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How about the possible voltage differentials btwn the primary (high) and secondary (low) when there is current flow present within the respectful system neutrals?

Sounds plausable, but I'm looking for more...coffee!

Just my first thought...

It is a common connection and a fault on one side can/will trip OC devices on the other, I have seen it happen.

Transformer manufacturers charge a premium to make one though and there is usually a phase shift, but I do not remember the details on that, though I'm sure someone will chime in.

Maybe I misunderstand the question but aren't we talking about the grounded conductor on each side of the transformer?

Don't you usually bond all the grounding conductors together?

I am still on my first coffee too.

You do, but with a typical wye/delta, except for the ground the two sides are isolated from each other. With a wye/wye the neutral passes straight through.

If you're not using it on one side no big deal, but if you are then there are many issues to overcome or at least to be aware of.

Vindi...

Delta/Wye transforms cause a 30 degree phase shift... with the Wye lagging the Delta... It's in Ugly's -- the fine print.

BTW, Delta and Wye are TENSOR diagrams. So you can visually see the math by placing the star inside the delta and note that the star tensors bisect the delta's wedge -- each wedge is 60 degrees -- and cutting it in half gives you the 30 degrees lagging shift.

But enough Vector Algebra...

Wye/Wye transformers use more wire -- to no good purpose.

The windings ratio in a 480 to 208Y120 is 4:1...

This reality is beyond many electricians, I've been on the end of many flaming posts because of such belief. Some j-man think that the ratio is 480/208 -- which is entirely false. For those disputing this... open up a transformer and trace the connections. ( Turn it off, first. )

The delta connection is H1-H2 : 480 volts

The wye connection is L1-N : 120 volts

Wye-Wye transforms also permit noise from either side to crossfeed.

There are occasions where you'll find wye-wye: massive DC power supplies used to drive electro-chemistry: aluminum reduction, chlor-alkalai plants and such. They use a mix of delta-wye and wye-wye to phase shift -- creating 12 phase power from 3 phase. At that point the AC is rectified with a massive low pass filter -- thence to the reduction line.

That's the big issue, the noise if you will.

This came up for me when using large step-up step-dn transformers on large scale PV projects, several 'other' engineers wanted to use a y/d-d/y configuration and it took forever to get across to them that there was no (easy/cheap) way to open a fault that way so they proposed a y/d-y/y configuration, which would work (noise was not an issue), but the additional cost and lead time was not worth it.

We standardized on a y/d-y/d configuration, but still wound up with one installation with a y/d-d/y configuration where we corner grounded the delta on the utility side of the run.

I still remember the jw and foreman calling me asking repeatedly, "this is going to work when I throw the switch right?" and the total surprise in their voice when they called back to say "damned if it doesn't work!".

Interestingly enough I discovered that a LOT of utilities use Wye/Wye transformers within their own distribution networks.

FPL does it all the time.



The center tapped transformer in front of my house is fed from a wye. Pretty much the same issue with the neutral bonded on both sides.

Greg's Image above looks like a MGCN setup (Multi-Grounded Common Neutral).

The Primary -appears- to be connected as a Wye, but I am not 100% sure.

The Secondary must be Wye connected, as the Left-Side Bushing of the Two visible Pots has only a single lead with no interties; and the Right-Side Bushings have Bare Leads, which appear to be heading to the Star Point + MGCN termination.

Greg, do you have a Non-marked Up version of this Pole's Setup?

-- Scott

Sorry about the big pictures but I want them to be easy to see.

You can see the common grounded wire going up the pole here



This is the original

Greg,

No apologies necessary!
I should have added more details.

Larger Images are so much nicer than small Images!
Small Images are too difficult to view, and are best used for Thumbnails!!!

The "2nd" Transformer setup (1st in the latest reply) looks to be a Wye-Wye, with MGCN.
More specific, it appears as 12,470V Primary x 208Y/120 3 Phase 4 Wire Secondary. Apparent Power Rating = 75 KVA (3x 25 KVA)

Per each 25 KVA "Pot" (Transformer):

Primary Split-Coil 7200/12470V Windings setup in Parallel - for 7200 Volts,

Secondary Split-Coil 120/240V Windings also setup in Parallel - for 120 Volts.
(BTW, I have seen only a few Pots where the Center Tap Bushing is used for 120V 2 Wire output - normally the outer Bushings are used only).

The "1st" Transformer Image looks like 7200V x 480Y/277V 3 Phase 4 Wire, Wye-Wye, rated for 150 KVA (3x 50 KVA).

In the areas near me, nearly all Utility Transformer arrays have Delta Primaries (unless the Service is 1 Phase 3 Wire... ...).
LADWP had some Open Wye / Open Delta Setups. Saw these in the '70's and '80's.

Wye - or more specific "Grounded" Primaries help stabilize Transient Surges (from Faults, Line Surges and Lightning Surges), help stabilize L-G Voltage (i.e.: Capacitive Coupling Effects) and help stabilize L-G Fault levels.

In addition, Primary Coils have a much lower Voltage across them, which lessens the Flash-over issues between Primary and Secondary sides.

As far as I know, MGCN Systems greatly outnumber Uni-Grounded Common Neutral Systems.

-- Scott

Distribution seems to be wye here pretty much everywhere and we have a lot of the 2 transformer "delta Vee" thing. I have a picture of one of those too if you like.
(Mr Bill wants me to resize it before I post it)

We see a bunch of it in the "light industrial bay" type places.

Gee,
You guys over there make things so difficult for yourselves.
Over here that would be one single transformer fed with 11 or 22kV (normally single phase).

That's not too bad.

I was on a site walk for a military building two weeks ago that had 3 services (building was only 40k sq ft. One of the services was an 800a, 240v/3p high leg from pole mtd transformers. All three transformers were physically different sizes and A and B were 50kVA while C was 75kVA........

It should be noted that the building was originally an agricultural packing facility.

Trumpy, I think the PoCos like multiple transformers (of essentially the same type) that they can mix and match in several ways to create all of the strange service arrangements we have. The service truck does not need to carry many different types to fix almost anything.
The 50 KVA unit is becoming the defacto standard around here.
There are several ways to tap them and it has a voltage select switch to enhance that adaptability.

http://gfretwell.com/electrical/Transformer%20connection%20label.jpg

If there is no neutral winding on the transformer (AKA wye connected), I wouldn't even bring the neutral there.

I like the bottom connection diagram. We just had this in another thread didn't we?

ok, why Greg, and which would be compliant?



sorry if i missed it

~S~

I don't think either are "violations".

This is pretty much your exact question

http://www.electrical-contractor.net/for...html#Post202937