Fortunately, interconnects cannot go out of phase easily. If one region is out of phase with another, massive currents flow down the transmission lines within milliseconds, causing generators at one side or the other to slow down or speed up. I have seen this effect live at a hydroelectric dam. We were watching the turbine shaft spin at the moment they switched on some giant interconnect. There was a phase-alignment dial and it just instantly pinged up to 0 while the spinning shaft shuddered, slightly altering its rate of rotation in the blink of an eye.