Transients in Power Systems

In power systems, transients refer to short-term disturbances or fluctuations in voltage, current, or power that can occur due to various factors, such as sudden changes in load demand, faults in the system, lightning strikes, switching operations, and generator or load tripping.

Transients can have different time durations, ranging from a few microseconds to a few seconds, and can cause various problems, such as equipment damage, system instability, and power quality issues. Transients can destroy computer chips and TV.

To mitigate the effects of transients, various protective devices are used in power systems, such as circuit breakers, fuses, surge arresters, and voltage regulators. These devices help to limit the magnitude and duration of transients, and protect the system and equipment from damage.

Simulation tools, such as transient stability analysis software, are also used to model and analyze the behavior of power systems during transients, and help to identify potential problems and optimize system performance.

Transients are usually classified into two categories:

  • Impulsive and
  • Oscillatory

Impulsive transient caused by a lightning stroke. Switching of lines with power factor correction capacitor banks, poor grounding, switching of inductive loads, utility fault clearing, disconnection of heavy loads, and electrostatic discharge. Impulsive transients can be very fast events (5 ns rise time from steady state to the peak of the impulse) of short-term duration (less than 50 ns), and may reach thousands of volts, even in low voltage.

Devices are needed to prevents damage to electrical equipment caused by impulsive transients from lightning strokes Utilities use lightning arresters mounted on their transmission and distribution systems and in their substations, while many utility customers use transient voltage surge suppression (TVSS).

Fig- Impulse transients

Oscillatory transients occur when switching inductive or capacitive loads such as motors or capacitor banks. An oscillatory transient occurs because the load resists the change. Lighting, utility fault clearing and transformer energization and Ferro resonance could also cause oscillatory transients.

Oscillatory transients do not decay quickly like impulsive transients. They tend to continue to oscillate for 0.5 to 3 cycles and reach 2 times the nominal voltage or current. Another cause of oscillatory

transients, besides lightning strokes going into resonance, is switching of equipment and power lines on the utility’s power system.

Fig - Oscillatory transients

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