Over Voltage Protection

1. Unit-IV: Overvoltage Protection

2. CAUSES OF OVERVOLTAGES
Over voltages arising on a power system can be generally classified into two main categories as
follows.
1. External Overvoltages
i. Direct lightning strokes
ii. Electromagnetically induced overvoltages due to lightning discharge taking place near
the line (commonly known as ‘side stroke’)
iii. Voltages induced due to changing atmospheric conditions along the line length
iv. Electrostatically induced overvoltages due to the presence of charge clouds nearby
v. Electrostatically induced overvoltages due to the frictional effects of small particles such
as dust or dry snow in the atmosphere or due to change in the altitude of the line

3. 2. Internal Overvoltages
i. Switching overvoltages (or transient overvoltages of high frequency)
ii. Temporary overvoltages (or steady-state overvoltages of power frequency)

4. LIGHTNING PHENOMENA
The discharge of the charged cloud to the ground is called lightning phenomenon
Lightning mechanism

5. WAVE SHAPE OF VOLTAGE DUE TO
LIGHTNING
Lightning sets up steep-fronted, unidirectional voltage waves which can be
represented as the difference of two exponentials.
Thus
Where-
a and b are constants which determine the
shape,
v is magnitude of the steep voltage,
V is equal to the crest (peak) value of the
impulse voltage wave.
The wave is
then referred to as t1/t2wave. The standard wave chosen for the testing purpose is a 1/50 wave which implies that t1
is 1 ms and t2 is 50 ms.

6. OVERVOLTAGES DUE TO LIGHTNING