1. The Physics of Lightning
Michael F. Stringfellow
2006 PQIG Workshop

2. The Physics of Lightning:
Introduction
The Physics of Lightning:
How lightning originates
Leader propagation
Strike mechanism
The return stroke
Subsequent strokes
Channel multiplicity
Lightning flash density
Lightning interaction with overhead power lines
2006 PQIG Workshop

3. The Thundercloud
2006 PQIG Workshop

4. How Lightning Starts Lightning starts in cloud
Around 0°C - that’s typically 15,000 ft above ground
Breakdown starts in high-field region
Branching discharge moves up and down
2006 PQIG Workshop

5. Leader Propagation
Ground flashes almost always start with downward (usually) stepped leader from high charge region
Steps m long
Pauses between steps
Lowers charge to earth
Negative in > 95% of ground flashes
2006 PQIG Workshop

6. Connecting Leaders
Launched upward by electric field of stepped leader as it approaches earth
Occur at many locations near descending flash
Most are unsuccessful
One or more connect with downward leader to provide final channel to earth
Not often seen, but frequently heard
2006 PQIG Workshop

7. Connecting Leaders
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8. Return Stroke Large current impulse flows to ground
Large electromagnetic pulse radiated
Leader charge neutralized
2006 PQIG Workshop

9. VHF Radio Picture - First Stroke
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10. Subsequent Strokes "Dart" leaders launched from cloud
Follow path of first return stroke
Tap new cloud charges
Cause subsequent return strokes
Often depart from old path
2006 PQIG Workshop

11. VHF Radio Picture Subsequent Stroke
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12. Video Stills of Multi-Stroke Flash
2006 PQIG Workshop

13. Multiple Stroke Flashes
Typically 2-4 strokes per flash
Stroke intervals milliseconds
Reach ground at 1 to 5 points
Severe flashes have >4 strokes
Continuing currents likely
2006 PQIG Workshop

14. Multiple Ground Channels
Multiple ground channels are common
Root branching
Simultaneous leader branches
Successive strokes may depart from "main" channel
Three major channels for every two flashes
2006 PQIG Workshop

15. Multiple Ground Channels
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16. Currents & Voltages Cloud charging current a few amps
Cloud voltages 50 MV to 500 MV
Leader currents 10 A to 1000 A
Return stroke currents 5kA to 500 kA
Approximately log-normal distribution with 30 kA to 40 kA median
2006 PQIG Workshop

17. Electricity Production, Transmission & Distribution
TIE-LINE 2
TRANSMISSION
DISTRIBUTION
PRODUCTION
MEDIUM
VOLTAGE
EXTRA HIGH kV
HIGH kV
LOW V
24-69kV
PLANTS
POWER
INTERCONNECTING
SUBSTATIONS
SMALL INDUSTRIAL
COMMERCIAL
RESIDENTIAL
INDUSTRIAL
USER
HEAVY 1
2006 PQIG Workshop

18. Lightning and Overhead Lines
Direct strikes affect all voltage systems
Problems decrease with insulation level
Flashover when lightning strikes phase conductor
Also back flashover when tower or shield wire struck
Indirect strikes affect distribution and sub-transmission systems
Induced voltages up to 300 kV
2006 PQIG Workshop

19. Striking Distance Major influences Height of structure
Charge on lightning leader
Slenderness of structure
Random effects
2006 PQIG Workshop

20. Striking Distance Can be inferred from photographs
Point of last downward branch
Upward connecting leader path
Apparent junction
2006 PQIG Workshop

21. Voltages from Direct Strikes to Overhead Lines
Stroke to conductor
Conductor has surge impedance of about 400 ohms
Average return stroke current 30 kA
Conductor voltage = 400 x 15,000 V = 6 MV
Stroke to tower
Tower has footing resistance of 30 ohms
Tower voltage = 30 x 30,000 V = 900 kV
Shielding and grounding provide effective protection
Especially for higher voltage systems
2006 PQIG Workshop

22. Transmission Lines & Lightning
Characteristics
Shielded construction
High insulation levels
Good tower grounding
Effective protection
Well coordinated fast switchgear
Result
Excellent lightning performance
Permanent damage rare
Few flashovers quickly cleared by protection
2006 PQIG Workshop

23. Shielding Effectiveness
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24. Shielding Failure Likely low current strokes Less leader charge
Smaller striking distance
Flashover less probable
2006 PQIG Workshop

25. Distribution Lines & Lightning
Characteristics
Unshielded construction
Low insulation levels
Poor pole grounding
Less effective protection
Slower switchgear, autoreclosers and fuses
Result
Poor lightning performance
Permanent damage common
Many flashovers cleared
Some may take several shots
Nuisance fuse blowing
Many sags and short-duration outages
2006 PQIG Workshop

26. Voltages from Indirect Lightning Strikes
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27. Induced Voltage Flashover
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28. Lightning Transients on AC Power System
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29. Lightning Transients on AC Power System
2006 PQIG Workshop

30. Some Power System Lightning Problems
Multi-stroke flashes can stress switchgear
Transients occur when open
Multi-channel flashes can defeat system protection
Simultaneous faults occur on different parts of circuit
Frequent strikes in severe storm can overwhelm protection
“Weak-link” structures will flash over frequently
May limit line performance
2006 PQIG Workshop

31. Lightning Tracking Radio location used to locate lightning Real time
Storm warning
Allocation of resources
Archival data
Lightning flash density
Fault investigations
2006 PQIG Workshop

32. Lightning Incident Investigation
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33. US Flash Density
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34. Ground Flash Density Highest in southeast & Gulf coast USA
Tampa bay 60 per square mile per year
Houston 40 per square mile per year
Lower as you move north and west
Washington & Alaska < 0.1 per square mile per year
Phoenix area ~10 per square mile per year
Highly variable from year to year
Lightning “hot spots” or “lightning nests”
2006 PQIG Workshop

35. Lightning Hot Spots Local areas of high lightning incidence
Appear over several years’ recording
Important to ignore short-term random variations
May reflect surface features that steer or promote storms
Mountains & rivers
Cities
Industries
May be useful for line performance improvements
Shielding
Arresters
Enhanced grounding
2006 PQIG Workshop

36. Phoenix Lightning Ground Flash Density
2006 PQIG Workshop

37. Summary Overhead transmission lines are resistant to lightning
Shielded, grounded, high insulation levels
EHV systems are almost immune
Electricity distribution systems are vulnerable
Unshielded, poorly grounded, low insulation levels
Some newly discovered challenges from multi-channel flashes
Lightning location systems have many benefits
Real-time tracking
Archival flash density
2006 PQIG Workshop