Page 186 - Fluid Power Engineering

Advanced W ind Resource Assessment 159

1. Onsite 50-year extreme wind speed computed from 10-min
averages is less than the WTG’s V ref .
2. The probability density function of onsite 10-min average
wind speed at hub height is less than the probability den-
sity of Rayleigh distribution with V ave = 0.2V ref for all wind
speed in the range of 0.2 to 0.4 times WTG’s V ref . For most
Weibull distributions with the value of k close to 2, this may
be simplified to: The 10-min average wind speed at hub height
is less than 0.2V ref .
3. Onsite effective turbulence intensity (including wake effect)
is less than WTG’s I ref . The onsite turbulence intensity that is
of interest is for wind speed in the range of 0.6 times rated
wind speed to cut-out wind speed of the turbine.
4. Onsite average wind shear at hub height must be positive and
less than 0.2.
5. Onsite flow angle with respect to the horizontal plane must
be ±8 ◦

In complex terrain, the longitudinal TI must be multiplied by a
5
correction factor C ct :
⎧

2
1 σ + σ 2
⎪
⎨ 2 3
1 +
C ct = 1.375 σ 2 (8-20)
1
⎪
1.15 if σ i is not available
⎩
where σ 1 ,σ 2 ,σ 3 are longitudinal, vertical, and transversal standard
deviation of wind speed. Complex terrain is defined as one that does
not meet the conditions defined in Table 8-4.
Maximum Terrain Variation
Distance Range Max Slope of from a Disc with Radius 1.3 z hub
from Wind Turbine Fitted Plane Fitted to the Terrain
<5 z hub <10 ◦ <0.3 z hub
<10 z hub <0.6 z hub
<20 z hub <1.2 z hub

*z hub is the hub height. A terrain that does not meet these conditions is a complex
terrain
Source: From Wind Turbine, Part 1 Design Requirements. International
Electrotechnical Commission, Geneva, 2005. IEC 61400-1 Edition 3.

TABLE 8-4 Definition of Simple Terrain with Respect to a Fitted Plane that
Passes Through the Turbine Base*