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WHY PROTECTING WIND TURBINES FROM LIGHTNING IS CRITICAL-
LIGHTNING VS. WIND TURBINES
Introduction
Because wind turbines are usually isolated structures, characteristically located on otherwise
undeveloped hilltop regions, they are very likely to become victims of any and all lightning
strikes occurring in their vicinity.
This inherent vulnerability has been found to be only part of the problem. Research shows that
wind turbines can actually cause lightning under certain conditions. The relatively fast rotational
movement of the wind turbine blade tips can directly trigger electrical discharges, by selfgenerating upward streamers. (Energy Research Journal- 2014)
Wind turbine blades are generally electrical insulators, and insulating materials cannot easily
dissipate accumulating electrical charge. The spinning blades, many over 60 meters in length,
collect charge while brushing against air and precipitation, and even more charge is picked up
by electrostatic induction when charged thunderclouds are overhead.
Researchers believe that the motion of the blades allows them to outrun their corona (the
ionized air created by charge dissipation) that surrounds an object under a high electric field
potential. Electrical corona acts as a sort of buffer that dampens the electric field around
stationary objects and delays the generation of upward streamers. By escaping their covering of
ionized air, the turbine blades become more likely to experience an electrical discharge.
(Journal of Geophysical Research: Atmospheres, 2014)
Solution Providers for an Energized World™
It is reported that lightning damage is the single largest cause of unplanned downtime for wind
turbines. Utility generating equipment is normally located within purpose built shelters providing
ample protection. However, these substantial structures are not available for power generating
equipment located a few hundred feet above the earth. Lightning currents injected into these tall
structures will be subject to reflections at the top, bottom and at the blades junction, with the
fixed turbine base.
Downtime costs, repair and replacement of valuable equipment, and human safety factors must
be seriously considered.
There are a number of possible paths for lightning current to reach a wind turbine generator:
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Strike to Tower
Strike to Nacelle
Strike to Blade
Strike to Power Line
Strike to Transmission Line
A strike on the tower or nacelle can induce voltages and currents in the power cables and
generator windings causing premature winding failures. A strike on a blade will likely lead
currents to the generator through the blade, and to downstream equipment. Surges from
lightning and switching on the power lines may also damage the generator.
Computer Simulated Case Study
Lightning Risk Analysis: Rolling Sphere Model
A method of Lightning Protection Zone analysis may be accomplished using the “Rolling
Sphere” technique. In lightning physics, the “final” striking distance of a lightning stroke is
represented as a function of lightning return stroke current. Various electro-geometrical models
exist to calculate the striking distance corresponding to the return stroke current, and to
describe the anticipated radius of the predicted sphere wherein a lightning strike is likely to
occur. According to the IEEE standard 998-1996, the striking distance to an object is given by
the formula D = 8* I0.65, which is taken into consideration in the subsequent application of the
rolling sphere method. The following Figure shows the variation of striking distance with return
stroke current according to the IEEE Standard.
I
D
D
H
R
Figure 1: Striking Distance vs. Return Stroke Current
In the model under consideration, a typical 150 foot radius rolling sphere is considered to
determine the strike zones for a wind turbine. This radius represents a lighting strike with a 14.2
kA peak current value. The red dots indicate direct lightning strike attachment points.
Figure 2 - Wind Turbines and Rolling Sphere Analysis
Figure 3- Wind Turbine Blades and Nacelle Detail
Lightning Risk Analysis: Electro-Geometrical Model
At any moment, it is estimated that 1,800 thunderstorms are in progress somewhere on Earth.
About 100 lightning bolts hit the ground each second. Historical lightning data has been
compiled to create an Isokeraunic map showing the statistical average number of thunderstorm
days per year, for any area of the world. Figure 4 below shows a side-by-side comparison of
wind energy activity locations and thunderstorm activity in the United States.
