Compute the primary current and CT error for (a) I = 10 A and ZB=10 ohm and for (b) I=13 A and ZB=1.3 ohm. (c) For reliable relay operation, the fault to relay pickup current ratio with minimum fault current should be greater than 2. Determine the minimum fault current for application of this CT and relay with 5 A pickup setting.

Excitation curves of a C100 current transformer is provided below: 500 100 CT Secondary esistance ratio 6005 505 0.061 00:5 0.082 150:5 0.104 200:5 0.125 250:5 0.146 400:5 300:5 0.168 0.211 400:5 450:5 0.230 50:5 500:5 0.242 600:5 0.296 150:5 200:5 250:5 0.1 0.01 0.00 0.1 10 Secondary exciting amps-l. An equivalent circuit of a current transformer is shown below, where le is the excitation current shown in the Xaxis ofthe above figure, ZB is the burden and I' is the secondary current that flows thru the relay. 1 :n Excitation curves of a C100 current transformer is provided below: 500 100 CT Secondary esistance ratio 6005 505 0.061 00:5 0.082 150:5 0.104 200:5 0.125 250:5 0.146 400:5 300:5 0.168 0.211 400:5 450:5 0.230 50:5 500:5 0.242 600:5 0.296 150:5 200:5 250:5 0.1 0.01 0.00 0.1 10 Secondary exciting amps-l. An equivalent circuit of a current transformer is shown below, where le is the excitation current shown in the Xaxis ofthe above figure, ZB is the burden and I' is the secondary current that flows thru the relay. 1 :n