Transformer Connection 3PH Open Delta 480V w, 2-Bushing Xfmrs

[joozu6]

We have some 2-bushing 277/480V overhead transformers in stock and would like to use them to install a 480V open delta bank (versus using 3-bushing transformers that have a center tap, which we don't have in stock). I'm not a transformer connection expert, but I'm thinking I can delta-connect two 2-busing 277/480V transformers and run the 3 legs for a 3-wire 480V service. I took a stab at the attached electrical diagram, but I'm wondering if the winding polarity will give me the correct voltage.

 

[stevenal]

Your output from this type of transformer is 277 V. Connect 3 or 2 of them in wye or open wye, and you will have 480 line to line. Connect them in delta or open delta, and you will have 277 V line to line.

 

[joozu6]

Thanks. I thought so. If a customer is on a 480V open delta, is there any reason not to give them a 277/480V wye service and just run the 3-480V legs?

 

[jraef]

A reason NOT to? Cost. If all of their loads are simple 480V motors running full speed, or they are feeding a step-down transformer for single phase loads, then there is no reason to add more transformers.

But if ANYTHING in their facility is using 480V 3 phase input to a power conversion device, such as a VFD, servo, UPS, large 3 phase power supply etc., then open delta is a disaster waiting to happen, and the preventative medicine for that is to create a solidly grounded wye system, even if you never use a Line to Neutral connection on it.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[joozu6]

We're trying to phase out the open delta banks, so when an open-delta transformer fails the linemen are replacing them with 480V wye banks and not running the neutral. However, we're told this is a safety issue and could burn up the customers' equipment.

 

[waross]

I don't see an issue with changing open delta to grounded wye. Any equipment that has an issue is probably faulty.
Jeff, I understand that he is going from delta to wye. From that perspective, your answer will be correct in that safety issues will be with the existing delta system, not the proposed wye system.
The safety issue may be if a customer has a ground fault on an ungrounded delta system that he has neglected to repair.
The first warning of the failure will be the ground detector system.
If the customer chooses to ignore that indication of a failure, the second indication of failure will be more dramatic when the grounded wye system is connected.
You may want to have the customer sign a disclaimer before energizing the new transformer bank.
Sign now and the power will be on in a few minutes.
Decline to sign and the proper transformers are on back order and the power will be on in about 6 weeks when the new transformers arrive.

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

 

[stevenal]

You will need to run a neutral. First of all you will need it for metering. You don't need it if all loads are connected line to line, but free power can be obtained once they start illegally connecting loads line to ground. I believe code requires running a neutral even if it ends at the service without any further connection to load.