay to be adjusted to a predetermined operate time at a specified current. The time dial generally allowsa 15 to 1 or greater range of time adjustment. In the characteristic Equation (1) and Equation (2), the constants A, B, and t, are varied proportionally with time dial. Whereas the ratio ofA to B may vary to some extent with the time dial setting in induction relays, the ratio of A to B remains constant in microprocessor relays.
3.4 Standard time-current characteristics The constants and exponents in Table 1 when used in Equation (1) and Equation (2) define the shape of the standard moderately inverse, very inverse, and extremely inverse trip characteristics. The constant t, when used in Equation (1), defines the optional reset characteristic. These constants define the curve that represents the mean curve of the induction characteristics defined in Annex A.
The characteristics of a microprocessor-based protective relay conform to this standard when they are implemented according to Equation (3), where t(I) 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.IEEE Std C37.112 pdf download.