dry transformer to convert a 4wire closed delta secondary to a 277/480 1

[lowrider87]

A customer of ours is supplied witha closed delta 4 wire system 120/240v, they would like to know if they would be able to convert, using a dry transformer to a voltage of 277/480v to run 2 large pieces of equipment.

 

[YuriSarcas]

Does the transformer have multiple taps? If not I would think you would have to change coils in the transformer to get the higher voltage. If it comes down to changing coils, if it is possible to replace the whole thing I would.

 

[lowrider87]

they did not purchase a transformer yet so i just need to know if they can use a dry transformer to convert to 277/480v Someone had doubted if you could go from the 4 wire closed delta voltage supplied by the utility to a wye system using the dry transformer.

 

[DanDel]

If by 'closed delta system 120/240V' you mean a 3-phase delta system, 240V between phases, with a center tap midway between two of the phases so you have 120V between that tap and each of the phase ends on that coil, and 208V to the other phase, then you need a three phase, 240 delta-to-480/277V wye transformer.

 

[davidbeach]

What DanDel said.

 

[waross]

Do you need a neutral on the 480 V equipment?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[YuriSarcas]

As long as it is indoors in a well air conditioned room I would think a dry type would be fine.

I am not sure if you can have a 4 wire Closed Delta. It sounds like you want a Delta-Wye transformer?

 

[waross]

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
--------------------
"Why not the best?"
Jimmy Carter