Re: delta-delta, wye-wye, delta-wye, wye-delta transformer connections, which?

[Don Kelly]

I have a number of question why I hope electrical engineers can shed
light on:

1. What is generally the ideal 3-phase transformer connection for a
2400 primary and 240 secondary?
2. I am told that the advantage of the wye connections is its grounded
neutral. However, is it really necessary to have system grounding?
3. If loads are unbalanced, which connection is generally more
advantageous?
4. Safety-wise, which connection is more advantageous?

delta delta- possible and works well- no ground can mean that the neutral on
the secondary floats. That is not good from a safety basis. This is also
true of a wye delta. Single phase voltages will be line to line so if you
have a 240V line to line secondary you can handle 240V single phase loads.
If you center tap one winding to ground you can handle 120V loads but suh
loads are limited to one leg.

Wye-wye is not used as there is no path for triplen harmonics in the
magnetising current and there will be 3rd and multiples of 3rd harmonics in
the voltage waveform. Not wanted. (where wye-wye is used, there is usually
a tertiary winding which is in delta- this takes care of this problem)

Delta-wye with the delta on the high side provides for a grounded neutral
and elimination of floating voltages Single phase loads can be taken off any
leg to neutral and can be balanced between phases- this is also an
advantage. Thus you get 208/120V service. You can use either 3 single
phase transformers or a single 3 phase transformer for each connection. At
this voltage level single phase units are everywhere so that it is often
advantageous to use 3 single phase transformers.

Overall, except in very specific situations, a grounded secondary provides
safety benefits.
Unless there is a good reason to do otherwise- the delta HV/Wye LV is
generally the best choice for this level of distribution.

Backfeed has been mentioned (wye/delta transformer with one HV leg
open ) -possible but probably a non -issue in general. There will be an
associated inbalance due to impedance differences and, while there may be
some loading on the transformers, it will not appear on the energy bill.

For high voltage lines the HV is usually wye and the LV of the transformers
are delta -for several advantages which result -on a delta HV, atmospheric
charges can cause flashovers, ground faults can produce severe damage
without detectable changes in line currents (fault fed through line
capacitance), cost advantage to transformers as the insulation can be graded
from hot end to ground rather than be the same throughout.


Don Kelly
[email protected]
remove the urine to answer