b) Radial protection systems are protected by time-delay overcurrent relays. Figure Q3b)i shows a 50 Hz, single line diagram with 34.5 kV (line to line) at all busses. Assume 0.5 s is the time interval between breakers. Define the meaning of backup protection in single line diagram. i. ii. [2 marks] Examine the Current Tap Setting and Time Dial Setting for each relay connected to the breakers as shown in Figure Q3b)i by referring the data in the Table Q3b)i and Table Q3b)ii. [15 marks] Breaker B1 B2 B3 Bus 1 2 3 MVA 11 9 6 Table Q3b)i Max fault current (A) 5k 2k 1k Table Q3b)ii Breaker operating time CT ratio 7 cycles 600:5 400:5 200:5 5 cycles 3 cycles Relay CO-7 CO-7 CO-8 1 B1 B2 00:15:4 P2 Operating time in seconds L1 Figure Q3b)i 5 2 3 Current as a multiple of tap setting Time dial setting L2 B3 Current tap settings: 4,5,6,7,8,10,12 amperes P1 Typical time curves type CO-7 over current relay 50-60 cycles 6 7 8 9 10 12 14 16 18 20 3 L3