--On September 30, 2008 7:24 AM -0700 Darryl Thayer <daryl_solar@...
> NIce Job Rob
> This is a good education in lightning.
> Everything you say is consistant with my experiance and explanation,
Quite a bit of time and effort went into gaining a better understanding of
lightning and lightning protection. There is a great deal of conflicting
(and misleading) information on this topic out there.
> I only had an idea that lightning had these current levels and
> voltages. It is easy to see how when finding many paths to ground
> conductors and equipment can be effected anywhere in the structure.
Indeed, grounding structures doesn't mean they are at 'ground' potential
during a strike: For well-grounded towers the expectation is around 1,000
Volt for every foot in height. So, taking the turbine leads off the tower
at 3 feet from the ground means you just created a 3,000 Volt differential
going into the batteries/inverter, when lightning hits the tower.
> appears that stand alone towers are a better target than towers where the
> power lines are near.
> It apperas from experiance that a very good surge arrestor of the
> varistor type should be able to bind the building conductors together
> preventing damage to interior circuits.
Most varistor type devices can't handle full-blown strikes (ie. a 10/350
device to use the parlance). They are mostly for residual voltages/currents
(8/20 devices in other words). A possible exception may be the Delta surge
arrestors, these are silicon oxide varistors; they are not the greatest
surge arrestors, but they can handle the currents for most strikes. Their
downfall (for the Delta's) is the pass-through voltage; it's far too high
for most electronics to survive. Now, these days inverter manufacturers put
MOVs at the inputs of the electronics, and if you get lucky (sufficient
inductance between the Delta and the inverter to decouple the two
arrestors, and a strike that's not too powerful) it may just be enough to
have the electronics survive.
There are varistors that can handle the currents involved in a direct
strike, but they are large and expensive. Manufacturers of arrestors use,
for the most part, some type of spark gap device for that purpose. More
robust, and cheaper. Except for combo-devices those direct-strike arrestors
have a too high pass-through voltage to protect electronics. A second
(down-stream) device is needed (and sometimes a third-level device) to get
voltages down to manageable levels. There are combo devices that put those
in a single package, for example, Dehn makes them. Not cheap though.
> The analysis seems to say GROUND
> the tower extreamly well.
Anything you can divert to ground means less energy to deal with for the
arrestor/electronics. Grounding is definitely the no. 1 thing that can be
done to prevent damage. Still it's important to keep in mind that not all
ground is ground when it comes to lightning; during a strike the currents
are such that ground potentials can vary by many kV's from one place to
another (causing currents and voltages to go places where they will kill
electronics if nothing is done).
> Where I always will have a ground water table
> in the desert it may be to low to find, and other methods sucn as
> Faraday cage.
Take a look at the Ufer ground. This was developed during the 2nd world war
specifically to ground structures in sandy, poorly conducting soils. Very
effective, and cheap to do if you plan for it at the construction stage.
Faraday cages are expensive and normally overkill. From what I've seen they
are not normally used for commercial installations that require hardening
for lightning. Using proper grounding, wiring, single point ground, and
surge arrestors can make things virtually lightning-proof (that is not
cheap either though).
> There are a couple of aspects I did not see expressed, 1) the Faraday
> cage effect that could help prevent damage if considered in the
> construction phases. and 2) it mentioned the fall of potenical
> method but not the induced current method of ground testing.
I've not looked into ground testing much. The intend was to give people an
idea of how lightning does its damage, and what can be done to mitigate
that. Preferably with the means available. If those pages can help with
that I'm happy! :)
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