4.2.3 Relationships between setting parameter Base Current, CTrated primary current and minimum pickup of a protectionIED GUID-8EB19363-9178-4F04-A6AC-AF0C2F99C5AB v1Note that for all line protection applications (e.g. distance protection or linedifferential protection) the parameter Base Current (i.e. IBase setting in the IED)used by the relevant protection function, shall always be set equal to the largestrated CT primary current among all CTs involved in the protection scheme. Therated CT primary current value is set as parameter CTPrim under the IED TRMsettings.For all other protection applications (e.g. generator, shunt reactor, shunt capacitorand transformer protection) it is typically desirable to set IBase parameter equal tothe rated current of the protected object. However this is only recommended to doif the rated current of the protected object is within the range of 40% to 120% ofthe selected CT rated primary current. If for any reason (e.g. high maximum shortcircuit current) the rated current of the protected object is less than 40% of therated CT primary current, it is strongly recommended to set the parameter IBase inthe IED to be equal to the largest rated CT primary current among all CTs involvedin the protection scheme and installed on the same voltage level. This willeffectively make the protection scheme less sensitive; however, such measures arenecessary in order to avoid possible problems with loss of the measurementaccuracy in the IED.Regardless of the applied relationship between the IBase parameter and the ratedCT primary current, the corresponding minimum pickup of the function on the CTsecondary side must always be verified. It is strongly recommended that theminimum pickup of any instantaneous protection function (e.g. differential,restricted earth fault, distance, instantaneous overcurrent, etc.) shall under nocircumstances 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 individuallyverified for all current inputs involved in the protection scheme.Note that exceptions from the above 4% rule may be acceptable for very specialapplications (e.g. when Multipurpose filter SMAIHPAC is involved in theprotection scheme).4.2.4 Setting of voltage channels SEMOD55055-44 v4As the IED uses primary system quantities, the main VT ratios must be known tothe IED. This is done by setting the two parameters VTsec and VTprim for eachvoltage channel. The phase-to-phase value can be used even if each channel isconnected to a phase-to-earth voltage from the VT.4.2.4.1 Example SEMOD55055-47 v3Consider a VT with the following data:1MRK 505 393-UEN B Section 4Analog inputsLine differential protection RED650 2.2 IEC 55Application manual
