Structural parameter identification of a 2.4 MW bottom fixed wind turbine by excitation test using active mass damper

While the structural damping is an important parameter in the seismic resistant design, several problems exist in the current design codes such as that the recommended values vary largely between design codes, and that the second mode damping ratio which is required in the design is usually not described. In order to evaluate the damping ratios for both first and second modes of MW‐size wind turbines, a series of excitation tests using an active mass damper were performed in this study on a 2.4 MW offshore wind turbine. First, the sinusoidal test, which gives accurate and reliable results for linear systems, is performed, and results show that the damping ratio for the fore‐aft first mode is 0.2% and the fore‐aft second mode is 2.4% for the target wind turbine. Next the free decay test, which is applicable to systems with the effect from the aerodynamic damping, is performed, and results show that the damping ratios obtained for the fore‐aft first and second mode are similar to those from the sinusoidal test. The damping ratio is 1.2% for the side‐side first mode and 3.2% for the side‐side second mode. Finally, an empirical formula for the damping ratios of first mode is proposed for wind turbines with steel towers using the results from the previous researches and the excitation tests in this study.