Where:Itf Maximum primary fundamental frequency three-phase fault current that passes the CTsand the power transformer (A).RLsw The resistance of the single secondary wire and additional load (Ω).In impedance earthed systems the phase-to-earth fault currents often are relatively small andthe requirements might result in small CTs. However, in applications where the zero sequencecurrent from the phase side of the transformer is a summation of currents from more than oneCT (cable CTs or groups of individual CTs in Holmgren connection) for example, in substationswith breaker-and-a-half or double-busbar double-breaker arrangement or if the transformerhas a T-connection to different busbars, there is a risk that the CTs can be exposed for higherfault currents than the considered phase-to-earth fault currents above. Examples of suchcases can be cross-country faults or phase-to-phase faults with high fault currents andunsymmetrical distribution of the phase currents between the CTs. The zero sequence faultcurrent level can differ much and is often difficult to calculate or estimate for different cases.To cover these cases, with summation of zero sequence currents from more than one CT, thephase side CTs must fulfill the Requirement (17) below:2sr Ral alreq f ct Lpr rI SE E I R RI Iæ ö³ = × × + +ç ÷è øEQUATION2242 V2 EN-US (Equation 102)Where:If Maximum primary fundamental frequency three-phase fault current that passes the CTs(A)RL The resistance of the secondary wire and additional load (Ω). The loop resistancecontaining the phase and neutral wires shall be used.18.1.7 Current transformer requirements for CTs according to otherstandards SEMOD53771-1 v1M11623-4 v3All kinds of conventional magnetic core CTs are possible to use with the IEDs if they fulfill therequirements corresponding to the above specified expressed as the rated equivalent limitingsecondary e.m.f. Eal according to the IEC 61869-2 standard. From different standards andavailable data for relaying applications it is possible to approximately calculate a secondarye.m.f. of the CT comparable with Eal. By comparing this with the required rated equivalentlimiting secondary e.m.f. Ealreq it is possible to judge if the CT fulfills the requirements. Therequirements according to some other standards are specified below.18.1.7.1 Current transformers according to IEC 61869-2, class P, PR M11623-6 v3A CT according to IEC 61869-2 is specified by the secondary limiting e.m.f. Ealf. The value of theEalf is approximately equal to the corresponding Eal. Therefore, the CTs according to class Pand PR must have a secondary limiting e.m.f. Ealf that fulfills the following:2 max alreqE max E>EQUATION1383 V3 EN-US (Equation 103)Section 18 1MRK 504 158-UEN ARequirements270Application manual
