As we know that the secondary voltages of a 3-phase transformer may undergo a phase shift of either +30° leading or -30° lagging or 0° i.e. no phase shift or 180° reversal with respective line or phase to neutral voltages.
On the name plate of a three phase transformer, the vector group is written as Yd11, Dyn11 etc. Typical representation of the vector group could be Yd1 or Dy11 etc.
Here first Capital latter Y indicates that the primary is connected in star and the second lower case latter d indicates delta connection of the secondary side. The third numerical figure says the angle of phase shift based on clock convention.
The minute hand is used to represent the primary phase to neutral voltage and always shown to occupy the position 12. The hour hand represents the secondary phase to neutral voltage and may, depending upon phase shift, occupy position other than 12 as shown in the figure below.
The angle between two consecutive numbers on the clock is 30°.
Consider the figure below.
It can be seen from the figure that the angle between the primary side phase voltage and secondary phase voltage is zero and the Transformer is Star Delta connected. So its nomenclature will be
Yd1 mind that we are giving value 1 for primary which is fixed at 12 o’clock position.
Again, consider the figure below.
As primary – Star
Secondary – Delta
Angle between Primary and Secondary = 30° (leading)
Hence, it is Yd11
If suppose neutral is grounded then we would have Ydn11.