Key steps for wind turbine power performance testing

Contributed by Matt Cramer, ArcVera Renewables

Running power performance tests (PPTs) on operational assets is a highly complex task that requires a significant amount of experience. 

When should I carry out power performance tests?

Power performance testing is carried out to determine the economic value of a wind project and to ensure that projects are performing to expectations. Performance testing allows investors to identify project underperformance, ensure that projects are generating the expected return on investment, and manage investment risk.

By plotting the power generated against the wind speed, the power curve compares actual on-site results to the warranted power curve in order to identify any deviations or anomalies, which are then analyzed to pinpoint the root cause. The OEM will often recommend actions for improving performance, such as adding vortex generators, adjusting control parameters, or increasing the rated power to maximize the annual energy production and potential revenue of a wind farm.

Beyond turbine performance verification, power performance testing is also conducted for regulatory compliance and warranty verification. Performance testing is warranted for new turbine models or models with inconsistent PPT results. PPTs are also recommended for projects in simple terrain and are a must-do in certain site conditions, such as complex terrain, high elevation, low-temperature, and extreme precipitation, that have no proven performance for the selected wind turbine generator (WTG) model. Terrain deviations in wind projects can have significant impacts on the turbine performance over the project lifetime.

The effects of wakes, terrain-induced turbulence, up flow, veer, and shear due to the proximity of turbines to terrain features and to each other, as well as atmospheric conditions, all tend to have a negative impact on turbine performance. Newer methods for power performance testing can account for many of these conditions, as well as reduce the cost of testing, by utilizing remote sensing to quantify shear, veer, turbulence, inflow angles, and other factors to enable normalization of data for these atmospheric factors. The use of remote sensing can also reduce the cost of testing by enabling the use of shorter meteorological towers (or no towers at all in the case of nacelle-mounted LIDAR testing).

How do I select the right PPT provider?

Because testing is carried out according to a standard, PPT methodologies do not vary significantly among providers. However, this is not to say that all providers are equal in quality of service; differences will be apparent in a provider’s troubleshooting methodologies and execution recommendations are given for when to run a test and how to deploy instruments efficiently and accurately. Because the quality of a provider’s methodologies may be difficult to evaluate right off the bat, it is essential to be able to identify other markers of a good PPT provider beforehand. Listed below are two of the most critical parameters for turbine owners to look for when selecting their PPT provider:

1. The most crucial consideration is experience. Experienced PPT professionals will have established best practices for gathering all the relevant information to accurately scope, manage, and deliver the project. They can efficiently resolve or identify problems with the test and keep it on schedule. Experience with factors such as annual weather variations and their impact on PPT helps seasoned professionals determine optimal conditions for testing that can save clients time and money.
2. Experience with the physical deployment of hardware generally improves understanding of results. This is also true for fieldwork; field staff must possess the kind of data analysis experience that enhances the understanding of the test. Best practices in hardware deployment are informed by an understanding of the data, data quality, and uncertainty requirements of the test.

How can providers ensure high levels of confidence in their power performance test results?

A high level of confidence in test results is achieved by ensuring that on-site measurements have high data availability and low scatter.

The quality of the data collected (high versus low data availability) is partly dependent on the weather. For example, tests that run in weather windows with high winds and mild temperatures will generally be time-efficient due to high data availability. On the other hand, cold weather, ice, lightning, and other extreme weather can cause instrument failure and low data availability.

In addition to conducive weather, securing high data availability also requires the constant monitoring of sensors. Monitoring sensors around the clock allows for catching problems early, promptly troubleshooting, and repairing or replacing sensors as needed. Using reliable, high-quality equipment and having a responsive site manager and meteorological tower (met tower) crew available to troubleshoot are prerequisites for successful sensor monitoring.

Scatter is driven mainly by real-world conditions and wind turbine performance. It can be reduced by placing the met tower closer to the turbine (within the lower end of the 2D to 4D range allowed by the IEC). It should be noted, however, that moving the met tower closer to the turbine can also exaggerate the much-discussed “blockage effect” on the power curve test. This method of scatter reduction is indeed a trade-off, so the pros and cons must be weighed carefully.

Additionally, beyond the minimum requirements of the IEC standard, restricting the wind-direction sector can also help reduce scatter. This is done by utilizing the blockage effect, which can sometimes be quantified by segregating data into two different categories: 1) when the wind comes from a direction that places the met tower upwind of the test turbine and 2) when the wind comes from a direction that places the met tower to the side of the test turbine where it is not subjected to blockage from the turbine rotor.

