Generator and Electrical System - Small Wind Turbines: Analysis, Design, and Application - page 248

Fig. 11.14 Lightning
damage at the trailing edge
near the tip of a Westwind
20 kW wind turbine blade at
the CSIRO Energy Centre,
Newcastle. Photo courtesy of
Phil May, Solartec
Renewables

where T d is the number of thunderstorm days per year, 8 another value which can be
obtained from meteorological services.

Example 11.2 What is the worst case estimate for the number of lightning strikes
on the Aerogenesis 5 kW turbine in Australia?

Answer From the map from footnote 7, the maximum value of N g is 12 in the
tropical north close to Darwin. The tower height is 18 m, and the blade radius is
2.5 m so that H = 20.5 m. In the worst case, the turbine is on a hill. From ( 11.6 )
the value of N d is
N d ¼ 12 9p 20 : 5 2 2 10 6 ¼ 0 : 285 direct strikes per year
or nearly six strikes in the 20 year lifetime of the turbine.

Example 11.3 Figure 8 of McNiff [ 17 ] gives the maximum number of thunder
days per year for the USA as 100 (in Florida). Estimate the number of strikes per
year on the turbine of the previous example.

Answer From Eq. 11.7 N g & 10, leading to the estimate of nearly five strikes in
the 20 year lifetime.

These examples demonstrate a potentially serious risk. Lightning often strikes
the blades rather than the nacelle or tower, and can cause major damage even to
composite blades as documented in McNiff [ 17 ]. Lightning damage to the tip
region of a small turbine blade is shown in Fig. 11.14 . Most strikes occur near the
tip which is the least critical area of the blade's structure and so they can often
be repaired easily, provided attention is paid to maintaining the blade mass and
centre of mass for balance purposes: the blade in Fig. 11.14 was repaired at the
University of Newcastle and was returned to service.
Although small turbine blades do not require lightning protection, it is advisable
to include lightning conductors for other components where appropriate: for
example, between the generator casing and the tower, if the casing is mounted on

8

For the USA this information is shown in Fig. 8 of [ 17 ].