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Transformers
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Open Wye – Open Delta Banks
Open Wye – open Delta banks require only two primary phases and a system neutral and can be connected in one of two ways. Utilities commonly select one or the other when designing their standards. Determination is usually made based on construction methods, clearance requirements, squirrel/bird problems, and other similar factors.
Same-Opposite Connection
The same-opposite connection is so called because the “same” primary bushings are connected together (H2 to H2) and grounded, while the “opposite” secondary bushings are connected together (x1 to x3).
Chalk Talk:
Open Wye-Open Delta Same Opposite
lcvid.com/owodso
68 norTHWesT LIneman CoLLeGe
A B C
n p h h
“Lighter” transformer
h - h & h - p voltage 240
480
H1 H2 H1 H2
x3x2x1 x3x2 x1
Same
(H2 to H2)
Opposite (x1 to x3)
Nameplate Rating 120/240 240/480
n - p voltage 208 416
n - h voltage 120 240
h -h & h -p
Although 240/480 volts is listed in the table, open banks almost exclusively serve 120/240-volt three-phase loads. If a customer has a 480-volt motor, the load is almost always substantial enough to require three units.
n -p
voltage voltage voltage
(see (see (see table) table) table)
n -h