Note: Descriptions are shown in the official language in which they were submitted.
~ CA 0220~43S 1997-0~-14
The object of this invention is an apparatus for protecting large ground
areas from lightning by directing the lightning strikes to a safe ground spot.
In particular, it is intended to reduce the number of lightning initiated forest
fires.
PRIOR ART
Lightning rods used to protect buildings have a protection coverage limited
to a radius equivalent to their height. Conductive wires raised by tethered
permanent aerostats could provide protection over a large area, but would
be uneconomical and would present a hazard to aircra~ft navigation.
A laser produced conductive ionized path is claimed in U.S. Pat. 4,017,767.
U.S. Pat. 3,371,144 related to the protection of transmission lines and the
book "LIGHTNING" by M. A. Uman (McGraw-Hill Book Co.) give a broad
overview of the subject and the related scientific literature.
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Rocket launched wires are routinely used in scientific research for artificially
triggering strikes in order to study one component of the strike, the return
stroke which happen to be similar to the return stroke of a natural strike.
The use of rockets is however impractical for lightning protection since the
timing and angle of launch is very critical and the wire does not stay up long
enough. In addition, copper wire is used; because of its relatively low
melting temperature, the high current pulses frequently cause it to melt
prematurely failing to leave behind a conductive ionized path for the
lightning strike to occur.
SUMMARY
In natural lightning, a strike is preceded by a downward negatively charged
leader that emerges from a negatively charged bottom portion of the cloud
progressing in steps toward the positively charged ground. As this leader
gets closer to ground, a short positive upward leader emerges from the
ground and meets the downward leader establishing a conductive path for
the actual strike.
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Grounded objects in the presence of the electric field created by a
thunderstorm charge emit a glow of positively charged ions. The intensity
of this glow increases with height in the field and intensity of the field. From
studies of glow from tall structures and rocket launched wires, it is known
that once the field across the length of the grounded wire exceeds a critical
value, the glow will transform into an arc or a leader that will continue to
propagate as long as the field gradient is sufficient The combination of the
grounded wire and propagating leader can be viewed as a lightning rod
rapidly growing in length toward the charged portion of the cloud As the tip
of the leader approaches the cloud charge, the field gradient between the
two of them exceeds a critical value and a downward negatively charged
leader emerges from the cloud charge to meet the upward leader. The
distance between the meeting point and the ground connection is the
effective protection range; it is almost the height of the bottom of the cloud,
about 4 km for a typical thunderstorm cloud.
Computer simulations of a rocket launched grounded wire in an electric field
with a gradient of 40 kV/m at altitude 100 m and 80 kV/m at altitude 1000 m,
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typical when an average thunderstorm is directly overhead, indicate that the
progress of the upward leader becomes continuous and self propagating
above an altitude of 180 m as the ambient electric field across the length of
the wire exceeds about 6.5 MV.
Therefore with a wire raised to a rather modest height, we can obtain the
lightning protection range of an equi\lalent lightning rod having 20 times that
height. This range can be further extended by having a mid-portion of the
wire non conductive and extending the overall wire length. A range of close
to 10 km could thus be reached. Thus large ground area could be
protected with quite sparsely distributed wires. Such wires could present
a hazard to aircraft navigation however, unless they can be made so fine as
to be easily and safely ruptured. This requirement is conflicting with the
need of being able to sustain large current peaks and not melt prematurely
before a conductive ionized path is established. I have determined that
high temperature melting wire material and carbon fibers in particular can
carry a sufficiently large current and reach a sufficiently high temperature
to ionize thermally the surrounding air and reliably create a conductive
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ionized path for the leader to continue propagating after the wire has
disintegrated and still be made so fine as not present a hazard to aircraft
navigation. In addition, carbon fibers being very light requires smaller
balloon and less lifting gas.
The balloon borne wires can be manually launched from the ground in
advance of thunderstorm clouds or at the beginning of the lightning or dry
season. The combination of balloon/wire and grounding rod can also be
dropped from an aircraft so that the grounding rod implants itself into the
ground, for inaccessible locations or in advance of a thunderstorm.
