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GATE 2016 EE – Question 52

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

If the star side of the star-delta transformer shown in the figure is excited by a negative sequence voltage, then

A star-delta transformer with star terminals A, B, C and neutral N on one side and delta terminals a, b, c on the other, with the winding polarity dots marked.
  1. $V_{AB}$ leads $V_{ab}$ by 60°
  2. $V_{AB}$ lags $V_{ab}$ by 60°
  3. $V_{AB}$ leads $V_{ab}$ by 30°
  4. $V_{AB}$ lags $V_{ab}$ by 30°

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Correct answer: (D) $V_{AB}$ lags $V_{ab}$ by 30°

Explanation

In a star-delta transformer of this connection, the positive sequence voltages on the star side lead those on the delta side by 30°. For negative sequence quantities the phase shift reverses, so the star side voltage $V_{AB}$ lags the delta side voltage $V_{ab}$ by 30°.