Although utilities try to deliver clean well-regulated power to their customers, events beyond their control frequently make this impossible. Lightning strikes, large load and power factor compensating capacitor changes, noisy motor controllers and arc welders, downed distribution lines, breaker tripping and reclosure, and other similar randomly occurring events about which the utility can do little all "dirty up" the customer's power line. If something is not done about this situation the result may be scrambled digital data in memories and on data transmission lines, over-stressed semiconductor components, damaged disc drives, and other events which can cause, at minimum, faulty electronic equipment performance and, at maximum, expensive hardware damage.
The types of power line disturbances include over-voltages and under-voltages, voltage sags and surges, transients or impulses, and dropouts or line interruptions. These are all voltage phenomena which affect the magnitude and waveform of the AC sine wave. Frequency variations can also be a problem, but this is uncommon in the United States where utility networking results in tight frequency control.
Over-voltages and under-voltages are slow changes in the steady-state RMS voltage which exceed upper or lower limit thresholds for more than 2.5 seconds. Sags and surges are shorter-term voltage variations which exceed established thresholds for less than 2.5 seconds. In general, sags and surges are characterized by greater voltage variations than over-voltages and under-voltages. Impulses are spike-like distortions of the AC sine wave which are characterized by high peak amplitudes, extremely short durations, and sometimes oscillation. They last from a fraction of a microsecond to a few milliseconds and can reach hundreds or even thousands of volts. Dropouts and line interruptions are just that - the voltage drops to zero for any time period from an instant on up.
Motor-Generator Sets
Power Line Monitoring
Uninterruptible Power Supplies
