Applicability Limits of Operational Modal Analysis to Operational Wind Turbines

Operational Modal Analysis (OMA) is one of the branches of experimental modal analysis which allows extracting
modal parameters based on measuring only the responses of a structure under ambient or operational excitation
which is not needed to be measured. This makes OMA extremely attractive to modal analysis of big structures
such as wind turbines where providing measured excitation force is an extremely difficult task.
One of the main OMA assumption concerning the excitation is that it is distributed randomly both temporally and
spatially. Obviously, closer the real excitation is to the assumed one, better modal parameter estimation one can
expect. Traditionally, wind excitation is considered as a perfect excitation obeying the OMA assumptions.
However, the present study shows that the aeroelastic phenomena due to rotor rotation dramatically changes the
character of aerodynamic excitation and sets limitations on the applicability of OMA to operational wind turbines.
The main purpose of the study is to warn the experimentalists about these limitations and discuss possible ways
of dealing with them.