2 ms·

> a single shaft could run through all of them, and the shaft would not twist I think this assumes that every AC generator has the same number of poles per rev

by nayuki 4y ago

> a single shaft could run through all of them, and the shaft would not twist I think this assumes that every AC generator has the same number of poles per revolution. > If you did it wrong, there's a tremendous amount of torque exerted on the rotating machinery by the grid power, which may simply force it into compliance, or in more extreme cases, may shred millions of dollars of equipment in an instant. Indeed. When I read news articles during the Texas blackout of 2021, the authorities were watching in horror the system frequency dropping and saying they had to shed load before the generators experienced permanent damage. It took me several days and reading Reddit comments to understand that the entire AC grid is one big synchronized machine, and if generators fall out of frequency or phase with each other, they will drive each other instead of the load, thus damaging the generators.

myself248 4y ago
> I think this assumes that every AC generator has the same number of poles per revolution. Oh yeah. An oversimplification, and one that need not exist (all the generators _could_ be wound with the same number of poles), and I think would've complicated the point beyond GP's level. But yes, you are right. > and if generators fall out of frequency or phase with each other, they will drive each other instead of the load, thus damaging the generators. That's one reason, another is that some of the prime movers simply can't exert full torque at significantly-wrong RPM. I'm not a turbine engineer but it's my understanding that even in a standalone situation with no grid at all, if you apply full power to a turbine and then slow the rotor to a stop, somewhere before you even reach a full stop, some of the blades will break, because they can only handle that gas pressure when moving and experiencing the rotating flows of a running machine. The point where that occurs depends on the engineering of each individual machine, but a highly-optimized design may have a tolerable operation range that is narrow indeed. (When a new unit is started up, it does so unloaded, so it only takes a relative whisper of fuel to get it moving. Only once synchronized and loaded do they really hit the gas.) In my mind this seems very similar to the back-EMF developed by a rotating motor, and the phenomenon of locked-rotor current burning out the windings when the shaft isn't allowed to turn at some appreciable fraction of rated RPM. Just the back-pressure is fluid rather than voltage. Any actual turbine engineers here?

by nayuki 4y ago

myself248 4y ago

> I think this assumes that every AC generator has the same number of poles per revolution. Oh yeah. An oversimplification, and one that need not exist (all the generators _could_ be wound with the same number of poles), and I think would've complicated the point beyond GP's level. But yes, you are right. > and if generators fall out of frequency or phase with each other, they will drive each other instead of the load, thus damaging the generators. That's one reason, another is that some of the prime movers simply can't exert full torque at significantly-wrong RPM. I'm not a turbine engineer but it's my understanding that even in a standalone situation with no grid at all, if you apply full power to a turbine and then slow the rotor to a stop, somewhere before you even reach a full stop, some of the blades will break, because they can only handle that gas pressure when moving and experiencing the rotating flows of a running machine. The point where that occurs depends on the engineering of each individual machine, but a highly-optimized design may have a tolerable operation range that is narrow indeed. (When a new unit is started up, it does so unloaded, so it only takes a relative whisper of fuel to get it moving. Only once synchronized and loaded do they really hit the gas.) In my mind this seems very similar to the back-EMF developed by a rotating motor, and the phenomenon of locked-rotor current burning out the windings when the shaft isn't allowed to turn at some appreciable fraction of rated RPM. Just the back-pressure is fluid rather than voltage. Any actual turbine engineers here?