ÇUKUROVA UNIVERSITY INSTITUTE OF NATURAL AND APPLIED ...

2. POWER QUALITY Mustafa İNCİ
2.2.1. Voltage Sag
Voltage sags are now one of the most important power quality problems in
the distribution system. A voltage sag is a momentary decrease in the RMS ac
voltage (10%–90% of the nominal voltage), at the power frequency, of duration from
0.5 cycles to a few seconds. Voltage sags are normally caused by short-circuit faults
such as a single-line-to-ground fault in the power system or by the starting up of
induction motors of large rating. Voltage sags may cause the malfunction of voltagesensitive
loads in factories, buildings, and hospitals (Kangarlu et al., 2010).
Voltage sags can cause tripping of contactors, motor starters, relays, restarting
expense of computers and shutdown of an entire production line. The sources of
voltage sags are basically faults on adjacent feeders, lighting, short circuit event, start
up of heavy loads, transformer energizing and motor starting (Bollen, 2001).
2.2.2. Voltage Swell
Voltage swell is defined as a short duration increasing in RMS supply with
increase in voltage ranging from 1.1 pu to 1.8 pu of nominal supply. The main causes
for voltage swell are switching of large capacitors or removal of heavy loads
(Kangarlu et al., 2010).
Voltage swells might not be as common as voltage sags, however are much
more harmful and disruptive to static power converters. In fact, they may severely
damage or trip them, causing shutdowns of entire processes. This overall situation
has become even more critical given the recent industrial trend to increase operating
voltages of power converters (a practice that has pushed semiconductor devices up to
their limit. A closer look to this phenomenon is, hence, required (Burgos et al.,
2005).
2.2.3. Voltage Fluctuations
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