So, i have a larger system set up to work as an island system for a Quinta in Portugal. approx 80kwh Sunwoda batteries, 6x multiplus 5000 (as 3 phase system). enough solar panels distributed on the island grid (all SMA inverters).
For legal rasons my only ‘grid tie’ are 3 phase AC chargers which i can command to charge the batteries from the grid in case there is little availability from the inverters and batteries.
So far so good, this works well. The system uses frequency shifting to control the inverters in case there is more production than power can be used (it works well).
But now to my problem
I also have a tinyhouse with its own installation (1 Multiplus 5000, frequency shift controlled inverter with its own solar panels).
The ‘grid’ (the quinta island grid) is connected to the line input of the Multiplus.
My hope was that i can go ‘on grid’ (onto the quinta island) if batteries are low, but otherwise stay ‘off grid’ (transfer switch open, i control this via modbus/tcp).
To my astonishment, even with the transfer switch disconnected/open, the ‘island’ frequency follows the ‘grid’ frequency. as the main network (the grid) has batteries usually full early, the subsystem will follow the frequency of the ‘grid’ and block the AC coupled inverter of the subsystem with that despite the batteries being half empty and ample sunshine.
Of course i can add an AC contactor into the ‘grid’ connection and disconnect if from the grid input of the Multiplus, but shouldn’t the transfer switch do that ? If there is no grid, the Multiplus behaves as expected …
Any advice how to solve this with a software command or config ?
Johannes