It is to be carried out to check whether the transformer
under test is suitable for parallel operation with another
transfomer having same vector group and voltage ratio but
difference in KVA rating.

1,Vector group is done on transformer High voltage winding
with respect low voltage winding.
2,To find the phase angle/difference between different
connections in high voltage windings.
3,In three phase winding transformer there are 36 vector
groups.(1u1v1w 2u2v2w)

vector group are made to know at what position the secondary
winding of transformer we get star and delta voltages . and
it indicates the type of winding in primary and secondary
which helps in parallel operation of transformer with
different ratings having same vector group .

Mostly using transformer phasing arrangements are
DYn11,DYn7,DYn5.This is related to phase
displacements.vector group test is doin for analaysing
transformer phaseangles as per the phasing arrangements
like DYn11 or DYn7 or ...

Vector Group can be used as basis in setting the phase
compensator factor for numerical transformer differential
relays, i.e. SEL387.
A 3-Ph Transformer is wired internally in different ways to
produce a desired phase position between primary and
secondary terminals.
IEC standards develop a transformer circuit configuration
and a corresponding vector group code which follows a clock
that represents a 30 degrees shift in every hour.
The CT's of a numerical transformer differential relays are
usually wired with its polarity mark facing away from the
protected winding. Because of the phase shift between the
primary and secondary terminals of the transformer it is
necessary to set a phase compensator factor to the
differential relay so that it will not trip on normal load.

Transformer vector relationship is tested by shorting the
primary A phase to secondary A phase, then applyying a
three phase voltage to the primary side of the transformer.
Voltage measurements are then taken as follows:

A-B
B-C
C-A
B-b
C-b
B-c
C-c

These values will then allow you to draw the vector diagram
of the transformer.