bigger than the user settable level. Both of these monitoring features are phasesegregated and they give out binary signals, which can be either used to triggerdisturbance recorder or for alarming purposes.en06000071.vsdIdAlarmLev a>babttIdAlarmALDIFFALIINIdIin a>babIinAlarmLevIEC06000071 V1 ENFigure 56: Simplified logic for Zone supervision features5.1.5.2 Open CT detectionThe three input quantities into the open CT detection algorithm are:• Id = RMS value of the differential current• Iin = RMS value of the incoming current• Iout = RMS value of the outgoing currentIt is to be noted that the open CT detection algorithm does not know the number ofconnected CT inputs into the IED.The open CT detection algorithm is completely phase-segregated. Therefore, it isexplained for one phase only.Fast operating open CT detection logic instantly detects the moment when ahealthy CT secondary circuit carrying the load current is accidently opened (that is,current interrupted to the differential relay). The logic is based on the perceptionthat the total busbar through-load current is the same before and after that CT isopen circuited.In order to prevent false operation of this logic in case of a fault or disturbance inthe power system, the total through-load current must not have big changes threeseconds before the open CT condition is detected.When one CT secondary circuit is open circuited during normal through-loadcondition one measuring point is lost and, therefore, the following should hold true:• values of Iin and Iout were equal one cycle before• value of Iin remains constant (that is, unchanged)• value of Iout drops for more than pre-set value of OCTOperLev• value of Id rises for more than pre-set value of OCTOperLev• value of the sum Iout + Id is equal to value of Iin one cycle beforeSection 5 1MRK 505 208-UEN BDifferential protection122Technical reference manual
