The Physics of Lightning Michael F. Stringfellow 2006 PQIG Workshop

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

The Thundercloud 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 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 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

Connecting Leaders 2006 PQIG Workshop

Return Stroke Large current impulse flows to ground Large electromagnetic pulse radiated Leader charge neutralized 2006 PQIG Workshop

VHF Radio Picture - First Stroke 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

VHF Radio Picture Subsequent Stroke 2006 PQIG Workshop

Video Stills of Multi-Stroke Flash 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

Multiple Ground Channels 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 2006 PQIG Workshop

Electricity Production, Transmission & Distribution TIE-LINE 2 TRANSMISSION DISTRIBUTION PRODUCTION MEDIUM VOLTAGE EXTRA HIGH 345-765 kV HIGH 115-230kV LOW 120-600V 24-69kV PLANTS POWER INTERCONNECTING SUBSTATIONS SMALL INDUSTRIAL COMMERCIAL RESIDENTIAL INDUSTRIAL USER HEAVY 1 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 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 Effectiveness 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 2006 PQIG Workshop

Induced Voltage Flashover 2006 PQIG Workshop

Lightning Transients on AC Power System 2006 PQIG Workshop

Lightning Transients on AC Power System 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

Lightning Incident Investigation 2006 PQIG Workshop

US Flash Density 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

Phoenix Lightning Ground Flash Density 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 2006 PQIG Workshop