Ieee std 1159-1995, ieee recommended Practice for Monitoring Electric Power Quality

4.4.3 Long duration variations 
Long duration variations encompass rms deviations at power frequencies for longer than 1 min. The steady-
state voltage tolerances expected on a power system are speciÞed in [B1] . These magnitudes are reßected in
table 2. Long duration variations are considered to be present when the ANSI limits are exceeded for greater
than 1 min.
Long duration variations can be either overvoltages or undervoltages, depending on the cause of the varia-
tion. Overvoltages and undervoltages generally are not the result of system faults. They are caused by load
variations on the system and system switching operations. These variations are characterized by plots of rms
voltage versus time.
Figure 7ÑTemporary voltage sag caused by motor starting

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IEEE
MONITORING ELECTRIC POWER QUALITY
Std 1159-1995
19
4.4.3.1 Overvoltage
Overvoltages can be the result of load switching (e.g., switching off a large load), or variations in the reac-
tive compensation on the system (e.g., switching on a capacitor bank). Poor system voltage regulation capa-
bilities or controls result in overvoltages. Incorrect tap settings on transformers can also result in system
overvoltages.
4.4.3.2 Undervoltage
Undervoltages are the result of the events that are the reverse of the events that cause overvoltages. A load
switching on, or a capacitor bank switching off, can cause an undervoltage until voltage regulation equip-
ment on the system can bring the voltage back to within tolerances. Overloaded circuits can result in under-
voltages also.
The term brownout is sometimes used to describe sustained periods of low power-frequency voltage initiated
as a speciÞc dispatch strategy to reduce power delivery. The type of disturbance described by brownout is
basically the same as that described by the term undervoltage deÞned here. Because there is no formal deÞ-
nition for the term brownout, and because the term is not as clear as the term undervoltage when trying to
characterize a disturbance, the term brownout should be avoided in future power quality activities in order to
avoid confusion.