sr Ral alreq rt ct L 2pr rI SE E 30 I R RI Iæ ö³ = × × × + +ç ÷è øEQUATION1412 V2 EN-US (Equation 135)where:Irt The rated primary current of the power transformer (A)22.1.6.2 Distance protection M11619-3 v5The current transformers must have a rated equivalent limiting secondary e.m.f. Ealthat is larger than the maximum of the required rated equivalent limiting secondarye.m.f. Ealreq below: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 136)kzone1 sr Ral alreq ct L 2pr rI I SE E k R RI I× æ ö³ = × × + +ç ÷è øEQUATION1081 V2 EN-US (Equation 137)where:Ikmax Maximum primary fundamental frequency current for close-in forward andreverse faults (A)Ikzone1 Maximum primary fundamental frequency current for faults at the end of zone 1reach (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). In solidly earthedsystems the loop resistance containing the phase and neutral wires should beused for phase-to-earth faults and the resistance of the phase wire should beused for three-phase faults.In isolated or high impedance earthed systems the resistance of the singlesecondary wire can always be used.SR The burden of an IED current input channel (VA). SR=0.020 VA/channel for Ir=1A and SR=0.150 VA/channel for Ir=5 ATable continues on next page1MRK 505 393-UEN B Section 22RequirementsLine differential protection RED650 2.2 IEC 429Application manual
