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ECN publication
Soft foundation control design and its aero-servo-elastic stability
Published by: Publication date:
ECN Wind Energy 6-6-2018
ECN report number: Document type:
ECN-E--17-075 ECN publication
Number of pages: Full text:
35 Download PDF  

With the increasing size of wind turbines the height of the support structure increases both for onshore and for offshore wind turbines. This will reduce the natural frequencies of the support structure. These lower natural frequencies can be closer to and be excited by turbulence, waves and the rotor rotational frequency. Within the ECN internal project “Soft tower control”, an ACT controller has been designed for a wind turbine with a very low first tower frequency. The purpose for this design is to investigate if there are specific challenges related to controller design for turbines with so-called soft-soft towers, towers with first frequency below the rotational frequency of the rotor (1p). To this end, the D4REL turbine model has been modified by only reducing the tower parameters to achieve a first tower frequency of 0.0855 Hz, according to a structural tower design performed using TURBU. The support structure consists of a steel monopile together with a composite transition piece and tower. A controller is designed for a turbine with a soft-soft tower. Besides baseline rotor speed and power control loops, tower fore-aft damping and tower sidewards damping control loops are designed. Based on the performed analysis in ACT, no issues were encountered in the controller design related to the very low tower frequency. It is concluded that a standard controller design achieved acceptable performance with respect to the rotor speed and power regulation. Tower top oscillations were significant with baseline control, but adding tower damping control achieved dramatic reduction the tower oscillations. Finally an aero-servo elastic stability analysis is performed. The focus of the analysis was on the first tower bending modes. However the dynamics of the complete wind turbine system are considered. Modal analysis are performed with and without the controller included in the analysis. For 11m/s wind speed and the controller included the case without structural tower damping shows instable behaviour when a load simulation with TURBU is performed. It is therefore recommend always to check structural dynamic behaviour and control behaviour with an aeroelastic simulation. The work of the current project is continued in the TKI innovation project IDL Tower.

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