Ir The rated current of the protection IED (A)Rct The secondary resistance of the CT (Ω)RL The resistance of the secondary wire and additional load (Ω). The loopresistance containing the phase and neutral wires shall be used.SR The burden of a REx670 current input channel (VA). SR = 0.020 VA / channel forIr = 1 A and SR = 0.150 VA / channel for Ir = 5 AIn case of three individual CTs connected in parallel (Holmgren connection) on thephase side the following additional requirements must also be fulfilled.The three individual phase CTs must have a rated equivalent limiting secondarye.m.f. Eal that is larger than or equal to the maximum of the required ratedequivalent limiting secondary e.m.f. Ealreq below:22 sr Ral alreq tf ct Lswpr rI SE E I R RI Iæ ö³ = × × × + +ç ÷è øEQUATION2241 V2 EN-US (Equation 68)Where:Itf Maximum primary fundamental frequency three-phase fault current that passesthe CTs and 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 relativelysmall and the requirements might result in small CTs. However, in applicationswhere the zero sequence current from the phase side of the transformer is asummation of currents from more than one CT (cable CTs or groups of individualCTs in Holmgren connection) for example, in substations with breaker-and-a-halfor double-busbar double-breaker arrangement or if the transformer has a T-connection to different busbars, there is a risk that the CTs can be exposed forhigher fault currents than the considered phase-to-earth fault currents above.Examples of such cases can be cross-country faults or phase-to-phase faults withhigh fault currents and unsymmetrical distribution of the phase currents betweenthe CTs. The zero sequence fault current level can differ much and is often difficultto calculate or estimate for different cases. To cover these cases, with summation ofzero sequence currents from more than one CT, the phase side CTs must fulfill theRequirement 69 below:2sr Ral alreq f ct Lpr rI SE E I R RI Iæ ö³ = × × + +ç ÷è øEQUATION2242 V2 EN-US (Equation 69)Section 19 1MRK 504 169-UEN ARequirements370 Transformer protection RET650 2.2 IECApplication manual
