Lightning & Surge Protection in the Real World
By: Rob Beckers
Most of us do not have unlimited budgets, and lightning & surge protection tends to be an afterthought much of the time anyway. However, with a little thought and planning it is absolutely possible to provide a real level of surge protection without breaking the bank. This section tries to give you some ideas how to get there.
The ideal ground system described in the previous sections is of course not for everyone. It can be simply too expensive in relation to the cost of the wind turbine and electronics, or it can be impractical to install. As always, you have to balance the cost of installation vs. the benefits of a better ground system and surge protection. Part of that equation is an assessment of the risk for your location; if your area sees only a few thunderstorms a year, and your wind turbine is not the most attractive target for a lightning bold in the area, then there may be little reason to spend much money on lightning protection. On the other hand, if you are on a mountain top, or in an area with frequent thunderstorms, then spending a few thousand more on a good ground and professional lightning arrestors can make a great deal of sense. Either way, you should make an effort to create at least a decent ground system, even if it is not perfect, and much can be done with relatively small means.
Copper ground strap can be hard to find, and even though it contains less copper than #6 AWG solid copper wire, it somehow ends up being much more expensive. For long runs of ground wire, such as radials and perimeter loops, the next best choice is solid #6 AWG copper wire. Solid wire is preferred over stranded for buried applications because it lasts much longer in case of galvanic corrosion (depending on the soil ph).
A Ufer ground is a proven way to create a good ground. Wind turbines use lots of concrete, so make it do double duty and create Ufer grounds where possible. This goes for the tower base, and in case of a guyed tower also for the guy anchors. Be sure not to use any wood or polyethylene for forms, pour directly into the ground! In fact, if you have a guyed tower you also have the makings of a radial ground system: Those guy points have to be electrically bonded together anyway, so run #6 AWG radials to the tower base. If you have some money left in the budget, pound ground rods every 16 or 20 feet (for 8 ft. resp. 10 ft. rods) and connect them to the radials. Still have some money left? Run a ground wire around, from guy anchor to guy anchor. All those ground wires need to be buried at least 8″, better yet 18″ deep. For Ufer grounds have the ground connections come out of the concrete on top, not underground, from there ground the guy wires as described in an earlier section. This helps prevent possible corrosion damage to the concrete. Remember, gentle curves when it comes to ground wire.
If a full perimeter ground for your house or structure is not an option, try to run at least a buried (solid) #6 AWG ground wire between the place where the tower ground comes to your house or equipment room, the well casing (if you have one close to the house, and it is made of metal), and the house ground. This will keep part of the ground current that would otherwise flow through the structure outside, where it cannot do damage. Pound in a few ground rods along the perimeter ground, at least at the corner points, and connect them to the perimeter ground wire. The current going through the perimeter ground in case of a lightning hit will be much less than that going through the tower’s ground radials, so fewer ground rods here should be OK to do the job of dispersing the charge.
The turbine power wires coming down the tower really should be inside metal conduit. If you have a tubular tower, use it and run them inside, if not, use EMT. Similarly, those wires should really run underground to the house or equipment shack. If that is not possible then EMT is once again required. While there is some advantage to running the ground wire that accompanies those power wires at least 3 feet away, this can get expensive since it requires a separate trench. If the ground wire has to go in the same trench as the turbine wires, just run it outside of the plastic conduit that contains the power wires. Maybe space it away a foot or so by putting the turbine power wires a little deeper and partially backfilling before placing the ground wire.
A single-point ground is usually impossible for a renewable energy retrofit, but with a little effort it is possible to create ‘islands’ of equipment that all see the same single-point ground on a local scale. What it takes is a proper layout, putting all the electronics in close physical proximity, and running (if possible) a ground strap that all the devices connect to. Better still, mount all the electronics on a metal (aluminum) back plate. Simply cladding plywood (so you have something to screw inverters and other devices onto) with a sheet of metal will do (aluminum flashing can be a good source of cheap sheet metal). Then ground the back plate, and make sure all the devices that are mounted on it are also electrically connected (grounded) to it. Simply screwing through it can take care of this. You still have to use bare copper wire to connect all devices together, code requires it, plus we do not want to solely rely on mechanical connections to a metal back plate, so do not leave this out. This will make an awesome single-point ground on a budget!
