Lightning protection for alternate energy systems
Ah, what a sight to behold! you're the proud owner of a new wind
system. you and a bunch of dedicated friends labored all weekend
to get the tower up, the wind generator in place, and the wiring
completed. and now it's done. The wind is crankin' and she's pumping
amps through the inverter. but wait! what's this? the sky is blackening!
everything has gone still! lightning is striking the ground fast
and furious on the horizon! and it's all coming your way.
send a chill down your spine? It was meant to. Too often,
lightning protection is an afterthought, if it's thought at all.
While lightning protection should be considered in the planning
stages of any renewable energy project, certain measures can be retrofitted
at any time. Whatever stage
you're at, plan now, before the great electrons in the sky start
eyeballing your tower.
note: while this article is about wind generators, the principles
can applied to any renewable electric system.
know your enemy
the surface of the earth carries a negative charge, while the ionosphere
is positively charged. What we have here, on a global scale, is one
great capacitor. under certain circumstances, this "mother of all
capacitors" will discharge. We call these occurrences thunderstorms.
at any given time, 2000 to 3000 thunderstorms are in progress somewhere
around the earth.
For a cloud to ground discharge to occur, what we call lightning,
the electrical resistance of the atmosphere has to be overcome. the
electrical potential necessary to jump from ten to a hundred miles
is on the order of hundreds of millions of volts. The current in
a bolt of lightning averages 20,000 amps. the amount of energy in
a single lightning strike is about 100 million joules. suffice it
to say that this is enough energy to melt a ton of copper in about
one tenth of a second.
Double trouble
There are two problems associated with lightning. the obvious problem
presented by thunderstorms is a direct lightning strike to the tower
and wind generator. Lightning is looking for a path to ground, and
a wind generator tower, being tall and a conductor, helps it out.
a second but lesser known problem is the electrostatic transients
caused by a nearby strike that can be set up in towers, generators,
and wiring. these transient voltages can be just as damaging to generators
and electronic equipment as a direct lightning strike.
When lightning strikes near a wind generator, a current can be induced
in towers, wire runs, and utility and telephone lines. These induced
currents can set up voltage spikes that are very harmful to electronic
equipment associated with wind generators, such as inverters and
control boxes. in addition, the voltage spikes and induced currents
can degrade wire insulation in the wind generator and tower wiring
over time, resulting in shorts to ground. Both problems need to
be addressed, but in different ways. Lightning rods
In my part of the country, we have many houses and barns constructed
with "tin" roofs. back in the '30's, a door-to-door lightning rod
salesman would come around and hawk his wares. the idea was to put
up a lightning rod
that was well grounded to earth to take the lightning strike, rather
than the tin roof. This was supposed to keep your house or barn from
burning down. These lightning rods work, but not for the originally
conceived reason. Remember that the earth carries a negative charge.
this negative charge extends to all objects on the earth, such as
houses, barns, and towers, as well as all things living on the earth,
such as trees and people. lightning rods work because they bleed
off any static charge that builds up on whatever
they're attached to. If a given surface has less of
a static charge, it is less attractive to lightning. the important
part of a lightning rod is not the rod itself, but the grounding
system. a lightning rod is only as good as its ground.
Grounding towers
Wind generators and towers are made of metal. metal has a low resistance,
or low impedance, to the flow of electricity. they are excellent
conductors. they are, therefore, a logical path for lightning to
follow in attempting to get to the earth. all towers should be grounded.
a ground is simply a metal rod driven into the earth. This rod is
attached to the tower by a heavy gauge wire. a properly installed
ground will bleed off any static charge present on the wind generator
and tower making a direct strike
less likely, as well as conducting a direct lightning strike to
the earth where it belongs. Upon being hit, the entire tower structure
will shunt or discharge the energy in the lightning harmlessly into
the ground.
Commercially available grounding rods are 8' long steel rods coated
with copper. if bedrock is less than 8' below the surface, drive
the ground rod at an angle, rather than shorten the rod. this will
maximize the surface area of the rod in contact with the earth.
The wire used to connect the rod to the tower should be either a
very thick single strand of copper wire, or stranded wire with a
few strands of heavy gauge copper wire. a few heavy strands of wire
are preferable to many fine strands of light gauge wire, such as
the type used for welding cables. this is due to the fact that the
copper is going to oxidize and corrode. Fine wire, having a larger
cumulative surface area, corrodes much faster than heavy wire. corroded
wire between a grounding rod and a grounding rod and a tower will
soon cease to exist, and along with it any grounding protection.
Some countries prohibit the use of stranded wire for lightning grounds.
for the same reason, aluminum wire should never be used for grounding
purposes.
