I currently have a Multiplus II 2x120v (3kva) installed in a standard DC coupled system on my RV. I already have a Cerbo GX, and several MPPT solar controllers with batteries. All works well.
I am looking to add additional solar panels to a separate trailer and would like to use a grid tie inverter such as a growatt MIC 3-5kva to make an AC coupled system. I would also like to be able to connect the entire system to an external local microgrid, and have the solar augment the local microgrid (in my case the microgrid is a 45kva diesel generator, or alternately multiple similar small solar systems).
I think that this is possible but forsee several problems.
The Victron AC coupling page says to connect the Grid Tie inverter to the output side of the Multiplus II. https://www.victronenergy.com/live/ac_coupling:start
But, when I look at the manual for the multiplus 2x120 (https://www.victronenergy.com/media/pg/MultiPlus-II_12V-24V_3kVA_2x120V/en/UUID-1ff22c09-d3c7-a2e1-1f4e-4fd90f9a5e89.html) it looks like there is a relay state where the L1 and L2 lines are bonded. This would obviously be a problem if they are being fed from a standard split phase 120/240v grid tie unit (e.g. L1 120/ L2 120/ N).
0) Is the multiplus II 2x120 capable of back/grid feed if it is reconfigured? The wiring diagram above suggests this should be possible. It is not clear in the documentation if this feature is only available with the 240v units or if 2 separate 120v units are needed to function in this manner.
1) Can this relay that bonds L1 to L2 in the multiplus be disabled? None of my RV systems utilize this L2 line at the moment so it currently adds zero function.
2)Is this actually a problem (i.e., would this relay trip if it were energized on L1 and L2)?
3) Would connecting the Grid Tie inverter to the L2 line of the Output2 bus fix this?
4) Could I absolve these issues (1-2) by using the Victron Autotransformer to step down the 120/240v split phase off the grid-tie unit into a single 120v phase that I then connect to my L1 on the multiplus? Will any errors with the grid tie inverter's frequency matching capability be introduced by the autotransformer?
5) If none of the above are possible, is there a way to use a grid-tie inverter on the grid (AC input) side of the multiplus? Since most grid-tie inverters have anti-islanding function, how could I work around this to get 'dark start' functionality (i.e., how do I get the grid tie to start producing power in an off grid situation?) This would be preferable if there is an easy work around.
Thanks in advance!