We have covered the technical aspects of lightning and surge arrestors, what has not been discussed is the financial side of it. Good surge arrestors are very expensive. Especially 10/350 rated devices, they can easily ring in at $1000 or more for a 3-phase arrestor suitable for a turbine alternator. It should be clear that a single surge arrestor is not going to cut it; The wires from the turbine need an arrestor on both ends, as well as all the other lines connected to the electronics (grid, PV panels etc.). That can bring the tab for serious surge arrestors to $2000 or so without trying too hard. That may not make sense for a small wind turbine worth the same, or for a location that rarely sees lightning. By the same token, it may make perfect sense for an exposed wind turbine with $50K invested in it. What is a person to do, besides turning to religion or drinking? As stated before, the first thing is to provide a good ground system. This can be done with relatively modest means, plus lots of sweat. What a good ground system buys you is that more of the surge energy goes directly to ground, leaving less for the equipment to deal with. This also means that you can use lesser/cheaper surge arrestors and still have a good chance they will be able to do their job when lightning hits. Do not forgo surge arrestors altogether though, they are still needed.
There is a line of surge arrestors that, while not the greatest, provides some protection for a relatively small price. They are made by Delta, ubiquitous in renewable energy installations (and Solacity stocks them, so let us know if you need any). These are silicon oxide based, basically pure fine-grained sand. They are a form of MOV (or SOV as the manufacturer likes to call it, due to the silicon in them). Delta arrestors are rated for 8/20 surges and have good current handling characteristics. If luck is on your side they will even eat a direct-strike. Once (you can tell a Delta arrestor has failed when their housing is blown open). What they do not do well, is their let-through voltage. Despite the manufacturer’s specs, others have measured far higher voltages before the arrestor starts conducting, and higher-yet voltages when large currents are passed through the device (in the thousands-of-Volts range). The other side of the coin is that they are cheap, and do offer a real level of protection on a budget. If this is enough to make the electronics survive is going to depend on the severity of the surge. In case of a direct strike the chances are slim, for indirect strikes your mileage may be better. In any event, if you are not going to invest in industrial-strength lightning protection then please do get a few Delta arrestors and place them in strategic locations. Try to stick to the wiring suggestions presented here, so the best possible use is made of them. You can see the use of Delta arrestors in our Scirocco wiring diagram. They are specified there as a bare minimum in surge protection.
There is another type of surge arrestors that works well, are a little more expensive than Delta but still modestly priced, and with very predictable (and low) let-through Voltages: Those are MidNite Solar’s line of MNSPD sure protectors. These are made for several rated Voltages, both AC and DC, and are internally made up from a number of large MOVs that eat the surge. They have an LED that lights up when the unit is functioning, and when the internal fuse is blown the LED stays dark, that is how you know the unit is no longer functioning. MidNite does not specify what waveform these units are tested or certified for. While their current ratings are high, 50+ kA, it will not be at more than an 8/20 waveform (if that). Still, their let-through Voltage is low enough to provide real protection for electronic devices. They make excellent secondary (surge) arrestors, at a good price (and Solacity stocks them, just contact us if you need any). Make sure to keep the leads short, when connecting them, and don’t coil up any excess wire!
The Delta surge arrestors are good for outside; at the foot of the tower with the stop-switch (for a wind turbine), or at the solar array. Combine this with the MidNite MNSPD arrestors inside, or in the equipment room, on the AC wiring coming in from a turbine, the DC from solar panels, and lastly the AC again that comes out of the inverter (to protect against secondary surges coupling into things like well-pump wiring). If there is a reasonably long line from your inverter to your house distribution panel you could also add another Delta or MidNite arrestor right at the house panel. With lightning it is a matter of divide-and-conquer, and a combination of devices at strategic locations will give you a level of real protection on a budget.
We hope you have found our lightning protection information helpful. If you have any questions, suggestions, corrections, or other comments please let us know. and may Wodan, the old Germanic god of lightning, be smiling upon you!