Hmmm.. are we talking direct hits? The lightning literature is fairly
full of the electrical properties of lightning strokes, and the current
pulse typical stroke is substantially less than 1 ms long (classic model
is 2 us rise (10%-90%), 50 us fall to half max magnitude). There are
some super strokes with longer durations and/or large "continuing
currents" but they constitute substantially less than 1% of the overall
strikes at a given location.
Yes, we are talking direct hits, and those numbers I gave were for the
very large positive strikes which are possible, but rare in comparison
to negative strikes. I also stated that you have to decide what the
source waveform should be for the calculations. There are several
different "standards" for various situations. You don't want to use a
standard not intended for direct hits. There are some that are intended
for direct hits like ANSI/IEEE C62.41 (50KA 10X350ms) but when you look
at the details even that assumes currents much less than that of the
very large positive strikes. For purposes of calculating numbers, you
have to pick a standard, a number, a waveform, or something you feel
safe with before starting the calculations. That will probably be
something that doesn't cover all the possibilities, so there is an
element of risk involved.
The current pulse you choose for the excitation is only good at the
point at which it is applied. At other parts of the system the current
waveforms will be different. It's somewhat like pinging a complex
electronic circuit consisting mostly of many inductors which also have
mutual coupling, and asking what is the current waveform in this one,
but it is more complex than that. If you put a current into a SPICE
model of an inductor you instantaneously get the same current out the
other end of the inductor. If you do that to a physical inductor of
significant size, like an antenna, you do not get the same instantaneous
current out the other end. To decide what you have at any point in the
system you have to either analyze the whole system or make some good
decisions about reducing the complexity by eliminating the insignificant
items. If you get any arcing, waveforms change significantly. It's not
enough to just know the peak currents in the system. These have to be
applied over a time period. (i^2)t would be good information.
There is one other major element of this system that requires a huge
simplification, modeling the ground system and its impedance. That will
be very non-linear and frequency dependent. There are such things as
ground saturation occurring that I don't think are very well
understood. Just choosing a resistive value for ground impedance is a
gross simplification. Even if you can't model ground saturation, wire
and rod lenghts should be modeled.
I don't know of any single tool that can perform all the calculations
needed for this system. If NEC didn't have its limitation of handling
closely spaced wires, and intersection of widely different size wires,
and could handle an arbitrary source instead of just a sine wave, and
could perform transient analysis instead of only AC frequency analysis,
then it could do the calculations. That's a lot of if's. If you
neglect all inductive coupling and non-linear ground effects, SPICE
could do the calculations, but that would generate only a rough guess
for an answer.
Also don't forget that the typical strike usually consists of more than
one stroke. That may be important depending on what you are calculating.
You can assume a current in a tower, and assume a rise time, and assume
a tower inductance, and then calculate a voltage, but the answer doesn't
give you any useful information.
Jim Lux wrote:
> K4SAV wrote:
>> If you just use di/dt for calculating voltage drops on lightning
>> conductors you can get some very misleading answers. Sure if you
>> have a known rise time and inductance you can calculate a voltage,
>> but that doesn't convey much information. It isn't even the fast
>> edges on the lightning pulses that cause the most damage. Those can
>> punch holes in you coax jacket, but they won't melt the coax (unless
>> it happens to be in the center of an arc). The huge voltages cause
>> big arcs, but most of the damage is done by the energy contained
>> within the wider pulses. Those wider pulses may be up to 10 ms wide
>> or more, at hundreds to tens of thousands of amps.
> Hmmm.. are we talking direct hits? The lightning literature is
> fairly full of the electrical properties of lightning strokes, and the
> current pulse typical stroke is substantially less than 1 ms long
> (classic model is 2 us rise (10%-90%), 50 us fall to half max
> magnitude). There are some super strokes with longer durations and/or
> large "continuing currents" but they constitute substantially less
> than 1% of the overall strikes at a given location.
> The longer pulses with high currents show up as fault currents in
> power systems. A lightning stroke causes a flashover of the line to
> something else, and the continuing arc burns until something
> interrupts it (an upstream circuit breaker, for instance). A 1000 Amp
> surge with many millisecond duration wouldn't be unusual (I can't
> recall what the "standard" surge is off hand). However, this would be
> very unlikely for a tower scenario (while actually being more likely
> for a power line connected scenario).
