Two Methods for Estimating Aeroelastic Damping of Operational Wind Turbine Modes from Experiments

The theory and results of two experimental methods for estimating
the modal damping of a wind turbine during operation are
presented. Estimations of the aeroelastic damping of the
operational turbine modes (including the effects of the
aerodynamic forces) give a quantitative view of the stability
characteristics of the turbine. In the first method the esti-
mation of modal damping is based on the assumption that a turbine
mode can be excited by a harmonic force at its natural frequency,
whereby the decaying response after the end of excitation gives
an estimate of the damping. Simulations and experiments show that
turbine vibrations related to the first two tower bending modes
can be excited by blade pitch and generator torque
variations.However,the excited turbine vibrations are not pure
modal vibrations and the estimated damping is therefore not the
actual modal damping. The second method is based on stochastic
subspace identification,where a linear model of the turbine is
estimated alone from measured response signals by assuming that
the ambient excitation from turbulence is random in time and
space.Although the assumption is not satisfied, this operational
modal analysis method can handle the deterministic exci- tation,
and the modal frequencies and damping of the first tower and first
edgewise whirling modes are extracted.