Put them in a black box and you probably couldn't tell from the outside what was in the box. Consider the secondary at 240V, 3-phase:
In the open delta, two transformers with 240V secondaries are connected end-to-end and will match the primary if it is connected in open delta.
In a T-T connection, the secondaries would be (1) at 240V with a center tap, and (1) at 208V without a center tap. One end of the 208V secondary is connected to the center tap of the 240V secondary. Primaries would be connected similarly and would need similar relationship of voltages.
I used to see TT transformers years ago in dry types. One project used a lot of them for 120/208 volt lighting panels.
As I remember there were two transformers side by side in the one enclosure for three phase. I think they were inspired by the old Scott connection. The one winding would be 208 volts with an internal (not always accessible) center tap. The other winding would be 180 volts and have one end connected internally to the internal center tap. It would have a tap brought out from the 120V, 60 V position.
The primary would be a 480 volt winding with a 416 volt winding connected to the center tap.
I have always wondered what the advantages and disadvantages were compared to conventional transformer connections.
The transformer I saw were rated for 120/208 volts, the transformers that davidbeach is familiar with are for a 240 volt system. Other than that I think we are in agreement.
If you want to go from 480 volts 3 phase to a 120/240 volt 4 wire delta TYPE system, you can use two 240/480:120/240 volt transformers. One transformer is under utilized in that it is operating at 87% of rated voltage. This connection is feasible for the size and voltage range of transformers that are normally availlable with center tapped primaries and secondaries. Other voltages and sizes requirea special transformer with a center tap on each winding.
The 120/208 volt lighting transformers were supplied as a complete unit in one enclosure. As davidbeach said about the black box, if you didn't look at the vector diagram on the nameplate you probably wouldn't notice that it was a TT unit. You couldn't tell from the voltages in the panel.
respectfully
Although this type of connection is feasible, is rarely done in the power utility industry because there are very few two-phase systems in use today. This is limited to special cases and occasionally for control applications. See the enclose link for more details.
cuky2000, I beg to disagree. The T-T connection can be used on 3-phase systems, and is still commonly used for small (sub 9kVA) three phase transformers.
