Easy conversion from 3 phase to 2 phase. The first power systems in the US were 2 phase. Latter as networks upgraded to 3 phase some method had to be devised to convert 3 phase to 2 phase for customers who already had 2 phase motors and switchgear. 2 phase power is an extreme rarity, most systems being at least 100 years old.
Of note, a lot of people even a collage professor on U tube was calling split phase 2 phase power claiming 2 phase power is 180 degrees apart. Its not, that's split phase or center tap power. 2 phase is 90 degrees apart. Power can be distributed a number of ways 3, 4 or even 5 wire. When run as a 3 wire the common wire acts as a partial neutral but not a real one since balanced loads wont give you zero like split or 3 phase.
I think some are now even museums. Most hydro electric if I remember.
Of note, the T connection is actually not a rarity. Its still used to step down 3 phase power where economy is desired. A scot T transformer can be configured with the right taps to convert 2 phase to 3 and latter 3 phase to 3 phase. TT connected windings are used in many low cost 3 phase pole pigs. One of those connections like an open delta, not perfect for heavy loads but light 3 phase doesn't care.
Philadelphia is I think the only place still using true 2-phase power in the US now, and only in the really old industrial sectors. Niagara used to have it too, but I recently heard that it's only in a museum now. Tesla's first power generators at Niagara Falls were 2-phase.
In Europe by the way, they use the term "2 phase" all the time, but for them it means 2-out-of-3 phases, i.e. 2 pole. Confused the heck out of me when I worked for Siemens, I thought it was weird that they sold so many different products that could only be used in Philadelphia!
There are still some plants in New England that are almost exclusively 2 phase. One of our customers is even 2Ø 40Hz. The whole plant runs off a massive DC-link inverter yard because it was cheaper than upgrading all the specialized gear in the plant itself.
There are still two villages here with their own generation plants and utilities running two phase 5 wire services in ancient buildings. I upgraded one to 3 ph.
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