question

All feasible, but why not just connect the PV inverter at the input side of the Victron inverter?

R&D indeed, it's risky and unsupported, but it sounds like you already know that.

I presume you have another reason for not wanting to use DC bus solar on an MPPT?

This issue being where does the power go when there is a sudden loss of load.

I would consider a Voltage/Frequency relay and a dump load.

But in full knowledge, that you are in unsupported territory.

Probably not the solution but maybe an idea in which direction you could go.

For my ESS system with Fronius Galvo inverter and MPPT, I wrote a bash-script that is running every 10 minutes on the Venus GX.

This script reads out the battery SoC, determines the actual solar power and according to this I change a few ESS settings (Max. inverter power, Grid setpoint) AND the script also switches my Fronius Inverter to Standby if its output power is below 15 Watt. I need this because I get strange humming and other resonance noise if the pv output power is only a few watts.

I simply used one of the Venus GX relais and connected it directly to an input of the Fronius inverter.

Additionally, in your case I would definitely think about a few fail-safe features too, because I would not rely on cron-jobs and scripts with a few dbus commands.

Best regards

Rainer

Hi all,

Thanks for the replies. This is indeed for off grid customers, so ESS does not apply. I've wondered about having one PV inverter on the output side of the Multi and one on the input side of the Multi. We'd somehow need to make that PV inverter see a "grid" though... not sure how to do that.

The controller that controls the relay/contactor for a dump needs to respond very fast... and maybe it can't be fast enough.

Going DC coupled would indeed be a straight forward solution (and in fact for one customer we've already done that). For another customer though the additional array will be on a different building (and far from the batteries) so for that customer we're considering using an AC coupled system (since there's already AC between the buildings) where the combined power of the (two) PV inverters is limited to the rated power of the Multi.

It means that we won't be able to use all available power to charge both the domestic and the car battery but it will provide the customer with the needed power in the winter. Both PV inverters will have their max power set to combined 5KW (as they use a 5KW Multi) and the ratio will be determined by the panel layout.

Yet another way to do this would be to increase the capacity of the domestic battery but that too is not always feasible.

Jan

Hi Rainer,

I think I'm going to turn the SB off too when it's output power drops below a certain level... lots of relays clicking inside too often for my liking. The question remains though how to decide when to turn the PV inverter (Fronius in your case, SB in my case) back on. How are you doing that?

cheers,
Jan

Hi Guy,

Is having a general purpose watchdog "assistant" that can be enabled and disabled in the Multi something feasible to implement? The enabling and disabling could be implemented in VE.Configure or perhaps even via a writeable ModbusTCP register or MQTT.

It would be very useful for any custom activity/functions in Venus that the Multi would (need to) rely on, including but not solely for the one discussed in this thread.

cheers,
Jan

I was using an unsupported PV inverter ( APS 1000 Watt) on AC out 2 of Quattro and programmed the Quattro AC out 2 to switch on and off based on battery Voltage. through assistants

As an additional safety relay added a contractor on the AC inverter switched by BMV NO contact ( Contactor on only when Battery is below 90 % SOC)