Dry type transformers in hi-leg delta configuration

[PHILIP J]

A client has a 1200 amp 120/240 volt 3 phase 4 wire service fed from a utility owned bank of 3-167KVA pole type transformers in a closed delta configuraton. In an effort to get the customer on primary metering, we would like to get him off of the above mentioned service and provide a bank of dry type transformers fed from an existing lightly loaded 1600 amp 480/277 Volt 3phase 4 wire service which is already on primary metering. The single phase load is such that a 75 kva transformer will handle it and the three phase load requires 225 KVA (3@75 KVA). Therefore I want to propose a closed delta configuration of (1) 167-1/2 KVA and (2) 75 KVA 480-120/240 single phase dry type transformers. I have used mismatched KVA transformers in open delta and closed delta configurations before but only in the 25 to 37-1/2 kva size - in another words, not anything this large. My questions are as follows:

(1) Will eddy currents or circulating currents be a problem?
(2) How should the configuration be fused on the transformer primary?
(3) How should the configuration be fused on the secondary?

 

[jim dungar]

Open delta configurations are used often with mismatched transformers, closed deltas are not as forgiving. Why do you want closed delta?

Have you thought about providing two different banks, one for the single phase load and a different one for the three phase?

 

[PHILIP J]

Hi Jim-

I am certainly open to the open delta configuration. I will have to derate the transformers to .866 of their KVA value which may push me to the 225 KVA on the single phase side and obviously larger on the three phase side. I think in this case the customer would have much less total purchased KVA with the closed delta. However, I certainly don't want a configuration that is "unforgiving". I have actually been unable to find any reference material on the plus's and minus's of either configuuration. When the utilities discuss these concepts, they usually discuss it in terms of the open delta being used in an emergency when one of the closed transformers has gone bad or when they might be expecting more three phase load in the future- so go open delta now and then add the third transformer in the future. This doesn't seem to be a popular question but i really need to find the correct answer.

 

[jim dungar]

I can not remember any time that I designed a closed delta system for a 'transition' system like this. These systems rarely get added to, so future capacity is usually not an issue. The cost of three vs two transformers should also be considered.