Current restraint feature M13751-338 v3The overcurrent protection step operation can be made dependent of a restrainingcurrent quantity (see table 191). Practically then the pickup level of the overcurrentstep is not constant but instead increases with the increase in the magnitude of therestraining current.IsetHighIsetLowIMeasuredRestraintatan(RestrCoeff)en05000255.vsdOperate areaI>RestrCoeff*IrestrainIEC05000255 V1 EN-USFigure 118: Current pickup variation with restraint current magnitudeThis feature will simply prevent overcurrent step to pickup if the magnitude of themeasured current quantity is smaller than the set percentage of the restrain currentmagnitude. However this feature will not affect the pickup current value for calculationof trip times for IDMT curves. This means that the IDMT curve trip time will not beinfluenced by the restrain current magnitude.When set, the pickup signal will start definite time delay or inverse (IDMT) time delayin accordance with the end user setting. If the pickup signal has value one for longertime than the set time delay, the overcurrent step will set its trip signal to one. Reset ofthe pickup and trip signal can be instantaneous or time delay in accordance with theend user setting.11.1.6.4 Built-in undercurrent protection steps M13751-231 v2Two undercurrent protection steps are available. They are absolutely identical andtherefore only one will be explained here. Undercurrent step simply compares themagnitude of the measured current quantity (see table 189) with the set pickup level.The undercurrent step will pickup and set its pickup signal to one if the magnitude ofthe measured current quantity is smaller than this set level. The pickup signal will startdefinite time delay with set time delay. If the pickup signal has value one for longerSection 11 1MRK 511 408-UUS AMultipurpose protection344 Phasor measurement unit RES670 2.2 ANSITechnical manual