The balloon borne wires can be remotely released upon the approach of a
thunderstorm by rangers manning the lookouts. They can be automatically
launched by having for instance a capacitor charged by the electric field of
the thunderstorm discharge at a preset voltage and trigger a helium filled
cannister to inflate one or more balloons, the inflating of the balloon in turn
triggering a guillotine to cut off the balloons free. Additional balloons can
be automatically launched to replace stricken ones by having lightning
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~ CA 02205435 1997-05-14
striking previously kaunehed wires sequentially trigger additional launchings.
The preferred embodiments of this invention are illustrated in the following
drawings, of which:
FIG. 1 is a view of a balloon borne continuously conductive wire;
FIG. 2 is a view of a balloon borne wire having a mid portion non conductive
for extended range;
FIG. 3 shows the means for launching balloon borne wires either remotely
or automatically;
FIG. 4 shows means for sequentially launching addition~l balloon borne
wlres;
FIG. 5 shows an aircraft dropped wire a~er implanting itself into the ground.
~n an err bodiment of the invention, FIG. 1 shows a conductive wire 1 of
substantial length having a lower end grounded to earth through the
grounding means ~. and an upper end raised to altitude by balloon 3 thereby
extending the wire up to altitude. A high temperature resistant thermally
and elec,trically insulating connector 5 joins the upper end of wire 1 to
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CA 02205435 1997-05-14
balloon 3 so as to thermally protect balloon 3.
Balloon 3 is a disposable balloon filled with helium and may be made of
metalizecl film or n~bber. The wire length is determined by the expected
ambient field encompassing the length of the wire. An encompassing field
of about ~.5 MV is required for a self propagating leader to occur. For an
average thunderstorm directly overhead the wire, the length may be as
short as 180 m; however for a thunderstorm some distance away, the
weaker field will ne!cessitate a longer wire. However the longer wire may
trigger a strike from a cloud passing overhead that would not normally have
produced natural lightning strikes, thereby wasting a wire and balloon
unnecess,arily. Therefore a tradeoff has to be made between longer range
longer wires widely separated and shorter range shorter wires more closely
spaoed. The optimum choice depends on the expected number of strikes
per storm ~nd the economics of the gridwork deployment of the wires.
Generally, the length should be sufficient for the wire to an altitude such that
when in the electric field of a thunderstorm, it attracts lightning and direct it
to the safe ground area.
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However, it is preferable to strive for a length such that when in the electric
field of a thunderstorm not quite sufficiently strong for a natural lightning
strike to occur, the ambient electric field encompassing the wire is
sufficiently strong to cause an upward leader to emerge from the top end of
the wire and self-propagate indefinitely toward the thunderstorm cloud until
a lightning strike is triggered from the cloud to the tip of the leader and
along the ionized conductive path left after the wire is thermally vaporized.
For a more extended lightning protection range, wire 1 may have a non
conductive mid-portion as shown in FIG. 2 wherein wire 1 has a conductive
upper portion 7, an intermediate non-conductive portion 9 of substantial
length relative to the upper portion and a bottom conductive portion 11
whose lower end is grounded to earth. Not being grounded, the upper
conductive portion 7 needs a stronger field across its length for leader
discharges to emerge from its ends. Therefore it has to be both longer and
extend closer to the cloud, where the field gradient is stronger.
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Such a combination of wire has been rocket launched to simulate and study
intra cloud lightning strike between the upper positive charge and lower
negative charge of a cloud; here we use the same combination for a
different application, namely discharging the negative bottom charge of the
cloud safely to the ground. The scientific literature reports that when the
upper conductive portion is in a field approximately 10 MV across its length,
an upward positive leader emerges from its top end self-progressing toward
the negative cloud charge and a few milliseconds later, a downward
negative leader emerges from its bottom end and quickly jumping across
the non-conductive intermediate portion 9 joinsa short positive leader
emerging from the top end of the bottom grounded conductive portion 11;
thus a continuous conductive path is established from the ground to the still
progressing top leader. Very quickly though the current increases so much
that the conductive wires melt leaving behind an ionized conductive path
that keeps the leader progressing for many hundreds of seconds toward the
thunderstorm cloud until it gets close enough to trigger a lightning strike
along the conductive path.
