Synchrophasors sit in electricity substations and report on conditions on the lines. The current new breed of synchrophasors reports back 30 times a second rather than every 2-4 seconds as the older ones do.
Better yet, it will report back with a time stamp, which it will generate by listening for a GPS signal, so that all the reports can be synchronized and computers at the control center can be clear on the sequence of events. Hence the term “synchrophasor.”
Thus, grid conditions can now be monitored and reacted to in real-time. 2-4 seconds really can be too long to wait. The US DoE is funding seed synchrophasor projects now.
The North American SynchroPhasor Initiative has more.