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Transient disturbances may be caused by:
1. Faults
2. Load variations
3. Faults, Load variations or Switching operations
4. Switching operations

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Correct Answer - Option 3 : Faults, Load variations or Switching operations

Transient: Transients are an instantaneous change in the waveform of the system or an instantaneous change in the energy in the power system for a very short duration.

The duration of the transient is from a few milli-second to a maximum of 1 second.

Internal Causes of Transient:

Switching Surges: The over-voltages produced on the power system due to switching operations are known as switching surges.

Insulation Failure: The most common cause of insulation failure in a power system is the grounding of the conductor (i.e. insulation failure between line and earth) which may cause over-voltages in the system.

Arcing Ground:

  • The phenomenon of intermittent arc taking place in the line-to-ground fault of a 3-phase system with consequent production of transients is known as arc ground.
  • This happens when there is the presence of a sporadic arc in line-to-ground fault belonging to a three-phase system.
  • Here, short-lived oscillations are produced in the system due to some changes in the voltage and the current load.
  • This phenomenon may lead to serious problems like the breakdown of the insulation and may harm equipment connected to the power system.

Resonance: This one occurs when the value of the inductive resistance in the power system becomes equal to the value of capacitive resistance.

External Causes of Transient:

This type of transient over-voltage originates from atmospheric disturbances, mainly due to lightning. This takes the form of a surge and has no direct relationship with the operating voltage of the line. It may be due to any of the following causes:

  • Direct lightning stroke.
  • Electromagnetically induced over-voltages due to lightning discharge taking place near the line, called 'side stroke'.
  • Voltages induced due to atmospheric changes along the length of the line.
  • Electrostatically induced voltages due to the presence of charged clouds nearby.
  • Electrostatically induced over-voltages due to the frictional effects of small particles like dust or dry snow in the atmosphere or due to change in the altitude of the line.

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