Figure 4- Wind Turbines vs. Lightning
A three (3) dimensional model was developed for a typical Wind Turbine. Applying an ElectroGeometric model to the 3-dimensional representation of the facility, all of the lightning
attachment points are calculated and displayed visually. The software searches a sky grid for
possible sources of lightning strikes. The possible sources and termination points are evaluated
for probability and a graphical representation of these locations is created.
The probability of a direct lightning strike to the wind turbine was determined with consideration
of its possible location. The following graphics show the total number of expected strikes per
year for the modelled turbine located at two different Isokeraunic values.
Figure 5Top: Modeled Wind Turbine Located in California, Isokeruanic Value = 10;
Expected Number of Strikes = Slightly More Than Once Every 30 Years
Bottom: Modeled Wind Turbine Located in Texas, Isokeruanic Value = 70;
Expected Number of Strikes = Slightly More Than Once Every four Years
Of course, the wind farm facility will likely have a greater number of wind turbines than the
single one modeled. This allows for some useful calculations. For example, if an owner/operator
maintains approximately 30 turbines in California, then that would be equivalent to 4 turbines in
Texas, in that each site would likely suffer one strike to a turbine per year. Since it cannot be
known beforehand which individual wind turbine will be struck, it is highly suggested that all
wind turbines should be protected.
The relationship between lightning strike probability and the number of wind turbines is very
linear, and for a good approximation the change in risk doubles each time the number of wind
turbines double. The modelled examples are recreated below, with an increase from one wind
turbine, to nine wind turbine structures.
Figure 6Top: Nine (9) Modeled Wind Turbines Located in California, Isokeruanic Value = 10;
Expected Number of Strikes = Slightly More Than Once Every Three Years
Bottom: Nine (9) Modeled Wind Turbine Located in Texas, Isokeruanic Value = 70;
Expected Number of Strikes = Slightly More Than Two Times Every Year
Protection Recommendations
Wind turbine blades should be supplied with receptors (lightning strike attachment points) and
adequate lightning strike down conductors. Receptors should be located as close as possible to
both sides of the tip. All direct strike lightning protection components and conductors should be
as low resistance and low impedance as practical. A Faraday cage of air termination devices
(lightning rods) and conductors should be in place around all exposed nonmetallic structural
elements. Isolated and/or insulated metal should be bonded together. Lightning protection
standards allow the use of >/= 3/16” continuous bolted/welded metal as natural down
conductors, and specify copper conductors of >/= 115 kcmil cross sectional area (Class II,
structures over 75’ tall). Corrosion, gaps, sharp bends or loops in conductors can all be a source
of flashover, arcing and blade penetration/damage.
A wind turbine site should include the proper installation of correctly sized and cascaded Surge
Protection Devices (SPDs). The communication systems, generator, transformer and power
collection system should all be protected. SPDs should be connected to a common ground
reference.
In addition, the utility distribution lines connected to the turbines can collect huge lightning
currents, a particular concern in high lightning risk areas. It is suggested that the cable links
from turbines be installed underground and be protected with Medium Voltage surge protection
devices.
Low resistance-low impedance stable grounding systems, using common/shared bonding with
foundation steel and turbine ground electrodes and conductors, are important for wind turbines.
A turbine grounding system should form a counterpoise (loop) around the base of the structure
and should be linked to other similar grounding systems in the wind farm, forming a common
ground grid.
The Systematic Surge Protection Device (SPD) Solution
The time to review possible lightning effects upon wind turbines is during the site selection and
design stages. This important process is often overlooked until later when problems arise. Then,
a specialized lightning safety engineer is required to analyze the effects of lightning during
operations and provide a rationale for “safety-through-redesign” modification to the wind farm
facilities. The cost of creating a stable lightning mitigation system for wind turbines is estimated
to be merely 0.50%-0.75% of the total capital costs.
Important SPD Protection Locations Include:
The following graphic illustrate typical locations where properly installed surge protection
devices are required to safeguard different aspects of the wind turbine, from base to nacelle.
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AC power distribution transformer – connection of the utility medium voltage feeder to
the wind turbine site.
