IBase set in primary Amp M12541-95 v3Set a common IBase in Global base for the protected line (circuit). Most currentsettings for the protection function are then related to the IBase. The setting ofIBase is normally made so that it corresponds to the maximum rated CT in any ofthe line terminals. For example, if all CTs used in a protected circuit are 1000A/1A,that is, the primary current rating is 1000 A, then IBase = 1000 A is a good choice.NoOfUsedCTs M12541-98 v8NoOfUsedCTs indicates to the function the number of three-phase CT sets includedin the protected circuit. Note that one IED can process one or two local currentterminals of the protected circuit. This is the case, for example, in 1½ breakerconfigurations in the line bay, where one of the two CT sets will be represented asone separate current terminal. A two-terminal line with 1½ breaker configurationsat one line end must consequently have NoOfUsedCTs = 3.Current terminalsIEC15000450-2-en.vsdxIED IEDIEC15000450 V2 EN-USFigure 38: Example when setting NoOfUsedCTs = 3 in 1½ breakerconfigurations6.1.3.2 Percentage restrained differential operation M12541-104 v4Line differential protection is phase-segregated where the operate current is thevector sum of all measured currents taken separately for each phase. The restraincurrent, on the other hand, is considered the greatest phase current in any line endand it is common for all three phases. Observe that the protection may no more bephase-segregated when there is an in-line power transformer included in theprotected circuit. These are usually of the Dy or Yd type and the three phases arerelated in a complicated way. For example, a single-phase earth fault on the wyeside of the power transformer is seen as a two-phase fault on the delta side of thetransformer.Operation: Line differential protection function is switched on or off with thissetting. If the parameter Operation is set to Off this IED is switched over to Slavemode and trip is initiated by the remote end IED.The characteristic of the restrained differential function is shown in Figure 39. Therestrained characteristic is defined by the settings:1MRK 505 393-UEN B Section 6Differential protectionLine differential protection RED650 2.2 IEC 87Application manual
