Power quality is a term used to discuss events on electric power grids that can damage or disrupt sensisitive electronic devices.

It is often useful to think of power quality as a compatibility problem: is the equipment connected to the grid compatible with the events on the grid, and is the power delivered by the grid, including the events, compatible with the equipment that is connected? Compatibility problems always have at least two solutions: in this case, clean up the power, or make the equipment tougher.

Ideally the power would be supplied as a sine wave with the amplitude and frequency given by national standards (in the case of mains) or system specifications (in the case of a power feed not directly attached to the mains) with an impedance of zero ohms at all frequencies.

No real life power feed will ever meet this ideal. It can deviate from it in the following ways (among others):

• Variations in the peak or rms voltage (both these figures are important to different types of equipment) When the rms voltage exceeds the nominal voltage by a certain margin, the event is called a "swell". A "dip" (in British English) or a "sag" (in American English - the two terms are equivalent) is the opposite situation: the rms volage is below the nominal voltage by a certain margin.
• An "undervoltage" occurs when the low voltage persists over a longer time period
• Variations in the frequency
• Variations in the wave shape - usually described as harmonics
• Quick and repetitive variations in the rms voltage. This produces flicker in lighting equipment.
• Abrupt, very brief increases in voltage, called "spikes", "impulses", or "surges" (see surge protective devices). Generally caused by lightning or large inductive loads being turned off.
• Nonzero low frequency impedance (if the appliance draws more power the voltage drops)
• Nonzero high frequency impedance (if the appliance demands a large amount of current or stops demanding it suddenly there will be a dip or spike in the voltage due to the inductances in the power supply line)

See also

• Electricity distribution

Electricity distribution