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The wire portions could simply have substantial length, however in order to
avoid unnecessary artificial lightning strikes, the length of the upper
conductive portion 7 is preferably made and the length of the intermediate
portion 9 is such as to raise the upper portion 7 to suffcient altitude so that
when in an electric field of a thunderstorm not quite sufficiently strong for a
natural cloud to ground lightning strike to occur, the electric field
encompassing the upper portion is sufficiently strong to cause an upward
leader to emerge from the upper end of the upper portion and self-
propagate indefinitely toward the thunderstorm cloud and a downward
leader to emerge from the lower end and jump across the intermediate non-
conductive portion 9 to the bottom conductive portion 11, so as to bridge
conductively the gap between the upper portion 7 and the bottom portion 11
and thereby establish a continuous conductive path from the grounding
means to the tip of the self-propagating upward positive leader.
The bottom conductive portion 11 may be rather short, about 50 to 100 m
and serves to direct the strike to the designated safe ground spot.
Insulating spacers 13 insulate the lower melting temperature intermediate
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portion from the hot upper and bottom portions, keeping it from melting
prematurely.
WIRE MATERIAL
Very early during the leader self propagation, the current increases so much
that the wire quickly melts. If prior to melting the temperature is high
enough to thermally ionize the surrounding air, then a conductive ionized
path will survive the wire and allow the leader to continue progressing
toward the thunderstorm cloud and establish an ionized conductive path
reaching close enough to the thunderstorm cloud to trigger an artificial
cloud to ground lightning strike along the conductive path.
At 1000~K, air is a good insulator; at 4000~K, it is in an ionized state and is
a good conductor. Therefore a high melting temperature material is
preferred. As a matter of fact, copper commonly used in rocket launched
wire has a rather low melting temperature and has a high rate of failure in
triggering artificial lightning.
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The highest current carrying capacity of a wire prior to melting is achieved
when the resistive power generated within the wire can be radiated at the
surface according to the following formula for a unit length round wire:
4 p I2 , or
d2
3.74. 10 ( ) d K
p
where I = current, A p = resistivity, Qm
~ = emissivity d = diameter, m
6 = 5.67 x 1 o-8 W/m2/~K4 K = absolute temperature
(Boltzmann constant) (~C + 273)
This equation shows that temperature and diameter, hence density, are the
dominant controlling factors for the highest current carrying capacity prior
to melting. The highest temperature conducting materials are carbon fibers,
tungsten and molybdenum.
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~ CA 0220~43~ l997-0~-l4
The following table summarizes the relevant properties near the melting
temperature, comparing the required wire diameter and weight of a 100 m
long wire capable of carrying 10 A prior to melting:
MELTINGRESISTIVITYDENSITYEMISSIVITYDIAMETERWEIGHT (g)
TEMP (oK)(~S2m) (g/cm3) (cm) PER 100 m
PAN CARBON FIBER 3925 32 1.8 0.85 0.048 33
TUNGSTEN 3683 1.2 19.3 0.35 0.024 85
MOLYBDENUM 2883 1.1 10.2 0.2 0.038 119
The table shows carbon fibers to be definitely the optimum material. Not
only is it the highest temperature resistant, it sublimates instead of melting
making it survive much longer and thereby ionize more of the surrounding
air. For the same current carrying capacity, it is almost three times lighter
than tungsten. In addition, its greater resistivity helps slow the rapid current
increase in the electric field.
In the above table, the candidate materials were evaluated on the basis of
a single round wire. In fact, a carbon fiber of such diameter cannot be
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made because of process constraints. Instead they are only produced in
extremely small filaments in the range of 7 to 10 microns in diameter. They
are commercially available at an acceptable price as a minimum tow of
1000 filaments (1 K).
