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Lightning Protection Mick Sagrillo
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? Good! 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.
There are two problems associated with lightning. The obvious problem presented by thunderstorms is a direct lightning strike to the tower and wind generator. Lightening 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.
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. Lightening rods work because they bleed off any static charge that builds up on whatever they're attached to. This is good! 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!
Wind generators and towers are made of metal. Metal has a low resistance, or low impedance, to the flow of
©1991 Mick Sagrillo
Home Power #24 • August / September 1991 53
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