Grounding an ungrounded Delta-Delta distribution transformer

[power2engineer]

thread238-262723

https://www.eng-tips.com/viewthread.cfm?qid=262723#:~:text=Grounding an ungrounded,thread238-2

I just ran into this NOT so desirable 3750KVA, 34.5KV/480V Delta-Delta transformer. I have attached the transformer Nameplate and 480V Main switchboard Photos.
The Main Switchboard has A voltmeter with Voltage Selector switch which read 480V. In addition it has 3 separate voltmeters marked "Ground Voltmeter #1, #2 and #3 respectively They all read 277V.

1) I am wondering if the system is grounded in a way Mr.David Beach in the referenced thread 238-262723 indicated on 12 Jan 2010 using a WYE (Neutral grounded)-delta Instrument PT for the Ground voltmeters OR can the Instrument PTs be connected using DELTA-Wye (Neutral grounded) to read 277 Phase to neutral

2) How to size this Delta-Wye or Wye Delta transformer for Phase ground fault monitoring voltmeters or for providing system grounding on 480V side?

3) Can this i) Delta-wye or ii) WYE delta solidly grounded neutral be used for all secondary loads requiring neutral / ground connections?

4) In case of WYE-Delta connected grounding transformer the Ground Voltmeters could be connected to each Phase to Neutral and the secondary Delta should be left not connected Open circuit or form at least a delta of its secondary winding?

5) What would be the right way to ground the Main ungrounded Delta-Delta transformer A) by connecting Wye (with solidly grounded neutral) to 480V secondary terminals of Main power transformer or B) by connecting Delta-Wye PTs Delta to the 480V Secondary terminals of the main transformer and using Wye with solidly grounded neutral for Ground detection voltmeters??

 

[jghrist]

If you have voltmeters measuring phase-neutral voltages of 277 V, you most likely have a grounding transformer on the system. It could be either wye-delta or zig-zag.