ABSTRACT
Vibration can be a good indicator of the operating conditions of a range of mechanical components and structures and thus can support condition monitoring of important wind turbine components such as the rotor, drive train and tower [1,2]. Analysis of vibration signals in both the time and frequency domain can be used to identify incipient failure of these components, but the vibration sensor and analysis methods for tower and drive train are different. For the tower, because the vibration frequency is quite low, a low frequency sensor (0-200 Hz) and an appropriate model based analysis method are used, while for the drive train bearing and gearbox, a high frequency acceleration sensor (3-20 kHz) and fast Fourier transform (FFT), Cepstrum methods are used [3]. However, there are two difficulties in the application of vibration analysis to wind turbines. First, large-scale wind turbines operate these days in a variable speed mode to optimize performance in relation to time changing wind speed and so the rotational
speed of the rotor, gearbox and generator are changing significantly in time. Because the rotation speed of the gearbox is changing, the width of the vibration sidebands is not fixed, and this creates difficulties in locating the exact locations of gear or bearing faults. It is conventional as in [4] to use order analysis to deal with this problem, or equivalently azimuthal data sampling (rather than fixed time interval sampling) in which the rotor vibration is analyzed based on samples recorded at equidistant rotational angles instead of time equidistant samples. Second, there is strong aerodynamic and vibrational coupling between different turbine components and thus many interconnected factors may influence the vibration signatures. Rotor dynamics and control can for example, significantly influence tower vibration (TV). When the wind speed is above the rated one, the blade angle will normally be adjusted to maintain the rated power. This will result in changes to the aerodynamic forces acting on the rotor, and thus can lead directly to changes in tower vibration (both frequencies and amplitudes). It therefore makes sense to analyze vibration in wider context.
