A transient is a voltage event that has an extremely short duration, typically less than half a cycle or a single cycle. Many transients show both impulsive and oscillatory transient behavior.

Impulsive Transients

An impulsive transient is a sudden unidirectional variation in the steady-state condition of a voltage or current waveform. Lightning strikes close to power lines are the most common source of impulsive transients. Lightning strikes create an electromagnetic field in the vicinity of power lines, which can lead to a transient overvoltage in power lines.

Oscillatory Transients

Oscillatory transients show a damped oscillation, with a voltage or current waveform continuing to oscillate for half a cycle to three cycles and reaching a new steady-state value. Oscillatory transients arise because of switching actions in circuits, namely the energizing and de-energizing of capacitors and inductors. A sudden change in an electrical power system when a component is energized or de-energized leads to high-frequency damped oscillations until the system reaches a new steady state.

Effects of Transients

Transients can cause temporary or permanent damage to electrical equipment. A transient overvoltage can damage components, especially semiconductors, which are not designed to withstand high voltages. Transient overvoltages of sufficient duration can dissipate an amount of energy that is larger than a component is designed to withstand, which causes permanent damage.

Transients can cause data errors in data processing and storage equipment because transients in electrical power circuits can couple to communications and control circuits or cause adjustable speed drives to trip accidentally.

Measurement of Transients

Because transients intrinsically have a very short duration, the measurement of transients requires wide bandwidth circuits with high sample rates.