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

Introduction

For quantifying impacts of grid outages, limitations or delay of supply, procedures for calculating related energy yield loss are required and need to be applied in agreement with the transmission system operators. Such calculations have been performed by the presenter for more than 2200 MW offshore wind farms in Germany, covering periods between several days and some years. The results have been applied for quantifying compensation payments for the delay of the offshore grid supply.

Approach

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. 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 applicability of nacelle anemometer for the described purpose.


Main body of abstract

The German Federal Network Agency (BNetzA) has defined detailed procedures for calculating the impact of outage or delay of supply of the grid for offshore wind farms. Extensive experience has been gained by the presenter in application of these procedures for more than 2200 MW offshore wind farms in Germany during the last years. These procedures have been relevant for a huge amount of compensation payments in the past, and will remain important for future incidents related to the grid compensation. Generally, such or similar procedures are highly relevant for assessment of any other kind of losses.

The special requirements of such calculations will be explained on basis of the experiences gained within these calculations. This includes the specific task of providing a 15-minutes time series calculation of the turbine specific energy yield within the wind farm, while several turbines may be in operation already, and match this with the energy yield counter values.

A special emphasize is given to the utilisation of the nacelle anemometer readings for this purpose, which is part of the applicable procedure in Germany. While the nacelle anemometer can be a helpful instrument for relative performance assessment and operational data evaluation, problems arise when a reference to the absolute value is made, especially in untypical operational situations. Indications for such problems could be derived from operational data analyses of several offshore wind farms.

While nacelle transfer functions may have been measured for some turbine types, 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 relating to the nacelle anemometer in general, is discussed.


Conclusion

The presentation shows the special requirements on procedures for calculating energy yield losses due to grid related issues. 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.


Learning objectives
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 which relates to the nacelle anemometer.