Delta-Delta (grounded phase) vs. Wye-Wye (grounded phase)

[LBolt]

We currently have a 15kVDelta-480VDelta transformer feeding our plant. We ground the "B" phase of our 480V system so there are no fuses in "B" phase throughout the plant. The transformer failed and the utility wants to replace with Wye-Wye transformer. They plan to ground the HV neutral at the transformer. We would still have to ground the "B" phase on the 480V and leave X0 unconnected. As long as the secondary winding can handle the full voltage on the other two phases indefinitely I figure this would work under normal conditions.
I am just not sure of the zero sequence current that will flow on a primary system ground fault. Has anyone ever had any experience with this application?

 

[jghrist]

Grounded wye - ungrounded wye connections have problems. See

http://www.geindustrial.com/publibrary/checkout/38652.30055.7762.44019/generic/GET-3388B.pdf

The transformer would normally have the primary and secondary neutral tied together internally. This would obviously cause major problems if you corner ground the secondary (like a Ø-grd fault). I doubt seriously that the utility would allow disconnecting the internal connection and corner grounding the secondary.

 

[LBolt]

Thanks jghrist,
You are exactly correct. The H0 and X0 are internally connected by a removable link. And the utility has already agreed to doing this versus giving us the transformer we want. So this would not be a typical grounded wye-ungrounded wye situation since H0 would be grounded on the primary and "B" phase would be grounded on the secondary. Under normal conditions I think it would work. However, I may have some voltage balance issues if my loads are not balanced and the utility voltage is not balanced. I think primary ground faults will affect me a lot differntly on the secondary (i.e. voltage collapse versus voltage dips) on certain scenarios. Not sure how the zero sequence current would flow through on a primary fault, but that would be a utility issue.