with Implications for Air Terminals

Transcription

1 Recent Research on Lightning, with Implications for Air Terminals William Rison Professor of Electrical Engineering New Mexico Institute of Mining and Technology Socorro, New Mexico

2 Recent Lightning Research with Implications for Air Terminals Computer Modeling of the Lightning Attachment Process High Speed Videos of the Lightning Attachment Process

3 Video by Tom Warner, June 17, 2007, near Devil s Tower, Wyoming High Speed Video of Negative Cloud to Ground Discharge 7,100 Frames per Second

4 Video by Tom Warner, June 17, 2007, near Devil s Tower, Wyoming

5 Video by Tom Warner, August 4, 2007, near Rapid City, South Dakota High Speed Video of Attachment Process for a Negative Cloud to Ground Discharge 7,100 Frames per Second 500 foot tall tower

6 History of Attemptt to Pass a Standard for ESE Air Terminals 1980 s --- Laboratory experiments indicate 100 µs time advantage for ESE air terminals Early 1990 s --- Push by ESE manufacturers for adoption of an ESE stan ndard 1990 s --- Prolonged procedural and legal fight over proposed NFPA standard Early 2000 s --- NFPA decides to reject ESE standard and retain NFPA through Federal court rules against ESE manufacturers

7 History of Attempt to Pass Standard for ESE Air Terminals ESE based on three assumptions 1) Laboratory time advantage of about 100 µs will also present in real lightningg 2) Initial early streamer will develop into successful upward leader 3) Upward leader speed is about 1 x 10 6 m/s If these three assumptions are true, they will give ESE terminals a 100 meter capture radius No input from scientificc community on development of standardd

8 History of Attempt to Pass Standard for ESE Air Terminals No scientific consensus on ESE assumptions at when the standard was proposed

9 Validity of ESEE Assumptions 1) Laboratory time advantage of 100 µs also present in real lightning No scientific research indicating that this is the case Golde (1941), The Validity of Lightning Tests with Scale Models there is a question regarding the validity of any laboratory test designed to simulate the physics of the lightning stepped leader and the attachment process, since typical leader step lengths, tens of meters, exceed the size of the laboratory experiment (Rakov & Uman, 2003) Becerra and Cooray (2008), Laboratory Experiments Cannot be Utilized to Justify the Action of Early Streamer Emission Terminals

10 Validity of ESEE Assumptions 2) Initial early streamer will develop into successful upward leader No research at time standard proposed Moore et al (2000) M Measurements of Lightning Rod Responses to Nearby Strikes shows early streamers ineffective

11 Validity of ESE Assumptions Early streamers do not develop into leaders

12 Validity of ESEE Assumptions 3) Upward leader speed is about 1 x 10 6 m/s Not many measurements of upward leader speeds at time standard proposed Now, several measurem ments of upward leaders show speed of about 1 x 10 5 m/s Tom Warner tower strike video: average speed of 1.2 x 10 5 m/s Speed of 1 x 10 5 m/s gives only 10 meter capture radius

13 Validity of ESEE Assumptions Research since the ESEE standard was proposed shows all three ESE assumptions are incorrect There have been no independent field studies showing ESE terminal ls are effective There is now scientificc consensus that ESE terminals are ineffective It was premature to adopt a standard for ESE terminals in early 1990 s

14 CVM History Idea initially proposed by Eriksson (1979) Theory was further developed by D Alessandro and Gumley(2000) and D Alessandro (2002) Tested by D Alessandro in Kuala Lumpur and Hong Kong (Petrov & D Alessa andro, 2001, D Alessandro and Petrov, 2006)

15 CVM Assumptions (D Alessandro and Gumley, 2000) 1) Electric field intensification by structures affects formation of upward leader 2) Critical radius of 38 cm from laboratory measurements a) Any sharp geometrical fe eatures with a radius of less that of the critical value must be rounded off to this value b) [T]he criterion for the initiation of corona and consequently a stable upward leader is the attainment of 3.1 MV/m at the critical radius 3) Ratio of velocity of downward leader to upward leader is about 1.2

