Line frequency is the most tightly controlled parameter in the electrical distribution system.
Since almost all generation in a region is connected essentially in parallel, the frequency is inherently synchronized through most utilities.
Most generators increase or decrease their power with slight changes in frequency (speed up the generator for more power, slow it down
for less), so on a system-wide scale, generators are adjusting to real-time changes in aggregate load by slightly adjusting their frequency, in a coordinated and distributed fashion.
Consequently, measuring line frequency during a PQ investigation or for routine monitoring is not often required...