Off grid AC PV coupling with generator

DIY
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1

The system consists of the following components:

  • 3x Quattros 15000VA 48V 230V on 3phase configuration
  • 2x SmartSolar mppt 250/100 connected via ve.direct
  • 1x Fronius Verto 33.3kW
  • 1x CCGX
  • 6x 16.6kWh poweroad Tervol L16
  • 1x 3 Phase 20kVA generator

The Quattros will form a 400/230 V, 50 Hz island network. The PV Inverter Support Assistant will regulate the Fronius through frequency shifting during inverter operation, with the Fronius configured for compatible MicroGrid operation (MG50 grid code). As expected, the Fronius inverter will connect to the AC output of the Quattros because no grid is present. The MPPTs are there for maximum reliability and also for breaking the deadlock with the AC-coupled PVs in the event the BMS locks out due to low SoC.

During normal off-grid operation, the Quattros regulate the Fronius through frequency shifting, and the system works as expected.

The concern arises when the generator is connected to the Quattros’ AC inputs. Once the Quattros synchronise with the generator, they can no longer independently adjust the AC frequency to regulate the Fronius.

If the batteries cannot accept further charging, the loads are low, and PV production is high, surplus power could flow back into the generator, potentially damaging it. I therefore need a reliable way to stop or disconnect the Fronius before the Quattros connect to the generator.

I have considered three options, but each raises some questions:

  1. Connect the Fronius to AC-out-2 and reverse the default relay logic.
    The outputs would remain enabled during inverter operation and disconnect when the generator is connected. However, the Fronius produces approximately 48–49 A per phase at nominal voltage, while AC-out-2 is rated for 50 A. Is this arrangement approved for continuous, bidirectional operation at that current? I am concerned about the limited current margin and long-term contact reliability.

  2. Connect the Fronius to AC-out-1 through an external three-phase contactor rated for the application, for example 100 A.
    The contactor would be controlled by a programmable relay on the master Quattro. Does VEConfigure support the necessary logic to disconnect the Fronius before the generator is accepted and reconnect it after the generator has been disconnected? Can this be coordinated reliably across the three-phase system?

  3. Control the Fronius over the local network through the CCGX.
    The Fronius will already be connected to the same LAN as the CCGX for monitoring. Victron documents Fronius zero-feed-in control for ESS installations, but does this functionality also work in a fully off-grid system using the PV Inverter Support Assistant? If so, can it reliably prevent reverse power into the generator, including during sudden load rejection or loss of communication?

Which approach would you recommend? Is there an official Victron-supported solution or application diagram for this arrangement, including the required settings and switching sequence?

2

Possibly a 4th option would be to disconnect the generator when the battery SoC reaches or exceeds a threshold?

3

That could help, but I would not rely on an SoC threshold alone to protect the generator. SoC does not indicate how much surplus power the batteries can absorb at any given moment.

For example, suppose the Fronius is producing 30 kW, the loads consume 35 kW, and the generator supplies the remaining 5 kW. If a 25 kW load suddenly switches off, consumption drops to 10 kW, leaving a 20 kW PV surplus.

If the batteries cannot absorb that surplus, it could flow towards the generator before an SoC-based stop sequence reacts—even if the SoC is still below the threshold. Sudden load rejection therefore needs to be addressed independently of the SoC limit.

And in the end I won’t sleep well knowing that a potential corner case can cause that much damage

4

The generator is probably meant to provide backup if the batteries have a low SOC and the PV system isn’t currently generating enough power to prevent the SOC from dropping below the minimum, right? Bad weather… or it’s night. Then the generator can be set to start at, say, 25% SOC and stop again at 35%. And it will keep doing that until the sun comes out again. Or is your idea different? Best, Jörg

5

Hello,

In line with that fourth option. The battery’s CCL parameter could be added for greater safety.

For example, if the battery accepts 100A and the SOC is below 90%, allow the generator to run. Or better still, switch off the generator if the thresholds are exceeded. Check the conditions every few seconds or whenever the CCL or SOC changes.