16 CVM Assumptions (D Alessandro and Gumley, 2000) 1) Electric field intensification by structures affects formation of upward leader This is correct, from basic freshman physics

17 CVM Assumptions (D Alessandro and Gumley, 2000) 2) Critical radius of 38 cm from laboratory measurements a) Any sharp geometrical features with a radius of less that the critical value must be rounded off to this value Moore et al (2000) showe ed air terminals with radius of much less than a few centimeters behave quite differently from those with radius of a few centimeters

18 CVM Assumptions (D Alessandro and Gumley, 2000) 2) Critical radius of 38 cm from laboratory measurements b) [T]he criterion for the initiation of corona and consequently a stable upward leader is the attainment of 3.1 MV/m at the critical radius It is necessary for the field between the critical radius and the downward leader to be strong enough to sustain propagation No field measurements to confirm this Becerra & Cooray (2007) modeled development of upward leader and found this not to be the case: It is shown that the collection volume concept overestimates the lightning protection areas of air terminals placed on complex structures

19 CVM Assumptions (D Alessandro and Gumley, 2000) 3) Ratio of velocity of downward leader to upward leader about 1.2:1 During initial upward leader, velocity ratio is about 10 to 1 Overall, velocity ratio is about 3 to 1 D Alessandro: [A] higher velocity ratio yields a smaller attractive radius

20 CVM Assumptions (D Alessandro and Gumley, 2000) 3) Ratio of velocity of downward leader to upward leader is about 1.2 During critical initial leader development (first 100 feet of upward leader) velocity ratio is about 10:1 This is when other objects on the structure are competing with the air terminal D Alessandro & Gumley (2001) In fact, if one attempts to use a ratio of 4 [or higher], Eq. (9) does not have a real solution.

21 Field Validation of CVM (Petrov & D Alessandro, 2000) (D Alessandro and Petrov, 2006) Studies in Hong Kong and Kuala Lumpur, Malaysia Buildings protected by ER RICO Dynaspheres placed according to CVM calculations Claim to show protection levels in the 85-98% range 2 to 15% of low intensity flashed under 10 ka could bypass lightning protection system

22 Field Validation of CVM (Petrov & D Alessandro, 2000) (D Alessandro and Petrov, 2006) Study in Kuala Lumpur Data in study has been disp puted by Hartono & Robiah of Lightning Research Pte. Ltd. of Kuala Lumpur This review shows that the field data given in the above study is invalid and should not be used to validate the CVM method of air terminal placement. No independent verification of CVM method

23 Field Validation of CVM (Petrov & D Alessandro, 2000) (D Alessandro and Petrov, 2006) Two problems with methodology of field study: 1) Two different techniques were tested with one study, with no way to differentiate between the two 2) There was no control

24 Field Validation of CVM (Petrov & D Alessandro, 2000) (D Alessandro and Petrov, 2006) 1) Two different techniques tested with one study, with no way to differentiate between the two Study used ERICO Dynaspheres placed according to CVM technique If there was an effect, was it due to the Dynasphere or to the CVM?

25 Field Validation of CVM (Petrov & D Alessandro, 2000) (D Alessandro and Petrov, 2006)

26 Field Validation of CVM (Petrov & D Alessandro, 2000) (D Alessandro and Petrov, 2006) 2) There was no control Only CVM placement wa as tested. Would the results have been different with another placement method?

27 Field Validation of CVM (Petrov & D Alessandro, 2000) (D Alessandro and Petrov, 2006) Data from field study have been disputed. If there were an effect, there is no way to determine if the effect was due to CVM or Dynasphere No control No independent verification of results

28 Critique of CVM Lightning research does not support most of the assumptions of CVM No field study confirming leader will develop when field at critical radius reaches 3.1 MV/m Contrary to laboratory studies, field studies show radius does matter when smaller than critical radius Recent high-speed video is in conflict with the assumption that leader velocity ratios are about 1:1 Problems with field verification need to be resolved Having only two field tests, with same methodology, by proponents of CVM, is not sufficient to meet scientific consensus

29 Critique of CVM There is no consensus in the scientific community that CVM is valid IEEE standards should be based on principles accepted by the scientific community It is premature to adopt CVM into IEEE standard