To Bruce V .
Hi ,
Three phase generating and distribution systems display the fascinating property of self synchronisation – sometimes also known as sync lock or phase lock .
When a new generator is to be linked in to an already running grid with other generators connected the operators of the new generator go through a procedure of setting the running speed of their machine so as to give an output frequency nearly the same as that of the grid and then switch it in .
If the matching of frequencies is within acceptable limits the new generator will lock on to the grid frequency without further adjustment . Basically errors between generator and grid frequencies create a (crudely described) back EMF into the generator which tends to speed it up or slow it down to correct for frequency mismatch . This effect is very powerful and very stable .
Traditionally generator operators watched a three bulb display when bringing generator to sync speed . Bulbs were difference connected between generator and grid wth one for each phase . When frequency error was big bulbs lit brightly . As sync was approached they became dimmer and were seen to pulsate either clockwise or anticlockwise . When dim and nearly still phase difference was within acceptable limits for connecting in .
They did of course have local shaft speed and frequency meters as well . In modern times the system is automated but still watched carefully by operators at connect in times .
When frequencies are not matched well enough a generator will hunt dreadfully and uncontrolled large currents can flow . A growling sound is often heard on small systems . Large systems have to be shut down within a couple of second otherwise massive damage will result .
So the answer to your actual question is that all generators (within a zone at least) are locked together on frequency and the adjustment for timekeeping is both feasible and commonly done .
Notes :
Like all things in the real world there are some exceptions possible . One of these touches on mention of the American grid . Where a lot of generators and loads in one place and a lot of generators and loads in another place are connected but a long way apart there will be some error in the synchronisation between the two places though even then not usually very much .
Regards ,
Michael Williams