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Lightning Discharge

Lightning Discharge. 1 Introduction 2 Physical Phenomena 3 Effects and Typical Parameters 4 Lightning protection 5 Conclusions. Lightning Discharges. 1 Introduction 2 Physical Phenomena 3 Effects and Typical Parameters 4 Lightning protection 5 Conclusions. since at ancient times

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Lightning Discharge

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  1. Lightning Discharge 1 Introduction 2 Physical Phenomena 3 Effects and Typical Parameters 4 Lightning protection 5 Conclusions

  2. Lightning Discharges 1 Introduction 2 Physical Phenomena 3 Effects and Typical Parameters 4 Lightning protection 5 Conclusions

  3. since at ancient times the lightning phenomena have impressed the mankind ** often explained by ** mystical reasions

  4. from the Bible´s history up to the weapon of Thor … Moses at the announcement of the god´s law on Sinai

  5. … and to Benjamin Franklin (1752) and his experiments, in which were demonstrated the electrical nature of lightning

  6. … such dangerous experiments were stopped after the tragical death of Prof. Richmann (1753) …  serious explanations of lightning phenomena exist until the 19th/ 20th century

  7. Lightning Discharges 1 Introduction 2 Physical Phenomena 3 Effects and Typical Parameters 4 Lightning protection 5 Conclusions

  8. Atmospheric effects • fine- weather- field ( ) • global current (air ions, ~ 2pA/m2) • lightnings lead mostly to a negative charge on earth • global exchange of charge by conductivity of soil and middle atmosphere („charge generator“)

  9. Statistics of lightning (northern hemisphere) during a year during a day

  10. Lightning- distribution along Europe lightnings per year (a) and area (km2) • 1-7 / (a⋅km2) • north- south- gradient • Maxima on alps, Italy • and Adria lightning- location- systems (BLIDS, EUCLID, …)

  11. Lightning- distribution along Europe www.euclid.org actually at 21th april, 11h

  12. Global distribution • (NASA Satellite 1996-2000) • 50 flashes per second (in 1000 aktive thunderstorms) • Maximum about tropical areas (shore)

  13. Generation of lightning separation of charges into a cloud polarization friction gravitation moisture in air & (vertical) wind --> intensive soil heating by sun --> cold weather front under warm air- sheet

  14. Electrical structure of a thunderstorm- cloud • positive charge in upper part (T < -25°C) • negative charge in lower part (T > -25°C) • (small) positive charge near the rain area • influenced (positive) charge on the earth + + +++ + + + +

  15. for direct breakdown between cloud and earth an (average) value of E = 3000 kV/m would be necessary ---> impossible ionization processes into the cloud • increased values of E on the top of space charge channel lead to ionisation processes

  16. Generation of lightning (neg. CG) • charge channel moves in direction to the ground •  ~ 10 m, high ionized core  ~ 1 cm -> LEADER • velocity ~ 3· 105 m/s, stepwise 10 .. 60 m, ~ 10 s --> „stepped“ leader

  17. Generation of lightning (neg. CG) the most important part for the target point is the last step (before ground) final striking distance

  18. Generation of lightning (neg. CG) This behaviour can be used for evaluation of target point (rolling sphere model)

  19. Types of lightning Intra- Cloud(s) (IC) Cloud- Ground (CG) • IC/ CG = 7/ 3 (50th latitude) • positive/ negative polarity Classifikation of CG- strokes is determined by the direction of „stepped“ leader Ground- Cloud (5%) Cloud- Ground (95%)

  20. Types of lightning negative/ positive CG- stroke (positive CG 5 .. 10%) negative/ positive GC- stroke

  21. Generation of lightning (neg. CG) After the meeting of „catching discharge“ with the stepped leader a main discharge will be initiated imax ~ 10 … 100 kA , T ~ (10 … 30) 103 K (!) W ~ (109 … 1010) J Q ~ (1 … 60) nC  of charge channel ~ 15 cm p ~ 100 bar time duration ~ (10 .. 100) s

  22. possible current types • single current • multiple currents μs

  23. Generation of multiple lightning (flash) stepped leader dart leader main discharge (return stroke) velocity: ~ 108 m/s (c/3)

  24. Lightning Discharges 1 Introduction 2 Physical Phenomena 3 Effects and Typical Parameters 4 Lightning protection 5 Conclusions

  25. Effects of lightning lightning effects can be distinguished into: • indirect effects ---> overvoltages • direct effects ---> electrical, thermal, chemical, biological

  26. Indirect Effects • Overvoltages on overhead lines caused by „liberation“ of influenced charges or as „back- flashover“ from tower to phases • Overvoltages in loops (installation, equipment) by inducing effects of lightning current

  27. Generation of overvoltages on overhead lines 1 2 3 1 - influenced charged on overhead lines (o.l.) at thunderstorm 2 - due to admittance of lines remains positive charge only on o.l. 3 - occuring a stroke the charge is not further fixed --> travelling overvoltage wave along the line

  28. lightning overvoltages can damage electrical equipment for power transmission which connected on overhead lines e.g power transformers, switch- gears, … --> all equipments for power transmission must be tested before going in operation (test against „atmospheric overvoltages“)

  29. Lightning Impulse Testing (LI) Test parameter: --> front time - 1.2 (+ - 30%) s LI - 1.2/ 50 s --> half-to-value time - 50 (+ - 20%) s

  30. Parameter for LI- testing testing voltage (1.2/50) s/ phase- ground- insulation

  31. Direct Effects • Lightning current effects by • current flows through the object • voltage drop on the ground • thermal action into the object (overheating, radiation, melting, explosion)

  32. Damages in nature (trees) exploded, broken by vaporized water typical spiral- structure

  33. Damages in airplanes Space-shuttle stroke in radar hole by melting

  34. Damages in human beings or animals • Damage by direct stroke, over-step or induced step- voltage • burning, overheating • bio- electrical disturbance • heart- interruption

  35. Typical parameters of lightning • maximal current imax  25 ... 100 kA • time- gradient di/dt  10 ... 200 kA/s • charge  idt  3 ... 100 As • specific energy  i2dt  2.5 ... 10 (kA)2s

  36. Effects by maximal current U = imax·RE

  37. Effects by current gradient U = M · Δi/ Δt

  38. Effects by charge (of lightning) Q = ∫ i dt W = Q · UA,K

  39. Effects by specific energy (of lightning) W = R · ∫ i2 dt W/ R = ∫ i2 dt

  40. temperature rising ΔT (in K)

  41. Lightning Discharges 1 Introduction 2 Physical Phenomena 3 Effects and Typical Parameters 4 Lightning protection 5 Conclusions

  42. Protection against lightning overvoltages Insulation design for withstanding to LI (electrical strength of insulation > LI- strength) Limiting of overvoltages by • lightning arresters (Ventil-, MOA- type) • Surge Protection Devices (SPD)

  43. Protection against lightning current Ligthning protection by • ligthning conductor • screening („Faraday Cage“)

  44. historical overview 1752 B.Franklin 1754 P.Divis

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