Tormentas Electricas - NASA

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Proteccin ContraTormentas Elctricas en

el Centro EspacialKennedy

Pedro J. Medelius, Ph.D.


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Lightning Protection at Pad 39B

Lightning protection is primarily provided by the tall mast on top of the

FSS, the RSS, and the catenary wire running in a North/South direction

to ground points 1000 ft. away on each side of the mast


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Ground Flash Density


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Central Florida Is Lightning Alle

TampaTitusvil


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Lightning Initiation


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Considerations

The Kennedy Space Center is located in a region withsignificant lightning activity. The possibility of a lightningstrike at the surface or aloft is a hazard that must beavoided during launches and during ground activities.

Lightning activity at the Space Shuttle launch pads ismonitored in several ways. Local instrumentation andremote measurements provide an indication of lightningstrike locations and induced voltages or currents into

conductors.


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Lightning Locator

A network of magnetic direction finder antennas is used

to locate ground strikes in the KSC area. Individual

antennas provide an azimuth angle to a particular

lightning strike, and the location is determined by the

intersection of the azimuth lines from different antennas.The accuracy of the system is in the order of a few

hundred meters within the KSC area.


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LDAR

The Lightning Detection andRanging (LDAR) system has shownto be highly useful to the spacecraftlaunch community in reducing thehazards associated with lightningactivity, through a combination of

effective detection and displaytechnologies.

The system was designed to mapthe location of intracloud and cloud-to-ground lightning based on the

time of arrival of theelectromagnetic radiation. Thelocation system consists of 7 VHFradio receivers centered at 66MHz., and displays intracloudlightning activity in real-time.


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LDAR


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Cameras

A lightning strike location method currently in use at the

pads involves the use of a set of video cameras pointing

at different locations within the Pad. If a lightning strike

occurs within the field of view of three or more cameras,

the location of the strike can be determined. However, ifthe cameras are not pointing in the correct direction, or

their field of view is obscured by either the Pad structure

or by a heavy downpour, the determination of the striking

point becomes difficult, and in some cases impossible.There is often an uncertainty as far as the location of the

exact point of contact to the ground.


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SOLLO

The Sonic Lightning Location (SOLLO) system is basedon the use of both electric field and thundermeasurements, and can achieve 5-meter accuracy forlightning strikes within the pad perimeter. The SOLLO

System will enable the users to accurately determine theattachment point of a lightning strike. This will provideimportant information that can be used to determine ifany equipment and/or instrumentation


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L l C l ti Di t ib ti


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Lognormal Cumulative Distribution

Function of First Strokes Peak

Current

1st Stroke Median: 27.7kA, =0.461 [CIGRE, Uman 87]

0 10 20 30 40 50 60 70 80 90 1000

20

40

60

80

100

Current, kA

Percent

Lognormal Cumulative Distribution Function P(I > I0)


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Cone of Protection MethodNot applicable for tall structures

An angle of protection close to

39 would be required to

effectively protect the Space

Shuttle.

Although widely used in the past,the cone of protection approach is

not suitable for tall structures. A

tall structure is one whose height

is larger than the striking distance

of the lightning strike.

International Standards and the

NFPA code recommend the use of

the Rolling Sphere method for

determining areas at risk.


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The use of the Cone of Protection approach was an accepted way of

protecting facilities against direct lightning strikes back through the

1970s. Further scientific studies have proven that the cone of

protection is not an effective way to provide protection for tall

structures, although it is still adequate for short structures whose

heights are less than the striking distance. The striking distance is thedistance between the tip of the lightning leader and the object being

struck.

Since the 1990s, the use of the Rolling Sphere method has been

recommended by international and U.S. standards as a preferred

method for designing lightning protection systems for tall and forcomplex structures.


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The IEEE Standard 998-1996 recommends using a

protective angle of 40-45 degrees for heights up to 15

m, 30 degrees for heights between 15-25m, and less

than 20 degrees for heights up to 50 m.

It is important to realize that since the height of the

Space Shuttle and supporting structures (FSS, RSS,

lightning mast, catenary wires, etc.) is larger than 50

m, an alternate method such as the Rolling Spheremust be used to determine the regions where

lightning protection is lacking.


