In the SLD shown in next page, one transformer is connected close to 6.6kV bus B (negligible
Question:
In the SLD shown in next page, 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
Fundamentals of Electric Circuits
ISBN: 9780073301150
3rd edition
Authors: Matthew Sadiku, Charles Alexander