A single 7-micron polyacrylonitrile (PAN) carbon filament can carry 0.017 A
before starting to sublimate; 600 such filaments could carry 10 A if spread
apart and would weigh only4.3 g per 100 m, less than 3% of the weight of
tungsten for the same current carrying capacity.
The 1 K tow could be used as is with some reduction in the radiative
capacity, or it can be subdivided into smaller tows. Spreading the filaments
in a ribbon-like arrangement increases the radiating surface. However as
the filaments carry the current in the same direction, the electromagnetic
force would tend to bunch them together. To overcome this tendency, the
filaments could be glued together in a ribbon-like arrangement or held in
this arrangement by crosswise glued filaments. An appropriate adhesive
could be phenolic based as at elevated temperature it would also carbonize.
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It can be appreciated from the above table that the use of high melting or
sublimating temperature material allows extremely fine diameter wire to be
used for the application. Such fine wire would therefore have such a low
strength as not to present any hazard to a flying aircraft.
Wire as defined here may be construed as being round or flat, being
conductive or having conductive and non conductive portions, being made
of one or more filaments loosely assembled or twisted or glued in a ribbon-
like arrangement. A high temperature resistant wire is preferred for the wire
of FIG. 1 and at least the upper conductive portion 7 of FIG. 2; the bottom
conductive portion 11 needs not be since it is involved only at a later stage
of leader development.
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MEANS FOR AUTOMATIC LAUNCHING
The automatic launching a balloon borne wires may be desirable for
replacing wires struck by lightning and/or for remote or unmanned stations.
The basic sequence of events may include:
1. Detection of conditions warranting the launch, i.e. the approach of a
thunderstorm and in the case of forest protection determining the
dryness condition of the forest. Although this could be done by human
observation, for instance the rangers manning the lookouts, they could
also be done by on site electric field sensors and hygrometers;
2. Triggering the launch sequence. This can be done remotely by human
intervention or it could be done automatically by the field sensor;
3. Inflating one or more balloons with lighter than air gas such as helium;
4. Setting the balloons free to rise.
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The means for automatically launching one or more balloon raised wires are
illustrated in FIG. 3. As a field sensor, capacitor 13 has on the upper side
an extension rod 15 extending up in the electric field of a thunderstorm,
while the lower side 14 is grounded to earth through contactor 17 being
closed by optional hygrometer 19 sensing a dry forest condition. A pointed
electrode 21 is placed a calibrated gap 23 away from the upper side of the
capacitor and mounted on insulator 18.
The gas filling apparatus is made of a pressurized helium cylinder 25
mounted inside guide 26 and preferably weld sealed; the lower end of the
cylinder has a necked extension 27 with an easily piercable diaphragm 28.
The necked extension 27 engages the upper end of flexible sleeve 29. The
lower end of sleeve 29 engages one end of elbow 31 having a piercing
pin 33 in proximity with diaphragm 28. The other end of elbow 31 is
engaging the neck 34 of one or more balloons 3. A conductive weight 35
is held near the top of guide 26 attached by fusible link 37 to insulating
cover 39. Electrode 21 is electrically grounded via wire 22, fusible link 37,
weight 35 contacting guide 26 which is grounded to earth.
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A V-shaped lever 43 has a first leg resting on the balloon and has a
weight 45 attached near the end of the second leg and is pivoting around
fulcrum 44 at the apex of the V. A blade 49 is held at the top of guillotine 47
by pin 51 linked to the end of the second leg of lever 43 by a loose
string 53. Guillotine 47 is mounted over the neck 34 of balloon 3. Wire 1
is coiled and has one end attached to balloon 3 and the other end
grounded.
As the thunderstorm approaches, the electric field gradient builds up across
the length of extension rod 15. If a dry hazardous condition exists,
hygrometer 19 will close contact 17 grounding the lower plate of
capacitor 13. As the electric field builds up, capacitor 13 becomes charged.
