Why don't you tow a 500 foot tower? That should work.
How many Navy ships tow a lightning distraction bouy?

Bwahahahahah what a fool.

Joe

No Joe, you're the fool:


ROCKET-TRIGGERED LIGHTNING EXPERIMENTS
AT CAMP BLANDING, FLORIDA

Vladimir A. Rakov
University of Florida, Gainesville, USA

1. Introduction

Many aspects of the interaction of lightning with power systems are not yet
well understood and are in need of research that requires the termination of
the lightning channel on or in the immediate vicinity of the power system.
The probability for a natural lightning to strike a given point of interest
on the Earth's surface is very low, even in areas of relatively high
lightning activity. The simulation of lightning in a high-voltage
laboratory has limited application in that it does not allow the proper
testing of large distributed systems such as power lines since the
laboratory current path is different from that of lightning and since the
laboratory discharge does not produce lightning-like electric and magnetic
fields. The most promising tool for studying both the direct and the induced
effects of lightning on power systems is artificially initiated (triggered)
lightning that is stimulated to occur between an overhead thundercloud and a
designated point on the power system or on nearby ground The lightning
triggering techniques and various lightning discharge processes involved are
outlined in Section 2. A relatively new facility for triggered-lightning
experiments is the International Center for Lightning Research and Testing
at Camp Blanding, Florida, located about 40 km north-east of Gainesville and
described in Section 3. The facility was constructed in 1993 by Power
Technologies, Inc. (PTI) under the funding and direction of the Electric
Power Research Institute (EPRI), and has been operated by the University of
Florida (UF) since Fall 1994. In Section 4 we give some examples of the
results of the studies conducted there. Additional information on the
triggered-lightning studies at Camp Blanding can be found in Uman et al.
(1994a,b, 1996a,b, 1997), Rakov et al. (1995a,b, 1996a,b, 1998), Ben Rhouma
et al. (1995), Barker et al. (1996), Fernandez (1997), Fernandez et al.
(1998a,b,c,d), Wang et al. (1999a,b,c,d), Crawford (1998), and Crawford et
al. (1999).

2. Lightning triggering techniques

The most effective technique for triggering lightning involves launching a
small rocket trailing a thin grounded wire toward a charged cloud overhead.
This triggering method is sometimes called "classical" triggering and is
illustrated in Fig. 1. The cloud charge is indirectly sensed by measuring
the electric field at ground, with values of 4 to 10 kV/m generally being
good indicators of favorable conditions for lightning initiation. When the
rocket, ascending at about 200 m/s, is about 200 to 300 m high, the field
enhancement near the rocket tip launches a positively charged (for the
common summer thunderstorm having predominantly negative charge at 5 to 7 km
altitude) leader that propagates upward toward the cloud. This leader
vaporizes the trailing wire and initiates a so-called "initial continuous
current" of the order of several hundred amperes that effectively transports
negative charge from the cloud charge source via the wire trace to the
instrumented triggering facility. There often follows, after the cessation
of the initial continuous current, several downward dart leader/upward
return stroke sequences traversing the same path to the triggering facility.
The dart leaders and following return strokes in triggered lightning are
similar if not identical to dart leader/return stroke sequences in natural
lightning, although the initial processes in natural and classical triggered
lightning are distinctly different. The reproduction of the initial
processes in natural lightning can be accomplished using a triggering wire
not attached to the ground. This ungrounded-wire technique is called
"altitude" triggering and is illustrated in Fig. 2 which shows that a
bi-directional (positive charge up and negative charge down) leader process
is involved in the initiation of the first return stroke. Properties of
altitude triggered lightning are discussed by Laroche et al. (1991), Lalande
et al. (1996, 1998), Uman et al. (1996a), and Rakov et al. (1996b, 1998).

