Answer / avnish

above answers are correct but as u asked shraddha that will
hv lead or lv so i clear that capital D is for secondary and
small forthe primary and again 11 ....means that hv have the
30 degree leading phase diffrence..

Answer / ghost rider

Abobe two reply explained correctly "what is vector group".
The answer to the last question "why it is needed" is as
follows:-
By virtue of constrction & terminal connections of the
transformer, a difference between primary vector &
secondery vector occurs. We need to know the same for the
following reasons:-

1)Matching of vector group is required (to avoid
circulating current/ fault ), when 2 transformer need to be
operate parallal on a common bus/system.

2) to provide transformer differential protection correctly.

etc

Answer / strateia

The Dyn11 is interpreted as follows:
D-primary winding is delta connected (i.e the three primary
phase windings within the transformer)
y-the secondary winding is delta connected

11-is a multiplying factor for the phase displacement
between the primary and secondary voltages
i.e. for the Dyn11, phase displacement is
= 11*30degrees
= 330 degrees

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