ECN Forum Forums Technical Discussion Electrical Theory and Applications Xformer theory Q

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

https://www.electrical-contractor.n...e_to_a_detached_building.html#Post202937