A transient can be a unidirectional impulse of either polarity or a damped oscillatory wave with first peak occurring in either polarity.
The term transients has 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.
Transients can be classified into two categories, impulsive and oscillatory. These terms reflect the wave-shape of a current or voltage 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, which can also be revealed by their spectral content.
Figure 1.2 shows a typical current impulsive transient caused by lightning. 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.
ii. Oscillatory Transient:
An oscillatory transient is a sudden, non-power frequency change in the steady- state 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. 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 with duration 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. Cable switching results in oscillatory voltage transients in the same frequency range. Medium-frequency transients can also be the result of a system response to an impulsive transient.
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. The most frequent is capacitor bank energization, which typically results in an oscillatory voltage transient with a primary frequency between 300 and 900 Hz. The peak magnitude can approach 2.0 pu, but is typically 1.3 to 1.5 pu with a duration of between 0.5 and 3 cycles depending on the system damping.
Oscillatory transients with principal frequencies less than 300 Hz can also be found on the distribution system. These are generally associated with ferroresonance and transformer energization.
Transients involving series capacitors could also fall into this category. They occur when the system responds by resonating with low frequency components in the transformer inrush current (second and third 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 system with a separate neutral conductor can be either common mode or normal mode, depending on whether it appears between line or neutral and ground, or between line and neutral.
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Electrical transients cover a wide range of interference and have the potential to impact the operation of electrical systems and equipment. These events can be caused by power grid switching, sudden disconnection or connection, lightning, as well as generated within or by associated electrical equipment. They commonly impact commercial, automotive, and military, electrical systems and equipment, and are often transmitted by power and data lines.
Transients can be broken down into two main categories: impulses and oscillatory.(2) The associated image shows each common type of transient, an electrical fast transient (EFT) in accordance with IEC 61000-4-4 and a ring wave transient in accordance with IEC 61000-4-12. The IEC 61000-4 series of standards are some of the most commonly used requirements for defining these events and will be referenced throughout the article.
These potentially high voltage and current events typically have failures categorized by hard and soft failures, with complete degradation of function being considered a hard failure. A soft failure can be remedied by recycling the power to the equipment. It is crucial that the design of electrical systems be sufficiently immune, having no impact on the operation, to ensure both product reliability and compliance to underlying standards.
This type of conducted electrical interference can come from both internal and external sources which has the potential to impact AC and DC systems. The associated table provided by Cadence System Analysis(3) specifies common sources of electrical transients. It is estimated that between 60-80%(4) of surges, a common transient, are created within a facility as opposed to externally.
There are many potential sources of these events and the waveforms used to simulate these events can vary significantly. It is also worth noting that how the associated transient is defined can vary by application and the underlying standard.
There are a variety of both international and manufacturer based standards that are used to describe, define, and test the impact of transients on equipment. The most commonly used commercial requirements include the IEC 61000-4 series as well as CE Mark and the ANSI/IEEE C series. The breakout below shows the most common sections and the associated transient waveforms.
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