Using a Wye-Wye Transformer for A Delta-Wye connection

[chevycrawler]

I have recently beeen giving the task of hooking up a 380 piece of equipment the requires a 380V power source at a facility that has 480V. This machine came from a different plant and was hooked up with a Wye-Why transformer. I don't know the conditions of that plant, but very familiar with this one. The power source here is an ungrounded delta system. The question is it possible to use this transformer. The equipment requires a neutral because I believe there are some single phase loads inside, ie exahaust fans. The manager seems to be some sort of electrical engineer and is telling me not to bond the transformer. It is a dry type and has several leads for different inputs. H1-H2-H3 & X1-X2-X3 with a neutral marked HOXO. There are no neutrals present for the primary side. Would bonding X0 to equipment gound work? He seems to think it would cause interference and wants to keep it isolated from the rest of the plant. Appearently it is a $800,000 piece of equipment.

 

[davidbeach]

Nope, won't work. Not enough time right now to go into all the reasons why, but you won't have any voltage stability on the 380V side, if the transformer is in fact a 480Y-380Y transformer. H0 must be connected to the neutral of the 480V system. Neither H0 nor X0 (same terminal) is permitted to be connected to ground as you can not form a separately derived system from a wye-wye transformer.

 

[chevycrawler]

Ok that is very helpful. I was always under the impression that a transformer had to be grounded in order to be stable. Unless it is a perfectly balanced load, it wont be and that is practicaly impossible. Any further info would be appreciated as I am trying to learn a little more about this situation. It is indeed a 480V Wye - 380V Wye transformer. All of the other existing in the plant are 480V Delta - 208/120Y.

 

[davidbeach]

You need a 480V Delta - 380V/220Y transformer. Or you need to come off a solidly grounded 480V wye system and include the neutral. No other choices.

 

[chevycrawler]

Thank you very much for your input. It is greatly appreciated!