GATE 2016 EE (EE2) – Question 61

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

Three single-phase transformers form a 110 kV/11 kV delta–star bank delivering 8 MW at 0.8 lagging pf at 11 kV. Neglect losses. The ratio of delta-side phase current to star-side phase current is

$1:10\sqrt3$
$10\sqrt3:1$
$1:10$
$\sqrt3:10$

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Show answer and explanation

Correct answer: (A) $1:10\sqrt3$

Explanation

The line-voltage ratio is 10, so line currents have inverse ratio 1:10. In delta, phase current equals line current divided by √3; in star, phase and line currents are equal.

Therefore IΔ,phase/IY,phase=(1/10)/√3. The ratio is **1:10√3, A**. The load magnitude and pf cancel in this ratio.

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