Question 3 A proposed 132kV/33kV substation is to be equipped with two 90MVA Star/Delta transformers which will operate in parallel at the 33kV busbar and will be supplied as transformer feeders from a 400/132kV Grid supply point, as shown in the diagram below. The connection will be by 20km of 175mm² ACSR double circuit overhead steel tower line; the line has positive sequence reactance of 0.225% per km and a zero sequence reactance of 0.587% per km. The positive sequence impedance of the transformers is expected to be 25% and the zero sequence impedance 21.25%. All impedances are quoted to a base of 100MVA. The negative sequence impedance equals the positive sequence impedance for all plant. The star point of each transformer will be connected directly to Earth. The fault level at the grid supply point is 2500MVA and the earth fault current is 13122A (equivalent to 3000MVA). Calculate for a line to ground short circuit on the red phase 132kV bushing of one transformer the current in amperes that would flow in: (a) each phase of the high voltage windings of each transformer; (b) each transformer star point, and (c) the red phase of the supply circuit. Calculate the zero sequence reactance in ohms of zigzag connected earthing transformers connected to the 33kV bushings of the main transformers which would limit the earth fault current on the 33kV busbar to 3150A. HELPING ADVICE: • The zero sequence impedance of the source is derived from the Earth Fault current and the fact that the impedance for an earth fault is the sum of the positive, negative and zero sequence impedances. • Use of the relationship (a+a)= -1 where a is the 120 degree operator should simplify your calculations. 132kV 33kV

Fig: 1