Power performance - Assessment of terrain at test site

[Cunhae]

Dear all,
I ´d like to understand how WAT perform the calculation of the maximum slope and variation. It is not quite clear how the software handle this. Tks

[Morten]

Hi Cunhae,


WAT is trying to support the terrain assessments in IEC 61400-1, IEC 61400-12-1 and IEC 61400-12-2 and they are all different. Furthermore, there are variations among different editions of these standards, and sometimes it is not always clear how to interpret them.


When you say maximum slope and variation, I assume that your question refers to IEC 61400-1. First thing to notice is that according to editions 2 and 3 (but not the coming edition 4) the fitted planes must be forced through the tower base. The WAT help file will tell you how this is done in WAT by a least-square-error minimization. Two alternative sets of reference points can be used: 1) all points in the orthogonal grid map lying inside the segments or 2) new points interpolated to a polar grid. I recomend the first method for high-resolution maps and the second method (interpolation) for low-resolution maps. The fit will generally have a tangential component unless you select the option called 'force slope of fitted plane along sector centreline'. The standard does not specify whether this is needed or not, so I found it best to make it optional.


Edition 2 (1999) and the original edition 3 (2005) defined the variation as the maximum distance between the fitted plane and auxilary planes fitted to a small disc at every point of terrain assessment. I suppose the intention was to filter out effect of small terrain features. The rules changed in the ammended edition 3/A1 (2010) so now we check the vertical variation at all points, but only consider the criteria to be failed when exceedance is found for points corresponding to an area larger than 5*sqr(z_hub). The WAT project options will allow you to decide whether you want to calculate the variation as specified in the ammendment or not.


I admit that this can be confusing. There are many options to consider, so please consult the WAT help file and the edition of the IEC standard you want to follow. You can also right-click the root element in the WAT object hierarchy and open the project option window, then you can select file>default option for the relevant edition.


Best regards,
Morten Nielsen