Electrical Bus

An oscillatory transient is a sudden, non–power frequency change inthe steady-state condition of voltage, current, or both, that includesboth positive and negative polarity values.An oscillatory transient consists of a voltage or current whose instan-taneous value changes polarity rapidly. It is described by its spectralcontent (predominate frequency), duration, and magnitude. The spec-tral content subclasses defined in Table 2.2 are high, medium, and lowfrequency. The frequency ranges for these classifications are chosen tocoincide with common types of power system oscillatory transient phe-nomena.Oscillatory transients with a primary frequency component greaterthan 500 kHz and a typical duration measured in microseconds (or sev-eral cycles of the principal frequency) are considered high-frequencytransients. These transients are often the result of a local systemresponse to an impulsive transient.Atransient with a primary frequency component between 5 and 500kHz with duration measured in the tens of microseconds (or severalcycles of the principal frequency) is termed a medium-frequency transient.Back-to-back capacitor energization results in oscillatory transientcurrents in the tens of kilohertz as illustrated in Fig. 2.2. Cable switch-ing results in oscillatory voltage transients in the same frequencyrange. Medium-frequency transients can also be the result of a systemresponse to an impulsive transient.
Atransient with a primary frequency component less than 5 kHz,and a duration from 0.3 to 50 ms, is considered a low-frequency tran-sient.This category of phenomena is frequently encountered on utilitysubtransmission and distribution systems and is caused by many typesof events. The most frequent is capacitor bank energization, which typ-ically results in an oscillatory voltage transient with a primary fre-quency between 300 and 900 Hz. The peak magnitude can approach 2.0pu, but is typically 1.3 to 1.5 pu with a duration of between 0.5 and 3cycles depending on the system damping (Fig. 2.3).Oscillatory transients with principal frequencies less than 300 Hzcan also be found on the distribution system. These are generally asso-ciated with ferroresonance and transformer energization (Fig. 2.4).Transients involving series capacitors could also fall into this category.They occur when the system responds by resonating with low-fre-quency components in the transformer inrush current (second andthird harmonic) or when unusual conditions result in ferroresonance.It is also possible to categorize transients (and other disturbances)according to their mode.Basically, a transient in a three-phase systemwith a separate neutral conductor can be either common modeornor-mal mode,depending on whether it appears between line or neutraland ground, or between line and neutral.

Labels: Transient

Other definitions in common use are broad in scope and simply state
that a transient is “that part of the change in a variable that disappears
during transition from one steady state operating condition to
another.”8 Unfortunately, this definition could be used to describe just
about anything unusual that happens on the power system.
Another word in common usage that is often considered synonymous
with transient is surge. A utility engineer may think of a surge as the
transient resulting from a lightning stroke for which a surge arrester
is used for protection. End users frequently use the word indiscriminantly
to describe anything unusual that might be observed on the
power supply ranging from sags to swells to interruptions. Because
there are many potential ambiguities with this word in the power quality
field, we will generally avoid using it unless we have specifically
defined what it refers to.

Broadly speaking, transients can be classified into two categories,
impulsive and oscillatory. These terms reflect the waveshape of a current
or voltage transient. We will describe these two categories in more detail.

Labels: Transient

Because of the high frequencies involved, the shape of impulsive
transients can be changed quickly by circuit components and may have
significantly different characteristics when viewed from different parts
of the power system. They are generally not conducted far from the
source of where they enter the power system, although they may, in
some cases, be conducted for quite some distance along utility lines.
Impulsive transients can excite the natural frequency of power system
circuits and produce oscillatory transients.
Figure Lightning stroke current impulsive transient.

Labels: Transient