Question (a): An 11 kV, single phase system is protected by a scheme using 1000/5 CTs and an IDMT relay having a plug setting of 150% and time multiplier setting of 0.68 (FIGURE 3).

Using the standard PSM/time characteristic, find the relay operating time (TMS = 1) for a 236 MVA fault.

(b) The circuit breakers CBI, CB2, CB; and 034 are part of a scheme which uses 1000/5 CTs and IDMT relays.

If the maximum fault current capability is 6700 amps, clearance of which must be initiated in 2.25 secs, select, using the standard characteristic:

(i) plug setting multipliers for CBI at plug settings of 100% and 50%

(ii) the time multiplier setting of each IDMT relay to afford effective discrimination of 0.4 sec at a plug setting of 50%.