Correct Answer - Option 3 : arcing ground
Causes of overvoltage:
The abnormal overvoltages may be caused due to various reasons such as sudden interruption of heavy load, lightning impulses, switching impulses, etc.
The overvoltage of a power system may be broadly divided into two main categories viz.
Internal Causes:
Switching Surges: The overvoltages 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 overvoltages in the system.
Arcing Ground:
- The phenomenon of intermittent arc taking place in the line-to-ground fault of a 3phase 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 with the value of capacitive resistance.
External Causes:
This type of overvoltages 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 overvoltages 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 overvoltages 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.
