10-2 L90 Line Current Differential System GE Multilin10.1 CT REQUIREMENTS 10 APPLICATION OF SETTINGS10(EQ 10.3)3. This gives a total burden of:. (EQ 10.4)4. This is less than the allowed 3 :, which is OK.The following procedure verifies the kneepoint voltage.1. The maximum voltage available from the .2. The system X/R ratio .3. The CT voltage for maximum phase fault is:(EQ 10.5)4. The CT voltage for maximum ground fault is:(EQ 10.6)5. The CT will provide acceptable performance in this application.10.1.3 CALCULATION EXAMPLE 2To check the performance of an IEC CT of class 5P20, 15 VA, ratio 1500:5 A, assume the following values:• maximum Ifp = 14 000 A• maximum Ifg = 12 000 A• impedance angle of source and line = 78°• CT secondary leads are 75 m of AWG 10.The IEC rating requires the CT deliver up to 20 times the rated secondary current without exceeding a maximum ratio errorof 5%, to a burden of:(EQ 10.7)The total Burden = Rr + Rl = 0.008 + 0.52 = 0.528 :, which is less than the allowed 0.6 :, which is OK.The following procedure verifies the kneepoint voltage.1. The maximum voltage available from the .2. The system X/R ratio .3. The CT voltage for maximum phase fault is:(EQ 10.8)4. The CT voltage for maximum ground fault is:(EQ 10.9)5. The CT will provide acceptable performance in this application.R CT 0.75 :R r0.2 VA5 A 2------------------ 0.008 :R L 2 75 m 3.75 :1000 m--------------------uu 2 0.26 :u 0.528 :Total Burden R CT R r R L 0.75 : 0.008 : 0.52 : 1.28 :CT 1500 2000e 400u 300 V78qtan 4.71V 14000 Aratio of 300:1----------------------------------- 4.71 1 0.75 0.26 0.008 :uu 271.26 V (< 300 V, which is OK)V 12000 Aratio of 300:1----------------------------------- 4.71 1 0.75 0.52 0.008 :uu 291.89 V (< 300 V, which is OK)Burden 15 VA5 A 2---------------- 0.6 : at the 5 A rated currentCT 1500 2000e 400u 300 V78qtan 4.71V 14000 Aratio of 300:1----------------------------------- 4.71 1 0.75 0.26 0.008 :uu 271.26 V (< 300 V, which is OK)V 12000 Aratio of 300:1----------------------------------- 4.71 1 0.75 0.52 0.008 :uu 291.89 V (< 300 V, which is OK)
