Tue., Mar. 22, 2011
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Welcome back to ATMO/ECE 489/589. I hope you had a nice
Spring Break.
We'll try to slowly ease back into school mode with Homework #5 pt. 1. The
single question deals with the effects of the 1986 Chernobyl nuclear
reactor explosion on atmospheric conductivity.
Today and Thursday we'll be discussing lightning return stroke
currents. Today we'll look at what current properties or
parameters are of interest (mostly from a lightning damage or
protection point of view) and look at ways of measuring lightning
currents. On Thursday we'll look at how return stroke currents
can be determined from remote measurements of the electromagnetic
fields radiated by lightning.
The outline below was on one side of a class handout.
We'll start with some examples or situations where current parameters
such as peak current amplitude or peak current derivative value are of
interest. Then we'll will look at some of the techniques, both
older and more modern, that are used to measure lightning current
directly.
The superscripts on the outline above refer to specific references that
were on the back side of the handout. This list of references is
shown below.
Links to all but references 3,4, and 5 can be found in the class articles folder.
Many of these studies are still the source of some of the best
lightning return
stroke current data available. Table 2.1, for example, in Uman's
2008 book "The
Art and Science of Lightning Protection" is adapted largely from
Reference #3 above.
Now a quick look at the engineering parameters and situations
where
they are important.
Peak current is of interest when lightning strikes an object that
presents a resistive load to the lightning current. Uman, in the
lightning protection book mentioned above, uses the example of a phase
conductor on a power line. Power lines, apparently, have a
characteristic impedance of about 500 ohms.
A 1st return stroke typically has a peak current of about 30,000
Amps. Assume that the current divides in half as shown above and
then travels through 500 ohms to ground. This will produce a
voltage of 15,000 Amps x 500 ohms = 7.5 Mvolts. This is high
enough to spark across to a ground wire or one of the other phase
conductors.
Lightning rods on the house sketched below are connected to ground rods
that are driven into the soil.
Depending on the soil type and moisture content, the ground resistance
can range from a few 10s of ohms to a few 100s of ohms. A peak
current of 30,000 Amps would produce a voltage of 750,000 volts across
25 ohms and 7.5 Mvolts across 250 ohm. The latter value almost
certainly would be enough to spark across to nearby plumbing or
something like that (the lightning rod should be electrically connected
(bonded) to a nearby water pipe to avoid this kind of occurrence).
The peak value of the return stroke current derivative is of interest
if lightning strikes something with an inductive impedance.
The straight down conductor in the sketch above has an impedance, L, of
roughly 1 microHenry per meter. Lightning return strokes (both
1st and subsequent strokes) have peak current derivatives values of
about 100 kA/usec or about 1011
A/sec.
The voltage produced here will be L x dI/dt = 1011 A/sec
x
10-6 H =
100,000 volts.
The lightning dI/dt will produce a time varying magnetic field that can
couple into nearby electronics (loops are of primary concern).
The time integral of the return stroke current gives the total
charge transferred during the strike. This apparently determines
whether the lightning current will burn through a sheet of metal (or
perhaps the thin metal skin of an airplane).
Here it is the continuing currents that are of concern.
That's where most of the charge transfer occurs. I like to think
of
the lightning strike acting as arc welder in cases like this.
The lightning will burn through the metal sheet unless heat can be
carried away from the strike point quickly enough. Thin sheets
won't be able to dissipate heat as quickly as thicker sheets.
The last parameter of interest is something called the action integral
The instantaneous power dissipated by a resistive load is
so the energy deposited is R times the action integral
This will cause heating or can cause vaporization of materials
with low electrical conductivity that are struck. Lightning can
vaporize the sap in a tree, for example, and cause the tree to explode.
Some of the earliest estimates of return stroke peak currents come
from measurements of the residual magnetism in nephelitic basalt
(whatever that is) near trees that were struck by lightning (within
centimeters of the tree perhaps). The data were published by
Pockels in 1897 & 1898 (the publications are in German or I would
try to find them and put them in the articles folder - they'd be
interesting to look at).
Pockels had determined, in laboratory experiments, that magnetism of
the
basalt depended on the peak current and wasn't affected by the shape of
the current waveform or its duration.
The figure below shows a klydonograph (on one side of a class
handout). A high voltage will produce a Lichtenberg figure on the
film. The diameter of the pattern depends on the peak voltage
which can then be related to peak current if the impedance of the
arrangement is known. Different patterns are produced depending
on the polarity of the applied voltage.
Here's
an
interesting web site with some interesting historical background on
Lichtenberg figures.
Another handout showed a Lichtenberg figure-like burn pattern left
in the grass on a golf course green (in Tucson). The
photograph is on the cover of the June 1977 issue of Weatherwise
magazine (I don't think I should put it on the online notes because of
copyright
restrictions). Similar burn patterns are, apparently, sometimes
seen on lightning strike victims (something we'll definitely not put in
the online notes).
"Magnetic links" have been used extensively by the electric power
industry to estimate peak currents in lightning strikes to power lines.
Magnetizable material is positioned perpendicularly to a straight
conductor. The strength of the magnetization in the link can be
related to the peak current in the conductor. Often two or more
links are mounted with different orientations and at different
distances from the conductor.
An inexpensive version was at one time used at the Kennedy Space Center
to estimate peak currents in certain launch facilities. A strike
to a launch complex would probably require time consuming and expensive
testing to ensure the facility hadn't been damaged and was still fully
operational.
A loop of prerecorded magnetic tape (on a plastic support and sealed
inside a capped piece of PVC pipe) was positioned perpendicularly to
conductors that might carry large lightning currents. A portion
of the signal on the tape would be erased by the magnetic fields
produced by lightning currents. The magnetic tape wouldn't be
removed and analyzed unless the photo bulb had flashed, indicating that
a lightning strike had occurred.
A wire loop placed close to a straight conductor can be used to
determine the current derivative.
This is the principle behind a Rogowski Coil
used to measure time varying currents moving through a conductor.
Multiple loops of wire on a toroidal support surround the conductor.
The multiple loops of wire create inductance that can limit the high
frequency response of a sensor like this.
A sketch of a faster dI/dt sensor is shown below.
The induced voltage is measured across the gap on the inside
surface of the sensor. Sensors like this are used to measure
lightning dI/dt signals and also are used in nuclear
electromagnetic pulse testing.
Breaking the current conductor and adding a resistive element is
perhaps a more obvious way of measuring lightning currents.