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Know Thine Enemy: What is Lighting?

There is always a charge difference between the earth and upper atmosphere. In a lightning cloud the friction between ice crystals causes this to separate charges, and set up a very large electric field between the top and bottom of the storm cloud. The top is positively charged, the bottom negatively (with respect to earth potential).

Most lightning strikes are from cloud to cloud. This can do damage to electronics by coupling into overhead power and phone wires. Some strikes are from cloud to ground. Most ground strikes are from the bottom of the cloud, resulting in a negative strike (because the cloud bottom is negatively charged). Typical currents during a negative strike are in the order of tens of thousands of Ampere. Rarer are strikes from the cloud top to ground, resulting in a positive strike. Positive strikes carry much more current, in the order of a hundred thousand Ampere!

 

1% of strokes exceed 200kA
10% of strokes exceed 80kA
50% of strokes exceed 28kA
90% of strokes exceed 8kA
99% of strokes exceed 3kA

Since current only flows in one direction during a strike, lightning is in essence a DC event. However, the current rises very quickly to its maximum value, in 10 micro seconds or less, causing a large part of its energy to have AC components in the lower-MHz spectrum.

A lightning strike usually lasts several hundred micro seconds (350 μs is typical), and is actually not a single charge exchange but several in rapid succession. Our eyes are too slow to react, so they appear as one strike.

From an electrical point of view, lightning behaves like an almost perfect current source. The current follows a discharge profile with a rapid rise (10 μs) to a maximum value, then a slower decay (300 μs) back to zero. It will create whatever voltage is needed to follow that current profile. Many tens of millions of Volt is no problem. To put this in perspective: After just having jumped 4 km (2 mi) from the cloud bottom to your wind turbine tower, it is no problem for lightning to draw a spark of a few meters (10 ft) from your inverter to ground!

Lightning Protection

Lightning looks for the path of least resistance to ground. That tall metal tower with your wind turbine on top, at least 60 feet high, and sticking out at least 40 feet over everything else in a wide area around it (that is, if you sited your turbine properly!), may very well be that easiest path. While insurance companies lump lightning in with "Acts of God", we prefer not to rely on higher powers to keep our wind turbine, and especially those associated expensive electronics, from getting converted to scrap metal when lightning hits. Fortunately there is quite a bit that can be done to make your wind turbine, and the electronics such as the inverter, safe from lightning damage!

By using a combination of proper grounding, proper wiring, plus lightning and surge arrestors you can change the odds decidedly in your favour. Not only for wind turbines, but for any item susceptible to lightning or surges, such as solar (PV) panels, and your grid service entrance. This information is not only useful in mitigating lightning damage, it works just as well for 'regular' grid surges. Contaminated grid power is a much more common source of stress or failure for electronic equipment than lightning. Since insurance was mentioned: Most home owner insurance policies will insure renewable energy sources such as solar panels and wind turbines against lightning damage. It is important to talk to your insurance broker about this, since there is no iron-clad guarantee when it comes to lightning that there will be no damage from a strike. Even when you do all the right things.

The information in these pages was extracted from many different sources on lightning protection and grounding of wind turbines, professional communications towers, and HAM radio towers and equipment. It gives an overview of 'best practices' and should not be regarded as a blue-print for your particular installation. Rather, it is intended to convey a way of thinking when it comes to lightning protection that allows you to do the best job you can with the means available to you.

Protection against lightning consists of several major parts that all need to be done right to be effective:

The map on the right shows the average number of days that have lightning for a given area. It gives some idea of what to expect for your area, but it is pretty coarse. If you live in a mountainous area you already know that terrain can play its part to make lightning more likely for your location. For Canadian locations close to the US border just extrapolate from the map.

The number of lightning days is half the story. The tower height also determines how likely a lightning strike is. The next figure puts a number to that. It should really be stressed that this is a crude approximation of the real odds of a wind turbine getting hit by lightning. It does give a number though, and therefore some measure of what to expect. As always, your mileage (or hits, as it were) may vary. Lightning does not read the charts and your turbine may get hit tomorrow, or not al all for years to come. It is all about statistics.

Protecting against lightning and, more in general, power surge damage is not an all-or-nothing choice. There are many shades of gray, or should we say the color of the currency of your choice. The spectrum runs from doing nothing (in which case you better be on friendly terms with the God(s) of your choice), all the way to a grounding network around your tower and structures, with industrial quality lightning and surge arrestors to top it off. Unfortunately, as the quality of protection increases, so does the money needed to get there. When done right, it is quite amazing what level of protection money can buy; the communications towers used by the pros get hit by lightning on a regular basis, and those towers can almost always take direct hits without any damage to the tower, antennas, or electronics. Note the word "almost". There are no iron clad guarantees when it comes to lightning, only degrees of protection. Even the Pros have the occasional bad day. Still, designing and installing with lightning protection in mind will greatly improve your odds of surviving a direct or indirect hit without damage.

By the way, did you know that making your renewable energy installation lightning proof will help 'harden' it to some degree for certain types of EMP. That is electromagnetic pulse, the by-product of a nuclear explosion designed to knock out electronics and communications. Lightning and EMP do very similar things to equipment, and proper lightning protection will go a long way towards EMP protection. The difference is that lightning strikes have an effect up to a few miles, while EMP can destroy equipment thousands of miles away.

 

Links, Notes, and Papers

Polyphaser/Transtector white papers

MTL Surge Technologies white papers

Surge arrestor manufacturers

 

 
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