rattus said:
Larry, that is educational, but I still have a couple of questions:
How do yout get a single phase 120/208 service?
Are virtually all distribution systems 4-wire wye?
Are delta-delta xfmrs ever used in distribution?
That's three, but who's counting?
1) You can get 208v single-phase line-to-line and 120v line-to-neutral single phase simultaneously, similarly to the 240/120v single-phase system in your home. This is most often seen in larger apartment complexes, and is why most major household appliances come with both voltage ratings.
The timing difference between 120 deg. and 180 deg. manifests itself as the reduction (208 rather than 240) in the line-to-line voltage. A dual-trace oscilloscope with each channel referencing one line to the neutral would show two normal sine waves with a 120-deg. timing offset.
2) I'll bow to 72's experience, but I do know that it is distributed both ways. If you're receiving all three phases, the transformers can be what are called dual-bushing units, which can be connected either line-to-line (Delta) or line-to-neutral (wye), but the latter requires a system neutral.
The wye system is almost always grounded, whereas the Delta can be floating. The secondaries, which are what we really care about, can also be either Delta or wye, and as we know, either can have a grounded neutral conductor, but the latter really requires one, or why bother with it?
The high-leg system is really an abomination, because of how it came about. If you ignore the high-leg conductor and its transformer for a moment, what you have left is a single-phase 240/120v secondary with a grounded center tap, exactly, and I mean exactly, like the typical 240/120v 1-ph system.
That single-phase distribution system, one phase and a system neutral (the latter of which is already run with the phase wire), is obviously a line-to-neutral system, meaning that it originated from a wye sub-atation transformer. By adding one more phase, you can create an open-Delta secondary.
If there happens to already be two phases run, sure, a double-bushing transformer could be used to derive a single-phase secondary, but since the system neutral is almost always also present, again what's the advantage? Using up old stock, perhaps? Why not create a 3-phase system instead?
This is very suitable to upgrading an existing 1-phase service, where a relatively small 3-phase load needs to be added to a relatively large, but existing, single-phase system. There is almost no interruption to the delivery of power, and the cost to the POCO is rather small: the second transformer.
Where the customer requires three-phase from the get-go, there's no advantage to an open-Delta service over either a standard Delta or wye service. Three phase distribution would be run when the installation is made; whether the POCO opts for a Delta or wye primary is of no consequence to us.
In reality, there is no need for a high-leg service, open-Delta or not, except for re-supplying an existing installation, as the two jobs I mentioned earlier. Why would anyone intentionally specify a service that renders one phase relatively useless? Compare a hi-leg service to any other.
A wye system, either 480/277 or 208/120, obviously provides for the most efficient line-to-neutral loading, such as lighting (277v) and receptacle (120v) loads. A Delta system provides the most efficiency for large motor installations, as well as power distribution within a facility.
Why would anyone want a high-leg service? Where does it have an advantage over any other service type? It makes panel filling wasteful, and uses orange tape that nobody has any other use for. Wye services are becoming the
de facto standard, except in industrial systems. Why ask wye?
3) I think so, but again, where the primary distribution system is already run in 3-phase. It's likely that the POCO would run a Delta primary where they know the secondary system will be Delta, because wye-Delta is known to be problematic, as has been discussed in other threads in this forum.
How's that?
