Power frequency variation is any deviation of the fundamental frequency (60-Hz).
Usually caused by a single generator, and/or inverter system like a UPS not connected to the utility grid.
Power frequency variations may confuse logic systems and affect the operating speed of rotating machinery. Significant power frequency variations in utility systems are very rare and in most conditions would cause a large power failure on the utility side.
Power frequency variations may be most easily detected using a frequency analyzer.
Addressing it at the source i.e. generator or inverter system