Make these parameters configurable in the Dashboard so they can be adjusted as desired, with due care.

I’d feel more reassured if the battery’s CCL were checked, since its BMS tells you whether it will accept the charge you calculate.

And for greater safety, if you detect a reading error or the batteries disconnect, and you can’t access these parameters, switch off the generator anyway.

Regards, :waving_hand:

6

The generator is partly there to support the batterries whenever they fall bellow a certain SoC but it is also there to provide a boots in power for some big loads. In any case the end user that will use the system will have control so the system needs to be fool proof

7

The generator is simply controlled by the Cerbo.

Here you can set various switching thresholds (SOC, power).

More information about hybrid systems

Switch to a Cerbo; the CC is end of life.

8

Is this “power boost” needed because there is one heavy kW device that is used occasionally, or simply when lots of normal devices combine together?

If the former, maybe consider moving that device onto ACin and start/stop the generator when you use it?

If the latter, maybe consider DC coupling the generator and switching it based on mid-range. SoC as @_m0nsam suggests?

Disclaimer: I’m no expert in generator systems, I run a solar-only off-grid system and another on-grid system.

9

If this system is installed and programmed by a professional, it’s foolproof. You have everything you need for it! I’d probably also swap the cc for a Cerbo, as this greatly expands the control options, but that’s not essential either.

10

Hello @vkyriafi,

If you don’t want to stop the generator and give it priority over the Fronius, and only want to stop the Fronius, then I can see how your option 2 could work, using a relay to activate a suitably rated contactor that can withstand any transient load that might occur while the Fronius is disconnected.

Personally, though, I would look for a more integrated solution using commands, so that the Fronius shuts down in a controlled manner. I think Fronius integrates with Victron in Venus OS, and that you could create a Node-RED flow to send a shutdown command to the Fronius when you detect power coming in on AC In. But that’s just what I think; I don’t know for sure.

One thing I do know you could do is activate the Fronius shutdown by telling Venus OS that the manual CCL limit is 0 A. Even if the battery isn’t full, you can tell Venus OS not to accept charge, which then activates the frequency increase to prevent surplus power from the AC grid being accepted.

Another option that comes to mind is to connect the Fronius to AC In 2. I don’t know whether it’s possible to use one of the AC In inputs for the generator and the other for the Fronius. But if that’s not a problem, you could then control the AC In port perfectly through Node-RED. Not only that, but you wouldn’t have the problem of an AC grid outage or where to send the load. You should consider this option, though, because it’s only an idea. I have a MultiPlus II, not a Quattro.

Based on what I know, I would use Node-RED in Venus OS to control a flow that sets CCL to 0 in Venus OS, thereby activating frequency shifting to shut down the Fronius even if the battery isn’t full. For example, when AC In is accepting power, that means the generator is connected and running. Then I would check whether the battery’s CCL could handle a blackout on the AC out side; if it couldn’t, I would set the Venus OS CCL to 0 to activate frequency shifting and shut down the Fronius in a controlled manner. In addition, I would install a relay with a contactor, such as a Shelly, so it can be integrated into Venus OS. That Shelly would serve two purposes. First, if you want to keep charging the battery with the generator or the MPPTs, after shutting down the Fronius with frequency shifting, I would safely open the Shelly and then restore the Venus OS CCL to -1 or whatever is appropriate. Second, it would let you cut power to the Fronius immediately, manually or under certain exceptional conditions, for safety. For example, if the battery is at 100% and the battery CCL is 0, you could open the Fronius circuit with the Shelly (this shouldn’t happen if the previous algorithm is implemented correctly).

In short, in my case, I would use Node-RED and set up different logic based on my priorities, whether that means prioritising the generator or the Fronius.

I hope I’ve at least given you another idea.

Regards, :waving_hand:

11

Thank you all for your suggestions. After considering everything I think I will go for a hybrid solution. I will program the fronius for zero feed in via the ccgx and use an external relay controlled by the Quattro for disconnecting the fronius completely in case there is a comm failure or any other abnormal situation.