Page 231 - Wind Energy Handbook

ERRORS AND UNCERTAINTY 205

ties in the measurements are converted into uncertainty in the measurands by
means of sensitivity factors following the ISO methodology (1995).
Table 4.1 lists the minimum selection of parameters which must be included in
the analysis; all are category B except the variability of electrical power which is
determined from the scatter in the bins.

4.11.2 Sensitivity factors

The sensitivity factors indicate how changes in a particular measured parameter
affect the relevant measurand. For example, temperature measurements are used to
calculate the air density used in the power curve calculation through correction of
the wind speed or power. We are interested in the rate of change of power (the
measurand) with temperature, i.e. the gradient @P=@K. From the correction formu-
la, this factor is P i =288:15 (kW=K). Similarly the sensitivity factor for air pressure
measurement is P i =1013 (kW=hPa).
The sensitivity factor for the impact of wind-speed error on the power curve is
given directly by the local gradient of the power curve, calculated from:

P i P i 1
(4:16)
U j U i 1

Table 4.1 Uncertainty Components

Measured parameter Uncertainty component
Electric power Current transformers
Voltage transformers
Power transducer
Data acquisition system
Variability of electric power
Wind speed Anemometer calibration
Operational characteristics
Mounting effects
Data acquisition system
Flow distortion due to terrain
Air temperature Temperature sensor
Radiation shielding
Mounting effects
Data acquisition system
Air pressure Pressure sensor
Mounting effects
Data acquisition system
Data acquisition system Signal transmission
System accuracy
Signal conditioning