Industrial facility with multiple grounding configuration on XFMR's?

[MrJLH]

I was recently working in a facility which all the transformers which were originally configured in a un-grounded delta-delta which were all fed from a utility at a single point outside the plant fence. (When I mean un-grounded I mean, only three wires, one for phase A-B-C for the secondary side, with no common ground grid in the facility)

The owner recently decided to demo two of the existing transformer, replace the transformers and install two new utility taps (so three utility taps total). One of the new transformers is now a primary wye-grounded with a secondary Corner grounded delta and the other transformer a primary delta with a solidly grounded wye.

Since all the existing plant distribution feeders only has three wires to the plant distribution switchgear, one for each phase, what happens with these transformer grounds? Both of these new transformers are hundreds of feet apart with the existing transformer even further, and there is no common ground grid in the facility.

Hope I'm clear

Thanks

 

[jim dungar]

I...what happens with these transformer grounds?

Nothing really. An isolation transformer creates a separately derived system, so what happens on one transformer secondary has little to no impact on the grounding conductor of another.
However, I hope you have a very strictly followed conductor color coding scheme. You do not want the grounded conductors to become intermixed as it can make troubleshooting difficult. Technically the corner grounded conductor should be 'white' or 'grey' just like the solidly grounded neutral is.

 

[synchro]

On the existing feeders supplied by the new corner-grounded delta secondary, I wonder if they verified that all existing overcurrent devices meet 240.22 (which prohibits overcurrent devices on grounded conductors unless they disconnect all conductors together). ... with the exception of any necessary motor overload protection devices.
If fuses were used they may have to insert dummy fuse slugs in the slots for the grounded phase conductor.