3. The Camp Blanding lightning triggering facility

The Camp Blanding lightning triggering site (see Fig. 3), called the
International Center for Lightning Research and Testing (ICLRT), occupies a
flat, open field with dimension of approximately 1 km by 1 km and since Fall
1994 has been operated under an agreement between the University of Florida
and the Camp Blanding Florida Army National Guard Base. The site includes a
0.8 km test underground power cable, a 0.7 km test overhead power line, four
instrumentation stations, IS1, IS2, IS3, and IS4, located along the
underground cable and containing padmount transformers, a simulated house
fed by one of the transformers, a test runway with operational lighting
system, a number of other test structures, including a lightning protected
shelter, two launch control complexes, a number of rocket launchers, an
office building, and storage facilities. The existing elements of the power
system (overhead line and underground cable), presently unenergized,
can be connected in a variety of configurations. The facility allows the
measurement of the total lightning current injected into the power system's
conductors or to nearby ground and the monitoring of voltages and currents
at various points of the system. Electric and magnetic field measurements,
video recording, and still and high-speed photography are also performed,
making the Center a unique facility for studying simultaneously and
synergistically various aspects of atmospheric electricity, lightning, and
lightning protection. Examples of still photographs of lightning flashes
triggered at Camp Blanding, Florida, are shown in Fig. 4. During summers
1995 through 1998 over 30 scientists and engineers (excluding UF faculty,
students, and staff) from 13 countries representing 4 continents performed
experiments at the Center.


4. Results

The results of triggered-lightning studies provide new insights into the
physics of the lightning discharge and the mechanisms of lightning
interaction with various objects and systems. Some examples are presented
in Sections 4.1 through 4.3.

4.1. Close lightning electric fields

Characterization of the close lightning electromagnetic environment is
needed for the evaluation of lightning induced effects and for the
validation of various models of the lightning discharges.

4.1.1. Electric field waveshapes. Leader/return stroke vertical electric
field waveforms appear as asymmetrical V-shaped pulses, the bottom of the V
being associated with the transition from the leader (the leading edge of
the pulse) to the return stroke (the trailing edge of the pulse), as
described, from earlier Kennedy Space Center (KSC) measurements, by
Rubinstein et al. (1995). Examples of leader/return stroke electric fields
simultaneously measured at 30, 50, and 110 m from the 1993 Camp Blanding
experiment are shown in Figs. 5 and 6. From the 1993 experiment the
geometric mean width of the V at half of peak value is 3.2 ?s at 30 m, 7.3
?s at 50 m, and 13 ?s at 110 m, a distance dependence close to linear. This
waveshape characteristic can be viewed as a measure of the closeness of the
leader electric field rate of change to that of the following return stroke.
As seen in Fig. 5, at 30 m the rate of change of leader electric field near
the bottom of the V can be comparable to that of the return stroke field,
while at 110 m the two rates differ considerably, with the leader rate of
change being appreciably less. This observation, in conjunction with the
fact that within some hundreds of meters the leader and return stroke
electric field changes are about the same in magnitude (Uman et al. 1994a,
Rakov et al. 1998) (see also Figs. 5 and 6), suggests that induced voltages
and currents on power and other systems from very close (a few tens of
meters or less) lightning subsequent strokes can contain an appreciable
component due to the leader.

