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.