Transcript The Physics of Lightning

The Physics of Lightning
Michael F. Stringfellow
2006 PQIG Workshop
Introduction
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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
The Thundercloud
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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
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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 > 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 >4
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 lognormal distribution
with 30 kA to 40 kA
median
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Retu r
K ilo a mp
1 2 0
1 0 0
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2 0
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M ic r o
Electricity Production, Transmission
& Distribution
PRODUCTION
MEDIUM
VOLTAGE
TRANSMISSION
EXTRA HIGH
VOLTAGE
345-765 kV
HIGH
VOLTAGE
115-230kV
DISTRIBUTION
MEDIUM
VOLTAGE
24-69kV
LOW
VOLTAGE
120-600V
1
TIE-LINE
2
HEAVY
INDUSTRIAL
USER
MEDIUM
INDUSTRIAL
USER
SUBSTATIONS
TRANSMISSION
SMALL INDUSTRIAL
TRANSMISSION
SUBSTATIONS
COMMERCIAL
RESIDENTIAL
POWER
INTERCONNECTING
DISTRIBUTION
PLANTS
SUBSTATIONS
SUBSTATIONS
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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
Number per Year
1000
100
10
1
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Induced Voltage kV
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1000
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 < 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 multichannel flashes
• Lightning location systems have many benefits
– Real-time tracking
– Archival flash density
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