2006 PQIG Workshop The Physics of Lightning Michael F. Stringfellow
2006 PQIG Workshop The Physics of Lightning Michael F. Stringfellow.
Dec 23, 2015
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2006 PQIG Workshop
The Physics of Lightning
Michael F. Stringfellow
2006 PQIG Workshop
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
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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2006 PQIG Workshop
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
Breakdown zone
--
- -- -
- -
--
2006 PQIG Workshop
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
Return Stroke
•Large current impulse flows to ground•Large electromagnetic pulse radiated•Leader charge neutralized
2006 PQIG Workshop
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
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
2006 PQIG Workshop
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
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
0 50 100 150 200 250 300
Microseconds
0
20
40
60
80
100
120
Kiloamperes
Return Stroke Current
2006 PQIG Workshop
TIE-LINE
2
TRANSMISSION DISTRIBUTIONPRODUCTION
MEDIUMVOLTAGE
EXTRA HIGHVOLTAGE345-765 kV
HIGHVOLTAGE115-230kV
LOWVOLTAGE120-600V
MEDIUMVOLTAGE
24-69kV
PLANTS
POWER INTERCONNECTING
SUBSTATIONS
DISTRIBUTION
SUBSTATIONS
TRANSMISSION
SUBSTATIONS TRANSMISSION
SUBSTATIONS
SMALL INDUSTRIAL
COMMERCIAL
RESIDENTIAL
MEDIUM
INDUSTRIAL
USER
HEAVY
INDUSTRIAL
USER
1
Electricity Production, Transmission & Distribution
2006 PQIG Workshop
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
Striking Distance
•Major influences– Height of structure– Charge on lightning
leader– Slenderness of
structure– Random effects
Striking Distance
Capture Radius "Rc"
Charge "Q"Current "I"
Height "H"
--
----
-
2006 PQIG Workshop
Striking Distance
•Can be inferred from photographs
– Point of last downward branch
– Upward connecting leader path
– Apparent junction
2006 PQIG Workshop
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
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
Shielding Failure
•Likely low current strokes
– Less leader charge
– Smaller striking distance
– Flashover less probable
2006 PQIG Workshop
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
Voltages from Indirect Lightning Strikes
1
10
100
1000
10 100 1000
Induced Voltage kV
Nu
mb
er
pe
r Y
ea
r
2006 PQIG Workshop
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
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
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
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
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
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