Are T-T Connected Transformers for 3-phase utility power commonly used?
They were "Kind-Of Commonly Used", back when some customers still utilized 2 Phase Equipment, and I do remember seeing a few for low demand 3 Phase customers too.
Have not seen anything lately (last 10 years) new from a PoCo - on a pole - which was connected in a "Tee" format.
How do they compare to Open Delta setups?
They are Open Delta Configurations!!!
A "Common" Open Delta "Vee" uses "Standard" type Transformers, and the Configurations are achieved by connecting the Two Transformers in Parallel at the end of each Transformer - and if wanted, using the center tap of one winding (secondary) for a multiwire system, or just a center tapped grounded system as only 3 Phase 3 wire.
On an Open Delta "Tee" configuration, one Transformer may be of a "standard" type - with center tap abilities on both the Primary and Secondary windings, however, the second Transformer requires Primary and Secondary winding taps at 86.6% of th full winding length.
This is the typical "Scott - T -"
setup, and may be used for 2 Phase 3 Wire outputs from 3 Phase 3 Wire systems.
Two Phase systems also used the "Taylor"
configuration, which used Three separate single phase Transformers - one being the typical "Center Tapped Type", and the remaining two being the type with "86.6% Taps".
I have seen a few "simple" 3 Phase 3 wire Open Delta Tee systems, on poles - here and there, mostly in older commercial areas of L.A., but that was like between 1976 and 1995 - so who knows if these systems are still in existence.
Not very sure about other Utilities across the Country - can only describe the most commonly seen stuff in my area from the following Utilities:
- Southern California Edison (SCE),
- Pacific Gas and Electric (P,G, & E),
- San Diego Gas and Electric (SDG & E),
- Anaheim Public Utilities (APU),
- Los Angeles Dept. of Water and Power (LADWP),
- A few others in Southern California.
Many Polyphase Low Voltage (up to 600 Volts) Dry-Type Transformers, with Apparent Power ratings between 5 KVA and 15 KVA, are setup as Open Delta "Tee" configurations.
In fact, Square D uses Open Delta Tee setups for 208/120V 3Ø 4 Wire Dry Type Step-Down Transformers of 3 KVA thru 15 KVA.
The "86.6%" tap point is where terminal "X0" is made - which is the "Common Neutral Conductor" terminal location.
These setups are listed as "Wye" configurations - mainly because the output yields similar Voltages of a Star Connected 3 Transformer coil setup with a Common Neutral Conductor tapped to the Star point (208/120V), however they are actually Open Delta "Tee" configurations.
No big deal - they work fine for most basic multiwire stuff.BTW
There is another "Open" type configuration that is commonly used - by Utilities and for Customer Owned Transformers...
This one is known as the "Open Wye" setup.
Open Wye configurations resemble an Open Delta "Vee" configuration, only it typically carries a "Common Conductor" across the Primary and Secondary sides, which is terminated at the Star point, and is most always a Grounded Conductor.
Some Open Wyes are setup isolated from the Primary side, but most are setup in a basic Autotransformer fashion, using some Conductor which is common to both the Primary and the Secondary sides.
An example for a Low Voltage setup:
Using only 2 Ungrounded Conductors + the Grounded Conductor of a 208Y/120 3Ø 4 Wire Wye system, to feed the Primary side of an Open Wye configuration (using only 2 Transformers), we may achieve a 3 Phase output on the Secondary side.
In fact, the Primary Feeders of the above noted example - IS, IN FACT - a polyphase system.
Additional information may be found in the Technical Reference area, and from post by other members.
edited fer spelin blunderz!
[This message has been edited by Scott35 (edited 04-03-2006).]