GATE 2018 EE – Question 61
A 3-phase 900 kVA, $3\text{ kV}/\sqrt3$ kV ($\Delta/Y$), 50 Hz transformer has primary (high voltage side) resistance per phase of $0.3\ \Omega$ and secondary (low voltage side) resistance per phase of $0.02\ \Omega$. Iron loss of the transformer is 10 kW. The full load % efficiency of the transformer operated at unity power factor is ________ (up to 2 decimal places).
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Correct answer: 96.91 to 97.89
Explanation
The primary phase current is $\frac{900\times10^3}{3\times3000}=100$ A, so the primary copper loss is $3\times100^2\times0.3=9$ kW. The secondary phase voltage is $1000$ V and its current is $\frac{900\times10^3}{3\times1000}=300$ A, giving $3\times300^2\times0.02=5.4$ kW. The total loss is $10+9+5.4=24.4$ kW, so $\eta=\frac{900}{900+24.4}=97.36\%$.