GATE 2026 EE – Question 38
Three single-phase 11 kV/3.3 kV 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:
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Show answer and explanation
Correct answer: (D) Dd10
Explanation
Both sides are delta-connected. Tracing the HV delta, each winding's dot end is at A, B, C and its other end goes to the next phase, so the winding voltages are $V_A-V_B$, $V_B-V_C$, $V_C-V_A$. On the LV side, the dotted ends are at $a,b,c$ and the other ends go to $c,a,b$ respectively, so the winding voltages are $V_a-V_c$, $V_b-V_a$, $V_c-V_b$. Equating corresponding windings, $V_a-V_c\propto V_A-V_B$. For positive sequence, $V_A-V_B=\sqrt3V_Ae^{j30^\circ}$ and $V_a-V_c=\sqrt3V_ae^{-j30^\circ}$, so $V_a$ leads $V_A$ by $60^\circ$, i.e. the LV lags by $-60^\circ$, which in clock notation is 10 o'clock: Dd10.