> There's some experimental data collected in, I believe, Canada, where
> they did a lot of statistics on actual surges. It was done in the
> last 20 years or so to inform updates on testing standards and
> methods, which were based on 50-100 year old measurements of fairly
> large uncertainty.
>> So if you are trying to calculate voltages and currents on things you
>> need a tool that considers the whole waveform. Also you need to
>> decide what the source waveform should be. It will vary a lot at
>> different points on the tower and it will vary a lot between
>> different types of lightning strikes. Many times you will see
>> references to a standard lightning waveform, but that is for a
>> specific purpose, usually cables on a power pole or in a conduit or
>> something like that, with different levels for different conditions.
>> A tower will be completely different. It's out there in the open
>> being hit directly so it is subject to the entire wide variety of
>> lightning waveforms that mother nature can generate, and they are not
>> nearly all the same. Even the max current pulse that should be used
>> is questionable. Current pulses up to 340,000 amps have been
>> recorded. Those are apparently rare (recording is also rare), but
>> what number should you use a design criteria? I would think it should
>> be something more than a typical value. A tower will also ring like
>> a bell when hit with a strike. Not only do you get the initial
>> pulses but you get some sort of distorted damped sinusoid following
> In fact, folks have modeled this sort of thing with NEC. A 300kA
> stroke would be very unusual. One might design for "fail safe" in
> that case. Typical guidelines design for things like 30kA or 50kA.
>> To complicate the problem even more, you have to calculate the
>> induced currents. In a lightning strike these are NOT negligible.
>> Just ask anyone who has some long cables near a strike, or has a
>> house wired-intercom and lightning happens to hit near the house.
>> Near the tower, induced currents can be huge.
> probably, actually, a bigger problem than direct hits. Lots more
> "nearby strikes" than direct hits, in any given area. And much
> tougher to model. However, the magnitude is, almost certainly, less
> (the induced current has to be less than the stroke current, because
> the magnetic coupling has to be something <1)
>> Once you analyze all this you can start to get a feeling of what is
>> really happening. It's not a trivial analysis. When you see cable
>> lightning currents calculated by very simple formulas, you can bet
>> the answers are completely worthless.
> Not at all. You can use a simple formula to give yourself some
> bounds, or to estimate general magnitude. If one uses a simple
> formula and finds that the estimate is, say, 10 Volts, and your
> "danger threshold" is 1000 Volts, you can walk away and be done with
> it. Likewise, if the simple formula returns 100kV and your threshold
> is 1000 Volts, you stop your analysis right there. It's when you're
> trying to compare the estimate of 500Volts against the threshold of
> 1000 Volts wherein the danger of simple approximation lurks.
>> A couple of other points:
>> 1. Obvious but worth stating: For the high frequency edges on
>> lightning strikes, inductance always dominates the calculations for
>> conducted currents. However, as stated above those aren't the
>> currents that cause the most damage. Also induced currents have to
>> be added and those may not take the same paths.
>> 2. You can reduce conducted currents on a cable by coiling the cable
>> to make a choke, but it is a two edged sword. Reducing the currents
>> increases the voltage drop across the choke and it may increase
>> susceptibility to induced currents.
> The latter from what mechanism? Because the choke is a coil
> intercepting the changing field? A quick and dirty fix would be to
> wind the choke with two windings, in different directions, side by
> side (i.e. the coils are coplanar, so they don't have much mutual
> coupling). or to form the coils in a toroidal form.
> Obviously, one can't get away from issues about high voltage across
> the choke. In the long run, putting the cable underground or in a
> conduit or cable tray is probably a better solution than flailing away
> with chokes.
>> It is obvious that this gets very complicated when you try to
>> calculate anything. It would be really nice if everyone was able to
>> optimize his tower system by calculating the expected currents on
>> things, but it just isn't going to happen for the average guy, at
>> least not with any dependable accuracy. I'm afraid we are limited to
>> the (hopefully) overdesign by rules method.
> I agree.
> Jim, W6RMK
>> Jerry, K4SAV
TowerTalk mailing list