OSCILLATORY TRANSIENTS IN THE POWER SYSTEM

Friday, May 13, 2011

Oscillatory Transient is described as a sudden, non–power frequency change in the steady-state condition of voltage, current, or both that has both positive and negative polarity values (bidirectional).

In other words, the instantaneous voltage or current value of an oscillatory transient varies its polarity quickly. It is described by its spectral content or predominant frequency, magnitude and duration.

Just like the impulsive type, the oscillatory transient is subdivided into three classes. These were based on selected frequency ranges, which correspond with common types of power system oscillatory transient phenomena. It should also be noted that the frequency of the oscillation gives a trace to the origin of the disturbance.
Oscillatory Transients Classification
Oscillatory Transients Classification
Types and Examples

Low-frequency oscillatory transient

This type is normally encountered on subtransmission and distribution systems, which could originate primarily due to capacitor bank energization. Electric distribution utilities use capacitor banks to improve power factor, as well as lower system losses. For better results, capacitor banks have to be switched in and out of the system to match with changes in the load profile. However, capacitor bank energization yields an oscillatory voltage transient with frequencies between 300 and 900 Hz. Theoretically, its magnitude can go as high as 2.0 pu, but system damping typically limits the values from 1.3 to 1.5 pu, with a duration of 1/2 to 3 cycles.

Also, oscillatory transients with fundamental frequencies less than 300 Hz can be observed on the distribution system due to transformer energization and ferroresonance. In addition, series capacitors may also produce this transient type when the system resonance causes the magnification of low-frequency components in the transformer inrush current or when unusual conditions lead to ferroresonance.

Medium-frequency oscillatory transient

An example of this transient type is the back-to-back capacitor switching. It occurs when a capacitor bank is switch in close electrical proximity to another capacitor bank that is already energized, which sees the deenergized bank as a low impedance path.
Oscillatory Transient Due to Back-to-Back Capacitor Switching
Oscillatory Transient Due to Back-to-Back Capacitor Switching 
Other causes of medium-frequency oscillatory transient include cable switching and as a system response to an impulsive transient.

High-frequency oscillatory transient

These transients are linked with power electronics and switching events (e.g. line or cable energization). Power electronics, like the switching power supply in computers, generate oscillatory voltage transients that repeat several times per 60 Hz cycle. Usually, they are also the result of a local system response to an impulsive transient.

References:

IEEE 1159-1995. Recommended Practice For Monitoring Electric Power Quality. New York: IEEE, Inc.
Rogers, B. (2005). An Overview of Power Quality Issues.

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I am an Electrical Engineer with a Masters Degree in Business Administration. My interest is in Power Quality and Protective Relaying. I have been working in an electric distribution utility for about seven years now as a Planning & Design Engineer. I handle PQ studies, power system analysis and capital budgeting for company projects.