I may be explaining it wrong, but here is a reference, particularly 22.8:

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.