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.

mick sagrillo avoids being zapped at lake michigan wind & sun, e 3971 bluebird rd., forestville, wi 54213