4.1.2. Leader electric field versus distance. From measurements at 30, 50,
and 110 m at Camp Blanding in 1993 (Uman et al. 1994a, Rakov et al. 1998)
the variation of the leader electric field change with distance was observed
to be somewhat slower than the inverse proportionality theoretically
predicted by using a uniformly-charged leader model by Rubinstein et al.
(1995). The uniformly charged leader model, although clearly a crude
approximation, is supported by experimental data, as explained next.
Thottappillil et al. (1997) showed that the modified transmission line
return stroke model with linear current decay with height (MTLL), developed
using the assumption that there exists a uniform distribution of leader
charge along the channel, predicts a ratio (R) of leader to return stroke
electric field between +0.81 and +0.97 at distances between 20 and 50 km,
assuming a total channel length of 7.5 km (see their Table 2). These values
of R are consistent with the mean value of R = +0.8 determined
experimentally (97 measurements) for this distance range by Beasley et al.
(1982, Fig. 23d). On the other hand, the return stroke model that is
derived assuming that there exists a distribution of leader charge
exponentially decreasing with height (MTLE) predicts values of R between
+2.6 and +3.0 (see Thottappillil et al. 1997, Table 2), while the lightning
model that is derived assuming that there exists a vertically symmetrical
bidirectional leader process (positively charged part propagating upward and
negatively charged downward) predicts values of R approximately between +0.2
and +0.3 (see Mazur and Ruhnke 1993, Fig. 25), in both cases inconsistent
with the experimental data of Beasley et al. (1982). From the 1993 Camp
Blanding experiment, individual leader electric field changes for six
strokes, simultaneously recorded at the three distances, are given in Table
1. Arithmetic mean values of the leader electric field changes for the six
events in Table 1 are 25, 21, and 16 kV/m at 30, 50, and 110 m,
respectively. Using the 50-m value, 21 kV/m, as a reference and assuming an
inverse distance dependence, we estimate values of 35 (versus 25) and 10
(versus 16) kV/m at 30 and 110 m, respectively. A relative insensitivity of
the leader electric field change to distance was also observed from
measurements at 10 and 20 m at Fort McClellan, Alabama (Fisher et al. 1994).
An electric field versus distance dependence that is slower than an inverse
proportionality, observed within 110 m of the channel, is consistent with a
decrease of line charge density with decreasing height near the bottom of
the channel. Such a leader charge distribution near ground might be due to
the incomplete development there of the radially formed corona sheath that
surrounds the channel core and presumably contains most of the leader
charge. Some support for this speculation comes from the observation that
the propagation speeds of radial corona streamers from conductors subjected
to negative high voltage in the laboratory are about 105 m/s (0.1 m/?s)
(Cooray 1993), so some microseconds are required for the development of a
corona sheath with a radius of the order of meters. Since for dart leaders
the downward propagation speeds (107 m/s) are about 2 orders of magnitude
higher than the radial-streamer speeds, the delay in corona-sheath formation
may be appreciable. On the other hand, Depasse (1994) observed, from
triggered-lightning experiments in France, that seven simultaneously
measured vertical electric fields due to return strokes at 50 and 77 m,
expected to be approximately equal in magnitude to the fields due to the
corresponding leaders (Uman et al. 1994a, Rakov et al. 1998) (see also Figs.
5 and 6), exhibited an inverse distance dependence, consistent with a
uniform distribution of charge density along the channel. Further, electric
field measurements at six distances ranging from 10 to 500 m at Camp
Blanding in 1997 suggest that leader field change varies approximately
inversely proportional to distance (Crawford et al. 1999). Additional
multiple-station data and modeling are needed to interpret the observed
variations of leader field change with distance. It is worth noting that,
as shown by Rubinstein et al. (1995) based on a uniformly charged leader
model, the presence of a triggering structure of about 5 m has a very small
effect on the leader field at distances of 30 m and greater. They computed
an error of about 1% at 30 m, with fields at greater distances being even
less sensitive to the presence of the triggering structure.

