Lightning Phenomena is an electric discharge in the form of spark originated in the charge cloud. This natural phenomenon occurs due to the negative charge which located in the lower part of the cloud. generally, charge cloud has the field of 1000v/m field. Under the influence sufficiently strong field large voltage drops in the rainy cloud become elongated in the direction of the file and it’s also become unstable. Streamer also developed in this scenario while the lightning occurs.
Some Technical Details of Lightning Phenomena
When the stream field exceeds about 9000v/cm which is less than 30 kV/cm required to initiate the breakdown of the dry air and air with moisture with the air will break down. The breakdown process in the air also continues several numbers of times.
Each steps increasing the channel length 10-200m. Because of the steps like sequence which this stream travel to the earth this process is named step leader stroke. When eventually the step leader approach within 20-60 m to the earth feel intensities at earth is sufficient to a form an upward streamer and bridge the remaining gaps.
The Vertical separation of the positive and negative charge centers of the cloud is about 2 – 5 km, and the charges involved are 10 – 30 C at temperature.
The approximate time durations of the various components of a lightning stroke are summarized as follows.
- Stepped leader = 10 ms
- Return stroke = 40 ms
- the period between strokes = 40 ms
- duration of dart leader = 1 ms
This will also cause a large neutralizing current flow along the already ionizing path processed by the step leader and this called as an upward leader or return stroke.
This return stroke can have about 20Ka current but it can go even higher as 200 kV. After this neutralizing current about (40ms) a second leader, stroke propagates to the earth in a continuous and rapid manner.
This leader stroke called as the draft leader. The composition of all these appears as the style flash or lightning.
Frequency of Lightning Flash Occurrence
Having a good knowledge of the lightning stroke frequency of occurrence is really important in the design process of the lighting protection.
We have to define the frequency of the current of logging per unit area per year. However, measurement of the frequency of occurrence is not really easy.
Instead of the frequency of occurrence the number of thunder days per year which thunder is mainly defended and names by keraunic-level.
Experimentally it has been found that keraunic level is relatively related to the lightning flash per unit area of the year.
Most of the countries have developed a map indicating the ketonic levels at a different location. A map which shows the same amount of keraunic level is named as an isoceraunic map.
Lightning is a natural phenomenon and therefore it is highly probabilistic. So it is highly unpredictable to define when the lighting occurs and how much of energy contains in that lighting. However, the frequency occurring in the lighting is very important in power system design process.
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