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Power quality is a concern for today's power grid and the loads it serves. Computer equipment, in particular, is sensitive to power quality problems, and the ubiquity of computers in today's manufacturing environment means high power quality is important to many commercial and industrial firms as well as the average homeowner.
Alternating current can be illustrated as a sinusoidal wave, as shown in the diagram. Over time, the voltage oscillates between a positive and negative value that is slightly more than the average voltage.
 Alternating current wave form. |
The most severe power quality problem is a large voltage surge caused by a lightning strike. Other power quality problems include:
- Voltage sags and swells — The amplitude of the wave gets momentarily smaller or larger because of large electrical loads such as motors switching on and off. Voltage sags are the most commonly experienced power quality problem among electronic and computer equipment users.
- Impulse events — Also called glitches, spikes, or transients, these are events in which the voltage deviates from the curve for a millisecond or two (much shorter than the time for the wave to complete a cycle). Impulse events can be isolated or can occur repeatedly and may or may not have a pattern.
- Decaying oscillatory voltages — The voltage deviation gradually dampens, like a ringing bell. This is caused by banks of capacitors being switched in by the utility.
- Commutation notches — These appear as notches taken out of the voltage wave. They are caused by momentary short circuits in the circuitry that generates the wave.
- Harmonic voltage waveform distortions — These occur when voltage waves of a different frequency—some multiple of the standard frequency of 60 cycles per second—are present to such an extent that they distort the shape of the voltage waveform.
- Harmonic voltages — These can also be present at very high frequencies to the extent that they cause equipment to overheat and interfere with the performance of sensitive electronic equipment.
Other power quality problems may also be considered reliability problems because they occur when the transmission system is not capable of meeting the load on the system.
- Brownouts are a persistent lowering of system voltage caused by too many electrical loads on the transmission line.
- Blackouts are, of course, a complete loss of power. Unanticipated blackouts are caused by equipment failures, such as a downed power line, a blown transformer, or a failed relay circuit. Although normally limited by design to a small geographic area, blackouts have been known to affect wide regions of the United States.
- "Rolling" blackouts are intentionally imposed upon a transmission grid when the loads exceed the generation capabilities. By blacking out a small sector of the grid for a short time, some of the load on the grid is removed, allowing the grid to continue serving the rest of the customers. To spread the burden among customers, the sector that is blacked out is changed every 15 minutes or so—and hence, the blackouts "roll" through the grid's service area.
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