Page 241 - Wind Energy Handbook

EXTREME LOADS 215

5.4.2 Non-operational load cases—machine fault state

Examples of load cases in this category are ones involving the failure of the yaw or
pitch mechanisms. On the assumption that there is no correlation between such a
failure and extreme winds, the design wind for this load case is normally taken as
the gust speed with a return period of 1 year. GL rules lay down that this is to be
taken as 80 percent of the 50 year return gust speed, whereas IEC 61400-1 specifies
the lower ratio of 75 percent.

5.4.3 Operational load cases—normal machine state

Several load cases have to be investigated in this category, so that the effects of
extremes of gust loading, wind direction change and wind shear can be evaluated
in turn. Two types of deterministic discrete gust models are used for the gust
loading: the ‘rising gust’ and the ‘rising and falling gust’. In the former case, the
wind speed rises sinusoidally over the time interval t ¼ 0to t ¼ T=2 to a new value,
according to the formula U(t) ¼ U þ ˜U(1 cos 2ðt=T), and remains there. In the
simple version of the ‘rising and falling gust’, the sinusoidal wind speed variation
continues over the full cycle until the wind speed has returned to its original value.
However, IEC 61400-1 defines a more sophisticated version, incorporating a brief
dip in the wind speed before and after the rising and falling gust, the complete
cycle being defined by

ó u
U(z, t) ¼ U(z) 0:37â sin 3ðt=T[1 cos 2ðt=T] (5:1)
1 þ 0:1(D=¸ 1 )
where ó u is the standard deviation of the turbulent wind speed fluctuations, and
the factor â takes the values 4.8 and 6.4 for gusts with recurrence periods of 1 and
50 years respectively. D=¸ 1 is the ratio of the rotor diameter to the turbulence scale
parameter (see load case 1.7, below). The duration of the gust, T, is specified as
10.5 s for the 1 year and 14 s for the 50 year return gust. The gust profile is
illustrated in Figure 5.1.
The ultimate-load cases during normal running defined in IEC 61400-1 are
described below. The standard points out that the cases listed are the minimum that
should be investigated, and that other cases may need to be considered in relation
to specific turbine designs. Note that an inclination of the mean air flow of up to 88
to the horizontal is to be considered in each case. The acronyms in capitals are those
used by the code to identify the types of gust and/or direction change.

Load case 1.1: Hub-height wind speed equal to U r or U o , with turbulence (Section
5.1.2), where U r is the rated wind speed, defined as the wind speed at which the
turbine’s rated power is reached, and U o is the upper cut-out speed.

(Load case 1.2 is a Fatigue Case).

Load case 1.3: Gust and direction change (ECD). Hub-height wind speed equal to U r