GATE EE 2026 Set 1 — Question 38
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Electrical Machines → Three-Phase Transformers → Connections & Vector Groups
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Question
Three single-phase transformers are connected to form a three-phase transformer bank with connections as shown.
Considering ABC phase sequence, the vector group of the transformer is:
Correct answer
(D) Dd10
Solution
1.HV Side (Delta): Phase A is connected to the dot of the first winding, and Phase B is connected to the non-dot. Thus, the voltage across the first HV winding is .
2.LV Side (Delta): Terminal 'a' is connected to the dot of the first winding, and terminal 'c' is connected to the non-dot. Thus, the voltage across the first LV winding is .
3.Phase Relationship: Since the windings are on the same core, (LV) is in phase with (HV). Let (Reference at 12 o'clock). Then .
4.Line Voltage Calculation: In a balanced delta, . For the second winding, terminal 'b' is at the dot and 'a' is at the non-dot, so is in phase with . Since , we have , which implies .
5.Vector Group: The LV line voltage leads the HV line voltage by . In clock notation, a lead corresponds to 10 o'clock. Therefore, the vector group is Dd10.
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