DC side of the inverter – typically 750V
Aviation Warning Light – typically 120V single phase for older lamps, system
requirements may be different for LED lamps.
Data and Control Circuits – may require surge protection if they are exposed to surge
environments.
If the wind farm has a UPS System as part of the meteorological instrumentation package may
require SPD protection. A typical MET station operates at 24 Vdc, possibly with a solar panel,
an AC trickle charger and batteries.
Figure 7- SPD Protection Locations (Example)
Lightning Strike Counters
A large wind farm may be subject to thousands of lightning strikes. Quickly and easily
determining which wind turbines have actually received a lightning strike can make the
difference between an initial repair cost of around $10,000 and a later one of $100,000 or more.
Installation of a lightning strike counter can assist with inspection and maintenance at very little
cost.
Conclusion
Exactly when and where lightning will strike is an unpredictable act of nature. If scientists could
calculate the exact location of future lightning, the United States would be able to avoid the $2
billion in annual property damage due to this awesome yet terrifying natural phenomenon. The
fact that lightning usually strikes the highest point is indisputable. Height, shape and isolation
are all leading factors in determining the place where lightning will choose to strike. One of the
worst possible courses of action during a storm is to stand under a tree; in fact, this is the
number one leading cause of death or injury from lightning strikes. Wind turbines act in a similar
manner, as they are the tallest point and sustain a high probability of being struck by lightning.
Therefore, it is often not a matter of “if” but “when” a wind turbine will be struck by lightning or
experience other types of overvoltage and currents.
With the US presently ranking as the world leader in both wind energy generation and new wind
energy capacity installation, it is now more important than ever to mitigate the chance of
lightning strike damage to wind farms. A properly installed lightning protection system will
dramatically improve both the cost-effectiveness and reliability of a wind turbine. Without the
system a lightning strike on an unprotected blade can lead to temperature increases up to
54,000 degrees Fahrenheit and result in an explosive development of air within the blade.
According to the updated National Fire Protection Association handbook, “While physical blade
damage is the most expensive and disruptive damage caused by lightning, by far the most
common is damage to the control system.” Wind turbines have a concentrated amount of very
expensive technology installed in a relatively small space, and the presence of many different
voltages in a wind turbine installation can easily lead to overvoltage and surges within the
system.
The Department of Energy’s “20% Wind Energy by 2030” report examined the realistic
achievability of using wind energy to generate 20% of the nation’s demand for electricity by
2030. With wind turbine projects accounting for nearly 42% of all new energy generation in the
United States, it is becoming more probable that the United States will make that goal a reality.
For this reason, it is now more important than ever to carefully consider lightning protection,
grounding, and surge suppression technologies companies. ALLTEC has the knowledge,
experience, reputation, and superior quality products to provide the optimal solution for your
wind farm’s power quality, grounding, and lightning protection needs.
The ALLTEC “Protection Pyramid”
As an ISO 9001 registered full-service company, ALLTEC specializes in engineered solutions
which reduce the risks associated with direct and indirect lightning strikes, as well as
diminishing the hidden effects of surge events. Offering decades of knowledge and experience,
ALLTEC’s recommendations advise the best methods for risk mitigation, and ultimately apply
these evaluations as comprehensively engineered solutions. Our global experience has yielded
specialized knowledge and system expertise for a wide range of applications, and our
organization maintains one of the most knowledgeable and experienced technical staffs in the
world.
ALLTEC offers a strategic approach to meeting our clients’ needs under the ALLTEC Protection
Pyramid™. This approach looks at all aspects of a facility and works in a holistic method to
make sure all areas are protected with an effectively interlocking defense. We make sure your
operation is the safest possible facility, backed by our team of dedicated risk-mitigation experts.
If you have a current facility or future project that needs grounding/bonding solutions, surge
suppression or lightning protection please contact ALLTEC at either 1-828-646-9290 or onlineinfo@alltecglobal.com