Correct answer: A — Dd1
Decode the vector-group notation
A transformer vector group identifies the winding connections and the phase displacement between the high-voltage and low-voltage voltage systems. The capital letter identifies the HV winding, the lowercase letter identifies the LV winding, and the clock number expresses displacement in multiples of 30°.
Step 1: Interpret Dd. Both windings are delta connected. Each delta line voltage is directly associated with a winding phase voltage, so the connection does not introduce the star–delta 30° conversion.
Step 2: Interpret clock number 1. With the HV reference at 12 o’clock, clock 1 represents the LV reference lagging by 30°:
Phase displacement = 1 × 30° = 30°
Step 3: Compare the required and available displacement. The common delta–delta groups are Dd0 and Dd6, corresponding to 0° and 180°. Conventional delta–delta winding polarity and phase arrangements do not produce the odd 30° displacement demanded by Dd1.
Therefore Dd1 is the invalid connection intended by the question. The clock number must agree with the actual winding phasors; it cannot be attached arbitrarily to any letter combination. Zigzag connections require their own split-winding phasor construction, so the delta–delta argument should not be applied to a group containing z.
