IEEE Std C37.112:2018 pdf download.IEEE Standard for Inverse- Time Characteristics Equations for Overcurrent Relays.
The scope of IEEE Std C37.112 includes the review of various existing analytic techniques used to represent relay operating characteristic curve shapes and proposes analytical (formula) representation of typical operating characteristic curve shapes to foster some standardization of available inverse-time relay characteristics provided in microprocessor or computer relay applications.
1.2 Purpose
The purpose of this standard is to provide an analytic (formula) representation of typical relay operating characteristic curve shapes of various inverse-time relays to facilitate representation by microprocessor-type relays and promote a degree of standardization in the inverse shape of a selected curve.
2. Definitions
For the purposes of this document, the following terms and definitions apply. The IEEE Standards Dictionary Oiiline should be consulted for terms not defined in this clause.’
inverse-tinie overcurrent relay: A current sensing relay that produces an inverse time-current characteristic by integrating a function of current F(I.) with respect to time. The function F(I,) is positive above and negative below a predetermined input current called the pickup current. Pickup current is therefore the current at which integration starts positively and the relay produces an output when the integral reaches a predetermined positive set value.
For the induction relay, it is the disk velocity that is the function of current F(I) that is integrated to produce the inverse time characteristic. The velocity is positive for current above and negative for current below a predetermined pickup current. The predetermined set value of the integral represents the disk travel, required to actuate the trip output.
reset characteristics: The time versus current curve that defines the time required for the integral of the function of current F(I) to reach zero for values below current pickup.
reset: The state of an inverse-time overcurrent relay when the integral of the function of current F(i, that produces a time-current characteristic is zero.
time dial: The time dial is the control that determines the value of the integral at which the trip output is actuated, and hence controls the time scale of the time-current characteristic produced by the relay. In the induction-type relay, the time dial sets the distance the disk must travel, which is the integral of the velocity with respect to time.
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 Pt 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.IEEE Std C37.112 pdf download.