There has been a tendency to attribute
disturbances and failures to power surges, a term often used by the media but
rather ill-defined. The ambiguity results in part from an unfortunate dual
definition of the word surge.
a) To some people, a surge is indeed the
phenomenon being discussed here, that is, a transient voltage or current
lasting from microseconds to at most a few milliseconds, involving voltages
much higher than the normal (two to ten times).
b) To other people, a surge is a momentary
overvoltage, at the frequency of the power system, and lasting for a few
cycles, with voltage levels slightly exceeding the five to ten percent
excursions that are considered normal occurrences.
The term swell has been adopted by this
recommended practice to describe this second type of overvoltage; perhaps one
day it will supplant the usage of surge for that meaning. It would be a mistake
to attempt protection against these long-duration power frequency swells with a
surge protective device that is designed to absorb large but short impulses of
energy.
There is a growing recognition that the
horror tales of surge protective device failures are more likely
to be caused by swells rather than by large
surges.
Nonlinear loads draw nonsinusoidal currents
from the power system, even if the power system voltage is a perfect sine wave.
These currents produce nonsinusoidal voltage drops through the system source
impedance, which distort the sine wave produced by the power plant generator.
A typical nonlinear load is a dc power
supply consisting of rectifiers and a capacitor-input filter, such as used in
most computers, drawing current only at the peaks of the voltage sine wave.
This current has a high third harmonic content that has also created a new
concern, that of insufficient ampacity of the neutral conductor in a
three-phase system feeding power supplies.
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