b) A 33 kV busbar having a short-circuit level of 4X0 MVA is connected to a 5 MVA 33/11 kV transformer as shown in Figure Q4.1. A three-phase fault on the 11 kV feeder produces a short circuit current of 12.XY KA. In the previous information, X and Y are the last two digits of your student number. 33 kV 11 kV CT 400/1 CT 500/1 33/11 kV CB CB 5 MVA Relay Relay Fault Figure Q4.1. Network for problem 4b). For example, if your student number is c1700123, then: Short circuit level is 420 MVA and short circuit current is 12.23 kA. (i) Calculate the three-phase short circuit current for a fault on the 33 kV terminals of the transformer. (ii) What is the short circuit level of the 11 kV busbar? (i) The 11 kV relay is a standard 2 A, IDMT over-current relay with a plug setting of 150% and a time multiplier setting of 0.75. Use theomally inverse curve characteristic shown in Figure Q4.2 to calcufate the operating time for a three-phase fault on the 11 kV feeder. 30 TMS-1.0 10 1.0 2.0 3.0 5.0 10 20 Current as multiple of the relay setting (plug setting multiplier) Figure Q4.2. Normally inverse characteristic of a relay (time multiplier setting of 1.0). Time (s)