Transients - An Overview

A Transient can be defined as a momentary change in a parameter such as voltage, current or frequency that occurs when a system changes from one steady state operating condition to another. Transients occur during switching on and switching off loads, during starting motors, etc.

Transients can be classified into

Impulsive and
Oscillatory transients

Impulsive Transients
They are usually caused by lightning. These transients do not have any impact on the system frequency. They cause a very sharp change in either the voltage or the current. However, the change is in only one direction, the positive or the negative side i.e. they are unidirectional. They are characterized by a sharp rise followed by a decay. For instance a 1.5 x 60 microsecond, 2500V surge cause the voltage to rise to 2500V in a period of 1.5 microseconds. This will be followed by a decay to 50% of the voltage value in 60 microseconds.

The impulsive transient may appear differently in the waveform from different points in the system as it is a fast-changing phenomenon. This is because the transient can be modified by various components of the power system.

Oscillatory Transients
They differ from impulsive transients in that they are bi-directional, they occur on both the positive and negative sides of the waveform. Oscillatory transients can be classified into High, Medium, and Low Frequency Transients depending on the primary frequency of the transient.

High frequencies, with a primary frequency greater than 500 kHz, are caused by a reaction of the system to an impulsive transient. Impulsive transient can excite the natural frequency of the power system which can cause oscillations.

Medium Frequency Transients, with a primary frequency ranging from 5 to 500 kHz, are known as Medium frequency transients. They are generally caused by switching capacitor banks or charging large cables.

Low frequency transients have a primary frequency less than 5 kHz. They are usually caused by transformer energization and Ferro resonance.