Quantification of grid related energy yield losses: Experience gained in German offshore projects

Martin Strack
Deutsche WindGuard Consulting GmbH, Varel, Germany


For assessment of operational performance as well as quantification of lost revenues of a wind farm, procedures for calculating the target yield and quantifying energy yield losses are required. Such assessment can relate to losses from non-availability, service and maintenance, as well as outages, limitations or delay of external grid.

The presenter has performed assessments of operational performance or lost energy for more than 2200 MW offshore wind farms in Germany, based on SCADA and external measurement data, partially for the purposee of compensation claims towards the transmission system operators.


The special characteristics of the calculations and relating requirements on the methodology will be explained, and the experience gained with the performed calculations will be presented. Emphasize will be given to the usability of the nacelle anemometer for such calculations, which is part of the German relevant legislation for grid related compensation. Detailed investigations and measurement campaigns have been carried out to evaluate the perturbations of the nacelle anemometer in specific situations. Conclusions are drawn on the general applicability of nacelle anemometer such purpose.


Nacelle transfer functions may have been measured for some turbine types, but these are only valid for regular operation. For untypical operational situations, no measurements or experience on the perturbation on the nacelle anemometer was available earlier. As these situations became relevant for the described calculations, big effort has been spent on investigation of these, and on two German offshore wind farms detailed measurement campaigns have been performed in order to measure the perturbation of the nacelle anemometer reading in situations like idling rotor and controlled yaw-error of the turbine.
The results of the measurement campaigns are presented in detail, and the impact on the described procedure for quantification of energy yield loss, but also for operational data assessment in general, utilizing nacelle anemometer, is discussed.


The presentation shows the special requirements on procedures for performance assessment or calculation of energy yield losses. The investigation and performed measurement campaigns on two offshore wind farms show the problems and limitations of the application of the nacelle anemometer for this purpose.


The requirements and detected problems can be transferred on any other procedure for quantifying energy yield losses, and are of high relevance for any operational data assessment in general, but especiall all methods which are related to the nacelle anemometer.