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```Q3. How do you calculate the induced voltage in a
power distribution network caused by an indirect
lightning strike?
The induced overvoltage caused by a lightning strike can breakdown a
distribution network. To prevent this failure and install proper
insulation, we need an accurate estimate of lightning induced
overvoltage.
Lightning induced overvoltage would depend on various parameters:



Rate of rise of lightning current(inductance)
Magnitude or peak value of current
Soil to strike point resistance
Indirect Strokes mainly causes over voltages and thereby causing line
outages. In case of indirect Stroke, the return stroke in nearby lines
generates electromagnetic field which produces lightning induced
over voltages.
There are several methods through which over voltages induced by
indirect lightning strokes can be calculated. The methods employed
practically can be subdivided into two main categories:
1. Time varying horizontal and vertical electric fields are calculated
from the magnetic fields, generated by the lightning channel,
using analytical formulas. Afterward, lightning induced
overvoltage are computed from these computed electric fields
using coupling models that are based on telegraph equations of
2. Maxwell’s electromagnetic equations are numerically solved to
calculate the electromagnetic fields radiated from the lightning
channel and, hence, the lightning-induced overvoltage are
computed. Different numerical methods are employed for these
calculations such as the method of moments, finite-element
method, hybrid electromagnetic model, and finite-difference
time-domain (FDTD) method.
#Evolution of Rusck formula:
Rusck relation is the main formula for calculating
induced over voltages caused by an indirect lightning strike
in power transmission lines. It includes two basic
assumptions, perfect ground conductivity and a vertical
stroke channel.
where v, c are the velocities of the return stroke and
light, respectively; Ip is the peak value of the lightning
current, h is the height of the overhead line and d is the
distance between the return stroke channel to the overhead
line.
Since Rusck formula is applicable to only idea soil, Hence, for it
to be valid for an actual soil, an additional term is added to
eq.(1):
Where Pg is the ground electrical resistance and k is a
numerical factor between 0.15 and 0.25 according to IEEE
standard 1410-2010.
Reference:
[IEEE 2019 27th Iranian Conference on Electrical Engineering (ICEE) Yazd, Iran (2019.4.30-2019.5.2)] 2019 27th Iranian Conference on
Electrical Engineering (ICEE) - Investigation of the Induced Overvoltages
Caused by the Indirect Lightning Strike in Power Distribution Lines
```