CTs involved in the protection scheme. The rated CT primary current value is set as parameterCTPrim under the IED TRM settings.For all other protection applications (e.g. generator, shunt reactor, shunt capacitor andtransformer protection) it is typically desirable to setIBase parameter equal to the ratedcurrent of the protected object. However this is only recommended to do if the rated currentof the protected object is within the range of 40% to 120% of the selected CT rated primarycurrent. If for any reason (e.g. high maximum short circuit current) the rated current of theprotected object is less than 40% of the rated CT primary current, it is strongly recommendedto set the parameterIBase in the IED to be equal to the largest rated CT primary currentamong all CTs involved in the protection scheme and installed on the same voltage level. Thiswill effectively make the protection scheme less sensitive; however, such measures arenecessary in order to avoid possible problems with loss of the measurement accuracy in theIED.Regardless of the applied relationship between theIBase parameter and the rated CT primarycurrent, the corresponding minimum pickup of the function on the CT secondary side mustalways be verified. It is strongly recommended that the minimum pickup of any instantaneousprotection function (e.g. differential, restricted earth fault, distance, instantaneousovercurrent, etc.) shall under no circumstances be less than 4% of the used IED CT input rating(i.e. 1A or 5A). This corresponds to 40mA secondary for IED 1A rated inputs and to 200mAsecondary for IED 5A rated inputs used by the function. This shall be individually verified for allcurrent inputs involved in the protection scheme.Note that exceptions from the above 4% rule may be acceptable for very special applications(e.g. when Multipurpose filter SMAIHPAC is involved in the protection scheme).4.2.4 Setting of voltage channels SEMOD55055-44 v3As the IED uses primary system quantities the main VT ratios must be known to the IED. This isdone by setting the two parametersVTsec andVTprim for each voltage channel. The phase-to-phase value can be used even if each channel is connected to a phase-to-earth voltage fromthe VT.4.2.4.1 Example SEMOD55055-47 v2Consider a VT with the following data:132 1103 3kV VEQUATION2016 V1 EN-US (Equation 1)The following setting should be used:VTprim=132 (value in kV)VTsec=110 (value in V)4.2.4.2 Examples how to connect, configure and set VT inputs for mostcommonly used VT connections SEMOD55055-60 v5Figure 15 defines the marking of voltage transformer terminals commonly used around theworld.1MRK 505 355-UEN A Section 4Analog inputs45Application manual
