The 4 wire delta is quite common in some areas. It doesn't matter if it is open or closed.
If you do not need 277 volts, you will be able to generate 480 volts quite economically with an open delta auto transformer connection.
You only need two transformers and the size may be reduced.
Standard 120/240 by 240/480 dry type transformers will do fine.
The transformers are connected for 240 volts on both the primary and secondary. The primary and secondary are then connected in series as an auto transformer, 240 in, 480 out.
Now draw your delta as "A-B", "B-C", "C-A". with "C-A" as the center tapped phase.
Label your auto transformers as 0a, 240a, and 480a and 0c, 240c, and 480c.
The first auto transformer is connected O to "B", and 240 to "A"
The second auto transformer is connected 0 to "B", and 240 to "C"
You now have 480 volts three phase from 480a to "B", from "B"to 480c and from 480c to 480a.
Issues; Some delta supplies do not like unbalance loads, however if you have 4 wire delta, your system must be compatible with unbalanced loads.
You will be loading only two phases but they will be the phases that do not have the single phase loads. It is a possibility that the new loads will improve the load balance of the system.
Your neutral will be offset. "B" will be 208 volts to neutral/ground instead of 277 V. 480a and 480b will be greater than 277 V to ground.
If you can accept the offset neutral this is a very economical solution.
Sizing; The 240/480V section of the windings must take full load current and should be sized accordingly.
Example.
1> A three phase 45 KVA load at 480 volts = 45000VA/480V/1.73 = 54A per phase.
2> Each auto transformer must carry 54 amps through the 240V secondary winding. 54A x 240V = 12990VA, = 13KVA.
With auto transformers, the job may be done with two 15 KVA transformers at 87% loading instead of three 15 KVA transformers at 100% loading.
Bill
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"Why not the best?"
Jimmy Carter