Terrain assessments in WAT
IEC 61400-1 prescribes turbulence corrections factors for site assessment in complex terrain and rules for evaluation of terrain complexity. The basic idea is first to fit planes to the terrain in various areas around each turbine sites and then to check their slope and maximum deviation from the terrain. This terrain assessment is both implemented for the old rules based on circular areas and for the new sector-based ones
Another standard, the IEC 61400-12-1 for in-situ power-curve measurements, includes similar though different terrain and obstacle assessment. The main issue is whether the terrain significantly alters the flow at the tested turbine compared to the reference mast, which would call for site calibration and extra field measurements. With WAT it is easy to try different reference mast positions, accept or adjust the measurement sector, and evaluate the need for site calibration.
The IEC 61400-12-2 standard for performance measurements using nacelle anemometry prescribes yet another variant of the terrain assessment. Site calibration is not used in this standard but the terrain assessments are needed for uncertainty estimates. The obstacle assessment in this standard is done for obstacle parts with no dimension larger than 50m.
