Hydroturbines are the key equipment in hydroelectric power stations, and their monitoring and diagnosis play an important role in the asset management and maintenance. Hydraulic pressure fluctuation is inevitable during the operation of hydroturbines. It is characterized by nonstationarity during transient processes. The hydraulic pressure fluctuation signal measured in the draft tube of an axial-flow hydroturbine during a start-up transient process is analyzed based on Hilbert-Huang transform. The frequency components and their time evolution with the operating conditions are identified. A dominant low frequency component waxing and waning around a constant smaller than the rated rotating frequency exists throughout the transient process, indicating the possible occurrence of vortex core. Both the amplitude and the frequency are time varying, and are closely related to the operating conditions, especially the guide vane opening and switch, but they are not dominated by the speed. The closure and opening of guide vane and the on-off switching of hydroturbine can result in large amplitude pressure fluctuation, even the pressure surges or impulses in draft tube, which may influence the strength of some parts and destroy the operating stability. Compared with conventional time-frequency analysis methods, such as short time Fourier transform and wavelet transform, Hilbert-Huang transform is more suitable to analyze the nonstationary hydraulic pressure fluctuation signals of low frequency.
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