Using the IEC Simple Load Model for Small Wind Turbines - Small Wind Turbines: Analysis, Design, and Application - page 176

9.2.4 Load Case D: Maximum Thrust

This is a problematic load because it increases as the square of the wind speed,
so that selecting a sufficiently high speed, will produce very high loads. The
compromise adopted in IEC 61400-2 is to use 16

F x shaft ¼ 3 : 125C T qU ave pR 2 ð 9 : 14 Þ

where C T is the thrust coefficient, taken to be 0.5. The factor of 3.125 arises from
applying ( 9.14 ) at a wind speed U = 2.5U ave , which is claimed to give thrust loads
comparable with those obtained from aeroelastic modeling of small turbine blades.
The thrust must also be transmitted to the tower, making it, along with the turbine
weight, and the gyroscopic loads described above, the main ways in which the
turbine loads influence the tower loads.

9.2.5 Load Case E: Maximum Rotational Speed

The centrifugal loading on the blades and especially the blade hub can become
extremely large at high rotational velocities. The centrifugal load in the blade root
is calculated using 17 :

F zB ¼ m B X 2 max R cog

ð 9 : 15 Þ

and the bending moment in the shaft due to small blade unbalances is found
using 18 :

M shaft ¼ m r gL rb þ m r e r X 2 max L rb ð 9 : 16 Þ

9.2.6 Load Case F: Short at Load Connection

If a direct short occurs across the output terminals of the generator, a large moment
is created on the turbine shaft as a result of an increase in the generator torque,
known as the short circuit torque. The following moments arise from this increase
in torque 19 :

16

(IEC 32).

17

(IEC 33).

18

(IEC 34).

19

(IEC 35).