Category A uncertainty in electrical power was calculated by statistical analysis based on the standard deviation of power outputs in bin i divided by the square root of the number of sampled data in bin i. Category B uncertainties in the power output, air density and method were obtained by applying the uncertainty estimates provided in IEC 61400-12-2. Category B uncertainty in the method is the uncertainty associated with air density correction, dynamic power measurement, seasonal variation, variation of rotor inflow, and the turbulence effect on averaging and binning. Category B uncertainty in wind speed will follow in the next section.
4.1. Uncertainty in Free-Stream Wind Speed from Nacelle LIDAR
Category B uncertainty in wind speed includes the uncertainty in NTF, which contains the uncertainty in free-stream wind speed. Although the uncertainty component in free-stream wind speed from a met mast, uFS,Cup, can be calculated in compliance with IEC 61400-12-2, the uncertainty in free-stream wind speed from nacelle LIDAR, uFS,NL, cannot be computed because of no regulation in current IEC standards. Thus, the following components were taken into account according to the following references:
- -
The statistical uncertainty of the nacelle LIDAR measurements (u
FS,NL1) [
8];
- -
The uncertainty caused by flow distortion due to terrain (u
FS,NL2) [
8];
- -
The uncertainty related to the measurement height (u
FS,NL3) [
27];
- -
The uncertainty of the tilt inclinometers (u
FS,NL4) [
27].
uFS,NL1 was calculated from the standard deviation of nacelle LIDAR measurements in bin i divided by the square root of the number of data in bin i. uFS,NL2 was estimated to be 2% of the wind speed given in IEC 61400-12-2 because the distance between the test wind turbine and the measurement point was less than three times the rotor diameter, and no site calibration was undertaken.
Because the nacelle LIDAR tilts due to the motion of the wind turbine nacelle by wind variation, the tilt of the nacelle LIDAR should be set at −2.5% of the hub height to measure wind conditions. In addition, nacelle LIDAR measurements should be performed within ±2.5% of the hub height.
Figure 5 illustrates the nacelle LIDAR measurement height relative to the hub height along with the wind speed ratio. The mean values of the bin interval of 0.5 m/s are presented as well. The measurement height increased with an increase in the wind speed until the rated speed and then steadily decreased. This is because of blade pitching, which decreases the thrust force on the rotor after the rated wind speed. It was confirmed that the nacelle LIDAR measurement was carried out within ±2.5% of the hub height.
The uncertainty caused by variation in measurement height due to tilting motion, u
NL,FS3, can be calculated using Equations (4) and (5) [
27]:
where V
NL,i is the average nacelle LIDAR wind speed in bin i, and V
hub,i is the wind speed extrapolated to the hub height. z
NL,i is the average nacelle LIDAR measurement height in bin i, and z
hub is the hub height. In Equation (5), the power law exponent was assumed to be 0.5.
Figure 6 shows the relative uncertainty in the nacelle LIDAR wind speed due to the tilt motion, u
NL,FS3, with wind speed ratio. Uncertainties of approximately 0.12% were distributed in low wind speed regions and were very close to zero at 56% rated wind speed. Next, the uncertainty rapidly increased until the rated wind speed, and it gradually decreased after the rated wind speed due to blade pitching.
To evaluate u
FS,NL4, the calibration of the tilt inclinometers was carried out in accordance with the procedure in DTU Wind Energy E-0020 [
32,
33]. The opening angle, α, was first calibrated by an iterative process of blocking and unblocking using a jig designed for beam detection at a distance of 29.85 m. The measured opening angle was confirmed to be 30.06°, and the maximum error of beam detection pointing, ΔH, was 21 mm. Next, the tilt value was measured using a theodolite Leica TM50. The uncertainty for tilt, u
β, was estimated using the following equation:
where Δβ
T2 is the standard uncertainty of the theodolite associated with the tilt measurement, which was 0.03° according to the instrument calibration report. u
β can be used to obtain the vertical length at a measurement distance of 2.5 times the rotor diameter, Δz
1, using Equation (7) [
27]:
Finally, u
FS,NL4 can be determined by the following equation assuming that wind shear follows a power-law profile with a shear exponent of 0.2:
Additionally, the two sensitivity factors for category B uncertainty in wind speed for estimating power curve uncertainty, c
V,PC,i, and AEP uncertainty, c
V,AEP,i, were calculated using the following equations [
22]:
where P
i + 1, P
i and P
i − 1 are bin-averaged power output in bins i + 1, i and i − 1, respectively. V
i + 1, V
i and V
i − 1 are bin-averaged wind speed in bins i + 1, i and i − 1, respectively.
The sensitivity factors for category B uncertainties in the air density and method were also evaluated using other related equations presented in IEC 61400-12-2.
4.2. Combined Standard Uncertainty
Finally, the combined standard uncertainties of the power curves, u
c,i, were evaluated using the following equation:
where u
i is the combined category B uncertainty. u
T,i and u
B,i are the respective uncertainties in air temperature and pressure in bin i. c
T,i and c
B,i are the respective sensitivity factors of air temperature and pressure in bin i.
Figure 7 shows the combined standard uncertainties in PC
NTF, NL and PC
NTF, Cup of wind turbine no. 15 with that in PC
Cup, which was a reference complying with IEC 61400-12-1 1st edition. For all the uncertainties, higher uncertainties were generally found between wind speed ratios of 0.5 and 0.9, while lower uncertainties were estimated for the other wind speed ratios. The uncertainties of power curves from NTFs were higher than that of PC
Cup. Because the uncertainty of PC
NTF, NL was similar to that of PC
NTF, Cup calculated in compliance with IEC 61400-12-2, the NTF derived from the nacelle LIDAR measurements could be utilized to estimate power curves without a met mast.
Figure 8 presents the individual uncertainties of components in category B uncertainties for PC
NTF, NL of wind turbine no. 15. It was clear that the combined standard uncertainty of PC
NTF, NL originated mainly from wind speed. The high uncertainties of PC
NTF, NL and PC
NTF, Cup after the rated wind speed (
Figure 7) resulted from the uncertainty in the method, which was not taken into account when the uncertainty of PC
Cup was estimated. The power output, the temperature and the atmospheric pressure did not have a significant impact on u
c,i.
The individual uncertainties of each component in wind speed in
Figure 8 are further presented in
Figure 9. The uncertainty of NTF derived from the nacelle LIDAR measurements was the highest among the uncertainty components. All other uncertainties related to the nacelle anemometer, such as operational characteristics and mounting effects, had comparatively low uncertainties of less than 100 kW. From
Figure 7,
Figure 8 and
Figure 9, the combined standard uncertainty of PC
NTF, NL, was confirmed to result mostly from NTF because the uncertainty for each bin of NTF in
Figure 9 was slightly lower than the combined standard uncertainty for each bin in
Figure 7.