Lightning over-voltages

Lightning Phenomenon 

 The lightning is a natural phenomenon generated during thunderstorm by electrostatic
discharge which produced electromagnetic radiations. The lightning is occasionally associated
with thunder due to the electric current passing through the lightning channel.

Types of Lightning
 Lightning discharges can be classified into three main types:
1. Intracloud discharge,
2. Cloud-to-cloud discharge and
3. Cloud-to-ground discharge.
 From the power system point of view, cloud-to-ground discharge is the frequent type that

generates lightning over-voltages on overhead distribution lines.

Lightning Process

 Figure 1.1 shows induced charges on transmission line when the thunder cloud located above
the transmission line. In Figure 1.1.a, the distribution of negative and positive charges of the
charged thunder cloud is depicted with different temperature values. In the base of the thunder
cloud, the negative charges center is at temperature -5 οC and located in the lower part of the
cloud. At higher elevation, there is a positive charging center with temperature less than -20 οC.
In the circumstances of many neighbor storm clouds, there are generated positive charges in a
small region at the cloud base as depicted in Figure 1.1.a . This small region temperature is
around 0 οC.

Figure 1.1.b shows the induced charge in the line through the down coming leader charge. 

These induced charges travel along the line with approximately the light velocity when the 

cloud is discharged. Otherwise, the induced charges remain at their positions without any 

traveling. The travelling of the induced charges along the line generates voltage surge in both 

sides of the line. This voltage is dependent on the lightning stroke location where it can be 

evaluated using the form: 

where E is the peak value of the voltage waves on the line, C1 is the capacitance between 

the cloud and the line, C2 is the capacitance between the line and the ground, q is the bound 

charge per unit length of the line, C is the capacitance per unit length of the line. 

 The induced overvoltage process can be simply interpreted considering principles of 

electrostatic induction and electromagnetic induction .