Current-Aided Order Tracking of Vibration Signals for Bearing Fault Diagnosis of Direct-Drive Wind Turbines

Vibration monitoring is one of the most popular, effective, and reliable methods for bearing fault diagnosis. A key issue in the vibration monitoring for the bearings used in variable-speed wind turbines is the elimination of the effect of the turbine shaft speed fluctuation in the vibration signals measured under varying-rotating-speed conditions. This paper proposes a new current-aided vibration order tracking method for bearing fault diagnosis of variable-speed direct-drive (i.e., no gearbox) wind turbines. The method explores a new simple and effective approach to acquire the reference signal from a current signal measured from the stator of the generator for vibration order tracking. First, the instantaneous fundamental frequency of the current signal is estimated in the time-frequency domain to obtain the shaft rotating frequency. Then, the shaft phase-time relationship is established. With this information, the envelope of the synchronously recorded vibration signal is subsequently resampled at the equal-phase-increment time points. Finally, bearing fault diagnosis is performed by observing the peaks at bearing characteristic frequencies in the power spectrum of the resampled vibration envelope signal. The proposed method is validated by successful diagnosis of different bearing faults in a direct-drive wind turbine under varying-speed conditions.