Approaches to verifying the power curve of a wind turbine

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The power curve of a wind turbine is the relationship between its available electrical power and the wind speed to which it is subjected. Its measurement is a valuable tool, as it allows the evaluation of the performance of the wind turbine and determining whether production is above or below expectations.

Wind turbine manufacturers provide, for each model, a theoretical curve obtained in the laboratory, which they give as a guarantee for the performance of that same wind turbine. However, sometimes it is necessary to check if the wind turbine, in a specific location with specific atmospheric conditions, complies with the curve guaranteed by the manufacturer.

But how to characterize the power curve? The analysis of the energy performance of a wind turbine is, as a rule, done through a standardized method, widely used, but also expensive. This is why INEGI's team of consultants sought to create an alternative that would help wind farm operators to extract more value from their projects.

Now let's see the difference between the normalized and simplified approach:

There is a standardized methodology for the process of verifying the power curve of a wind turbine (published in the IEC 61400‑12-1[1] standard). This provides for the installation of a meteorological station in the vicinity of the wind turbine, to measure several meteorological variables, such as the wind speed at the height of the rotor axis, for direct comparison with the electrical power of the wind turbine under test.

This standard aims to provide uniform methodologies that guarantee consistency, precision and reproducibility in determining the energy performance of wind turbines, which is why it is divided into three main steps:

In a first phase, the evaluation of the terrain makes it possible to assess the relevance of the obstacles present, and of the variations of the terrain in the measurement of the wind characteristics in the place where the performance of the energy produced by a particular wind turbine will be evaluated in the future. Through this evaluation, the need to carry out the calibration test of the site is verified.

When the terrain is considered complex, the site calibration test serves to evaluate the distortion of the terrain between the place where the station is installed where the wind characteristics will be measured and the place where the future wind turbine will be installed, resulting in the determination of distortion factors between the two locations. These factors will later be used in the verification of the wind turbine power curve. This test is carried out before installation of the wind turbine, as it requires the use of a temporary weather station installed at the location of the future wind turbine, necessary to obtain the aforementioned factors of atmospheric flow distortion caused by the shape of the terrain.

Finally, the verification of the power curve is carried out with the objective of evaluating the performance of the wind turbine in the specific place where it was installed. This assessment is carried out using an on-site measurement campaign, through the simultaneous measurement and collection of the wind characteristics and the power produced at the output of the wind turbine (using equipment installed for this purpose), for a sufficiently long period to guarantee significant  coverage of range of speeds, wind directions and weather conditions. In addition to obtaining the measured power curve, the annual production energy is also estimated, both complemented by the analysis of the uncertainties associated with the measurement.

Power curve measurement is an expensive process whose need must be well-founded. But the aspect that most limits the impact of this test in terms of performance evaluation is the fact that the results refer exclusively to the wind turbine that is the target of the test.