Civil Engineering Reference
In-Depth Information
FIGURE 14-9
Allowable voltage deviation in utility-grade power versus time duration of the deviation.
(Adapted from the American National Standards Institute.)
which can be used as a quick approximate check on the quality of power at
any location in the system.
14.7.2
Voltage Transients and Sags
The bus voltage can deviate from the nominally-rated value due to many
reasons. The deviation that can be tolerated depends on its magnitude and
the time duration. Small deviations can be tolerated for a longer time than
large deviations. The tolerance band is generally defined by voltage versus
time (v-t) limits. Computers and business equipment using microelectronic
circuits are more susceptible to the voltage transients than the rugged power
equipment such as motors and transformers. The power industry has devel-
oped an array of protective equipment. Even then, some standard of power
quality must be maintained at the system level. For example, the system
voltage must be maintained within the v-t envelope shown in
Figure 14-9
,
where the solid line is that specified by the American National Standard
Institute (ANSI). The right hand side of the band comes primarily from the
steady state performance limitations of motors and transformer-like loads,
the middle portion comes from visible lighting flicker annoyance consider-
ations, and the left hand side of the band comes from the electronic load
susceptibility considerations. The left hand side curve allows larger devia-
tions in the microsecond range based on the volt-second capability of the
power supply magnetics. The ANSI requires the steady state voltage of the
utility source to be within 5 percent, and short-time frequency deviations
less than 0.1 Hz.
