This is the same as home photovoltaic or backup petroleum generator. The home kits don't have their own frequency generator. They lock on the grid frequency and just follow it. A system with own frequency generator needs to be able to bias its own frequency to get into phase with the grid before the breaker is closed. You need a controller that measure both the local and the grid frequency to control this synchronization.

Once you have one such frequency source in your island grid, then you can operate it together with the simpler home kit. Just need one source of frequency. On a generator, look for isochrone vs droop operation.