Question 1 - High-impedance voltage-differential relays are to be applied to protect a three-breaker bus, as shown in Figure 10.9. The CTs are all 600:5 multiratio type with characteristics per Figure 5.10. For this application, determine the relay-pickup setting voltage and the minimum primary-fault current for which the relays will operate. The maximum external fault is 8000 A rms. Assume that the lead resistance RL = 0.510 Ω for the maximum resistance from any CT to the junction point.
For the particular relays applied, the pickup setting voltage is
VR = 1.6k(Rs +pRL) (IF/N)V
where 1.6 is a margin factor, k is a CT performance factor (assume k= 0.7 for this problem), p =1 for three-phase faults and p= 2 for single-phase-to-ground faults (Figure 5.9), IF is the primary rms external maximum fault current, and N and CT ratio. Rs is the CT resistance. p= 2 should be used to determine the value of the VR setting.
The maximum setting of the relay voltage element should not exceed 0.67 times the secondary exciting voltage of the poorest CT in the differential circuit at 10 A exciting current.
The minimum internal fault primary current to operate the relays is
Imin = (nIe + IR + IT)N primary amperes,
where n is the number of circuits, Ie is the exciting current of the individual CT at the pickup voltage, IR is the relay current at the pickup setting voltage, and IT is the current required by a high voltage protective device across the relay coil (not shown in Figure 10.9). For this problem, assume IT = 0.2 A. The relay impedance and generally negligible resistance of the leads from the junction to the relay is 1700 Ω. nIe is applicable in this problem since all three breaker CTs are the same; otherwise this is a summation of the different CT exciting currents at the VR pickup voltage.
Question 2 - A feeder circuit is added to the bus of Problem 10.1, making a four-circuit bus. The new breaker has the same type 600:5 multiratio CTs. With this addition, the maximum external fault increases to 10,000 A rms. All other circuit values remain the same. For this change, calculate the relay-pickup setting voltage and the minimum primary-fault current for which the relays will operate.
Textbook - Protective Relaying Principles and Applications Third Edition, by J. Lewis Blackburn.
Attachment:- Assignment Files.rar