11 SENSITIVE EARTH FAULT PROTECTIONWith some earth faults, the fault current flowing to earth is limited by either intentional resistance (as is the casewith some HV systems) or unintentional resistance (e.g. in very dry conditions and where the substrate is highresistance, such as sand or rock).To provide protection in such cases, it is necessary to provide an earth fault protection system with a setting that isconsiderably lower than for normal line protection. Such sensitivity cannot be provided with conventional CTs,therefore the SEF input would normally be fed from a core balance current transformer (CBCT) mounted aroundthe three phases of the feeder cable. The SEF transformer should be a special measurement class transformer.11.1 SEF PROTECTION IMPLEMENTATIONThe product provides four stages of SEF protection with independent time delay characteristics.Stages 1, 2 provide a choice of operate and reset characteristics, where you can select between:● A range of IDMT (Inverse Definite Minimum Time) curves● A range of User-defined curves● DT (Definite Time)This is achieved using the cells● ISEF>(n) Function for the overcurrent operate characteristic● ISEF>(n) Reset Char for the overcurrent reset characteristic● ISEF>(n) Usr RstChar for the reset characteristic for user -defined curveswhere (n) is the number of the stage.Stages 1 and 2 also provide a Timer Hold facility. This is configured using the cells ISEF>(n) tReset.Stages 3 and 4 have definite time characteristics only.11.2 NON-DIRECTIONAL SEF LOGICISEF>1 Timer BlkISEF>1 StartISEF>1 TripI2H Any StartISEF> BlockingV006152H Blocks ISEF>1Inhibit SEFISEF>1 CurrentAR Blk Main ProtISEF> BlockingAR Blks ISEF>3ISEF>1 DirectionNon-directional& IDMT/DTTimer Settings&&&AR blocking available for DT-only stagesNote: This diagram does not show all stages . Other stages follow similarprinciples.AR blocking is only available for stages 3 and 4ISEFFigure 58: Non-directional SEF logicThe SEF current is compared with a set threshold (ISEF>(n) Current) for each stage. If it exceeds this threshold, aStart signal is triggered, providing it is not blocked. This can be blocked by the second harmonic blocking function,or an Inhibit SEF DDB signal.Chapter 6 - Current Protection Functions P14x130 P14xEd1-TM-EN-1