Scenario testing, or “what if” analyses, can be used to troubleshoot inconsistent results and thus increase confidence in those results. By restricting data or, for example, comparing data between different turbine operating states, it becomes easier to identify what might be driving any discrepancies in turbine performance results. A large, geographically diverse wind turbine operational database is required to support this analysis. ArcVera’s extensive PPT database for wind farms across the United States is one of the tools that aids ArcVera in its ability to proficiently perform such detailed analyses.

What are the key steps involved in the delivery of successful power performance tests?

While power performance testing comprises several processes, in general, a PPT requires three overarching phases:

Test planning

The test plan is a highly detailed document that is written in accordance with the IEC 61400-12 standard and depicts all details concerning test methodology. The test plan is a preliminary document that helps the client and test supplier make sure that the test methodology will meet the client’s requirements and that the goals of the test are mutually understood. It requires significant effort and input from multiple parties and involves reducing risks related to compliance with the Turbine Supply Agreement (TSA), IEC 61400-12 standard, test laboratory accreditation requirements, and turbine vendor requirements.

Hardware selection and deployment

Hardware must meet wind turbine vendor, client, and IEC requirements. In addition, additional requirements are sometimes placed on the hardware by the grid operator. Selection and specification of appropriate and compliant hardware is a crucial capability acquired through comprehensive experience with test execution. Particular attention should be given to calibration requirements. Calibration certificates are required by many accreditation bodies, so ensuring that the supplier has those certificates during procurement will help to avoid problems later. Hardware field performance and compatibility with the DAS are other important considerations. Equipment shipping must be coordinated and the enclosure setup tested. Hardware should be shipped directly to the site and installed according to procedures so that the test hardware installation meets the accreditation requirements.

Data collection analysis and reporting

Clear and transparent reporting is key to a successful test. Data must be regularly checked for errors and immediately input into a database that calculates bin completion. Errors due to meteorological events are often challenging for unseasoned analysts to recognize, which is another reason prioritizing a PPT provider’s experience level is so important. For example, one weather condition that is often not identified until after data analysis is icing, which can often lead to detrimental sensor failures. Icing is likely to be filtered out during post-processing, but it can lead to undetected sensor damage if the analyst does not apply deliberate scrutiny. Sensor damage can invalidate test results if unchecked or not addressed promptly.

Experience level also comes into play if the wind turbine vendor requirements include filtering out periods of precipitation, as this involves a degree of professional judgment to decide if conditions such as heavy fog or light mist should be regarded as meaningful precipitation.

Automated data processing and reporting tools exist to allow experts to recalculate the power curve quickly and easily. These tools will not, however, remove the need for experts. For example, subsets of the collected data can be selected by the expert analyst to restrict the test period, valid direction sector, turbine availability definition, wind shear filters, turbulence filters, and other data filters to help troubleshoot inconsistent results. With the proper expertise, DAS and sensor failures due to lightning strikes or other causes can be accurately diagnosed by examining the data.​​

Despite the fact that troubleshooting is a regular occurrence that requires experienced personnel, it is not always possible to require such a stipulation within a time-bound proposal process. Therefore, it is critical for turbine owners to do their due diligence beforehand and seek out PPT providers with enough experience to be able to effectively troubleshoot any unexpected issues, should they arise.

Should I opt for lidar or met towers?

There are two main ways of assessing a site’s wind resource: met towers and lidar.

Meteorological towers (met towers) configured with industry-standard anemometers to gather wind-speed measurements are the wind industry’s most widely accepted wind measurement methodology. Anemometer-based measurements have been used for decades for wind energy measurement application purposes to technically support project financings.

This long history of use translates to an accepted level of measurement uncertainty for project finance technical due diligence requirements. Met towers are particularly cost-effective as tilt-up versions; however, tilt-up towers above 80 meters are typically more expensive and are more difficult to permit. In the United States and many other countries, reliable and expensive aviation lighting is required for towers higher than 60 meters (200 feet), and depending on the structure, higher than 60-meter towers may have decreased weather ratings.

Higher met towers are also structures of unique designs, using steel or aluminum welded lattice structural geometry that introduces wind flow obstruction properties that are not well understood and require unique sensor mounting hardware to mitigate.

Adding to the cost equation is met towers are simply difficult to reuse – the decommissioning and re-shipment of the tower is costly; adding more expense if higher towers often require a concrete foundation that must be removed, and the sensors must be either replaced or recalibrated after each use. The highest quality anemometers use a bearing system that is not User serviceable, requiring the time-consuming and expensive process of sending the sensor back to the factory for bearing replacement and calibration.

