If there are 3 transformers connected wye the neutral is required or
else the neutral point will float and the voltage across the 3
transformers will be different depending on the secondary load.
(If there are 2 transformers - Scott or T connection - I don't think you
need the neutral.)
The secondary system will be ungrounded?
It's a single 3-phase transformer labeled for use as 480 (delta) -to- 208
(wye) step-down. Which, reading your statement above, means that phase
voltages will be different in the primary and secondary depending on loading.
Common ground between supply and load, at the transformer chassis.
If the transformer has 3 sets of coils I'm going to change my answer. I
think the connection on the delta side would force the voltages on the
wye side to be substantially equal. I think the voltages on the delta
side would be a little closer to equal with a connected neutral and
losses would be lower since the delta side is not forcing a balance on
the wye side. Anyone with a better theory?
Particularly some small 3 phase transformer units are made with 2 sets
of coils with a Scott/T connection which could be connected without a
The transformer 480V delta side would be ungrounded unless you "corner"
ground one of the phases.
I think that is correct. Any 3-phase transformer
with a delta on one side (pri or sec) has the benefit
that the delta winding internally circulates third
harmonic currents and balances the wye-side so that
the neutral is reasonably stable. This means that
the star point may or may not be earthed and, (if it
is), does not require a heavy gauge connection. It
also means that a wye-connected secondary allows a
4-wire supply that can have simultaneous balanced and
unbalanced loads without much penalty.
Contrast that with a wye-wye transformer, where the
neutrals can be very unstable, requiring heavy gauge
In the UK the delta-wye is the standard 11KV 3-wire
to 240V (line-neutral) 4-wire configuration for
domestic and industrial distribution.
Be aware that doing this results in a wye-delta transformer configuration.
This has a problem that if you lose a phase, the transformer will try to
power the lost phase from the other two. Therefore, expect the breaker to
pop whenever you lose a phase, when it attempts to backfeed the local
On the other hand, if you don't connect the wye common point to the
neutral, it won't do this. Even if the load on the delta side is
unbalanced, the configuration won't try to generate any neutral current,
it happens to work out. You'll still have single phasing, of course.
But before you do this, run it by someone with more knowledge of these
things than myself, as well as whether code allows this. I do know
that a local electric company does this when powering an old segment of
delta-connected distribution system from a higher voltage wye system.
The common connection of the primarys don't connect to the neutral.
Neutral on the wye is mandatory.
There is no neutral on the delta side.
Each delta phase is feed from two of the hots.
Instead of having three power circuits like AN, BN, CN, you get AB, BC, CA.
If you tie any delta phase to ground you will be chasing voltages on the
WELL BY PROVIDING THE NEUTRAL ON THE STAR POINT,WE CAN REMOVE THE 3RD
HARMONICS .THE THIRD HARMONICS IF NOT GROUNDED COULD RESULT IN HEAVY
CURRENT FLOW THAT CAN DAMAGE THE TRANSFORMER WINDINGS AND MAY ALSO
TRIP THE WHOLE POWER SYSTEM
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