Lightning is caused by the separation of positive and negative charges within thunderstorm clouds. As ice crystals collide within the clouds, electrons are transferred, leaving the top of the cloud positively charged and the bottom negatively charged. When the negative charge at the bottom of the cloud is attracted to the positive charge in the ground, a stepped leader forms to connect them. This stepped leader travels from the cloud to the ground in sections, with pauses in between. Once connected, a return stroke allows the flow of electrons from the cloud and positive charges from the ground, producing a visible lightning strike. The rapid heating and expansion of air during this process causes thunder.

1. HIGH VOLTAGE ENGINEERING
Lightning
Dr.S.Sangeetha,
Associate Professor /EEE
Sri Ramakrishna Institute of Technology
Coimbatore-10.

2. Lightning
• Lightning is a discharge of electricity. A single stroke of
lightning can heat the air around it to 30,000°C (54,000°F)
• This extreme heating causes the air to expand explosively fast.
• As ice crystals high within a thunderstorm cloud flow up and
down in the turbulent air, they crash into each other.
• Small negatively charged particles called electrons are knocked
off some ice and added to other ice as they crash past each
other.
• This separates the positive (+) and negative (-) charges of the
cloud. The top of the cloud becomes positively charged while
the base of the cloud becomes negatively charged.
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3. Lightning
• Lightning happens
when the negative
charges (electrons) in
the bottom of the
cloud are attracted to
the positive charges
(protons) in the ground.
Pic Courtesy: scied.ucar.edu
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4. Lightning
• The accumulation of electric
charges has to be great
enough to overcome the
insulating properties of air.
• When this happens, a stream
of negative charges pours
down towards a high point
where positive charges have
clustered due to the pull of
the thunderhead.
Pic Courtesy: scied.ucar.edu
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5. Lightning
• The connection is made and
the protons rush up to meet
the electrons.
• It is at that point that we see
lightning and hear thunder.
• A bolt of lightning heats the
air along its path causing it to
expand rapidly. Thunder is the
sound caused by rapidly
expanding air.
Pic Courtesy: scied.ucar.edu
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Pic Courtesy:University of Waikato

7. Stepped Leader
• The area of negative charge at the base of the
thundercloud builds up, it induces a region of positive
charge to develop on the ground below.
• A potential difference or voltage is created across the
cloud-to-ground gap.
• Once the voltage reaches a certain strength, the air
between the base of the cloud and the ground develops
an electrical conductivity.
• At first a channel, known as a stepped leader, is formed
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8. Stepped Leader
• It is invisible to the naked eye, this allows electrons to
move from the cloud to the ground.
• It is called a stepped leader because it travels in 50 to 100
m sections, with a slight pause in between, to the ground.
• As it nears the ground, a positively charged streamer fires
upwards from the ground to connect with it.
• Once connected, electrons from the cloud can flow to the
ground and positive charges can flow from the ground to
the cloud.
• It is this flow of charge that is the visible lightning stroke.
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9. Steps in Formation of Lightning
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10. Mechanism of lightning flash
• Pilot streamer and Stepped leader
• Ground streamer and return stroke
• Subsequent strokes
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11. Characteristics of lightning
strokes
• Current-time characteristics
• Time to peak or Rate of rise
• Probability distribution of current and time
• Wave shapes of lightning voltage and current
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12. Lightning current
• Short front time - 10µs
• Tail time – several ms.
• 50% lightning stroke current – greater than 7.5kA/µs.
• 10% lightning strokes current – exceeds 25 kA/µs.
• Stroke current above half value – more than 30µs.
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13. Surge voltage
• Maximum surge voltage in transmission line – 5MV
• Most of the surge voltage is less than 1000 kV on line.
• Front time – 2 to 10 µs
• Tail time – 20 to 100 µs
• Rate of rise of voltage – 1MV/ µs
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14. Thank you
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