GATE 2015 EE (EE2) – Question 61

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

The two three-phase transformer banks are built from single-phase units with the shown winding polarities. Find the phase-difference magnitude between V1 and V2 in degrees.

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Correct answer: 30

Explanation

Both banks have the same primary delta connection. The upper secondary is delta, so V1 follows a secondary winding voltage. The lower secondary is star, so V2 is the difference of two phase winding voltages.

For a balanced sequence, the difference of equal phasors separated by 120° has √3 times the magnitude and a 30° displacement. The shown polarities therefore give an inter-bank line-voltage phase difference of **30°**.

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