Investigation on operational stability of main shaft of a prototype reversible pump turbine in generating mode based on ensemble empirical mode decomposition and permutation entropy

Evaluation of the operational stability of the main shaft is of great significance to ensure the security and reliability of the prototype reversible pump turbine (RPT). In the present paper, the experimental study was carried out using the sensors with high accuracy to obtain the shaft displacement signals under different load conditions of the RPT in the generating mode. A set of signal extraction procedure based on ensemble empirical mode decomposition, permutation entropy (PE) and modified wavelet soft-threshold de-noising method is proposed to reduce the influences of the random noises and extract the effective components within the signal. The PE values of the extracted shaft displacement signals are all below 0.3, illustrating that good extraction results have been achieved. Meanwhile, the typical shape evolution of the extracted shaft orbit with load variations at the turbine guide bearing is also depicted in detail. And the PE analysis result of the extracted shaft orbit can effectively reflect the evolution of different internal flow patterns under different load partitions of the RPT, which are 0.33-0.36 for low partial load partition, 0.22-0.30 for medium load partition and 0.30-0.32 for high partial load partition, respectively.