Generators all stay in sync. Generator owners have expensive devices that sync the phase before the generator is connected to the grid. Once a generator is connected to the gird, it will stay in sync - in fact that is why they have the expensive devices to make sure that they are in sync before they connect them, as if they are not, it will instantly jump to being in sync, which may destroy the generator.
As a matter of fact, it may destroy the generator, the housing, the building, the damn, and more. An out-of-sync generator becomes a motor until it is in sync. lt can be a graphic and dramatic event.
Big generator are synchron maschines, as they can generate also reactive power. If a out of sync synchron maschine is connected to the grid, theres a big "kawumm" and then the maschine is in sync or dead. Only the angle between the rotor and the magentic field make the difference between generator and motor. A synchron motor can not self-start and only run at fixed grid freuency / rpm's. A overloaded motor suddenly stops. Smaller generators are asynchron maschines, that can run faster or slower than network frequency - ie run as generator or motor - but they always consume reactive power. They can self-start. Synchronising maschines to a grid is not a big problem, the bigger problem is to syncronise 2 disconnected grids. Some years ago in europe a grid operator violated the n+1 redundancy rule as he needed to switch of a big power line over the river "Ems" - to allow a big ship to leave the shipyard. The result was a netsplit trough whole europe - a lot of "big" line-breakers flipped and switched of north-west and south-east power lines. The whole european grid was split into 3 parts, running at higher and lowet frequencies. Details: http://www.bundesnetzagentur.de/SharedDocs/Downloads/EN/BNetzA/Areas/Electri... Kind regards, Ingo Flaschberger