Victron - Fronius - control by Venus

Please have a look at this image:

  • At this moment, the Fronius is delivering about 4KW more than the load (AC + Critical) needs. This 4KW is thus charged into the battery, as can be seen (The battery is receiving about 4kw more than the DC MPPT is delivering)

  • At this moment, the Multiplus input and output is connected - that is the only way that you will get the Fronius’ power into the AC loads.

  • With the Fronius connected to the grid, no frequency control is possible - it is running at full power.

  • I want to understand what will happen when the battery is full:

    • The Fronius still delivers more than the (AC + Critical) and has to be throttled down
    • The AC coupling feedback mechanism means that the AC frequency of the Fronius needs to be increased
    • The only way that the Multiplus can change the frequency is to disconnect from the grid - the “Critical Loads” side is then driven by the Multiplus + Fronius.
      • The Multiplus then varies the frequency of its output to throttle down the Fronius
    • But the main AC load is still 10.9 KW, and since the Fronius is disconnected from the grid / Main AS load, it can not contribute power to the main load.

Am I right in this description? The main AC load will then have be switched to the grid and draw full power from the grid? All PV power will then only be used for the critical loads?

Why does it need to change the frequency? There are other ways to throttle a Fronius :slight_smile:

Specifically, the Fronius inverters (all the newer ones at least) implement SunSpec model 123 (immediate controls) which implements (among others) a modbus-tcp way to limit the power production.

If you disable feeding in of AC-coupled PV, then there is a particular part of the software that wakes up and starts controlling this limit.

If there is a grid failure while this is going on, then this limit is removed (the PV-inverter is returned to unthrottled status). The Multi then controls it via frequency control, just as it would in an off-grid situation.

1 Like

Thanks for the explanation. I have been trying to understand that Fronius feedback mechanism in ESS mode, but only found the frequency control in the documentation.

Can you refer me to a document where this SunSpec feedback by Venus is explained? Eg - if this Fronius is very far from the Venus - can it be connected over ethernet?


Yes, of course.

I don’t know of a document that explains this sunspec feedback thing. You can google for the sunspec documentation itself (if I recall they want your email address and who you work for, then you can download it), and that explains the various models.

There is one particular document, the SunSpec model reference, which is a spreadsheet (I have an old one from 2017… but they don’t change much). That has all the models each on their own sheet, and when you look at model 123, you get:

And the bit I marked in red… that is what makes it work: WMaxLimPct, and WMaxLim_Enabled.

If you see power going into the grid, you bring the Pct (percentage) number down. The other number you simply write a 1 repeatedly to enable the limiter. When you want to unlimit the PV-inverter, you set that back to a zero and you’re done.

SMA supports this kind of thing too, but there are some caveats towards getting it right (either firmware has to be new enough, or something else… I’m not 100% sure). And ABB/Fimer also has a decent implementation of model 123.

But because some implementations are dodgy, we specifically only enable limiting with Fronius and ABB.

Welcome Francois…

Thanks for the explanation. I just think Victon’s own documentation for Fronius integration can explain that there is a non-AC coupled control mechanism for ESS with Fronius.

So here is the reason why I am investigating this:
I have a 5kva Multiplus II with 6kWp PV and a BlueSolar MPPT 250/100. Pretty well matched when there is good sunshine.

But many days the sun does not shine enough. Eg: I got 26KWh two days ago, and today I got 17 KWh.

So I want to add another 6kWp PV panels.

But the only location where I can install the second set of panels is about 80m from the batteries.
So I see three options

  1. 80m heavy cables that can take 150V DC at 32Amp to another 250/100 MPPT (R16k + cables…?)
  2. 80m lighter cables that takes 360V at 12Amp to a new high voltage 450/100 MPPT (R21k + cables…?)
  3. 4kw Fronius close to panels, connected to 220v AC, with one Ethernet cable. (R25k + cables…?)

The Fronius will be the most expensive, but it will also double my peak current consumption when the sun shines, but then there is the question of what is legal to connect to a single phase 80 amp grid connection in Johannesburg.

Can’t you just run each string individually with (for example) 4mm cable to the DC combiner box where you put them in parallel?

I didn’t go do the maths whether 4mm is enough, but I don’t think it should be necessary to get the amps up before you want to plug into the MPPT?

My current panels are about 25m away. I am using 2 x 6mm DC cables in parallel to carry 32A from the combiner box to the 250/100. For 80m I will need at least 3 x 6mm cables.

That might be simply because these two methods are part of different worlds, so to speak.

Frequency-shifting is typically used in off-grid systems. On these systems, you don’t use ESS.

Sunspec-limiting is used on grid-tied systems. On these you use ESS. This is part of the ESS ecosystem and won’t even work unless you have a Multi with ESS installed.

Victron has way more off-grid installations in the world than grid-coupled :slight_smile:

But isn’t that because you already put the panels in parallel before you start the cable run? @JacoDeJongh wired my panels so that you bring down each string in series to the DC combiner box, there you parallel them, adding the amps, and then a short run to the MPPT.

That way, you have a bit more control over your individual strings and don’t need to run very thick wires (just more wires).

Exactly what I did … and the bonus is, when you want to test each string, switch them on, one by one, and check the volts/amps under load, to ensure all strings are still preforming as expected.

For 80 meters I’d use a PV-inverter. No question.


Can you share with us your complete system design and layout? How many multiplus’s do you have? There is also settings on the actual Fronius that you need to do to activate the ability of the victron to control the fronius.

Indeed, but that is all documented. Essentially you just have to enable modbus-TCP and you’re done. It is off by default.

The way the GX device scans for a Fronius is by first connecting to port 80, where there is a small web server (the same one you use to configure the inverter). It uses a protocol called SolarAPI to query the DataManager, which is a little box that sits between the PV-inverter and your network. In many inverters the DataManager is built into the PV-inverter already so you don’t even know that there is this distinction.

SolarAPI is mostly JSON requests and responses. Boring and not worth too much detail. Part of the response contains the number of PV-inverters connected to the DataManager (you can have multiple PV-inverters per DataManager, most residential installs will have just one). Each of these are on a different modbus address, starting from one (so for most people, the pv-inverter has unitid=1).

It then connects to the DataManager on port 502 (the default for modbus-tcp) and queries the sunspec support for each PV-inverter. And then it continues talking to the inverter using only sunspec.

If you don’t turn on modbus-tcp support on your Fronius/DataManager, then it still works… but it uses the older SolarAPI to get the data, which is a little slower, and cannot do any power limiting. Power limiting only works with sunspec (aka modbus-tcp) enabled.


Sometimes I wonder what we did to deserve Izak.

1 Like

I had the privelidge to install a similar system this week. It has 112 x 390 mono panels on the roof. 96 feeding a 27kw Fronius and the other 16 feeding a 250/100 Mppt. Also has 64kwh lithiums, 3 x 15kva Quattro’s and a 40kva generator as a final backup.


Soos altyd: baie netjies gewerk, Jaco.

1 Like

Can you tell us about the strings feeding the Fronius? Vmppt and I per string for 27kw?

Currently driving home, not sure of VOC, but string voltage just over 600vdc while producing.

16 x 390 watt mono panels in series per string and six strings connected to 1 mppt. Just over 10 amps per string. .