Lasten - Anhang V: Sektormanagement
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This appendix describes the curtailment or “wind sector management” calculations in SITE COMPLIANCE and LOAD RESPONSE. The detailed curtailment calculations require curtailment settings to be defined on the WTG objects individually and that the Curtailment option has been checked on the main setup tab of SITE COMPLIANCE.
Curtailment in SITE COMPLIANCE
In SITE COMPLIANCE, the curtailment settings will NOT influence the calculation of the ambient wind climate parameters. The ambient wind shear or turbulence is the same no matter if the turbine is in operation or not. However, the wake induced turbulence behind a wind turbine depends strongly on, whether or not the turbine is operating or shut down (or running in a reduced mode). Hence, in SITE COMPLIANCE, the curtailment will only influence the “Effective turbulence” IEC main check, as it combines both the ambient and wake contributions to turbulence. In the Effective turbulence check, the effect of curtailment is quite simple. For each wake situation between two turbines, it is considered whether the wake generating turbine is curtailed or not for each direction wind speed bin. In case the wake generating turbine is curtailed, its wake contribution is omitted for the speed and direction in question. The wake of any further upstream turbines might become important and contribute to the turbulence as the Frandsen wake model only considers the wake turbulence of the nearest upstream turbine (in operation). The wake signature, sometimes called “view angles”, typically has a fixed width of 22 degrees in the Frandsen model (see Appendix II - Frandsen Effective turbulence model). Hence, a simplifying assumption is needed to handle partial curtailment within the direction interval generating the wake. A basic assumption is adopted saying that each degree where the wake generating turbine is curtailed within the 22 degree directional interval, leads to one degree less wake at the receiving WTG.
Curtailment in LOAD RESPONSE
The effect of curtailment on the load calculation for a wind turbine is more intuitive and directly influences the turbine itself in contrast to the effect of curtailment on the wake effects. For directions and wind speeds where the turbine is shut down, it will experience much reduced fatigue compared to being in operation. On the other hand, there is a minor contribution to fatigue from the additional starts and stops required from the implementation of the curtailment rules. The current version of LOAD RESPONSE (windPRO 3.1) implements the main fatigue design load case DLC1.2 “Normal operation”. To calculate the effect of the additional starts and stops enforced by the curtailment would require implementation of the load cases DLC3.1 “Start up” and DLC4.1 “Normal shut down”, but also a detailed time series calculation to estimate the needed number of additional stops and starts. This is not possible with the current version of SITE COMPLIANCE and LOAD RESPONSE, but it is a clear goal to include these elements in the future versions. To compensate for the simplification of not directly accounting for the fatigue of the additional starts and stops, a simple assumption has been made. Whenever a turbine is shut down, the fatigue load (DEL) contribution of wind speed and direction bin is not decreased to zero (or nearly zero), but reduced by a fixed fraction. If the fatigue of the starts and stops was explicitly accounted for, the fraction would be very close to zero, but since they are not accounted for, the fraction is set to 0.5.
This may seem as a very conservative assumption, but, since the DEL is used in the very non-linear fatigue calculation (see Appendix IV - Theory of LOAD RESPONSE and Fatigue, page 105) using the Wöhler exponent, this is not the case. For a Wöhler exponent of m=10, the fatigue damage contribution of a curtailed wind speed and direction bin is reduced to 0.1% and for m=5 it is reduced to 3%.