With 3 phase power, there are 6 points within the cycle where
sync can occur, so power phase will never be more than +- 30° out of
sync
I may have misspoken a bit about the purpose of the GPS. It's for
keeping the phasor* measurement unit (PMU) in sync. These units
report back to the system operator so that early signs of grid
problems (and by early, that would be a few seconds) can be managed
remotely. PMUs were required after the 2003 blackout: there was no
shortage of diagnostic information after that outage, but most of it
had imprecise time stamping, so was of little use to the postmortem
investigation.
It's not as pretty as the Synchronome clock that hangs in the board
room of the IESO in redacted (one is not supposed to know
where their HQ is: it has no sign, is heavily armoured, runs on
positive pressure, etc.) which was used to manage The Hydro's grid.
The SEI unit is a little easier to interface to than the
Synchronome's invar pendulum, though.
Large solar inverters synthesize their power waveform by copying the
incoming waveform. Unless they are tightly controlled, they'll
happily act as little repeaters for any sync problems.