On 9 Sep 2004 01:57:56 -0700 johnn wrote: | 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 explanation.
| 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 systems.
| 3. If loads are unbalanced, which connection is generally more | advantageous?
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).