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Switching PV to off-grid inverter when grid fails


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I'm planning on getting a Solis 4G single string grid tied inverter (Probably the 2kW model) and 6x345W JA Solar 60-cell panels. I plan on wiring the panels 3S2P but connect the two strings in series before going into the Solis. I’ll use my current Axpert 3kVA MKS Plus as a backup/off-grid inverter. If Eskom fails I plan to disconnect the PV from the Solis and wire them in parallel to the MPPT on my Axpert to keep the batteries charged. When the grid comes back, I just switch the PV back to the Solis. Can this work? See the attached diagram.

I'm thinking of using Solid State Relays as the switches (something like this) and control it possibly using the ICC Raspberry Pi , which should be able to detect a grid presence on the AC in of the inverter.

I'm under Stellenbosch municipality but getting power directly from Eskom, if that has any bearing on the COC/SSEG registration.

Any thoughts?

Screenshot 2021-04-07 at 12.11.55.png

Edited by gooseberry
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Why don't you just use your AXPert as a UPS with it's batteries and let your Solis be grid tied between the Axpert and the loads? 

It will feed in any load that it can and the Axpert will provide it with the frequency to sync too. 

That way you don't have to swop any panels around.

 

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Posted (edited)
45 minutes ago, Sc00bs said:

Why don't you just use your AXPert as a UPS with it's batteries and let your Solis be grid tied between the Axpert and the loads? 

It will feed in any load that it can and the Axpert will provide it with the frequency to sync too. 

That way you don't have to swop any panels around.

 

I want to be able to power non-essential loads like the geyser from the Solis.  I think my Axpert and batteries are too small to do AC coupling safely and I doubt the Axpert supports frequency shifting.

Edited by gooseberry
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Not very familiar with the Axperts, but if the Axpert is setup in UPS mode, doesn't it only switch over if there is a power failure from Eskom?  

If so, the only problem you would then have would be if the geyser is running when load shedding kicks in, the Axpert inverter would be overloaded and would trip out as a result. If there is enough solar power being fed in by the Solis you would just carry on running of solar panels + batteries.  

Would definitely be a simpler solution than trying to switch you solar panels between inverters.  

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4 hours ago, gooseberry said:

Any thoughts

I hope that the ends of the panels are not earthed, as your diagram suggests to me.

Also, I don't know if you can get changeover SSRs, and if so, whether they guarantee break before make. You might need a lot of SSRs and software delays between breaking and making.

Edited by Coulomb
Added last sentence.
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Posted (edited)
39 minutes ago, Coulomb said:

I hope that the ends of the panels are not earthed, as your diagram suggests to me.

Also, I don't know if you can get changeover SSRs, and if so, whether they guarantee break before make. You might need a lot of SSRs and software delays between breaking and making.

Haha no that's just the negative... it's been a while since drawing circuits.

Perhaps instead of doing something so fancy I can just use 2 of these https://thepowerstore.co.za/products/k-n-dual-string-dc-switch-disconnector-25a-220v-11a-460v-per-string and switch manually. Would just need to make sure that you switch both off before switching over.

 

Edited by gooseberry
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4 minutes ago, gooseberry said:

I only have a 200ah 24V lead acid bank at the moment. I'm not aware of any 24V Hybrid inverters and don't want to buy more lead acid.

Could the battery bank not be reconfigured to 100ah 48v? Assuming its not a single 200ah 24v battery. 

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@gooseberry People here are adamant that you should spend at least double what you were planning to, so my suggestion is to just become rich and buy Tesla... yes, the whole company, why not? It'll definitely solve your problem, but also not in the way that you asked :P

 

Just an observation: You might want to over-spec your SSR's slightly if you're using them for critical switching. Not sure the 12A model you linked to will be up to the task. Bearing in mind that the MOSFET types fail open (do they still do this? 🤔). Just remember that you can end up getting more out of your panels than you're expecting, in cold bright weather. Also note that the minimum control voltage (3.5V) is probably a bit high for the 3.3V Pi, so you'll also need a level shifter of some sort to control that relay reliably. I'm not familiar with the Solis 4G 2kW, but are you sure this would be able to power a 3.5kW geyser? 

 

Good luck on your project!

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3 hours ago, ThatGuy said:

@gooseberry People here are adamant that you should spend at least double what you were planning to, so my suggestion is to just become rich and buy Tesla... yes, the whole company, why not? It'll definitely solve your problem, but also not in the way that you asked :P

 

Just an observation: You might want to over-spec your SSR's slightly if you're using them for critical switching. Not sure the 12A model you linked to will be up to the task. Bearing in mind that the MOSFET types fail open (do they still do this? 🤔). Just remember that you can end up getting more out of your panels than you're expecting, in cold bright weather. Also note that the minimum control voltage (3.5V) is probably a bit high for the 3.3V Pi, so you'll also need a level shifter of some sort to control that relay reliably. I'm not familiar with the Solis 4G 2kW, but are you sure this would be able to power a 3.5kW geyser? 

 

Good luck on your project!

Thanks! I think I'll just go with the manual switch for now. And perhaps at first only connect to the Solis for getting signoff then add the switch and connection to the Axpert.

The Solis won't power the geyser alone as is but I can probably replace the element with a 2kW one, otherwise just it will just pull a bit from the grid too.

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