Jun 8, 2013

Lightning Phenomenon In High Voltage Engineering



Lightining
The discharge of the charged cloud to the ground is called lightning phenomenon.
It is a peak discharge in which charge accumulated in the clouds discharges into a neighboring cloud or to the ground.

The large spark accompanied by light produced by an abrupt, discontinuous discharge of electricity through the air, from the clouds generally under turbulent conditions is called lightning.

Representative values of a lightning stroke are as below:
Voltage : 2 x 108 volts; 200 MV (peak)
Current :  4 x 44 amperes
Duration:  l0-5 Second
KW      : 8 x 109
KWH     : 22
The influencing factors for formation or accumulation of charge in the clouds are many and they are uncertain. However, during thunder storm conditions positive and negative charges in the clouds get separated by heavy air currents with ice crystals in the upper part and rain in the lower parts of the cloud.

The distance of separation between cloud to ground is very large, may be even 10000 m; but it depends on the height of the clouds. The maximum voltage gradient reached at the ground level due to a cloud could be as high as 300 V per cm.

A probable charge distribution model as shown in the fig. below with the corresponding field gradient near the ground:

simpson's theory
A theory by Simpson explains that there are three essential regions in the cloud, for charge formation. Below region A (shown in Fig) air currents travel above 800 cm/s and no rain drops fall through. In region A, air velocity is high enough to break the falling rain drops causing a positive charge spray in the cloud and negative charge in the air. The spray is blown upwards, but as the velocity of air decreases, the positively charged water drops combine with the larger drops and fall again. Thus region A, eventually becomes predominantly positive charged, while region B above it, becomes negatively charged by air currents. In the upper regions of the cloud, the temperature is as low as freezing point and only ice crystals exist. The impact of air on these crystals makes them negatively charged, thus the distribution of the charge with in the cloud becomes as shown in top of the Fig.


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