Lightning Protection

I have two wind turbine installations (ARE 110s) and need to do something about lightning protection. Our turbines have never been struck (and I know it's dangerous just writing that), and the tower is set in clay soil and appears to be adequately grounded. I have ARE's lightning protection recommendations, but the equipment they recommend seems expensive. Do you have guidelines and cost estimates for protecting a tower and turbine from lightning?

Paul Kenyon - Bridport, Vermont

First, I need to state that I am with ARE, and we offer the expensive system that you refer to. Grounding systems first reduce the attractiveness of the tower to lightning, by reducing the charge buildup in the tower. Second, they provide an easy path for the lightning to ground.

If you have clay soil at your site that stays moist all year and you have a ground rod at each tower leg or at the guy anchors and tower base, you probably have an adequate ground. We recommend that all of the ground rods be connected together with a buried, bare conductor and that the same conductor continue on to the controller's grounding system.

A good grounding system is necessary but not sufficient for protecting your system from lightning. Even with good grounding, your tower can still be hit, and even a nearby strike can induce high voltage in the tower and wires. A good lightning protection system (LPS) helps protect your wires and electronics from these induced voltage surges and from most direct hits.

An LPS provides a path between your electrical wires that limits the voltage difference between any two wires and between any wire and ground at that location. The insulation on your wires should withstand 2,500 volts. We first place a lightning arrestor at the top of the tower to protect the wire run down the tower, which should be inside a tower leg or inside a grounded metal conduit. This will reduce the induced voltage from a lightning strike coming down the tower or a nearby lightning strike.

Because there will still be a voltage surge in the wires, an arrestor is needed at the base of the tower to dissipate it. If the tower receives a direct hit, there will be thousands of amps (20,000 A, on average) in the strike. The first lightning arrestor will only limit the voltage to about 2,000 volts at that current, so further protection is needed.

We also place a lightning arrestor just before the controls. At this point, the voltage and current from even a direct hit have been greatly reduced by the first two devices, and the wire run will have slowed the rate at which the voltage rises. This last device can now limit the voltage and current to a value that the capacitors in the controller will absorb without damage.

The whole system is designed to protect the inverters and other controls from 90% of direct hits. Each device is rated to absorb 80,000 amps. In addition to the three devices between the wind generator and the controls, an arrestor at the AC panel is installed to protect from voltage surges coming from the utility or home wiring.

Our LPS was designed to survive 90% of direct lightning strikes without damage to the equipment, and offers a warranty for lightning damage to the inverters and controls we sell with our turbines. Some customers elect to purchase a single device and place it at the base of the tower or at the controller. This approach will provide some protection, but has no warranty. The LPS for the ARE110 lists for $2,500, and includes four high-quality arrestors and extended warranty coverage. A single arrestor lists for $594. You can purchase other devices for less than $100 each, but when we tested a couple of inexpensive units, they did not pass current until approximately 7,000 volts — much too high for adequate protection.

When purchasing a lightning arrestor, make sure that it is MOV- or SOV-based. (Metal-oxide and silicon-oxide varistors are voltage-dependent resistors, designed to break down at specific voltages, shunting a lightning strike to ground.) Also find out the device's current rating. The average lightning strike is 20,000 amps, so a 10,000-amp-rated arrestor will not protect your system from the "average" lightning strike. Place your bets.

Robert Preus, Abundant Renewable Energy - Newberg, Oregon