Displacement of shaft during hydropower generator air gap measurements

Abstract

Abstract. The objective of the study is to discuss the specific requirement of the hydropower generator health condition monitoring standard, which suggests taking into account displacement of the hydropower generator shaft when measuring the air gap. This paper quotes some research, where EMF current is used to evaluate the air gap variation considering the shaft displacement. To verify the proposed methodology, three steps were completed in this paper. Firstly, the correlation between EMF and the air gap was studied in different modes and the percentage of the change of the EMF determined by the change in the air gap was calculated. Secondly, the force vector resulting from uneven air gap is obtained using imaginary vector summation. Finally, the real values of the shaft displacement are presented and discussed. The results show that correlation between EMF and the air gap is strong, but there are also other factors that affect the EMF current, not only the air gap size. The force vector could change depending on the generation mode, but not for all units. Finally, the research shows that for the generator in healthy condition (using ISO standards as a reference for acceptable shaft displacement) the shaft displacement is only a small part of the air gap variation, approximately 2 %. However, the discussion part shows that it matters a lot, which evaluation standard is used, since 2 % of variation is a great value, when only 3 % is acceptable. To address this problem an alternative standard for dynamic air gap measurement and evaluation is quoted.