Three single-phase 11kV/3.3kV transformers are connected to form a three phase transformer bank with connections as shown. Considering ABC phase se quence, the vector group of the transformer is:

The correct vector group of the transformer is Dd0.

To determine the vector group, we analyze the connections of both the High Voltage (HV) and Low Voltage (LV) sides of the transformer bank step-by-step:

1. Analysis of the HV Side Connections:
Let the three windings on the HV side be on Leg 1 (top), Leg 2 (middle), and Leg 3 (bottom) respectively. Each winding has a polarity dot marked on its left end.
- The dotted terminal of the top winding is connected to phase line A, and its non-dotted terminal is connected to phase line B (which is also connected to the dot of the middle winding).
- The dotted terminal of the middle winding is connected to phase line B, and its non-dotted terminal is connected to phase line C (which is also connected to the dot of the bottom winding).
- The dotted terminal of the bottom winding is connected to phase line C, and its non-dotted terminal is looped back to phase line A.
This forms a closed delta connection. Since it is on the HV side, we represent this configuration with the capital letter D.

2. Analysis of the LV Side Connections:
On the LV side, the three windings on the corresponding legs also form a delta connection. The terminals are labeled a, b, and c. Since it is on the LV side, we represent this configuration with the lowercase letter d.

3. Determination of Phase Displacement:
To find the phase displacement between the HV and LV line voltages, we compare the voltages across corresponding windings on the same core leg:
- On the top leg (Leg 1), the HV winding is connected between terminals A and B, with the polarity dot at A. Thus, the voltage across the HV winding is:
$V_{\mathrm{AB}}$
- On the same top leg (Leg 1), the LV winding is connected between terminals a and b. The polarity dot is on the right side at terminal a. Thus, the voltage across the LV winding is:
$v_{\mathrm{ab}}$
Since the polarity dots of both the HV and LV windings on Leg 1 are connected to the corresponding terminals of the phase sequence (A to a, B to b), the voltage
$v_{\mathrm{ab}}$
is exactly in phase with
$V_{\mathrm{AB}}$
without any phase shift.

Because there is a 0° phase displacement between the primary and secondary line voltages, the vector group corresponds to the 12 o'clock position (0). Combining the connections and the phase shift, we get the vector group designation Dd0.