| I have a number of question why I hope electrical engineers can shed
| light on:
I'm not an EE, but I've been reading a bunch of stuff here and other
places, and maybe my different perspective could help make a better
| 1. What is generally the ideal 3-phase transformer connection for a
| 2400 primary and 240 secondary?
Is your power coming in 2400 delta, or 4160/2400 wye (it matters)?
| 2. I am told that the advantage of the wye connections is its grounded
| neutral. However, is it really necessary to have system grounding?
A grounded neutral is definitely a benefit, at least on the last transformer.
It's not essential, and there are tradeoffs that can in many cases favor
not having one. Only qualified personnel should ever work on ungrounded
or resistive grounded systems. They can be made safe, but customary
practices for grounded systems can put people at risk on ungrounded
| 3. If loads are unbalanced, which connection is generally more
I'm not aware that this would make that much of a difference. But a delta
secondary would let the other phases supply some power to an overloaded
phase, at the tradeoff of a poor power factor on those phases. But I don't
know that this is a good thing. If your loads are (nearly) all single
phase, then a third solution (see below) might be better.
| 4. Safety-wise, which connection is more advantageous?
I'd go for the wye secondary. You can always connect a delta load to a
wye secondary if the voltages are right. For a 240 volt delta load, you'd
have to have a 240Y/139 volt system to do it, which is probably why the
240 delta has not been phased out as rapidly as 480 delta.
When you have a wye-delta, you can also get backfed power when one phase
goes out. The 2 live secondary windings will energize the 3rd, which in
turn energizes the coupled primary phase at primary voltage, feeding the
primary lines. Then anyone else who has a delta-wye transformer on that
same primary circuit will be drawing that power from your transformer.
If there's enough current in that, your transformer primary fuse will
blow, and you'll have to wait for additional power company service before
your power is restored. If your choice of wye-delta is overriding the
power company choice of delta-wye, you may be obligated to pay for the
fuses and labor to replace them at a few hundred dollars a pop (the fuses
are not cheap, nor are the people).
If you can better describe your power load and usage patterns, I think
we all can give you a better idea of your choices, and make a better
recommendation. There are scenarios where just about everything could
be a best choice.
If all your loads are SINGLE phase, and you just need to supply it all from
a three phase service, then my recommendation would be 3 separate SINGLE
phase transformers producing 120/240 with their neutrals all solidly tied
together and grounded at one point just before the 3 separate single phase
panels. This will handle extreme imbalances due to crazy single phase usage
patterns better. Tranformer costs go up while panel costs go down.
If you also have some three phase needs, you can also get them from that
bank of 3 transformers. You'll have 2 separate 208Y/120 systems there
(actually a 6-point star system in total).
If you have 240 volt motors that cannot be rewired or reconfigured to 208
volts, you'll need some other solution. A separate smaller delta-delta
transformer just for that would be one way to go (since 240Y/139 transformers
are next to impossible to find).
| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
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