(reposted now under: https://victron.discourse.group/t/mqtt-for-ac-coupling-on-ac-in-using-an-old-stecagrid-3600-pv-inverter/877 )

Hi,

I have a fairly fresh installation of MultiPlus-II (3P) and GobelPower Batteries running now quite well after my latest addion of a VM-3P75CT as a grid meter. I'm still working on the additional PV using SmartSolar MPPTs (DC-coupling), but my 12yr-old 4.3 kWp is not visible to my GX/ESS as single phase AC-Coupled-PV.

The StecaGrid3600 is quite old and only provides readings via a proprietary RS485 interface. I did reverse engineer the Steca RS485 protocol some time ago ( https://github.com/Aeroid/stecagrid_rs485 ), so I do get the momentary inverter AC output (W) and DC (panel) input (V/A/W) and publish that via MQTT to my Home Assistant at short invervals.

My primary goal is of course to use and charge my batteries, but I do have a fairly nice fixed price to sell this energy (14c/kwh), but only from that old PV! So there is no need to limit the Steca production: my grid is fairly reliable and it would also make sense commercially. Allthough my Victron installation doesn't know anything about this PV, it does limit grid use and redirects oversupply to the batteries... works fine, as far as I can tell from the last few days.

But here comes my challenge:
Assuming my new PV (additional 8kWp) will "outshine" and become my primary supply (batteries and day consumption), I might as well want to primarily supply the Steca PV output (amount) to the grid as much as possible, when it's not needed to be fully charged at the end of the day.

One very simple idea was to simply add an alternative clamp to be switchable to 3P75CT on Phase 1 behind and infront of the PV AC coupling. That would exclude the AC-in-coupled and 3P75CT & GX would assume that to be part of the grid supply completly. This very crude but simple hardware approach might be automated via a relais I'm sure.

But how can I possibly make my ESS and GX aware of this PV supply and possibly solve this via software by actively managing the "Grid Setpoint"? This might not be as simple as the fourth clamp, but might be more complete as the ESS would know about the extra 4.4kWp PV. And yes, I've planned to use dynamic ESS for the winter when all of that PV won't fully charge my batteries.

Can I partially emulate a Fronius/SMA/etc inverter by software? (I already looked into emulating the 3P75CT UDP stream, but that's just a meter, not sure if ESS/GX will treat this as PV supply properly)

Can I publish MQTT data to make the GX aware of the Steca PV power?

Thanks, Gunnar