Nacelle-mounted lidar for Power Performance Testing

Webinar Q&A – Nacelle-mounted lidar for Power Performance Testing: Industry prepares to scale up PPT in advance of pending IEC standard

On September 28, 2021, Leosphere, a Vaisala company, along with DNV, ENGIE North America, Inc., and GE Renewable Energy hosted a webinar where we discussed:

• the ongoing IEC 61400-50-3 classification of the WindCube® Nacelle, performed by DNV, plus the results of a joint industry onshore PPT campaign where nacelle-mounted lidar was implemented
• changing PPT practices and what they mean for the acceptance and commercial application of nacelle-mounted lidar for PPT

Many good questions were posed during the webinar and we didn’t have time to answer all of them, so we are happy to answer the ones that were not addressed here:

IEC 61400-50-3 classification and nacelle-mounted lidar calibration

1) IEC 61400-50-3

Question: When will nacelle-mounted lidar be considered IEC-approved?
Answer: The IEC 50-3 is expected to be approved and published in early 2022. When the standard is published, nacelle lidars which have successfully passed the classification will be officially considered as IEC-compliant measurement instruments for Power Performance Testing. However, nacelle lidars are already accepted (“considered bankable”) and are regularly used for Power Performance Testing, following industry best practices such as EUDP guidelines and IEC 50-3 draft version (CDV). Moreover, WindCube Nacelle is already included in several Turbine Supply Agreements between turbine manufacturers and turbine owners.

Question: Is the IEC-50-3 technology agnostic?
Answer: IEC 50-3 is applicable for any type of nacelle lidars. However, every lidar model has to comply with measurement range and successfully pass the Classification in order to be compliant with the standard.

Question: Are environmental sensitivities studied on a white-box level, per beam? Are there therefore, 5 campaigns (two on-nacelle, three off-nacelle) per beam?
Answer: The sensitivity study is done on a white-box level - for each Line of Sight of the lidar. This sensitivity study is performed at DNV calibration site in Germany. The five campaigns (two on-nacelle and three off-nacelle) are required only for building the evidence base which is another part of the classification exercise. Therefore, the five campaigns are not required for the sensitivity study.

2) Nacelle lidar calibration/validation

Question: Was your lidar validated for each of these distances? If not, how much could validation impact these results?
Answer: The nacelle lidar used for this campaign was not calibrated at a 3rd party test site. However, the lidar was verified against our "golden" nacelle lidar reference. This procedure is done for every WindCube Nacelle lidar at the end of the manufacturing process and after maintenance. It consists of validating the lidar against another "golden" lidar which was calibrated at a 3rd party test site. This validation is done for every Line of Sight (LOS) at 7 range gates. The lidar used for this campaign passed this verification meaning that the wind speed deviation for each range gate of each LOS was between +/- 0.1m/s. Therefore, we do not expect to observe significant variations between measurements at different range gates of the lidar. Moreover, thanks to its pulsed lidar technology, WindCube Nacelle has the same probe length at all ranges meaning that the accuracy is not deteriorated at long distances.

Question: How is the calibration done for WindCube Nacelle?
Answer: The 3rd party calibration of the lidar is usually done following IEC 61400-50-3 CDV and previous industry guidelines such as EUDP. It consists of a calibration of the inputs of the lidar Wind Field Reconstruction (WFR) algorithm, i.e., radial wind speed measurements along the Lines of Sights (LOS) of the lidar. The first step consists of:

• A verification of the scan geometry and inclinometer of the lidar
• A verification of the measurement range
• A calibration of the LOS wind speed and assessment of the LOS wind speed uncertainty

All steps should be detailed in a calibration report provided by the 3rd party at the end of the campaign. Other calibration methods can also be applied, such as black box calibration, and in that case needs to be well documented to obtain a thorough assessment of the Nacelle Lidar measurement uncertainty.

Question: What will be the requirements in terms of verification procedures and intervals?
Answer: For contractual Power Performance Testing, the verification of the performance of the nacelle lidar will be done following IEC 61400-50-3 white box calibration guidelines. Most of the terms & conditions of the contractual PPT campaigns state the calibration state and validity duration of this specific campaign. Based on our experience, we generally state that the 3rd party calibration is valid for two years, provided that the lidar has been properly stored, manipulated and installed following Leosphere and turbine manufacturer guidelines. This means that a calibrated nacelle lidar can potentially be used for several Power Performance Tests.

WindCube Nacelle measurement performance

3) Nacelle lidar data availability

Question: How did the nacelle lidar compared to the other measurement devices in terms of data availability?
Answer: Horizontal wind speed data availability observed for the WindCube Nacelle was approximately 88% 2.5D (317.5m) and approximately 79% at 4D (508m) over the 3-month measurement campaign. There was in fact a type-o in the data counts presented in the slides during the webinar. Finally, the valid data for each concurrent period was 780 hours for WindCube Nacelle, 568 hours for WindCube and 715 hours for the Met Mast suggesting in fact the shortest test period would be with the nacelle lidar. DNV did not specifically investigate availability independent of the IEC power curve measurement filters.

4) Nacelle lidar measurement range

Questions: What is the recommended measurement range for Power Performance Measurements with nacelle mounted lidars?
Answer: 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 that at 2.5D, the free wind speed might not be reached, meaning that 3D might be more representative. Further than 3D, there is a risk of de-correlation between the nacelle lidar wind measurements and the actual wind hitting the rotor, especially for large offshore wind turbines.

Question: Can WindCube Nacelle measure at several distances at once?
Answer: Yes, WindCube Nacelle can measure at 10 range gates simultaneously, from 50 up to 450m; WindCube Nacelle Long Range PPT can measure at 20 range gates simultaneously, from 50 up to 700m. Thanks to its pulsed lidar technology, the probed length and measurement accuracy of WindCube Nacelle is the same at every range gate. This key feature allows to accurately study the wind profile in front of the turbine: it can be used to simultaneously measure the wind speed at values of interest such as 2.5D while accurately evaluating the blockage effect of the turbine.