4.2. Lightning channel termination on ground

In examining the lightning current flowing from the bottom of the channel
into the ground, it is convenient to approximate lightning by a Norton
equivalent circuit (Carlson 1996), i.e., by a current source equal to the
lightning current that would be injected into the ground if that ground were
perfectly conducting (the short-circuit current) in parallel with a
lightning-channel equivalent impedance Zch assumed to be constant. The
lightning grounding impedance Zgr is a load connected in parallel with the
lightning Norton equivalent. Thus the "short-circuit" lightning current Isc
effectively splits between Zgr and Zch so the current measured at the
lightning-channel base is found as Imeas = IscZch/(Zch + Zgr). Both source
characteristics, Isc and Zch, vary from stroke to stroke, and Zch is a
function of channel current, the latter nonlinearity being in violation of
the linearity requirement necessary for obtaining the Norton equivalent
circuit. Nevertheless, if we are concerned only with the peak value of
current and assume that for a large number of strokes the average peak value
of Isc and the average value of Zch at current peak are more or less
constant, the Norton equivalent becomes a useful tool for studying the
relation between lightning current peak and the corresponding values of Zch
and Zgr. For instance, if the measured channel-base current peak statistics
are similar under a variety of grounding conditions, then Zgr must always be
much lower than Zch at the time of the current peak.
Camp Blanding measurements of lightning currents that entered sandy soil
with a relatively poor conductivity of 2.5 x 10-4 S/m without any grounding
electrode resulted in a value of the geometric mean return-stroke peak
current, 13 kA, that is similar to the geometric mean value, 14 kA, from
measurements at KSC made using a launcher of the same geometry which was
much better grounded into salt water with a conductivity of 3-6 S/m via
underwater braided metallic cables. Additionally, a fairly similar
geometric mean value, about 10 kA, of return stroke current peak was found
from KSC measurements using a well-grounded ground-based launcher of
significantly greater height, and fairly similar geometric mean values were
found from the Fort McClellan measurements using a relatively small-height,
poorly grounded launcher (10 kA) and the same launcher well grounded (11
kA). Additionally, Ben Rhouma et al. (1995) give arithmetic mean values of
return stroke current peaks in the range from 15 to 16 kA for the
triggered-lightning experiments at Camp Blanding in 1993 and at KSC in 1987,
1989, and 1991. The geometric mean values of peak current indicated above
along with other pertinent information on the measurements are summarized in
Table 2. The values of grounding resistance (probably the dominant
component of Zgr) given in Table 2 should be understood as the initial
values encountered by lightning before the onset of any breakdown processes
in the soil or along the ground surface. Note from Table 2 that the
grounding resistance varies from 0.1 ? to 64 k?, while Zch was estimated
from the analysis of the current waves traveling along the 540-m high tower
to be in the range from hundreds of ohms to several kiloohms (Gorin et al.
1977; Gorin and Shkilev 1984). The observation that the average return
stroke current is not much influenced by the level of man-made grounding,
ranging from excellent to none, implies that lightning is capable of
lowering its grounding impedance to a value that is always much lower than
the equivalent impedance of the main channel. On the basis of the evidence
of the formation of plasma channels (fulgurites) in the sandy soil at Camp
Blanding (Uman et al. 1994b, 1997) and on optical records showing arcing
along the ground at Camp Blanding and at Fort McClellan, Alabama (see Fig.
7), we infer that surface and underground plasma channels are the principal
means of lowering the lightning grounding impedance, at least for the types
of soil at the lightning triggering sites in Florida and Alabama. Injection
of laboratory currents up to 20 kA into loamy sand in the presence of water
sprays imitating rain resulted in surface arcing that significantly reduced
the grounding resistance at the current peak (M. Darveniza, personal
communication, 1995). The fulgurites found at Camp Blanding usually show
that the in-soil plasma channels develop toward the better conducting layers
of soil or toward buried metallic objects that, when contacted, serve to
further lower the grounding resistance. The percentages of return strokes
producing optically detectable surface arcing versus return stroke peak
current, from the 1993 and 1995 experiments at Fort McClellan, Alabama, are
shown in Fig. 8. The surface arcing appears to be random in direction and
often leaves little if any evidence on the ground. Even within the same
flash, individual strokes can produce arcs developing in different
directions. In one case it was possible to estimate the current carried by
one arc branch which contacted the instrumentation: approximately 1 kA or 5%
of the total current peak in that stroke (Fisher et al. 1994). The observed
horizontal extent of surface arcs was up to 20 m, which was the limit of
photographic coverage during the 1993 Fort McClellan experiment. No
fulgurites were found in the soil (red clay) at Fort McClellan, only
concentrated current exit points at several spots along the 0.3- or 1.3-m
steel earthing rod (see Table 2). It is likely that uniform ionization of
soil, usually postulated in studies of the behavior of grounding electrodes
subjected to lightning surges, is not a valid assumption, at least in the
southeastern United States, where distinct plasma channels in the soil and
on the ground surface appear to be the principal means of lowering the
grounding resistance.

