A070980 V1 ENFigure 63: Numerical overcurrent protection functionality for a typical sub-transmission/distribution substation (feeder protection not shown).Blocking output = digital output signal from the start of a protectionstage, Blocking in = digital input signal to block the operation of aprotection stageThe operating times of the time selective stages are very short, because the gradingmargins between successive protection stages can be kept short. This is mainly dueto the advanced measuring principle allowing a certain degree of CT saturation, goodoperating accuracy and short retardation times of the numerical units. So, for example,a grading margin of 150 ms in the DT mode of operation can be used, provided thatthe circuit breaker interrupting time is shorter than 60 ms.The sensitivity and speed of the current-selective stages become as good as possibledue to the fact that the transient overreach is practically zero. Also, the effects ofswitching inrush currents on the setting values can be reduced by using the IED logic,which recognizes the transformer energizing inrush current and blocks the operationor multiplies the current start value setting of the selected overcurrent stage with apredefined multiplier setting.Finally, a dependable trip of the overcurrent protection is secured by both a properselection of the settings and an adequate ability of the measuring transformers toreproduce the fault current. This is important in order to maintain selectivity and alsofor the protection to operate without additional time delays. For additionalinformation about available measuring modes and current transformer requirements,refer to section where general function block features are described in the IEDtechnical manual.1MRS756378 D Section 5Protection functionsREF615 137Application Manual
