Neutral/Ground Bond Clarification with Victron Autotransformer

[WolfbaneAries]

Setup: 2x MPII in split phase configuration running to Autotransformer 100A for load balancing, feeding a main breaker panel in a shed. Neutral and Ground are bonded inside the main panel. The Autotransformer instructions say that the ground relay in the AT only should be used and the ground relay(s) in the MPs should be disabled. However isn't there only supposed to be one location where N and G are bonded? Since they are bonded in the main panel, does that mean none of the ground relays should be used?

 

[pvgirl]

Does your system have a grid power inlet? If so you should use the N/G relay in the autotransformer and remove the bond in the main panel. This way when off grid relay will make the N/G bond, and when on grid will the relay will open and the N/G bond on the grid feed will be used.

If you have no grid power feed you can use the bond in the main panel and disable any of the relays in the Victron equipment.

 

[WolfbaneAries]

No it's completely off grid. Great, that's what I thought!

 

[Hedges]

If you wire this correctly, it should allow full 240V power of inverters to loads, full autotransformer current for imbalanced load, protect the transformer from overload, disconnect all loads if needed to protect transformer and not subject loads to brownout or lost neutral.

https://www.victronenergy.com/upload/documents/AT-1-split-phase-240V-to-120-240V-with-Quattro-240V.pdf

We've seen other autotransformer implementations that had problems. I think this one does everything correctly at least if no grid input. In the event there is grid or other input with its own neutral/ground bond, then I might have issues with this design.

 

[WolfbaneAries]

I have two MPs stacked in split phase so I'll be following this diagram https://www.victronenergy.com/uploa...20V-to-120-240V-with-2xQuattro-120V-REV-B.pdf

 

[Hedges]

Yes, I think that's functionally equivalent, just happens to stack two 120V inverters and takes 240V from them to autotransformer, instead of using one (or more) 240V transformers.

But what's the point of using autotransformer to derive neutral in this case, if you already have neutral between the two inverters?
Looks to me like 5000W Quatros stacked can deliver 40A imbalance between L1/L2. (3000W can deliver 25A)
Using autotransformer, you can get 28A. Sounds like a downgrade, with some reduction in efficiency, for extra $$.

https://www.emarineinc.com/Shared/pdf/Victron/Datasheet-Quattro-3-5-kVA-120V.pdf

 

[WolfbaneAries]

In this case the AT is being used to the balance the loads between the two inverters.