Page 271 - Fluid Power Engineering

238 Chapter Eleven

Wind turbine
tower
Foundation (reinforcement
bar normally bonded to
lightning protection earth)

Side view

Top view

Earth electrode interconnecting
next turbine
Lightning protection
earth electrode
FIGURE 11-10 Representation of a grounding system around the foundation
of a turbine. 16 (With permission from IEC 61400-24.)

Lightning Protection
16
IEC61400-24 provides guidance on design of wind turbine for light-
ning protection. In order to design a system that is safe during
lightning strikes, pathways must be designed for the lightning cur-
rent to be safely conducted down to the earth without causing unac-
ceptable damage or disturbances to the systems. Traditional tall struc-
tures like buildings and transmission lines use conductors that are put
above and around the structure to provide lightning attachments
points. In a flat area, an attachment point of height h protects an area of
radius 3h. Wind turbines do not have separate conductors, other than
blades, that provide attachment points. Since most modern blades are
made of composites, the blades are not good conductors. In order to
conduct lightning currents, blades have embedded conductors with
attachment points starting a few centimeters from the tip of the blade
and placed all along the length of the blade. Steel wires of 12 mm or
more are used to conduct the lightning current safely to ground and
without damage to the blade. Serious damage to blade occurs when
arcs are formed inside the blade as the lightning current jumps from
the conductor to air chamber or between layers of composite mate-
rial.Theinternalarcscausepressureshockwavesthatcancausesevere
damage, including explosion of blade. From the blade, lightning may
pass through pitch bearings, hub, main shaft, main shaft bearings,
gears, generator bearings, bedplate, yaw bearing, and tower. As the