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Lightning Strike Frequency

This section describes two methods that can be used to estimate the

lightning strike frequency to a structure.

The calculations are based on the Equivalent Collection Method

described in the International Standard IEC 61024-1; and onErikssons equation, widely used for distribution and transmission

lines.


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Equivalent Collection AreaInternational Standard IEC 61024-1


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Lightning Strike FrequencyUsing the Equivalent Collection Area

H=347ft=105.8m

3H

Equivalent collection area Ae (as per IEC 61024-1):

222 316.03 kmHrAe

Lightning strike frequency Nd (as per IEC 61024-1):

116.31316.0102

21C

year

flashesCkm

yearkm

flashesCANN egd

Where Ng is the average flash density in the region where thestructure is located and C

1is the environmental coefficient

(equal to 1 for unshielded structures).


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Lightning Strike FrequencyUsing Erikssons Equation

Using the flash collection rate formula used for distributionand transmission lines (Erikssons equation):

yearkm

flashesbhNN g

10010

286.0

Where h is the height of the pole, b is the width of the line,and Ng is the flash ground density (flashes/km2/year).

Converting the the pad catenary system into an equivalenttransmission line (See figure on following page), and usingthe equivalent height and length we obtain:

year

f lashes

yearkm

flashesHN 85.1

10085.302

10

02/2810

6.0


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Lightning Strike FrequencyUsing Erikssons Equation

H

H/2

Equivalent Line

2000ft

Equivalent power line of the pad lightning protection system


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Rolling Sphere Method

The Rolling Sphere method is widely accepted by the scientific community,

and its use is recommended by International and National Standards:

NFPA 780

IEC 61024

IEEE Std 998-1996

Additional endorsement for the Rolling Sphere method will be included in

the IEC 62305 standard, soon to be released. IEC standards are based on

scientifically proven theories and technical experimentation worldwide

taking into account the international expertise in the matter.


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Rolling Sphere MethodUman and Rakov


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Shaded area represents the protected region


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Shaded area represents the protected region


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The most accepted equation that relates dandI is:

d= 10 xI0.65 (1) [Golde, 1977; IEEE Std 998-1996]:

Where:

d = striking distance in meters

I = peak lightning current in kA



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Rolling Sphere MethodStep length vs. peak lightning current (using

Eq. 1)Estimated Step Length (meters)

0.00

50.00

100.00

150.00

200.00

250.00

300.00

350.00

0 50 100 150 200 250

Peak lightning current (kA)

Step

length


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Rolling Sphere MethodNFPA 780

NFPA 780 code recommends using a sphere with a 46 m radius.

Where the sphere is tangent to earth and resting against a strike termination

device, all space in the vertical plane between the two points of contact and

under the sphere shall be considered to be in the zone of protection


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Rolling Sphere MethodInternational Standard IEC 61024-1


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Monte Carlo Analysis


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References

Eriksson, A. J.: A Discussion on Lightning and Tall Structures. CSIR Special Report

ELEK 152, National Electrical Engineering Research Institute, Pretoria, South

Africa, July, 1978. See also, Lightning and Tall Structures. Trans. South Afr. Inst.

Electr. Eng., 69:2-16 (1978).

Golde, R. H.: The Lightning Conductor.InLightning, Vol. II, LightningProtection (R. H. Golde, ed.), pp. 545-576. Academic Press, New York, 1977.

Uman, M.A. and V.A. Rakov, A Critical Review of Nonconventional Approach to

Lightning Protection,Bulletin American Meteorological Society,pp 1809-1820,

December 2002.

Uman, M.A., The Lightning Discharge, Academic Press, Inc., 1987.

Working Group 01 (Lightning) of Study Committee 33 (Overvoltages and Insulation

Co-ordination), Guide to Procedures for Estimating the Lightning Performance of

Transmission Lines, CIGRE Brochure#63, Oct. 1991, Paris.


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Contact Information

Pedro J. Medelius

M/S ASRC-19

Kennedy Space Center, FL 32899

E-mail: [email protected]

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Tormentas Electricas - NASA