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GATE 2014 EE (EE1) – Question 26

Electrical Machines · Three-phase transformers: connections, vector groups, parallel operation · 1 mark · Multiple choice

The two-bus system and its zero-sequence network are shown. Select transformer winding connections, ordered from left to right in each transformer.

Diagram for GATE 2014 EE (EE1) question 26
  1. T₁: star–star; T₂: star–delta
  2. T₁: grounded star–star; T₂: grounded star–delta
  3. T₁: grounded star–delta; T₂: delta–grounded star
  4. T₁: delta–grounded star; T₂: star–grounded star

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Show answer and explanation

Correct answer: (B) T₁: grounded star–star; T₂: grounded star–delta

Explanation

At T1the grounded bus-side star provides a zero-sequence path through its leakage reactance, but the ungrounded line-side star blocks continuation to the line. At T2the line-side grounded star has a return path through its delta winding, while zero-sequence line current cannot enter the delta-connected bus side. This gives **T1=grounded-star/ungrounded-star and T2=grounded-star/delta,B**. The tiny gaps in the zero-sequence diagram are open connections.