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SMA Sunny Island grid-tie / battery backup with Sunny Boy inverters design discussion

Each Sunny Island just makes one 120V phase. Each can pass through up to 56A to its phase, or supply something less inverting from battery.

With 2x SI wires 2s, you get 120/240V split-phase.
Sunny Boy is 240V. Say it delivers 20A (4800W), and you have 4800W load all one phase 1. Half that load is driven by SB (although SB doesn't put any current into neutral, connection may be optional); SB puts 20A into phase 1 and 20A into phase 2.
No loads on phase 2, so the SI on phase 2 sucks down 20A, 2400W to 48V.
On phase 1, loads are 4800W but only 2400W, 20A, coming from SB (on phase 1), so SI on phase 1 draws from 48V and delivers another 2400W, 20A on phase 1.

That behavior is similar to what an auto-transformer would do.
You might get a benefit from connecting an auto-transformer. It would try to balance load on the two phases, and Sunny Island would also try to balance. I don't know how much each would do.
There may be a problem having auto-transformer connected while on-grid. If grid is imbalanced and you receive different load on phase 1 vs. phase 2, the autotransformer would try to balance the grid.
I don't know if SI has an on/off grid signal. I've thought about that for my system.

An isolation transformer, 240V in, 120/240V split-phase out, would perfectly balance loads. It comes at an efficiency cost.
 
I used a 2-pole 70A breaker QO270 to connect two SI, wired series or 2s when I first brought the system up.
Then 2x QO270 for four SI wired series/parallel 2s2p. That was when I observed current imbalance something like 3:1, tracked down to different in resistance between two QO270 on a given phase.

Which Surge protector - Midnight? Those seem good.
I had been using Delta, on both AC and DC. Until I tested one, found it didn't clamp even with 5000 Vrms HiPot (7000V peak). Apparently they are a joke in the industry. Midnight has videos comparing their on surge protector to an unidentified grey cylinder.

For the 225A QO breaker panel, I'm aware of 200A and 150A main breakers, don't think there is a 100A. So I could see interlocking 150A main with 100A or 125A backfeed.

The smaller 125A QO panel has 125A, 100A, 70A main breakers.

Without PV to backfeed it, the critical loads panel isn't subject to 120% rule. Smaller panel may meet your needs.
It is the Solar PV Landing Panel where larger panel would be needed if expanding to 4x SI and 4x SB.
 
I used a 2-pole 70A breaker QO270 to connect two SI, wired series or 2s when I first brought the system up.
Then 2x QO270 for four SI wired series/parallel 2s2p. That was when I observed current imbalance something like 3:1, tracked down to different in resistance between two QO270 on a given phase.

Which Surge protector - Midnight? Those seem good.
I had been using Delta, on both AC and DC. Until I tested one, found it didn't clamp even with 5000 Vrms HiPot (7000V peak). Apparently they are a joke in the industry. Midnight has videos comparing their on surge protector to an unidentified grey cylinder.

For the 225A QO breaker panel, I'm aware of 200A and 150A main breakers, don't think there is a 100A. So I could see interlocking 150A main with 100A or 125A backfeed.

The smaller 125A QO panel has 125A, 100A, 70A main breakers.

Without PV to backfeed it, the critical loads panel isn't subject to 120% rule. Smaller panel may meet your needs.
It is the Solar PV Landing Panel where larger panel would be needed if expanding to 4x SI and 4x SB.
Yes on the Midnight SPDs.

We are on a new page ... so thank you again for your patience and detailed explanations. I've reread the thread at least 10 times. No telling what I haven't yet absorbed though. ;)
 

I don't know if SI has an on/off grid signal. I've thought about that for my system.
It does. That is what the DIGIN and the relay is for. When the grid is gone, the relay contact is closed. The SI is in off grid mode and will not back feed a generator. Display changes from a "power pole" symbol to a "square root" symbol. When the grid is there, relay is open and SI can backfeed the grid (as I understand it anyway).
 

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  • TechNDIGIN-HUS123112.pdf
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I'm referring to an output, not an input.
So I can do something different when SI has disconnected from the grid. Maybe connect a balancing transformer, maybe shed loads differently.

This seems to be it:

"GdOn ... Utility grid ... Relay switching when utility grid is available and connected."

Not sure if it is reliable/fast enough to use for an external grid relay (e.g. switch L2 of grid for a system with single Sunny Island on L1 and auto-transformer on output), but it would work for my other purposes.

I have a 3-phase setup with just one phase connected to grid. I was thinking I could synthesize a split-phase in addition to the 3-phase output, and if that could be switched onto grid, twice as much power could flow to-from grid.
My earlier thought was a current transformer on the L1 grid connection. Above 30A, connect the split phase relay to grid. Below 10A, disconnect. If this signaling relay works, that could be the control.
 
That is beyond my knowledge. I just know about the DIGIN part. Can't connect a generator without it and the tech sheet info is not in the manual, so it is not obvious how to set it up.
 
That would be can't switch between generator and grid input without DIGIN.
If off-grid, input can be configured for generator only, and it won't allow backfeed.


Sunny Island can be configured to have just Master running and Slave sleep if not needed for more power. But reportedly, that results in second phase lost in 2phase2 and 2phase4; SMA failed to address split-phase systems for this feature. So I figure I could connect an auto-transformer to allow it. But might want that disconnected when on-grid to avoid trying to balance the grid. This output relay could control it.
 
That would be can't switch between generator and grid input without DIGIN.
If off-grid, input can be configured for generator only, and it won't allow backfeed.


Sunny Island can be configured to have just Master running and Slave sleep if not needed for more power. But reportedly, that results in second phase lost in 2phase2 and 2phase4; SMA failed to address split-phase systems for this feature. So I figure I could connect an auto-transformer to allow it. But might want that disconnected when on-grid to avoid trying to balance the grid. This output relay could control it.
Right, forgot about that . So the DIGIN relay saying the grid is there or not does not help with what you are tring to do then.
 
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