4.3. Testing of MOV arresters

One of the projects at Camp Blanding in 1996 was concerned with the
performance of 10 kV MOV arresters. Given below are selected results, taken
from Fernandez et al. (1998b), for one negative lightning stroke in Flash
9632 whose current was directed to the phase conductor of the overhead line
between Poles 9 and 10 (see Fig. 3) which were separated by about 50 m. The
test power distribution system was configured so that the underground cable
was connected to the overhead line at pole 9, as shown in Fig. 3. The
transformer in IS1 was connected to the cable, and the simulated house
service entrance was attached to the secondary of the transformer. MOV
arresters were installed at the transformer primary (Cooper elbow arrester)
and at Poles 9 and 10 (GE Tranquell arresters). Additionally, there were
MOV surge protective devices (SPDs) installed at the service entrance of the
simulated house. The neutral conductors were grounded at each arrester, at
the terminal poles, at the service entrance, and at IS4.
The waveform of the arrester voltage at Pole 9 for Flash 9632 is shown in
Fig. 9 along with the total lightning current (recorded in the channel
having +/- 7.5 kA measurement range in order to resolve the structure of
relatively low level current after the initial peak). The lightning current
peak was about 12 kA, typical of subsequent strokes in natural lightning.
First strokes in natural lightning have current peaks two to three times
larger. The waveforms in Fig. 9 are displayed on a 10-ms time scale. The
discharge current of the arrester is not shown because it has insufficient
amplitude resolution after some hundreds of microseconds.
After the initial spike (probably associated with both the arrester lead
inductance and magnetic coupling to the voltage measuring circuit), the
voltage waveform in Fig. 9b is clamped near 20 kV for about 4 ms, then
begins to return to zero. The falling trend of the waveform is interrupted,
and the voltage exhibits a hump with amplitude of about 3 kV. (A similar
feature is also seen in the waveform of the total lightning current in Fig.
9a). After the hump, the voltage further decreases, crosses zero, and
produces an opposite polarity overshoot lasting several milliseconds. The
overshoot has peak value of about 8 kV.
At 200 ?s, the arrester discharge current is about one-half of the total
lightning current. Thus, if we assume that the same fraction (one-half) of
the lightning current in Fig. 9a is flowing through the arrester at Pole 9
after 200 ?s and is causing the voltage response in Fig. 9b, we can estimate
the energy absorption by the arrester at Pole 9 as a function of time, shown
in Fig. 10. As seen in Fig. 10, the energy absorbed during the initial 200
?s is about 8 kJ or about one-third of the total energy of 25 kJ absorbed
during the voltage-clamping stage of the arrester operation lasting 4 ms or
so.
For GE Tranquell MOV arresters, the maximum energy capability is 4.0 kJ/kV
of rating (Greenwood, 1991) or 40 kJ for this case. Therefore, during the
first 4 ms of the event considered here, the arrester was subjected to about
60% of its maximum energy capability. Video and photographic records, along
with visual inspection, indicate that physical damage was not sustained to
any of the MOV arresters as a result of this test. Barker et al. (1993),
who measured voltages across and currents through 10 kV MOV arresters
installed in actual four-wire multi-grounded power distribution systems,
estimated the dissipated energy for one negative lightning event to be in
excess of 80 kJ. Although this energy value exceeded the maximum energy
capability of the arrester, the arrester did not fail. The measured
lightning current through the arrester exhibited a slow tail lasting for
about 2 ms at an average level of about 2 kA, and the bulk of the dissipated
energy was associated with this tail.

5. Summary

1. Rocket-triggered lightning appears to be in many respects a controlled
analog of natural lightning. The results of triggered-lightning studies
provide new insights into both the physics of the lightning discharge and
lightning interaction with various objects and systems.

2. At very close ranges (a few tens of meters or less) the time rate of
change of the final portion of the dart leader electric field can be
comparable to that of the return stroke. This observation, coupled with the
fact that within some hundreds of meters the leader and return stroke
electric fields are about equal in magnitude, suggests that very close dart
leaders can make a significant contribution to induced voltages and currents
in power and other systems.

3. The variation of the close dart leader electric field change with
distance can be somewhat slower than the inverse proportionality predicted
by the uniformly charged leader model, perhaps due to a decrease of leader
charge density with decreasing height associated with an incomplete
development of the corona sheath at the bottom of the channel. However, the
bulk of the presently available data on leader electric field changes within
500 m suggests a more or less uniform distribution of charge along the
leader channel. More data and modeling are needed to interpret the observed
variations of leader field change with distance.

4. Judging from the similar average current peak values in dissimilar
grounding situations, lightning appears to be able to reduce the grounding
impedance which it initially encounters at the strike point so that at the
time of channel-base current peak the reduced grounding impedance is always
(regardless of the initial grounding impedance) much lower than the
equivalent impedance of the channel (hundreds of ohms to several kiloohms).
Breakdown processes forming distinct plasma channels in the soil and on the
ground surface are probably the principal means of lowering the grounding
impedance, at least in the case of poorly conducting (of the order of 10-3 -
10-4 S/m) sandy and clay soils.

Acknowledgments. The experiments at Camp Blanding, Florida, reviewed here
were made possible through the efforts of many individuals including M. A.
Uman, K. J. Rambo, M. V. Stapleton, T. W. Vaught, J. A. Versaggi, J. A.
Bach, Y. Su, M. I. Fernandez, D. E. Crawford, C.T. Mata, G. H. Schnetzer,
and R.J. Fisher (UF); A. Eybert-Berard, J. P. Berlandis, L. Barret, and B.
Bador (CENG); P. P. Barker, S. P. Hnat, J. P. Oravsky, T. A. Short, and C.
A. Warren (PTI); R. Bernstein (EPRI), and J. L. Koepfinger (Duquesne Light
Co.).

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