Page 221 - Wind Energy Handbook

WIND-TURBINE PERFORMANCE MEASUREMENT 195

ity also be measured, and corrected for. Accuracy should be such as to give the air
density to a precision of 1 percent. The IEC Draft states that pressure should be
measured at hub height or corrected to that height using ISO 2533. The IEC require
that the data be normalized to two different reference air densities, the average
3
measured air density at the test site (rounded to the nearest 0:05 kg=m ), and
standard conditions at sea level defined as 158C and 1013:3 mbar which, for dry air,
3
corresponds to a density of 1:225 kg=m .
The standard requires that the measured power is corrected by multiplying it by
the ratio of the standard air density and the test air density, calculated from the
temperature and pressure, but only when the average air density lies outside the
3
standard value, plus or minus 0:05 kg=m .
The above procedure is appropriate for stall-regulated machines, but where pitch
regulation is active the results of the correction can be misleading. In these
circumstances it is more appropriate to correct the wind speed using the one third
power of the ratio of the test air density to the standard density. This approach was
adopted by the IEA (1990) and is specified for power levels above 70 percent of
rated. The IEC standard follows this line, and the formulae to be used for the
correction according to the standard are given below.

Density corrections

For stall-regulated wind turbines each 10 min averaged power value should be
corrected as follows. The test air density r T , is given by:

288:15 B
r T ¼ 1:225
T 1013:3
where T is the test air temperature in degrees absolute and B is the barometric
pressure in mbar. The power corrected to standard condition, r S is given by

r S
P S ¼ P T
r T
3
where P T is the measured power and r S is the standard air density of 1:225 kg=m .
For pitch-regulated wind turbines the above approach is applied below 70 percent
of rated power but above that value the following correction should be used:

1=3
r T
U S ¼ U T
1:225
Here the averaged wind-speed value is corrected rather than the power.
Where U T is the measured wind speed in m/s and U S is the value corrected to
standard conditions. After considerable debate, the exponent of 1=3 appearing in
Equation (4.13) has been adopted in the IEC standard, following the IEA precedent.