Q&A: WindCube® Nacelle new IEC classification and enhancements

Vaisala recently announced the new IEC classification and enhancements for WindCube Nacelle, which paves the way for increased adoption and acceptance for state-of-the-art Power Performance Testing (PPT). In our recent webinar, experts from Vaisala, DNV and GE discussed the enhancements and classification and verification processes, along with their industry implications. Today’s blog answers several valuable questions from the webinar.

Classification

I thought "classification" in the IEC classification number ending in 50-3 was replaced by the evidence-based concept to avoid confusion with the classification indication in ground-based lidars and some other sensors.
That is correct. Vaisala and DNV are using the "classification" term for simplification. It refers to the evidence-based approach and sensitivity study as described in Chapter 8 of IEC 61400-50-3 (2022).

Does the new IEC standard regarding lidar for PPT require a met mast calibration, or can we use nacelle mounted lidar alone?
The IEC 61400-50-3 requires calibrating the nacelle lidar laser beams against a mast-mounted reference sensor (such as a cup or sonic anemometer). This calibration has to be performed prior to the power curve measurement campaign. After calibration, the nacelle lidar can be used as a standalone wind measurement device for power curve verification.

How is calibration performed?
WindCube Nacelle third-party calibration is usually completed following IEC 61400-50-3 (2022) and previous industry guidelines such as EUDP. The process consists of calibrating the inputs of the lidar Wind Field Reconstruction (WFR) algorithm, e.g., radial wind speed measurements along the lidar’s Lines of Sights (LOS). The steps are:

1. Verify the scan geometry and inclinometer
2. Verify the measurement range
3. Calibrate the LOS wind speed
4. Assess the LOS wind speed uncertainty

The third party should detail the steps in a calibration report at the end of the campaign.

Should the classification uncertainty used in PPT take the full or actual measurement range?
You can use both the maximum range and the calculated range for your specific measurement campaign. If the air density and wind shear have only been measured during the calibration campaign, the maximum range should be used for the uncertainty calculation. If the air density and wind shear have been measured during both the calibration campaign and the specific measurement campaign, the calculated range can be used for the uncertainty calculation.

Using the calculated range will lead to lower classification uncertainty and therefore lower horizontal uncertainty. Note: WindCube Nacelle can be equipped with an optional weather sensor to measure air density without an additional data logger. In addition, the windshear can be measured with the lidar itself. Therefore, no additional sensors are required to measure the significant Environmental Variables (EVs) and use that information for the uncertainty calculation.

What is the criteria used to define if two EVs are interdependent or not?
The criteria to define if an EV is interdependent is based on the correlation quality between the two EVs.

What are the reference values of the independent EVs?
Air Density [kg/m³]: Min = 0.9, Max = 1.35, Bin width = 0.05, Range = 0.45
Wind spear exponent: Min = -0.4, Max = 0.8, Bin width = 0.05, Range = 1.2

Was the sensitivity study carried out on a single lidar unit at a single test site? Is there no requirement in IEC classification ending in 50-3 for the sensitivities to be derived from multiple lidar units at multiple test sites?
The sensitivity study of the “Classification” does not require multiple lidars or locations, and a single nacelle mounted lidar was used for the sensitivity analysis. However, the evidence-based study requires multiple tests (noted on slide 11 and 14 of the DNV webinar presentation).

What are the reasons for the WindCube Nacelle rotor diameters (97.7-238.5m) and hub height ranges (68.5-171.4m)? Will Vaisala get larger ranges?
The IEC-50-3 classification results are applicable to specific rotor diameters (RD) and hub height (HH) ranges. The RD and HH tested in a new power curve campaign shall be within 30% of the RD and HH of the turbines used for the on-nacelle evidence base.

For the WindCube Nacelle classification, two on-nacelle tests have been performed on two turbines of 127m / 167m RD and 89m / 120m HH, respectively. This means the classification results are applicable to new turbines within 97.7m-238.5m RD and 68.5m-171.4m HH. This suits the size of all recent turbine models. New on-nacelle tests can be added to the evidence base to match future turbine size requirements.

Complex terrain

Is it possible to use WindCube Nacelle for complex terrain sites according to the IEC classification?
Complex terrain measurements are not part of the scope of IEC classification number ending in 50-3. However, procedures for complex terrain measurements already exist with promising results. There are different approaches such as site calibration using nacelle lidar or induction zone measurements.

GE presentation

Did you compare turbine intensity (TI) at the same heights as shear? Have you compared the TI correlation mast vs. lidar when the lidar points at the same cup anemometer heights?
For both campaigns, the nacelle mounted lidar TI was taken from the same elevation as the cup anemometry: at hub height.

What is the influence of the induction zone in the database? How far must we measure to get free wind speed?
IEC standards recommend measuring the wind at between 2 and 4 rotor diameters. Following best practices, the 2.5D distance is generally used. However, recent studies show the free wind speed might not be reached at 2.5D, meaning that 3D might be more representative. Further away, there is a risk of de-correlation between the wind measurement location and turbine position.

One key benefit of WindCube Nacelle is its capability of simultaneously measuring the wind at up to 20 distances in the rotor. This enables power curve measurements at several distances in order to better study the blockage effect of the turbine.

Nacelle mounted lidar acceptance

Where do you think efforts should be focused now to further encourage the adoption of nacelle lidar technology?
GE is gaining experience with calibration of the units following the standard. We measured discrepancies between the devices and the met mast without the calibration: would calibrations close these gaps, thus give a closer match to the existing benchmark: met mast cup anemometry?

Additional campaigns comparing nacelle mounted lidar with met mast will give further confidence in the differences between them. We also aim to see precision and accuracy in the comparisons, and gain the confidence that future campaigns will yield predictable differences between nacelle mounted lidar and a cup anemometer.

There seems to be a comparison between confidence in lidar and the fact that wind turbine OEMs are including more advanced controls so the rotor acts as an anemometer and does not need lidar.
This is an important comparison. GE looks at the nacelle lidar as a necessary independent wind speed measurement. Accurate, independent wind speed measurements—whether they are from met masts, ground-based lidars or nacelle mounted lidars—are critical to validating both performance and loads on the turbines.