Help understanding 3 phase electrical systems

[randywright]

Can someone explain the difference between a 3-phase 3-wire delta system and a 3-phase 4-wire wye system that does not utilize a neutral?

[SecondGen]

Phase relationships are produced by the service transformer winding configuration. D-Y and Y-D transformers have a 30 degree phase shift but a D-D or Y-Y has no phase shift. This is very important for metering and relays because you need to know your Phase to Phase relationships. If you have a 3W service you need to know if it's 3W Delta or 3W Wye.

[randywright]

what considerations are taken when choosing between a 3W delta vs 3W wye?

[McDaniel8402]

what considerations are taken when choosing between a 3W delta vs 3W wye?

I can't think of any valid reason to use a 3-wire WYE source over a 3-wire delta source for 3-phase loads, and if you're serving single phase loads, you'd need a 4-wire source, generally speaking. Deltas come in a few different flavors - 3-wire delta, 4-wire center-tap delta, and 4-wire corner grounded delta - with the last of those being rather uncommon, not to mention dangerous.

For a given amount of kVA delivered, the insulation requirements would be less, but the amount of copper would be more for the wye-connected source. This is because each coil would see line-to-neutral voltage, but full line current.

For the delta connected source, the insulation requirements would be greater (each coil sees L-L voltage), but each coil (internal to the delta) would see (line current / SQRT(3)).

For 4-wire delta systems (most commonly center tap, B-phase is high-leg), you can serve single phase loads easily, but you wind up with B-phase being useless for serving single phase loads. Wye sources don't have that issue because all 3 phases have the same line-to-neutral voltage.

I'm not real sure if i've gone off on a rabbit trail, or actually answered any of your questions here.

[randywright]

This does help a lot, McDaniel8402 thank you. My question came up because I seen this once in the field; it was a 3-phase 4-wire WYE distribution board with only three phase loads being fed from it, the neutral bar had no feeder or loads. It made me wonder why they wouldn't have gone with a DELTA configuration. Have you ever seen something like this?

[McDaniel8402]

This does help a lot, McDaniel8402 thank you. My question came up because I seen this once in the field; it was a 3-phase 4-wire WYE distribution board with only three phase loads being fed from it, the neutral bar had no feeder or loads. It made me wonder why they wouldn't have gone with a DELTA configuration. Have you ever seen something like this?

I've seen several WYE connected panels that served nothing but 3-phase loads. This does not mean that it is not a 4-wire source, it simply means that the "4th wire", aka "neutral", is not being used by the loads that are served form the panel.

You can not physically have a wye connected source that doesn't have a neutral point. Such an animal does not exist. You can certainly have a wye connected source where the neutral is not being used, or the neutral point is not accessible to be used.