GATE 2017 EE (EE2) – Question 55

Electrical Machines · Three-phase transformers: connections, vector groups, parallel operation · 2 marks · Numerical answer

A 25 kVA Y–delta transformer has primary line voltage 3.3 kV on the Y side and per-phase turns ratio 5:1. Its secondary line-current rating in amperes is ________.

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Correct answer: 37 to 39

Explanation

The primary winding voltage is 3300/√3 V. Dividing by the phase turns ratio gives secondary winding voltage $3300/(5\sqrt3)$ V.

On the delta side winding voltage equals line voltage. Use three-phase apparent power: $I_{L2}=25000/[\sqrt3\cdot3300/(5\sqrt3)]$.

This simplifies to 25000(5)/3300=**37.8788 A**, in the official 37–39 interval.

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