13.8kV Bus Grounding with station service transformer?

[3winding]

I have an ungrounded 13.8kV Bus I would like to ground. The 13.8kV bus connects wye connected generators to a Delta - Delta step up transformer for export to the utility. When the generators are disconnected the bus becomes ungrounded. The station service transformer connects to the this 13.8kV bus. Does anyone see any inherent problems with using a Wye-Wye station service transformer (13.8/8kV - 480/277V) as the ground point? The generators are high impedance grounded.

 

[busbar]

One reference may be worth reviewing as sort of a “checklist” of things to watch out for—

IEEE Std C62.92.3-1993 …Application of Neutral Grounding …Generator Auxiliary Systems

A grd·ZZ—grd·Y configuration may be easier to apply, for an unloaded 13.8kV-wye winding on its own has high zero-sequnce impadance.

 

[dpc]

As busbar mentions, a wye-wye transformer is not a good choice for a grounding transformer. A delta-wye or zig-zag (special order) would be much better.

You also don't want to compromise your generator high impedance grounding, so I assume you'd use high-impedance grounding on the transformer as well, or provide a means of switching out the transformer ground when the generators are running.

 

[3winding]

A Delta/Wye would not work as it would not ground the 13kV bus. I require the 480/277 for station service. I haven't used a Grd Zig Zag - Ground Wye before. What are your thoughts on leaving the 13kV bus ungrounded? It would only operate ungrounded when both generators were offline.