Page 244 - Wind Energy Handbook

218 DESIGN LOADS FOR HORIZONTAL-AXIS WIND TURBINES

Equation (5.1), superimposed on hub-height wind speed of U r or U o (EOG 1 ) : The
duration of the gust, T, is specified as 10.5 s.

5.4.5 Operational load cases—machine fault states

As noted in Section 5.2.2, it is commonly assumed that the incidence of machine
faults is uncorrelated with extreme winds, so only normal wind conditions need to
be considered in the relevant load cases. IEC 61400-1 specifies two load cases as
follows.

Load case 2.1: Control system fault, with steady hub-height wind speed equal to U r
or U o and normal wind shear. Partial safety factor: normal.

Load case 2.2: Protection system fault or preceding internal electrical fault, with
steady hub-height wind speed equal to U r or U o and normal wind shear. Partial
safety factor: abnormal.

5.4.6 Start-up and shut-down cases

IEC 61400-1 also specifies start-up and shut-down cases with a 1 year return rising
and falling gust, a start up case with a 1 year return direction change and an
emergency shut-down case. In each, hub-height wind speeds of U r and U o are to be
considered.

5.4.7 Blade/tower clearance

In addition to checking the acceptability of the stresses arising from the above load
cases, the designer should also check that none can result in a collision between the
blade and the tower.

5.5 Fatigue Loading

5.5.1 Synthesis of fatigue load spectrum

The complete fatigue load spectrum for a particular wind turbine component has to
be built up from separate load spectra derived for turbine operation at different
wind speeds and from the load cycles experienced at start-up, normal and
emergency shut-down and while the machine is parked or idling. First the cycle
counts for each stress range for 1 h operation in a particular wind speed band are
calculated and scaled-up by the predicted number of hours of operation in that
band over the machine lifetime. IEC 61400-1 specifies that this prediction is to be
based on the Rayleigh distribution (Section 2.4), with the annual mean wind speed