Emerg Gen & XFMR to share the HRG. Any Issues?

[Mikey44]

I plan to install a 750KVA 2400V-480 Delta-Wye XFMR with a high resistance ground. There will also be a 200KW emergency generator that feeds the emergency panel through an ATS on loss of the 750KVA XFMR power. I have limited space to add an additional HRG and would like to use a single HRG, connect the neutrals from both the generator and the XFMR together, then run through the HRG. I realize there is a safety concern when performing maintenance on either the generator or the transformer that up to 277V to ground could be seen on the neutral if there is a ground fault on the operating power source. I plan to deal with the safety issue with warning labels on the equipment and notes on the drawings and may also add a disconnect to each neutral to isolate during maintenance. Are there any issues/concerns/recommendations with this approach? Thanks in advance

 

[waross]

You may not use a High resistance ground when there are any line to neutral loads.
With no line to neutral loads, ground the transformer through the HRG device. Connect the generator neutral to the transformer neutral.
If the generator is grounded as well as the transformer, only one ground system may be connected at any time. You will need a transfer switch that b switches the neutral as well as the phase conductors.
Much easier to connect the generator neutral to the transformer neutral and let one HRG device service both the normal and the standby supplies.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mikey44]

Thanks for the reply Bill. I do not have any 277V (line to neutral loads). You mention only one ground system may be connected at any time. I do have an open transition ATS however the emergency panel will be fed from the generator at times while the rest of the system will be from the transformer so both sources would ground through the HRG at the same time. You mention the transfer switch should also switch the neutral and I currently have a quote with an ATS that switches the neutral however I was considering changing this since I do not have line to neutral loads. Are you recommending switching the neutral because of the safety concern that I mention for maintenance on either source there could be 277V present on the neutral caused by the a ground fault on the operating source? If so then I can see where I could remove the neutral disconnects that I was considering but I would think Danger signs and notes on the drawings would still be necessary at the transformer neutral as it could see 277V on the neutral with the XFRM primary and secondary breakers locked out. ATS neutral could also be hot but with one source operating the ATS would be hot anyway so warning labels may not be necessary there. I have not done calculations yet but I assume charging current would not change much for the different configurations because the same loads are being served whether powered from the Generator or the transformer. Please comment on the need for switching the neutral on the ATS.

 

[waross]

I avoid switching neutrals whenever possible.
I have several times done repairs when the neutral switching failed and left the neutral open while the power was connected.
Consider the main system without the generator installed.
Yes in the event of a ground fault, the neutral will be at 277 Volts. You need signs warning of the possibility of a hot neutral. Under some safety codes you may have to interlock access covers and doors in the same manner that line connections are protected.
When you install the generator I suggest the same level of protection be applied to locations at both the transfer switch and the generator where the neutral conductor and connections may be accessed.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mikey44]

I considered using the 4th pole of the ATS to do the disconnecting of the neutrals for maintenance however when drawing it out there are conditions where the pole will not isolate the neutrals (See attached Sketches). One example is the generator running with maintenance on the transformer, the 4th pole would be closed and the transformer neutral could be hot. For this reason I plan to place two disconnects near the HRG, one for generator neutral and one for the transformer. There will be warning labels at the disconnects, the generator, the transformer, and the single line.
I will also use a 3 pole ATS as there will be a place other than the 4th pole to land the generator neutral.
Thank you for the comments.
Mike