Microgrid Grounding

[kingpb]

When the behind the meter microgrid (with solar, BESS, and other generation) disconnects from the utility either at MV or LV to operate in island mode, i.e. without utility power, the utility phase conductors are disconnected but the ground is still present. If that is the case, a reference ground is still present. Why would I then need to provide a reference ground? But is that safe? Would it be best to add an electrically operated switch on the ground to isolate the ground as well and make our own reference ground? Or, can I utilize the utility ground and essentially make a ground grid and keep everything on the same reference. There seems to be a lot of confusion around grounding these facilities.

 

[jaggedben]

Ground remains ground and non-current-carrying conductors and equipment would still need to be grounded, first of all. And assuming you have a grounded conductor then it still needs to be bonded and grounded. Certainly if you don't maintain that for grounded systems then things will be less safe. On the systems I install (resi single phase) the grounded conductor remains bonded and grounded in the service equipment. I've heard of larger three-phase systems using a grounding transformer but I believe that is a design choice. Without more specifics I think it's going to be difficult to answer your questions for the general case.

 

[kingpb]

Ground remains ground and non-current-carrying conductors and equipment would still need to be grounded, first of all. And assuming you have a grounded conductor then it still needs to be bonded and grounded. Certainly if you don't maintain that for grounded systems then things will be less safe. On the systems I install (resi single phase) the grounded conductor remains bonded and grounded in the service equipment. I've heard of larger three-phase systems using a grounding transformer but I believe that is a design choice. Without more specifics I think it's going to be difficult to answer your questions for the general case.

I should have stated that this is for large/larger, 3-phase electrical systems.

 

[jaggedben]

..Why would I then need to provide a reference ground?

You wouldn't, broadly speaking, if you're talking about a service that is already grounded.

But is that safe?

At least for LV I think it's safe.

Would it be best to add an electrically operated switch on the ground to isolate the ground as well and make our own reference ground?

Are you speaking of an EGC or a grounded conductor at a service? I think this is where details matter. If the MID disconnects the neutral then you'll need the neutral to be grounded on the load side of the MID. The situation is not so unlike whether an ATS switches a grounded conductor or not. This may depend on how the neutral reference is provided in your system, i.e. is it provided by the inverters or does it require a transformer. A transformer that is only connected when the system is off-grid is a common approach.

Or, can I utilize the utility ground and essentially make a ground grid and keep everything on the same reference. There seems to be a lot of confusion around grounding these facilities.

If we are talking about premises under NEC jurisdiction, the NEC has you ground the service and not make assumptions about utility grounding. I think that holds here. You cannot utilize utility grounding under the NEC.

 

[kingpb]

Thanks, that does help reaffirm my thoughts on this.