Testing the power performance of wind turbines is essential to verify that they operate efficiently under site-specific conditions, ensuring that both the expected energy performance and the economic viability of the project are met.
The test evaluates the electric power generated by the turbine based on wind speed and other meteorological factors, using accurate measurement systems like meteorological masts or LiDAR. Key steps and components of the process include characterizing the site-specific power curve, measuring wind and electric power output, assessing performance parameters such as availability, and applying corrections for air density to ensure reliable annual energy estimates.
Standards and methodologies, such as IEC 61400-12-1, provide the framework to ensure accuracy, traceability, and consistency in measuring wind turbine power output.
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The performance test of a wind turbine generator (WTG) consists of the verification of its performance under the site-specific conditions. This means: the WTGs are characterized by a Power Curve (PC) that describes the output of the turbine in terms of generated electric power depending on the wind speed reaching the turbine. That PC does not only depend on the wind speed, but also on other site specific wind conditions, such as: turbulence intensity, wind shear, wind veer, air density, upflow…. that vary from one location to another one. Based on that, a generic PC does not apply to each specific location, but a site-specific PC needs to be considered for each specific project during the development phase and Energy Yield Assessment (EYA), and that site-specific PC needs to be verified once the turbine is working under those site-specific conditions. This is the only way to ensure that both the PC considered during the EYA phase and the economic viability of the wind project are met during the operational life.
The power output is measured in terms of wind speed reaching the WTG rotor and also the electric power generated by the turbine. The wind speed needs to be measured with accurate and precise measurement systems, such as meteorological mast or remote sensing devices as LiDAR (WTG anemometry, such as nacelle anemometry, does not provide sufficient accuracy for the purposes of correctly characterizing the wind conditions reaching the WTG). The electric power generated by the turbine is measured by electric measurement sensors which are installed by the Independent Tester in the turbine. This measurement system is aimed to measure the net power generated by the turbine.
The main parameter related to the evaluation of the turbine performance is the generated electric power, that is measured as a function of the wind speed reaching the turbine. However, that power also depends on other meteorological factors, such as turbulence intensity, wind shear, upflow… that are evaluated during the power curve test.
Availability is also one critical parameter to be evaluated when assessing the performance of the turbines, and can also be evaluated during a power curve test.
The IEC 61400-12-1 is a set of standards developed under the scope of the IEC scheme (International Electrotechnical Commission) that rules how the power curve test of a WTG is to be developed and performed in order to ensure accuracy and traceability of the power curve evaluation. In this way, it is ensured the traceability between the Power Curve informed by the OEMs and the Power Curve that is measured by the different Independent Testing Entities that work under the IEC scheme.
The power output of the turbine depends on the kinetic energy of the wind flow, that is determined by the air density of that wind flow. The IEC 61400-12-1 air correction is the procedure to correct the Power Curve measured under a specific air density to another air density that has been agreed upon as the site-specific air density. The power curve test is performed during a limited period of time that could not gather the yearly average air density. In order to assess the yearly performance of the turbine, a correction from the power output at the measured air density and the yearly air density is required, and that is obtained through the power curve density correction.
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