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