50 This part of IEC 61400-12 specifies a procedure for measuring the nacelle transfer function of 51 a single electricity-producing, horizontal axis wind turbine, which is not considered to be a small 52 wind turbine per IEC 61400-2. It is expected that this standard shall be used when a valid 53 Nacelle Transfer Function is needed to execute a power performance measurement according 54 to the IEC 61400-12-2 standard. 55 56 A wind speed measured on the nacelle or hub of a wind turbine will be affected by the turbine 57 rotor (i.e. speeded up or slowed down wind speed). Therefore, the nacelle-measured wind 58 speed shall be corrected for this flow distortion effect. Procedures for determining that 59 correction will be included in the methodology. In IEC 61400-12-1, an anemometer is located 60 on a meteorological tower that is located between two and four rotor diameters upwind of the 61 test turbine. This location allows direct measurement of the free wind with minimum 62 interference from the test turbine s rotor. In this IEC 61400-12-6 procedure, the anemometer is 63 located on or near the test turbine s nacelle. In this location, the anemometer is measuring a 64 wind speed that is strongly affected by the test turbine s rotor and the nacelle. The procedure 65 in this standard includes methods for determining and applying appropriate corrections for this 66 interference. However, it must be noted that these corrections inherently increase the 67 measurement uncertainty compared to a properly-configured test conducted in accordance with 68 IEC 61400-12-1. 69 70 This IEC 61400-12-6 standard describes how to characterise a wind turbine s nacelle transfer 71 function. The nacelle transfer function is determined by collecting simultaneous measurements 72 of nacelle-measured wind speed and free stream wind speed (as measured on a meteorological 73 mast) for a period that is long enough to establish a statistically significant database over a 74 range of wind speeds and under varying wind and atmospheric conditions. The procedure also 75 provides guidance on determination of measurement uncertainty including assessment of 76 uncertainty sources and recommendations for combining them.