Flicker, also known as voltage fluctuations, describes rapid, sequential voltage changes whose magnitude does not vary more than 10% from the nominal amount. The voltage variation is sufficient in duration to allow visual observation of a change in an incandescent electric light source intensity.
Large load swing equipment such as arc furnace, car crushers, certain cranes, large chipper mills, and others can cause flicker problems. These large load swings cause the voltage to vary over time.
Flicker, most notably, causes rapid dimming and brightening of lights. Since it varies less than 10% from nominal voltage, it should not affect equipment, but will cause nuisance dimming of lights that is noticeable to humans.
A Flicker Meter is used to measure flicker. These meters now have a PST index. This is calculated based on a scale of the frequency and the magnitude of the change in the voltage. If the number is less than one then the flicker is not considered a problem.
Static Var Converters and in some cases capacitors for the var loading, larger transformers and/or load controlling of the equipment
Best Source(s) of Information
IEEE Flicker Standard