General

A Power Curve is a relationship between free wind speed at the turbine location and the associated expected power being produced. Power curve warranties are included in contracts with the OEM, to assure the wind turbine performs according to specification. In an energy production calculation, a power curve supplied by the turbine supplier is used to determine the amount of energy a project can expect over 8,760 hours per year of operation. However, the provided power curve for the specific turbine model is only a representation of what could be achieved by the turbine model on simple (flat) terrain whereas more complex terrain may yield a different result in energy production. High wind hysteresis for example, an algorithm the machine is programmed with to avoid continuous start-stop sequences that would fatigue the turbine when wind conditions fluctuate around higher or lower winds, may be different from site to site and impact this loss factor in the energy proforma.

All projects require Power Curve Testing for contractual purposes.  Provided there's not an additional requirement for Site Calibration, Power Curve Testing commences near COD and concludes after 6 months.

Flat Terrain

IEC 61400-12-1 without site calibration is the method used on flat terrain.

Basically, a meteorological tower (met mast) is positioned near a turbine to measure the wind, and a uniform flow at mast and turbine is assumed. The normal uncertainty is 5-8 %, depending on annual wind.

The best position for the mast is in the middle of sector, and it has to be between 2 and 4 diameters in front of the turbine (for instance for a V90 it would be between 180 and 360 meters). It is often erected at a 2.5 diameters distance.

The met mast can be erected before or after turbines are on site.

In cases of complex terrain, site calibration is needed. 

During power curve verification in flat terrain, an undistorted, free sector is determined that assumes that the wind speed measured at the wind mast is precisely the same free flow wind speed that hits the wind turbine at hub height, just before blockage effect of the wind turbine comes into force.  That relation of undistorted, free flow wind speed between mast and turbine cannot be maintained in complex terrain where wind measurements between the met mast and turbine might be affected by topographical and surface roughness characteristics of the terrain.  To calibrate the power curve for this difference, two masts are used to measure the wind and to subsequently derive the relationship between both masts. Having derived the proper statistical relationship enables removal of one mast, and putting a wind turbine exactly at that same position where the measurement mast was located before.  IEC 61400-12-1:2005 describes this procedure.

Depending on the site characteristics as well as in cases where a post-construction site calibration may be impossible (e.g. as a turbine is already operating and economic constraints may oppose dismantling of that turbine for site calibration), we perform Power Performance Verification by aid of the nacelle anemometer as described in IEC 61400-12-2. Preferably the Nacelle Transfer Function (NFC) of another WT and/or site can be taken into account during that process, significantly lowering measurement term and cost.

Not all wind projects need Site Calibration.  Consult your Wind COE lead for what is needed for your project.

Perform before the foundations are Constructed!

Site Calibration must be planned and scheduled prior to the roads and foundation work planned for a given turbine site. 

Complex Terrain

IEC 61400-12-1 with site calibration is the method used on complex terrain.

In this case, a temporary met mast is placed in the future position of the selected turbine, and at the same time the wind farm definitive met mast is erected.

The 2 masts works together for a certain amount of time, until a relationship between the wind measured by the definitive met mast and the temporary met mast can be found.

After, the temporary met mast is disassembled, the WTG is erected in his position and it can be defined if the energy produced is in line with the expected generation with the wind speed (extrapolated using the data of the wind farm met mast and the previously defined correlation).