Understanding Grounding/Bonding in a Microgrid

[Matt.HH]

Hello, newbie here. I'm trying to comprehend grounding and bonding in an offgrid system involving parallel generators and/or inverters.

First of all, I flagged what I believe to be a distinct violation on a job our company did half a year ago: We installed a 190 kW and a 275 kW generator for parallel operation, with no POCO service present. The paralleling switchgear was a simple arrangement we built ourselves. (We are a UL891 shop.) It is not service entrance rated, so it was not equipped with an MBJ. The install tech bonded gen neutral and ground in the generators' respective breaker panels mounted on the units. The GEC lands on the ground bus in the switchboard and EGC's return to each generator. The inspector did not even comment on it. Since all the neutrals are solidly connected, that could result in potential objectionable current or harmonics circulating between the generators, not? I recommended removing that bonding jumper in one of the generators.

Now my question: Which sections apply to such a system? Personally, I'm of the opinion that we should be rating our switchboard for service equipment use and bonding N-G there. On the other hand, there is no "service" per the NEC 2023 definition. What would be the argument against a non-service-rated switchboard containing the MBJ?

The second and stickier scenario, concerning hybrid microgrids: Almost all the AHJs we have worked with require a N-G bond both at the inverter and at the generator. (Here again, no utility service.) They state that two separate power sources each need their own main bonding jumper. This would be true but for the fact that the inverters are combination inverter-chargers that do not isolate their input and output. Conversely, the N terminal is a single bus bar on which both the gen feed and outbound load feeders land their grounded conductors. Not a true SDS. I believe this arrangement would also be a parallel fault path. The install crews find the inspectors more friendly if they take the "yes man" approach, so they aren't about to argue. Thoughts?

To reiterate, I'm a new kid to the wonderful electrical world, not even an experienced electrician. Perhaps I'm missing the obvious.

 

[jaggedben]

I think this is something of a blind spot in the code but your N-G bond is called a System Bonding Jumper in this case and in my opinion it belongs either at the switchboard or at only *one* source.

250.24 applies to 'service supplied' systems, and doesn't apply here because the definition of a service involves a utility. (Also that means you can stop worrying about having anything be service rated from a code point of view.)

So, that has us looking at 250.30, separately derived systems, and/or at 705 if we treat one source as 'primary' and the rest as interconnected.

In 250.30 the key sentence is at the end of the first paragraph: "Multiple sources of the same type that are connected in parallel to form one system ... shall be treated as a single separately derived system...". The definition of an SDS further confirms that you have one system. I'm gonna say that inverter and generator are the same type if they output the same nominal voltage and phase, but certainly it can apply to two generators.

So then, treating all sources as one system, when we look at where 250.30(A)(1) tells us to put the SBJ, the only place that makes sense is the switchboard where (I presume) we have the first disconnecting means for 'the system'. We can't put it at 'the source' when there is no single source.

The other option I see, code wise, is to declare one generator the 'primary source' and the other one(s) 'interconnected power sources' and follow article 705. In this case system bonding happens at the primary source according to 250.30, and you don't bond N-G anywhere else.

The AHJs who require bonding at multiple places are certainly wrong.

 

[Matt.HH]

I think this is something of a blind spot in the code

Tell me about it!

In 250.30 the key sentence is at the end of the first paragraph: "Multiple sources of the same type that are connected in parallel to form one system ... shall be treated as a single separately derived system...". The definition of an SDS further confirms that you have one system. I'm gonna say that inverter and generator are the same type if they output the same nominal voltage and phase, but certainly it can apply to two generators.

I was looking at that sentence in particular.

So then, treating all sources as one system, when we look at where 250.30(A)(1) tells us to put the SBJ, the only place that makes sense is the switchboard where (I presume) we have the first disconnecting means for 'the system'. We can't put it at 'the source' when there is no single source.

Exactly.

The other option I see, code wise, is to declare one generator the 'primary source' and the other one(s) 'interconnected power sources' and follow article 705. In this case system bonding happens at the primary source according to 250.30, and you don't bond N-G anywhere else.

Which is essentially what we're doing at the generator microgrid I mentioned.

Thank you for your kind explanation, as well as your recommendation on how to apply the Code.