Page 23 - Power Quality in Electrical Systems

Introduction 7

Equally authoritative, the qualification is made in the Standard
Handbook of Electrical Engineers, 14th edition, (2000) ([1.43] pp. 18–117):
Good power quality, however, is not easy to define because what is good
power quality to a refrigerator motor may not be good enough for today’s per-
sonal computers and other sensitive loads. For example, a short (momen-
tary) outage would not noticeably affect motors, lights, etc. but could cause
a major nuisance to digital clocks, videocassette recorders (VCRs) etc.

Examples of poor power quality
Poor power quality is usually identified in the “powering” part of the def-
inition, namely in the deviations in the voltage waveform from the ideal
of Figure 1.1. 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 descrip-
tions of poor power-quality “events.” Throughout, we shall paraphrase
the IEEE definitions.

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■ A voltage sag (also called a “dip” ) is a brief decrease in the rms line-
voltage 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 switch-
ing 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 volt-
age suppressors such as Zener diodes and MOVs (metal-oxide varistors).
■ An oscillatory transient is a brief, bidirectional variation in voltage, cur-
rent, 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.

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Generally, it’s called a sag in the U.S. and a dip in the UK.