Answer Posted / ramachandran n

Vector Groups are the IEC method of categorizing the
primary and secondary winding configurations of 3-phase
transformers. Windings can be connected as delta, star
(wye), or interconnected-star (zig-zag). Winding polarity
is also important, since reversing the connections across a
set of windings affects the phase-shift between primary and
secondary. Vector groups identify the winding connections
and polarities of the primary and secondary. From a vector
group one can determine the phase-shift between primary and
secondary.
The first criterion to consider in choosing a vector group
for a distribution transformer for a facility is to know
whether you want a delta-star or star-star. Utilities often
prefer star-star transformers, but these require 4-wire
input feeders and 4-wire output feeders (i.e. incoming and
outgoing neutral conductors). For distribution transformers
within a facility, often delta-star are chosen because
these transformers do not require 4-wire input; a 3-wire
primary feeder circuit suffices to supply a 4-wire
secondary circuit. That's because any zero sequence current
required by the secondary to supply earth faults or
unbalanced loads is supplied by the delta primary winding,
and is not required from the upstream power source. The
method of earthing on the secondary is independent of the
primary for delta-star transformers.
The second criterion to consider is what phase-shift you
want between primary and secondary. For example, Dy11 and
Dy5 transformers are both delta-star. If you don't care
about the phase-shift, then either transformer will do the
job. Phase-shift is important when you are paralleling
sources - you want the phase-shifts of the sources to be
identical. If you are paralleling transformers, then you
want them to have the same the same vector group.
If you are replacing a transformer, use the same vector
group for the new transformer, otherwise the existing VTs
and CTs used for protection and metering will not work
properly.

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