In the SLD shown in figure, one transformer is connected close to 6.6kV bus B (negligible line impedance) and two transformers are connected away through a line having impedance of 0.3 ohms on another 6.6 kV bus C. Calculate and provide the following: a) Nominal currents at breakers 1 to 7. b) Short circuit currents at buses A, B, C. (c) CT ratio with 5A secondary for relays/ breakers 1 to 7 with CT saturation at around 20 times the primary current. d) Recommend the values of the current pick-up values and time dial settings of all phase relays to ensure a co-ordinated protection arrangement with discrimination margin of minimum 0.3 sec between successive relays. Assume the following: - Relay characteristics to be followed as per graph below (or any other with which student is familiar) with time dial settings as shown - Transformer loading almost 100% - Relay current pickup available in 1 to 12A in steps of 1A - For the overcurrent relays consider instantaneous trip at 8 times CT secondary value

5 of 5 lo - + Hit to width ID Page view A Read aloud Add notes 6 P X Bus-A 33kV 500 MVA Bus * 1 T1: 33/6.6 kV 6300KVA 7.5% Z X 2 Bus-B x 4 - 0.3 ohms -6.6kv T2: 6.6kV/440V 1600kVA 6% Z Bus-C XX X 7 T3, T4: 11kV/440V 2000kVA 7.5% Z Time (seconds) FOOOOO Time dial setting 0.4 0.3 0.2 0.08 0.07 0.05 0.04 0.03 0.02 20 30 40 3 4 5 6 7 8 9 10 Multiples of tap value current 5 of 5 lo - + Hit to width ID Page view A Read aloud Add notes 6 P X Bus-A 33kV 500 MVA Bus * 1 T1: 33/6.6 kV 6300KVA 7.5% Z X 2 Bus-B x 4 - 0.3 ohms -6.6kv T2: 6.6kV/440V 1600kVA 6% Z Bus-C XX X 7 T3, T4: 11kV/440V 2000kVA 7.5% Z Time (seconds) FOOOOO Time dial setting 0.4 0.3 0.2 0.08 0.07 0.05 0.04 0.03 0.02 20 30 40 3 4 5 6 7 8 9 10 Multiples of tap value current