There is a lot of concern for safety when DGs (distributed generators) are connected to the power grid. The grid operators must ensure that none of the DGs feed back into the grid when the grid generator is not operating. They go through several scenarios in this article.
From what I see the DG must be stopped from feeding upstream during a fault – normally the DG only feeds downstream to the loads. This assumes that the connection of the DG and grid AC is bidirectional – the DG power can go both downstream to the load and upstream to the main AC. This is typical of a transformer: it works both ways.
The telecom industry has been doing high power backup for much more than a century. They use 48VDC but there is no reason that the voltage can’t be higher. They use batteries and generator backup but the batteries don’t have to be used and the generator can be replaced with with solar or wind power. I assume that an island in a DG grid would use solar, wind or similar for its power.
The industry offers UPSes (uninterruptible power supplies) in sizes up to thousands of watts. There are two types: one has the battery and inverter in standby mode until the grid power is interrupted when they take over. The other UPS changes the grid AC to DC to charge the battery and the inverter is always connected to the battery and supplies all of the downstream power all the time.
Since converting the grid power from AC to DC is a one way action, there is no concern for the island power to go back into the grid. The power from the solar array is already DC so it can be connected through a one way rectifier to prevent the grid power from going into the array. The two DC sources can then be inverted to AC for use downstream. When the solar power is not enough, the grid power supplies downstream. This does not have any backup batteries; it just allows the solar to replace grid power during the daytime. But both the grid and solar power can be used to charge batteries, which then can be used to power the inverter for power 24/7 and there is no concern for the solar power going back into the grid.
This is essentially a Wire OR circuit. The grid power, the solar power OR both supply downstream, where they are inverted back to AC.