Gap 23 is adjusted so that as the capacitor becomes charged to a
predetermined voltage, arcing will occur across gap 23 and the capacitor
will discharge to ground via electrode 21, fusible link 37, weight 35 and
guide 26 and melt fusible link 37, releasing weight 35 onto cylinder 25.
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~ ' CA 0220~43~ 1997-0~-14
The energy of the falling weight causes the sleeve 29 to collapse allowing
pin 33 to puncture diaphragm 28 releasing the pressurized helium and
inflating the balloon 3. The inflating balloon tilts lever 43 around fulcrum 44.
As the weight 45 tilts past fulcrum 44, it builds up enough momentum at the
end of its travel to pull out pin 51 thereby releasing blade 49. String 53 has
enough slack to delay the pulling of the pin, allowing weight 45 time to build
up energy and balloon 3 to fill up.
The falling blade 49 slices the neck 34 of balloon 3 releasing it. A check
valve 55 in the neck of the balloon retains the helium. The freed balloon
raises one end of wire 1 to altitude while the other end of the wire is
connected to the earth grounding rod.
The above sequence of events initiated by sensing the presence of a
thunderstorm is used to release initially one or more balloons. Means for
sequentially launching additional balloons may be provided for replacing
wires already struck by lightning using lightning itself as a trigger, as shown
schematically in FIG. 4.
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Wires 1 and 1a have the bottom end grounded and the upper end raised to
altitude by balloons and wires 1b and 1c awaiting to be launched are
attached to a common grounding rod 60. A coil 63 surrounds the grounding
rod 60 at a safe distance; one end of the coil is grounded and the other end
is connected to spring blade 64. Weight 35b is suspended through fusible
link from the end of spring blade 64 and deflecting it down away from
contactor 68. Electrical continuity from the fusible link to the ground is
provided through the grounded guide 26b. As lightning strikes wire 1 a, the
current induced in coil 63 melts fusible link 37b dropping weight 35b to
launch a replacement balloon raised wire 1 b. Being freed of weight 35b,
spring blade 64 springs up, establishing contact with contactor 62
connected to spring blade 70 to which weight 35c is attached through
fusible link 37c in a similar arrangement as the previous one, establishing
continuity to ground via guide 26c and making the apparatus ready for the
next lightning strike to wire 1b to trigger the release of balloon raised
wire 1c. When the launching is manually triggered, the capacitor is not
used, instead fusible link is directly fused using on electrical impulse from
a battery for instance.
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CA 02205435 1997-05-14
The earth grounding means may be the bottom end of the wire being buried
into the ground or a rod driven into the ground, or connected to an
underground grid; it may be a conductive grounded structure or any other
grounding means known to the trade.
Referring to fig. 5, when the terrain is inaccessible or for tar~eting a
thunderstorm, instead of being launched from the ground, the balloon, wire
and grounding rod could be dropped from an aircraft from a sufficient
altitude for the grounding rod to implant itself into the ground and establish
sufficientlly effective grounding.
The method comprises coiling a conductive wire 1 in container 12, attaching
one end to helium filled balloon 3, attaching the other end of the wlre to the
groundin~3 rod 60 and dropping the bal~oon, wire and grounding rod from an
aircra~. Optionally a bottom portion 1 d of wire 1 of length extending up past
the fine combustibles on the ground or in the tree cover may either have a
much larger wire gauge than the rest of the wire so that it does not heat up
suffciently to start a firel or i~ can be surrounded with a sheath 62 filled with
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CA 02205435 1997-05-14
a light non combust:ible filler 64 such as fiberglass wool so as to keep wire
1d away from the combustible material 66 on the ground or in the tree
cover. The drop altitude should be such that the grounding rod reaches the
~round before rising balloon 3 unwinds wire 1 substantially to altitude in
order not to break the fine wire off the balloon and not cause a premature
artificial lightning s~.rike.
Alternatively sheath 62 and filier 64 can be replaced by a tumescent coating
expanding when he!ated by the lightning strike and insulating the rod and/or
the boffom wire portion from the fine combustible materiaJ.
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