Vector Group Transformer

Transformer nameplates carry a vector group reference such at Yy0, Yd1, Dyn11 etc.  This relatively simple nomenclature provides important information about the way in which three phase windings are connected and any phase displacement that occurs.
Winding Connections
HV windings are designated:   Y, D or Z (upper case)
LV windings are designated:    y, d or z (lower case)
Where:
Y or y indicates a star connection
D or d indicates a delta connection
Z or z indicates a zigzag connection
N or n indicates that the neutral point is brought out
Phase Displacement
The digits ( 0, 1, 11 etc) relate to the phase displacement between the HV and LV windings using a clock face notation.  The phasor representing the HV winding is taken as reference and set at 12 o'clock.  It then follows that:
Digit 0 means that the LV phasor is in phase with the HV phasor
Digit 1 that it lags by 30 degrees
Digit 11 that it leads by 30 degrees
etc
All references are taken from phase-to-neutral and assume a counter-clockwise phase rotation.  The neutral point may be real (as in a star connection) or imaginary (as in a delta connection)
When transformers are operated in parallel it is important that any phase shift is the same through each.  Paralleling typically occurs when transformers are located at one site and connected to a common busbar (banked) or located at different sites with the secondary terminals connected via distribution or transmission circuits consisting of cables and overhead lines 
Basic Theory
An ac voltage applied to a coil will induce a voltage in a second coil where the two are linked by a magnetic path.  The phase relationship of the two voltages depends upon which way round the coils are connected.  The voltages will either be in-phase or displaced by 180 deg as below:

 in phase

180 deg displacement

When 3 coils are used in a 3 phase transformer winding a number of options exist.  The coil voltages can be in phase or displaced as above with the coils connected in star or delta and, in the case of a star winding, have the star point (neutral) brought out to an external terminal or not.
Example -  Dyn11
We now know that this transformer has a delta connected primary winding (D) a star connected secondary (y) with the star point brought out (n) and a phase shift of 30 deg leading (11).  Connections and vector diagrams are as follows::

HV

LV

Other Configurations
By connecting the ends of the windings in other ways a wide range of options becomes available as set out below.

Phase shift (deg)	Connections
0	Yy0	Dd0	Dz0
30 lag	Yd1	Dy1	Yz1
60 lag	Dd2	Dz2
120 lag	Dd4	Dz4
150 lag	Yd5	Dy5	Yz5
180 lag	Yy6	Dd6	Dz6
150 lead	Yd7	Dy7	Yz7
120 lead	Dd8	Dz8
60 lead	Dd10	Dz10
30 lead	Yd11	Dy11	Yz11