A study of rotational loads on the supporting structure of an onshore wind turbine by time-frequency analysis

The structural health monitoring (SHM) is of primary importance among all the monitoring systems, because the integral functionality of the wind turbine relies heavily on the robustness of the supporting structure. From one aspect of SHM, traditional frequency domain tools limit the precise interpretations of the in-situ vibration signals of an onshore wind turbine. In this study, therefore, by time-frequency analysis, we try to identify the structurally, as well as mechanically, dynamic characteristics of an onshore wind turbine (Zephyros Z72, 1.5 MW), based upon the vibration signals of the supporting structure under parked (idling) and operational states. The dynamic signatures of the structure could be estimated from the vibration signals by system identification tools. However, several non-structurally dynamic patterns appear unexpected on the time-frequency representation while analysing the vibration signals by Short-Time Fourier Transform (STFT). Hence, STFT, equipping with the concepts of mechanical vibration and rotor dynamics, are used as the tools to push the implicit clues discovered on that representation to the explicit illustrations of events probably occurred on the wind-power generating machinery, such as the run-up of the rotor, blade shadowing effects and the subsequent beating phenomena over its supporting structure.