What is overvoltage? Discuss the causes of overvoltage.

Overvoltage:

An overvoltage is a voltage pulse or wave which is superimposed on the rated voltage of the network or When the voltage in a circuit or part of it is raised above its upper design limit, this is known as overvoltage (See Figure- 1). 

Figure-1: Example of Overvoltage

Causes of overvoltage:

The overvoltage of a power system may be broadly divided into two main categories viz.

A) Internal Causes

1. Switching Surges: The overvoltages produced on the power system due to switching operations are known as switching surges. 
2. Insulation Failure: The most common case of insulation failure in a power system is the grounding of conductor (i.e. insulation failure between line and earth) which may cause overvoltages in the system.
3. Arcing Ground:  The phenomenon of intermittent arc taking place in 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 three-phase system. Here, short-live 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 breakdown of the insulation and may harm equipment connected to the power system.
4. Resonance: This one occurs when the value of the inductive resistance in the power system becomes equal with the value of capacitive resistance.

B) 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:

1. Direct lightning stroke.
2. Electromagnetically induced over voltages due to lightning discharge taking place near the line, called 'side stroke'.
3. Voltages induced due to atmospheric changes along the length of the line.
4. Electrostatically induced voltages due to presence of charged clouds nearby.
5. 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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