where:Irt The rated primary current of the power transformer (A)Itf Maximum primary fundamental frequency current that passes two mainCTs and the power transformer (A)Ipr The rated primary CT current (A)Isr The rated secondary CT current (A)Ir The rated current of the protection IED (A)Rct The secondary resistance of the CT (W)RL The resistance of the secondary wire and additional load (W). The loopresistance containing the phase and neutral wires must be used for faultsin solidly earthed systems. The resistance of a single secondary wire shouldbe used for faults in high impedance earthed systems.SR The burden of an IED current input channel (VA). SR=0.020 VA/channel forIr=1 A and SR=0.150 VA/channel for Ir=5 AIn substations with breaker-and-a-half or double-busbar double-breaker arrangement, thefault current may pass two main CTs for the transformer differential protection withoutpassing the power transformer. In such cases and if both main CTs have equal ratios andmagnetization characteristics the CTs must satisfy equation 92 and equation 94.sr Ral alreq f ct L 2pr rI SE E I R RI Iæ ö³ = × × + +ç ÷è øEQUATION1414 V2 EN-US (Equation 94)where:If Maximum primary fundamental frequency current that passes two main CTs without passing thepower transformer (A)18.1.6.2 Distance protection M11619-3 v5The current transformers must have a rated equivalent limiting secondary e.m.f. Eal that islarger than the maximum of the required rated equivalent limiting secondary e.m.f. Ealreqbelow:k max sr Ral alreq ct L 2pr rI I SE E a R RI I× æ ö³ = × × + +ç ÷è øEQUATION1080 V2 EN-US (Equation 95)kzone1 sr Ral alreq ct L 2pr rI I SE E k R RI I× æ ö³ = × × + +ç ÷è øEQUATION1081 V2 EN-US (Equation 96)Section 18 1MRK 504 158-UEN ARequirements266Application manual
