&Start signalTrip Signal&Timer Blocking signals1V00654Timer SettingsThreshold IDMT/ DTTimer Blocking settingsStage Blocking signals1Stage Blocking settingsFunction inhibitDirectional Check&Energising quantityVoltageCurrentFigure 33: Principle of protection function implementationAn energising quantity is either a voltage input from a system voltage transformer, a current input from a systemcurrent transformer or another quantity derived from one or both of these. The energising quantities are extractedfrom the power system. The signals are converted to digital quantities where they can be processed by the IEDsinternal processor.In general, an energising quantity, be it a current, voltage, power, frequency, or phase quantity, is compared with athreshold value, which may be settable, or hard-coded depending on the function. If the quantity exceeds (forovervalues) or falls short of (for undervalues) the threshold, a signal is produced, which when gated with thevarious inhibit and blocking functions becomes the Start signal for that protection function. This Start signal isgenerally made available to Fixed Scheme Logic (FSL) and Programmable Scheme Logic (PSL) for furtherprocessing. It is also passed through a timer function to produce the Trip signal. The timer function may be anIDMT curve, or a Definite Time delay, depending on the function. This timer may also be blocked with timerblocking signals and settings. The timer can be configured by a range of settings to define such parameters as thetype of curve, The Time Multiplier Setting, the IDMT constants, the Definite Time delay etc.In General Electric products, there are usually several independent stages for each of the functions, and for three-phase functions, there are usually independent stages for each of the three phases.Typically some stages use an Inverse Definite Minumum time (IDMT) timer function, and others use a Definite Timetimer (DT) function. If the DT time delay is set to '0', then the function is known to be "instantaneous". In manyinstances, the term 'instantaneous protection" is used loosely to describe Definite Time protection stages, evenwhen the stage may not theoretically be instantaneous.Many protection functions require a direction-dependent decision. Such functions can only be implemented whereboth current and voltage inputs are available. For such functions, a directional check is required, whose output canblock the Start signal should the direction of the fault be wrong.Note:In the logic diagrams and descriptive text, it is usually sufficient to show only the first stage, as the design principles forsubsequent stages are usually the same (or at least very similar). Where there are differences between the functionality ofdifferent stages, this is clearly indicated.2.2.1 TIMER HOLD FACILITYThe Timer Hold facility is available for stages with IDMT functionality , and is controlled by the timer reset settingsfor the relevant stages (e.g. I>1 tReset, I>2 tReset ). These cells are not visible for the IEEE/US curves if an inverseChapter 6 - Current Protection Functions P14x96 P14xEd1-TM-EN-1