Ideally, the copper grounding wire should be physically bonded to
both the ground rod and the tower. this can be done by brazing or
silver soldering the metals to one another. because few people have
this capability, most folks choose to use some sort of clamping device
to connect the ground rods and tower. Make sure that these clamps
are made of either brass or copper. Dissimilar metals should avoided
where ever possible. when wet, dissimilar metals act as a very weak
battery. It is speculated that this galvanic phenomenon may actually
attract lightning strikes.
Keep the grounding wires as short as possible. we want to make it
easy for the lightning to reach the earth. For the same reason,
avoid sharp kinks in the grounding wire between the tower and ground
rods. lightning likes to follow a straight path. If possible, run
the grounding wires in the same line as the tower legs. If this is
impractical, then bend the grounding wires in a very gentle curve
from the tower leg to the ground rod.
make sure you ground all of the tower legs of a freestanding or guyed
tower. This not only guarantees adequate grounding but also prevents
second guessing which piece of your real estate will provide the
best ground. If you have a guyed tower, ground the guy wires where
the guy anchors enter the soil.
moist earth first!
We all know that a wind generator tower should be grounded. but not
all grounds are created equal. The purpose of a grounding system
is to provide
a low impedance path for lightning to follow to earth. and not just
any earth, but moist earth. Moist earth conducts electricity much
better than dry earth does. soggy spring-time soil
dries our in august, you loose 9/10th's of your grounding protection.
It also means that someone with drier, rockier soil will only have
1/10th the grounding protection of another person with wetter soil,
given the exact same grounding systems. what to do?
Extend the grounding system. this can be done by connecting the ground
rods together with bare copper wire buried below the soil's surface.
you can even build a grounding grid with a network of concentric
wire rings. the more elaborate the grid, the more surface contact
you have between the grounding system and the earth. If you have
a well nearby, run a ground wire over to the well casing. wells,
being in constant contact with water, make excellent grounds.
a trick that is often used to increase the conductivity of the soil
is to dig a hole with a post hole digger where you intend to locate
a ground rod. dig the hole a deep as you can. Then drive the ground
rod in the center of the hole. mix
the soil from the hole with salt, then replace the soil/salt mixture
back into the hole around the ground rod. the salt adds electrolytes
to the soil, making it much more conductive than it was originally.
Other things to ground...
your inverter, control box, and battery rack should also be grounded
inside your house. a water pipe will always make a good ground. a
dedicated
ground rod for this equipment is even better. use only one ground
rod with all connecting wires from individual electronic equipment
running to it. This will eliminate lightning flashing over from
a poor ground to a better one right inside your house.
...And not to ground
never, under any circumstances, ground any electrical
system to a gas pipe or an oil pipe.
also, i do not recommend that either the positive or negative leg
of a wind electric system be grounded. (i realize that this is heresy
to some, because it's "the law" of the national electric code. but
i would remind readers that many "laws" have been reversed. an excellent
example is the water hook-up law in chicago. tens of thousands of
chicago homes still have a 1" lead water pipe connecting the house
to the water main. for obvious reasons, this practice has been discontinued.
I rest my case on blindly following "laws".) for one thing, these
wind generators constitute "floating" systems. connecting one leg
of a dc source of electricity to ground will result in what is known
as a "ground loop" or "ground fault". a synchronous inverter connected
to the dc electricity of a wind electric plant will short out with
a ground fault present, blowing the scr's.
another reason not to connect a dc leg to ground is that an electrolytic
reaction will take place between the generator and ground. Back in
the the 30's, some manufactures of wind electric plants would connect
the negative leg of the generator to ground, thereby eliminating
one slip ring and brush. The negative was then picked up at the
tower base and carried to the batteries. after five or ten years,
these towers would fall over. Close inspection revealed that the
metal at the soil line was soft and spongy. what happened was that
the electric current present in the tower leg set up a weak battery
with the earth. ever so slowly, various metal ions would disassociate
from the tower and migrate into the earth. the result was usually
sudden and always catastrophic. For the same reason, I do not recommend
permanently grounding any battery bank. however, for safety considerations,
a battery bank should be temporarily grounded while working on it.
"Spark" arrestors
in 1935, a patent was issued for a "spark gap" type of lightning
arrestor. the purpose of these arrestors was to shunt a lightning
strike traveling down the tower wires safely to ground before it
reached the battery bank. while still commercially available, this
type of arrestor is easily fabricated, as follows. An automotive-type
of spark plug is needed for each wire to be protected. the gap of
the spark plug is widened to about 1/8". all of the spark plugs used
should be mounted onto one metal plate and placed inside of a weatherproof
metal enclosure, like a hoffman box. The metal mounting plate should
be grounded with wire in a straight line down to a ground rod. the
wind generator wires should come into the hoffman box from the top
and be connected to the terminals of the spark plugs. From there,
the wind generator wires take a sharp right angle bend out of the
hoffman box and continue on their way to the battery bank.