Lidar, on the other hand, is small, mobile, and reusable but is not generally accepted as a permanent monitoring system. Lidar has only recently been allowed for commercial tests under Edition 2 of IEC 61400-12-1, though the standard does not allow for the use of lidar in complex terrain. With lower test data scatter, good uncertainty numbers, much lower costs than traditional power curve testing, and faster test time due to reduced time for planning met tower logistics, it is quite likely that lidar will be more widely accepted in the future.

The upcoming IEC 61400-50-3 standard for measuring wind using nacelle-mounted lidar in flat terrain could soon become a game-changer, as the need for met towers could be eliminated entirely for many sites.

While lidar may remove the need for a hub height tower and thereby reduce costs and permitting complications, there are nevertheless other issues and logistical challenges to consider, such as power supply. As well, lidar verification must be done at intervals, potentially leading to longer, more expensive test campaigns. Unlike the anemometer, the lidar data recovery rate is generally not 100 percent, which reduces data availability and could induce delays in test completion, so any cost benefits may be lost.

How do larger turbine sizes impact performance testing requirements?

Larger rotors lead to more scatter in the data, which has partly inspired some changes in Edition 2 of the IEC 61400-12-1 standard, including allowing the use of lidar to measure shear and veer. Because of this new allowance and of the growing frequency of larger rotors, lidar measurement is likely to gain popularity in the coming years.

Since performing PPTs on individual WTGs is likely to become more expensive with higher hub heights, owners will end up testing fewer machines per megawatt of energy production since IEC and warranty requirements do not change with increased size. The number of tests required is typically calculated as a percentage of the number of turbines rather than the overall project capacity. For example, a project consisting of one hundred 1 MW turbines would require more tests than a project of the same size with two 5 MW turbines.

The use of a short met tower and lidar (as prescribed under Edition 2 of the IEC standard) is less expensive and reduces some of the logistical challenges associated with the installation of a hub height met tower; however, there is also higher uncertainty in this type of test. Because of this, ArcVera clients would generally be advised to stick with hub height met tower installation, as prescribed under Edition 1 of the IEC standard, in order to minimize test uncertainty and maximize coverage under the power curve warranty.       

Hub height met towers are likely to continue to be the gold standard, especially since they are still feasible for hub heights of up to at least 120 meters. Nacelle anemometry can be accurate if used together with one or more met towers to create a nacelle transfer function. This method has been successfully used in the past both for warranty verification and to allow evaluation of the performance of all wind turbines at a wind farm. 

Going forward, it is likely that nacelle lidar will be used instead of nacelle-mounted anemometers for many applications. This is particularly probable in the case of offshore wind farms. Offshore sites differ significantly from onshore sites in terms of layout and turbine size. The installation of met towers in the ocean has proven to be costly, time-consuming, and as such, impractical. As the terrain and elevation are consistent, nacelle-mounted lidar tests have become standard practice. 

Conclusion

Power performance testing is executed in order to determine the economic value of a wind project and to ensure that projects are performing as they are supposed to, in turn allowing investors to identify project underperformance and manage investment risk. Power performance testing is also conducted for regulatory compliance and warranty verification and is recommended for projects in simple terrain and other types of site conditions.

Although PPT methodologies do not vary significantly among providers due to the guidance of industry-utilized IEC standards, not all providers are equal in quality of service; differences are most apparent in a provider’s troubleshooting methodologies, test execution recommendations, and instrument deployment approach. Because informed judgment in making those pivotal decisions is what sets providers apart, and because power performance testing is such a complex task, depth of experience is the most crucial consideration for owners to look for when choosing a provider. The best PPT professionals will have used the totality of their experience to thoughtfully establish best practices for accurately scoping, managing, and delivering projects.

PPT services, while standardized, are complex and nuanced. Experienced providers recognize the planning value as well as the most expensive, time-delaying pitfalls. Procurement of PPT services should include careful attention to the firm’s experience doing PPT tests in a variety of locations. It is also critically important to find out who will actually be engaged in the assignment, since some firms tend to field their PPT services work scope using inexperienced, lower-cost staffing, presumably to maximize their margins. Acquiring an experienced PPT provider means having effective field personnel who know how to think through issues, attain reliable data, and maintain the plan-defined PPT schedule, lowering the risk of weather-related and other cost runups.

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About the author:

Matt Cramer, ArcVera Renewables’ Business Development Manager & PPT Technical Services Lead