I was assigned a research project for college and the question is .... Why is a Wye primary favored over a Delta. it is in reference to the volts and the amps in the system. it is an extra credit project the instructor has stated ot doesnt matter where or who we find it from we merely need to back it up with a reference in hard copy. i figured i would post for all of the tech expertise out there and learn as the posts come in . my instructor saw it in a book he was reading and it is merely 1 small paragraph long i have been able to find 2 things and they are : 1 line and phase amps are the same in a WYE and 2: that the voltage to ground ina wye is the same on all phases for easier balancing of the load. Any help will be greatly appreciated.

PS graduate in May BIG party all invited

One advantage
A Y can be used for 120 loads, a Delta is primarly for 3 phase loads.
On a 208/120Y system.






[This message has been edited by Dnkldorf (edited 03-30-2005).]

Did you mean to say wye or delta secondary instead of primary ?

If you meant WYE primary, the only reasons I can think of are cost for the power company (3 single bushing pole transformers are cheaper than 3 dual bushing pole transformers), and special phasing requirements.

A WYE primary with a DELTA secondary has feedback problems. If the primary loses a phase, all the WYE-DELTA transformers are now re-powering that phase in reverse. Other loads on that primary circuit that aren't re-powering it are using it, and generally this results in blown primary fuses all over the place. The power company doesn't like that, so where you see a WYE primary it's going to have a WYE secondary, generally.

Perhaps you meant to ask about WYE secondary. The points you mention would apply to the secondary. It is generally safer because the voltage relative to ground is lower. A 480Y/277 system can supply power to 480 volt delta loads, while only having 277 volts relative to ground (which often gets used for lighting circuits in industrial or large commercial locations). A 208Y/120 system is a compromise; it underpowers 240 volt delta loads, but it does have 120 volt circuits in balance over three phases (suitable for very large services that don't have much need for three phase or can transform or adapt as needed). A 240/120 volt delta system is more suited for many three phase loads and not too many 120 volt single phase loads, but one of the wires is 208 volts to ground.

In Europe and other places, the 400Y/230 system fits well for both single phase and three phase needs because they don't have legacy loads with 2x voltage ratios from Edison style 3-wire single phase circuits. Delta secondaries are rare in Europe.

Here in England they're almost non-existent. There might be one or two used in specialized applications, but a 415Y/240 distribution system (or thereabouts) has been the norm for many, many years.

Onboard naval ships, Delta power is used almost exclusively for safety. (Considering the risks of saltwater flooding, steel hulls and all.) Even the equivilent of 1 phase receptacle power is really just a leg of the delta- 110V 3-phase delta only requires each wire to be at 65V to get 110V through the circuit, which is a lot safer for shock risk than if someone were to touch a 115V hot wire.

The drawback of this is that, unlike wye, a load on one leg of a delta circuit will affect the other legs. If you've got a 100A delta circuit and load one phase-pair with 100A, you're limited to 17A on the other two phase-pairs. It does make it difficult to balance the load! Also, some COTS equipment doesn't like having both the "hot" and "neutral" wires above ground; really gotta be careful how a lot of it is wired internally.

[This message has been edited by SteveFehr (edited 03-31-2005).]

[This message has been edited by SteveFehr (edited 03-31-2005).]