Remember that power stations do not generate the same single phase signal that you tap into in your house. They generate three phase AC current. They also don't link up directly: there are transmission substations in between.
Think of the phase as a distributed signal that indicates the current state of the local system taking into account all connected generators. It's weird but true!
https://www.scribd.com/doc/118169687/Network-Protection-Auto...
This makes it sound more complex than what I learned when I last tried to figure it out =)
They seem to be describing that where two grids meet they go through a synchronizer that disconnects one or the other if they aren't in sync, so implies manually disconnections throughout the network as needed locally.
In early days of electrification the physical topology usually was spanning tree and in fact this was the motivation for research into minimal spanning tree algorithms at the time.