SMA Sunny Island + Sunny Boys

[reimhagen]

Hi,

I have a grid-tied 15KW PV system powered by two SMA Sunny Boy 7.7KW inverters. It's a ground mount system, and the inverters are located inside a shed away from my home. I'm thinking of adding a LifePO4 battery backup system in my home with a Sunny Island inverter. I'm trying to understand how the AC coupling works. Would I be able to use my solar panels in the case of an outage if I have the Sunny Island?

I have a split 400A service with 200A going into the house (where I want to install the Sunny Island). The other 200A panel receives the output from my PV system. Not sure if the sunny boys are supposed to be electrically "downstream" from the Sunny Island to do what I described above.

Thanks in advance

 

[Hedges]

Yes, it works.
Each SI can only pass through 56A, 6.8kW per 120V, 13.6 kW for a pair of SI.

You might be able to reduce max wattage setting of your two SB.

Otherwise, put one always on AC1 downstream of 2x SI and set up a manually interlocked transfer switch (e.g. interlocked "generator" breaker in your main panel) to move second SB onto SI during power failures, with a 5 second "off" before reconnecting.

Automatically protected loads go on a breaker panel connected to AC1. AC2 grid input of SI is fed from your main breaker panel (turn that off when turning on interlocked generator breaker to backfeed the panel, so SI doesn't see itself in the mirror and think the grid returned.)

SB should be set to Rule-21, per SMA's written instructions. I saw SMA America video recommending something different (island mode) but I'm not comfortable with the safety of that. Maybe there is an incompatibility issue they are trying to address, they didn't explain.

I use SI with SB 5000US for grid-backup, with RS-485. Other people here use SI with SB 7.7 -40 offgrid. Not sure how many are using the -40 or -41 for grid backup.

 

[reimhagen]

Would I be able to achieve such a setup without making the SBs downstream from the SI? It'll be somewhat tough to change the wiring. I've attached a quick sketch of how things would be hooked up. Worst case I suppose is I'd just use the SI as a battery backup independently of the SBs/PV array.

 

[Hedges]

Yes, but that's just a UPS and when batteries run down it shuts off.

1) move SI (your need 2x SI for 120/240V split phase) AC2 grid input over to the other main panel which has 2x SB.
2) Install interlocked "generator" breaker
3) Wire SI AC1 island output to "generator breaker"

I'm thinking of attaching a second type of interlock, the one that works across two branch circuit breakers, to enforce turning off breaker for AC2 when breaker for AC1 is turned on.

4) Put an interlocked breaker on backup panel, so if SI is down you can feed straight from grid.

If SI shuts down due to low battery, it will never wake up again until battery has charge (well, maybe if you turn off and turn on, but battery could get too low). Separate battery charger would be one approach. Backfeeding AC1 and enabling emergency charge is another.

I have SB tied directly to AC1, then "load shed" relay, then protected loads panel. Loads get shed at 70% DoD, SI turns off at 80% DoD. At least those are the default settings for lead-acid.

If you only use one SI, you can use auto-transformer to make split phase and power up SB. There are small auto-transformers available, or you can use the two 240V primary windings of a utility transformer, labeled 240/480, for 120/240.

But I recommend 2x SI. You can buy two in a box, with Midnight 250, for $4000 delivered on eBay.

 

[Hedges]

Which battery are you considering?

 

[reimhagen]

I was thinking of buying a bunch of those EVE 3.2V cells and building a battery bank out of them.

It's harder to move the SB/SIs around; i simplified the layout a lot in my drawing. The SBs and second panel are located at a shed 300 feet away from the main house; and only a pair of feeders connect the two buildings. So if I want to achieve a downstream setup I'll have to pull another set of feeders, but the conduit is at its fill already, so that would necessitate re-trenching and putting in another pair of feeders.

I might do it in the future but probably will just start with a simple battery backup system without the solar component yet, to make it simpler.

 

[Hedges]

You can put the SI upstream. They do need to be out of the rain. Just need one set of wires from grid to SI, and one set form SI to SB. It is convenient to have split wires SI to SB vs. SI to loads.

You can have SB physically far away from SI, SB where loads are located. It would be useful to put in a load-shed relay to disconnect loads but keep SB connected, need to run a cable to power the coil. This keeps battery from getting over discharged.

You can build batteries and either run open loop with VRLA settings, or use a compatible BMS for closed loop. Reportedly REC works well, but is expensive. EG4 PowerPro people report works closed loop.

Is the battery for daily cycling? LiFePO4 would be good for long life.
I used AGM because only for backup. Cost is about same for same kWh capacity, much more expensive per cycle life, but I only cycle during grid failures.