Victron GX AC-coupling PV Inverters power control

I’ve been reading up on PV Inverters with different PV systems on a GX setup.
For AC-coupling these PV inverter is installed on the output side of the Multiplus II and then act as a mini-grid when there is a power failure (loadshedding for instance).

What I don’t understand is the “power control function” vs. “Frequency shift power control”. The SMA inverter for instance does not support the power control function, but does support the Frequency shift power control. Both seem to be regulating the output power on the PV inverter.

What am I missing? Is the one to do with feedback into the grid and frequency shifting for a microgrid not connected? That seems to be the only logical explanation I can figure out.

The distinction is that power control through the frequency-shifting mechanism cannot be achieved when connected to the grid. Whereas a Fronius, for example will still have power control functionality when the grid is present.

So in other words the result is that the Fronius can limit power and thus don’t feedback, while the SMA will always feed back?

Yes, an SMA will always try to go full tilt at 50Hz. Whether it feeds back or not depends on the prevailing loads and SOC.

An SMA has its own smart meter that can change this behaviour, just like a Solis has a CT. Then the same thing as Fronius’s control could be achieved independently of the Venus operating system.

Remember also that the moment the grid fails, the GX device will stop controlling a Fronius PV-inverter over sunspec/modbus-TCP. The system instantly becomes a microgrid and the Multi/Quattro has to do the regulation.

When the grid returns, the GX device will start controlling the limit again.

Two cooks in the kitchen is a bad idea, you see.

That means all inverters that support frequency shifting can be used on the output of a Multi, and it will work fine in a microgrid setup. But if there is grid, then only the ones with limiter support can be used (unless grid feedback is not an issue).

You can put a Solis on the output of a Multi. You can also put a Huawei SUN on the output. They all work fine, they just cannot be measured (unless you install an energy meter for that) or limited.


“the moment the grid fails, the GX device will stop controlling a Fronius PV-inverter over sunspec/modbus-TCP. The system instantly becomes a microgrid and the Multi/Quattro has to do the regulation”

@plonkster : Is this for true both input side and output side coupled Fronius? Would you care to explain why please?

When the grid is out, PV inverters on the input side are shut down (they are sent a zero limit). PV-inverters on the output are set to unlimited, so the Multi can do the limiting.

When the grid is out, the PV-inverters on the input side will disconnect in any case, so what we do with them really doesn’t matter. Historically we’ve always shut it down. I think that is so they don’t ramp up uncontrolled when the grid does return (typically they remain connected to the network as long as there is sunshine).

It wasn’t always like this. It changed in July 2018 :slight_smile:

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I know that the AC-coupled inverters on the input side shut down when utility power cuts, and that they don’t on the output side. But I don’t get why the output side inverters are set to unlimited so that the multi can do the limiting. Why does the GX not keep managing the output side Fronius as it’s still powered and connected over Modbus? What’s the logic behind this?

I have never had a Modbus linked Fronius downstream of a Multi.
Mainly because:

  1. Even used Modbus-capable Fronius inverters are comparatively expensive.
  2. After enquiry, frequency-shifting control was a faster control algorithm.
  3. F-S is more reliable and works without comms channels or during comms channel failure.

In my off-grid system, I have used two PV inverters in parallel on a Multi output, namely a Solis and an older ABB Aurora. These inverters work seamlessly together as one with frequency-shifting. So I assume a Fronius (using the Frequency-shifting mechanism) would also.

So, to flip to your question, what is the logic of keeping the Modbus control in an off-grid situation?
Why use a slower control algorithm when the alternative is superior?
And secondly:
Why introduce the additional complexity of making a stable feedback loop for both Modbus and frequency-shifting stimuli when another PV inverter may/may not be involved?

This is what @plonkster implied about system stability when saying:

Again, why when the proven alternative is universal and simpler?

Correct. Also, there is about a 5 second delay after instructing an inverter (over modbus) to reduce power, but it moves slightly faster and smoother via frequency shifting. The frequency-based limiting also has priority over the modbus one. So why do the limiting twice, if just one of them works so well, and has priority anyway?

Maybe the language is confusing. When I say it sets it to “unlimited” I mean it takes the software control out of the way so that the hardware (frequency-based) limiting is the only cook in the kitchen. Why complicate things… they are complicated enough :slight_smile:

I understood that the the Fronius did autonomous frequency alignment (as in the Victron had no part in it) with the Multi/Quattro output via shifting, and that the Modbus connection with the GX was purely so that data could be displayed on VRM. So when the grid dropped a Fronius connected on the output side of a Multi/Quattro would already be synced and nothing would change. Right?

