How is grid phase synchronised?
[1] Synchronizing AC generators -- Part 1 (introduction and sync lamps)
[2] Synchronizing AC generators -- Part 2 (strobe light view)
[3] Synchronizing AC generators -- Part 3 (sync and unsync)
[4] Synchronizing AC generators -- Part 4 (phase deviation)
The first video explains all the concepts and shows a sync, but the later videos bring additional conditions and detail.
[1] https://www.youtube.com/watch?v=RGPCIypib5Q [2] https://www.youtube.com/watch?v=sFohkp2aaU4 [3] https://www.youtube.com/watch?v=GRk_qJxaxh8 [4] https://www.youtube.com/watch?v=RT1ySBc-Bls
Phase is monitored on both sides of the connection, and high voltage contactors complete the circuit when tolerances are met.
E.g., will an out of phase turbine have a physical resistance against it, as a consequence of the rest of the grid?
This kills the generator. Also, less amusingly, anyone nearby at the time.
If it's a larger mismatch, then that's more likely to fry the wiring before too much other damage happens. On rare occasions generators get connected while they are (exactly) 180 degrees out of phase; this is a dead short, and its internals will instantly vaporize. The vaporized copper may then proceed to explore the outer limits of the generator's physical integrity.
(Disclaimer, etc: I've only worked with DC generation, AC would be somewhat different especially if it's three-phase.)
Diesel units in hospitals will sync to each other as much as 30 degrees out of sync because just get thing online because it the load is critical.
I've also been told two second hand reports of the breaker closing drastically out of sync. One was a person closing the breaker manually when the unit was stopped. It essentially just puts a fault on the machine and it trips right away. Another case the close circuit was poorly designed and has only one relay, whose contacts eventually failed but happened to fail welded closed, and in conjunction with a poorly maintained breaker that developed a variable delay between the close coil being energized and actually closing. It made a big boom like faults do but the unit synced after the relay was replaced and breaker maintained and is fine.
Maybe medium and larger units with more inertia have tighter tolerances. Over certainly heard of broken shafts and generators being ejected through concrete walls.
A nice party trick is to do phase matching instead of syncing with slip so I can keep the unit exactly in phase with the grid as long as the grid doesn't wobble too much. This keeps the sync scope needle exactly at 12 o clock so you can close the breaker to put the unit online at will. Unfortunately i don't often get the budget for The additional inputs to get the nice phase difference signal so I can't perform my trick.
Well, when it was connected about 30 degrees out of phase, the entire unit pretty much exploded. It tore out of the concrete base, and was found to have rotated nearly halfway to the "correct" position.
I haven't seen anything nearly that dramatic myself, so I can't be sure how much it's exaggerated. There's probably a report somewhere.
And I love that party trick. There's no direct equivalence in DC transmission, of course, but I've done similar things with voltage-matching in Electrical Age. It always amazes me how much that game ends up reflecting reality.
In case of phase mismatch, you have a short circuit and the weakest element along the short gets fried.
Generally systems try to avoid too much loop power in the first place for this reason, setting up a directed but acyclic graph with some redundancy links carrying zero or near-zero power as standby.