On this subject, how do they actually keep turbines synced with the rest of the grid, while also maintaining the correct voltage? If you spin it at a certain speed it will produce a certain voltage, the faster you spin it, the higher the voltage, but the frequency also changes. Of course the voltage also changes based on load. Do they also have very large variacs for this? As you almost would need to control the speed just to keep the voltage constant based on load. Say you're feeding a 500kv line, it needs to actually have 500kv added to it, not more not less, if you add more you will surge the rest of the grid. Or is there actually lot of room for error as the extra voltage translates into extra current, and it just self regulates back down?
Or do most turbines actually do DC, then get inverted into correct AC voltage/frequency? I know they also have HVDC lines but I'm talking about AC ones. So basically turbine -> rectifier -> 60hz inverter with fixed voltage output -> step up transformer to bring fixed voltage to proper line level -> grid.
Once synced, you can't really un-sync it.
The generator spins at exactly 3600 RPM, The only time faster or slow is when he main breaker is opened and it being started up or shutdown.
Voltage control is handled by the amount of current in the rotor which controls the strength of the rotating magnetic field inside the generator. The part of the unit that makes the rotor current is called the exciter. Older units were different, they might have a MG set to supply the field current. New units have an gener-ex, or an alter-ex. on the end of the generator shaft.
If you have unit that is synced to the grid and you take away the steam feed, the unit then becomes a large motor.
Most units put out 22KV 3 phase. That then goes to the main power transformer which then takes it up to 365KV and into the system. Just before the main power transformer are two smaller Aux transforms that take the 22KV and provide to busses of 4160V. These run all the pumps, motors, lights for the unit. Cold start, power can come back n from the grid, and give you the aux feeds, or their are buss ties to other units in the plant. All else fails, a backup diesel can power things to get the unit back online.
As someone else said, how much power you make depends on how hard "push" not how fast you spin. With AC, the direction of power flow is complicated. A higher voltage at one end doesn't always imply the direction of flow.
Think of it this way. You have one unit, and fixed load. Thing is spinning at 3600RPM. Now add just a bit more load and and spinning slows down just a fraction of an RPM, the controls kick in and open the control valves a tad more to push harder and the speed comes back up to 3600RPM.