Using the IEC Simple Load Model for Small Wind Turbines - Small Wind Turbines: Analysis, Design, and Application - page 177

M x shaft ¼ GQ design ð 9 : 17 Þ

and 20

M xB ¼ M x shaft = N ð 9 : 18 Þ

where the numerical factor G = 2.0 unless a more accurate value is established.
This factor obviously relates the short circuit torque to the turbine's design torque.
Note that the order of treatment of the loads used in cases A to E the blades first
and then the shaft—has been reversed in this load case and the next. This is done
to follow the order in IEC 61400-2.

9.2.7 Load Case G: Shutdown (Braking)

This load case applies to turbines with some form of mechanical or electrical
braking within their drive train. It is necessary to know M brake , the extra moment
produced by the braking system, either by testing or calculation. M brake must be
multiplied by the gearbox ratio if the braking system acts on the high-speed shaft.
The shaft moment is 21 :

M x shaft ¼ M brake þ Q design ð 9 : 21 Þ

If the turbine has a brake and a gearbox, M x-shaft should be increased to account for
the drive train dynamics. If the necessary factor cannot be determined accurately, a
value of 2 is to be used.
The blade loading caused by the shutdown is calculated by 22 :

M xB ¼ M x shaft = N þ m B gR cog ð 9 : 22 Þ

where the value of M x-shaft is determined according to Eq. 9.21 .

9.2.8 Load Case H: Parked Wind Loading

This load case considers several separate loads acting on a parked turbine, that is, a
turbine whose rotor is not producing power. It is not necessary for the blades to be
stationary while parked. The loads are calculated using a wind speed of U e50 , the
3s 50-year extreme wind speed. The main loading on a stationary rotor is due to
drag 23 :

20

(IEC 36).

21

(IEC 37).

22

(IEC 38).

23

(IEC 39).