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
