The%20Physics%20of%20Lightning

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The Physics of Lightning

• Michael F. Stringfellow

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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

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The Thundercloud
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How Lightning Starts

• Lightning starts in cloud
• Around 0C - thats typically 15,000 ft above
  ground
• Breakdown starts in high-field region
• Branching discharge moves up and down

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Leader Propagation

• Ground flashes almost always start with downward
  (usually) stepped leader from high charge region
• Steps 10-100 m long
• Pauses between steps
• Lowers charge to earth
• Negative in gt 95 of ground flashes

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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

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Connecting Leaders
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Return Stroke

• Large current impulse flows to ground
• Large electromagnetic pulse radiated
• Leader charge neutralized

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VHF Radio Picture - First Stroke
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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

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VHF Radio Picture Subsequent Stroke
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Video Stills of Multi-Stroke Flash
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Multiple Stroke Flashes

• Typically 2-4 strokes per flash
• Stroke intervals 5 -100 milliseconds
• Reach ground at 1 to 5 points
• Severe flashes have gt4 strokes
• Continuing currents likely

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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

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Multiple Ground Channels
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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

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Electricity Production, Transmission
Distribution
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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

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Striking Distance

• Major influences
• Height of structure
• Charge on lightning leader
• Slenderness of structure
• Random effects

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Striking Distance

• Can be inferred from photographs
• Point of last downward branch
• Upward connecting leader path
• Apparent junction

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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

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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

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Shielding Effectiveness
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Shielding Failure

• Likely low current strokes
• Less leader charge
• Smaller striking distance
• Flashover less probable

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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

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Voltages from Indirect Lightning Strikes
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Induced Voltage Flashover
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Lightning Transients on AC Power System
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Lightning Transients on AC Power System
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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

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Lightning Tracking

• Radio location used to locate lightning
• Real time
• Storm warning
• Allocation of resources
• Archival data
• Lightning flash density
• Fault investigations

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Lightning Incident Investigation
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US Flash Density
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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 lt 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

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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

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Phoenix Lightning Ground Flash Density
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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