Okay. I will go back to basics, so please excuse me if I sound simplistic.
GTI's are slaves, they need to see and synchronise with an existing AC voltage and correct HZ. Either from your Main Grid utilities company or a stand alone Inverter say the OzInverter and its created Mini Grid.
That AC voltage and HZ should be stable and not sag or go out of range of the GTI's internal settings.
Once the GTI is putting into the Grid, it pushes up the Grid voltage a little and hence your Power supply meter starts going backwards. ie your pushing into the localised community with Main utilities Grid, or your pushing the OzInverter backwards which turns the AC voltage from your GTI and Mini Grid into raw DC voltage at about 60v into charging your batteries. (my previous post shows how we control that).
The GTI's are designed so that above a certain voltage, this varies across the World, the GTI shuts it self down and awaits suitable AC voltage to restart.
With the Main Utilities Grid this happens a fair bit when its sunny and there are several PV GTI systems on the consumer side of the Utilities power Transformer and no one is using up that localised power.
At present you need some sort of machine to create a stable AC Voltage and correct HZ, and here I use the OzInverter. The 48v DC battery bank get the Inverter going and producing a MINI GRID that the GTI wants to see.
The GTI's turn on, and that Mini Grid starts using up the power from the GTI's, and backs away from using the 48v DC batteries, and then starts pushing DC back into the batteries, that's if you have spare power from your GTI's.
So theoretically the GTI's do all the work and the OzInverter just gives a stable reference AC voltage and HZ reference. However, in real practical terms to keep the GTI's on board, the OzInverter must be able to handle many loads instantaneously backwards and forwards without the AC voltage sagging or going high, hence those parameters I have set in my previous posting.
Sunshine is just not consistent, its remarkable to see so much fluctuation even on a clear bright sunny day.
At Present there are some whizz kids out there working on a small Inverter that can instantly match loads and excess to the GTI's output, without the need for the main Mini Grid Inverter to be so robust as the OzInverter. As I said, 'this concept shows great potential', and I await the results with interest.
So at present you need 2 systems of AC Grid for your GTI to work on each.
Get some good change over switches, ON OFF ON, at your side of your consumer/switch board unit, and have your Main utilities Grid on one side and Your own Mini Grid on the Other. see photo.
We have many Main utilities Grid EDF outages here, and I only use the Grid at night time as its a cheap rate and more cost effective than installing a decent Generator.
That's a big used/second hand Krauss & Namier, fleebay, big 90amp change over switch, or known as a generator Change over switch. All poles are switched and there must be an OFF central position. Earthing here in this country is a Long copper earth rod at each consumer unit.
This system can be automated with AC contactors.
I trust this helps.