Fronius Symo Gen24 Plus with multiple DC connections to MPPT1

f2calv

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Jun 10, 2021
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My solar installer* has recommended a Fronius Symo Gen24 Plus 6.0;
The inverter has 2 MPPTs and according to the above documentation there are 2 DC connections possible to MPPT1.
1632625969433.png
I will have 3 strings because of a shading problem and I presume the 2 partially shaded strings (red & orange below) will connect to MPPT1 and the final unshaded string (yellow) to MPPT2.

I can't find any documentation/explanation on how these multiple DC connections to a single MPPT work (or will address the shading problem).

Can anyone explain to me, or direct me to some more information/documentation on this specific feature?

1632625837452.png

*With my Installer there is a language barrier, hence I'm asking here...
**Plus Fronius support seems to only want to talk to installers, not pre-purchase enquiries.
 

wattmatters

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I can't find any documentation/explanation on how these multiple DC connections to a single MPPT work (or will address the shading problem).
It just means you can put two strings of panels in parallel into one MPPT and one string into another MPPT. The two strings in parallel must have the same number of panels (each of the same wattage rating) in each string.

What is the shading problem?

I will have 3 strings because of a shading problem and I presume the 2 partially shaded strings (red & orange below) will connect to MPPT1 and the final unshaded string (yellow) to MPPT2.
I'm not really following your diagram. It seems to show 68 panels to be connected? That makes no sense. Unless you are talking about more than one inverter.

The 10kW Gen24 Symo inverter and has an effective limit of ~13.3kW of panels you can connect. It says the max is 15kW but it's hard to get modern panels to match that theoretical limit and remain inside other MPPT limits. To get close to 15kW of panels requires older panel types, of ~300W or lower. And that means a lot more panels.

What panels are you going to be using or is the installer suggesting?

e.g. if you use 375W panels you might have:
2 strings of 11 panels into MPPT 1 and one string of 13 panels into MPPT 2 for a total of 13.13kW of panels.
 

f2calv

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Jun 10, 2021
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@wattmatters thanks for your input, that makes sense regarding two strings in parallel with the same number of panels going into MPPT1. After some more googling I found a external discussion thread where someone has 2 strings connected in parallel to MPPT1.

The modules themselves are actually Planum PV solar roof tiles from Nelskamp @ 90W each so the total capacity of the roof is ~6kW.

The shading problem I talked about in an older thread, so I won't duplicate here;
I had asked the installer about possible optimisations of the panels via a combination of series/parallel string wiring (my electronics knowledge is 20+ years old) but they said basically 'nobody does that sort of wiring anymore'. They also said Tigo optimisers were too complicated/troublesome I guess because if an optimiser fails you can't get at the optimiser because it's hidden under the tiles. I guess they just relay on the inverter handling the shading issue.
 

wattmatters

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The modules themselves are actually Planum PV solar roof tiles from Nelskamp @ 90W each so the total capacity of the roof is ~6kW.
Ah, I've not heard of those. No wonder I got confused by the number of panels in the diagram!

I have a Fronius Symo 10kW (not the Gen 24) and I have two strings in parallel into each MPPT. My Symo current limits for each MPPT are 16A and 25A, whereas the Gen24 model reduces MPPT 2 current rating to 12.5A, hence the single string limit.

I see those panels have a working current ~9A. So for MPPT 2 then only one string of those makes sense.

MPPT 1 could manage two strings in parallel no problem, and you can probably put three strings in parallel. At times you'll hit the 25A MPPT limit but that's OK, the inverter just clips production at the current limit. This is only going to happen when all three strings are in optimal solar PV conditions.

I get current clipping on good solar days on my MPPT2:

Screen Shot 2021-09-21 at 7.57.17 am.png

It's no issue for the inverter - it will clip production if it needs to.

As for shade management, if each panel has one or more bypass diodes then that will also help a lot, as will parallel arrays.

Optimisers are OK for strategic deployment on a few shade affected panels in a single string only and are probably more suitable for larger wattage panels than the small wattage panels you are using.

You can't use Tigos in a paralleled array unless you put them on every panel across all strings connected the MPPT, and that would be ridiculously expensive. Which is why you really only consider them with single string arrays. If you want them on every panel then you are better to consider micro inverter solutions.
 

f2calv

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Jun 10, 2021
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Thanks for all of that information, much appreciated! :)

There is a supply shortage of the Fronius Symos (like everything in this Corona world) so unfortunately I won't get the inverter/battery installation added until early next year, when I do I will update this thread with output details.

I checked my discussion notes with the module manufacturers and apparently there is 1-bypass diode per module/panel.

I contacted Tigo support in Europe and they were very helpful, but I also had a small feeling it was going to over-complicate the setup because they are designed for big panels and not small modules/tiles like I have... and yes you are right, it all adds to the expense!

And here is the roof :)

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