Fronius on AC-out keeps shutting down every few minutes - ESS instability on weak French grid

Q&A and troubleshooting
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1

Hi,

I have the following setup:

  • Victron MultiPlus-II 48/10000/140-100/100
  • Cerbo GX (firmware v3.73)
  • 2x Fronius Primo (4.6kW and 5.0kW) on AC-output
  • 4x Voltsmile V10 batteries in parallel (20kWh / 400Ah)
  • Carlo Gavazzi grid meter between Linky and MultiPlus
  • Single phase, French rural grid (Enedis), max feed-in contract 12kW

Current ESS settings:

  • Grid metering: External meter
  • Grid setpoint: 100W
  • Max feed-in power: 1300W (limited by voltage rise on weak grid)
  • Max charge voltage: 55.2V
  • Min SOC: 25%
  • ESS mode: Optimized with phase compensation
  • AC-coupled PV feed-in excess: enabled
  • Fronius: Cos φ = 1.000, Dynamic power reduction: No Limit

Problem:

On sunny days with variable cloud cover, the Fronius inverters shut down every few minutes. This causes the MultiPlus to switch to Inverting mode, the battery discharges at -70A to compensate, and after 60-90 seconds the Fronius restarts — then the cycle repeats.

I have identified two separate causes from Home Assistant history graphs:

  1. Grid frequency instability — the Enedis grid frequency regularly drops to 49.1Hz or rises to 50.6Hz. This is a known issue with weak rural French grids.

  2. Overproduction when battery is full — when battery reaches ~90-95% SOC and PV production fluctuates rapidly due to clouds, the MultiPlus applies frequency shifting to reduce Fronius output. The frequency shift causes the Fronius anti-islanding protection to trigger and shut down.

What I have already tried and fixed:

  • Fixed grid metering from “Inverter/Charger” to “External meter” — this solved the main oscillation issue
  • Lowered max charge voltage from 57.6V to 55.2V to maintain battery buffer
  • Disabled Fronius Smart Meter (was incorrectly placed between Fronius and MultiPlus, not at grid connection point)
  • Set Fronius Cos φ from -0.900 to +1.000
  • Set grid setpoint to 100W
  • Weak AC input: disabled

Remaining problem:

On days with rapidly changing cloud cover and battery above 85% SOC, the system becomes unstable with Fronius shutdowns every few minutes. Maximum feed-in is limited to 1300W due to voltage rise on the weak rural grid (voltage exceeds 253V above 1300W feed-in).

Questions:

  1. Is there a way to slow down the frequency shift ramp rate in ESS to give the Fronius more time to respond gracefully instead of triggering anti-islanding?
  2. Are there any ESS Assistant settings in VEConfigure that could improve stability for AC-coupled PV on AC-output with a weak grid?

Any help appreciated.

Thanks

2

During grid / ESS operation, the MP2 cannot use frequency shifting. It shall control the Fronius via ModBus/TCP via you LAN connection.

If the grid is unstable, the loss of mains (LOM) detection might trigger a disconnect. In that case the MP2 builds a micro grid and after potential raising frequency to 53Hz it will lower to achieve desired amount of AC-PV generation.

Usually the Fronius connected to MP2 ACout should be set to grid code “micro grid 50Hz” (MG50).

The frequency ranges in the Victron ESS assistant and the Fronius need to match. MG50 uses wider frequency range for softer control.

If the Fronius uses official country grid codes, the ramp rate and recovery rate are defined by the grid code and cannot be changed.

The 60-90 seconds of Fronius to restart seems to be the time to monitor the (micro) grid to be stable before reconnect. Both, MP2 and Fronius have LOM detection.

You could check the Fronius if you can configure to accept a less stable grid to not disconnect.

With that high voltages and frequency changes I think the LOM detection disconnects your house from unstable grid. With this unstable grid you are risking sensitive electronics like SmartTV or computers.

If you tell more about your main requirements and yearly consumption und production, maybe we can suggest some solutions.

Do you need to feed back energy into the grid?
How much consumption do you take from the grid per year?

3

This should only be done if you have an external meter. Which i am going to assume is the carlo gavazzi meter.

This should not be used at all.

Usually just the basic M50 works. We also have unstable grid with frequency that looks like a bad generator at times.

The fronius itself may be the actual fussy one, and it may not be the system at all tripping its anti island protection.