GATE 2017 EE – Question 58
A three-phase, three winding $\Delta/\Delta/Y$ (1.1 kV/6.6 kV/400 V) transformer is energized from AC mains at the 1.1 kV side. It supplies 900 kVA load at 0.8 power factor lag from the 6.6 kV winding and 300 kVA load at 0.6 power factor lag from the 400 V winding. The RMS line current in ampere drawn by the 1.1 kV winding from the mains is ________. (Give the answer up to one decimal place.)
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Correct answer: 620 to 630
Explanation
The two loads add as complex powers. The 6.6 kV load is $900(0.8 + j0.6) = 720 + j540$ kVA, and the 400 V load is $300(0.6 + j0.8) = 180 + j240$ kVA. The total is $900 + j780$ kVA, whose magnitude is $\sqrt{900^2 + 780^2} = 1191$ kVA. The primary line current is $\frac{1191 \times 10^3}{\sqrt{3} \times 1100} = 625$ A.