Lightning -definition,, causes,effects in the power system

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.
2

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
3

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
4

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
5

6. 6
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
7

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.
8

9. Steps in Formation of Lightning
9

10. Mechanism of lightning flash
• Pilot streamer and Stepped leader
• Ground streamer and return stroke
• Subsequent strokes
10

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
11

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.
12

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
13

14. Thank you
14