Transformer Selection?

[MidwayJohn]

We have a 3-phase 240V high-leg delta service at our facility, but I have some utilization equipment that requires a 3-phase wye configuration. I have a transformer readily available that is marked for Delta 480V L-L on the HV side and Wye 480V L-L/277V L-N on the LV side. Would there any problems with using this transformer in my 240V application?

 

[winnie]

You can use a 480V rated transformer at 240V, but the effective KVA rating of the transformer will be halved. Essentially the transformer has a maximum current rating, and the reduced voltage wont significantly change this value.

-Jon

 

[vector1234]

zig zag

zig zag

You may be able to put a zig zag transformer in to creae a neutral for the 240 load if your neutral currents are low.

 

[Cold Fusion]

... I have some utilization equipment that requires a 3-phase wye configuration. ...

The transformer output voltage would be 240/139 Y. Is that what you need?

cf

 

[winnie]

You may be able to put a zig zag transformer in to creae a neutral for the 240 load if your neutral currents are low.

It is quite true that a zig-zag transformer can be used to derive a neutral from a delta system.

The difficulty in the current situation is that you already have a grounded conductor in the system (the mid point tap on one of the windings which makes this a 'high leg' delta, so you would not be able to ground this derived neutral. If you attempted to ground both the mid point tap and the zig-zag derived neutral, you would have a short circuit.

If the other loads being supply by this high leg service can tolerate the change, then the original ground could be removed, and the system re-grounded using a zig-zag transformer. As cf notes, this would create a 240/139V wye system, which straight 240V loads can tolerate but which would cause problems for any 120V loads on the original high leg system.

-Jon