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### Transients

*The term transients has long been used in the analysis of power system variations to denote an event that is undesirable and momentary in nature. The notion of a damped oscillatory transient due to an RLC network is probably what most power engineers think of when they hear the word 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. 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 indiscriminately 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 wave shape of a current or voltage transient. We will describe these two categories in more detail.

*Impulsive transient*

*Impulsive transient*

**An impulsive transient is a sudden, non–power frequency change in the steady-state condition of voltage, current, or both that is unidirectional in polarity (primarily either positive or negative).**

*Impulsive transients are normally characterized by their rise and decay times.*

*For example, a 1.2 × 50-μs 2000-volt (V) impulsive transient nominally rises from zero to its peak value of 2000 V in 1.2 μs and then decays to half its peak value in 50μs.*

*The most common cause of impulsive transients is lightning.*

*Oscillatory transient*

*Oscillatory transient*

**An oscillatory transient is a sudden, non–power frequency change in the steadystate condition of voltage, current, or both, that includes both positive and negative polarity values.**

*An oscillatory transient consists of a voltage or current whose instantaneous value changes polarity rapidly.*

*It is described by its spectral content (predominate frequency), duration, and magnitude.*

*The frequencies of spectral content are classified as high, medium and low frequencies.*

*Oscillatory transients with a primary frequency component greater than 500 kHz and a typical duration measured in microseconds (or several cycles of the principal frequency) are considered high-frequency transients. These transients are often the result of a local system response to an impulsive transient.*

*A transient with a primary frequency component between 5 and 500 kHz withduration measured in the tens of microseconds (or several cycles of the principal frequency) is termed a medium-frequency transient.*

*Back-to-back capacitor energization results in oscillatory transient currents in the tens of kilohertz as illustrated in Figure below.*

*A transient with a primary frequency component less than 5 kHz, and a duration from 0.3 to 50 ms, is considered a low-frequency transient. This category of phenomena is frequently encountered on utility sub transmission and distribution systems and is caused by many types of events.*

*Capacitor bank energization results in low frequency oscillatory voltage transient with a primary frequency between 300 and 900 Hz.*

*Oscillatory transients with principal frequencies less than 300 Hz can also be found on the distribution system. These are generally associated with ferro resonance and transformer energization.*

*Transients involving series capacitors could also fall into this category.*

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