IEEE C37.12-1996 pdf download IEEE Standard Inverse-Time Characteristic Equations for Overcurrent Relays
3. Definitions
3.1 inverse-time overcurrent relay: A current operated 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.
3.2 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.
3.3 reset characteristics: The time vs. current curve that deÞnes the time required for the integral of the function of current F(I) to reach zero for values below current pickup when the integral is initially at the trip value.
3.4 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.
4. The time-current equation
4.1 Coordination of inverse time-current characteristics Coordination practice is inßuenced by the type of grounding used in distribution systems. Notably, in Europe and Japan the practice is to operate impedance grounded or ungrounded relatively short three-wire primary distribution systems. Since there are no single-phase laterals protected by fuses, coordination can be achieved using deÞnite-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 I 2 t log-log plot of a fuse minimum melting time is often visualized as the basic time-current characteristic. However, a deÞnite time must be added to emulate the maximum clearing time of the fuse. This illustrates the fundamental concept that whenever Þxed 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 deÞnite time is added to the straight line of a log-log plot.
4.2 The analytic equation Equations (1) and (2) deÞne 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 deÞned. Equation (2) is similar to the IEC equation (see IEC 255-03 [1989-05]) except for the addition of constant B. The constant B deÞnes the deÞnite time component that is the result of core saturation of an induction type relay.
IEEE C37.12-1996 pdf download
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