Lightning Protection: History and Modern Approaches

Transcription

1 86 th AMS Annual Meeting 2 nd Conference on Meteorological Applications of Lightning Atlanta, Georgia, January 29 February 2, 2006 Lightning Protection: History and Modern Approaches Vladimir A. Rakov Department of Electrical and Computer Engineering University of Florida, Gainesville

2 Lightning Protection: History and Modern Approaches 1. Franklin rod system 2. Faraday cage approach 3. Placement of air terminals 4. Behavior of grounding systems under direct lightning strike conditions 5. Bonding Requirements 6. Non-conventional approaches to lightning protection 2 2

3 1. Franklin rod system (first described in 1753) Air terminal Down conductor Ground terminal Lightning protection system for houses proposed (most likely by G. Ch. Lichtenberg) in Adapted from Wiesinger and Zischank (1995). 3

4 1. Franklin rod system Air terminal locations (UL 96A, Fig. 6.2, 1998) Air terminal Down conductor Ground terminal A= 20 feet (6 m) maximum spacing for 10 inch (254 mm) air terminal height or 25 feet (7.6 m) maximum spacing for 24 inch (610 mm) air terminal height. B= 2 feet (610 mm) maximum spacing from corner, roof edge or ridge end. Metallic roofs whose thickness is 4.8 mm (3/16 in.) or greater do not require air terminals (NFPA 780). 4

5 2. Faraday cage approach In 1876, James Clerk Maxwell proposed that a gunpowder building be completely enclosed with metal of sufficient thickness, forming what is now referred to as a Faraday cage. Zone 1 If lightning were to strike a metalenclosed building, the current would be constrained to the exterior of the metal enclosure, and it would not even be necessary to ground this enclosure. In the later case the lightning would merely produce an arc from the enclosure to earth Zone 2 Zone The Faraday cage effect is provided by all-metal cars and airplanes. Modern steel-frame buildings with reinforcing metal bars in the concrete foundation connected to the building steel provide a good approximation to a Faraday cage. Zone 0 Zone 1 SPD = Surge Protective Device The general principles of topological shielding. Adapted from Vance (1980) 5

6 2. Faraday cage approach Lightning strike to a car with a live rabbit inside. Courtesy of S. Sumi. 6

7 2. Faraday cage approach Video frame of a lightning strike to an aircraft on takeoff from the Kamatsu Air Force Base, Japan, during winter. Courtesy of Z. I. Kawasaki. 7

8 2. Faraday cage approach Cape Canaveral Air Force Station, Launch Pads 40/41. Courtesy of R. Kithil. 8

9 2. Faraday cage approach Cape Canaveral Air Force Station, Launch Pad 41. Courtesy of R. Kithil. 9

10 3. Placement of air terminals Electrogeometrical model (EGM) N g =const Capture surfaces r s r s r s Illustration of capture surfaces of two towers and earth s surface in the electrogeometrical model (EGM). r s is the striking distance defined as the distance from the tip of the descending leader to the object to be struck at the instant when an upward connecting leader is initiated from this object. Vertical arrows represent descending leaders, assumed to be uniformly distributed (N g =const) above the capture surfaces. Adapted from Bazelyan and Raizer (2000). 10

11 3. Placement of air terminals 4 3 { r s = 10 I 0.65, m where I is in ka 1 2 I, ka r s, m Striking distance, r s, versus return-stroke peak current, I [curve 1, Golde (1945); curve 2, Wagner (1963); curve 3, Love (1973); curve 4, Ruhling (1972); x, theory of Davis (1962);, estimates from twodimensional photographs by Eriksson (1978);, estimates from three-dimensional photography by Eriksson (1978). Adapted from Golde (1977) and Eriksson (1978). 11

12 Q 3. Placement of air terminals Finding r s = f(i) 10 2 Assume leader geometry, total leader charge Q, and distribution of this charge along the channel. Assume critical average electric field between the leader tip and the strike object at the time of initiation of upward connecting leader from the object ( kv/m) I = 10.6 Q 0.7 For Q = 5 C I = 33 ka I peak/ Q impulse neg. first strokes n=89 Finding r s = f(q) Use an empirical relation between Q and I to find r s = f(i) I Scatter plot of impulse charge, Q, versus return-stroke peak current, I. Note that both vertical and horizontal scales are logarithmic. The best fit to data, I = 10.6 Q 0.7, where Q is in coulombs and I is in kiloamperes, was used in deriving r s = 10 I0.65 Adapted from Berger (1972). 12

13 3. Placement of air terminals Rolling-Sphere Method r s = 46 m (150 ft) (NFPA 780, 2004), corresponds to I = 10.1 ka (95% of currents exceed this value) r s r s r s Illustration of the rolling-sphere method (RSM). The shaded area is that area into which, it is postulated, lightning cannot enter. Adapted from Szczerbinski (2000). 13

14 4. Behavior of grounding systems under direct lightning strike conditions Photograph of surface arcing associated with the second stroke (current peak I=30 ka) of flash 9312 triggered at Fort McClellan, Alabama (R g =260 Ω). The lightning channel is outside the field of view. One of the surface arcs approached the right edge of the photograph, a distance of 10 m from the rocket launcher. Adapted from Fisher et al. (1994). V= I x R g =7.8 MV. 14

15 5. Bonding Requirements D soil = I Z g / E b LPS air terminal If I = 60 ka, Z g = 25 Ω and E b = 300 kv/m, D soil = 5 m D air = 0.12Z g +0.1L D air LPS down conductor Wooden pole L Protected Object I = the lightning peak current Z g = the grounding impedance E b = the breakdown electric field in the soil LPS grounding system D soil Buried Services Bond whenever you cannot adequately isolate Illustration of safety distances in the soil (D soil ) and in air (D air ). Adapted from Kuzhekin et al. (2003). 15

16 5. Bonding Requirements 1 P I = 1 + (I /31) 2.6 (1) (2) D soil = IZ g /E b, m The IEEE peak current distribution given by equation (1) and corresponding values of D soil given by equation (2). Peak current, I, ka Percentage exceeding tabulated value, Ρ I 100% D soil, m (Z g = 25 Ω, E b = 300 kv/m)

17 5. Bonding Requirements structure clamp gap arrester information line pipe power line Bonding of external services near ground level. Taken from Wiesinger and Zischank (1995). 17

18 6. Non-conventional approaches to lightning protection Early Streamer Emission (ESE) systems Lightning struck point B (without air terminal), which was within the claimed protection radius of 30 m of the ESE air terminal at point A. The distance between A and B is 18 m. After the strike the manufacturer installed an additional ESE Terminal at point B. Taken from Chrzan and Hartono (2003). 18

19 6. Non-conventional approaches to lightning protection Lightning elimination systems Evolution of lightning elimination (dissipation) system claims: Corona current can discharge the thundercloud. Corona charge can neutralize an approaching lightning leader. Taken from the European Power News, February 2005 Corona charge can suppress or delay the formation of an upward connecting leader from the protected object. However, Multipoint corona systems (dissipation arrays) provide only local lightning protection. They reduce the number of lightning strikes to their own surface and the object components directly covered by them. The question of extending the protection area of such systems still remains open (Aleksandrov et al., 2005). 19

20 Thank You Lightning strike to the Washington Monument (169 m high) on July 1,

21 1. Franklin rod system 21

22 22

23 23 Surface arcing