How phases of electric voltages coming from different power plants is synchronized?

Question

Before distributing electric power to consumers?

"Then when the phases match they throw the switch"

I doubt that this approach can work.
Before generatior is disonnected the phase in the circuit is that of open-end standing wave. When they throw the switch, the phase becomes that of a wave propagating from generaotor to consumer. At 60Hz 1radian of phase is about 1000km of transimission line.

Answer 1

A very astute question. Not many people realize that this could cause some real problems.

All they have to do is watch the phase of the power as they're bringing a generator (or plant) on line. They can adjust the generator speed slightly to get the phase right. Then when the phases match they throw the switch.

Answer 2

Re your additional details, I think there isn't much more than that, i.e, "get in sync and throw the switch". It's very easy to electronically control the "connect" switch to not engage switch until match within a fraction of a degree is attained. And even if it wasn't that close when engaged, they would "sync" immediately (two voltages, once connected, obviously yield only one voltage). If the generator was spinning slightly ahead of the grid, the grid would load the generator, causing it to slow until it was in sync; if the generator were running a bit behind, the grid would briefly speed it up. Get it?

The most important monitor point in the grid is frequency. Even the slightest dip, on the order of 0.01Hz, is a dead giveaway that either a source is turning into a load, or the area is being overloaded.

I used to design and build, amoung other things, power factor and kilo/mega - watt/VA/VAR meters for utilities.