Application of a Speed Exclusion Zone Algorithm on a Large 10MW Offshore Wind Turbine

To optimize tower design it is desirable to relax the constraint of having to place natural frequencies outside the rotor and blade passing frequency-bands. A speed exclusion zone algorithm is tested to assess its effectiveness on preventing tower resonance caused by 3p thrust oscillations. The effect of the speed exclusion zone algorithm on power output is also considered. A simulation model adequate for qualitative testing of the algorithm is developed and implemented in the simulation tool Simulink by MathWorks. The model is based on a 10MW offshore wind turbine and includes a simplified representation of rotor aerodynamics, drive-train and generator dynamics and a two-dimesional finite element representation of the tower and support structure. To support simulations a rotor speed controller is implemented together with wind models accounting for wind shear, tower shadow, turbulence and rotational sampling. A significant reduction in tower vibration is seen with a ±10% speed exclusion zone. A exclusion zone width of ±5% is shown to increase tower vibrations in some cases. The speed exclusion zone algorithm will lead to a reduction in power output for a ±10% and ±15% speed exclusion zone. For the ±5% speed exclusion zone no significant change in power output is observed.