Isaac Shannos avatar image

Is Off-Grid AC Coupling any good with Fronius or Solar Edge?


From what I can tell Frequency shifting isn't a perfect solution for an off grid ac coupled solar control.

Does Victron support both Solar Edge and Fronius AC coupling options 100%? Are there any limitations to frequency shift control? Other than the 1:1 rule.

Is Selectronic Frequency shifting (generic AC coupling) more advanced than Victron? I heard Victron was 25% steps up or down in power.

offgridAC PV CouplingFroniussolaredge
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1 Answer
Guy Stewart (Victron Community Manager) avatar image
Guy Stewart (Victron Community Manager) answered ·

Hi @Isaac Shannos,

Sorry for the delay in getting to this one, I wanted to have some hard data to back it up, and that took a little while to get organised.

Firstly, if you are considering AC coupling with Victron;

Please do the introduction to Multiplus coupled with AC PV inverter training here - -

Read the general Victron coupled with AC PV documentation here.

And read the Specific manuals for each AC PV inverter that enjoys specific 'Victron supported status' are here.

I know you are an experienced professional, and a lot you will already know, (So you can watch the video at double speed and then jump straight to the exam ;)

When well configured, Frequency shifting is inherently the fastest response (and hence the most accurate) way to regulate an AC PV inverter.

It is possible to use alternative regulation, such as Modbus TCP commands, and Victron does use these as well with our compatible Zero-Feed in partners (e.g Fronius, ABB, etc) in ESS systems when it is not possible to shift the frequency but AC PV regulation may still be required.

It was a VERY deliberate decision to NOT use Modbus TCP as a control protocol in off grid regulation as an alternative to Frequency shift power control. Frequency shifting power control has some known issues (explained in the training), but where it counts (speed and certainty of adjustment) it is superior, and always will be over an additional network/communications protocol. Compatibility with a wide range of AC PV inverters is another big advantage.

I don't know where you heard 25% power adjustment steps for Victron, but that is not correct.

Hard Data

Using the Venus Docker Grafana tools, I can get a system reading with about 2 second sample rate resolution, unfortunately this is a limitation to our data collection, and shows the system to be less responsive than it actually is. But it will have to do for now.

The frequency adjustment is precise to equal or less than 0.15 hz.

The useful regulation band of 51 hz < - > 52.7 hz gives us at least 1.7 hz of range.

11 steps at 0.15 hz accuracy is ~9% steps. Again, it is actually better than that, but that is close to what I can demonstrate with the data collection tools I have in front of me.

The transition in this system example from 100% to 0% takes place over 51 seconds from 13:51:06 to 13:51:57 (this could be longer or shorter depending on the urgency, battery voltage response, etc). It was simulated by turning a 1 kW electric heater off with a 100% full lithium battery.

In a little under 2 minutes, you can see here the complete transition from an unregulated Fronius Primo 3.0 (with a partially shaded array), outputting 1620 W at 50.90 hz to a fully regulated output of 20W at 52.65 hz.

This demonstrates 10 actual power adjustment steps by the Fronius AC PV in 120 seconds,

The Fronius data has a sample rate frequency of ~5 seconds over Modbus TCP.

The MultiPlus data has a sample rate frequency of ~2 seconds, via dbus and then MQTT.

A better test to show actual accuracy would expand the time frame, or connect ACTUAL test equipment - but I think it's enough to show what is going on.

The Victron and Fronius support partnership is 100%. If you have everything designed, configured and installed according to the documentation, there will be no finger pointing to each other.

Now knowing that, in all cases, configurations, and brands, AC PV is inferior to DC PV for battery charging efficiency, fidelity and accuracy, which is a priority for long term off grid system reliability. Victron STRONGLY recommends the use of DC PV MPPTs for off grid applications.

VVV Click the jpg link here for a full resolution version of the chart VVV

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