Improving stability between Multiplus-ii and Fronius GEN24

[stirlsilver (Stirling)]

Hello everyone, I have recently commissioned an off grid solar system per the below description. I am experiencing some stability issues between the Fronius GEN24 and the Victron when there is a fast reduction of load - Every time there is a load reduction of say 2kW, the Fronius disconnects (due to Frequency going to 53Hz) and then it stays offline for about 2 mins before reconnecting. This is causing many disconnect events every day as household loads fluctuate.

• 6kW Fronius Gen24 Primo Plus
  • Connected to 60kWH 400V BYD Atto 3 battery back via Dala Battery Emulator
  • Connencted to 4.6kW Solar Array
• 5kVA Victron Multiplus-ii
  • Connected to 170Ah Lead Acid batteries (charged via 120A 400V-48V Charger connected to BYD battery pack) I am aware the batteries are a bit small
  • Connected to Cerbo GX Device

The Fronius is AC coupled to the Victron and is configured as follows:

• Grid Settings set to 50Hz Microgrid (Documentation keeps mentioning MG50 but that isn’t on the list!)
• Power limit set to 3kW (due to limited capacity of the SLA battery bank on Victron)
• Solar API on
• Smart meter placed on the load side

The Victron is configured as follows:

• No grid code selection
• Charger enabled
• PV Inverter Support Assistant enabled, PV inverter restart after battery voltage below 55V. Reduce power 50.2Hz, Min 52.7Hz, Disconnect 53Hz. Installed PV inverter power 5kW, same for solar array

Does anyone have any advice on how to make the load transients a little smoother so that they don’t disconnect the Fronius every time?

Also, I read that the Cerbo GX is supposed to be controlling the Fronius via Modbus, but I don’t think that is happening. The GEN24 shows under Settings/Devices with all the right information displayed there, but in the summary page, the information is not visible.

Below is the image of the Fronius dashboard, where grid is shown, is basically the Multiplus-ii. When there is a load drop, the fronius inverter shuts off, battery disconnects and solar generation stops and doesn’t reconnect until about 2 minutes later.

Here is the view just after the load reduced:

Any advice would be greatly appreciated!

Thanks.

[Courage2000]

Modbus control, see the picture.

MG50 is for Snap Inverter

[MondeoMan]

Modbus control is for export limitation in on-grid hybrid systems.
When the grid is connected, everything runs at grid frequency and the Multiplus can’t use frequency shifting to communicate so it uses Modbus.

For off-grid systems, PV inverter power modulation is via frequency shifting because it provides near-instantaneous and 100% reliable communication (no data loss).

———
I think the disconnects may be caused by the missing SoC (state-of-charge) on the Cerbo GX.
You could try to enable the battery monitor in VE.Configure or install a SmartShunt external Battery monitor.

The MultiPlus needs to know if there is available capacity in its battery to dump transient excess energy instead of disconnecting the PV inverter.

I think if the MultiPlus sees the battery is not yet fully charged (below 80% or 90%), it will not send the 53Hz disconnect signal.
It might also help to disable the DC/DC 400V-48V charger to allow more room in the battery to dump the transients on load disconnect into the battery instead of disconnecting the PV Inverter.

[stirlsilver (Stirling)]

Thanks for the information all. Thinking about this issue further, I think indeed it has to relate to the capacity of the SLA batteries. When the house load suddenly drops, the energy is dumped through the multiplus-ii and into the SLA battery. Depending on the voltage rise of the battery, the frequency is adjusted up. Because the batteries are only 170AH, the voltage spikes quite easily causing the multiplus-ii to frequently go to 53Hz.

I think I either need to get larger batteries, install some supercapacitors in parallel to the battery or shift more of the load off the Fronius (by limiting it max output) and use the Victron only for the large loads. As far as I can tell, the multiplus-ii can’t dump power into itself.