Multiplus II supply emergency power through generator interlock?

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jameshowison asked Oct 1, '23

Multiplus II supply emergency power through generator interlock?

I’m in the US. We have a generator inlet connected to an interlock switch. In an emergency we can connect a generator and power the house through this inlet and interlock. The interlock makes it so that the main breaker must be off for power to enter through the generator inlet. The generator inlet is connected to our main panel (not a critical loads panel). As one would expect there is a neutral-ground bond in that main panel. In this arrangement one is supposed to use a generator that does not have a neutral-ground bond.

We also have a camper with a Multiplus II, solar, and batteries. This is connected to grid power through an outlet on the house (for charging in addition to solar). The power outlets in the camper are connected to AC-out-1.

I would like to use the camper to provide emergency power through the generator inlet. Essentially this would mean "plugging the house into the camper" and having that power distributed through the main panel (we would switch large loads in the house off by turning off their breakers).

In the event of a grid power failure I would unplug the multiplus II from the house. I would isolate the house from the grid by flipping the interlock, then I would then connect AC-out-2 to the generator inlet. I would then turn on the Inverter on the Multiplus II.

Two questions:

1. Given there is a neutral-ground bond in the main panel, my understanding is that there should not be one in the Multiplus II. By default, though, when disconnected from the grid and the inverter on, the ground relay makes a neutral-ground bond. I can turn that off using veconfigure. Is that the right thing to do?

2. If I turn off the neutral-ground bond, will the camper outlets (connected to AC-out-1) be grounded through the neutral-ground bond in the main panel? ie. does the neutral-ground bond "downstream"
of AC-out-2 also work for things connected to AC-out-1, if the ground-relay is disconnected. I think it probably does?

And then two more questions:

1. I think I can use a programmable relay to turn on AC-out-2. I've tested that via veconfigure assistant and indeed power is present at AC-out-2 when the inverter is active (unlike the default).

2. Can the ground relay also be controlled by a programmable relay? I can't see a way to do that in veconfigure. If it can't, then I'd have to hook up veconfigure when the grid went down?

3. Can both of those relays be controlled by the Cerbo GX? Does that require wiring from the Cerbo GX relay to the Multiplus II aux relay? ie the programmable relays can't be triggered through VE.Bus?

Essentially I'd want two modes for the camper, one like the current default setup (AC-out-1 active, ground-neutral bond present if AC-in is unplugged ("camper mode"), and one where AC-2 is active and the ground-neutral bond is never present ("generator mode").

I understand that it would be bad if the system was in "generator mode" but the AC-2 connection to the house main panel was not made (as there would be no neutral-ground connection but the inverter would be providing power). I would avoid that, but I think it helps that all of the outlets connected to AC-out-1 are GFCI protected?

Multiplus-II Grounding

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Answer 1 · 2023-10-01T17:49:23Z

Alexandra answered · Oct 01, 2023 at 05:49 PM

@jameshowison

1. No sure what the question is, but it is easy to test. If it is inverter and there is no voltage then it is off. I am sure you have that handled.

2. The ground relay is controlled by the inverter and the base programming - not interactive otherwise; it should be an automatic protection if required.

3. No the AC 2 programmable relay is not controllable from the GX, only the main grid input one is. You can use the other relays (aux temp etc) to switch on and off other contactors using assistants and daisy chain them through the GX relay.

Local regulations are really what you need to look at for this one.

How are you hooking up the AC2? With 3 core? 2 core?

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Answer 2 · 2023-10-02T20:22:53Z

Thanks!

Point noted on regulations etc. I'm trying to get the Victron system to act like a gas/propane generator with a floating neutral (supplying power but not bonding the neutral-ground). That is a standard and code compliant thing in the US when used with generator interlock and inlet. e.g., https://www.electricgeneratorsdirect.com/stories/1021-How-to-Avoid-Tripping-a-Generator-With-a-Bonded-Neutral.html and https://diy.stackexchange.com/questions/247189/bonded-neutral-generator-connecting-to-house-and-removing-bond-in-main-panel-i

This is also discussed here: https://diysolarforum.com/threads/multiplus-ii-with-voltage-on-ac-output-ground.50936/

I had another question above:

If I turn off the neutral-ground bond, (and connect AC-out-2 to a panel that has a neutral-ground bond) will the outlets connected to AC-out-1 be grounded ie. does the neutral-ground bond "downstream" of AC-out-2 also work for things connected to AC-out-1.

I'm pretty confident now that the answer to that is yes. I confirmed (with a multimeter) that all the Grounds are connected together (it's also shown in the block diagram on the unit and in the manual).

I was able to run the system with AC-out-2 connected the main panel and the ground relay off (thus floating ground system). Once connected to the interlock all the outlets in the camper (connected to AC-out-1) show as having the neutral-ground bond from the panel. So that's great. Once I flipped the interlock braker the system supplied power to the house just fine.

For controls to swap between modes, given your answers my understanding is:

I will have to use a wire between the Cerbo GX and the Multiplus II to toggle AC2 on and off via an assistant.
The ground relay can only be toggled through veconfigure (either via USB Mk3 or via VRM upload of a veconfigure file).

Does that continue to be the case even if I have VenusOS installed on the Cerbo? Can VenusOS running on the Cerbo enable one to upload a veconfigure file? Or that can only happen from VRM (which means that the Cerbo has to have an internet connection)?

That isn't great as now the whole system is dependent on my ancient windows laptop continuing to work (as it _still_ seems to be the only way to run veconfigure?).

(small note for future searchers: I did spot the "external ground relay" on the unit but more reading shows me that is not relevant to controlling the ground relay on/off, it is just used to pass on the message to close an external relay (as used with an Auto-transformer for example).

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