Poor power quality is usually identified in the “powering” part of the definition, namely in the deviations in the voltage waveform from the ideal. A set of waveforms for typical power disturbances is shown in Figure 1.5. These waveforms are either (a) observed, (b) calculated, or (c) generated by test equipment.

The following are some examples of poor power quality and descriptions of poor power-quality “events.” Throughout, we shall paraphrase the IEEE definitions.

■ A voltage sag (also called a “dip”9) is a brief decrease in the rms linevoltage of 10 to 90 percent of the nominal line-voltage. The duration of a sag is 0.5 cycle to 1 minute [1.44–1.50]. Common sources of sags are the starting of large induction motors and utility faults.

■ A voltage swell is the converse to the sag. A swell is a brief increase in the rms line-voltage of 110 to 180 percent of the nominal line-voltage for a duration of 0.5 cycle to 1 minute. Sources of voltage swells are line
faults and incorrect tap settings in tap changers in substations.

■ An impulsive transient is a brief, unidirectional variation in voltage, current, or both on a power line. The most common causes of impulsive transients are lightning strikes, switching of inductive loads, or switching in the power distribution system. These transients can result in equipment shutdown or damage if the disturbance level is high enough. The effects of transients can be mitigated by the use of transient voltage suppressors such as Zener diodes and MOVs (metal-oxide varistors).

■ An oscillatory transient is a brief, bidirectional variation in voltage, current, or both on a power line. These can occur due to the switching of power factor correction capacitors, or transformer ferroresonance.

■ An interruption is defined as a reduction in line-voltage or current to less than 10 percent of the nominal, not exceeding 60 seconds in length.

■ Another common power-quality event is “notching,” which can be created by rectifiers that have finite line inductance. The notches show up due to an effect known as “current commutation.”

■ Voltage fluctuations are relatively small (less than 5 percent) variations in the rms line-voltage. These variations can be caused by cycloconverters, arc furnaces, and other systems that draw current not in synchronization with the line frequency [1.51–1.61]. Such fluctuations can result in variations in the lighting intensity due to an effect known as “flicker” which is visible to the end user.

■ A voltage “imbalance” is a variation in the amplitudes of three-phase voltages, relative to one another.

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