When the grid returned things may have changed as the Multi/Quattro may be slightly out of sync, and then it would sync with the grid and on the output side the Fronius would be syncing with the Multi/Quattro, right?

When the grid is there, everything is forced to run at the grid frequency, which is normally just about bang on 50Hz, but up to 50.7Hz no power reduction is done anyway. So while on the grid, the only way to stop a PV-inverter from feeding too much energy into the grid (and spilling out past your prepaid meter, which will bill you for it, trip, or go into tamper mode) is to tell the PV-inverter using software to throttle down.

In other words, while connected to the grid, frequency shifting cannot be used, so you use software.

Once disconnected from the grid, the setup changes to a microgrid. The Multi is a low-frequency design, which means that PV inverter basically feeds directly into the battery. Any delays in throttling down power production will overcharge the battery in seconds. Software control is simply too slow. So the software control takes a back seat and we play by microgrid rules until the grid comes back.

Just in case it’s being lost in translation, and I do apologise if I’m stating the obvious. When the grid is on, and you have the ESS assistant configured, the Multi connects its input to its output. It is essentially in “passthru” mode, except that the inverter is also in parallel with everything and can feed in energy, or take energy from the grid and/or other sources. You are running at grid frequency, and since we cannot shift the grid frequency, not without significantly more power anyway, that’s how it must be.

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Frequency control on the input of an inverter is not to control the grid. I think that it is physically impossible. However it does allow for the grid to control inverters. May not be the case in locations where feed back into the grid is not allowed. Here in Aus they have talked about the grid throttling back all the roof top solar during the peak of the day to simply the job of controlling the utility generators.

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You could, if you wanted to, make the inverter with a DC bus in the middle, so that the output frequency could be different to the input (ie, they are not latched together). Then it would be possible to use only frequency shifting on the output.

For example, I seem to think there was an Axpert inverter (King?) that worked a bit like that. Well, it couldn’t frequency shift, but I mean the other half: The incoming AC was rectified and placed on a high voltage DC bus (along with energy from PV and the battery) and then turned back into AC.

This allowed that inverter to have a 0ms changeover time… because it doesn’t change over. Came at a slight efficiency cost of course, and also meant that you are always limited to the maximum capacity of the inverter. But it would also, technically, allow running the output at any frequency you want, even while on the grid.

Of course, that is not how the Victron inverters work, so all of this is academic :slight_smile:

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When I said impossible I was not concerned about the ability of the inverter to produce a phase/frequency difference between input and output. I was referring to the inability to shift the frequency of the utility network. A small inverter trying to shif the frequency of a huge rotating machine/generator would be impossible.

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I know… sorry about that. Wasn’t disagreeing, merely used it as a connection point for elaborating!

Basically, the reason it is not done… is because compromises are made by engineers (and marketing people). There are advantages to closing that relay and running everything directly from the grid, but you have to compromise a little on other things :slight_smile:


I see this is an old thread, but I hope still alive.

I hope that you or anyone else will be able to help me with the scenario below.

  • Fronius inverter is connected to the output of a Victron 3-phase cluster.
  • After a grid outage and grid power returns, a Victron cluster will align its output frequency to the grid.
  • The Fronius will ramp up production as the frequency has been lowered to the grid frequency.
  • The Victron cluster has not yet connected to the grid.
  • MODBUS control of the Fronius production is not yet operational as the Victron system has not yet connected to the grid and is still in Micro Grid mode.

Question: how is the Fronius production controlled so that full batteries are not overcharged?

The Victron system will periodically ramp up to 53Hz while waiting the mandatory 60 seconds (according to grid code) to prevent the PV-inverter from starting up. Since the PV-inverter also has to watch the grid and can only reconnect once it has been in range long enough, that keeps the PV-inverter shut down until it’s reconnected to the grid.

There was another question here, about why this is done, and the Multi/Quattro has to carry the load on its own during this time, and this is the answer that came from above: That they use this method to prevent overcharging.

Thank you for the speedy reply.

What I have seen is Fronius instantly ramping up production when the frequency drops, resulting in the battery tripping due to overvoltage.

I have not seen the Victron change to 53Hz, I have only observed it tracking the grid.

The Fronius is set to MG50, and under normal grid conditions, the control of feed-in to grid works as expected via MODBUS.

In the chart above, the thin black should be Red as it is the Output frequency.

Do you have the ESS assistant configured so that it knows there is a PV-inverter on the output?