The idea behind the spark arrestor is that lightning likes to run
in a straight line ground. the spark plugs, connected in a direct
line to ground, provide that path. lightning, traveling down the
tower wiring, would rather jump the 1/8" gap and continue to ground
than make a right turn.
Lightning arrestors
The modern version of the spark arrestor is called the silicon oxide
varistor, or sov. in an sov, current-carrying electrodes are insulated
by silicon oxide, a material related to sand. the silicon oxide is
rated to insulate against a given voltage. with high voltages, the
silicon oxide is changed from an insulator to a conductor. The lightning
bolt is shunted safely to ground. when the voltage returns to its
normal level, the silicon oxide changes back to an insulator. Unless
the bolt of lightning is strong enough to blow them completely apart,
sov's can be used forever. While not cheap, they are the most effective
lightning protection that can be added to a system. sov's can be
used on both ac and dc systems.
disconnects
the most effective way of preventing lightning from playing havoc
with your batteries and electronic equipment is to physically disconnect
them from the tower wiring. but this must be done right to work properly.
I once connected a kilowatt-hour meter on the ac side of one of my
wind generators for this purpose. the idea was that i could remove
the meter from its socket and thereby prevent lightning from getting
into my inverter. My opportunity came with the next thunderstorm,
and i rushed down to the cellar to pull the meter from its socket.
a moment later, lightning hit my tower, and right before my eyes,
i saw the current arc across the terminals of the meter socket. I
foolishly thought that the five inch gap left by the removal of the
meter would stop a lightning bolt that had just traveled ten miles
or so across the open sky.
Needless to say, any system using a gap in a straight line will not
work. this means that a disconnect switch or large knife switch
is useless. what does work, however, is to use an electric range
or dryer plug and cable on the inverter end, and a matching outlet
on the generator end. If the outlet is level with or higher than
the plug and cord, then the plug and cord will dangle harmlessly
away from the outlet when unplugged. while it is a pain to unplug
your wind system whenever a thunderstorm rolls by, this system works
for high strike locations and it's cheap.
transients.
Remember that when lightning strikes the area near a wind generator
(or a utility or phone line), electrostatic pulses are induced in
wire runs and towers. these electrostatic pulses are known as voltage
transients, and can be as powerful as several thousands volts. Transients
can be very detrimental to wire insulation and electronic components
as they bounce around inside generators and inverters trying to dissipate
their energy. while they can be just as harmful as lightning, transients
are much easier to deal with than lightning.
Shielding
The best way to keep transients from entering your renewable energy
system is by shielding the wires. shielding begins on the tower.
all wires should be run down the tower in metal conduit, or emt (electrical
metal tubing). plastic conduit won't do. the conduit should be grounded.
electrostatic pulses caused by lightning will run through the conduit
safely to ground, never
making it into the tower wiring. If you have a metal tower with tubular
legs, you can run the wires down through one of the legs and eliminate
the conduit. by grounding the chassis or metal boxes that your inverter
and controls live in, you also shield them from lightning-induced
transients.
mov's
an effective way to shunt transient voltage spikes to ground is to
use metal oxide varistors, or mov's. An mov is a device that will
bleed off transients above a given voltage that are bouncing around
electronic circuitry or ac power lines. The gang plugs/surge suppressors
sold by electronics stores for tv, stereo, and computer equipment
are chock full of mov's. they are fast, cheap, and available at most
electronics outlets. Anyone connected to the utility grid should
add mov's to sensitive equipment, i.e., your synchronous inverter.
The static brush
wind generator blades will develop a static charge as they pass through
the air. rotating electrical equipment also builds up a static charge.
wind generators have both. As stated earlier, it is speculated that
this static charge may actually attract lightning. the best way
to dissipate static charges is to connect the rotating armature,
rotor, or blades to ground. this is done with a static brush.
A static brush is merely a carbon or metal graphite brush fitted
onto the generator shaft so that the brush runs continuously on the
rotating generator shaft. The other end of the static brush is grounded.
This continuous ground works to eliminate static charges from building
up.
Static brushes are also used to bypass generator and yaw bearings
and thereby prevent static charges from dissipating through the bearings
and pitting them.
1991 mick sagrillo
mick sagrillo avoids being zapped at lake michigan wind & sun, e
3971 bluebird rd., forestville, wi 54213 |