Kind of makes you wonder how they keep the many generating plants synched up, eh. Well, here's how that works in NA. It's a little dry, but if you want to know............
http://www.nerc.com/docs/oc/rs/NERC Balancing and Frequency Control 040520111.pdf
Note: This document was created in 2011, and if memory serves, there have been or at least some are under discussion, changes in the process.