where:Z Zs Zl Zs ZlZs Zl Zs Zl ZresA B BA a B Btrfa= +( ) × +( )+ + + + = 198ΩEQUATION14000039 V1 EN-US (Equation 24)The numerical value for Zres input in the formula for If138 gives If138 = 403 ATo avoid unwanted operation of the differential protection for a fault on the LV sideof the transformer, IdMin must be set to:> ×1 2 138. IfIBaseEQUATION14000038 V1 EN-US (Equation 25)IdMin IfIBase= × =1 2 11 5138. .EQUATION14000040 V1 EN-US (Equation 26)To allow the differential protection to act as a backup protection for internal faultsand faults on the LV side of the transformer, AddDelay is set to On, and a suitablesetting is calculated for Imax AddDelay. Differential currents below the set valuefor Imax AddDelay are time delayed. Value two times the rated current of thetransformer on the HV side is chosen. The setting is calculated as:ImaxAddDelay SU Inn Base= × × × =2 3 2 0.EQUATION14000041 V1 EN-US (Equation 27)Table 12:Backup protection example dataStation Tap transformer 10 MVAType of fault Three phase faultObject 1 = 10 kV Line A 2 = T1 10 kV 50/51 3 = 130 kV 87L(ImaxAddDelay)Setting 2 MVA 14 MVA 20 MVACharacteristic IEC Extr.inverse, k=0.5,A=80, p=2IEC Norm.inversek=0.12, A=0.14, p=0.02IEC Norm.inversek=0.18, A=0.14, p=0.02High set stage 14 MVA - - - - - - - -It is important to achieve a proper back-up protection. The short circuit protectionon the outgoing bays and on the LV side of the transformer are set according to aprepared selectivity chart. An example in Figure 48 shows that the setting of theshort circuit protection on the LV side is 14 MVA, and Normal Inverse has k=0.12to give back-up to outgoing bays' relays which are extremely inverse and selectiveto remote fuses.1MRK 505 393-UEN B Section 6Differential protectionLine differential protection RED650 2.2 IEC 103Application manual
