46 This part of IEC 61400-12 specifies a procedure for verifying the power performance 47 characteristics of a single electricity-producing, horizontal axis wind turbine, which is not 48 considered to be a small wind turbine per IEC 61400-2. It is expected that this standard shall 49 be used when the specific operational or contractual specifications may not comply with the 50 requirements set forth in IEC Publication 61400-12-1. The procedure can be used for power 51 performance evaluation of specific turbines at specific locations, but equally the methodology 52 can be used to make generic comparisons between different turbine models or different turbine 53 settings. 54 55 The wind turbine power performance characterised by the measured power curve and the 56 estimated AEP based on nacelle-measured wind speed will be affected by the turbine rotor (i.e. 57 speeded up or slowed down wind speed). The nacelle-measured wind speed shall be corrected 58 for this flow distortion effect through the use of a Nacelle Transfer Function (NTF). In IEC 61400- 59 12-1, an anemometer is located on a meteorological tower that is located between two and four 60 rotor diameters upwind of the test turbine. This location allows direct measurement of the free 61 wind with minimum interference from the test turbine s rotor. In this IEC 61400-12-2 procedure, 62 the anemometer is located on or near the test turbine s nacelle. In this location, the anemometer 63 is measuring wind speed that is strongly affected by the test turbine s rotor and the nacelle. The 64 NTF is applied to provide an appropriate correction for this interference. However, it must be 65 noted that such corrections inherently increase the measurement uncertainty compared to a 66 properly-configured test conducted in accordance with IEC 61400-12-1. 67 68 This IEC 61400-12-2 standard describes how to characterise a wind turbine s power 69 performance in terms of a measured power curve and the estimated AEP. The measured power 70 curve is determined by collecting simultaneous measurements of nacelle-measured wind speed 71 and power output for a period that is long enough to establish a statistically significant database 72 over a range of wind speeds and under varying wind and atmospheric conditions. In order to 73 accurately measure the power curve, the nacelle-measured wind speed is adjusted using a 74 transfer function to estimate the free stream wind speed. The procedure to measure such a 75 transfer function is given in IEC 61400-12-6 The AEP is calculated by applying the measured 76 power curve to the reference wind speed frequency distributions, assuming 100% availability. 77 The procedure also provides guidance on determination of measurement uncertainty including 78 assessment of uncertainty sources and recommendations for combining them into uncertainties 79 in reported power and AEP.