Generator and Electrical System - Small Wind Turbines: Analysis, Design, and Application - page 247

Note that car electrical cables are often only rated at 32 V DC due to their
insulation properties. Most cables have a voltage rating, but usually any mains-
rated cable will have a voltage rating high enough for most wind turbines.
However, the current ratings may be based on intermittent use and should be
calculated using Table 11.4 .
In addition to resistance, cables have inductance. The inductance reduces the
speed at which currents can change, and can cause over-voltage spikes. The
inductance can be reduced by keeping the cables near to each other, so as to reduce
the enclosed area of the cable.

11.8 Lightning Protection

Lightning strikes tall earthed items, as they provide a lower impedance pathway
for the electric charge to earth than the air. Wind turbines can be particularly
susceptible to lightning strikes, as the windiest site is often an exposed hilltop. IEC
61400-2 [ 14 ] specifically excludes lightning protection as a requirement for small
wind turbine blades whereas protection is mandatory for large blades by IEC
61400-24 [ 15 ], even though they are made of composites. This standard is
unusually comprehensive with details on calculating risk, and advice on protection
for large blades with metallic lightning conductors embedded in the blade. Annex
N gives valuable information on protection of small grid-connected turbines. Some
of the design and operational issues related to lightning protection are discussed by
Rachidi et al. [ 16 ]. McNiff [ 17 ] reports on measurements of strikes on large
turbines and gives examples of blade damage.
IEC 61400-24 [ 15 ] (Eq. 2) gives an expression for N d , the annual average
number of direct lightning strikes to a turbine as:

N d ¼ N g A d C d 10 6

ð 11 : 6 Þ

where N g is the annual average ground flash density per km 2 ,A d is the equivalent
''collection area'' for lightning strikes on the turbine in m 2 , so the factor of 10 -6 is
a conversion factor. The determination of A d is described in Sect. 7.1 of the
standard. Briefly and for the simplest case, A d = 9pH 2 , where H is the effective
height of the turbine, which is the sum of the hub height and blade radius. C d is an
''environmental'' factor whose maximum value is 2 for a turbine on a hill.
Values of N g can usually be obtained from meteorological services. 7

Alterna-

tively, Eq. 1 of IEC 61400-24 [ 15 ] gives an estimate for N g of

N g 0 : 1T d ð 11 : 7 Þ

7 For example, the map for Australia can be downloaded from: http://www.bom.gov.au/jsp/
ncc/climate_averages/thunder-lightning/ (accessed 30 Sept 2010).