Reconsider case (b) of Problem 10.5. Let the load impedance \(4.9+j 0.5 \Omega\) be the input impedance to a \(\mathrm{CO}-7\) induction disc time-delay overcurrent relay. The CO-7 relay characteristic is shown in Figure 10.41. For a tap setting of \(4 \mathrm{~A}\) and a time dial setting of 2, determine the relay operating time.

Problem 10.5

A CT with an excitation curve given in Figure 10.39 has a rated current ratio of 500:5 A and a secondary leakage impedance of \(0.1+j 0.5 \Omega\). Calculate the CT secondary output current and the CT error for the following cases:

(a) The impedance of the terminating device is \(4.9+j 0.5 \Omega\) and the primary CT load current is \(400 \mathrm{~A}\).

(b) The impedance of the terminating device is \(4.9+j 0.5 \Omega\) and the primary CT fault current is \(1200 \mathrm{~A}\).

(c) The impedance of the terminating device is \(14.9+j 1.5 \Omega\) and the primary CT load current is \(400 \mathrm{~A}\).

(d) The impedance of the terminating device is \(14.9+j 1.5 \Omega\) and the primary CT fault current is \(1200 \mathrm{~A}\).

Transcribed Image Text:

Operating time in seconds 7 6 10 A 3 2 ||||| 1/2 2 3 14 8 10 landam 4 Typical time curves type CO-7 overcurrent relay 50-60 cycles Current tap settings: 4,5,6,7,8,10,12 amperes Time dial setting 3 Current as a multiple of tap setting TT al manstur sunt mult 5 6 7 8 9 10 12 14 16 18 20