[snowwindwaves]

I've synced 10 mw 900 rpm hydro units as much as 7 degrees out of phase. There is a bit more of a thunk as the unit snaps in to sync but it's not the end of the world.

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.

[Filligree]

It depends entirely on how much inertia there is on the shaft, in various forms. The example I was told about was a 200MW peltier, constructed such that rotating it backwards implies pushing back against the water. The shaft itself also had some sort of extra equipment on it, but I'm not sure of the details.

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.

[sunkensheep]

Water is a bit less forgiving than steam, it has great resistance to changes in position or velocity. I suspect that closing the generator created a water-hammer, which physically or mechanically destroyed the machine.