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With AC Coupling, would it be fair to say......


Delta9
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With AC Coupling would it be fair to say that if you want to be able to run connected to the grid but also be able to use solar to power the AC loads but with the twist (and its a BIG twist to the story) that because the local grid is not that robust and suffers from loads of load shedding, that to be able to do this from inverters successfully one needs a big battery connected to the inverters simply to be able to technically power those AC loads when the grid is down? Not to be able to power those large AC loads from battery when the grid is down but because the inverters provide the sine wave for the Symo to operate when the grid is down.

By this I mean with an electrical system of say, a 100 kVA 380 volt transformer from the grid feeding into the main DB plus 3 Victron 15kVA inverters and a 20 kW Symo PV inverter connected together to supply the AC loads on the AC output bus from the inverters. When the grid is down (because its almost useless or because they load shedded us YET again for another 12 hours - take your pick because either is likely at any given time) and we need to use solar to power the AC loads because of the Victron 1:1 rule it means we have to have this huge battery bank in order for the system to work correctly. 

So even though the HEAVY AC loads wont ever be run from batteries we need the battery bank in place in order to keep the mini AC grid up that the Symo needs to sync too....and for all that to work right we need this massive battery bank in place to keep it all stable....even though those batteries will never carry those big AC motor loads.

So to me it seems like batteries with a max of 1C discharge rate with a "normal" continuous discharge rate of 0.2C would be good enough for this task as the main requirement of the batteries is to be big enough to absorb "shock loads" from the Victron AC / Symo system if a big AC load suddenly gets shut down. There is no need to discharge large amps from the battery to supply the big AC motors because the solar panels connected to the Symo (Fronius 3 phase PV inverter) are supporting the load directly (which is what AC coupling is all about - right? ... power AC loads directly from the panels through a PV inverter connected to the AC output BUS of the Victrons).....so in essence the batteries in this case are merely there as a "buffer" for sudden AC loads going on and off.

Is that a fair appraisal of the situation? This is the cost of running large AC motors from solar using both AC from the grid and AC coupling on an inverter system. When the AC grid is down the Symo needs an AC waveform from somewhere to lock too to carry on producing AC from the solar panels. In grid down scenario's that AC wave form is coming from the Victron inverters which in turn need the big battery bank behind them to keep everything stable.

 

Edited by Delta9
Clarify a point because I even confused myself
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