Transformer

If there are 3 transformers connected wye the neutral is required or else the neutral point will float and the voltage across the 3 transformers will be different depending on the secondary load.

(If there are 2 transformers - Scott or T connection - I don't think you need the neutral.)

The secondary system will be ungrounded?

-- bud--

Bud sez:

It's a single 3-phase transformer labeled for use as 480 (delta) -to- 208 (wye) step-down. Which, reading your statement above, means that phase voltages will be different in the primary and secondary depending on loading. Yes?

Common ground between supply and load, at the transformer chassis.

Sparky

Grounding of the neutral is preferred for safety, and it prevents non-symmetrical loading of one phase from shifting the voltage and phase of the others.

No if the load on those three phases is equilibrated (symmetrical). Yes if you want to keep the wye node at the same potential measured between the node and every phases on the 208 side.

Be aware that doing this results in a wye-delta transformer configuration. This has a problem that if you lose a phase, the transformer will try to power the lost phase from the other two. Therefore, expect the breaker to pop whenever you lose a phase, when it attempts to backfeed the local grid.

On the other hand, if you don't connect the wye common point to the neutral, it won't do this. Even if the load on the delta side is unbalanced, the configuration won't try to generate any neutral current, it happens to work out. You'll still have single phasing, of course.

But before you do this, run it by someone with more knowledge of these things than myself, as well as whether code allows this. I do know that a local electric company does this when powering an old segment of delta-connected distribution system from a higher voltage wye system. The common connection of the primarys don't connect to the neutral.

Neutral on the wye is mandatory. There is no neutral on the delta side. Each delta phase is feed from two of the hots. Instead of having three power circuits like AN, BN, CN, you get AB, BC, CA. If you tie any delta phase to ground you will be chasing voltages on the grounds.

Donald

If the transformer has 3 sets of coils I'm going to change my answer. I think the connection on the delta side would force the voltages on the wye side to be substantially equal. I think the voltages on the delta side would be a little closer to equal with a connected neutral and losses would be lower since the delta side is not forcing a balance on the wye side. Anyone with a better theory?

Particularly some small 3 phase transformer units are made with 2 sets of coils with a Scott/T connection which could be connected without a neutral.

The transformer 480V delta side would be ungrounded unless you "corner" ground one of the phases.

-- bud--

This is the opposite of what you said in your original post. You said 480 wye to 208 delta. Now, you're saying 480 delta to 208 wye. Which of these it is makes a lot of difference.

Whichever side ends up with the Wye connection needs the neutral throughout, and it will probably be grounded at the entrance panel (If it's the input side) or a "common neutral" if it's on the equipment side. Delta has no neutral; both need the safety (Earth, Frame) ground in any case.

Hope This Helps! Rich

Not always, local regulations vary. It is, however, customary and typical. It has advantages as other responders have noted.

That is fine, always works OK and is always legal if all leads are of sufficient size.

I think that is correct. Any 3-phase transformer with a delta on one side (pri or sec) has the benefit that the delta winding internally circulates third harmonic currents and balances the wye-side so that the neutral is reasonably stable. This means that the star point may or may not be earthed and, (if it is), does not require a heavy gauge connection. It also means that a wye-connected secondary allows a 4-wire supply that can have simultaneous balanced and unbalanced loads without much penalty.

Contrast that with a wye-wye transformer, where the neutrals can be very unstable, requiring heavy gauge neutral wiring.

In the UK the delta-wye is the standard 11KV 3-wire to 240V (line-neutral) 4-wire configuration for domestic and industrial distribution.