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A synchronous interconnect provides not just a source of truth but also stabilizes your grid frequency. If you have an isolated grid you have to match generation to demand to keep the grid frequency stable. If you have a 1GW interconnect that means you can mismatch generation and demand by up to a gigawatt and still be fine. I imagine that makes for a much faster startup procedure
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If the isolated grids all each synced to a source of truth first (GPS?), would it be possible for them to immediately connect afterwards?
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You can connect two running grids. Earlier this year the Baltic countries disconnected from the Russian grid, and synchronized and then connected to the European grid.

I imagine you can get close enough by syncing to a shared time source like GPS or the DCF77 signal, as long as you communicate how the phase is supposed to match up to the time source. Or at least you could get close enough that you can then quickly sync the islands the traditional way.

The question is if it's worth the effort and risk. Cold starting a power grid is a once in a lifetime event (at least in Europe, I imagine some grids are less stable) and Spain seems to plan to have everything back up again in 10 hours. Maybe if the entire European grid went down we would attempt something like that by having each country start up on their own, then synchronize and reconnect the European grid over the following week.

https://en.wikipedia.org/wiki/Baltic_states_synchronization_...

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