5) Nacelle lidar measurement accuracy

Questions: How does the nacelle lidar compare to met masts in terms of wind speed measurement accuracy?
Answer: We observe very good correlation factors between cup anemometers mounted on a met mast and WindCube Nacelle lidar measurements. During the calibration of the nacelle lidar, the radial wind speed linear regression is generally represented by a slope within 1% of unity, with a coefficient of determination (R²) > 0.99. During the measurement campaign, with R² >0.99 for horizontal wind speed measurements.

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Source: ACP Resource & Project Energy Assessment Virtual Summit 2021 – Onshore PPT Campaign DNV, GE, ENGIE - Leosphere Presentation

Question: Is there a significant difference in terms of scatter between the power curves measured with the met mast, WindCube vertical profiler and WindCube Nacelle?
Answer: As presented in the graphs bellow, the power curve scatter is lower for nacelle lidars. This can be explained by the fact that, unlike met masts and ground-based lidars, nacelle lidars are always measuring the wind coming in front of the turbine. Due to this better spatial correlation, we can generally observe a lower statistical uncertainty and a less scattered power curve for nacelle lidars.

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Source: ACP Resource & Project Energy Assessment Virtual Summit 2021 - Nacelle Lidar Update - DNV Presentation

Question: Is there a significant "LOS inaccuracy" derived from nacelle movement and vibrations at longer ranges? How do you cope with that?
Answer: The impact of tilt of the lidar caused by the tower bending is considered negligible as it is taken into account in the WindCube Nacelle wind field reconstruction algorithm. Further, this is addressed through the body of evidence requirements defined in IEC 50-3, where it must be proven that a nacelle lidar measurement at a specific range is generally the same as a met mast or ground lidar measurement.

6) Nacelle lidar power curve uncertainty

Question: What is the approximate uncertainty of a power curve verification when using a nacelle lidar?
Answer: Recent studies performed by DNV show that the AEP uncertainty measured by the lidar is in the range of 3-4%. It depends on specific wind conditions at the test site, but these recent results show that the uncertainty is generally lower for nacelle lidars than it is for met masts or ground-based lidars.

Question: In the presented campaign, where the uncertainties calculated based on IEC 12-1 and IEC 50-3?
Answer: Correct, IEC 50-3 was used for the nacelle lidar wind speed uncertainties however the classification values (uncertainty due to environmental conditions) were estimated since the evaluation is not complete. IEC 12-1 uncertainty was used for all other measurement systems, power and air density measurements.

7) Nacelle lidar for operational Power Performance Testing and other applications

Question: Could you characterize a new Nacelle Transfer Function (NTF) based on the WindCube Nacelle data?
Answer: WindCube Nacelle can be used to verify OEMs’ NTF or produce a new NTF. NTF sometimes is an important component in quantifying the proxy generation and revenue for the project in certain PPA structure.

Question: For the NTF: What was done to accommodate how the nacelle lidar itself will distort the flow over the nacelle, thus potentially changing the nacelle transfer function?
Answer: Provided that the lidar is not installed in from of the turbine anemometer, the nacelle lidar itself causes negligible disturbance to other wind sensors on the turbine.

Question: Can we install the lidar sensors for wind farm in operations? What are the benefits of utilizing PPT for operational wind farms?
Answer: WindCube Nacelle PPT can also be installed on currently running (operational) wind farms, it can be used for various applications: verification of turbine yaw alignment, characterization of the Nacelle Transfer Function (NFT), Power Curve verification before and after turbine upgrades. The WindCube Insights Analytics software tool can be used to leverage nacelle lidar data and perform such turbine performance optimization analyses. Additionally, WindCube Nacelle TC (Turbine Control Configuration) can be used for LAC (Lidar Assisted Control) of wind farms.

Question: Assuming that having a calibration for NML on a wind turbine which is from a given wind farm, can we use this NML for power curve verification on the same wind turbine type of another wind farm?
Answer: The NML calibration is performed prior to turbine installation and is generally independent from turbine type. It will be more important to check that the measurements used in the body of evidence for the classification are within certain rotor diameter and measurement range conditions defined within clause 8.3 of IEC 50-3. The validity of the calibration duration is up to guidance associated with the use case and advice of the lidar manufacturer.

8) Complex terrain

Question: Can nacelle lidars be used in complex terrain?
Answer: Complex terrain measurements are not part of the scope of IEC-50-3. However, procedures for complex terrain measurements already exist and show promising results. There are different approaches: site calibration using nacelle lidar or induction zone measurements. A specific, industry accepted roadmap has not been defined, but it would be critical to get some complex terrain measurements in parallel to fully IEC compliant met mast measurements at various sites to test, validate and refine the existing methods.

9) Turbulence Intensity

Questions:

• How are Turbulence Intensity measurements handled?
• How does someone have to cope with the TI measurements considering only NML without any met mast support?

Answer: WindCube Nacelle PPT can be used for Turbulence Intensity (TI) measurements as a standalone device. Different TI values are measured by the nacelle lidar, as direct output in the 10mn excel files:

• Radial TI along the 4 Lines of Sight (LOS). Field results and DNV studies show that this value is well correlated with TI measured by cup anemometers.
• Reconstructed TI at hub height. These values follow cup anemometer trends but tend to underestimate the TI value. The reason for this difference is that nacelle lidars are doing a volume averaging while cup anemometers are measuring at a single point.

There are some ongoing projects to improve the TI reconstruction at hub height which already give promising results.