A 22 kV switchboard receives power supply from the Power Grid and the supply voltage is stepped down to supply a 400 V switchboard as shown in Figure 1. A 400 V feeder is used to feed to Sub-Switchboard. Determine the short circuit withstand current rating of the main incomer of the sub-switchboard using the information below: (a) The short time withstand current rating at the 22 kV switchboard is 16 kA. The source resistance is negligible compared with the source reactance. (b) Impedance of 22 kV cable r = 0.2 m2/m and x = j0.3 m2/m, length of 22 kV cable is 50 m. (c) Percentage reactance of the 1 MVA, 22 kV / 400 V transformer is j5%. (d) 400 V feeder to the sub-switchboard uses 2 sets of 300 mm four-core XLPE- insulated copper cables. Length of cable is 80 m. (30 Marks) Ignore all impedances not stated in the question. Standard short circuit breaking capacities of Circuit Breaker are: 10 kA, 16 kA, 25 KA, 32 KA, 40 KA, 50 kA, 75 kA, 100 kA. --X- 22 kV switchboard Short circuit withstand current rating = 16 kA 22 kV cable, 50 m, r = 0.2 mo/m, x= j0.3 m/m 1 MVA 22 kV/400V Impedancel=j5% 400 V switchboard Figure 1: Electrical distribution system 2 sets of 300 mm XLPE, four-core, copper cable, 80 m long Legend Sub- switchboard 22 kV / 400 V transformer Distribution Board X Circuit Breaker